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7 Nanotechnology: Perspective for Environmental Sustainability M.H. Fulekar, Bhawana Pathak, and R.K. Kale Abstract “Environmental nanotechnology” is considered to play a key role in the shaping of current environmental engineering and science. The conven- tional environmental remedial techniques seem to be relatively ineffectual in the face of currently extensively expanding load of pollutants that permeate the air, water, and soil environment. Nanotechnology can provide a way to purify the air and water resources by utilizing nanoparticles as a catalyst and/or sensing systems. In the present research chapter, the potential of nanotechnological products and processes and their applica- tion to clean up the environment contaminants have been discussed. Water treatment and purification techniques based on nanotechnology have been highlighted. These also include the environmental and energy application of nanotechnology which focuses on clean technology, reducing global warming, eco-friendly and efficient energy-generating techniques, eco- friendly surface coating, remediation techniques, and environmental mon- itoring. Environmental nanoscience products, devices, and processes have an impact on socioeconomic aspects for maintaining a clean environment for sustainable development. Keywords Environmental nanotechnology • Nanoproducts • Clean technology • Eco-friendly coating • Environmental monitoring • Remediation Introduction Nanotechnology is a broad and interdisciplinary area of research and development activity that has been growing exponentially worldwide. M.H. Fulekar () • B. Pathak • R.K. Kale School of Environment and Sustainable Development, Central University of Gujarat, Sector 30, Gandhinagar, Gujarat 382030, India e-mail: [email protected]; [email protected] Nanotechnology will provide the capacity to create affordable products with dramatically improved performance. This will come through basic understanding of works to control and manipulate matter at the nanometer scale through the incorporation of nanostructures and nanoprocesses into technological innovations. A revolution has been occurring in science and technology, based on the recently developed ability to measure, manipulate, and organize matter on this scale. Nanotechnology is M.H. Fulekar et al. (eds.), Environment and Sustainable Development, DOI 10.1007/978-81-322-1166-2 7, © Springer India 2014 87

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Page 1: Environment and Sustainable Development || Nanotechnology: Perspective for Environmental Sustainability

7Nanotechnology: Perspectivefor Environmental Sustainability

M.H. Fulekar, Bhawana Pathak, and R.K. Kale

Abstract

“Environmental nanotechnology” is considered to play a key role in theshaping of current environmental engineering and science. The conven-tional environmental remedial techniques seem to be relatively ineffectualin the face of currently extensively expanding load of pollutants thatpermeate the air, water, and soil environment. Nanotechnology can providea way to purify the air and water resources by utilizing nanoparticlesas a catalyst and/or sensing systems. In the present research chapter, thepotential of nanotechnological products and processes and their applica-tion to clean up the environment contaminants have been discussed. Watertreatment and purification techniques based on nanotechnology have beenhighlighted. These also include the environmental and energy applicationof nanotechnology which focuses on clean technology, reducing globalwarming, eco-friendly and efficient energy-generating techniques, eco-friendly surface coating, remediation techniques, and environmental mon-itoring. Environmental nanoscience products, devices, and processes havean impact on socioeconomic aspects for maintaining a clean environmentfor sustainable development.

Keywords

Environmental nanotechnology • Nanoproducts • Clean technology •Eco-friendly coating • Environmental monitoring • Remediation

Introduction

Nanotechnology is a broad and interdisciplinaryarea of research and development activity thathas been growing exponentially worldwide.

M.H. Fulekar (�) • B. Pathak • R.K. KaleSchool of Environment and Sustainable Development,Central University of Gujarat, Sector 30, Gandhinagar,Gujarat 382030, Indiae-mail: [email protected]; [email protected]

Nanotechnology will provide the capacity tocreate affordable products with dramaticallyimproved performance. This will come throughbasic understanding of works to control andmanipulate matter at the nanometer scalethrough the incorporation of nanostructures andnanoprocesses into technological innovations. Arevolution has been occurring in science andtechnology, based on the recently developedability to measure, manipulate, and organizematter on this scale. Nanotechnology is

M.H. Fulekar et al. (eds.), Environment and Sustainable Development,DOI 10.1007/978-81-322-1166-2 7, © Springer India 2014

87

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88 M.H. Fulekar et al.

fundamentally changing the way materials anddevices will be produced in the future. The abilityto synthesize nanoscale building blocks withprecisely controlled size and composition andthen to assemble them into large structure withunique properties and functions will revolutionizematerials, manufacturing, and their applications.

Nanoscience and engineering could sig-nificantly affect molecular understanding ofnanoscale processes that take place in theenvironment: the generation and remediationof environmental problems through control ofemissions, the development of new “green”technology that minimize the production of unde-sirable by-products, and the remediation of exist-ing waste sites and streams. Nanotechnology willalso afford the removal of the smallest contami-nants from water supplies and air and the contin-uous measurement and mitigation/prevention ofpollution to clean up the environment.

New technologies offer interesting opportu-nities for sustainable solutions, depending onthe area of application and the framework con-ditions given. The research results show thatthere are varying implications at different life-cycle stages and application levels concerningthe resource consumption. Therefore, NT appli-cations and their implications need to be care-fully studied, taking a life-cycle-wide perspec-tive as well as looking at different levels (e.g.,the production of NT products, the use phaseincluding systemic effects). Using the experiencegained in investigating the sustainability impactsof emerging technologies, nano tech offer greatpromise for delivering new and improved Envi-ronment technology (Carvajal 2004). Nanotech-nology can make a considerable contribution tosustainable development through its combina-tion of economic, ecological, and social bene-fits. Sustainable development means balancingeconomic, ecological, and social requirements inorder to meet present needs without restrictingthe opportunities available to future generation.

Potential of Nanotechnology

Nanoscience is associated with the intimate un-derstanding and control of matter at dimensions

of roughly 1–100 nm and is intrinsically impor-tant because of the genuine expectation that theproperties of materials at this scale will differ sig-nificantly from those of their bulk counterparts.Nanotechnology involves the imaging, measur-ing, modeling, and manipulation of materials atthis length scale. The synthesis and fabrication offunctional nanomaterials with predictive, rationalstrategies is a major focal point of research formany groups worldwide (Xia et al. 2008; Xu et al.2009).

In practice, this effort has entailed thedesign, production, and characterization of amyriad of nanostructures including nanoparticles,nanocubes, nanorods, nanowires, and nanotubes,which maintain fundamentally interestingsize-dependent electronic, optical, thermal,mechanical, magnetic, chemical, and otherphysical properties. From the perspectiveof applications, these structures have wide-ranging utility in areas as diverse as catalysis,energy storage, fiber engineering, fuel cells,biomedicine, computation, power generation,photonics, pollution remediation, and sensing.Tables 7.1 and 7.2 demonstrate nanomaterialsand its properties and applications.

Nanotechnology: MultidisciplinaryAspects

Nanotechnology is the design, characterization,production, and application of structures, devices,and systems by controlling the shape and size atthe nanometer scale; environmental nanotech-nology (E-nano) products can be developed fora wide array of urgently needed environmentalremediation. The nanoparticles/nanostructuresmade by mechanical and/or microbial actionwith fundamental building blocks are among thesmallest human-made objects and exhibit novelphysical, chemical, and biological properties,which has wider application for pollution preven-tion, detection, monitoring, and remediation ofpollutants. Environmental nanotechnology wouldbe the new innovation to remediate and treat thecontaminants to acceptable levels. Environmentalscientists and engineers are already working with

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Table 7.1 Potential nanomaterial, materials, and manufacturing

Nanomaterials Properties/application

Nanotubes (type of nanoparticulate with atleast a roughly cylinder shape and a diameterthat is nanometer scale)

Can be used both as raw materials and as productsMultiwalled nanotubes are already going into compositesIncreased conductivity at much lower filler loadsCarbon nanotubes are in commercial production and said to fit awide range of applications

Nanocatalysis Potentials for speeding up reactionsImproving efficiency of many processes to many foldsIn controlling emissions in industrial processes and automobilesWide applications in fossil fuel industriesOther areas of applications like material production, reducingpollution, medicine production, and chemical industry

Nanocomposites Mainly clay-based for structural application (increased strength) orwith novel propertiesDeveloping nanospore composite materials, synthesis of polymericoptical display substrate materials, nanostructural surfacecomponents, polymer and clay organic–inorganic nanocomposites,embedding and filling technologiesThese are already penetrating the automotive and aerospaceindustries

Nanopowders Nanoparticles of metals/compounds/ceramicsHighly dense materialsPotentials for magnetic, electronic, and structural applications

Nanofilms They can be organic or inorganicHave a wide range of properties, like chemically active, wearresistant, high erosion resistance in hostile environments, ability tofocus x-rays, and the prevention of the transmission of laser lightMonolayers deposited on semiconducting substrates that emitelectrons when sunlight fallson them form the basis of solar energycell applicationsCan be deposited in multilayers, for example, specific magneticproperties useful in magnetic recording with high packaging

Table 7.2 Nanoengineered advanced materials and manufacturing

Advanced materials DescriptionSuper Super materials are “nano-pure” or in other words every atom is exactly where it is supposed to

be. ‘This type of developed material is free from defects and is super strong and could also beNano engineered’ for enhancement of material properties, like diamond bolts and even wingsfor aircrafts

Smart Smart materials have the ability to react to commands. They could change shape, color, density,and physical property. For example, smart paint has the ability to rearrange its atoms to refractthe light differently

Active Active materials are full of nanoscale sensors, computers, and actuators. Using thesecomponents the material can probe its environment, compute a response, and act

Swarms Swarms consist of large numbers of simple nanomachines that work together to achievecomplex goals. These are types of active materials

nanoscale structure to manipulate matter of theatomic or molecular scale that has cut acrossdisciplines of chemistry, physics, biology, andeven engineering (Fig. 7.1).

In research communities, there are statementsthat NT is still not well defined. Some defines it

on the basis of physical dimension of structuresranging from 0.1 to 100 nm; it means differentfor different disciplines of people. Definitions ofNT are as the applications of it. The followingtable outlines the set of definitions related to itsdiscipline (Table 7.3).

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90 M.H. Fulekar et al.

Fig. 7.1 Demonstration of nanotechnologies’ multidisciplinary aspect

Table 7.3 The set of definitions related to its discipline

S. no. Discipline Definition of nanotechnology1. Physics The ability to design and control the structure of an object at all length scales

from the atom up to macroscale2. Biology The core of nanotechnology consists of system in the size range of

nanometers3. Chemistry It is the assembly of molecules or atom to make use of their unique properties

existing at nanoscale (Curtin University of Technology)4. Science fiction Manipulation of individual atoms and molecules to form desired structures or

patterns with novel functionality

Nanotechnology is a result of convergence oftraditional fields of physics, biology, and chem-istry. It depends on the contribution from amongthese and other disciplines. The convergence ofthese traditional fields in the environment andenergy is well illustrated in Fig. 7.2 which alsoshows the scales of the areas of interaction. Thereare considerable debates in the scientific commu-nity about the boundaries of the new disciplinesemerging from this convergence, e.g., betweenmicrotechnology and nanotechnology, but it isbecoming clear that in practice no clear divisioncan be made. Thus, for example, sensors andbiochips at nanoscale need to be packaged forcommercial application using microtechnology.

Nanotechnology and Energy

Nanotechnology has been shown to directlycontribute to the long-term energy sustainabilitythrough the development of energy-saving andrenewable energy technologies. Nanoscience

accounts for the development of a vast varietyof high-performance materials for thermal insu-lation as well as lighter and stronger materialsfor vehicle production and LEDs for applicationin energy-efficient lighting devices and displays.In the field of renewable energy, nanotechnologyhas contributed to the development of a varietyof solar photovoltaic (PV) technologies forelectricity production. PV nanotechnologieshave lower production costs than the expensive,conventional silicon crystalline technology.The thin-film copper–indium–gallium diselenide(CIGS) technology is considered as the mostpromising PV nanotechnology. CIGS cells showvery high conversion efficiency compared tothe rest of the nano-PV technologies, and theirmanufacturing costs are relatively low. One ofthe promising areas of NT is under increasedusage of renewable energies like hydrogen,solar, and cleaner fuels. Currently, conventionaltechnologies provide hydrogen as energy carrierin fuel cells, but there are drawbacks in it,because unfortunately, a convenient hydrogen

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Fig. 7.2 Environmentaland energy applicationsof nanotechnology

storage system does not exist today. The currenttechnologies include compressed gases (likenatural gas, propane), which occupy largervolumes and metal hydrides, which are veryheavy and therefore result in a reduced drivingrange for automobiles (National RenewableEnergy Laboratory, n.d.). CNTs are essentiallytiny, lightweight cylinders with diameters thesize of several hydrogen molecules, and they canprovide a solution to the storage problems.

Research carried showed that these CNTsare capable of adsorbing hydrogen (Dillonet al. 1999), and this indicates that CNTsare the ideal building blocks for constructingsafe, efficient, and high-density adsorbents forhydrogen storage application like fuel cellsand batteries in electronic and automobileapplications. For example, these nanotubes havebeen reported to store up to 20 l of hydrogenper gram. This has significant implicationfor reducing costs – transporting and storinghydrogen (Khan et al. 2003). CNTs absorbtrace quantities of CO2 present in natural gas,so researchers are currently developing CNTmembranes that could revolutionize naturalgas purification technology. And also thereare ongoing research to make use of CNTs fordesalination and treatment of wastewater. Thesenanotubes can enhance water treatment and alsoout complete reverse osmosis or distillationprocesses using at least 10 times less energythan reverse osmosis and at least 1,000 times

less energy than distillation (Bruns 2000). Itis envisaged that NT will show potentiallysignificant impacts in environmental and energyapplications. These include developing newgreen processing technologies that minimizethe generation of undesirable by-products. Itcan be used to monitor and remediate existingwaste sites and polluted wastewater resources.Nanotechnology, in particular nanofabrication,offers a variety of tools to contribute for solvingthe energy crisis, since creating materials anddevices smaller than 100 nanometers (nm) offersnew ways to capture, store, and transfer energy.The level of control that nanofabrication providescould help to solve many problems that the worldis facing related to current energy generationtechnologies, including the array of alternative,renewable energy approaches. The conversionof primary energy sources, i.e., the sun intoelectricity, heat, and kinetic energy, can be mademore efficient and environmentally friendly usingnanotechnology. Producing electricity throughthe conversion of sunlight, known as solarphotovoltaics (or solar cells), is a field wherenanostructured materials and nanotechnologyare contributing greatly. Successful researchcould result in a significant reduction of themanufacturing cost of these solar cells and alsoimprove efficiency. Cell types being investigatedinclude thin-layer solar cells, dye solar cells,or polymer solar cells. The use of a layer ofquantum dots – tiny blobs of one semiconductor

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grown on the surface of another, added behindthe conventional multilayer compound – isalso being investigated. It is anticipated thatnanotechnology will help to develop the idealsolar cell, incorporating optimum structure anddesign.

Nanofabrication of materials is also beingused in other energy conversion processeswhere specific, extreme conditions need tobe withstood, such as heat-resistant turbinematerials. Coal-fired power stations can also bemade more environmentally friendly using nano-optimized membranes, which separate out andstore the carbon dioxide. Thermoelectric energyconversion using nanostructured semiconductorspromises increases in efficiency that couldpave the way for a broad application in theutilization of waste heat, e.g., from car or humanbody heat for portable electronics in textiles.Hydrogen fuel cell technology is another areawhere nanotechnology can be applied to improveefficiency. Other renewable energy sources arealso being “improved” using nanotechnologyincluding wind energy, using lighter, moredurable nano-based materials for rotor blades;geothermal, using nano-coatings and compositesfor wear-resistant drilling equipment; hydro-/tidal power, using nano-coatings for corrosionprotection; and biomass energy (“biofuels”),using nano-based precision farming to optimizeyields.

Benefits of Nano-Energy

• Reduced energy consumption – By op-timizing/increasing efficiency in energystorage, generation, and conservation,energy consumption will decrease throughnanotechnology applications.

• Environmentally friendly – Nanotechnologycan contribute to “cleaning” up and reducingthe environmental impact throughout the valuechain of the energy sector.

• Cheaper – The use of nanotechnology canreduce the cost of energy production, distri-bution, and storage, since it has the advantageof reducing the amount of power outputs.

Through miniaturization, nanotechnology alsoprovides an opportunity to tailor-make solu-tions.

• Independent power sources – The applicationof nanotechnology in the energy sector couldcontribute to providing alternative sources ofenergy to the national grid.

• Facilitate transition to renewable – The appli-cation of nanotechnology in the energy sectorcould facilitate the transition from fossil fuelsto renewable energy.

Clean Technology

Nanotechnology involves atom-by-atom con-struction; it will be able to create substances andfinished objects without producing the dangerousand messy by-products that most currentmanufacturing processes produce. Nanodeviceswill operate in a liquid containing the necessaryraw materials usually carbon or silicon, withtrace amounts of other elements as needed, andwill simply plug the appropriate atoms in theappropriate places to produce the desired endproduct. Such processes should produce few by-products, and those by-products can be purifiedreadily by other nanodevices and recycled backinto the feedstock (Reynolds 2001). Toxicwaste generated during the various industrialprocesses consists of harmless atoms arrangedinto noxious molecule. With inexpensive energyand equipment able to work at the molecularlevel, these wastes can be converted into harmlessforms (Drexler et al.1991).

In addition, most products of NT and abundantelements like carbon in diamond or diamondoidform will be the basis of most nanomanufac-turing. Products made of such materials will bestrong, which uses smaller amounts of materials,and carbon is an abundant material, meaningthat little in the way, exploration and extractionwill be needed. As a greenhouse remediationmeasure, nanodevices could extract carbondioxide (CO2) form in the air, if desired(Reynolds 2001). Presently, the atom-by-atom-based manufacturing in the industriesis achieving efficient, environment-friendly

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processes by using some nanomaterials likenanocatalysts. The nanocatalysts are improvingthe efficiency and specificity of chemicalreactions, which results in the design of lightand strong materials that can lead to savingenergy and raw materials (ION 2001). Apart fromthis, NT could contribute to cleaner industrialproduction process or products, mainly throughthe use of raw materials and energy. These trendsincludes miniaturization of devices design andnew catalyst at the manoscale level.

Nanomaterial could also help to developgreener technologies, hence reducing CO2

emissions and energy consumption for buildingsand transport. For example, US buildings accountfor about 40 % of the country’s total energyconsumption and are responsible for 39 %of CO2 emissions. Transport’s contributionamounts to another 33 %. By combiningthe use of light-emitting diodes (LEDs) andsuper-insulating and self-cleaning windows,together with the implementation of morepowerful rooftop photovoltaic (PV) panels andnanosensors, monitoring energy usage, energyefficiency, and environmental sustainability ofbuildings would be enhanced. In particular,the improvement of solar photovoltaic (PV) islinked to the development of new materials withhigher efficiency in solar energy conversion.Nanostructured organic photovoltaics can nowreach a power efficiency of 8 %. Althoughstill lower than traditional silicon-based PV,given their high flexibility and relatively cheapindustrial production, these organic PV panelshave found applications in many sectors, rangingfrom buildings to transport and electronicequipment. Improved recycling in industrialprocesses would enable the recovery of precioustransition metal ions from aqueous solutions andmixtures, in parallel to a decrease in solid wasteand greenhouse gas (GHG) emissions. Suchrecycling could be achieved through recoveryand reuse of magnetic nanoparticles via magneticseparation.

Furthermore, highly porous nanomaterialsencompassing metal organic and zeolitic imida-zole frameworks capture large amounts of CO2,in parallel to metallic iron nanoparticles which

degrade organic contaminants like chlorinatedhydrocarbons. The development of new catalyticnanoparticles for transport technology, at lowercost than the currently used platinum-basedcatalytic converters, would guarantee cleaneremissions. New nanotechnologies based oncarbon nanotubes (CNTs), leading to newmaterials between 10 and 56 times stronger thansteel but significantly lighter, would reduce fuelconsumption (PNNL 2002).

Nano-Enhanced Green IndustryTechnologies

Environmentally friendly catalysis is possibleusing cooperative catalytic systems incorporatedin mesoporous silica nanoparticles (MSNs).Nano Scientists have described how catalyst workinside the pore of MSNs and tailored to performa series of reactions – for instance, to synthesizebiodiesel from free fatty acids in vegetableoils. These catalytic MSNs are environmentallybenign and easy to recycle. It is possible to tailorthe activity and stability of nanoparticle catalystsby modifying their port structure. Researcher’sgoal is to make stable catalysts for use inremoving poisonous carbon monoxide from theair in places such as mining shafts. Scientists havedescribed a promising method for synthesizingnanogold and nanopalladium catalysis in anordered mesoporous support structure withcontrollable pore sizes. The synthesis can bedone at low temperatures using microwaves. Anovel nonporous sorbent effectively removesmercury and other toxic heavy metals fromwastewater generated during offshore oil andgas platform drilling. The material, which iscalled thiol-SAMs, removes 99 % of mercuryfrom gas-condensate liquid containing 800 ppmof mercury.

Polymer nanospheres offer a new way to se-lectively detect hazardous materials in aquaticenvironment. Scientists have described how toprepare polymer nanospheres that change theirshape and optical properties whenever a certainchemical is present. The change can be measuredby surface plasma on resonance, which enables

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94 M.H. Fulekar et al.

the detection of pollutants at the level of parts perbillion (ppb). The nanospheres can be tailored tosense a specific chemical by molecular imprint-ing of the polymers. The method could proveparticularly useful for detecting pharmaceuticalsand other emerging pollutants in waterways thatare currently difficult to detect.

Nano-Enhanced CleanupTechnologies

Magnetic nanoparticles could become an im-portant green tool for removing arsenic fromdrinking water, particularly for point-of-use treat-ment in developing countries. Nanoparticles ofiron oxide bind strongly and specifically to ar-senic and can be separated out of solutions usingmagnets. Scientists determined that particles of12 nm size removed 99.2 % of the arsenic insolution. Zerovalent nanoparticles of iron andmagnesium more effectively degrade heavy met-als and organic solvents in water sediments whenthey are combined with emulsion liquid mem-branes. The membranes increased the contactbetween these catalytic nanoparticles and the tar-geted pollutants. This is a promising remediationtechnology.

Eco-Friendly Coating: PollutionPrevention

Pollution prevention is reducing or eliminatingwaste at the source by modifying productionprocesses, promoting the use of nontoxic or less-toxic substances, implementing conservationtechniques, and reusing materials rather thanputting them into the waste stream. The uniqueand potentially useful properties of nanomaterialshave dramatically increased surface areas andreactivity that improved strength and weightratios and increased electrical conductivityand changes in color and opacity. Materialsdesigned to take advantage of these propertiesare finding application in a variety of areas,such as electronics, medicine, and environmentalprotection.

Nanoproducts

Examples of products with potential for prevent-ing pollution include coatings that are free ofvolatile organic compounds and diisocyanates,safer surfactants, and self-cleaning surfaces. Nan-otechnology and nanomaterials can help to createalternatives to light-emitting or absorbing appli-cations that previously relied upon heavy metal-based semiconductors. Nanocomposites may beused in a variety of products, resulting in reducedneed for addition of flame-retardant chemicals. Inaddition, products including a variety of tools, au-tomobile and airplane components, and coatingscan be made harder and more wear, erosion, andfatigue resistant than conventional counterparts.Examples are as follows:(a) Alternatives to Chrome VI Coatings: Chrome

and cadmium plating provides very effectivesurface treatments for wear and corrosionreduction. Electrolytic hard chrome (EHC)is applied using the reduction of hexavalentchrome in a solution of chromic acid. TheEHC plating process is a less efficient pro-cess which results in vigorous gas evolu-tion at the electrode. This results in airbornemist which contains hexavalent chrome, aknown carcinogen. Several alternative tech-nologies exist to coat a substrate with metalwithout using electrolytic solutions or plat-ing baths. These technologies do not elimi-nate the use of metal coatings, but they doeliminate the use of nonmetal toxic com-ponents such as cyanide from the platingprocess. They can also reduce the amountof metal-contaminated wastewater and sludgethat is generated from plating. These alterna-tive technologies include thermal spray coat-ing, vapor deposition, and chemical vapordeposition and can be used to apply alternatematerials.• Powdermet, Inc., has developed a

combined top-down/bottom-up approachto this problem, utilizing a novel processfor producing nano-featured feedstockmaterials for the alternative technologiesmentioned above. This approach reducesboth the amount of waste produced and

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the amount of post-plating processingrequired resulting in a hard, corrosion-resistant coating.

(b) Energetics, Including Safer Non-leadPrimers to Prevent Pollution: Researchersare currently developing nanoengineeredthermites (nanothermite) with tailoredproperties that are expected to replace leadazide and lead styphnate primers in the nearfuture. Nanothermites are comprised of amixture of fuel and oxidizer nanostructures.Typically Al nanoparticles are used as fueland metal oxides like CuO, MoO3, andBi2O3. Environmentally safe chemicals areutilized for the preparation of these materials.Nanothermites are self-assembled and coatedwith polymers or explosives in nanoscale forapplications such as primers, reactive materi-als, and propellant initiators. These materialsare also integrated with microdevices forvarious applications such as microthrusters,power generators, micro-shock generators fordrug delivery, and microinitiators.

(c) Nanoscale Thermoelectric Materials andDevices: Increasingly, the ability to providepower and cooling is a major factor in howlarge-scale computer systems, as in Internetdata centers, are designed and managed.This stems from the fact that advanced high-performance electronics in many situationsare currently limited by the ability to removeheat from the operating chip. This heat istypically concentrated in hot spots in the chip,experiencing very high heat fluxes. Suchthermal problems are also relevant to small-scale computationally intense, wireless, andcommunication devices that rely on advancedalgorithms for voice, video, and error-freedata operations. Component- and system-level heat flux densities are projected toincrease as computers and other digitaldevices add increased functionality. Strate-gies to manage these problems will have aprofound impact on future technology andeconomic growth. Concomitant with thesetechnology issues are the emerging problemsof rising energy costs that are demandinghigher efficiency in automobiles to cooling

systems, as well as in other energy-intensiveindustries. It is worth noting that nearly 60 %of the world’s useful energy is wasted as heat.Thus, it is worth considering the recovery ofeven a fraction of this heat by converting touseful electric power. Complicating this mixis the urgent need for limiting CO2 levels inthe atmosphere to contain global warmingand minimizing ozone-depleting refrigerants.This shows how nanoscale thermoelectricmaterials and device technology can have animpact on such a broad spectrum of emergingtechnological, energy, and environmentalissues.

Nanoprocesses

Processes that could prevent pollution includemore efficient industrial chemical productionthrough the use of nanoscale catalysts and thebottom-up self-assembly of materials, resultingin processing efficiency, reduction of waste inmanufacturing, and stronger materials with fewerdefects. In addition, the ability to enhance andtune chemical activity can result in catalysts thatimprove the efficiency of chemical reactionsin automobile catalytic converters, powergeneration plants, and manufacturing facilities.Examples are as follows:(a) Nanoengineering Materials for Pollution

Prevention: Concrete and Reduced CO2

Emissions. Protecting the built environmentfrom the forces of the natural world withdams and seawalls is an important work,as is protecting the natural environmentfrom the engineered world. But the twenty-first-century engineer should also look tothe natural world as a powerful designpartner and a source of sustainable solutions.A good place to begin is by studying theway natural materials are constructed atthe nanoscale and drawing inspiration fromthem as we engineer our own materials. Forexample, the civil engineer’s constructionmaterial of choice includes concrete, theoldest engineered building material and oneof the most widely consumed materials on

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earth. Each year, 2.1 billion tons of cement,the primary component of concrete, ismanufactured, enough to produce one cubicmeter of concrete for every person alive.Unfortunately, cement is a major source ofatmospheric carbon dioxide largely becauseit is made by burning fossil fuel to heat alimestone and clay powder to 1,500ıC, whichchanges its molecular structure. When thecement powder is later mixed with water andgravel, the invested energy is released intochemical bonds that form calcium silicatehydrates, the glue that binds the gravel tomake concrete. The production of cementaccounts for 7–8 % of all human-generatedcarbon dioxide emissions. If novel cementcan be engineered, whose manufactureproduces only half as much CO2, a significantreduction in total CO2 emissions will beachieved. The application of nanotechnologymay make it possible for all this tohappen.

(b) Stereoselective Green Chemistry StrategiesUsing Crystal-to-Crystal Reactions: TheAdvantages of Molecular Nanocrystals. Ithas recently been shown that photochemicalexcitation of crystalline ketones bearingradical stabilizing substituents on their two ’-positions leads to efficient photodecarbony-lation reactions. Experiments with crystalsof carefully designed carbonyl compoundscan be used to accomplish the formation ofsigma bonds between adjacent quaternarystereogenic centers in good chemical yieldsand with remarkable chemical control. Theexceptional simplicity, chemoselectivity, andstereospecificity of the solid-state reactionshave been combined with well-developedketone chemistry for the preparation ofseveral natural products, including thesesquiterpenes cuparenone, herbertenolide,and touchuinyl acetate. Simple scale-upstrategies based on nanocrystals and the useof sunlight suggest a remarkable potentialfor specialty chemical applications for thesecrystal-to-crystal reactions.

(c) Nanotechnology as a Tool to AdvancePollution Prevention in the Semiconductor/IT

Industry. Nanotechnology is not new to thesemiconductor/IT (information technology)industry. Many features in semiconductorsand integrated circuits have been at thenanoscale for decades. The InternationalTechnology Roadmap for Semiconductorsidentifies nanotechnology, both traditionalsemiconductor nanotechnology and novelnanomaterials, as a critical key to the futuredevelopment of the industry, both in ex-tending the current CMOS (complementarymetal oxide semiconductor) scaling and inpositioning beyond the current CMOS plat-form. The expanded use of nanotechnologyand the incorporation of nanomaterials intosemiconductor/IT processes and productsnot only provide potential solutions to manytechnical and business challenges facing theindustry, but they also offer a tremendousopportunity to advance pollution prevention.Examples of nano-applications in semicon-ductor/IT processes and products that mayresult in energy and resource conservation,waste minimization, and energy-efficientproducts include bottom-up self-assembly,nanoarchitecture, and nanophotonics.

Environmental Monitoring

Environmental monitoring plays an importantrole in natural and resource conservation. It iscrucial for environmental policy and research,because it provides policymakers, scientists, andthe public with the data needed to understand andimprove the environment.(a) Air Monitoring: Conventional air pollution

monitoring employs large, fixed systems,which often fail to detect “hot spot” pollutionpeaks (Court et al. 2004). Solid-state gassensors (SGSs), based on nanocrystallinemetal oxide thin films, provide fasterresponse with real-time analysis capability,higher resolution, simplified operation, andlower running costs compared to conven-tional methods, such as chemiluminescenceand infrared spectrometry (Rickyerby andMorrison 2006).

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(b) Water Monitoring: When the EU WaterFramework Directive was implemented in2000, new regulations for water monitoringof organic substances were imposed andmeasurements had to be done downto microgram-per-liter (�gL�1) levels.Such accuracy, however, was difficult toachieve with conventional water monitoringtechnologies, and a necessity for faster,more sensitive systems appeared. This ishow the automated water analyzer computer-supported system (AWACSS) was created.

(c) Microbial Monitoring and Detection: Theuse of QDs as a fluorescence labeling systemin microbial detection has been successfullydemonstrated. Thiolated CdSe-core QDscould be conjugated with wheat germagglutinin (WGA), a lectin that is commonlyfound in gram-positive bacteria (Kloepfer etal. 2003). By reacting with bacterial cells,this QD-conjugated WGA can bind to sialicacid and N-acetylglucosaminyl residues onbacterial cell walls. QDs can also be biocon-jugated with a substrate such as iron, whichis essential for the growth of pathogenicorganisms inside a human host. Pathogenicbacteria normally contain receptors for ahuman host’s own shuttle protein transferrinand can harvest iron from transferrin (Modunet al. 2000). Using this metabolism-specificapproach, transferrin-conjugated QDs canbe transported through the membrane intometabolically active cells of iron-deprivedStaphylococcus aureus and detected underfluorescence excitation. In contrast, no QDsignal is observed in nonpathogenic bacteria.QDs can be further conjugated with specificantibodies to detect pathogenic microorgan-isms such as Cryptosporidium parvum andGiardia lamblia (Zhu et al. 2004).

The detection and monitoring of microor-ganisms can be further accelerated usingnanoparticles in a fluorescence labeling system inmicrofluidic devices. Nanoparticle-based fluores-cence reporting systems can be further developedto achieve rapid bacterial detection at the single-cell level (Zhao et al. 2004). To do so, thefluorescence intensity of individual nanoparticles

was greatly increased through the encapsulationof thousands of fluorescent dye molecules ina protective silica matrix. After modifying thesurface of these silica nanoparticles, they wereconjugated with antigens specific to Escherichiacoli strain O157 and used in immunologicalassays. With the improvement in the fluorescencereporting system, the fluorescence intensityemitted by one E. coli O157 cell was sufficientto be detected using a normal spectrofluorometerin a conventional plate-based immunologicalassay or to be accurately enumerated using alaboratory-made flow cytometer within 60 s ofsample preparation.

Nanoparticles can be further used to immobi-lize microbial cells that can degrade or biorecoverspecific chemicals (Shan et al. 2005). Unlike con-ventional cell immobilization on micron-sizedmedia or a fixed surface, magnetic nanoparti-cles (i.e., Fe3O4) were functionalized with am-monium oleate and coated on the surface ofPseudomonas delafieldii. By applying an externalmagnetic field to these microbial cells, thesemagnetic nanoparticle-coated cells were concen-trated at a specific location on the reactor wall,separated from the bulk solution, and recycledfor the treatment of the same substrate. Thesemicrobial cells were added into a bioreactor ata high biomass concentration and were demon-strated to desulfurize organic sulfur from fos-sil fuel (i.e., dibenzothiophene) as effectively asnon-nanoparticle-coated cells (Shan et al. 2005).In environmental research, these nanoparticlesfurther enhance the detection sensitivity of micro-bial monitoring and the degradation and recoveryefficiency of chemicals.

Water Treatment–BasedNanotechnologies

Water Treatment TechnologiesInvolves the Following

(a) Filtration: Filtration involves removing thepollutants through a straining mechanism.Nanoparticles enhance traditional filtration

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98 M.H. Fulekar et al.

methods because they are more stronglyattracting the pollutants as they pass throughthe filter.

(b) Desalination: Desalination is a very specificwater treatment that focuses on creatingfreshwater from saltwater. Given that mostof the Earth is covered with saltwater,desalination could be an important processto ensure that the world has enough drinkingwater. Desalination is often conducted usingreverse osmosis.

(c) Disinfection: Waterborne pathogens are themajor cause of diseases and infection. A com-mon disinfection technique uses powders orchlorine-based chemicals to kill bacteria inwater. Nanotechnology can improve the dis-infection capabilities of these chemicals, andnew nanotechnology applications can kill or-ganisms that are developing resistance to tra-ditional disinfection techniques.

(d) Photocatalysis: This application uses light toclean the water. As opposed to other tech-niques that simply collect pollutes from adirty source, photocatalysis actually degradesthe pollutants. In this process, a catalyst, oftentitanium dioxide, is placed into the dirty wa-ter. When UV light hits the particle, it begins achemical reaction that degrades the pollutantsin the system.

(e) Remediation: In general remediation refers toremoving heavy metals and other pollutantsfrom water that was contaminated due toindustrial processes. Remediation processeshave to be quick and cheap in order to cleanthe effluent as it leaves industrial plants.

(f) Sensors: It is important that pollutants bedetected in water sources so that they can betreated. Nanosensors promise to be a cheapand easy method to detect whether a watersource has been contaminated.

Water Purification

Water must be purified in order to remove harm-ful materials and make it suitable for human uses.Contaminants can include metals like cadmium,copper, mercury, lead, nickel, zinc, chromium,

and aluminum; nutrients include phosphate, am-monium, nitrate, nitrite, phosphorus, and nitro-gen, and biological elements such as bacteria,viruses, parasites, and biological agents fromweapons. UV light is an effective purifier, but itis energy intensive, and application in large-scalesystems is sometimes considered cost prohibitive.Chlorine, also commonly used in water purifica-tion, is undesirable because its production is oneof the world’s most energy-intensive industrialprocesses, consuming about 1 % of the world’stotal electricity output. Nanotechnology is open-ing new doors to water decontamination, purifica-tion, and desalinization and providing improveddetection of waterborne harmful substances.

For example, iron nanoparticles have highsurface area and reactivity and can be used todetoxify carcinogenic chlorinated hydrocarbonsin groundwater. They can also render heavy met-als like lead and mercury insoluble, reducing theircontaminations. Dendrimers, with their sponge-like molecular structure, can clean up heavy met-als by trapping ions in their pores. Nanoscalefilters have a charge membrane enabling themto treat both metallic and organic contaminantions via both steric filtration based on the size ofopenings and Donnan filtration based on electri-cal charge. They can also be self-cleaning.

Gold nanoparticles coated with palladiumhave proven to be 2,200 times better thanpalladium for removing trichloroethylenefrom groundwater. In addition, photocatalyticnanomaterials enable ultraviolet light to destroypesticides, industrial solvents, and germs.Titanium dioxide, for example, can be usedto decontaminate bacteria-ridden water. Whenexposed to light, it breaks down bacterialcell membranes killing bacteria like E. coli.Purification and filtration of water can alsobe achieved through nanoscale membrane orusing nanoscale polymer “brushes” coated withmolecules that can capture and remove poisonousmetals, proteins, and germs (Elvin 2007).

A new sterilizer, the RVK-NI, mixes ozonenanobubbles with oxygen microbubbles to pro-duce, according to manufacturer Royal Electric,almost completely bacteria-free water for foodprocessing. Ozone gas is a naturally occurring

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type of oxygen that is formed as sunlight passesthrough the atmosphere; it can be generated artifi-cially by passing high-voltage electricity throughoxygenated air. Because ozone is an unstablehighly reactive form of oxygen, it is 51 timesmore powerful than chlorine, the oxidizer usedby most food processers. With it, manufacturerscan forego the use of environmentally harmfulchlorine or other chemicals used in conventionalwater disinfection process. The ozone process isalso said to kill bacteria and other microbes 3,000times faster than chlorine. Desalination is anothercritical area of water purification. Dais AnalyticCorporation, for example, is currently preparingits nanoclear desalinization process for commer-cialization. Low-cost techniques for water purifi-cation, self-cleaning, evaporation, reduction, anddesalination could have tremendous impact byproviding adequate supplies of clean water.

Water Treatment

Conventional water treatment technologies in-clude filtration, ultraviolet radiation, chemicaltreatment, and desalination, whereas the nano-enabled technologies include a variety of differ-ent types of membranes and filters based on car-bon nanotubes, nanoporous ceramics, magneticnanoparticles, and other nanomaterials.

In a recent study, several polymeric nanofiltra-tion and reverse osmosis membranes were testedfor the treatment of brackish groundwater (waterthat is salty, but less so than seawater). The testsshowed that nanofiltration membranes can pro-duce potable water from the brackish groundwa-ter. As expected, the reverse osmosis membranesremoved about 99 % of all the solutes, but theconcentrations of essential nutrients, such as cal-cium and magnesium ions, were reduced to levelsthat were below the specifications of the WorldHealth Organization standard for drinking water.The product water therefore had to be spiked withthese nutrients to provide drinking water of therequired quality.

These studies also underline the importance ofmaking communities aware of the actual qualityof their drinking water because it is not possible

to detect contaminants, for example, by simplyobserving the physical properties of the water(i.e., smell, taste, and color).

However, it is not enough to develop technicalsolutions to these problems; the technology mustalso be transferred to the country that needsit. To be effective, technology transfer must beaccompanied by technology adaptation and tech-nology adoption to take account of the technicalcapability, infrastructure, and market potential ofthe developing country that needs the technology.

Nanofiltration

Nanofiltration is a relatively recent membranefiltration process, which holds promise to delivercost-effective water and air treatment solutions.Nanomembranes (NMs) are used not only toremove contaminants from polluted water and airbut also for desalination of salty water. Thereare two types of NMs currently available onthe market: nanofilters, using either carbon nan-otubes (CNTs) or nanocapillary arrays to me-chanically remove impurities, and reactive NMs,where functionalized NPs chemically convert thecontaminants into safe by-products.

Ordered arrays of densely packed, verticallyaligned CNTs are used as membranes to filterout water impurities while letting the water flowfreely through the filter. Carbon nanotube mem-branes (CNMs) are able to remove almost allkinds of contaminants including bacteria, viruses,and organic pollutants. CNMs are also effectivein desalinating salty water.

Until now, several NM types, which effec-tively remove CO2 from industrial flue gases,were developed from both polymeric and inor-ganic materials (i.e., carbon-based membranes,mesoporous oxide membranes, and zeolite mem-branes). These membranes show better selectivityand higher removal capacity than their conven-tional alternatives, and they are often more cost-efficient (Fujiokya et al. 2007). The large-scaleindustrial application of the CO2-removing NMswould contribute to the reduction of the anthro-pogenic CO2 emissions in the atmosphere andimpede climate change.

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Recently, enzymatic quorum quenching (inthe form of a free enzyme or an immobilizedform on a bead) was successfully appliedto a submerged membrane bioreactor witha microfiltration membrane for wastewatertreatment as a novel approach to controlmembrane biofouling. In this study, a quorum-quenching enzyme (acylase) was directlyimmobilized onto a nanofiltration membrane tomitigate biofouling in a nanofiltration process. Ina flow cell experiment, the acylase-immobilizedmembrane with quorum-quenching activityprohibited the formation of mushroom-shapedmature biofilm due to reduced secretion ofextracellular polymeric substances (EPS). Theacylase-immobilized membrane maintained morethan 90 % of its initial enzyme activity formore than 20 iterative cycles of reaction andwashing procedure. In the lab-scale continuouscrossflow nanofiltration system operated at aconstant pressure of two bar, the flux withthe acylase-immobilized nanofiltration (NF)membrane was maintained at more than 90 %of its initial flux after a 38 h operation,whereas that with the raw NF membranedecreased to 60 % accompanied with severebiofouling. The quorum-quenching activityof the acylase-immobilized membrane was

also confirmed by visualizing the spatialdistribution of cells and polysaccharides on thesurface of each membrane using confocal laserscanning microscope (CLSM) image analysistechnique.

Metal Ion Recovery from Solutionsby Dendrimer Filtration

Diallo et al. (2005) have developed a dendrimer-enhanced ultrafiltration (DEF) system that canremove dissolved cations from aqueous solutionions using low-pressure membrane filtration(Fig. 7.3). DEF works by combining dendrimerswith ultrafiltration or microfiltration membranes.Functionalized and water-soluble dendrimerswith large molar mass are added to an incomingaqueous solution and bind with the target ions.For most metal ions, a change in solutionacidity and/or salinity causes the dendrimersto bind or release the target metal ions. Thus,a two-stage filtration process can be used torecover and concentrate a variety of dissolvedions in water including Cu(II), Ag(I), andU(VI). A key feature of the DEF process is thecombination of dendritic polymers with multiplechemical functionalities with the well-established

Fig. 7.3 Recovery of metal ions from aqueous solutions by dendrimer filtration (Adapted from Diallo 2008)

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separation technologies of ultrafiltration (UF)and microfiltration (MF). This allows a newgeneration of metal ion separation processesto be developed that are flexible, reconfigurable,and scalable. The flexibility of DEF is illustratedby its modular design approach. DEF systemswill be designed to be “hardware invariant” andthus reconfigurable in most cases by simplychanging the “dendrimer formulation” and“dendrimer recovery system” for the targetedmetal ions of interest. The DEF process hasmany applications including the recovery ofvaluable metal ions such as platinum groupmetals, rare-earth metals, and actinides frommineral/hydrometallurgical processing solutions,in situ leach mining solutions, and industrialwastewater solutions.

Research is under way to use advance nan-otechnology in water purification for safe drink-ing. Nanotechnology, the deliberate manipulationof matter at size scales of less than 100 nm, holdsthe promise of creating new materials and de-vices which take advantage of unique phenomenarealized at those length scales, because of theirhigh reactivity due to the large surface to volumeratio (Ichinose et al. 1992). Nanoparticles are ex-pected to play a crucial role in water purification(Stoimenov et al. 2002). The environmental fateand toxicity of a material are critical issues in ma-terial selection and design for water purification.No doubt that nanotechnology is better than othertechniques used in water treatment, but today theknowledge about the environmental fate, trans-port, and toxicity of nanomaterials (Colvin 2003)is still in infancy. Advances in nano scienceand engineering suggest that many of the cur-rent problems involving water quality could beresolved or greatly diminished by using nonab-sorbent, nanocatalysts, bioactive nanoparticles,nanostructured catalytic membranes, submicron,nanopowder, nanotubes, magnetic nanoparticles,granules, flake, high surface area metal parti-cle supramolecular assemblies with characteris-tic length scales of 9–10 nm including clusters,micromolecules, nanoparticles, and colloids havea significant impact on water quality in naturalenvironment (Mamadou and Savage 2005). Nanosensors are used for detection of pesticides (Nair

and Pradeep 2004) and biological agents includ-ing metals (e.g., cadmium, copper, lead, mercury,nickel, zinc), nutrients (e.g., phosphate, ammo-nia, nitrate, nitrite), cyanide organics, algae (e.g.,cyanobacterial toxins) viruses, bacteria, parasites,antibiotics, and biological agents are used for ter-rorism. Innovations in the development of noveltechnologies to desalinate water are among themost exciting and seem to have promise (Diallo etal. 2005). Opportunities and challenges of usingnanomaterials in the purification of surface water,groundwater, and industrial wastewater streamsare a matter of continuing concern. A miscon-ception and one of the many impressions thatpeople have about the future of nanotechnol-ogy is the expectation that nanoparticles can beused to kill harmful organisms, to repair bodytissue, to improve water quality, and to curedisease. Recent applications of nanoparticulatesilver have included open wound and burn treat-ment, and preliminary studies have shown thata 20 ppm silver colloidal suspension (v30 nmdiameter) in purified water has a 100 % curerate for malaria. Titanium dioxide, especially asnanoparticulate anatase, is also an interesting an-tibacterial, with notable photocatalytic behavior.But ultrafine anatase has also been identified ascytotoxic and in vivo studies have shown thatit can be severely toxic in the respiratory sys-tem (Oberdorste 2001; Ishibashi 2000). Nanocap-sules and nanodevices may present new possi-bilities for drug delivery, gene therapy, medicaldiagnostics, antimicrobial activity, etc. The effectof particle size on the adsorption of dissolvedheavy metals to iron oxide and titanium dioxidenanoparticles is a matter of laboratory-scale ex-periments. Iron oxide and titanium dioxide aregood sorbents for metal contaminants. Sphericalnanoparticle have a similar size and shape likeresin beads that are used in water purification.Ligands, fulvic acids, humic acids, and their ag-gregates have a significant impact on contaminantmobility, reactivity, and bioavailability. Nanopar-ticles can also be designed and synthesized toact as either separation or reaction media forpollutants. The high surface area to mass ratios ofnanoparticles can greatly enhance the adsorptioncapacities of sorbent materials. In addition to

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having high specific surface areas, nanoparticlesalso have unique adsorption properties due todifferent distributions of reactive surface sites anddisordered surface regions. Their extremely smallfeature size is of the same scale as the critical sizefor physical phenomena, for example, the radiusof the tip of a crack in a material may be in therange 1–100 nm. The way a crack grows in alarger-scale, bulk material is likely to be differentfrom crack propagation in a nanomaterial wherecrack and particle size are comparable. Funda-mental electronic, magnetic, optical, chemical,and biological processes are also different at thislevel.

Environmental Nanotechnology:Perspectives

Biogenic Synthesis of NanoparticlesUsing Microorganisms

Numerous microorganisms are reported tobiosynthesize gold and silver nanoparticlesby NADPH-dependent reductase enzymes thatreduce metal salts to nanoparticles throughelectron shuttle enzymatic metal reductionprocess (Table 7.4). The pollution detectionnanostructures and/or devices are being usedfor a variety of environmental applications.

Sensors

The characterization of environmental sensorsis based primarily on the physics involved andtheir operating mechanisms. For example, chro-matography relies on the separation of complexmixtures by percolation through a selectivelyadsorbing medium with subsequent detection ofcompounds of interest. Electrochemical sensorsinclude sensors that detect signal changes (e.g.,resistance) caused by an electric current beingpassed through electrodes that interact withchemicals. Mass sensors rely on disturbances andchanges to the mass of the surface of the sensorsduring interaction with chemicals. Opticalsensors detect changes in visible light or other

electromagnetic waves during interactions withchemicals. Within each of these categories, somesensors may exhibit characteristics that overlapwith those of other categories. For example, somemass sensors may rely on electrical excitationor optical settings. The nanostructure materialdeveloped for detection of pollution monitoringand remediation is highlighted in Table 7.5.

Sensors: Biosensors, Electrochemical Sen-sors, Mass Sensors, Optical Sensors, Gas Sensors

Nanotechnology and PollutionControl

Pollution results from resource production andconsumption, which in their current state arewasteful. Nanofabrication holds much potentialfor effective pollution control, but it currentlyfaces many problems that prevent it from masscommercialization, particularly its high cost.Nanotechnology plays a vital role in air and waterpollution control. Air pollution can be remediatedusing nanotechnology in several ways. One isthrough the use of nanocatalysts with increasedsurface area for gaseous reactions. Catalysts workby speeding up chemical reactions that transformharmful vapors from cars and industrial plantsinto harmless gases. Catalysts currently in useinclude a nanofiber catalyst made of manganeseoxide that removes volatile organic compoundsfrom industrial smokestacks. Other methods arestill in development. Another approach usesnanostructured membranes that have pores smallenough to separate methane or carbon dioxidefrom exhaust. John Zhu of the University ofQueensland is researching carbon nanotubes(CNTs) for trapping greenhouse gas emissionscaused by coal mining and power generation.CNT can trap gases up to a hundred timesfaster than other methods, allowing integrationinto large-scale industrial plants and powerstations. This new technology both processesand separates large volumes of gas effectively,unlike conventional membranes that can only doone or the other effectively.

As with air pollution, harmful pollutants inwater can be converted into harmless chemicals

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Table 7.4 Biogenic synthesis of gold and silver nanoparticles by various microorganisms

Microbe Location Size range (nm)Silver (Ag) nanoparticlesBacteriaPseudomonas stutzeri Intracellular 200Morganella sp. Extracellular 20–30Lactobacillus strains IntracellularPlectonema boryanum (cyanobacteria) Intracellular 1–10, 1–100Klebsiella pneumoniae Extracellular 5–32YeastMKY3 Extracellular 2–5FungiPhoma sp.3.2883 Extracellular 71.06–74.46Verticillium Intracellular 25˙12Aspergillus fumigatus Extracellular 5–25Trichoderma asperellum Extracellular 13–18Phanerochaete chrysosporium Extracellular 50–200Gold (Au) nanoparticlesBacteriaLactobacillus strains IntracellularShewanella algae Intracellular, pH 7 10–20Extracellular, pH 1 1–50Escherichia coli DH5 Intracellular 25–33Thermomonospora sp. Extracellular 8Rhodococcus sp. Intracellular 5–15FungiFusarium oxysporum Extracellular 20–40AlgaeSargassum wightii Extracellular 8–12Chlorella vulgaris 9–20

Table 7.5 Pollution detection and sensing – nanostructure material

Nanostructure material FunctionSilver nanoparticle array membranes Water quality monitoringCarbon nanotubes (CNTs) Electrochemical sensorsCNTs as a building block Exposure to gases such as NO2, NH3, or O, the electrical resistance

of CNTs changes dramatically, induced by charge transfer with thegas molecules or due to physical adsorption

CNTs with enzymes Establish a fast electron transfer from the active site of the enzymethrough the CNT to an electrode, in many cases enhancing theelectrochemical activity of the biomolecules

CNT sensors Developed for glucose, ethanol, sulfide, and sequence-specific DNAanalysis

Magnetic nanoparticles coated with antibodies Useful for the rapid detection of bacteria in complex matrices

through chemical reactions. Trichloroethene,a dangerous pollutant commonly found inindustrial wastewater, can be catalyzed andtreated by nanoparticles. Studies have shownthat these materials should be highly suitable ashydrodehalogenation and reduction catalysts forthe remediation of various organic and inorganicgroundwater contaminants. Nanotechnology

eases the water-cleansing process becauseinserting nanoparticles into underground watersources is cheaper and more efficient thanpumping water for treatment. The deionizationmethod using nanosized fibers as an electrodeis not only cheaper but also more energyefficient. Traditional water filtering systems usesemipermeable membranes for electrodialysis

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or reverse osmosis. Decreasing the pore sizeof the membrane to the nanometer rangewould increase the selectivity of the moleculesallowed to pass through. Membranes that caneven filter out viruses are now available. Alsowidely used in separation, purification, anddecontamination processes are ion exchangeresins, which are organic polymer substrate withnanosized pores on the surface where ions aretrapped and exchanged for other ions (Alchin2008). Ion exchange resins are mostly usedfor water softening and water purification. Inwater, poisonous elements like heavy metalsare replaced by sodium or potassium. However,ion exchange resins are easily damaged orcontaminated by iron, organic matter, bacteria,and chlorine.

Recent developments of nanowires made ofpotassium manganese oxide can clean up oil andother organic pollutants while making oil recov-ery possible (Yuan et al. 2008). These nanowiresform a mesh that absorbs up to 20 times itsweight in hydrophobic liquids while rejectingwater with its water-repelling coating. Since thepotassium manganese oxide is very stable even athigh temperatures, the oil can be boiled off thenanowires and both the oil and the nanowires canthen be reused (Yuan et al. 2008).

Removal of Pollutant by Adsorbent

The nanoparticles have been investigated as ad-sorbent for the removal of organic and inorganiccontaminants. The nanosized metal oxides andnatural nanosized clays have been investigatedfor the removal of metals and inorganic ions.Besides, oxidized and hydroxylated CNTs aregood absorbers for metals such as Cu, Ni, Cd,and Pb. Pristine multiwalled CNTs have beenfound to be stronger adsorber materials fororganometallic compounds. CNTs have alsobeen found as a powerful adsorbent for a widevariety of organic compounds from aquaticenvironment, which include dioxin, polynucleararomatic hydrocarbons (PAHs), DDT and itsmetabolites, PBDEs, chlorobenzenes andchlorophenols, trihalomethanes, bisphenol A and

nonylphenol, phthalate esters, dyes, pesticides,and herbicides such as sulfuron derivatives,atrazine, and dicamba. Nanoporous polymerswhich have cross-linked and copolymerized withfunctionalized CNTs have been demonstratedfor a high sorption capacity for a variety oforganic compounds such as p-nitrophenol andtrichloroethylene.

Nanoremediation Technologies

Environmental protection and pollution issues arefrequently discussed worldwide as topics thatneed to be addressed sooner rather than later.Nanotechnology can strive to provide and fun-damentally restructure the technologies currentlyused in environmental detection, sensing, and re-mediation for pollution control. Some nanotech-nology applications that are near commercializa-tion include nanosensors and nanoscale coatingsto replace thicker, more wasteful polymer coatingthat prevent corrosion, nanosensors for detectionof aquatic toxins, nanoscale biopolymers for im-proved decontamination and recycling of heavymetals, nanostructured metals that break downhazardous organics at room temperature, smartparticles for environmental monitoring and purifi-cation, and nanoparticles as novel photocatalystfor environmental cleanup.

Environmental cleanup has promoted the de-velopment of highly efficient photocatalysts thatcan participate in detoxification reactions. Envi-ronmental remediation by photocatalysts comeswith several advantages: direct conversion of pol-lutants to nontoxic by-products without the ne-cessity for any other associated disposal steps,use of oxygen as oxidant and elimination of ex-pensive oxidizing chemicals, potential for usingfree and abundant solar energy, self-regenerationand recycling of photocatalyst, etc. A significantamount of research on semiconductor-catalyzedphotooxidation of organic chemicals has beencarried out during the past 15 years. The abilityto catalyze the destruction of a wide variety of or-ganic chemicals and complete oxidation of organ-ics to CO2 and dilute mineral acids in many cases,the lack of inherent toxicity, and the resistance to

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photodegradation at low cost render this processhighly suitable for environmental remediation.

Degradation of Pollutants UsingSemiconductors: TiO2 and ZnOThe semiconductor TiO2 nanoparticles have beenextensively studied for oxidative transformationof organic and inorganic contaminants (Obareand Meyer 2004; Hoffmann et al. 1995). Theseare now used in a variety of products such asself-cleaning glass, disinfectant tiles, and filtersfor air purification (Fujishima et al. 1999). TiO2

electrodes have the capacity to determine thechemical oxygen demand of water and are usedas sensors for monitoring contaminated water(Kim et al. 2001). TiO2 nanoparticles can beimmobilized on different supports which areused for the solar detoxification of water andair. These engineered nanoparticles are knownfor their interaction with organic, inorganic,and biological contaminants such as heavymetals, organochlorine pesticide, arsenic, andphosphates in water, induced by ultraviolet light.TiO2 leads to pollutant degradation throughtwo everyday chemical reactions: reductionand oxidation. Once excited by UV and TiO2

electron–hole pairs develop, these electronshave sufficient oxidizing potential to oxidizepollutants in wastewater (Bahnemann 2004).Interestingly, the combination of UV and TiO2

generates bactericidal activity, which attacksseveral types of bacteria. This approach thusprovides a comprehensive treatment proceduresince chemical species and pathogens can beremoved from wastewater simultaneously. Othernanoparticles with semiconducting properties,such as ZnO, ZnS, F2O3, and CdS, can beused for photocatalysis oxidation. TiO2 isbiologically and chemically inert and hasdemonstrated great resistance to corrosion alongwith the capacity to be used repetitively withoutsubstantial loss of catalytic activity, and it istherefore inexpensive to use. In light of theseproperties, TiO2 is potentially more attractive forenvironmental applications than other oxidativenanoparticles (Pirkanniemi and Sillanpaa 2002).However, TiO2 requires ultraviolet light and,consequently, is effective only for treatment of

transparent wastewater. Several research groupshave attempted to overcome this limitation byextending the nanoparticle excitation into visiblelight with the doping of TiO2 with transitionmetal ions or sensitizing dye such as Ru (II)polypyridyl complex, or investigated an approachinvolving a tube reactor production based onhollow glass tubes (Kamat and Meisel 2003).The tubes are extremely coated with TiO2 andUV light passes through the hollow tubes.Preliminary results suggested that combiningultrasound processes such as sonolysis andphotocatalysis improves pollutant oxidation andcould be an effective approach. Several pilotprojects are under way to evaluate the potentialof solar reactors to generate the energy requiredfor TiO2 excitation and detoxification of pollutedwater. If these attempts are successful, sunlightcould play an economic and eco-friendly role inthe treatment of wastewater.

Nanoparticulate titanium dioxide (TiO2) hasbeen traditionally used in environmental reme-diation because of its low toxicity, high photo-conductivity, high photostability, availability, andlow cost (Wallington 2005). Novel technologiesand improved processes, however, enabled thedevelopment of a variety of Tio2 photocatalyticderivatives. Metals such as copper (Cu), silver(Ag), gold (Au), and platinum (Pt) have beentested for their ability to improve the decontam-ination activity of TiO2 with Cu, for example,accelerates the reduction of hexavalent chromium(CrC6)ions. The coupling of TiO2 with Au orAg results in similar reductive capabilities. TheTiO2-based p–n junction nanotubes (NTs) rep-resent the most recent innovation in the field ofnanophotocatalysts. The NTs contain platinum(Pt) (inside) and TiO2 (on the outside). The TiO2

coating of the tubes acts as an oxidizing surface,while the inside of the tube is reductive (Chengand Cheng 2005). The ability of p–n junction onNTs to destroy toluene was tested by Chen et al.,and the results showed that they exhibit muchhigher decontamination rates than non-nanotubematerials (Cheng and Cheng 2005).

Unlike TiO2, semiconductor nanoparticlessuch as ZnO are able to emit strongly in thevisible region, and the visible emissions of ZnO

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are usually very sensitive to whole scavengerssuch as phenols or iodide ions. ZnO particlesthus seem good candidates for use as sensorsfor chemical compounds. It has been claimedthat sensor system based on ZnO could reach adetection sensitivity of 1 ppm. Furthermore, ZnOhas the ability to induce contaminant degradationunder ultraviolet light (Kamat and Meisel 2003).Consequently, the use of ZnO can be consideredas a promising way to simultaneously sense anddestroy toxic chemicals. It has been reported thatnanostructured ZnO films could simultaneouslydetect and degrade organic compounds in water.Such a catalyst system is useful to inducecontaminant degradation where the system sensesa targeted molecule, thus avoiding destruction ofmolecules present in the environment.

Degradation of Pollutants Using IronNanoparticlesLaboratory research has established thatnanoscale metallic iron is very effective indestroying a wide variety of common contam-inants such as chlorinated methanes, brominatedmethanes, trihalomethanes, chlorinated ethane,chlorinated benzenes, other polychlorinatedhydrocarbons, pesticides, and dyes (Zhang 2003).The basis for the reaction is the corrosion ofzerovalent iron in the environment. Contaminantssuch as tetrachloroethene can readily accept theelectrons from iron oxidation and be reducedto ethane. However, nanoscale zerovalent iron(nZVI) can reduce not only organic contaminantsbut also the inorganic anions nitrate, which isreduced to ammonia (Sohn et al. 2006; Liouet al. 2006) perchlorate (plus chlorate or chlorite)and then reduced to chloride (Cao et al. 2005),selenate, arsenate (Kanel et al. 2006), arsenite(Jegadeesan et al. 2005), and chromate (Ponderet al. 2000; Manning et al. 2007). nZVI is alsoefficient in removing dissolved metals fromsolution, e.g., Pb and Ni (Li and Zhang 2006).The reaction rates for nZVI are at least 25–30 times faster and also sorption capacity ismuch higher compared with granular iron (Liet al. 2006). The metals are reduced to eitherzerovalent metals or lower oxidation states, e.g.,Cr (III), and are surface complexed with the iron

oxides that are formed during the reaction. Somemetal can increase the dechlorination rate oforganics and also lead to more benign products,whereas other metals decrease the reactivity(Lien et al. 2007). Most of the research usingnZVI has been devoted to groundwater and soilremediation. Nanoremediation methods entailthe application of reactive nanomaterials fortransformation and detoxification of pollutants.These nanomaterials have properties that enableboth chemical reduction and catalysis to mitigatethe pollutants of concern. For nanoremediationin situ, no groundwater is pumped out foraboveground treatment, and no soil is transportedto other places for treatment and disposal (Ottoet al. 2008).

Nanomaterials have highly desired propertiesfor in situ applications. Because of theirminute size and innovative surface coatings,nanoparticles may be able to pervade very smallspaces in the subsurface and remain suspendedin groundwater, allowing the particles to travelfarther than larger, macro-sized particles andachieve wider distribution. However, in practice,current nanomaterials used for remediation donot move very far from their injection point(Tratnyek and Johnson 2006). Many differentnanoscale materials have been explored forremediation, such as nanoscale zeolites, metaloxides, carbon nanotubes and fibers, enzymes,various noble metals [mainly as bimetallicnanoparticles (BNPs)], and titanium dioxide.Of these, nanoscale zerovalent iron (nZVI) iscurrently the most widely used. The differentnanomaterials, along with the pollutants theycould potentially remediate, are listed inSupplemental Material for a comprehensiveoverview of the chemistry and engineering ofvarious nanotechnology applications addressedin Supplemental Material used for remediation(Theron et al. (2008) and Zhang (2003)). nZVIparticles range from 10 to 100 nm in diameter,although some vendors sell micrometer-scaleiron powders as “nanoparticles.” Typically, anoble metal (e.g., palladium, silver, copper) canbe added as a catalyst. The second metal createsa catalytic synergy between itself and Fe andalso aids in the nanoparticles’ distribution and

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mobility once injected into the ground (Salehet al. 2007; Tratnyek and Johnson 2006; U.S. EPA2008). These BNPs may contain more than twodifferent metals. The second metal is usually lessreactive and is believed to promote Fe oxidationor electron transfer (U.S. EPA 2008). Somenoble metals, particularly palladium, catalyzedechlorination and hydrogenation and can makethe remediation more efficient (U.S. EPA 2008;Zhang and Elliott 2006).

In the 1990s, Fe at the nanoscale wassynthesized from Fe(II) and Fe(III) to produceparticles ranging from 10 to 100 nm, initiallyusing borohydride as the reductant, and examinedin laboratory studies. Zhang (2003) tested nZVIfor the transformation of a large number ofpollutants, most notably halogenated organiccompounds commonly detected in contaminatedsoil and groundwater. The author reported thatnanoscale Fe particles are very effective for thetransformation and detoxification of a varietyof common environmental pollutants, includingchlorinated organic solvents, organochlorinepesticides, and polychlorinated biphenyls(PCBs). According to Zhang (2003), Fe-mediatedreactions should produce an increase in pHand a decrease in the solution redox potentialcreated by the rapid consumption of oxygen,other potential oxidants, and the production ofhydrogen. Although batch reactors produce pHincreases of 2–3 and an oxidation–reductionpotential (ORP) range of �500 to �900 mV,it is expected that the pH and ORP would beless dramatic in field applications where othermechanisms reduce the chemical changes (Zhang2003). Previous work showing an increase ofpH by 1 and an ORP in the range of �300 to�500 mV supports this assessment (Elliott andZhang 2001; Glazier et al. 2003). Zhang (2003)also showed that modifying Fe nanoparticlescould enhance the speed and efficiency of theremediation process.

The first field application was reported in2000. Nanoparticles have been shown to remainreactive in soil and water for up to 8 weeks andcan flow with the groundwater for >20 m. In onestudy, Zhang (2003) produced a 99 % reductionof TCE within a few days of injection.

Because nanoscale particles are so small,Brownian movement or random motion, ratherthan wall effects, dominates their physical move-ment or transport in water. The movement ofmicrometer-scale particles, especially microscalemetal particles, is largely controlled by gravity-induced sedimentation because of their size andhigh density. In the absence of significant surfaceelectrostatic forces, nanosized particles can beeasily suspended in water during the design andmanufacturing stages, thus providing a versatileremediation tool that allows direct injection asa liquid into the subsurface where contaminantsare present. Coating the Fe particles to improvemobility and catalytic reaction rates is important.Some of the particles flow with the groundwaterand remain in suspension for various amounts oftime, whereas others are filtered out and bind tosoil particles, providing an in situ treatment zonethat could hold back emanating plumes (Hennand Waddill 2006).

The high reactivity of nZVI particles is in parta direct result of their high specific surface area.For example, nZVI produced by the borohydridemethod has surface areas in the range of 20–40 m2/g, which can yield 10–1,000 times greaterreactivity compared with granular Fe, which hasa surface area <1 m2/g (Wang and Zhang 1997).nZVI’s small particle size also allows more ofthe material to penetrate into soil pores, and itcan be more easily injected into shallow and deepaquifers.

Initially, Fe nanoparticles have a core of ZVIand an outer shell of Fe oxides, which suggest thefollowing redox reactions:

Fe0 .s/ C 2H2O .aq/ ! Fe2C .aq/ C H2 .g/

C 2OH� .aq/

2Fe0 .s/ C 4HC .aq/ C O2 .aq/ ! 2Fe2C .aq/

C 2H2O .l/ ;

where s is solid, aq is aqueous, g is gas, and l isliquid (Matheson and Tratnyek 1994).

Although Fe nanoparticles have been shownto have a strong tendency to form microscale

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108 M.H. Fulekar et al.

aggregates, possibly because of their weaksurface charges, coatings can be applied tochange the surface properties. These differentforms of Fe could be useful for the separation andtransformation of a variety of contaminants, suchas chlorinated organic solvents, organochlorinepesticides, PCBs, organic dyes, various inorganiccompounds, and the metals As(III) (trivalentarsenic), Pb(II) (bivalent lead), copper [Cu(II)(bivalent copper)], Ni(II) (bivalent nickel), andCr(VI) (hexavalent chromium) (Sun et al. 2006).

Nanoremediation, particularly the use ofnZVI, has site-specific requirements that mustbe met in order for it to be effective. Adequatesite characterization is essential, includinginformation about site location, geologicconditions, and the concentration and types ofcontaminants. Geologic, hydrogeologic, andsubsurface conditions include composition ofthe soil matrix, porosity, hydraulic conductivity,groundwater gradient and flow velocity, depth towater table, and geochemical properties (pH,ionic strength, dissolved oxygen, ORP, andconcentrations of nitrate, nitrite, and sulfate).All of these variables need to be evaluated beforenanoparticles are injected to determine whetherthe particles can infiltrate the remediation sourcezone and whether the conditions are favorablefor reductive transformation of contaminants.The sorption or attachment of nanoparticlesto soil and aquifer materials depends on thesurface chemistry (i.e., electrical charge) ofsoil and nanoparticles, groundwater chemistry(e.g., ionic strength, pH, and presence of naturalorganic matter), and hydrodynamic conditions(pore size, porosity, flow velocity, and degree ofmixing or turbulence). The reactions between thecontaminants and the nZVI depend on contact orprobability of contact between the pollutant andnanoparticles (U.S. EPA 2007, 2008).

Degradation of Pollutants UsingBismuth Vanadate (BiVO4)PhotocatalystOne of the promising non-titania-based visible-light-driven semiconductor photocatalyst isbismuth vanadate (BiVO4). It was applied ascatalyst in oxidative dehydrogenation of ethyl

benzene to styrene (Kudo et al. 1999) andas photocatalyst where O2 was successfullyevolved from aqueous silver nitrate solutionunder visible-light radiation (Kudo et al.2001). There are exactly three main crystalforms of BiVO4, known as tetragonal zircon-type structure, tetragonal scheelite structure,and monoclinic distorted scheelite structure(Gotic et al. 2005). The color of BiVO4

varies from inhomogeneously yellow brownto homogeneously lemon yellow and dependson many factors including phase composition,stoichiometry, particle size, and morphology.The photocatalytic activity of BiVO4 has alsobeen reported to be strongly influenced by thecrystal structure; monoclinic BiVO4 showedbetter photoactivity than tetragonal (Kudo etal. 1999).

Several synthesis methods have been used toprepare monoclinic BiVO4 such as sonochemicalmethod (Zhou et al. 2006), solid-state reaction(Gotic et al. 2005), reflux (Zhou et al. 2007),solgel (Hirota et al. 1992), and hydrothermalmethod (Liu et al. 2003; Zhang and Zhang 2009;Zhang et al. 2006, 2009). The photocatalytic ac-tivities of BiVO4 nanocrystals prepared by sono-chemical methods, evaluated by decolorizationof methyl orange under visible-light irradiation,showed better photodegradation rate (95 % in30 min) than that of sample prepared by solid-state reaction (8 %).

The photocatalytic activities of BiVO4 wereevaluated by degrading methylene blue (MB) dyesolution under visible-light irradiation. A 23 Wlight bulb was used as a visible-light source andthe experiments were carried out at room temper-ature for 4 h. Prior to photocatalysis experiment,the amount of MB dye removed via photolysisand adsorption process was determined. Elevenpercent of the dye was removed via photolysisafter 4 h of irradiation time. However, undervisible-light irradiation, significant increase inthe percentage removal of MB was observed.Even though higher percentage of MB photode-graded by BiVO4 has been reported, the authorshave used a much higher intensity of light (200 Wof Xe arc lamp) (Yu et al. 2009) compared tothese studies (23 W). This indicates the potential

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of the prepared BiVO4 to be used as photocatalystin photodegrading MB dye at low light intensity.The removal of MB dye by BiVO4 is influencedby the surface area and the crystal structure ofthe catalyst. The higher the surface area of thecatalyst, the higher is the removal by adsorptionprocess. Removal of MB via photocatalytic pro-cesses is more dominant than adsorption processfor a more distorted monoclinic scheelite BiVO4

(Abdullah et al. 2009).

Degradation of Pollutants Using CdSAs an important II–IV compound semiconductor,CdS, with direct bandgap energy of 2.4 eV atroom temperature, has attracted much attentionbecause of its unique properties and potentialapplication in light-emitting diodes, flat-paneldisplays, solar cells, photocatalysis, and thin-film transistors. Since a material’s propertiesdepend greatly on its morphological features,nanostructured CdS with different sizes, shapes,and dimensionalities has been fabricated andcharacterized. Extensive studies have preparedCdS nanostructures with various shapes, such asnanowires, nanobelts, and self-organized sphereson the scale of micro-/nanometers. The size,shape, and crystalline structure of semiconductornanocrystallites, which lead to considerablechanges in the recombination of electrons andholes trapped at spatially separated donors andacceptors, are important in determining theiroptical properties. For instance, Xu et al. reportedthe synthesis of nanostructured flower-like CdSby a solvothermal method using ethylenediamineas a structure-directing template. Qing Xia etal. successfully synthesized a wide range ofcadmium sulfide (CdS) three-dimensional (3D)polycrystalline walnut-like nanocrystals by asolvothermal method with polyvinylpyrrolidone(PVP) as stabilizer. Lin et al. synthesizeduniform-sized CdS hollow nanospheres viahydrothermal treatment of aqueous solutionsof cadmium acetate and thiourea and reportedthat the spheres could be made solid or hollowby manipulating the precursor Cd/S molar ratioin the synthesis system. Li et al. synthesizedCdS microspheres assembled from high-qualitynanorods by a hydrothermal method using PVP

as a surfactant. Danjun Wang et al. synthesizednovel 3D dendritic CdS nanoarchitectures viaa facile template-free hydrothermal processusing CdSO4 and thiourea as precursors andcetylpyridinium chloride (CPC) as a cappingreagent. Zhang et al. prepared CdS submicro-and microspheres by a convenient hydrothermalprocess through the reactions of CdCl2 andNa2S2O3 in aqueous solution at a relatively lowtemperature. The solution phase or hydrothermalor solvothermal technique has been demonstratedas an effective method for preparing low-dimensional nanomaterials from sulfides. Liuet al. (2012) have synthesized TiO2 film coatedon fiber glass cloth by solgel method and furthersensitized by CdS nanoparticles using a sequen-tial chemical bath deposition technique. Gaseousbenzene was adopted as a model pollutantto evaluate the photocatalytic performancesof the supported TiO2 films sensitized withvariable content of CdS nanoparticles, whichare obtained by adjusting the concentration ofCd2C or S2� precursor solution. Along with theincrease of the CdS amount, the photocatalyticactivity of these samples initially increasesand then shows a downward trend. When theconcentration of Cd2C or S2� precursor solutionachieved 0.005 M, the sample performs thebest photocatalytic property and the degradationefficiency reaches 92.8 % and 32.7 % underuv–vis and visible-light irradiation, respectively.The enhancement of the photocatalytic activitycould be attributed to the formation of micro-heterojunction. However, a thick CdS sheath-covered catalyst surface will form when a highconcentration of Cd2C or S2� solution is used.The low photogenerated electron–hole pairs’separation efficiency and the photo corrosionprocess of CdS decrease the photocatalyticactivity gradually.

Single-crystalline CdS nanoparticles weresynthesized for the first time by the composite-molten-salt (CMS) method, which had numerousadvantages including one step, ambient pressure,low temperature, template free, and low cost.The influence of temperature, growth time, andamount of salts on the morphology of CdSnanoparticles was systematically investigated.

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110 M.H. Fulekar et al.

It shows that a smaller size of CdS nanoparticlescan be obtained under lower temperature, lessgrowth time, and more composite salts. uv–vis reflection spectrum of the nanoparticlesreveals that the nanoparticles have a bandgapof 2.34 eV. Photoluminescence spectrum wasalso carried out to explore its optical property.Photocatalytic degradation of rhodamine B(RhB) and methylene blue (MB) in the presenceof the CdS nanoparticles was compared withthat in the presence of the commercial TiO2

nanoparticles under the simulated sunlight(Li et al. 2012).

Doping of Photocatalysts for EffectiveDegradationDoping of TiO2 has been an important approachin bandgap engineering to change the opticalresponse of semiconductor photocatalysts.The main objective of doping is to inducea bathochromic shift, i.e., a decrease of thebandgap or introduction of intra-bandgap states,which results in the absorption of more visiblelight. Doping may lead to photocatalytic systemsthat exhibit enhanced efficiency (Carp et al.2004). There are three different main opinionsregarding modification mechanism of TiO2 dopedwith nonmetals:1. Bandgap narrowing: Asahi et al. (2001) found

N 2p state hybrids with O 2p states in anataseTiO2 doped with nitrogen because their ener-gies are very close, and thus, the bandgap ofN-TiO2 is narrowed and able to absorb visiblelight.

2. Impurity energy level: Hiroshi et al. (2003)stated that TiO2 oxygen sites substituted bythe nitrogen atom form isolated impurity en-ergy levels above the valence band. Irradiationwith UV light excites electrons in both the VBand the impurity energy levels, but illumina-tion with visible light only excites electrons inthe impurity energy level.

3. Oxygen vacancies: Ihara et al. (2003)concluded that oxygen-deficient sites formedin the grain boundaries are important toemerge vis-activity, and nitrogen doped inpart of oxygen-deficient sites is important as ablocker for reoxidation.

Noble metals including Pt, Ag, Au, Pd, Ni,Rh, and Cu have been reported to be very effec-tive at enhancing photocatalysis by TiO2 (Rupaet al. 2009; Adachi et al. 1994; Wu and Lee2004). Because the Fermi levels of these noblemetals are lower than that of TiO2, photoexcitedelectrons can be transferred from the conductionband of TiO2 to metal particles deposited on thesurface of TiO2, while photogenerated holes inthe valence band remain on TiO2. This greatlyreduces the possibility of electron–hole recombi-nation, resulting in efficient separation and higherphotocatalytic activity.

Sustaining a Clean Environment

Mitigating Ultimate Climate ChangeImpact

Nanoscale metal organic frameworks (MOFs)and zeolite imidazole frameworks (ZIFs) arepromising CO2 sorbents with high adsorptioncapacity, selectivity, and reversibility. However,the first generation of nanoscale MOFs andZIFs can only perform a single function,i.e., CO2 separation. Thus, their use alonemight not lead to the revolutionary advancesneeded to significantly decrease the atmosphericrelease of greenhouse gases by capturingCO2 and converting it to useable products(e.g., fuels and chemicals). The vision forthe 5–10 year time frame is that convergencebetween nanotechnology, chemical separations,catalysis, and systems engineering will leadto revolutionary advances in CO2 capture andconversion technologies, including:• Nanoscale sorbents containing functionalized

size- and shape-selective molecular cages thatcan capture CO2 and convert it to useableproducts

• Nanoporous fibers and/or membranes contain-ing functionalized size- and shape-selectivemolecular cages that can capture CO2 andconvert it to useable productsIn addition to CO2 capture, transformation,

and storage, geoengineering is being considered

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7 Nanotechnology: Perspective for Environmental Sustainability 111

as a potential climate mitigation technology. Theultimate goal of geoengineering is to reduceglobal warming by developing and deployinglarge-scale “cooling” systems in the stratosphere.

Reducing Global Warming

As explained, clean nanotechnologies have thepotential to produce plentiful consumer goodswith much lower throughput of materials andmuch less production of waste and to reducesources of chemical pollution. According to thevision of Drexler, the natural feedstock will bethe input materials; using them will emit the samematerials, which can be reused again. With NT,energy through solar cells and clean fuels fromsolar energy, air, and water can be made. Withcheap solar and fuel energy, coal and petroleumcan be replaced, ignored, and left in the ground.Like this many sources, pollution can be step bystep eliminated.

The release of other gases, such as chloroflu-orocarbons (CFCs) used in foaming plastics, isoften a side of primitive manufacturing processes.Foaming plastics will hardly be used duringmolecular manufacturing. These materials canbe replaced or controlled, and they include thegases most responsible for ozone depletion.Nanotechnology based devices/systems alsoabsorb CO2 that could help mitigation of globalwarning and bring the planet’s ecosystem backinto balance.

Sustaining Biodiversity

In the next 10C years, it is expected that nan-otechnology will contribute significantly to thepreservation of biodiversity through the develop-ment and implementation of:• Advanced sensors and devices for monitoring

ecosystem health (e.g., soil/water compo-sition, nutrient/pollutant loads, microbialmetabolism, and plant health)

• Advanced sensors and devices for monitoringand tracking animal migration in terrestrialand marine ecosystems

• Cost-effective and environmentally acceptablesolutions to the global sustainability chal-lenges, including energy, water, environment,and climate change

Socioeconomic Aspects forSustainable Development

Nanoscience is at the unexplored frontiers ofscience and engineering, and it offers one ofthe most exciting opportunities for innovation intechnology. It is important to have social scien-tists study the processes by which nanoscienceis conducted and nanotechnology is developedeven at this early stage. The knowledge gainedwill help policymakers and the public under-stand how nanoscience and nanotechnology areadvancing, how those advances are being dif-fused, and how to make necessary course cor-rections. Insight into innovation process will alsogrow. Social scientists and scholars possess manyeffective ways of studying the development ofnew technology and its implications for society.Applications of a scientific idea to a technicalproblem, technology transfer, and an introduc-tion of products into the marketplace can betracked through statistics on research and de-velopment investments, patent applications, andnew products and services. The societal impactsof nanotechnology may be of great scope andvariety. The domains and measures of potentialsocial impacts include economic growth, em-ployment statistics, social transformations, andmedical statistics.

Nanoscience may also enable new materialsand technologies that reduce economic depen-dence on other kinds of natural resources. Asthe global economy continues to be transformedby new technology, a keen competition will de-velop for talent, intellectual property, capital, andtechnical expertise. Technical innovations willincreasingly shape economies and market robust-ness. Technology will continue to drive globaland domestic GDP. In the economic environment,nanotechnology comprehensively integrated intothe economy due to high readiness, effectivestrategic planning, and widespread investments

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112 M.H. Fulekar et al.

by business, education, labor, and government.The study of socioeconomic aspects of nanoscalescience and technology (NST) will provide greennanoproducts, nanoprocesses, and their applica-tions in science, engineering, and technology forsustainable development.

Priorities for PromotingNanotechnology

Educate and train a new generation of scientistsand workers skilled in nanoscience and nanotech-nology at all levels. Develop scientific curriculaand programs designed to:(a) Introduce nanoscale concepts into mathemat-

ics, science, engineering, and technologicaleducation

(b) Include societal implications and ethical sen-sitivity in the training of nanotechnologist

(c) Produce sufficient number and variety ofwell-trained social and economic scientistsprepared to work in the nanotechnologyarea

(d) Develop effective means for giving nanotech-nology students an interdisciplinary perspec-tive while strengthening the disciplinary ex-pertise they will need to make maximumprofessional contributions

(e) Establish fruitful partnerships between indus-try educational institutions to provide nan-otechnology students adequate experiencewith nanoscale fabrication, manipulation,and characterization techniques

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