Int. J. Environ. Res., 6(4):995-1006, Autumn 2012ISSN: 1735-6865

Received 16 Aug. 2011; Revised 19 June 2012; Accepted 26 June 2012

*Corresponding author E-mail:asokadak@gmail.com

995

Use of Surface Modified Silica gel factory Waste for Removal of 2,4-DPesticide from Agricultural Wastewater: A case study

Koner, S .1, Pal, A. 2 and Adak, A. 3*

1 Department of Civil Engineering, Jalpaiguri Government Engineering College, Jalpaiguri – 735102,India

2 Department of Civil Engineering, Indian Institute of Technology, Kharagpur – 721302, India3 Department of Civil Engineering, Bengal Engineering and Science University, Shibpur, Howrah –

711103, India

ABSTRACT: The silica gel waste (SGW), after its collection from a local factory (Kolkata, India) wasmodified with cationic surfactant and was utilized as an adsorbing media for the removal of 2,4-dichlorophenoxyacetic acid (2,4-D) from agricultural runoff of a tea garden. Characterization of base adsorbentand modified adsorbent was carried out. The efficacy of the adsorbent was evaluated through both batch andfixed bed mode. Kinetics and isotherm study were conducted and the parameters were compared with 2,4-Dbearing distilled water samples. The values of diffusion coefficients were determined. Presence of electrolyteand solution pH was found to affect the performance of the adsorbent significantly. The removal efficiency ofsurfactant modified silica gel waste (SMSGW) in case of wastewater was found to be lower than the 2,4-Dbearing distilled water sample. The data of column run was analyzed using Logit model. Batch desorptionstudy was carried out using ethanol and acetone. The studies revealed surfactant coated SGW to be an efficientadsorbing media for 2,4-D removal.

Key words: Adsorption, Herbicide, waste, Agricultural runoff

INTRODUCTIONThe large-scale application of pesticides in

agriculture field and forestry, fast growing ofagrochemical industries worldwide and the domesticactivity of controlling pastes cause various pesticidesand herbicides entering into the surface and groundwater resources (Ghaderi et al., 2012). These renderserious pesticide pollution in the water resources(Nasrabadi et al., 2011; Mhadhbi and Boumaiza, 2012).The leaching runoff and industrial discharges areresponsible for this contamination in surface water(Gupta et al., 2006; Ahsan and DelValls, 2011; Assassiet al., 2011). One of the widely used herbicides in theworld is 2,4-dichlorophenoxyacetic acid (2,4-D). It isconsidered as moderately toxic and potentiallycarcinogenic (Chao et al., 2008). Due to its low pKavalue (2.73), it exists predominantly in anionic formwhich can badly affect the aquatic life and hamper theecosystem and at the same time the poorbiodegradability makes 2,4-D environmentally stable(Carter 2000). As a consequence, it has been frequentlydetected in water bodies in various regions of the world.The removal of pesticides from water has thus become

one of the major environmental concerns these days(Hammed et al., 2009).

The various treatment technologies such aschemical oxidation (Chu et al., 2004), photo-degradation (Topalov et al., 2001; Kundu et al., 2005),miceller flocculation (Poras and Talens, 1999),biological process (He and Wareham, 2009; Elefsiniotisand Li, 2008) and sorption techniques (Xi et al., 2010,Salman and Hammed 2010; Hammed et al., 2009; Chaoet al., 2009; Chao et al., 2008; Gupta et al., 2006; Goyneet al., 2004; Aksu and Kebasakal; 2005 & 2004; Pradoand Airodi, 2001; Alam et al., 2000; Akcay andYurdakoc, 2000) are available for removal of 2,4-D fromwater environment. Among these methods adsorptionhas been proven to be an effective and attractivemechanism (Hameed et al., 2009; Aksu and Kebasakal,2010). Recently, adsorption of 2,4-D has been carriedout using various surfactant modified metal oxidesbecause the use of surfactant bearing sludge(surfactant exhausted sorbent) has become one of thecost-effective alternatives for organic pollutantremoval from wastewater (Xi et al., 2010; Akcay and

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Koner, S. et al.

Yurdakoc, 2000). The ionic surfactants form monolayeror bilayer structures on the charged metal oxide surfacedepending on surfactant concentration(Somasundaran and Fuerstenau, 1966). The surfactantlayers pose the ability to solubilize different organicmolecules within its structure. Again, during the lastdecade, initiative has been taken by the manyresearchers to use the industrial wastes as anadsorbing media for wastewater treatment. Such useof waste would not only reduce its disposal problembut also would reduce the discharge of liquid waste inthe environment. Therefore use of such waste as metaloxide would be useful. Silica gel is a material that iswell known for its high thermal and chemical stability,good selectivity, swelling resistance, possibility ofrepeated use and economic viability (Donia et al., 2009).So the waste generated from a silica gel factory couldbe used as metal oxide. Moreover, in a recent studycarried out by the present authors, it was observedthat the waste generated from a local silica gel factorywas effective in adsorptive removal of the cationicsurfactant from wastewater (Koner et al., 2011). Withthis in view, the present study aims at furtherinvestigation of the potential of that waste, after itsuse as a sorbent for cationic surfactant (CS), for theremoval of 2,4-D from real wastewater collected froman agricultural runoff of a tea garden.

MATERIALS & METHODSOrange II (Hi-Media, India),

cetyltrimethylammonium bromide (CTAB) (Hi-Media,India), technical grade 2,4-D (98% pure) (Merck,Germany) and chloroform (Merck, India) were used asreceived. All other chemicals used in this study wereof high purity and used without further purification.All the chemicals were of analytical reagent grade. Anelectrical balance (CP 225D, Sartorious GMBH) wasused for weighing. Digital pH meter (PB-11, SartoriousGMBH) was used for pH measurements. Aspectrophotometer (UV 1800, Rayleigh, Beijing) wasused for absorbance measurement. For SEM and EDXanalysis, JEOLJSM5800 scanning electron microscopewas used. The silica gel waste was supplied by localfactory located at Kolkata, India. After receipt of thematerial it was thoroughly washed with tap waterfollowed by distilled water to remove the foreignsubstances and dried at 100o C. It was then sieved andabout 70% of the total material lied in the particle sizebetween 150 and 300 µm and that was used for thestudy. The final material after its washing and sievingwas measured to be around 65% of the collected waste.The silica gel waste (SGW) was then treated with CTAB,a representative of CS to form micelle like structure ontheir surfaces and thus surfactant-modified silica gelwaste (SMSGW) was prepared. This material was then

for 2,4-D removal from water environment.Spectrophotometric method was used for thedetermination of CS in the concentration range 0-12mg/l (Few and Ottewil, 1956). Orange II (0.4 x 10-3 M)chemically known as p-(β-napthol-azo) benzenesulfonic acid as ion pairing agent with CS was usedand chloroform was used as an extraction solvent fromthe solution and the same was taken for absorbancemeasurement at 485 nm wavelength. 2,4-D wasquantified by spectrophotometric method in theconcentration range of 0-35 mg/l at 230 nm wavelength.The real wastewater containing 2,4-D was collectedfrom surface runoff of a tea garden situated in Jalpaiguridistrict, West Bengal, India . To control the weeds ofthe tea garden, 2,4-D herbicide is applied in the form of2,4-D sodium salt (80% w/w) which is commerciallyknown as SALIX salt. After application of SALIX saltin the garden, there was a heavy storm of duration 25min and the runoff was collected from one of the fieldchannels of the garden during the storm. Thecharacteristics of the wastewater are presented in Table1. The concentration of 2,4-D in the wastewater wasmeasured to be 32.90 mg/l. After collection, thewastewater was passed through ordinary filter paperin order to remove the suspended particles and then itwas taken for the study.

Table 1. Characteristics of real wastewater(agricultural runoff) collected from tea garden

Parameter Values 2,4-D 32.9 mg/l pH 8.5±0.1 Turbidity 213 NTU Total dissolved solids 136 mg/l Total suspended solids 256 mg/l Conductivity 148.8 µS/cm Total hardness 44 mg/l as CaCO3 COD 37 mg/l

The kinetic experiments were carried with a fixed

SMSGW dose of 8 g/l at 30oC taking the wastewaterand the same was compared using synthetic samplesof 2,4-D prepared in distilled water at the same initialconcentration and the samples were shaken in amechanical shaker at an agitation speed of 150 rpm.The shaking time was varied from 0 to 30 min. Theequilibrium time was found to be 20 min and was usedfor further studies. 2,4-D bearing real wastewatergenerally contains various dissolved salts which mayinterfere the performance of the adsorbent. Therefore,it is required to investigate the effect of variousinterfering substances on the 2,4-D removal process.Thus, experiments were carried out to see the effectsof various electrolytes and dissolved salts like sodium

Int. J. Environ. Res., 6(4):995-1006, Autumn 2012

997

chloride, sodium sulphate, calcium chloride,magnesium sulphate (0.01N and 0.02N). Effect of pH(in the range 1.47 - 10.88) was also studied. For thesestudies the initial 2,4-D concentration spiked in distilledwater was taken as 50 mg/l and the adsorbent dosewas 10 g/l. The temperature and contact time and pHwere 30oC, 20 min and 3.9±0.1 respectively. Batchdesorption study was conducted using organicsolvents. Four isotherm models, namely Freundlich,Langmuir, Temkin and Redlich-Peterson isotherm werealso tested for their ability to describe the experimentalresults at the temperature of 30oC and the isothermparameters obtained for wastewater were comparedwith distilled water spiked sample. Fixed bed columnstudy was conducted using a column of 2.5 cm internaldiameter and 100 cm length. The adsorbent was packedin the column between two layers of glass wool andthe depth of bed was 20 cm. The study was conductedin the up flow mode with a volumetric flow rate 10 ml/min (~1.22 m3/m2/hr). The samples were collected atcertain intervals and were analyzed for remaining 2,4-D concentration. In order to investigate the mechanismof solute adsorption onto the adsorbent, four kineticmodels viz., first order reaction model (Benefield andRandall 1980) based on the solution concentration,pseudo-first order equation of Lagergren (1898) basedon the solid capacity, second order reaction modelbased on the solution concentration (Benefield andRandall 1980) and pseudo-second order reaction modelof Ho and Mckay based on the solid phase sorption(Ho and Mckay 1999) were analysed and a comparisonof the best fit sorption mechanism was made.The linearized forms of different reaction models areshown below.

First order: tkCC ot 1lnln −= (1)

Pseudo first order: ( )teSt qqk

dtdq

−= 1 (2)

Second order: tkCC ot

211

=− (3)

Pseudo second order: tqqkq

t

eeSt

112

2

+= (4)

Where,Ct = solute concentration at any time tCo = solute concentration at time t=0qt = amount of solute adsorbed per unit weight ofadsorbent at any time tqe = amount of solute adsorbed per unit weight ofadsorbent at equilibrium

k1 = first order reaction rate constantkS1 = pseudo-first order reaction rate constantk2 = second order reaction rate constantkS2 = pseudo-second order reaction rate constant

The experimental reaction kinetic data were analysedusing the above four kinetic model.

RESULTS & DISCUSSIONBefore preparation of SMSGW, it is important to

find the maximum adsorption capacity and the initialCS concentration at which it would occur. In the presentstudy, CTAB was used as a representative of CS.Therefore, the adsorption isotherm study was carriedout in the initial CTAB concentration range of 0-20000mg/l. The adsorbent dose was kept fixed at 30 g/l. Thecontact time was taken as 30 min since this was theequilibrium contact time. The temperature and pH were302o C and 6.9±0.1 respectively. From this isothermstudy, the maximum adsorption capacity was found tobe 0.188 mmol/g (68.62 mg/g) and it occurred wheninitial concentration of CTAB was 7500 mg/l. Thiscondition was used for the preparation of SMSGW.Therefore, 180 g SGW (at a dose of 30 g/l) was shakenfor one hour with 6 litres of CTAB solution havingconcentration of 7500 mg/l. After shaking, thesupernatant was discarded and the SGW was washedthoroughly initially with tap water and finally withdistilled water and then dried at 60oC for 24 hours. Thusthe SGW was converted to SMSGW.

The characterization of the base adsorbent wascarried out after proper washing and sieving of thecollected waste. Table 2 summarizes the differentproperties of the base adsorbent (SGW) and modifiedadsorbent (SMSGW) such as size, bulk density, pHpoint of zero charge (pHpzc), BET surface area and porevolume. The surface morphology of the adsorbentwas obtained from the Scanning Electron Microscopy(SEM) studies and the elemental composition fromEnergy Dispersive X-ray (EDX) analyses. The SEMphotograph showed that the adsorbent particles areof ill defined shapes having irregular surfaces. It wasalso noted that after surfactant modification the surfaceof SGW became smooth (Fig. 1) and lowered the surfacearea. EDX analysis is a useful tool for identification ofthe kinds of elements contained in the solid specimenwithin short analysis period. The EDX analysis showedthat the SGW contained 96% silica.

Kinetics studies were conducted to find out theequilibrium contact time for 2,4-D uptake from realwastewater as well as distilled water spiked sample bySMSGW. An adsorbent dose of 8 g/l and the shakingtime ranging from 0 to 30 min was used to conduct thekinetic study. Fig. 2 shows the results of effect of

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Removal of 2,4-D Pesticide

Table 2. Characteristics of SGW and SMSGW

Parameter SGW SMSGW Partic le size (µm) 150 - 300 150 – 300

Bulk density (g/cc) 0.615 0.624

pHpzc 2 10.2

BET surface area (m2 /g) 264.6 194.4

Pore volume (ml/g) 0.66 0.38

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0 5 10 15 20 25 30 35

Contact time (min)

q t (

mm

ol/g

)

distilled water spiked sample

real wastewater

Fig. 2. Effect of contact time on 2,4-D uptake by SMSGW

Fig. 1. SEM image of (a) SGW and (b) SMSGW

(a) (b)

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Int. J. Environ. Res., 6(4):995-1006, Autumn 2012

contact time. The rate of adsorption is very rapidinitially. From the figure it is shown that in 15 min theequilibrium reached. But a contact time of 20 minconsidered to be sure that full equilibrium was reached.Similar trend was also observed in case of 2,4-D-spikedin distilled water sample. But in case of distilled waterspiked sample the removal efficiency was better thanwastewater due to presence of various electrolytes andprobably higher pH of the wastewater.

The equilibrium contact time, so found, indicatedvery quick reaction compare to other studies like 90min (Gupta et al., 2006), 450 min (Hameed et al., 2009)and 7 days (Aksu and Kebasakal, 2004). The practicalapplication of this adsorbent in continuous mode inthe field will be easier since it would facilitate to reducethe reactor volume due to such lower value ofequilibrium contact period.

The rate of sorption was determined by testingthe four reaction kinetics models as described earlier.After analyzing the four models, it was found that thepseudo-second order model (Fig. 3) fitted best amongall the models indicating the rate limit step to bechemisorption or chemical adsorption (Ho and Mckay1999). The equations, equilibrium uptake capacity,reaction rate constant and the determining coefficients

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35

Contact time (min)

t/qt

distilled water spikedsamplereal wastewater

Linear (real wastewater)

Table 3. Equations and R2 values of the linear fit lines of pseudo-second order reaction models for the removalof 2,4-D by SMSGW

(R2) of the pseudo-second order model are shown inTable 3. The value of reaction constant was found tobe 0.4425 g/mg.min for real wastewater and 0.2067 g/mg.min distilled water spiked sample. The reactionconstant, so found, could be used for the design ofdifferent types of batch reactors generally used in thefield.

The diffusion coefficients for the present studywere determined using first order reaction kinetics data.Assuming spherical geometry of the sorbents, the firstorder rate constant, K1 obtained from first orderkinetics profiles can be correlated to the pore diffusionand film diffusion coefficients (Helfferich, 1962).

pDr

t2

02/1 030.0= (5)

CC

Dr

tf

δ021 23.0= (6)

Where, t1/2 = half time ro = radius of adsorbent particle = 0.0225 cm Dp = pore diffusion coefficient (cm2/sec) Df = film diffusion coefficient (cm2/sec)

Fig. 3. Linear plot of pseudo-second order reaction kinetic model

Sample type Equation of linear fit line Reaction rate constant (g/mg?min)

R2

Real wastewater tqtt

3854.03365.0 += 0.4425 0.9999

Distilled water spiked

tqt

t

2701.03529.0 += 0.2067 0.9998

1000

Koner, S. et al.

−

C = concentration of adsorbate on the adsorbent =

27.68 mg/lC =concentration of adsorbate in solution = 5.22 mg/lδ = film thickness (cm)t1/2 can be calculated using the relation (Asher etal., 1990)

121

)]5.0[ln(K

t −= (7)

The value of K1 in case of real wastewater wasfound to be 0.0225 min-1. Using the value of K1, thevalues of t1/2 were calculated and it was found to be1848.39 sec. Assuming spherical geometry of thesorbents, δ = 0.001 cm (Helfferich, 1962) and usingcalculated values of t1/2 the film diffusion coefficients,Df and pore diffusion coefficients, Dp for realwastewater were calculated and these were 1.48x10-8cm2/sec and 8.21x10-9 cm2/sec respectively. Nowaccording to Michelson (1975) for film diffusion to bethe rate limiting step in sorption, the value of filmdiffusion coefficient (Df) should be between 10-6 to10-8 cm2/s, where as for pore diffusion to be rate limiting,the pore diffusion coefficient (Dp) should be in therange of 10-11 to 10-13 cm2/s. In the present study, thevalue of Df was found to be near to 10-8 that indicatedthat there is also a possibility for the film diffusion tobe the rate limiting step.

The effect of pH and electrolytes were studiedtaking initial 2,4-D concentration of 50 mg/l (spiked indistilled water sample). The adsorbent dose was 10 g/l. From the Fig. 4, it is shown that with increase of pH,the capacity of 2,4-D uptake by SMSGW decreasedsince at higher pH the surface of SMSGW (pHpzc =10.2) became more negative and therefore resulted

lower affinity of the adsorption of anionic 2,4-Dmolecules (Gupta et al., 2006; Chao et al., 2009;Hammed et al., 2009). But for the present case, as thedesorption of CTAB from the surface of SMSGWoccurred at lower pH, the adsorption of 2,4-D at lowerpH also found to decrease (Koner et al., 2011). Theresult for effect of electrolytes is plotted in Fig. 5. Therewas a reduction in removal efficiency in presence ofthese salts. This was due to competition of coexistenceof anions on positively charged surface. Chao et al.(2009) have studied that the negative effect of anionson 2,4-D removal was in the order of SO4

-2 > Cl-. Similarresult was obtained in this case also. It was alsointeresting to see that in presence of CaCl2 and MgSO4,the removal efficiency was slightly increased compareto the removal efficiency in presence of NaCl andNa2SO4 respectively. This was probably due topresence of multivalent cations that increased thepositive charge of SMSGW.

A proper contact between the adsorbate andadsorbent is necessary for any adsorption process. Theconcentration gradient at the adsorbent surface ismaximum initially. However, with time adsorbate speciesstart migrating to the adsorbent surface andconcentration gradient starts decreasing. After sometime concentration gradient becomes negligible and thereis no net transfer onto the adsorbent. This establishesthe dynamic equilibrium between adsorbate present insolid and liquid phase. The adsorption isotherm definesthis equilibrium state. Several models have beendeveloped to define the adsorption isotherm. Amongthem four widely used models such as Freundlich model(Freundlich 1906), Langmuir model (Langmuir 1918),Temkin model (Temkin and Pyzhev, 1940) and Redlich-Peterson model (Redlich and Peterson 1959) have beenstudied in the present study.

0.010.020.030.040.050.060.070.080.090.0

100.0

0 2 4 6 8 10 12

Initial pH

2,4-

D re

mov

al (%

)

Fig. 4. Effect of initial pH on 2,4-D uptake by SMSGW

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Int. J. Environ. Res., 6(4):995-1006, Autumn 2012

The Freundlich equation is an empirical expression thatencompasses the heterogeneity of the adsorbentsurface and the exponential distribution of sites andtheir energies. The expression for Freundlich model isgiven in equation 8.

nefe Ckq1

= (8)

Where,kf = Freundlich constant related to adsorption capacity

n1

= adsorption intensity

qe = amount of adsorbate on adsorbent at equilibrium(mg/g)Ce = equilibrium concentration (mg/l)The linear form of Freundlich isotherm model isrepresented below.

efe Cn

kq ln1lnln += (9)

Langmuir isotherm model is known as idealizedmonolayer model. The basic assumptions of Langmuirmodel (i) a fixed number of accessible sites having equalenergy (homogenous surface) and (ii) reversibility ofthe adsorption process. When the rate of adsorptionbecomes equal to the rate of desorption of moleculesfrom the surface, equilibrium is reached. Langmuirisotherm model may be expressed as

eL

eLme CK

CKqq

+=

1 (10)

Where,qm = maximum adsorption capacity (mg/g)

KL = constant related to energy of the sorption system(l/mg)The linearized form of the Langmuir isotherm model ispresented below.

eLmme CKqqq1111

+= (11)

Temkin isotherm considered the effect of indirectadsorbate adsorbent interactions on adsorption.Temkin isotherm also represents the bindingheterogeneity with a simple expression, which haspredictive power over a wide range of concentration.Temkin isotherm can be represented as

)ln( eTe CKb

RTq = (12)

Where, R is universal gas constant, T is absolutetemperature (K) and b and KT are models constants. Itcan be linearized as

)ln()ln( 11 eTe CBKBq += (13)

Where, bRTB =1 related to heat of adsorption and

KT is the equilibrium binding energy constant (l/mg)corresponding to the maximum binding energy.The Redlich-Peterson isotherm model incorporatedthree parameters and can be applied either inhomogeneous and heterogeneous systems. Jossenset al. (1978) included the basic concepts of Freundlichand Langmuir isotherms to make a single equation andexpressed as

βeR

eRe Ca

CKq

+=

1 (14)

50.0

55.0

60.0

65.0

70.0

75.0

80.0

85.0

90.0

Noelectrolyte

Sodiumchloride

Sodiumsulphate

Calciumchloride

Magnesiumsulphate

Added electrolytes (N)

2,4-

D re

mov

al (%

)Zero electrolyte 0.01(N) 0.02(N)

Fig. 5. Effect of electrolytes on 2,4-D uptake by SMSGW

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Removal of 2,4-D Pesticide

Freundlich constants Sample type Kf [(mg/g) (l/mg)1/n] 1/n R2 Real wastewa te r 0.258 0.856 0.9780 Distilled water spiked 1.67 0.657 0.9629 Langmuir constants Sample type qm (mg/g) KL ( l/mg) RL R2 Real wastewa te r 17.73 (0.08 mmol/g) 0.011 0.734 0.9797 Distilled water spiked 33.90 (0.153 mmol/g) 0.032 0.381 0.9767 Temkin constants Sample type KT (l/mg) B1 R2 Real wastewa te r 0.228 2.231 0.9527 Distilled water spiked 0.518 5.306 0.9973 Redlich-Peterson constant s Sample type KR (l/mg) aR β R2 Real wastewa te r 0.416 0.751 0.2260 1.0000 Distilled water spiked 1.656 0.188 0.7460 1.0000

Table 4. Isotherm constants for 2,4-D removal from real wastewater and distilled water spiked samples bySMSGW at 30oC

This equation can be linearized as

)ln()ln()1ln( eRe

eR Ca

qC

K β+=− (15)

Where, KR and aR are the model constants and βis the exponent which has the value between 0 to1.Depending on the value of exponent β either inLangmuir or Henry’s law of isotherm. A trial and erroroptimization technique was adopted to solve the aboveequation using MATLAB. Table 4 represents theconstants of the above four isotherm models for bothwastewater and distilled water spiked samplesrespectively. From the R2 values of determiningcoefficients (R2) it is seen that Redlich-Peterson modelfitted best in comparison to other models for the realwastewater. Similar result obtained in case distilledwater spiked sample also. The maximum adsorptioncapacity obtained from Langmuir isotherm was 17.73mg/g (0.08 mmol/g) at 30oC, which was found to belower than the distilled water spiked sample (33.9 mg/gor 0.153 mmol/g). Such lower value of adsorptioncapacity and higher value of optimum adsorbent dosethan distilled water spiked sample was due to thepresence of dissolved salts in the wastewater.Moreover, the solubility of 2,4-D sodium salt (45000mg/l at 25oC) in water is 50 times of 2,4-D acid (900 mg/l at 25oC). This higher solubility of the 2,4-D salt resultedlower affinity towards adsorption.

The capacity obtained for uptaking pesticides (fordistilled water spiked sample) was found to be greaterthan other sorbents like dioctadecyl dimethylammoniumbromide-modified polysagrite (Xi et al., 2010), blastfurnace sludge and dust (Gupta et al.,2006), activatedused tyres (Hamadi et al., 2004), oil shale ash (Al-

Quodah et al., 2007), bituminous shell (Ayar et al., 2008),3-trimethoxysilylpropylamine anchored silica gel (Pradoand Airodi, 2001) and waste rubber granules (Alam etal., 2000). Though this capacity was lower compare tocarbonaceous adsorbents derived from date stone(Hameed et al., 2009) and fertilizer waste (Gupta et al.,2006), granular and powdered activated carbon (Salmanand Hammed, 2010; Aksu and Kebasakal 2004 & 2005)and layered double hydroxides (Chao et al., 2008; Chaoet al., 2009). The important part, however, in case of2,4-D removal by SMSGW was the utilization ofexhausted waste after removal of CS, which is knownto be an environmental pollutant and resistant to bio-degradation. The use of an industrial waste for CSremoval and further use of exhausted waste for 2,4-Dremoval made the process not only cost-effective butalso it caused waste minimization.

It is also very important to be mentioned that theuptake capacity of SMSGW could be increased, if theadsorption of CTAB on the surface of SGW was carriedout in presence of electrolytes and with pH adjustmentduring the preparation of SMSGW, since the presenceof electrolytes and higher pH was found to enhancethe surfactant coverage on the surface of silica particles(Koner et al., 2011). So in actual application in the fieldthere is no possibility to achieve the adsorptioncapacity of SMSGW lower than the capacity obtainedhere. The essential characteristics of the Langmuirisotherm can be expressed in terms of a dimensionlessconstant separation factor RL (Hall et al., 1966), whichis given by

LoL KC

R+

=1

1(16)

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Int. J. Environ. Res., 6(4):995-1006, Autumn 2012

Where, Co is the initial concentration of 2,4-D (mg/l)and KL (l/mg) is Langmuir constant. The value of RLindicated nature of adsorption; irreversible (RL = 0),favourable (0 < RL < 1) and unfavourable (RL = 1). Thevalue of RL (Table 4) indicated favourable adsorption.The breakthrough curve for 2,4-D using SMSGW forthe column run is shown in Fig. 6. The breakthroughtime (corresponding to C/Co = 0.1) and exhaust time(corresponding to C/Co = 0.9) were found to be 12 and25 hours respectively. The corresponding volumes oftreated wastewater were 7.2 and 15 l respectively. Theproperties of adsorption zone or mass transfer zonewere calculated using the column data (Adak, 2006).Height of adsorption zone was found to be 16.45 cmand the rate of adsorption zone movement through thebed was 1.26 cm/h. The percentage of the total columnsaturated at breakthrough was found to be 42%.

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30

Time (h)

C/C

0

ln [C/ (C 0 -C )] = 0.3052 (t)- 5.4127R2 = 0.9612

-3

-2

-1

0

1

2

3

10 12 14 16 18 20 22 24 26

Time (h)

ln [C

/(C

0-C

)]

The fixed bed column was designed by Logitmethod (Oulman 1980). The logit equation can bewritten as:

tKCV

KNXCC

CCo

o

o +−=⎥⎦

⎤⎢⎣

⎡− /1

/ln (17)

Where,C = Solute concentration at any time tCo = Initial solute concentrationV = Approach velocityX = Bed depthK = Adsorption rate constantN = Adsorption capacity coefficient

Rearranging Eq. 17

Fig. 6. Breakthrough curve for 2,4-D uptake from wastewater by SMSGW

Fig. 7. Linearized form of Logit model

1004

Koner, S. et al.

tKCV

KNXCC

Co

o+−=⎥

⎦

⎤⎢⎣

⎡−

ln (18)

Plot of ⎥⎦

⎤⎢⎣

⎡−CC

C

oln vs. t gives a straight line with

slope KCo and intercept VKNX

− from which, value

of K and N could be calculated. Plot of vs. t is shownin Fig 7. The value of adsorption rate coefficient (K)and adsorption capacity coefficient (N) were obtainedas 0.0095 l/mg.h and 3473 mg/l respectively. Thesevalues could be used for the design of adsorptioncolumns.

2,4-D could be desorbed from exhausted SMSGWsurface using ethanol and acetone. It was observedthat in case of acetone and ethanol more than 80% and75% desorption was achieved respectively. 10 g ofexhausted SMSGW was taken in 50 ml of solvent (200g/l) and then agitated at 150 rpm for 60 min at atemperature of 30o±2 C. The regenerated 2,4-D couldbe collected and reused. The organic solvent could bedistilled off using a Soxhlet apparatus and the samesolvent could be recycled for the same purpose.

CONCLUSIONSMSGW could effectively be used for the removal

of 2,4-D from real wastewater. A very quick reactionwas found to occur for the adsorption process. Theremoval efficiency and adsorption capacity was foundto be lower for wastewater in comparison to distilledwater. The uptake capacity of SMSGW decreased inpresence of various electrolytes and at higher pH.Interestingly in a wide range of pH, the removal wasnot significantly affected. The value of diffusioncoefficients indicated that the film diffusion might bethe rate limiting step for real wastewater. Theadsorption of 2,4-D followed the R-P isotherm modelbest compared to other isotherm models likeFreundlich, Langmuir and Temkin isotherm model. Thecapacity obtained from Langmuir isotherm can becompared with many other adsorbents especially whichare derived from industrial waste. Therefore, anindustrial waste after its use for removal of surfactantcould be reused for pesticide removal from waterenvironment. Such effective use of a ‘waste’ made theprocess not only cost-effective but also environment-friendly.

ACKNOWLDEGEMENTWe are thankful to the SERC Division, Department

of Science and Technology, New Delhi, India for thefinancial support for this work. We are also gratefulto M/S Kalpataru Pvt. Ltd., Kolkata, India forsupplying us the silica gel waste material and Mr.Alakesh Chandra Mandal of Indian Institute ofScience, Bangalore, India for his kind help andvaluable advice.

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Removal of 2,4-D Pesticide