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BIOSORPTION: TECHNOLOGY

FOR COLOUR REMOVAL

INTRODUCTION

In most developed and developing countries, stricter environmental regulations, withregard to contaminants discharged from industrial operations, are being introduced. This

means that industries need to develop on-site or in-plant facilities to their own effluents andminimize the contaminant concentrations to acceptable limits prior to their discharge. The

discharge of these dye stuffs and metals from industries into rivers and lakes results in areduced dissolved oxygen concentration causing anoxic conditions, which subsequently

affect aerobic organisms (1). Apart from the toxicological properties of dyes, their colour is

one of the first signs of contamination recognized in a wastewater Biosorption is a technique

that can be used for the removal of pollutants from water, especially those that are not easily

biodegradable such as metals and dyes. A variety of biomaterials are known to bind these

pollutants, including bacteria, fungi, algae, and industrial and agricultural wastes. There are

three principle advantages of biological technologies for the removal of pollutants; first,

biological processes can be carried out in situ at the contaminated site; Second, bioprocess

technologies are usually environmentally benign (no secondary pollution) and third, they are

cost effective. Of the different biological methods, bioaccumulation and biosorption have

been demonstrated to possess good potential to replace conventional methods for the removal

of dyes. In this review, the biosorption abilities of bacterial biomass towards dyes areemphasized (9).

Water resources are of critical importance to both natural ecosystem and humandevelopments. Increasing environmental pollution from industrial wastewater particularly in

developing countries is of major concern. Many industries like dye industries, textile, paperand plastics use dyes in order to colour their products and also consume substantial volumes

of water. As a result they generate a considerable amount of coloured wastewater. Thepresence of small amount of dyes (less than 1 ppm) is highly visible and undesirable. Many

of these dyes are also toxic and even carcinogenic and pose a serious threat to living

organisms. Hence, there is a need to treat the wastewaters containing toxic dyes before they

are discharged into the water bodies (18). Many researches have been investigated in order

to remove dyes molecules before disposal of wastewater such as chemical coagulation and

ozonation. Recent research works had proved thatadsorption method is becoming the most

promisingalternative in this domain. The most common adsorbentis the activated carbon

but its use remains quite expensive. This method do not show significant effectiveness and

economic advantage and higher the quality greater the cost. Many low cost biosorbents were

tested for the removal of dyes from aqueous solutions such as wheat bran (ieket al., 2007),

Posidonia oceanica; agricultural waste, pine sawdust orange peel and rice husk(1)

Many physico-chemical methods like coagulation, flocculation, ion exchange, etc are

available for the treatment of heavy metals and dyes. Major drawbacks of these methods are

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high sludge production, handling and disposal problems, high cost, technical constraints, etc.

The increasing awareness and concern about the environment motivated research for new

efficient technologies that would be capable of treating inexpensively, waste waters polluted

by dyes. This search brought biosorption to the foreground of scientific interest as a potential

basis for the design of novel wastewater treatment processes. Several adsorbents are currently

used which are by-products from agriculture and industries, which include seaweeds, molds,yeast, bacteria, crabshells, agricultural products such as wool, rice, straw, coconut husks, peat

moss, exhausted coffee waste tea leaves, walnut skin, coconut fibres, etc.Thus

adsorption/biosorption using low cost adsorbents could be technically feasible and

economically viable sustainable technology for the treatment of wastewater streams (10).

Structures of some majourly used dyes are as follows-

(13)

(18)

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(15)

(17)

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(14)

(16)

KEYWORDS : Wastewater treatment, conventional methods, decolourization, biosorption,

biosorbent, dye.

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TOXICOLOGICAL ASPECTS OF DYES

Dyeing industry effluents are one of the most problematic wastewater to be treated notonly for their high chemical oxygen demand, but also for high biological oxygen demand,

suspended solids, turbidity, toxic constituents but also for colour, which is the firstcontaminant discernible by the human eye. Dyes may affect the photosynthetic activity in

aquatic life due to reduced light penetration and may also be toxic to some aquatic life due tothe presence of aromatics, metals, etc. in them. Dyes usually have a synthetic origin and

complex aromatic molecular structure, which make them more stable and more difficult tobiodegrade(19). Water soluble reactive and acid dyes are problematic; as they pass through

the conventional treatment system unaffected, posing problems. Dyes have generated muchconcern regarding its use, due to its toxic effects. It has been reported to cause

carcinogenesis, mutagenesis, chromosomal fractures, teratogenecity and respiratory toxicity

(12).

NEED FOR THE REMOVAL OF DYES

India produces 64,000 tonnes of dyes, 2 per cent of which - 7,040 tonnes - are directlydischarged into the environment. There are around 700 varieties of dyes and dye

intermediaries produced in India. In India only a third of the dyestuff producing industries are

in organised sector (17).

Dyes and colour pigments also contain metals such as copper, nickel, chromium,

mercury and cobalt. Metals are difficult to remove from wastewater and may escape the

capacities of the effluent treatment system. Moreover, the unused dyes and colour released in

effluent from dyeing vats, interferes with the transmission of light in the water bodies that

receives the effluent. This in turn inhibits the photosynthesis activity of aquatic biota besides

direct toxic effects on biota. Several textile and food dyes have been linked tocarcinogenicity, such as dye intermediaries like Benz dines. Hence the ubiquitous colour

needs to be regulated. But removing the colour from effluents is extremely difficult. There is

no universally applicable technique for all conditions (18).

CONVENTIONAL METHODS FOR THE TREATMENT OF DYES

Synthetic dyes often receive considerable attention from researchers interested in textile

wastewater effluents treatment processes. As discharge standards are becoming more

stringent, the development of technological systems for minimizing concentration of dyes and

their break down products in wastewater are nowadays necessary. The following are

generally used for the removal of colour from wastewaters (20).

1) Physicochemical methods for dye removal2) Photo catalytic decolourisation and oxidation of synthetic dyes3) Membrane processing

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4) Mi i l i l iti t ti) Enzymati mposition of synt ti dyes

6) Adsorption

DISADVANTAGES OF USING CONVENTIONAL METHODS FORDYE REMOVAL

(21).

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BIOSORPTION

The bio sorption process involves a solid phase (sorbet or biosorbent; adsorbent;

biological material) and a liquid phase (solvent, normally water) containing a dissolved

species to be sorbed (adsorb ate, dyes). Due to the higher affinity of the adsorbent for the

adsorb ate species, the latter is attracted and bound there by different mechanisms. Theprocess continues till equilibrium is established between the amount of solid-bound adsorb

ate species and its portion remaining in the solution. The degree of adsorbent affinity for the

adsorb ate determines its distribution between the solid and liquid phases. The pH of the

solution is an important controlling parameter in the sorption process(3). Salt concentration

also has an adverse effect on sorption process(6).

Bio sorption has advantages compared with conventional techniques. Some of these are

listed below:

Cheap: the cost of the biosorbent is low since they often are made from abundant or waste

Material. Dye selective: the dye sorbing performance of different types of biomass can be more or

Less selective on different dyes. This depends on various factors such as type of biomass,

Mixture in the solution, type of biomass preparation and physico-chemical treatment.

Regenerative: biosorbents can be reused, after the dye is recycled.

No sludge generation: no secondary problems with sludge occur with bio sorption, as is the

Case with many other techniques, for example, precipitation.

Competitive performance: bio sorption is capable of a performance comparable to the mostsimilar technique, ion exchange treatment. Ion exchange is, as mentioned above, rather

costly, making the low cost of bio sorption a major factor.Bio sorption by seaweed has been demonstrated to be a practical alternative to

conventional systems for the removal of dyes (7).

BIOSORPTION OF DYES BY MICRORGANISMS

A wide variety of micro organisms including bacteria, fungi and yeasts are used for the

bio sorption of a broad range of dyes. Textile dyes vary greatly in their chemistries, andtherefore their interactions with micro organisms depend on the chemical structure of a

particular dye, the specific chemistry of the microbial biomass and characteristics of the dyesolution or wastewater (11). Depending on the dye and the species of micro organism used

different binding capacities have been observed. Magnetically modified yeast cells can thus

be a promising magnetic affinity adsorbent which may be used to the removal of dyes (5).

Decarboxylation is a good option for pre-treatment of the raw biomass to make effective biosorbents able to remove reactive dyes from dye-containing wastewaters, even at low

concentrations(14).

There are certain inherent disadvantages of using micro organisms for the bio sorption of

heavy dyes and they are as follows: the protein rich algal and fungal biomass projected as dye

biosorbents have limitations as proteinious materials are likely to putrefy under moist

conditions. Further, most dye sorption reported in literature is based on algal and fungal

biomass, which must be cultured, collected from their natural habitats and pre-processed, if

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available as discards and transported under special conditions, thus introducing the factor of

additional costs (4).

ALTERNATIVE LOW COST ABSORBENTS

The disadvantages of using micro organisms can be overcome by using low costadsorbents. In general, a sorbent can be assumed to be low cost if it requires little

processing and is abundant in nature, or is a by product or waste material from another

industry (18), which has lost its economic or further processing values The by-products from

the agriculture and industries could be assumed to be low-cost adsorbents since they areabundant in nature, inexpensive, require little processing and are effective materials (16).

Palm kernel fibre, a very abundant agricultural by-production Nigeria has been shown to be agood sorbent for the anionic dye (2). Another agricultural solid waste, tamarind fruit shell is

also a potential sorbent. (13).Commercially available activated carbons (AC) are usually derived from natural

materials such as wood, coconut shell, lignite or coal, but almost any carbonaceous materialmay be used as precursor for the preparation of carbon adsorbents. Hazelnut shell, low cost

material, also has suitable adsorption capacity to the remove dyes from its aqueous solution(15). Raw agricultural solid wastes and waste materials from forest industries such as sawdust

and bark have been used as adsorbents. These materials are available in large quantities and

may have potential as sorbents due to their physico-chemical characteristics and low-cost.

Amino acid fermentation industry waste, Corynebacterium glutamicum, has been found to

possess excellent bio sorption capacity (8).

Water-pollution control thus is presently one of the major areas of scientific activity as

effluent discharge from textile and dyestuff industries to neighbouring water bodies and

wastewater treatment systems is currently causing significant health concerns to

environmental regulatory agencies. So colour removal, in particular by bio sorption is a

proven technique potentially for the removal of dyes from aqueous solution (12).

SUMMARY:

Biosorption is a technique that can be used for the removal of pollutants from water,

especially those that are not easily biodegradable such as metals and dyes. A variety of biomaterials are known to bind these pollutants, including bacteria, fungi, algae, and

industrial and agricultural wastes. There are three principle advantages of biologicaltechnologies for the removal of pollutants; first, biological processes can be carried out in situ

at the contaminated site; Second, bioprocess technologies are usually environmentally benign(no secondary pollution) and third, they are cost effective. Of the different biological

methods, bioaccumulation and biosorption have been demonstrated to possess good potential

to replace conventional methods for the removal of dyes. In this review, the biosorptionabilities of bacterial biomass towards dyes are emphasized.

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Reference:

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3) Augustine E. Ofomaja , Yuh-Shan Ho, (2008) Effect of temperatures and pH onmethyl violet biosorption by Mansonia wood sawdust, Bioresource Technology, (99)

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6) Juan Mao,Sung Wook Won,Yeoung-Sang Yun, (2009) Biosorption of reactive andbasic dyes using fermentation waste Corynebacterium glutamicum: the effects of pH

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