Pergamon

0956-053X(95)00025-9

Waste Management, Vol. 15, No. 4, pp. 271-282, 1995 Copyright C 1995 Elsevier Science Ltd Printed in the USA. All rights reserved

095&053X/95 $9.50 + 0.00

ORIGINAL CONTRIBUTION

ADSORPTION OF CHROMIUM (III), CHROMIUM (VI) AND SILVER (I) ON BENTONITE

Saad Ali Khan, Riaz-ur-Rehman and M. Ali Khan” Institute of Chemistry, University of the Punjab, Lahorr, Pakistan

ABSTRACT. The sorption of Cr(III), Cr(V1) and Ag(1) from aqueous solutions on bentonite has been studied by a batch technique. Distribution coefficients (&) were determined for sorption systems as a function of contact time, pH, sorbent and sorbate concentration and temperature. Sorption data have been interpreted in terms of Freundlich, Langmuir and Dubinin-Raduskevich (D-R) equations. Thermodynamic parameters for the sorption systems have been determined at different temperatures. The sorption of Cr(II1) and Ag(1) on bentonite is exothermic in nature while that of Cr(V1) is an endothermic process. The AH” values for Cr(III), Cr(V1) and Ag(1) were -51.94 kJ/mol at 298 K, 5.619 kJ/mol at 293 K and -7.64 kJ/mol at 303 K respectively. The mean free energy of sorption, E, for Cr(V1) is 10 kJ/mol which shows that chromate ions from aqueous solutions at 10 5 to 10m3 M are predominantly sorbed on bentonite by an ion-exchange pro- cess. The sorption data for Cr(II1) and Ag(1) do not fit in the D-R equation. Negative values of AC” show the spontaneity of the sorption processes; AGo values for Cr(II1) and Ag(1) become less negative at higher temperatures which shows that sorption is less favoured at higher temperatures, while the increase in the negative values of AC” for Cr(V1) with the increase in temperature indicates that sorption is favoured at higher temperatures.

INTRODUCTION

Uncontrolled industrialization’,’ in Pakistan, since its creation in 1947, has resulted in the degradation of the environment. The waste water effluents from industrial establishments containing all types of haz- ardous substances including heavy metals are being disposed of either onto the open land or into natural water bodies directly or through the municipal sewage system.3.4 These substances enter into the life cycle through various routes and have given rise to various diseases including cancer.5m7

Many leather tanning units, varying from the cot- tage scale to big industrial units, are in operation in and around many big cities of Pakistan. They use large quantities of chromium salts for leather tan- ning. They produce a good variety of exportable leather, but not a single unit has the conventional waste treatment facilities for treating their water effluents containing chromium (VI) salts by reducing the Cr(V1) and alkaline precipitation to Cr(OH),. The effluents are discharged onto the open land or into the sewage system. These industries are a major

RECEIVE~D 18 NOVEMBER 1994; ACCEFTED 5 JUNE 1995. *To whom correspondence may be addressed.

source of chromium pollution of the environment. The most common sources of silver pollution in the environment are silver mirroring, soldering, use of silver in photography, electrodes and ornaments.

Adsorption of soluble metallic species by clays, oxides and other colloidal matters appears to be an important means of controlling the soluble metal ions concentration in heterogeneous systems.8,9 Many adsorbents are of mixed oxide type, and vari- ation in composition profoundly influences their behaviour.‘O Adsorption of chromate ions on a single adsorbent,” mixed adsorbent” and 1:l mixture of fly ash-wollastonite has been studied by Panday and coworkers.‘3 Very recently chromate ion adsorption on MnO, has been studied by Bhutani and coworkers.‘4 Not much work appears to have been done on the adsorption of Cr(III), Cr(VI) and Ag(1) on natural clays and sediments.

Bentonite, because of its physical and chemical properties, is considered one of the most promising candidates for use as a buffer material in geological disposal systems for high level radioactive wastes.‘5,‘6 Laboratory studies on bentonite samples from vari- ous deposits in many countries have, therefore, been carried out”m2’ to evaluate the suitability of this material as a backfill. These studies provide some

271

272 SAAD AL1 KHAN ET AL.

information regarding physical characteristics and the sorption behaviour of bentonite, mostly under repository conditions.22-26

The soil and clay minerals from different locali- ties, because of the variations in their composition may behave differently, therefore, each soil or clay deposit, if it is to be used as a backfill or for the dis- posal of low level radioactive waste into the ground, requires specific study.27-29

To evaluate the feasibility of local soil and clays for the disposal of industrial waste water effluents containing heavy metals, adsorption behaviour of Ag(I), Cr(III), and chromate ions on local bentonite has been studied under various experimental condi- tions, i.e. shaking time, pH, varying concentrations of the adsorbate and effect of temperature. Adsorption isotherms have been analysed in terms of Freundlich, Langmuir and Dubinit-Raduskevich (D-R) equations. Thermodynamic parameters such as enthalpy of adsorption, AH”, free energy change, AG”, entropy change, AS”, and mean free energy of adsorption, E, have been calculated and interpreted. Optimal adsorp- tion conditions are reported.

EXPERIMENTAL

Adsorbent Various bentonite samples were obtained through local clay contractors. The samples were picked up at random from individual big consignments of ben- tonite collected by contractors from various loca- tions. The samples were individually processed. This was done by first crushing the bentonite sample in a thoroughly cleaned jaw crusher. A steel roller was then used to homogenize the sample. It was then subjected to sampling by coning and quartering. Finally it was pulverized in a ball mill and sieved through a -250 mesh screen. Various physical, chemical and other structural properties of all the samples were determined.

TABLE 1 Chemical Analysis of Bentonite from Shina Bagh

Metal Oxides Chemical Analysis (%)

Silica (Si02) 70.68 Alumina (Al,Os) 16.22 Lime (CaO) 4.62 Iron Oxide (Fe,O,) 3.21 Magnesia (MgO) 3.00 Potassium Oxide (K,O) 1.05 Sodium Oxide (Na20) 0.71 Titanium Oxide (TiO,) 0.33 Chromium Oxide (Cr203) 0.01 Vanadium Oxide (V,Os) 0.17

100.00

TABLE 2 Characteristics Data of the Bentonite from Shina Bagh

Particle Size

Texture

Surface Area (i) Normal Sample (ii) Sample heated at 110°C

Intrusion Data Summary (normal sample)

Total Intrusion Volume = Total Pore Area = Median Pore Diameter (Volume) = Median Pore Diameter (Area) = Average Pore Diameter (4VI.4) =

Porosity

Cation Exchange Capacity

Bulk Density

Carbonate (Alkalinity)

pH (2O:l solution/bentonite)

Organic Matter Moisture Content (heated up to 15O’C)

Major Minerals

Ranged between 1.29 and 37.9 pm, 50% were 7.52 pm

Sand - 2% Silt - 74% Clay - 24%

34.0 m*/g 27.4 m*/g

0.492 ml/g 34.0 sq-m/g 1.01 pm 0.006 pm 0.0579 pm

49.2%

77.1 meq/lOO g

0.75

6.5 (wt%)

7.3

1.59% 14.0%

Montmorillonite and Quartz

On the basis of X-ray diffraction (XRD) analysis and other characteristic properties shown in Tables 1 and 2 bentonite from Shina Bagh, Kala Chita For- est, Attack, Pakistan, which contained montmoril- lonite as a major constituent and also contained some quartz, was selected for sorption studies. All other samples were found to contain either mixed clays or had more sandy contents. Bentonite was used without any heat or chemical treatment for the sorption studies reported here.

Chemicals A 1 X 10e2 molar silver solution was prepared by dissolving 0.1698 g of AgNO, (Merck, extra pure) into 0.01 M HNO, and making the volume to 100 cm3. Preparation of 1.0 M Cr(II1) stock solution was made by dissolving 40.015 g of Cr(N0)3.9H20 (Merck) into 0.01 M HNO, solution and making up the volume to 100 cm3. A 1.0 molar Cr(V1) stock solu- tion was prepared by dissolving 48.550 g of potassium chromate (Merck) into 0.1 M HNO, and making up the volume to 250 cm3. Silver-1lOm (tl12 = 253 days) was obtained from the Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad. The total activity of the tracer in 4 cm3 of 0.1 M HNO, was 0.18 MBq. Chromium-51 (t,,, = 27.8 days) was

ADSORPTION OF Cr(III), Cr(VI) AND Ag(I) ON BENTONITE 273

supplied separately in oxidation states (III) and (VI) by the Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad. Radiotracer 5’Cr3+ was in 16 cm3 of 0.1 HNO, solution, the metal content in the solution was 70 mg and its total activ- ity was 74 MBq. Radiotracer 51Cr6+ was supplied as sodium chromate containing 70 mg of chromium in 5 cm3 of 0.1 M HNO, solution, the total activity of the solution was 55.5 MBq.

Various spiked solutions of silver, Cr(II1) and Cr(V1) were prepared by taking known aliquots of the appropriate stock solution, adding a known vol- ume of the tracer solution and making the volume up to a known volume.

Instrumentation The pH of the solution was measured using micro- processor pH meter model HI8417 by Hanna instru- ments. The shaking was carried out in a thermostatted shaker bath, Precision USA Model 25. The centrifugation was done with a Wirowka Type WE-l Centrifuge machine at 4500 rpm. NaI (Tl) detector crystal size 7.62 X 7.62 cm (3 X 3 inches), coupled either with a complete counting assembly or 2 K multichannel y- spectrometer and an IBM com- puter from EG&G ORTEC, USA, was used for gamma counting.

Method The sorption of Cr(III), Cr(V1) and Ag(1) on ben- tonite was studied by a batch technique. The general method used for these studies is described below.

A known weight, i.e. 0.5 g, of the bentonite was equilibrated with 10 cm3 of the metal spiked solution of known concentration in a stoppered Pyrex glass flask at a fixed temperature in a thermostatted shaker water bath for a known period of time. The flasks containing the weighed amounts of bentonite and metal spiked solutions were kept separately in the thermostatted water bath before mixing for a suffi- cient period of time to attain the desired experimen- tal temperature. After equilibration the suspension was centrifuged in a stoppered tube for 15 min at 4500 rpm. Gross gamma activity in 1 cm3 of the supernatant was determined by a scintillation counter. The activity of 1 cm3 of reference solution was also recorded at the same time. The counting of the sam- ple solutions was always carried out over a period of time in order to have counts which lie within l-2% of statistical accuracy. The solution to solid ratio in all these experiments was 20: 1. The pH of the solu- tion was that of the supematant obtained after equi- libration. The amount of metal adsorbed, x, and the equilibrium concentrations of Cr(III), Cr(V1) and Ag(1) in the solution, C,, were always determined radiometrically, i.e. on the basis of the initial activity in the reference solution and the activity left in the

solution after equilibrium. Adsorption of Cr(III), Cr(V1) and Ag(1) on the walls of the glass flasks and centrifuge tubes was determined by running blank experiments; this was found to be negligible. Sorption of these metals on bentonite was determined in terms of distribution coefficients, I&, percentage adsorp- tion (%), or amount sorbed per unit weight of the sorbent, x/m. These are described below.

The distribution coefficient, Kd, is defined as the concentration of a species sorbed per gram of the sorbent divided by its concentration per cm3 in the liquid phase.

xlm K,, = ~

Y/V cm3/g

where x and y are the masses of nuclide sorbed and in solution respectively, m is the mass of the sorbent in g, and v is the volume of the solution in cm3.

In the present study K,, values were determined radiometrically by using the following equation

Kd = A; - A,

A, X ” (cm3/g)

m

where Ai and A, are the activities of radionuclide in solution at the beginning and at the end of sorption respectively, v is the volume in cm3 of the solution used for equilibration and m is the mass of adsor- bent in g.

The percentage adsorption (o/o), was calculated from Kd by using the following equation

P= y=,; (%) d

The amount adsorbed per unit weight of the ben- tonite, x/m, was calculated radiometrically from the initial (known metal concentrations) and final activi- ties of the solutions.

RESULTS AND DISCUSSION

Effect of Shaking Time The sorption of Ag’, Cr3+ and chromate ions on bentonite was studied as a function of shaking time at 30°C. A 10 cm3 aliquot of the spiked solution for each metal was shaken separately with 0.5 g of ben- tonite at a particular pH for different intervals of time in a thermostatted shaker water bath at 30 f 0S”C. Figures l-3 show the variations of Kd and percentage adsorption with shaking time for Ag’, Cr3+ and Cr6+, respectively.

The uptake of Ag’ from 1 X 10m3 mol/dm3 solu- tion at pH 6.5 is slow and attains equilibrium in 1 h after mixing (Fig. 1); a shaking time of 120 min was used for all further studies. The uptake of Cr(II1)

214

40

G 30 .?

E Y

SC” 20

IC

I I I I 30 60 90 120

Time (min)

30

g 20 ;;;

a

IO

FIGURE 1. Variation in Kd (0) and percentage sorption (A) on Ag+ on bentonite as a function of shaking time at 298 K.

from 1 X 10e3 mol/dm3 on bentonite at pH 3.5 and at 30°C is almost instantaneous. However, it is inter- esting to note that both Kd and percentage adsorption later on slowly decrease with longer periods of shak- ing (Fig. 2). It appears that Cr(II1) is not firmly held by the sorbent and no real equilibrium was established up to 24 h. The uptake of Cr(V1) from a 1 X lOA mol/dm3 solution at pH 2.0 by bentonite at 30°C is

I I I I I 30 60 90 120 24

30

50

~ Time (min) Time(h) -

FIGURE 2. Variation in Kd (0) and percentage sorption (A) of Cr(II1) on bentonite as a function of shaking time at 303 K.

SAAD ALI KHAN ET AL.

I I I I 30 60 90 120

Time (min)

FIGURE 3. Variation in Kd (0) and percentage sorption (A) of Cr(V1) on bentonite as a function of shaking time at 303 K.

very rapid, the Kd and percentage adsorption reach a maximum value almost immediately after mixing, but both the Kd and percentage adsorption later on decrease within the first 15 min to values which remain almost constant up to 120 min. On the basis of these results a 30-min shaking period was selected for all further studies.

Effect of pH The effect of pH on the sorption of silver ( 10m3 M), Cr3+ (10m3 M) and Cr6+ ( 10m5 M) on bentonite was studied by varying the pH of the metal aqueous so- lution-bentonite suspension from 1.5 to 10 at 30°C. The results are shown in Figs 4 to 6. The Kd value for Ag’ sorption on bentonite remains almost con- stant up to pH 6.9, beyond which it increases very sharply (Fig. 4). The percentage adsorption increases from 14% to 25% when the pH increases from 1.5 to 2.4. The percentage adsorption beyond pH 2.4 remains almost constant up to pH 5.8, beyond which it increases very sharply and attains a 95% adsorp- tion at pH 8.2. Further increase in the percentage adsorption with increasing pH is slow and it gradu- ally attains 99.8% adsorption at pH 9.8. A similar type of pH dependence on the sorption of Cr3+ on bentonite has been observed (Fig. 5). The Kd remains almost constant up to pH 4.16 beyond which both Kd and percentage adsorption increase very sharply. Both Kd and percentage adsorption of chromate ions on bentonite decrease with this increasing pH (Fig. 6). There is a sharp decrease both in Kd and percentage adsorption when the pH of the solution increases from 1.45 to 2.87. A small decrease in the values of Kd and percentage adsorption is observed when pH of the suspension is increased from 2.87 to 5.65, but beyond pH 5.65 a sudden fall in Kd and percentage

ADSORPTION OF Cr(LII), Cr(VI) AND Ag(1) ON BENTONITE

5 6 PH

275

40

20

IO

FIGURE 4. Influence of pH on Ag’ sorption on bentonite at 298 K: Kd (0) and percentage adsorption (A).

adsorption occurs which reaches very low values of K,, and almost zero percentage adsorption when the pH of the suspension becomes 6.5. The low Kd and percentage adsorption of Ag’ and Cr3+ on bentonite at lower pH is due to strong H’ ion competition for the available exchange sites or a surface adsorption phenomenon in which the adsorbent surface does not favour the adsorption of positively charged ions at lower pH. The oxides of aluminium, calcium, mag-

9000 100

8UOO 90

7000 80

6000 70 Gi

0‘ g E Y 5000 60 ‘;;

G $

4000 50

3000 40

2Oal 30

1000 20

0 1 2 3 4 5 6 7 8

PH

FIGURE 5. Influence of pH on Cr(II1) sorption on bentonite Kd (0) and percentage adsorption (A).

nesium, iron and silicon are present in varying amounts in bentonite (Table 1). The hydroxylated surfaces of oxides develop a charge on the surface in aqueous solution through amphoteric dissociation,30 as shown below

where M stands for Al, Ca, Mg, Si, etc. The surface will be positively charged at lower pH and conse- quently will not favour the adsorption of positively charged ions, but will favour the adsorption of Cr(V1) in the anionic form3’ as HCrO, This is in agreement with previous results3’,33 which show that amphoteric oxides favour the adsorption of anions at lower pH while adsorption of cations increases with increasing pH. On the basis of these results pH 6.5 was selected for Ag’ sorption on bentonite to avoid chloride interference in the alkaline media, while pH 3.5 was selected for Cr3+ sorption studies because it starts precipitating at pH 4.8, and pH 2.0 was used for sorption studies of chromate ions.

EfSect of Sorbent Concentration The dependence of Ag’ ( 10m3 M) sorption on ben- tonite concentration was studied at 30°C by varying the sorbent amount from 0.1 to 0.5 g while keeping the volume, 10 cm3, of the metal solution constant. The results are shown in Fig. 7. The results indicate that Kd does not change much with the increasing amount of the sorbent. However, the percentage adsorption increases with the increasing amount of the sorbent. The increase in the percentage adsorption

276 SAAD ALI KHAN ET AL.

I I I - 1 I l.

3 4 5 6 7 8 9 10

PH

50

40

30 2

20 P

10

0

FIGURE 6. Influence of pH on Cr(VI) sorption on bentonite Kd (0) and percentage adsorption (A).

with increasing amount of the sorbent is due to the greater availability of the exchangeable/adsorption sites at higher concentrations of the sorbent. On the basis of these results 0.5 g of the bentonite was used for all further studies.

Eflect of Sorbate Concentration The sorption of Ag’ and Cr3+ on bentonite as a func- tion of their concentration was studied at 30°C by varying the metal concentration from 10e9 to lo-’ mol/dm3 while keeping all other parameters constant. The results are shown in Figs 8 and 9. Percentage ad- sorption for both Ag’ and Cr3+ decreases with increas-

_I”

-40

- 30

3 - 20 :

a

‘“:‘o 0.1 0.2 0.3 0.4 0.5

Clay (8)

FIGURE 7. Dependence of Agf sorption on the amount of ben- tonite Kd (0) and percentage adsorption (A).

ing bulk metal concentration in the aqueous solutions. These results indicate that energetically less favourable sites become involved with increasing metal concen- trations in the aqueous solution. These results are similar to those previously reported for the sorption of various metal ions on a variety of sorbents.3’36

Adsorption Isotherms The adsorption isotherms for Cr(II1) and Cr(V1) sorption on bentonite were obtained at three differ- ent temperatures at various metal bulk concentra- tions from 0.01 to 0.3 mol/dm3 for Cr(II1) and from lo-’ to 10m3 mol/dm3 for Cr(VI), while keeping all other parameters, i.e. solution to solid ratio, shaking time and pH, constant. Figures 10 and 11 show these results for Cr(II1) and Cr(V1) respectively. The sorption results were analysed in terms of Freund- lich, Langmuir, and Dubinin-Raduskevich (D-R) isotherms.

Data obtained for the sorption of metal ions in the concentration range from 10m9 to 10d2 mol/dm3 for Ag’, 10e8 to lo-* mol/dm3 for Cr(II1) and 10e5 to 10m3 mol/dm3 for Cr(V1) were fitted to the Freund- lich isotherm. The following linearized form of the Freundlich equation was used for this purpose

log x/m = log K + l/n log C, (5)

where x/m is the amount of Cr(III), Cr(V1) and Ag(1) sorbed per unit weight of the sorbent, C, is the equilibrium metal concentration, l/n is a constant for the system and K is a constant dependent on the temperature. A plot of log x/m against log C, gives a

ADSORPTION OF Cr(III), Cr(VI) AND Ag(I) ON BENTONITE 277

60

I I I I I I I I

10-9 10-g 10-7 conce2:n of‘4;“;Inol,dm1;” 10-3 10-2 + 3

FIGURE 8. Effect of Ag+ concentration on jts sorption on bentonite. K,, (0) and percentage adsorption (A) for solutionibentonite ratio 20: 1.

straight line, the slope and intercept of which dependent sorption of Ag’, Cr(III), and Cr(V1) on correspond to l/n and log K respectively. Freundlich bentonite in the concentration range used. These plots for Ag’, Cr(II1) and Cr(V1) are shown in results are similar to the sorption of some metal ions Figs 12-14. Linear regression gives a slope of < 1 on illitic soi127.28 and various other adsorption for all three plots, indicating a concentration systems.29,36,37

- 80

80 - - 70

-60

iz - so;;:

?

-40

- 30

- 20

2oL I I I I I I

10-g 10-7 10-h 1O-5 10-4 10-3 10-2 Concentration of Cr (III) (mol/dm3)

FIGURE 9. Effect of Cr(II1) concentration on its sorption on bentonite. Kd (0) and percentage adsorption (A) for solutioniben- tonite ratio 20: I.

The sorption data for Ag’, Cr?+ and Cr6+ in the concentration range used did not fit in the Langmuir equation.

Thermodynamic parameters Thermodynamic parameters, i.e. heat of adsorption, AH” and entropy change, AS”, for the sorption of Ag(I), Cr(II1) and Cr(V1) on bentonite were calculated for each system by using the following equation38

1nK =ds”_dH” d R RT

The plots of In Kd vs l/T for Ag’, Cr3+ and Cr6+ are shown in Figs 15-17, respectively. The values of AH” and AS” are obtained from the slope and the inter- cept of each plot. The free energy for the specific adsorption, AG”, was calculated by using the follow- ing well known equation

AC” = AH” - TAP (7)

The values of the thermodynamic parameters for the sorption of Ag’, Cr(II1) and chromate ions on ben- tonite are given in Tables 3-5.

278 SAAD ALI KHAN ET AL.

log C, (mol/dm’)

FIGURE 10. Sorption isotherms of Cr(II1) on bentonite at different temperatures. 298 K (0), 303 K (A), 308 K (0).

1O-6 IO-> W4 10-3

log C, (mol/dm3)

FIGURE 11. Sorption isotherms of Cr(V1) on bentonite at different temperatures. 293 K (O), 303 K (A), 313 K (0).

These results show that sorption of Ag’ and Cr3+ on bentonite is an exothermic process. The value of AG” for these processes becomes less negative with increasing temperature, which shows that sorption is less favoured at high temperatures. The decrease in the entropy, As”, in both these processes in Tables 3 and 4 shows that the sorbate ions are stable on the solid sur- face. Association, fixation or immobilization of Ag’ and Cr3+ ions as a result of adsorption is attributed to a decrease in the degree of freedom of adsorbate ions which gives rise to a negative entropy change.

The positive values of AH” (Table 5) show that the sorption of chromate ions on bentonite is an endothermic process. The values of AG” become more negative with increasing temperature which

indicates the spontaneity of the process; it further shows that the sorption is favoured at higher tem- peratures. For an endothermic process the entropy of the system must increase and the value of TAP should be more positive than the value of AH”, if the process is to occur spontaneously. The value of AS” for the sorption of Cr(V1) from 8 X lOA mol/dm3 potassium chromate solution on bentonite is positive. The reason for the positive value of AS” in the sorption process of Cr(V1) is probably due to an increase in the translational entropy of the ions displaced from the bentonite compared with the entropy decrease due to the removal of chromate ions from solution and their sorption on the solid. This behaviour is different from what was reported

ADSORPTION OF Cr(III), Cr(V1) AND Ag(1) ON BENTONITE 279

log Ce (molldm3) FIGURE 15. Plot of In Kd vs l/T for Ag+ sorption on bentonite.

FIGURE 12. Freundlich plot for Ag’ sorption on bentonite.

10-Z

10-3

t m4 - 0

8 2; 2 10-5 -

E $ 10-6 -

2 10“ -

10-8 -

10-y -

lo-'0' I 10-10 10-9 10-g 10-7 10-6 10-s 10-d 10-3 10-z

log Ce (molldm3)

FIGURE 13. Freundlich plot for Cr(lII) sorption on bentonite.

10-h

0

0 0

/

0

10-8 10-6 10-s 10-4

log C, (molldm3)

10-j

FIGURE 14. Freundlich plot for Cr(V1) sorption on bentonite.

2.:

2.4

2 2.3 c

2.2

0 m / 0

,-

I-

I I I I 3.0 3.1 3.2 3.3 3.4

~/TX IO”(K)

.O-

.9 -

.8 -

.I -

2

I

I

I

5 '

I

I

I

I

.6 -

.5 -

.4 -

.3

I 0

% 3.1 3.2 3.3 3.4 I /TX lO‘3 (K)

FIGURE 16. Plot of In Kd vs l/T for Cr(II1) sorption on bentonite.

for Cr(V1) sorption on Mn0,,14 but is similar to the previously reported sorption behaviour of some metal ions on various other materials.‘8,39

The adsorption mean free energy, E, was calculated by using the following equation40,41

E = (_2K)m”2 (8)

The value of K was calculated from the D-R equation

lnX=lnX,-KEZ (9)

280 SAAD ALI KHAN ET AL.

~/TX lO-3 (K)

FIGURE 17. Plot of In Kd vs l/T for Cr(VI) sorption on bentonite.

TABLE 3 Values of the Thermodynamic Parameters for Ag+ (10m3 M)

Sorption on Bentonite

AH” (kJ/mol)

As” (kJ/K mol) 303 K

AC” (kJ/mol)

313 K 323 K

-1.64 -5.08 -6.106 -6.049 -5.999

where X is the amount of solute adsorbed per unit mass of solid (mol/g). X,,, is the sorption of adsor- bent per unit mass (mol/g), E is Polanyi potential = RT In (1 + l/C), C is the equilibrium solute solution

TABLE 4 Values of the Thermodynamic Parameters for Cr+3 (0.08 moi/dm3)

Sorption on Bentonite

AH” (kJimo1)

-51.94

As” (kJ/K mol)

-0.1585

298 K

-4.706

A G" (kJimo1)

303 K

-3.913

308 K

-3.121

TABLE 5 Values of the Thermodynamic Parameters for Chromate (g X lOA

moi/dm3) Sorption on Bentonite

AH” (kJ/mol)

5.619

As” (kJ/K mol)

0.020

293 K

a.241

A G" (kJimo1)

303 K 313 K

-0.441 -0.641

TABLE 6 Values of the D-R Parameters for Chromate Sorption on

Bentonite

Temperature K %l WI (mol’/kJ’) (kJ/:ol) (kJ/mol) (r)

293 -5.02 x 10-j 9.98 6.35 x 10m6 0.99 303 4.81 x lO‘3 10.19 7.16 x lO-6 0.99 313 -5.08 x lo-’ 9.91 1.10 x 10-5 0.99

concentration (mol/dm3), R is the gas constant (kJ/K mol), T is temperature (K), K is a constant related to the sorption energy (mo12/kJ2).

13 (W )’ I (moB2

300 400 500 600 700 800 900 loal I I I I I I

4 C

FIGURE 18. D-R plots of Cr(V1) on bentonite at different temperatures. 293 K (0), 303 K (A), 313 K (0).

ADSORPTION OF Cr(III), Cr(VI) AND Ag(1) ON BENTONITE 281

The sorption data for Ag’ and Cr(II1) in the con- centration range studied did not fit the D-R equa- tion. The plot of In X against I? for Cr(VI) sorption on bentonite is shown in Fig. 18. The D-R para- meters X, and K are calculated from the intercept and the slope of the line in Fig. 18, respectively. The values of these parameters and the value of E calculated from the K value are given in Table 6.

The value of E for chromate sorption on ben- tonite (Table 4) is about 10 kJ/mol at all three temperatures studied, which is within the energy range for ion-exchange processes,42.43 i.e. 8-16 kJ/mol. The adsorption capacity, X,,,, of the bentonite for Cr(V1) sorption at 20°C is 0.63 mmol/lOO g which increases to 1.1 mmol/lOO g at 40°C. These results show that bentonite can effectively be used for the removal of chromate ions from waste water effluents if the metal concentration is low and the pH of the waste effluents is adjusted to about 2.

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