Indian Journal of ChemistryVol. 23A, June 1984, pp. 514-515

Fly Ash-China Clay for Removal of Cr(VI)from Aqueous Solutions

K K PANDA Y, GUR PRASAD & V N SINGH-Applied Chemistry Section, Institute of Technology, Banaras Hindu

University, Varanasi 221005Received 8 April 1983; revised and accepted 28 December 1983

The removal of Cr(VI) from aqueous solution by adsorption on ahomogeneous mixture offly ash and china clay (I: I) has been studiedat various Cr(VI) concentrations, retention time, pH andtemperatures of the solution. The Langmuir's constants have beendetermined graphically and by regression analysis. Maximumremoval of Cr(VI) is observed at pH 2.0 and 30± 1'C.

The use of various adsorbents for the removal ofCr(VI), one of the major pollutants, from aqueoussolution has been reported in some earlier studies 1 3.

In this note, we describe the efficacy of a mixture of flyash and chinaclay(l: I) as an adsorbent for the removalof Cr(VI) from aqueous solution.

Fly ash and china clay were analysed by chemicalmethods" and passed through 53 nm sieve. Ahomogeneous mixture (I g) of fly ash and china clay(1:1)was taken in stoppered glass bottles and agitated(125 r.p.m) with aqueous solutions (50 ml each) ofpotassium dichromate of different concentrations, pHand temperatures. The amount of the adsorbent wasfurther increased from 1.0 to 3.0 g and agitated with1.0 x 104 M potassium dichromate solution at pH 2.0and 30± L'C, The progress of adsorption wasdetermined spectrophotometrically using 1,5-diphenylcarbazide". Al1experiments were done induplicate with suitable reagent and mixed adsorbentblanks. The desorption studies were carried out inwater and in 0.01 N sodium hydroxide solution.

Both the adsorbents, fly ash and china clay, mostlycontain oxides of aluminium and silicon with tracesof oxides of iron, calcium and magnesium. Theadsorption data reveal that with decrease inconcentration of Cr(VI) from 1.0 x 10-4 M (10.4 mglitre' 1 of Cr) to 0.4 x 10 -4 M (4.16 mg litre -I of Cr),the adsorption of Cr(VI) by mixed adsorbent increasesfrom 53 to 100% at pH 2.0 and 30± roc. When theamount of the mixed adsorbent is further increased to3.0g, 100% removal is achieved at potassiumdichromate concentration of 1.0 x 10 -4 M. Withincrease in pH from 2.0 to 9.0, the adsorption ofCr(VI)decreases from 100 to 27% at concentration of 0.4x 10 -4 M, indicating that the acidic medium isfavourable for the removal of Cr(VI). Similar resultsfor fly ash-dichromate" and fly ash-phenol? systems

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have been reported earlier. The higher adsorption inacidic pH is due to the fact that metal oxides of mixedadsorbent acquire positive charge at low pH (refs 8,9)and attract negatively charged adsorbate ions. Thecontent of alumina in fly ash '" and clays 1 1 isimportant in the adsorption of anions. In acidicmedium, the edge surface of alumina is positivelycharged over broad pH range (isoelectric point at pH8.4). It interacts with dichromate ions to form surfacecompound at pH values below 8.4. Silica exhibits asteady negative charge at pH values above theisoelectric point (2.5) and thus has a little influence onadsorption ofCr(VI) as dichromate ions. However, atpH values below 2.5, its surface wil1be favourable foradsorption. The surface of mixed adsorbent becomesnegatively charged? and Cr(VI) is converted intoCr(III) in alkaline medium, affecting the nature of theadsorbate-adsorbent system and probably due to thisreason adsorption is reduced to 27~~at pH 9.0.

It is noted that the equilibrium data for the presentsystem obey the Langmuir isotherm:

C I C--=--+-(x/m) KIK2 K2

where C is equilibrium concentration (Jig litre -1), xlm,the amount of Cr(VI) adsorbed per unit weight ofadsorbent (/-lggl) at equilibrium and KI and K2 areLangmuir constants. The values of KI and K2 werecalculated from the Langmuir's plot (Fig. l) and thevalues are 2.74 x 10 3 litre Jig ...1 and 306.06 /-lgs'respectively. The applicability of isotherm was furthertested by regression analysis. The values of KI and K2calculated come out to be 2.02 x 10 -3 litre /-lg -I and309.61 /-lgg -I respectively. The agreement between thetwo sets of values is fairly good. This suggests the

... (1)

JO

0~~~~~~6~~a-L-~L-~'2CI~ ,"'; lit";'

Fig. I Langmuir isotherm for adsorption of Cr(VI) on flyash-china clay (1:1)

validity of the Langmuir isotherm for the presentsystem. The linearity of the Langmuir's plot indicatesthe formation of monolayer dichromate ions on theactive alumina surface of the mixed adsorbent as theother oxides might have a little contribution inadsorption of Cr(VI).

The thermodynamic parameters for the adsorptionofCr(VI) have been calculated atpH 2.0. The values off:.Go ( - 3.76 and - 3.58 k cal mol :' at 30° and 40°Crespectively) are negative, indicating the adsorption tobe spontaneous. The negative value of f:.HJ (-9.27 kcal mol -I at 30°C) indicates that the process isexothermic and the adsorption of Cr(VI) on mixedadsorbent occurs by long range attractive forces. Thenegative value of f:.so ( - 0.018 e.u. at 30°C) shows thatthere is no configurational change and the surfacecompound is stable.

The adsorptive force between adsorbate and mixedadsorbent is so strong that Cr(VI) does not desorb inwater. This is further confirmed by the IR spectra ofmixed adsorbent, before and after adsorption ofCr(VI). The characteristic IR bands of Cr 20~.. ion at750 and 860cm -I (ref. 12) were shifted to 780 and 900em -1 respectively after adsorption on mixedadsorbent surface, indicating the possibility ofbonding of dichromate ions with the active sites ofmixed adsorbent 13. Cr(VI) could be completely

NOTES

desorbed from the adsorbent on treating it withsodium hydroxide solution.

We are grateful to the CSIR, New Delhi for theaward of a senior research fellowship to one of us(KKP).

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water hy adsorption process (Symp Fundamental andApplied Electro-chemistry, Bombay) 1982.

3 Jain K K, Singh V N & Prasad G, Wollastonite for removal ofCr( VI) (Symp Nuclear and Radiation Chemistry, Varanasi)1981.

4 Indian standard methods of chemical analysis offireclay and silicarefractory materials, IS: 1527,1960.

5 Standard methods for the examination of water and wastewaters(American public health association, Washington. Dq1975, 192.

6 Gangoli N, Markey DC & Thodos G. Removal of heavy metalions from aqueous solution with ./ly ash (Proc Natl ConfComplete Wastereuse) 1975, 855.

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10 Chu T 1, Steiner G R & McEntyre C L, J Wat Poll Contr Fed, ~(1978) 2157.

II SifTert B & Espinasse P, Clays & Clay Minerals, 28 (1980) 381.12 Miller F A & Wilkins C H, Analyt Chern, 24 (1952) 1253.13 Hair M L, Infrared spectroscopy in surface chemistry (Marcel

Dekker, New York) 1967,49.

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