Desalination 259 (2010) 249–257

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Desalination

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Removal of methylene blue from aqueous waste using rice husk and rice husk ash

Pankaj Sharma a,⁎, Ramnit Kaur b, Chinnappan Baskar a, Wook-Jin Chung a

a Energy and Environment Fusion Technology Center, Department of Environmental Engineering and Biotechnology, Myongji University, San 38-2, Nam-dong,Cheoin-Gu, Yongin-Si 449-728, Republic of Koreab Department of Chemistry, Lovely School of Sciences, Lovely Professional University, Phagwara 144402, Punjab, India

⁎ Corresponding author. Tel.: +82 31 336 8471; fax:E-mail address: sharmapankaj47@yahoo.com (P. Sha

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.03.044

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 January 2010Received in revised form 2 March 2010Accepted 23 March 2010Available online 13 May 2010

Keywords:Rice huskDyeAdsorptionMethylene blueIsothermPretreatment

Present study investigates the potential use of pretreated rice husk (RH) and rice husk ash (RHA) for theremoval of methylene blue (MB) from wastewater. A series of batch experiments were carried out todetermine the influence of different system variables. Neutral pH was optimum for the removal of MB.Adsorption of MB on RH and RHA was favorably influenced by an increase in the temperature of theoperation. The comparative studies of these two adsorbents, RA and RHA with the earlier reportedadsorbents obtained from agricultural and industrial waste products, inorganic materials and bioadsorbents,reveals that RH and RHA have maximum adsorption capacity. Adsorption data was fitted to the Langmuir,and Freundlich adsorption model. The former model achieved best fit with the experimental data and itscalculated maximum monolayer adsorption capacity have a value of 1347.7 mg g−1 for adsorption on RHand 1455.6 mg g−1 for adsorption on RHA at a temperature of 323 K. The change in heat of adsorption (ΔHo)and entropy (ΔSo) values for MB adsorption on RH as well as RHA were positive. The high negative values forGibbs free energy (ΔGo) indicate the feasible and spontaneous adsorption of MB on RH and RHA.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Synthetic dyes have beenusedwidely in various industries includingpaper printers, dye houses and textile dyers. Considerable amounts ofsynthetic dyes are lost annually to waste streams during textileprocessing, which ultimately enters the environment. The presence ofsmall amounts of dyes inwater is highly visible and undesirable [1]. Theeffluents from textile, leather, food processing, dyeing, cosmetics, paper,and dye manufacturing industries are the major sources of dyepollution. Various kinds of dyes and their break down products aretoxic for the living organisms. Fifteen percent of the total worldproduction of dyes goes off during the dyeing process and have beenreleased in textile effluents [2]. The release of this coloredwastewater ishazardous for the aquatic life. At the receiving streams, the coloredwastes interfere with the transmission of sunlight into streams andreduce the photosynthetic activity. From the environmental point ofview, the removal of synthetic dyes is of great concern. Since someof thedyes and their degradation products may be carcinogenic and toxic,such that, their treatment cannot be left upon thebio-degradation alone.Recently, an increased interest has been focused on removing dyes fromthe water due to its refractory bio-degradation and toxic nature whichaffects the aquatic biota and food web [3]. The removal of color from

wastewater can be carried out by flotation, chemical coagulation,chemical oxidation and adsorption. Adsorption technique is quitepopular due to its simplicity as well as the availability of a wide rangeof adsorbents.Moreover it's beenproved tobeaneffective andattractiveprocess for removal of non-biodegradable pollutants (including dyes)fromwastewater [4]. A commonly used adsorbent activated carbon hasa good capacity for the removal of organic pollutants [5–11]. But someofits disadvantages are the high price of treatment and difficultregeneration, which increases the cost of wastewater treatment. Thus,there is a demand for the other adsorbents, which are made up ofinexpensivematerial and does not require any additional pretreatment,such that the adsorption process will become economically viable. Asuccessful adsorption process not only depends on dye adsorptionperformance of the adsorbents, but also on the constant supply of thematerials for this process. So it is preferable to use low-cost adsorbents,such as rice husk and rice husk ash. Rice husk is an agricultural waste,accounting for about one-fifth of the annual gross rice production (545million metric tonnes) of the world. Rice husk is used as a fuel by anumber of industries to produce steam, thus, conserving both energyand resources. During the burning of rice husk, the residual ash, calledrice husk ash (RHA) is collected from the dust collection device attachedupstream to the stacks of rice husk-fired boilers and furnaces. The RHApossesses good adsorptive properties and has been used previously forthe adsorptive removal of metal ions and dye from the water.

Methylene blue is a heterocyclic aromatic chemical compoundwith molecular formula: C16H18ClN3S3H2O. Due to its toxic nature it is

250 P. Sharma et al. / Desalination 259 (2010) 249–257

harmful if swallowed and may create some problems if inhaled ormade in contact with skin or eye.

As methylene blue is used in various studies, so during its useconsiderable amount of methylene blue goes with wastewater to theoceans and make hazardous for the aquatic lives. Therefore,decolorization of dyes is important aspects of wastewater treatmentbefore discharge.

Most of the adsorption studies have been focused on untreatedplant wastes because of low cost, easy availability, easy to handle[12,13]. A large number of studies on cellulosic agricultural wastematerials have been reported for the treatment of industrial wastewater [14]. The applicability of these low-cost adsorbents obtainedfrom plants waste, acts as an important replacement for costlyconventional methods of removing organic dyes as well as toxic heavymetal ions from different waste streams. In general, an adsorbent canbe assumed as “low cost” if it requires little processing, is abundant innature, or is a by-product or waste material from another industry.Thus, adsorption is a well known equilibrium separation process andan effective method for water decontamination applications. Severalkinds of agricultural by-product such as rice husk [15], cereal chaff[16], Azadirachta indica leaf powder [17], A. indica leaf powder [18],giant duckweed [19], and fly ash [20], have been reported for theremoval of methylene blue from waste water. Apart from theseadsorbents there are so many other natural adsorbents which hadbeen used to make waste water free from methylene blue [21–26].The importance of biomasses for the removal of colored/hazardousdye methylene blue from aqueous waste can be judged from therecent reported data in the literature [27–32]. In general, pretreatedand chemically modified plant wastes exhibit higher adsorptioncapacities than the unmodified forms [33–36]. The previous adsorp-tion studies of rice husk and rice husk ash reveal that these areprominent adsorbent for metal ions, oil products removal fromaqueous waste and have potential to remove arsenic from industrialwaste streams [37–41].

Themain objective of this work is to study the adsorption potentialof low cost bioadsorbent, rice husk and rice husk ash for the removalof hazardous dye methylene blue from the waste water. Apart fromthis, another objective of this work is to evaluate the effect of dyeconcentration, adsorbent dose, contact time, temperature, and pH ofthe medium, on the adsorption characteristic of pretreated RH andRHA in order to make a comparison between them. This paper alsodiscusses the Langmuir and Freundlich adsorption isotherm modeland various thermodynamic parameters such as Gibbs free energychange (ΔGo), heat of adsorption (ΔHo) and entropy change (ΔSo) asapplied to the adsorption of methylene blue onto the RH and RHA.

2. Materials and methods

2.1. Pretreatment of rice husk (RH) and rice husk ash (RHA)

Rice husk is a farming waste, obtained from the rice mills after theseparation of rice from paddy. In the present study rice husk withfresh biomass was collected from nearby rice mill of Dasuya, Punjab,India. This obtained rice husk was washed four to five times withdistilled water, then half the amount of RHwas dried for 48 h at 373 K

in the oven and rest of the RH dried at room temperature. The biomasswas sieved and 20–40 mesh was used for the batch experiment. RHAused as obtained from a nearby papermill (Barnala papermill, Punjab,India) and then properly grinded into fine powder for the removal ofmethylene blue from waste water by batch treatment process. Thefollowing were the starting materials used for the adsorption studiesof MB dye: pretreated RH and RHA, moreover before each adsorptionstudy these adsorbents are activated by placing the adsorbent inpreheated oven at 343 K for 4 h.

2.2. Infrared and electron microscopic studies of RH and RHA

Fourier transformation infrared (FTIR) spectra of RH and RHAwereobtained using KBr pellets in conjunction with Shimadzu FTIR 8400Sspectrometer. The pellet for infrared studies was prepared bymixing agiven sample with KBr crystals and pressed into a pellet. A spectrumwas recorded in the mid-IR range from 4000 to 400 cm−1 with aresolution of 1 cm−1. The resulting spectra were the average of 20scans. The surface morphology, textures, porosity and particle size ofRA and RHA were examined by scanning electron microscopy (SEM).For scanning electron microscopic studies, the sample was dispersedonto carbon tape and coated with gold using a sputter coater systemto prevent charge accumulation on the sample. The RH and RHA werethen observed on a JEOL JSM-5800 microscope operating in the SEMmode at 20 kV.

2.3. Batch adsorption experiments

The adsorption experiments were carried out in a batch process.Variables parameters such as, initial methylene blue concentration,adsorbent amount, contact time, temperature and pH of the mediumwere studied. All experiments were performed by using conical flaskof 100 mL capacity containing 50–1250 mg of RH/RHA suspended in25 mL of dye solution. The initial pH adjustments were carried outeither by hydrochloric acid or sodium hydroxide solutions beforeadding the RH/RHA. Digital pH meter was used for the pHmeasurements and was calibrated with two buffer solutions havinga pH value of 4 and 7 prior to use. All the adsorption experiments havebeen carried out in duplicate and the results given are the averagevalues. The reaction mixture has mixed with the help of a horizontalshaker with an intensity of agitation 4 rps for desired time and afterthat, samples were filtered out by using Whatman filter papernumber-42 and care has been taken to repeatedly centrifuge thesolution at high speed to avoid any solid particles in the solutionphase. The left out concentration in the supernatant solution afteradsorption process was analyzed using an ELICO SL 159 UV–VISspectrophotometer by recording the absorbance changes at awavelength of maximum absorbance (664 nm). The amount of dyeadsorbed on the adsorbent surface was calculated from the concen-trations difference in solution before and after adsorption, observedfrom absorption values. The obtained results are expressed in terms ofadsorption percentage.

The distribution of dye between the adsorbents and the solventgives the measure of the selectivity of the particular adsorbent forcorresponding dye. The distribution coefficient (Kd) can be used tocalculate the volume of adsorbent that is needed to achieve a desireddecontamination for a certain amount of liquid in a batch process. Thedistribution coefficient is expressed as:

Kd =Co−Ce

Ce

� �:VW

� �ð1Þ

where, Co represents the initial concentration of the dye solution(mg L−1) and Ce concentration of solution (mg L−1) at equilibrium.

The effect of initial dye concentration and adsorbent dose on theadsorption of MB dye was investigated by employing different dosage

251P. Sharma et al. / Desalination 259 (2010) 249–257

of RH and RHA varying from 250–1250 mg to different concentrations(50–2000 mg L−1) of dye solutions. This adsorption experiment isperformed at fixed values of pH equals 7, temperature equals 303±1 K, equilibration time 30 min. Batch adsorption studies were alsoconducted at different contact times (20–180 min) at initial concen-tration of MB 500 mg L−1 and adsorbent concentration 40 mg mL−1

in 25 mL solution. Adsorbent and solution were separated at pre-decided time intervals, filtered, centrifuged and analyzed for residualMB concentrations in the solution phase spectrophotometrically. Theinfluence of pH on the adsorption potential of RH as well as RHA wasstudied at different pH values, as it is an important parameter forinteraction that take place between adsorbent and adsorbate on theadsorbent surface. The role of pH on the adsorption capacity of RH andRHA was studied in the pH range 1.0–9.0 at MB concentration1000 mg L−1, temperature 303±1 K, equilibration time 30 min andadsorbent dose 500 mg (i.e. 20 mg mL−1). As we know thattemperature of the system have remarkable effect on the adsorptionprocess, therefore in order to optimize the system temperature formaximum removal efficiency, experiments were conducted at threedifferent temperatures viz. 303, 313, and 323±1 K by varying dyeconcentration and keeping other parameters constant such as pH 7,equilibration time 30 min and adsorbent dose 20 mg mL−1.

The Gibbs free energy change of the adsorption process is relatedto the equilibrium constant by the classic Van't Hoff equation:

ΔGo = −RT lnKd: ð2Þ

According to thermodynamics, the Gibbs free energy change is alsorelated to the entropy change and heat of adsorption at constanttemperature by the following equation:

ΔG� = ΔH�−TΔS�: ð3Þ

On combining Eqs. (2) and (3), we get

lnKd = −ΔG� = RT = ΔS� = R� �

−ΔH�= RT ð4Þ

where ΔGo is the free energy change (kJ mol−1), ΔHo the change inenthalpy (kJ mol−1), ΔSo the entropy change (J mol−1 K−1), T theabsolute temperature (K) and R the universal gas constant(8.314 J mol−1 K−1). Thus ΔSo can be recorded from the interceptand ΔHo can be determined by the slope of the linear Van't Hoff plot,i.e., as lnKd versus 1/T.

The adsorption isotherms were studied for the initial MBconcentration ranging from 100–1000 mg L−1. Using a method oflinear regression, the adsorption data were fitted to Langmuir [42]and Freundlich [43] adsorption model to describe the adsorptionprocesses between solid–liquid interface.

Langmuir isotherm;

Ce = qe = 1 = Kaqmð Þ + Ce = qm ð5Þ

where, qe is the amount of MB adsorbed per unit weight of adsorbent(mg g−1), Ce (mg L−1) is the equilibrium concentration of the MB inthe solution. Ka and qm are the Langmuir coefficients representing theequilibrium constant for the adsorbate–adsorbent equilibrium andthe monolayer capacity respectively.

Freundlich isotherm;

log qe = logKF +1nlogCe ð6Þ

where,KF and 1/n are the Freundlich constants. The value ofKF (mg g−1)can be taken as a relative indicator of adsorption capacity, while 1/nshows the energy or intensity of adsorption.

Furthermore the feasibility of the adsorption process is evaluatedby the method suggested by Weber and Chakraborti [44]. One of the

essential characteristics of Langmuir equation could be expressed by adimensionless constant called equilibrium parameter or separationfactor, RL which can be calculated by the following equation [45]:

RL = 1= 1 + KaCoð Þ: ð7Þ

Generally, a suitable non-conventional low-cost adsorbent for dyeadsorption should meet the following requirements: (i) efficient forremoval of dye; (ii) high adsorption capacity and rate of adsorption;(iii) high selectivity; and (iv) tolerant of a wide range of wastewaterparameters. By keeping all these points in mind we tried to make acomparative study between the naturally available low-cost adsor-bents such as agricultural by products, industrial waste materials,plants and fruit waste, naturally available inorganic materials andbiosorbents on the basis of their adsorption capacity for MB removal.Several research groups have investigated the application of com-mercial and non-commercial naturally available adsorbents for thetreatment of industrial effluent contaminated with organic dyes byadsorption process but no one have compared their adsorptioncapacities.

3. Results and discussion

3.1. Characterization of RH and RHA

FTIR spectra showed that rice husk and rice husk ash containedfunctional groups of the standard polymers α-cellulose, and coirpith-lignin (Fig. 1). The spectrum of RH-cellulose extracted from rice huskis very similar to that of α-cellulose. The FTIR spectrum of RH/RHAshows functional groups from Si–O–Si (1096 cm−1), Si–H (801 cm−1,469 cm−1) and Si–OH (3000–3700 cm−1 broad band). Heating RHAat 300 °C resulted in a loss of C–H stretching bands (2910 cm−1), C–Cand C–H (1021 cm−1), C–O and C–O–C (1060 cm−1 and 1115 cm−1),and C–O–H (899 cm−1) [14]. These were replaced by the primaryfunctional groups of NC O (1715 cm−1),NC O and/or aromatic ring(1611 cm−1) [15], and dominated the silica functional groups of Si–O–Si (1096 cm−1), Si–H (801, 469 cm−1) and OH and Si–OH (3000–3700 cm−1) [16]. FTIR spectra of rice husk and rice husk ash areshown in Fig. 1. The spectra are virtually identical. The electronmicroscopic studies (Fig. 2) of both the adsorbent reveals that RHhave fibrous character with particle size of 40–100 μm, but on theother hand the RHA have particles of comparatively smaller size,moreover these particles are of spherical shape.

3.2. Effect of dye concentration and adsorbent dosage

Before designing an adsorption treatment process it is very muchnecessary to optimize dye concentration and the amount of adsorbentto be used for that dye's removal. It has been found that (Fig. 3) withthe increase in concentration of the adsorbate, the adsorptionpercentage decreases, while the amount of dye removed at equilib-rium increased, with the increase in dye concentration in both thecases. This is so because the initial dye concentration provides thedriving force to overcome the resistance to the mass transfer of dyebetween the aqueous and solid phase. The increase in initialconcentration also enhances the interaction between adsorbent anddye. Therefore, an increase in initial concentration of dye solutionenhances the adsorption uptake of dye.

Fig. 3 also reveals that that the dye removal increases with increasein adsorbent dosage. An increase in the adsorption with adsorbentdosage can be attributed to greater surface area and the availability ofmore adsorption sites. It can be seen that 750 mg of RH in 25 ml of500 mg L−1, and 500 mg of RHA in 25 mL of 1000 mg L−1 initial MBconcentration after agitation time of 30 min respectively, the amountof dye being adsorbed was nearly 100%. With this data, it was decided

Fig. 1. Infra red spectra of RH and RHA.

252 P. Sharma et al. / Desalination 259 (2010) 249–257

that 750 mg of adsorbent in case of RH and 500 mg in case of RHA issufficient for these adsorption studies.

3.3. Effect of equilibration time study

For the evaluation of adsorption process as a function of contacttime between adsorbate and adsorbent, 20 to 180 min contact interval

Fig. 2. Scanning electron microscopic images of RH and RHA.

is applied for both the adsorbents. Effect of equilibration time onadsorption of MB dye onto RH and RHA is presented in Fig. 4. Thestudy of the graph reveals that as the contact time increases, rate ofadsorption first increases and then remains almost constant. Fig. 4illustrated that rate of adsorption ofMBwas very fast during the initialstage of the adsorption processes. After fast initial rate of adsorption,theMB uptake capacity for both the adsorbents RH and RHA increasedwith the time and reached equilibrium value at about 30 min, sincemaximum adsorption is attained during this period. The concentra-tion gradient will be responsible for these changes in the rate ofadsorption. In the beginning because of high concentration gradientthe driving force helped in this rapid adsorption. However as theconcentration gradient decreased with time the rate was reduced tillit achieved equilibrium. This may be the reason that after a particulartime interval adsorption percentage remains almost constant.

3.4. Influence of acidity/basicity of the adsorption medium

The pH value of the solution is an important factor that must beconsidered during adsorption studies. The pH has two kinds ofinfluence on dye: an effect on the solubility and speciation of dye inthe solution. The effect of pH on the adsorption of MB onto RH andRHA is shown in Fig. 5. It has been found that dye's adsorptionincreases with increase in pH and maximum adsorption is foundaround a pH value of 7. This may be due to the fact that at higher pHvalues, the surface of adsorbent becomes negative which enhancesthe adsorption of positively charged MB cationic dye throughelectrostatic force of attraction. At pH 9 there is slight decrease inadsorption due to repulsion between the adsorbent surface andpresence of partial negative charge on MB due to chloride ions.

3.5. Thermodynamic parameters

While keeping the importance of temperature in the studies ofadsorption of dyes, the present work was initiated with an aim ofusing thermodynamic approach to dyes adsorption. It was decided toconduct an experiment under controlled conditions to study howdifferent system temperatures affect the adsorption of dyes onto RHand RHA. The effect of temperature on the adsorption of MB on bothRH and RHA was investigated at 303, 313 and 323±1 K. Temperaturehas two major effects on the adsorption process. Increasing thetemperature is known to increase the rate of diffusion of the adsorbatemolecule across the external boundary layer and in the internal pores

Fig. 3. Effect of adsorbate concentration and adsorbent dose on the adsorption percentage of MB onto RH and RHA.

253P. Sharma et al. / Desalination 259 (2010) 249–257

of the adsorbent particle, owing to decrease in the velocity of thesolution. In addition changing the temperature will change theequilibrium capacity of the adsorbent for particle adsorbate.

Experimental results concerning the effect of temperature on thesystem of dyes adsorption at different initial dye concentration aredemonstrated in Fig. 6. In both the cases of MB adsorption on RH andRHA, the adsorption percentage increases when the temperature ofsolution increased from 303 to 323±1 K, indicating the process to beendothermic in nature. Better adsorption at higher temperature maybe either due to acceleration of some originally slow sorption steps ordue to creation of new active sites on the sorbent surface. Thus anincrease in temperature will reduce the electrostatic repulsionbetween the surface and the adsorbing species, allowing adsorptionto occur more readily. This could also be due to enhanced mobility ofdye molecules from solution to the adsorbent surface. Furthermore,increasing temperature may produce a swelling effect within theinternal structure of RH and RHA enabling large dye to penetrate

Fig. 4. Effect of time contact between the solid and liquid pha

further. The close examination of the Fig. 6 reveals that even though athigh initial dye concentration, we can achieve 100 percent dyeremoval by raising the temperature by 10–15 K, which is compara-tively low at lower temperature.

In environmental engineering practice, both energy and entropyfactors must be considered in order to determine which process willoccur spontaneously. The ΔGo is the fundamental criterion ofspontaneity. The calculated apparent thermodynamic parametersfor adsorption of MB onto RH as well as RHA are summarized inTable 1 and the Van't Hoff plot is represented in Fig. 7. The negativevalues of ΔGo are due to the fact that the adsorption process isspontaneous with high affinity of MB to RH and RHA. Moreover, thenegative value of ΔGo becomes more and more negative whichindicates that the adsorption process becomes more spontaneouswith rise in temperature, which favors the adsorption process.Positive value of ΔHo confirms the endothermic nature of the sorptionprocess. Hence, by increasing temperature, the degree of adsorption

se on the adsorption percentage of MB onto RH and RHA.

Fig. 5. Effect of pH of the aqueous phase on the adsorption percentage of MB onto RH and RHA.

254 P. Sharma et al. / Desalination 259 (2010) 249–257

will increase. The numerical value of ΔHo also, predicts thephysicosorption behavior of these adsorption processes. The positivevalue of ΔSo reflected the affinity of the adsorbent for particular dyeand confirms the increased randomness at the solid–solutioninterface during adsorption.

3.6. Equilibrium parameters

Equilibrium data, commonly known as adsorption isotherms, arebasic requirements for the design of adsorption systems. The mostwidely used isotherms equation for modeling of the adsorption dataare the Langmuir and Freundlich equations. Adsorption isothermsconstants depend on certain parameters, whose values express thesurface properties and the affinity of the adsorbent. They can also weused to compare the adsorptive capacity of different adsorbents fordifferent dyes. Experimental results obtained for MB adsorption ontorespective adsorbent (RH/RHA) at three different temperatures (303,313, and 323±1 K) were fitted to Langmuir isotherm model, the plotbetween Ce/qe and Ce yield straight lines (Fig. 8) with very highcorrelation coefficient values (R2) and low standard deviation (SD)

Fig. 6. Effect of system temperature on the adsorption percentage of M

value. Adsorption equilibrium parameters and statistical errorvariable (R2 and SD) for MB adsorption onto RH and RHA weredisplayed in the same order in Table 2. However, the Langmuiradsorption isotherms are presented in Fig. 8. The maximum value ofadsorption capacity obtained from Langmuir adsorption isothermwas1455.6 mg g−1 i.e. for MB adsorption on RHA. The adsorption capacitydata also reveals that an adsorption capacity increases with rise intemperature in both the cases. A high Ka value indicates the highaffinity of adsorbent for MB adsorption. Moreover in case of RHA,when the adsorption experiments were carried out at 313 and 323 K,the Ka values comes out infinity, as in these two cases 100% adsorptionoccur (means Ce was zero). Therefore higher the temperature, higherthe Ka and maximum adsorption capacity (qm), a higher temperatureis also favorable for adsorption. This increase in adsorption intensityand adsorption capacitymay be due to the change in pore size/volumeand enhanced rate of intra-particle diffusion of solute, as diffusion isan endothermic process.

Evolution of Freundlich adsorption isotherms is also an importantstep in characterizing the adsorption process as it provides anapproximate estimation of sorption capacity of the adsorbent. The

B onto RH and RHA studied at different initial dye concentrations.

Table 1Thermodynamic parameters for MB adsorption onto RH and RHA at initial dyeconcentration of 2000 ppm.

Adsorbent ΔGo (kJ mol−1) ΔHo

(kJ mol−1)ΔSo

(J mol−1 K−1)303 K 313 K 323 K

RH −09.95 −11.05 −12.14 23.28 109.66RHA −11.15 −12.16 −13.16 19.31 100.52

255P. Sharma et al. / Desalination 259 (2010) 249–257

Freundlich constants KF and n were calculated from the equilibriumdata for adsorption of MB on RH and RHA respectively at threedifferent temperatures 303, 313, and 323 K and are tabulated inTable 2 along with the correlation coefficient (R2) and standarddeviation (SD) values. From these data we can conclude that theFreundlich adsorption isotherm model unable to gave the best fittingof the experimental results as these plots have very low R2 value andlarge deviation in between the data points. This indicates that therewas only monolayer adsorption take place and no surface heteroge-neity and the exponential distribution of active sites and theirenergies.

Fig. 7. Variation of lnKd with temperature (1/T) for the adsorption of MB onto

Fig. 8. Langmuir adsorption isotherm for the MB adsorpti

Furthermore, RL values tabulated in Table 2 for the MB adsorptiononto the respective adsorbent, reveals that in all the cases the valuesof RL were positive and less than unity, indicating thereby a highlyfavorable adsorption in all the three cases at all the three temperatureconditions. The very low, separation factor values RL (0.0–0.0009) incase of MB adsorption onto RHA was, indicates that the dye preferredto remain bound on the adsorbent surface and shows the irreversiblenature of the adsorption process.

3.7. Comparison of adsorption efficiency

A number of industrial waste products, agricultural by products,natural materials and biosorbents have been tested and proposed formethylene blue removal and each adsorbent has its own specificphysical and chemical characteristics such as porosity, stability,surface area, surface morphology and physical strength, as well asinherent advantages and disadvantages in wastewater treatment. Inaddition, adsorption capacities of adsorbents also vary, depending onthe experimental conditions. Therefore, comparison of adsorptionperformance is difficult to make. However, it is clear from the presentstudy and the literature survey that non-conventional adsorbents RH

RH and RHA at initial dye concentration of 2000 ppm (Van't Hoff plot).

on onto RH and RHA at three different temperatures.

Table 2Adsorption isotherm constants for MB adsorption onto RH and RHA at three different temperatures.

Langmuir constants

Temperature RH RHA

Ka

(Lmg−1)qm(mg g−1)

R2 SD RL Ka

(L mg−1)qm(mg g−1)

R2 SD RL

303 K 0.266 1015.7 0.999 0.005 0.00188 0.525 1253.1 1.0 0.002 0.00095313 K 0.521 1191.5 0.999 0.003 0.00096 ∞* 1377.4 1.0 0.000 0.0323 K 0.618 1347.7 0.999 0.002 0.00081 ∞* 1455.6 1.0 0.000 0.0

Freundlich constants

Temperature RH RHA

KF n R2 SD KF n R2 SD

303 K 7.08E−13 0.205 0.705 0.774 1.74E−15 0.188 0.552 0.766313 K 3.89E−20 0.140 0.933 0.774 4.79E−15 0.194 0.591 –

323 K 2.63E−18 0.155 0.918 0.774 5.25E−16 0.175 0.817 –

*Ka=slope/intercept, and in these two cases intercept value is zero.

256 P. Sharma et al. / Desalination 259 (2010) 249–257

and RHA have maximum potential for the removal of methylene bluefrom aqueous waste. Out of these two RHA was better adsorbent. TheRH and RHA can be used for the treatment of industrial effluent. Boththese adsorbent were readily available, inexpensive and are effective.They also possess several other advantages that make them excellentmaterials for environmental purposes, such as high capacity (Table 2and Fig. 9) and rate of adsorption (Fig. 3), high selectivity for differentconcentrations (Fig. 3), and also rapid kinetics. Fig. 9 presents a viewof some of the highest adsorption capacities reported [46–73] andstudied. From the recent literature reviewed, adsorbents that standout for high adsorption capacities are commercially activated carbon(980.3 mg g−1), bark (914.0), and activated carbon prepared fromplum kernels by NaOH activation (828 mg g−1). These adsorbents areefficient and can be used effectively for the removal of MB fromaqueous solutions, but these adsorbent have even less adsorptioncapacity than that of RH and RHA. So after this study the first choice

Fig. 9. Compared maximum adsorption capacities for methyle

will be the RHA and RH. This demonstrated that RHA can be used forthe decontamination of effluents. It offers both a procedure of choicefor extraction processes and a lot of promising benefits for commercialpurposes.

4. Conclusions

The adsorption treatment process will provide an attractivetechnology, if the low-cost adsorbents are ready for use. Some timeadsorbents are physically and chemically treated to improve itsadsorption efficiency. These pretreatment processes were so chosenthat these should be cost effective. Generally these pretreatmentmethods are not cost effective when we apply at large scale, but thetreatment process we adopted will be cost effective as the rice huskash used for adsorption studies was also obtained from the paper mill.

ne blue on several low-cost adsorbents, and RH and RHA.

257P. Sharma et al. / Desalination 259 (2010) 249–257

So RHA is also an industrial waste product. In our study we also foundout that pretreatment of the adsorbent, improve its adsorptionefficiency. Rice husk has low removal efficiencies and adsorptioncapacities for MB removal, but on the other hand physical andchemical treatment of RH and RHA significantly improve itsadsorption potential. The present investigation shows that RH andRHA can be employed effectively for the treatment of dyeing effluentscontaining methylene blue dye. The waste RH and RHA is abundantlyavailable in northern India and hence its application to the treatmentof dyeing wastewater is expected to be economically feasible. Theymay also be useful in the treatment of other harmful and undesirablespecies present in the effluent. Higher adsorption capacities forselective removal of methylene blue with RH and RHA could becarried out in an adsorption treatment process by adjusting the pHaround 7, equilibration time of 30 min, and system temperature of323 K. The thermodynamic study indicated that the adsorption ofdyes was spontaneous and endothermic.

SymbolsΔGo the Gibbs free energy change (kJ mol−1)ΔHo the heat of adsorption (kJ mol−1)ΔSo the entropy change (J mol−1 K−1)Co the initial concentration of the dye solution (mg L−1)Ce the concentration of dye solution (mg L−1) at equilibriumqe the amount of dye adsorbed per unit weight of adsorbent

(mg g−1)Kd the distribution coefficient (mL g−1)V the solution volume (mL)W the amount of adsorbent (g)T the absolute temperature (K)R the universal gas constant (8.314 J mol−1 K−1)Ka the Langmuir constant representing the equilibrium con-

stant (L mg−1)qm the Langmuir constant representing the maximum mono-

layer capacity (mg g−1)KF the Freundlich constant as a relative indicator of adsorption

capacity (mg g−1)1/n the Freundlich constant as intensity of adsorptionRL the separation factor

Acknowledgements

A part of this work was supported by Priority Research CentersProgram through the National Research Foundation of Korea (NRF)funded by the Ministry of Education, Science and Technology (2009-0093816).

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