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Adsorption studies of chromium (VI) removal fromwater by lanthanum diethanolamine hybrid materialSandip Mandala, Manoj Kumar Sahua, Anil Kumar Giria & Raj Kishore Patelaa Department of Chemistry, National Institute of Technology, Rourkela-769008, Odisha, IndiaPublished online: 06 Nov 2013.

To cite this article: Sandip Mandal, Manoj Kumar Sahu, Anil Kumar Giri & Raj Kishore Patel (2014) Adsorption studies ofchromium (VI) removal from water by lanthanum diethanolamine hybrid material, Environmental Technology, 35:7, 817-832,DOI: 10.1080/09593330.2013.852627

To link to this article: http://dx.doi.org/10.1080/09593330.2013.852627

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Environmental Technology, 2014Vol. 35, No. 7, 817–832, http://dx.doi.org/10.1080/09593330.2013.852627

Adsorption studies of chromium (VI) removal from water by lanthanum diethanolaminehybrid material

Sandip Mandal, Manoj Kumar Sahu, Anil Kumar Giri and Raj Kishore Patel∗

Department of Chemistry, National Institute of Technology, Rourkela-769008, Odisha, India

(Received 12 July 2013; final version received 25 September 2013 )

In the present research work, lanthanum diethanolamine hybrid material is synthesized by co-precipitation method and usedfor the removal of Cr(VI) from synthetic dichromate solution and hand pump water sample. The sorption experiments werecarried out in batch mode to optimize various influencing parameters such as adsorbent dose, contact time, pH, competitiveanions and temperature. The characterization of the material and mechanism of Cr(VI) adsorption on the material wasstudied by using scanning electron microscope, Fourier transform infrared, X-ray diffraction, Brunauer–Emmett–Tellerand thermogravimetric analysis–differential thermal analysis. Adsorption kinetics studies reveal that the adsorption processfollowed first-order kinetics and intraparticle diffusion model with correlation coefficients (R2) of 0.96 and 0.97, respectively.The adsorption data were best fitted to linearly transformed Langmuir isotherm with correlation coefficient (R2) of 0.997. Themaximum removal of Cr(VI) is found to be 99.31% at optimal condition: pH = 5.6 of the solution, adsorbent dose of 8 g L−1

with initial concentration of 10 mg L−1 of Cr(VI) solution and an equilibrium time of 50 min. The maximum adsorptioncapacity of the material is 357.1 mg g−1. Thermodynamic parameters were evaluated to study the effect of temperature onthe removal process. The study shows that the adsorption process is feasible and endothermic in nature. The value of E(260.6 kJ mol−1) indicates the chemisorption nature of the adsorption process. The material is difficult to be regenerated. Theabove studies indicate that the hybrid material is capable of removing Cr(VI) from water.

Keywords: Cr(VI); adsorption; Langmuir isotherm; hybrid material

1. IntroductionThe synthesis of hybrid materials, nano materials, porousmetal oxides and super adsorbents that possess func-tional organic molecules embedded to the inorganic partand has been very active area of research over past fewdecades because of their versatile uses. Hybrid materi-als are composites, which includes two or more thantwo moieties blended on the molecular scale. Commonlyone of these moieties is inorganic and the other one isorganic.[1,2] The hybrid material may be crystalline oramorphous with possibilities of weak and strong interac-tion between two components. The organic moiety, whichcontains functional groups, that allows the attachment toany inorganic network or moiety. The most noticeableadvantage of inorganic–organic hybrid materials is thatthey can favourably combine to offer many propertiesof organic and inorganic components in one material.[1]The desired function can be delivered from the inorganicor organic or from both the components. These types ofmaterials have diverse prospective applications in the fieldof separation, adsorption, catalysis, drug delivery, sen-sors and hydrogen storage, which attracted much attentionin recent years.[3–6] Many hybrid materials are reported

∗Corresponding author. Email: rkpatel@nitrkl.ac.in

like chitosan-impregnated hybrid material,[7] magnetitepolyethylenimine montmorillonite,[8] iron oxide-modifiedpolyglycidyl methacrylate (PGMA) graft copolymer hybridadsorbent (PGMAFe),[9] amine-grafted alumino-siloxanehybrid porous granular media [10] and magnetic cyclodex-trin chitosan [11] which were used extensively for theremoval of different anions. Because of excellent ion-exchanger properties of hybrid material, researchers touse these materials for environmental application havedrawn significant attention. Chromium is spread into var-ious segments of environment by different natural andanthropogenic activities. Chromium is used in the chem-ical industry as an oxidizing agent, as chrome alloys, paintsand pigments, anti-corrosive agents, fungicides, electro-plating, manufacturing of photography plates and steelindustries.[12] The wide industrial applications of thismetal in last few decades have increase the concentrationsof chromium in water environment. Cr(VI) is a knowncarcinogen [13–15] which is listed in the 25 most toxichazardous elements posing the greatest risk to humanhealth and environment.[16] Elevated concentrations ofchromium in drinking water are linked to health problemssuch as lungs cancer, bronchial asthma, stomach cancer and

© 2013 Taylor & Francis

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Table 1. Comparison of the present material with the materials available in the literature.

Sl. no Hybrid materials Adsorption capacity (mg g−1) Isotherm models Reference

1 Magnetite–polyethylenimine–montmorillonite

8.8 Langmuir [8]

2 PGMA graft copolymer and iron oxide-modified polyglycidyl methacrylategraft copolymer (PGMAFe)

132.5 and 162.9 Freundlich [9]

3 Amine-grafted alumino-siloxane hybridsorbent

48 Sips Redlich and Langmuir [10]

4 Graphene oxide functionalized withmagnetic cyclodextrin–chitosan

67.66 Langmuir [11]

5 Ethylenediamine functionalized Fe3O4 61.35 Langmuir [21]6 Fe-cross-linked chitosan 65 Freundlich [22]7 La-DEA 357.1 Langmuir Present study

hepatotoxicity [17] in human health because of its high tox-icity, the Central Pollution Control Board (CPCB, India),US Environmental Protection Agency (USEPA) and WorldHealth Organization (WHO) [18–20] have set the maxi-mum permissible limit as 0.1 and 0.05 mg L−1, respectively.Current technologies for the removal of chromium, likemembrane filtration, reverse osmosis and nano-filtrationare not very popular due to high process cost and regu-lar maintenance. However, adsorption is found to be mostapplicable and promising method due to its low processcost and high removal efficiency. However, the success ofthis process depends on the nature of adsorbent. Since thereare not many suitable adsorbent available for the removalof chromium, an attempt has been made to use lanthanumdiethanolamine (La-DEA) hybrid material for adsorptionof Cr(VI). The synthesis of La-DEA hybrid material andits application for removal of Cr(VI) has not been reported.The aim of present work is to study the removal efficiencyof Cr(VI) on La-DEA using batch process from syntheticchromium solution and hand pump water. The material inthis work has a better adsorption capacity as presentedin Table 1. The material decolourizes [23] the colour ofdichromate solution immediately after the addition of thematerial. Though La is an uncommon metal, the salts ofLa are having good ion-exchange capacity and have avery strong tendency towards phosphate, fluoride and anionof chromium.[24,25]

2. Experimental2.1. Reagents and chemicalsAll the chemicals used for the studies are of analytical rangegrade obtained from Merck (Merck, India) and Sigma-Aldrich (India). All solutions are prepared in distilled water.About 1000 mg L−1 stock solution of Cr(VI) is preparedby dissolving 5.1 g of K2Cr2O7 in 1 L distilled water. Therequired concentrations of chromium solution are obtainedby serial dilution of stock solution. The experiments wereconducted by using Borosil glassware.

2.2. Synthesis of the La-DEA hybrid materialThe synthesis of La-DEA is carried out by theco-precipitation method at pH = 9. The addition of 1.0 Maqueous solution of hexahydrate lanthanum nitrate (La(NO3)3.6H2O) drop-wise from a burette to 2.0 M aque-ous solution of Diethanolamine (DEA) taken in a roundbottomed-flask with constant stirring at a speed of 200 RPMby using a magnetic stirrer at a temperature around 60◦Cfor 3 h. The pH of the solution is maintained by the additionof required amount of 0.1 M HCl or NaOH solution untilprecipitation occurs. The white coloured gel so obtained isallowed to stand for 24 h to ensure the complete formationof gel at room temperature. The gel is separated from themother liquor by decantation and washed well with dis-tilled water till it attains pH = 7 of the washing. A numberof materials are synthesized with the variation of molar ratioof precursor (Table 2). The material is then dried at 100◦C inan oven for 24 h and collected as powder and the material isactivated by using 1 M HCl. A initial investigation is carriedout to know the Cr(VI) removal capacity of the material with10 mg L−1 Cr(VI) solution with fixed quantity of adsorbentprepared by different molar ratios, at pH = 7 and ambienttemperature (25◦C). The material prepared with 2 M solu-tion of DEA showed comparatively better results, which isused for further detail adsorption studies.

2.3. Characterization of the La-DEA hybrid materialThe hybrid material is characterized by different analyti-cal techniques. The surface charge density (σ ) of sorbent isdetermined by a potentiometric titration method. The fol-lowing equation is used to determine the surface chargedensity [26]:

σo = ((CA − CB + [OH−] − [H+])F)

m, (1)

where CA and CB are the molar concentrations of acid andbase needed to reach a point on the titration curve, [H+] and[OH−] are the concentrations of H+ and OH−, F is the Fara-day constant (96,490 C mol−1) and m (g L−1) is the mass of

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Table 2. Ion-exchange capacity of various samples of La-DEA (W = 1 g), IEC in meq g−1.

Materials Molar ratio Particle size (nm) IEC KCl IEC Ca(NO3)2 IEC NaNO3 IEC NaCl IEC Mg(NO3)2

La-DEA-124 1:1 124 0.87 0.61 0.68 0.84 0.72La-DEA-117 1:2 117 2.12 1.81 1.91 1.87 2.01La-DEA-68 1:3 68 1.54 1.04 0.92 1.35 1.27La-DEA-86 1:4 86 1.20 1.32 1.18 1.32 1.11La-DEA-132 1:5 132 0.60 0.57 0.68 0.77 0.63

the sorbent. The acid–base titrations were conducted sep-arately with 0.1 g of adsorbent at room temperature, in a250 mL Erlenmeyer flask. Free CO2 Millipore water is usedthroughout the experiment. A known volume of 0.2 M nitricacid or 0.2 M sodium hydroxide is added into the mixturewhich is gently shaken on a rotary shaker (Remi model:RS – 24 BL, India) at 20 RPM. The pH of each additionof acid or base in a time interval of 10 min is recordedusing an Elico LI 614 pH (accuracy: ±0.001) metre duringtitration. The high purity nitrogen gas (99.85%) is purgedinto the solution throughout experiment to remove the effectof carbon dioxide. Brunauer–Emmett–Teller (BET, Quan-tachrome Autosorb I), X-ray diffraction (XRD, ShimadzuXRD-6000 diffractometer), scanning electron microscope(SEM, JEOL JSM-6480LV SEM) and Fourier transforminfrared (FTIR, Shimadzu IR Prestige-21 FTIR instrument)are used for the characterization of the material before andafter adsorption. Thermogravimetric analysis–differentialthermal analysis (TGA–DTA Shimadzu TGA 60H) andparticle size analyzer (Malvern Nano ZS 90) are used fordetermination of thermal stability and particle size of thehybrid material (La-DEA). Atomic absorption spectroscopy(AAS, Elico-176 atomic absorption spectrometer) is usedfor the determination of the concentration of Cr(VI). Thecarbon, nitrogen, hydrogen and sulphur are determined byusing a carbon, hydrogen, nitrogen and sulphur (CHNSelemental analyzer, Vario EL Cube, Elementar, Germany)analyser.

2.4. Batch experimentsThe Cr(VI) adsorption experiments from its aqueous solu-tion by La-DEA is conducted using standard 50 mL Cr(VI)solution in absence of other competing ions. The adsorp-tion experiments are carried out in a series of 250 mLErlenmeyer flask with stopper by adding 0.1–1.0 g of La-DEA in 100 mL of Cr(VI) solution. Stoppers are providedto avoid change in concentration due to evaporation. Allthe experiments are carried out at room temperature toavoid interference due to difference in temperature aftercontinuous stirring over magnetic stirrer at 400 RPM fora predetermined time interval, the solid product is sep-arated by filtration through Whatman-42 (2.5 μm) filterpaper. In the present study, the Whatman-42 (2.5 μm) filterpaper has no role in the adsorption process. The remain-ing Cr(VI) concentration is determined by AAS. pH of the

solution is maintained by the addition of required amount of0.1 M NaOH or 0.1 M HCl. The important parameters suchas dose (0.1–1 g/100 mL), contact time (10–100 min), pH(1–12) and temperature (20–110◦C) affecting the removal ofCr(VI) ion have been independently varied by keeping otherparameters constant during the experiments. The experi-ments are repeated thrice and the mean observation valuefor each parameter is taken and represented in the presentstudy. In order to know the effect of other competing ions onCr(VI) adsorption, studies are done by using anions (bicar-bonate, carbonate, phosphate, fluoride, chloride, sulphateand nitrate). Varying concentrations of solutions of theseanions are prepared from their sodium and potassium salts.The initial concentration of Cr(VI) is fixed at 10 mg L−1,while the initial concentration of other anions varied from10 to 300 mg L−1.

The adsorption capacity (Qe) of the material is obtainedby using the following equation:

Qe =(

(C0 − Ce)

m

)× V , (2)

where Qe is the amount of arsenic adsorbed onto unit mass ofthe adsorbent (mg g−1); C0 and Ce (mg L−1) are initial andequilibrium concentrations of total arsenic in the syntheticarsenic solution, V is the volume of arsenic solution (L)and m is the dry mass of the adsorbent (g). The adsorptionpercentage is calculated by using the following equation:

Removal% =[(

C0 − Ce

C0

)]× 100. (3)

The chemical stability of the material is studied by usingdifferent concentrations of acids and alkali, 1 g of material(La-DEA) is taken in 50 mL of different acids and alkali,kept for 24 h at room temperature and observed for changein the solution.

The Ion exchange capacity (IEC) of the material is deter-mined by the standard process; 1 g of the La-DEA materialis activated by keeping in suspension of 1 M HCl for 4–5 h and the material is collected by filtration and washingseveral times with distilled water. To know the magnitudeof elution, constant volumes (100 mL) of the eluents arepassed through constant weight of the activated material.The activated material of different compositions varied byparticle size is taken in a glass column of internal diameterof 1 cm with glass wool at its bottom. About 100 mL of 1 M

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solution of nitrate or chloride salts of K, Na, Mg and Ca isused as eluent with a flow rate of 0.5 mL min−1. The elu-ent is titrated against a standard alkali solution to determinethe total amount of H+ released which is equivalent to thecation captured by the material. The IEC is calculated usingthe following formula:

IEC = MVW

, (4)

where ‘M ’ is the molarity, ‘V ’ is the volume of alkali usedduring titration and ‘W ’ is the weight of the exchangertaken. The results of the above studies are presented inTable 2. The material La-DEA-117 is found to have maxi-mum ion-exchanger capacity and used in further studies.

2.5. Regeneration and desorption studiesThe loaded La-DEA material with Cr(VI) was kept understirring for 72 h at different pH (pH 5.0–11.0), after that thecontent of the flask is filtered and the concentration Cr(VI)measured in the filtrate.

2.6. Comparison of the removal of Cr(VI) in field waterand synthetic chromium solution

The chromium removal capacities of the material are testedin ground water of Baula-Nuasahi area of Keonjhar district(21◦N′ N: 86◦ 20′E) which is a well-known mining areafor chromite. The samples are collected in duplicate fromwater hand pump in an airtight bottle following the standardprocedure.[26] The existence of Cr(VI) depends on the pHof the water. The pH of hand pump water ranges from 7.6 to8.1, which is higher than the pH of the normal ground water,the total concentration of chromium, is found to contain3.052 mg L−1 (recorded in 11 December 2012) and otherphysico-chemical parameters are represented in Table 3.

3. Results and discussion3.1. Characterization of hybrid materialThe particle size of the material is 117 nm. The surface areaof the material is 398.42 m2/g. The material is chemicallystable as they are insoluble in 3 M solution of most of theacid (H2SO4, HNO3 and HCl) and alkali (NaOH, NH4OHand KOH) solution. The ion-exchange capacity of La-DEAwith particle size of 117 nm is found to be the best (Table 2).The SEM and corresponding energy-dispersive X-ray(EDX) spectrum of adsorbent before and after adsorptions ispresented in Figure 1(a) and 1(b), respectively. Small gran-ules are embedded in flat surface before adsorption but thereare shiny appearances above the small granules after adsorp-tion, which clearly manifests the adsorption of chromium inthe material. The EDX of material after adsorption confirmsthe presence of chromium in the material.

Powdered XRD diffractogram of the material is pre-sented in Figure 2, which was analysed by using Philips

Table 3. Physical and chemical constituents of hand pumpwater.

Sample (hand pump water) Values/concentration (mg L−1)

pH 7.6Temperature 26 ± 2◦CConductivity 328 μs cm−1

Total dissolved solids 119.76 mg L−1

Total chromium 3.052 mg L−1

Total arsenic 0.0028 mg L−1

Lead 0.00014 mg L−1

Chloride 68 mg L−1

Residual chlorine 0.68 mg L−1

Total hardness 64 mg L−1

Fluoride 1.8 mg L−1

Nitrate 5.38 mg L−1

Sulphate 24 mg L−1

Manganese 0.006 mg L−1

Iron 0.027 mg L−1

Carbonate 35 mg L−1

Bicarbonate 26 mg L−1

Note: Sample water collected from Baula-Nuasahi area ofKeonjhar district, Odisha, India.

X’pert High Score software and compared with Inter-national Center for Diffraction Data (ICDD-PDF-2) andJoint Committee on Powder Diffraction Standards (JCPDS)database to search the phases of starting materials andadsorbed materials. The material is found to be crystalline innature. The diffraction peaks for the material before adsorp-tion were observed at 2θ = 11.20◦, 17.23◦, 19.80◦, 27◦, 28◦,30◦, 33◦, 35.5◦, 36◦, 43◦ where the peak at 2θ = 11.20◦,17.23◦, 19.80◦, 27◦, 28◦, 33◦, 36◦ and 19.80◦, 27◦ are char-acteristic peak of La nitrate and DEA having monoclinicand anorthic phases. According to JCPDS database, peakat 2θ = 28◦ is a common peak for La nitrate, La hydrox-ide and La oxide (JCPDS#800748 and JCPDS#401279).The diffraction peaks for the material after adsorption areobserved at 20.78◦, 24◦, 26.22◦, 28.78◦, 30◦, 32◦, 36◦,44.57◦ and 49.37◦, whereas the peaks at 2θ = 30◦, 32.50◦,36◦ and 49.37◦ are prominent peaks for La chromium oxide(JCPDS#83-0256) having orthorhombic phase. The crystal-lite size of both fresh (for plane 111, at 2θ = 26.3157) andloaded material (for plane 110, at 2θ = 36.7099) is foundto be 17.39 and 23.74 nm, respectively, the increment in thecrystallite size is may be due to increment in space chargepolarization and crystal defects after adsorption. The crys-tallite size was calculated using the Debye Scherrer equation

D = 0.9λ

(B2θ . cos θmax), (5)

where D is the average crystal size in nm, λ is the specificwavelength of X-ray used, θ is the diffraction angle and B2θ

is the angular width in radians at intensity equal to half of themaximum peak intensity. The peak intensity is representedin Figure 2 which confirms the fresh material and chromiumadsorbed material.

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Figure 1. SEM and EDX spectrum of La-DEA hybrid material; before adsorption and after adsorption: (a) SEM images and (b) EDX(Energy dispersive X-ray spectroscopy) images.

The FTIR spectrum of the material is presented inFigure 3(a) and 3(b). The broad and strong absorption bandof fresh material in Figure 3(a) at 3881.27 cm−1 corre-sponds to the symmetric and asymmetric O–H stretchingvibration, indicating the presence of the hydroxyl groupin the material. Another broad peak at 3537.28 cm−1 indi-cates overlapping of O–H with N–H groups. The bandfrom 2033.54 to 2522.28 cm−1 is mainly because of com-bination stretching of C–H, N–H and O–H groups. Theband at 1474.40 cm−1 may be due to C–N stretchingand CH2 groups present in the material. The presenceof bands at finger print region (centred at 1048.77 cm−1)

corresponds to C–C and C–O groups. The band at811.04 cm−1 indicates the stretching vibration of La=Ogroups.

The FTIR spectrum of the Cr(VI) loaded material is pre-sented in Figure 3(b). There are many shifts in the bandspresent in fresh adsorbent but few important characteristicspeaks are presented. The band at 2354.86 cm−1 of adsor-bent has been shifted to 2346.30 cm−1 (blue shift) after itis loaded with chromium indicate the coordination of nitro-gen to metal ion. The band at 811.04 cm−1 is shifted to851.84 cm−1 may be due to stretching frequencies of Cr–O in Cr2O2−

7 groups. This spectrum clearly indicates theloading of the chromium to the adsorbent.

The TGA and DTA curves of the material are presentedin Figure 4. The weight loss with increase in temperature canbe described in the following three steps: (1) in the range of50–150◦C (weight loss of 9.84%) is due to loss of physicaladsorbed water; (2) the weight loss (9.09%) in the range of

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Figure 2. XRD of La-DEA hybrid material: (a) before adsorption and (b) after adsorption.

150–250◦C is attributed to the loss of ‘lattice water’ and COgroups present in material and (3) the biggest weight loss(25.5%) in the range of 250–500◦C to the decompositionof organic molecule present in the matrix at higher temper-ature. Correlating the data with DTA, an exotherm is alsonoticed at a temperature, of 224◦C, which supports the TGA.Four endothermic peaks at 100◦C, 160◦C, 300◦C, 330◦Cand two exothermic peaks 125◦C and 224◦C, respectively,in DTA support the TGA analysis.

The results of elemental analysis indicate that there isno sulphur content in the material. Carbon, hydrogen andnitrogen are found to contain of C = 10.75%, H = 2.48%,N = 3.13% and others = 83.64%, respectively. Based onthe studies of FTIR, TGA–DTA and the elemental compo-sition, the molecular formula of the adsorbent is deducedby using the Alberti equation [27]

18n = X (M + 18n)

100, (6)

where X is the percentage of water content and (M + 18)is the molecular weight of the material. It gives the value of

‘n’ as 5.419, the tentative empirical formula for the materialcan be suggested as [(LaNO3) (C4H11NO2)].nH2O.

Figure 5 shows the surface charge density of the mate-rial as a function of pH. The surface holds positive chargein lower pH and the surface charge density decreases whenthe solution pH increases. The pH of zero point surfacedensity charge (ZPC) is recorded around 5.9 (pHzpc). Thevalue of pHzpc represents the surface of the material is posi-tive charged when solution pH is less than 5.9 and acquiresnegative charge when the solution pH is higher than 5.9.

3.2. Removal study of Cr(VI) from water3.2.1. Effect of adsorbent doseThe removal efficiency of Cr(VI) versus the adsorptiondose is studied. The removal efficiency increases from‘75.43% to 99.31% and 65.58% to 98.59%’ with a varia-tion of adsorbent dose form 0.1 to 1.5 g/100 mL with initialCr(VI) concentration of 10 mg L−1 for synthetic solutionand 3.052 mg L−1 for hand pump water, respectively. Thereis no specific change in the percentage removal beyond the

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Figure 3. FTIR patterns of La-DEA hybrid material: (a) fresh hybrid material and (b) Cr(VI) loaded hybrid material.

Figure 4. TGA and DTA of La-DEA hybrid material.

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Figure 5. Surface charge density as a function of pH (m = 1.0 g L−1).

dose of 8 g L−1 and was considered as optimum dose andsaturation point of adsorption, used for further study. Thereaching of saturation point may also be due to overlappingof active sites beyond the optimum dosage and decrease inthe active surface area due to conglomeration of exchangerparticles.[28]

3.2.2. Effect of pHThe removal efficiency of the material is studied as afunction of pH in the range of 1–12 at room tempera-ture and the results are presented in Figure 6 for bothsynthetic solution and hand pump water. The percentageremoval increases ‘from 65.11% to 96.76% and 65.09% to98.58%, respectively’ for 10 and 3.052 mg L−1 of Cr(VI)with increasing the pH from 2 to 6. The highest adsorp-tion is at a pH 5.9 than decreases where the surface chargedensity is highest and attracted the negatively chargedspecies. When pH was approaching 7 and above, the con-centration of hydrogen ion in solution decreases and theextent of protonation of amino groups became smaller andadsorption decreases with increase in pH, moreover at ahigher pH there is a competition of the hydroxyl ions withCr(VI) ions towards the vacant adsorption sites and sub-sequently the mobility of hydroxyl ions increases withdecreased in mobility of chromium ions and reduces theadsorption.[27–29]

The chromate and dichromate anions simultaneouslyexist in equilibrium in water in acidic solution and theequilibrium towards the dichromate ion (pH < 6). Whenthe pH > 6, only CrO2−

4 is stable, and equilibrium shiftstowards chromate ion. The equilibrium between chromate

and dichromate with change in pH is presented in thefollowing equations:

H2O + Cr2O2−7 = 2CrO2−

4 + 2H+ [pH > 6],(7)

2CrO2−4 + 2H+ � Cr2O2−

7 + H2O [pH ≤ 6],(8)

CrO2−4 + H+ � HCrO−

4 [pH ≤ 6], (9)

2HCrO−4 � Cr2O2−

7 + H2O [pH ≤ 6],(10)

La-DEA + 2CrO2−4 + 2H+ → La-DEA—Cr2O2−

7

+ H2O [PH ≤ 6]. (11)

The dominant species are HCrO−4 and Cr2O2−

7 dependingon the pH of the solution. It is also be noted that there isno possibility arises of chemical oxidation or reduction asno oxidizing or reducing agent are added during the studiesand the solution only contain hexavalent chromium withthree species [i.e. (HCrO−

4 , Cr2O2−7 < 6)and (CrO2−

4 > 6)]depend on pH.

3.2.3. Effect of contact timeThe effect of contact time from 10 to 100 min is studiedby keeping all other parameters constant and the results arerepresented in Figure 7. The removal efficiency is foundto be increased from ‘60.02% to 98.48% and 59.85% to96.68%’ with increasing the time from 10 to 50 min for10 mg L−1 synthetic chromium solution and field watersample, respectively. It is observed that after 50 min, there

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Figure 6. Percentage removal of Cr(VI) by La-DEA of initial concentration 10 mg L−1 versus pH of the synthetic solution.

Figure 7. Time versus percentage removal of Cr(VI), La-DEA hybrid material, with initial concentration of 10 mg L−1.

is no noticeable removal takes place, and remains almostconstant which indicates the equilibrium time to be 50 min.Initially, the adsorption is high because of high initial solute

concentration gradient and availability of the vacant adsor-bent sites, which decreases with time; this may be due tooverlapping of the adsorption sites.

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Figure 8. Temperature versus percentage removal of Cr(VI) with La-DEA hybrid material, initial chromium concentration of 10 mg L−1.

3.2.4. Effect of temperatureTemperature is an important parameter to optimize theadsorption process as it affects the adsorption rate by chang-ing the molecular interactions and the solubility potentialof the adsorbate. The effect of temperature on the adsorp-tion is studied with initial chromium concentration of 10and 3.052 mg L−1 at optimum adsorbent dose 8 g L−1 in therange of temperature from 20◦C to 110◦C. The result is rep-resented in Figure 8. The removal efficiency increases withthe increase in temperature which indicate the endothermicnature of the process, may be due to (1) change in pore size,(2) intraparticle diffusion and (3) escaping tendency of theadsorbate species from the surface of the adsorbent. Thethermodynamic equilibrium constant of the ion interactionwith the material of adsorption is calculated using the Van’tHoff equation [29,30]

�G0 = −RT ln K , (12)

KC = C1

C2, (13)

log KC =[�S0

R− �H 0

RT

], (14)

�G0 = �H 0 − T�S0, (15)

where Kc is the equilibrium constant, C1 is the amount ofCr(VI) ion adsorbed per unit mass of adsorbent, C2 is theconcentration of Cr(VI) ion in aqueous phase, R is the uni-versal gas constant, T is the temperature (◦C) and �G0, �S0

and �H 0 are the changes in Gibb’s free energy, entropy andenthalpy of adsorption, respectively. A plot of log Kc versus1/T for initial chromium concentration of 10 mg L−1 wasfound to be linear and is presented graphically in Figure 9.Here, �H 0 and �S0 parameters are calculated from theslope and intercept of plot of log Kc versus 1/T , respec-tively. The results of the studies are presented in Table 4.The negative values of �G0 at all the temperature confirmedthe spontaneous nature of adsorption. The positive valuesof �H 0 confirm the feasibility of the reaction and indicatethe endothermic nature of the adsorption process.

3.2.5. Effect of competitive ionsThe results of adsorption can be affected by several otheranions presence in water. It is clear from the observa-tion that (Figure S1), with the increase in concentra-tion of these anions reduced the adsorption of Cr(VI)significantly. The anions reduced the Cr(VI) adsorp-tion in the order of carbonate > bicarbonate > nitrate >

phosphate > chloride > fluoride > sulphate. From theabove observation, it is clear that presence of these ionsreduced the adsorption of chromium.

3.2.6. Mechanism of adsorption of Cr(VI) on La-DEAhybrid material

A mechanism for the adsorption of Cr(VI) by the mate-rial is proposed by taking the results obtained from theexperimental investigations and computing the results using

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Figure 9. Van’t Hoff plots, log Kc versus 1/T for La-DEA hybrid material, initial chromium concentration of 10 mg L−1.

Table 4. Thermodynamic parameters using La-DEA synthetic chromium solution of 10 mg L−1.

�G0 (kJ mol−1)Initial Cr(VI)concentration (mg L−1) �H 0(kJ mol−1) �S0(kJ K−1 mol−1) 20◦C 30◦C 40◦C 50◦C R2

10 11.72068 0.9423 −7.12 −16.54 −25.97 −35.39 0.87083

Note: Experimental conditions: dose = 8 g L−1, pH = 5.6 and time = 50 min.

theoretical models. At lower pH values, the surface sites arepositively charged and therefore attract negatively chargeddichromate ion by an electrostatic force of interaction. Thematerial in alkaline condition the lanthanum surface com-pete the coordination shells with the available OH group.On the variation of pH, these surface active OH groupsmay further bind or release H+ where the surface remainspositive due to the reaction

MOH + H3O+ → MOH2+ + H2O. (16)

Thus when pH < 6.00, the overall dichromate adsorptionmechanism can be presented in three different forms:

(i) Electrostatic interaction between positively chargedcentre (nitrogen) and negatively charged dichro-mate molecule in solution.

(ii) Electrostatic attraction between positively chargedsurface hydroxyl group and dichromate:

MOH + H3O+ + Cr2O2−7 → MOH2+——Cr2O2−

7

+ H2O(electrostatic attraction). (17)

(iii) Ion-exchange reactions between positively chargedmetal centre and dichromate ion:

MOH + H3O+ + Cr2O2−7 → M+——Cr2O2−

7

+ 2H2O (ion-exchange). (18)

Furthermore, when the pH of the medium remains relativelyin acidic range (pH < 6.00) the Cr(VI) adsorption onto theneutral solid surface can be described by a ligand or ion-exchange reaction mechanism which is represented as

MOH + Cr2O2−7 → M+——Cr2O2−

7 + OH−. (19)

The modelling of the specific adsorption of Cr2O2−7 on any

material surface depends on a number of external factorssuch as temperature, pH, initial Cr2O2−

7 concentration aswell as the density of surface functional groups available forcoordination. In light of the above-mentioned mechanismof adsorption, it may be further noted that material showedadsorption capacity at a wide pH range which could be

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Figure 10. Proposed mechanism of adsorption and regenerationof the hybrid material: proposed mechanism of adsorption.

useful for commercial purpose. The role of DEA in thematerial is twofold. First, the attachment of DEA to thestructure develops nucleophilicity. Second, the attachmentof the DEA could be responsible for the development ofporosity in the structure and for imparting a large specificsurface area after the process of drying. Furthermore, theDEA may also combine with the La to form a layer where theneighbouring particles could be interconnected. A proposedmechanism of adsorption and regeneration of the hybridmaterial is presented in Figure 10.

3.2.7. Adsorption isothermThe adsorption isotherm is usually used to evaluate theadsorption capacity of an adsorbent for an adsorbate. The

linearized Langmuir adsorption isotherm equation is givenas follow:

1qe

= 1qobCe

+ 1qo

, (20)

where qe is the amount of Cr(VI) adsorbed at equilibrium(mg g−1), Ce is the equilibrium adsorbate concentration(mg L−1), b is the binding energy constant and qo is themono layer adsorption capacity (mg g−1). The values of qoand b are calculated from the slope and intercept, respec-tively, of the linear plot of 1/Ce versus 1/qe for differentpH values and the results are represented in Table 5 and inFigure 11.

To establish the relationship between dimensionlessparameter ‘r’ with Langmuir constant ‘b’ and the initial con-centration of the adsorbate solution C0, following equationis used:

r =[

11 + bC0

]r. (21)

The dimensionless equilibrium parameter ‘r’ indicates thetype of the isotherm, it is assume that if 0 < r < 1 thenthe isotherm is favourable, the value of the dimensionlessequilibrium parameter is represented in Table 5.

The value of the dimensionless equilibrium parameter‘r’ suggests that the process is favourable.

It is reported that the Langmuir and Freundlich adsorp-tion isotherm constants do not give any clear idea about themechanism of the adsorption,[28–30] in order to understandwell about the mechanism type, equilibrium data are fit-ted with the Dubinin–Radushkevic isotherm. The linearizedD–R equation is given in the following equations:

ln qe = ln qm − Kε2, (22)

ε = RT ln(

11 + Ce

). (23)

where ε is the Polanyi potential, qe is the amount ofadsorbate adsorbed per unit mass of adsorbent, Ce is theequilibrium solid concentration, qm is the hypotheticaladsorption capacity, K is the constant related to adsorp-tion energy, R is the gas constant and T is the temperaturein Kelvin. The value of K and qm for different pH valuesis obtained from the slope and intercept, respectively, fromthe plot of ln qe versus ε2 which is represented in Figure 12.The calculated value of K and qm at different pH values arerepresented in Table 5. The mean free energy of adsorption(E) was calculated from the constant K using the relation

E = −(2K)−1/2. (24)

The value of E is very useful in predicting the type ofadsorption and if the value is less than 8 kJ mol−1, thenthe adsorption is physisorption. If the value is between 8and 16 kJ mol−1 then the adsorption is due to ion exchangeand if the value is between 200 and 400 kJ mol−1, then theadsorption is chemisorption.[28,30] In the present study,

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Environmental Technology 829

Table 5. Langmuir and D–R isotherm constants obtained from the linear plot of La-DEA hybrid material.

Langmuir isotherm constant D–R isotherm constant

qo b K qm E(mg g−1) (L mg−1) r R2 χ2 RMSE (mol2 kJ−2) (g g−1) (kJ mol−1) R2 χ2 RMSE

pH = 2 53.2 .05 .65 .69 .093 .0043 −4.42 × 10−5 1.41 106.3 .93 2.95 × 10−6 4.312pH = 5 357.1 .35 .22 .99 .001 .0001 −7.36 × 10−6 1.77 260.6 .82 3.59 × 10−6 0.180pH = 8 8.41 .01 .92 .78 .047 .0025 −1.88 × 10−5 1.55 162.9 .88 7.20 × 10−6 0.077

Note: Experimental conditions: dose = 8 g L−1, pH = 5.6, temp = 25◦C, time = 50 min and RMSE = Root Mean Square Error.

Figure 11. Langmuir adsorption isotherm, 1/Ce versus 1/qe for La-DEA hybrid material: (a) pH = 2, (b) pH = 5 and (c) pH = 8.

the value of E was found to be 260.6 kJ mol−1 which is inbetween 200 and 400 kJ mol−1. Therefore, the adsorptioncan be best explained as chemisorption.

3.2.8. Adsorption kineticsThe rate constant of adsorption Kad is determined from thefirst-order rate expression given by Lagergren for initialCr(VI) concentration of 10 mg L−1 [27–33]

Log(qe − q) = Log qe − Kad

[t

2.303

], (25)

where qe and q (both in mg g−1) are the amount of Cr(VI)adsorbed at equilibrium and at time ‘t’, respectively, Kad isadsorption rate constant of pseudo-first-order rate equation.The plots of log (qe − q) versus t at different time intervalsare linear, indicating the validity of Lagergren rate equationfor first-order kinetics. The adsorption rate constant (Kad)calculated from the slope at different pH value is representedin Table 6.

3.2.9. Intraparticle diffusion rate constantsIn order to test the existence of intraparticle diffusion in theadsorption process, the amount of chromium adsorbed per

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Figure 12. D–R adsorption isotherm ln qe versus ε2 for La-DEA hybrid material: (a) pH = 2, (b) pH = 5 and (c) pH = 8.

Table 6. Rate constants (Kad and KP) obtained from the graph for different initial concentrations of Cr(VI).

La-DEA Lagergren first order Intraparticle diffusion rate constants

Initial concentration (mg L−1) Kad R2 SD RMSE KiP R2 C SD RMSE

pH = 2 10 .0315 .9453 .07795 .0304 .050 .975 .453 .0160 .0012pH = 5 10 .0669 .9676 .12588 .0792 .079 .974 .523 .0261 .0034pH = 8 10 .0271 .9426 .06866 .0236 .046 .948 .393 .0219 .0024

Note: Experimental conditions: dose = 8 g L−1, pH = 5.6 and temp = 25◦C.

unit mass of adsorbent, qe at any time t, was plotted as afunction of square root of time (t1/2).

The Weber–Morris rate constant for intraparticle diffu-sion model is represented[34–36]:

qe = Kipt1/2 + C, (26)

where qe is the amount of chromium adsorbed in mg g−1

of adsorbent, Kip is the intraparticle diffusion rate constantand ‘t’ is the agitation time in minutes. The value of Coffers evidence about the thickness of layer, i.e. the strongtowards to the external mass transfer. The larger the valueof C higher is the external resistant. The rate constants aredetermined from the slope presented in Table 6. There isdeviance of straight line from the origin due to the variationbetween the rate of mass transfer in the initial and final

stages of adsorption which indicates the pore diffusion isnot the solitary rate observing step.[28,37]

3.2.10. Desorption and regeneration studiesRegeneration and reusability studies are carried out inorder to know the efficiency of the material at pH 7.5–13with sodium hydroxide and the optimum pH for regenera-tion is found to be 11 (Figure S2). Normally chemisorp-tion exhibits poor regeneration.[30] It is due to the factthat in the chemisorption process, the adsorbent speciesare strongly bonded to the adsorbent with comparativelystronger bonds. Desorption of the adsorbed Cr(VI) in dis-tilled water resulted about 9.05% (Figure S3). From thestudy, it is clear that material has low desorption capacity

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Environmental Technology 831

because of chemisorption, which is in good agreement andsupports the previous data obtained for free energy study.Therefore, once the material completes the adsorption it isvery difficult to regenerate.

4. ConclusionThe La-DEA hybrid material is synthesized by co-precipitation methods. Synthesis is ascertained by theresults of various characterization methods such as SEM,XRD, FTIR, TGA–DTA, chemical analysis and BET stud-ies. The material exhibits BET surface area of 398.42 m2/gwith a particle size of 117 nm. The material is also crys-talline in nature. The maximum removal of Cr(VI) is99.31% at the optimum condition with adsorbent dose of8 g L−1, pH < 6, temperature = 70◦C and equilibrium time50 min with initial concentration of 10 mg L−1. The correla-tion coefficient and standard deviation (SD) value give bestfit to Lagergren first-order model and intraparticle diffusionmodel for the adsorption process. The adsorption data arebest fitted to linearly transformed Langmuir isotherm withcorrelation coefficient (R2) of 0.997 and χ2 analysis. Theendothermic nature of the process is ascertained from thethermodynamic parameters. The negative values of �G0

at all temperature confirm the spontaneity of the adsorp-tion. The positive value of �H 0 confirms the feasibilityof the process. The regeneration studies indicate that thepoor regeneration capacity. The effects of other compet-ing anions in the adsorption of Cr(VI) are in the followingorder: carbonate > bicarbonate > nitrate > phosphate >

chloride > fluoride > sulphate. The study concludes thatthe removal of Cr(VI) is very efficient by the material (La-DEA) and can bring down the chromium concentration toits permissible limit.

AcknowledgementsThe authors are thankful to Director, Prof. Sunil Kumar Sarangi,National Institute of Technology, Rourkela and Head of theDepartment, Prof. N. Panda, Department of Chemistry, NIT,Rourkela.

FundingThis work is financially supported by University Grant Com-mission, Government of India, New Delhi (UGC-RGNF) [F.14-2(SC)/2010(SA-III)] and National Institute of Technology,Rourkela.

Supplemental dataSupplemental data for this article can be accessed at10.1080/09593330.2013.852627.

References[1] Romero PG, Sanchez C. Functional hybrid materials. Wein-

heim: Wiley-VCH; 2004.

[2] Wen J, Wilkes GL, Garth L. Organic/inorganic hybridnetwork materials by the sol-gel approach. Chem Mater.1996;8:1667–1681.

[3] Soler-Illia GJDAA, Rozes L, Boggiano MK, Sanchez C,Turrin CO, Caminade AM, Majoral JP. New mesotexturedhybrid materials made from assemblies of dendrimers andtitanium(IV)-oxo organo clusters. Angew Chem Int Ed.2000;39:4249–4254.

[4] Kriesel JW, Tilley TD. Carbosilane dendrimers asnanoscopic building blocks for hybrid organic-inorganicmaterials and catalyst supports. Adv Mater. 2001;13:1645–1648.

[5] Haas KH. Hybrid inorganic-organic polymers basedon organically modified Si-alkoxides. Adv Eng Mater.2000;2:571–582.

[6] Laine RM. Nanobuilding blocks based on the [OSiO1.5]x (x = 6, 8, 10) octasilsesquioxanes. J Mater Chem.2005;15:3725–3744.

[7] De Castro Dantas TN, Dantas Neto AA, Moura MCPDeA,Barros Neto EL, De Paiva Telemaco E. Chromium adsorp-tion by chitosan impregnated with micro emulsion. Lang-muir. 2001;17:4256–4260.

[8] Larraza I, Gonzalez ML, Corrales T, Marcelo G. Hybridmaterials: magnetite polyethylenimine montmorillonite, asmagnetic adsorbents for Cr(VI) water treatment. J CollInterface Sci. 2012;385:24–33.

[9] Lua DD, Kayaa IGB, Bekera U, Bahire SF. Synthesisand adsorption properties of polymeric and polymer-basedhybrid adsorbent for hexavalent chromium removal. ChemEng J. 2012;181–182:103–112.

[10] Linsha V, Suchithra PS, Mohamed AP, Ananthakumar S.Amine-grafted alumino-siloxane hybrid porous granularmedia: a potential sol–gel sorbent for treating hazardousCr(VI) in aqueous environment. Chem Eng J. 2013;220:244–253.

[11] Leilei L, Fan L, Sun M, Qiu H, Xiangjun L, Huimin D,Chuannan L. Adsorbent for chromium removal based ongraphene oxide functionalized with magnetic cyclodextrin–chitosan. Coll Surf B: Bioin. 2013;107:76–83.

[12] Obrien TJ, Ceryak S, Patierno SR. Complexities ofchromium carcinogenesis: role of cellular response,repair and recovery mechanisms. Mutat Res. 2000;533:3–36.

[13] Ashraf AMM, Hala AMA. Ultrastructure changes in hepato-cytes of catfish Clarias gariepinus from lake Mariut. EgyptJ Environ Biol. 2010;31:715–720.

[14] Sidney KA, Harry S. The biological and environmentalchemistry of chromium. New York: VCH Publishers; 1994.

[15] Shi L-n, Lin Y-M, Zhang X, Chen Z-l. Synthesis, char-acterization and kinetics of bentonite supported nZVI forthe removal of Cr(VI) from aqueous solution. Chem Eng J.2011;171:612–617.

[16] USDHHS-USEPA. Notice of the first priority list of haz-ardous substances that will be the subject of toxicologicalprofiles. Federal Regis. 1987;52:12866–12874.

[17] Costa M. Toxicity and carcinogenicity of Cr(VI) in animalmodels and humans. Crit Rev Toxicol. 1997;27:431–442.

[18] U.S. Environmental Protection Agency. EPA issues guid-ance for enhanced monitoring of Cr(VI) in drinking Water.Washington (DC): Environmental Protection Agency; 2011.

[19] Central Pollution Control Board, (CPCB). India central pol-lution control board (CPCB). New Delhi: CPCB; 2011.Annual report.

[20] World Health Organization. World health organizationguidelines for drinking-water quality. 4th ed. Geneva: WorldHealth Organization; 2011.

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

20:

17 2

9 O

ctob

er 2

014

832 S. Mandal et al.

[21] Zhao YG, Shen HY, Pan SD, Hu MQ. Synthesis, charac-terization and properties of ethylenediamine-functionalizedFe3O4 magnetic polymers for removal of Cr(VI) in wastew-ater. J Hazard Mater. 2010;182:295–302.

[22] Zimmermann AC, Malabo A, Fagundes T, Rodrigues CA.Adsorption of Cr(VI) using Fe-cross-linked chitosan com-plex (Ch-Fe). J Hazard Mater. 2010;179:192–196.

[23] Ghassemi M, Park C, Recht HL, Northridge, Kleber EV, HillsCalif W. Water decolorization. United States patent office,US patent 3,736,255. 1973 May 29.

[24] Zhang L, Wan L, Chang N, Liu J, Duan C, Zhou Q, Li X,Wang X. Removal of phosphate from water by activatedcarbon fiber loaded with lanthanum oxide. J Hazard Mater.2011;190:848–855.

[25] Zhang L, Zhoua Q, Liu J, Chang N, Wan L, Chen J. Phosphateadsorption on lanthanum hydroxide-doped activated carbonfiber. Chem Eng J. 2012;185–186:160–167.

[26] Ma Y, Zheng Y-M, Chen JP. A zirconium based nanoparticlefor significantly enhanced adsorption of arsenate: synthesis,characterization and performance. J Colloid Interface Sci.2011;354:785–792.

[27] Weckhuysen BM, Wachs IE, Shoonheydt RA. Surface chem-istry and spectroscopy of chromium in inorganic oxides.Chem Rev. 1996;96:3327–3349.

[28] Islam M, Patel RK. Evaluation of removal efficiency of fluo-ride from aqueous solution using quick lime. J Hazard Mater.2007;143:303–310.

[29] Anirudhan Raji TS. Batch Cr(VI) removal by polyacry-lamide grafted sawdust: kinetics and thermodynamics. WaterRes. 1998;32:3772–3780.

[30] Mandal S, Padhi T, Patel RK. Studies on the removal ofarsenic (III) from water by a novel hybrid material. J HazardMater. 2011;192:899–908.

[31] Chakir J, Bessiere El, Kacemi K, Marouf B. A comparativestudy of the removal of trivalent chromium from aqueoussolutions by bentonite and expanded perlite. J Hazard Mater.2002;95:29–46.

[32] Gang D, Banerji SK, Clevenger TE. Cr (VI) removal bymodified PVP-coated silica gel. Pract Period Hazard ToxicRadioact Waste Manage. 2000;4:105–110.

[33] Tahir SS, Naseem R. Removal of Cr (III) from tannerywastewater by adsorption onto bentonite clay. Sep PurifTechnol. 2007;53:312–321.

[34] Guo X, Zhang FZ, Peng Q, Xu SL, Lei XD, EvansDG, Duan X. Layered double hydroxide/egg shell mem-brane: an inorganic bio composite membrane as an efficientadsorbent for Cr(VI) removal. Chem Eng J. 2011;166:81–87.

[35] Hena S. Removal of chromium hexavalent ion from aque-ous solutions using biopolymer chitosan coated with poly3-methyl thiopene polymer. J Hazard Mater. 2010;181:474–479.

[36] Mandal S, Tripathy S, Padhi T, Sahu MK, Patel RK.Removal efficiency of fluoride by novel Mg-Cr-Cl layereddouble hydroxide by batch process from water. J Env Sci.2013;25(5):993–1000.

[37] Li Q, Sun L, Zhang Y, Qian Y, Zhai JP. Characteristics ofequilibrium, kinetics studies for adsorption of Hg (II) andCr(VI) by polyaninline/humic acid composite. Desalination.2011;266:188–194.

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