adsorption of chromium by activated carbon from aqueous solution

PERGAMON Carbon 37 (1999) 1989–1997

Adsorption of chromium by activated carbon from aqueoussolution

*Diksha Aggarwal, Meenakshi Goyal, R.C. BansalDepartment of Chemical Engineering and Technology, Panjab University, Chandigarh, 160014, India

Received 11 November 19989 March 1999

Abstract

Adsorption isotherms of Cr(III) and Cr(VI) ions on two samples of activated carbon fibres and two samples of granulatedactivated carbons from aqueous solutions in the concentration range 20–1000 mg/ l have been studied. The adsorptionisotherms have been determined after modifying the activated carbon surfaces by oxidation with nitric acid, ammoniumpersulphate, hydrogen peroxide and oxygen gas at 3508C and after degassing at different temperatures. The adsorption ofCr(III) ions increases on oxidation and decreases on degassing. On the other hand, the adsorption of Cr(VI) ions decreaseson oxidation and increases on degassing. The increase of Cr(III) and the decrease of Cr(VI) on oxidation and the decrease ofCr(III) and the increase of Cr(VI) on degassing have been attributed to the fact that the oxidation of the carbon surfaceenhances the amount of acidic carbon–oxygen surface groups while degassing eliminates these surface groups. Thus whilethe presence of acidic surface groups enhances the adsorption of Cr(III) cations, it suppresses the adsorption of Cr(VI)anions. 1999 Elsevier Science Ltd. All rights reserved.

Keywords: A. Activated carbon; Carbon fibers; B. Oxidation; C. Adsorption; D. Surface oxygen complexes

1. Introduction range 250–300 mg/ l and found that the removal of Cr(VI)increased significantly at acidic pH values. These workers

Chromium is present in effluent waters of several also observed that at acidic pH values, a redox reactiondifferent industries. It is hazardous because it affects occurred on the carbon surface when Cr(VI) was reducedhuman physiology, accumulates in the food chain and to Cr(III). The activated carbon sites, in turn, werecauses several ailments. The stricter environmental regula- oxidised resulting in an increased adsorption capacity.tions related to the discharge of heavy metals make it Cici and Kales [5] studied the adsorption of Cr(VI) onnecessary to develop processes for their removal from rice husk charcoal activated with ZnCl at two different2

waste water. Activated carbons because of their high temperatures and observed that 30–99% Cr(VI) could besurface area, microporous character and the chemical removed from solutions containing between 20 and 200nature of their surface have been considered potential mg/ l of Cr(VI). Andreeva et al. [6] used several differentadsorbents for the removal of heavy metals from industrial fibrous activated carbons for the adsorption of severalwastewater. heavy metals under static conditions at different pH values

Narayana and Krishnaiah [1,2] and Kannan and Van- and suggested that fibrous activated carbons were veryangamudi [3] used activated carbon, lignite coal and effective for the removal of heavy metals from wastebituminous coal for the adsorptive removal of chromium water. The adsorption of chromium followed Freundlichfrom aqueous solutions at different pH values and ob- isotherm below 208C and Langmuir isotherm above 208C.served that both adsorption and reduction of Cr(VI) Dikshit et al. [7] used bituminous coal for the removal ofoccured. The adsorption of Cr(VI) was maximum at pH53 low concentration of Cr(VI) from highly acidic wastewhile the reduction was maximum at pH51. water by the batch technique. The rate of adsorption

Ouki and Newfeld [4] used column adsorption technique followed first order kinetics and the adsorption wasfor removal of Cr(VI) from water in the concentration suggested to be a diffusion process involving micropores.

Jayson et al. [8] studied the adsorption of chromium*Corresponding author. ions from aqueous solutions on a sample of activated

0008-6223/99/$ – see front matter 1999 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 99 )00072-X

1990 D. Aggarwal et al. / Carbon 37 (1999) 1989 –1997

carbon cloth using radioactive tracer techniques. The 3508C to enhance the amount of carbon–oxygen surfaceresults showed that the adsorption of chromium ions from chemical structures. The activated carbons were alsoaqueous chromate solutions was always greater by a factor degassed at different temperatures between 4008C andof ten than that from chromic solutions because chromic 9508C to gradually eliminate these surface chemical struc-ions are too large to enter parts of the microporous system. tures. The details of the oxidation and degassing treatmentsAs the amount of Cr(III) adsorbed exceeded the external are described elsewhere [16–19].surface, it has been postulated that the chromium ions on 0.2 g of each of the activated carbon sample was placedadsorption dehydrate, become smaller and migrate further in contact with 20 ml solutions of different concentrationsinto the micropores. Huang and Wu [9] observed that of potassium dichromate for the adsorption of Cr(VI) ions90–99% chromium present in wastewater could be re- and of chromium chloride for the adsorption of Cr(III)moved using activated carbon. ions. The change in concentration due to adsorption was

Yoshida et al. [10] examined the adsorption of Cr(III) determined spectrophotometrically using standard proce-and Cr(VI) from aqueous solutions using activated carbon dures.as a function of pH of the solution. Cr(VI) was adsorbedas an anionic species and the rate of its adsorption wasfaster than that of Cr(III). Huang and Wu [11] while 3. Results and discussionstudying the adsorption of Cr(VI) and Cr(III) on afiltrasorb carbon also observed that Cr(VI) was adsorbed 3.1. Adsorption of Cr(III)more readily than Cr(III). The optimum pH for theremoval was between 5.5 and 6.0 for Cr(VI) and 5.0 for Adsorption isotherms of Cr(III) ions from aqueousCr(III). These investigators found that Cr(VI) was readily solutions of chromium chloride in the concentration rangereduced to Cr(III) under acidic conditions in the presence 20 to 1000 mg/ l on two samples of granulated activatedof activated carbon. Huang and Bowers [12], however, carbons GAC-E and GAC-S and two samples of activatedfound that the removal of chromium ions by activated carbon fibers ACF-307 and ACF-310 are shown in Fig. 1.carbons involved reduction and adsorption consecutively. All the isotherms have been determined without addition

Grover and Narayana Swamy [13] and Viraraghavan of any buffer solution to avoid the addition of any external[14] used fly ash as an adsorbent for the removal of electrolyte, which may influence the adsorption processchromium from industrial wastewater. While the former [15]. It is seen that all the activated carbons, granular asobserved it to be a case of adsorption into the pores, the well as fibrous, adsorb appreciable amounts of Cr(III) ionslatter suggested the adsorption to be a case of chemisorp- and that the amount adsorbed at each concentration istion of Cr(VI). different for different carbons. In general, the granulated

Bautista–Toledo et al. [15] studied the influence ofoxygen surface complexes on the adsorption of chromiumions from aqueous solutions on a commercial activatedcarbon. The adsorption of both Cr(VI) and Cr(III) wasenhanced by the presence of surface oxygen complexes ofacid type. The adsorption was also enhanced by addition ofNaCl into the solution.

It appears from the above perusal of the literature thatthe use of activated carbons for the removal of chromiumfrom industrial wastewater has a great potential. However,a systematic approach to the mechanism involved has notbeen carried out. In the present work the adsorption ofCr(VI) and Cr(III) has been studied using commerciallyavailable granulated and fibrous activated carbons associ-ated with varying amounts of different types of carbon–oxygen surface chemical structures.

2. Experimental

Two samples of granulated activated carbons and twosamples of fibrous activated carbons have been used inthese investigations. The activated carbons were oxidisedwith nitric acid, ammonium persulphate and hydrogen Fig. 1. Adsorption isotherms of Cr(III) on different as-receivedperoxide in the solution phase and with gaseous oxygen at activated carbons.

D. Aggarwal et al. / Carbon 37 (1999) 1989 –1997 1991

activated carbons adsorb larger amounts of Cr(III) ions gas at 3508C. These treatments are known [21] to enhancecompared to fibrous activated carbons. The adsorption is the amount of chemisorbed oxygen on the carbon surface.maximum in the case of carbon GAC-S and minimum in The amounts of these two types of oxygen groups afterthe case of carbon ACF-307. The adsorption of GAC-S is oxidation with these different oxidising agents are recordedalmost three times the adsorption of ACF-307. This may in Table 2. It is seen that the increase in the amount ofpartly be attributed to the highly microporous nature of the chemisorbed oxygen on oxidation with nitric acid isactivated carbon fibers. Activated carbon fibers contain a considerably large than the increase on oxidation withlarger proportion of very small micropores, which may be ammonium persulphate, hydrogen peroxide and gaseousinaccessible to highly hydrated Cr(III) ions. oxygen. The adsorption isotherms of Cr(III) ions on the

31Cr(III) ion in aqueous solution exist as [Cr(H O) ] oxidised samples are presented in Figs. 2–5. The ad-2 6

which has a molecular diameter of 0.922 nm [20] and sorption increases on oxidation in all the cases althoughconsequently is accessible only to pores larger than 1 nm. the increase in adsorption depends upon the nature of theFurthermore, it is seen that the adsorption of Cr(III) on oxidative treatment. The increase in adsorption is at aACF-310 is less than the adsorption of GAC-E, although maximum in the case of the sample oxidised with nitric

2the surface area of ACF-310 (1184 m /g) is about the acid and at a minimum in the case of the oxidation with2same as that of GAC-E (1190 m /g) (cf. Table 1). This hydrogen peroxide. This may be attributed to the fact that

once again indicates that ACF-310 has a larger proportion the oxidation with nitric acid enhances the amount ofof the micropores inaccessible to hydrated Cr(III) ions. chemisorbed oxygen by considerably larger amounts com-

It is well known that all activated carbons are associated pared to other oxidative treatments.with varying amounts of chemisorbed oxygen, the amount The oxidised activated carbon samples were then de-depending upon the source raw material and the history of gassed at gradually increasing temperatures to removetheir preparations [21]. This oxygen in carbons is present varying amounts of the two types of carbon–oxygenin the form of surface- oxygen groups, some of which are surface groups and the adsorption isotherms were de-acidic in character [22]. Since the pH of the solution has a termined. The results are presented in Figs. 6–9. It is seengreat influence on the adsorption of Cr(III) ions from that the adsorption decreases gradually with gradual in-aqueous solutions and since these acidic surface groups crease in the temperature of degassing. It may be worth-

1ionise in water producing H ions [23], the presence of while to mention here that the oxygen on the surface ofacidic surface groups is expected to influence the ad- carbon is present in the form of two types of carbon–sorption of Cr(III) ions. oxygen surface complexes, one of which is acidic in

The amounts of these surface groups which are evolved character and is evolved as CO on evacuation in the2

as CO and CO on degassing were determined [18,19] and temperature range 4008–6508C. The other group which is2

are given in Table 1. It is seen that different carbons have evolved as CO in the temperature range 6008–9508C isdifferent amounts of the carbon–oxygen surface groups. non-acidic in character [18,19,22,23]. The formers areWhile the granulated activated carbons GAC-E and GAC- postulated as carboxylic or lactonic groups while the latterS have larger amounts of the surface groups evolved as are postulated as quinonic groups. When the carbon isCO , the fibrous activated carbons ACF-307 and ACF-310 degassed at 4008C only a small part of the acidic surface2

have larger amounts of the carbon–oxygen surface groups groups (cf. Table 3) is evolved. However, when theevolved as CO. As the amount of adsorption of Cr(III) is carbons are degassed at 6508C, a larger part of the acidiclarger in the case of granulated activated carbons, it surface groups is eliminated while the carbons retain aappears that the surface groups evolved as CO may have larger proportion of their non-acidic surface groups. The2

a larger influence on the adsorption of Cr(III) ions. 9508C-degassed carbon samples are almost completely freeIn order to examine the influence of carbon–oxygen of any associated oxygen (both acidic and non-acidic) (cf.

surface groups more clearly, the activated carbons were Table 3). It appears that the adsorption of Cr(III) isoxidised with nitric acid, ammonium persulphate and determined largely by the presence of oxygen surfacehydrogen peroxide in the solution phase and with oxygen groups and more so by the presence of those oxygen

Table 1Surface areas and amounts of oxygen evolved on degassing different as-received activated carbons at 9508C

2Sample Surface area (m /g) Oxygen evolved (g /100 g) as

CO CO H O Total2 2

ACF-307 910 1.00 5.30 2.30 8.60ACF-310 1180 1.90 4.20 2.40 8.50GAC-S 1260 2.10 1.05 1.24 4.39GAC-E 1190 2.13 1.66 1.73 5.52


Table 2Amount of oxygen evolved on degassing different oxidised samples at 9508C

Sample Oxygen evolved (g /100 g) as

CO CO H O Total2 2

ACF-307, oxidised with-HNO 12.90 7.47 2.40 22.773

(NH ) S O 5.40 7.51 4.91 17.824 2 2 8

H O 2.55 7.42 2.10 12.072 2

O 3.11 8.71 1.20 13.022

ACF-310, oxidised with-HNO 11.96 7.20 2.20 21.363

(NH ) S O 4.70 6.30 2.10 13.104 2 2 8

H O 1.70 6.20 2.10 10.002 2

GAC-S, oxidised with-HNO 12.20 7.60 1.50 21.303

(NH ) S O 6.34 6.53 1.25 14.124 2 2 8

GAC-E, oxidised with-HNO 12.40 11.20 1.92 25.523

(NH ) S O 4.63 5.61 1.73 11.974 2 2 8

O 2.17 5.97 1.13 9.272

groups which are acidic in character and are evolved as Cr(III) ions. Furthermore, the area covered althoughCO on degassing. increases on oxidation but is still much below the BET2

The surface area covered by Cr(III) ions has been surface areas. This indicates that the adsorption of Cr(III)calculated from the Langmuir linear plots and using 0.922 ions takes place on certain specific sites. These sites appearnm [20] as the molecular diameter of Cr(III) ion. These to be oxygen groups, more so the acidic oxygen groups,values are given in Table 4 for the as-received activated present on the carbon surface. The acidic surface groupscarbons. It is interesting to note that only a small fractionof the BET surface area in all the carbons is occupied by

Fig. 2. Adsorption isotherms of Cr(III) on ACF-307 before and Fig. 3. Adsorption isotherms of Cr(III) on GAC-E before andafter oxidation. after oxidation.


Fig. 4. Adsorption isotherms of Cr(III) on ACF-310 before andafter oxidation.

Fig. 6. Adsorption isotherms of Cr(III) on GAC-E before andafter oxidation and degassing.

Fig. 5. Adsorption isotherms of Cr(III) on GAC-S before and Fig. 7. Adsorption isotherms of Cr(III) on GAC-E before andafter oxidation. after oxidation and degassing.


Table 3Amount of oxygen evolved on evacuating different degassedsamples at 9508C

Sample Oxygen evolved (g /100 g) as

CO CO H O Total2 2

ACF-307, degassed at (8C)-400 0.72 5.24 2.11 8.07650 0.21 3.23 Tr 3.44950 – 0.07 Tr 0.07ACF-310, degassed at (8C)-400 1.47 4.03 1.92 7.42650 0.32 2.17 Tr 2.49950 – 0.23 Tr 0.23GAC-S, degassed at (8C)-400 1.67 0.97 1.12 3.76650 0.27 0.57 Tr 0.84950 – 0.04 Tr 0.04GAC-E, degassed at (8C)-400 1.72 1.52 1.24 4.48650 0.40 0.84 Tr 1.24950 – 0.09 Tr 0.09

Tr: Traces.

Fig. 8. Adsorption isotherms of Cr(III) on ACF-307 before andTable 4after oxidation and degassing.Data obtained from Langmuir adsorption isotherms of Cr(III) ionson different as-received activated carbons

31 2Sample x (mg/g) K (l /g) S (m /g)m Cr

GAC-S 13.31 0.07 103.1GAC-E 10.52 0.08 80.5ACF-307 7.08 0.10 58.8ACF-310 3.52 0.07 27.3

x is the maximum amount of Cr(III) ions adsorbed by activatedm31carbon, K is Langmuir’s constant and S is surface area ofCr

activated carbon covered by Cr(III) ions.

which have been postulated to be carboxylic and lactonic1groups ionise in aqueous solution producing H ions. As a

result of this ionisation, the activated carbon surface inaqueous solutions acquires a negative charge dependingupon the amount of acidic surface oxygen groups. Thesurface of carbon oxidised with nitric acid, therefore, has alarger negative charge than the surface of the carbonoxidised with hydrogen peroxide, ammonium persulphateand oxygen gas. When the oxygen surface groups whichimpart negative character to the carbon surface are re-moved on degassing, the carbon surface becomes lesserand lesser negatively charged. Furthermore, the presenceof acidic surface groups on the carbon surface changes thepH of the carbon suspension in aqueous solutions.

The chromic ions in aqueous solutions exist as31[Cr(H O) ] . These associated water molecules around2 6

the chromic ions are exchanged with the hydroxyl ions, thenumber exchanged depending upon the pH of the solutionFig. 9. Adsorption isotherms of Cr(III) on ACF-307 before and[24].after oxidation and degassing.


3.2. Adsorption of Cr(VI)

Adsorption isotherms of Cr(VI) from aqueous solutionsof potassium dichromate in the concentration range 20–1000 mg/ l on the four samples of activated carbonswithout adding any buffering reagent are shown in Fig. 10.Each carbon adsorbs appreciable amount of Cr(VI) ions.However, the amount of Cr(VI) adsorbed is comparativelymuch larger than the amount of Cr(III) ions adsorbedunder similar conditions. This larger adsorption in the caseof Cr(VI) may be attributed partly to the smaller size of

Thus a change in pH of the solution, as a result of thethe Cr(VI) ion in water so that it can enter a larger

change in amount of surface acidic carbon–oxygen surfaceproportion of the microcapillary pores and partly to the

groups will change the extent of the positive charge on thefact that Cr(VI) ion is adsorbed as an anion. As the pH of

chromic ion.the carbon suspension of as-received activated carbons is

These changes in the negative charge on the carbonin the range of 7.2–10.0, there is a possibility of the

surface as a result of oxidation and the changes in theexistence of some positively charged sites where nega-

positive charge on the Cr(III) ions in solution favour thetively charged Cr(VI) ions can be adsorbed.

adsorption of Cr(III) ions because the electrostatic attrac-In order to examine the effect of the nature of the carbon

tive interactions between the carbon surface and thesurface on the adsorption of Cr(VI) ions, the adsorption

chromium ions present in the solution are enhanced. Onisotherms of Cr(VI) ions on the oxidised carbons were

degassing, these electrostatic attractive interactions be-determined. Although adsorption isotherms were carried

tween the carbon surface and the Cr(III) ions in theout on all the four samples, the results on only two samples

solution gradually decrease and result in a decrease in theare given in Figs. 11, 12. It is interesting to note that in

adsorption of Cr(III) ions. When all the surface oxygenboth the carbons, the adsorption of Cr(VI) ions decreases

complexes are removed almost completely on degassing aton oxidation. The decrease in adsorption, however, is

9508C, there is little or no adsorption of Cr(III) ions.larger in the case of oxidation with nitric acid and smaller

Fig. 10. Adsorption isotherms of Cr(VI) on different as-received Fig. 11. Adsorption isotherms of Cr(VI) on ACF-307 before andactivated carbons. after oxidation.


In order to examine this aspect more clearly, adsorptionisotherms of Cr(VI) were determined on carbon samplesdegassed at gradually increasing temperatures. These ad-sorption isotherms for two samples are presented in Figs.13, 14. It is seen that the adsorption of Cr(VI) increases ondegassing, being maximum in the case of 6508C-degassedcarbon samples. This can be explained in terms of theamount and the type of the surface oxygen groups, whichare being eliminated at different degassing temperatures.

The evacuation data presented in Table 3 show that the4008-degassed carbon samples have lost only a part(around 20%) of the acidic surface groups (CO -evolving2

groups) while they retain most of their non-acidic surfacegroups (CO-evolving groups). 6508C-degassed carbons, onthe other hand, have lost most of their acidic surfacegroups (around 80%) and only a small part of the non-acidic surface groups. The 9508-degassed sample is almostcompletely free of any surface oxygen groups. As theremoval of acidic surface groups tends to decrease thenegative character of the carbon surface, the repulsiveinteractions between the carbon surface and the negativelycharged Cr(VI) ions decrease. Thus there is an increase inthe adsorption of Cr(VI) ions. As 4008-degassed carbonsamples lose only a small part of the acidic groups, there isa small increase in the adsorption of Cr(VI) ions.

Fig. 12. Adsorption isotherms of Cr(VI) on GAC-E before andIn the case of the 6508-degassed sample, almost all theafter oxidation.

acidic groups have been eliminated and the dominatinggroups on the carbon surface are quinonic groups. The pH

for oxidation with ammonium persulphate or gaseous of the carbon suspensions in these cases is around 5.5. Thisoxygen.

This decrease in the adsorption of Cr(VI) can beattributed to the formation of carbon-oxygen surfacegroups, which impart negative character to the carbonsurface. As oxidation with nitric acid is a stronger oxida-tive treatment, it results in the formation of larger amountsof acidic surface oxygen groups compared to other oxida-tive treatments and hence causes a larger decrease in theadsorption of Cr(VI) ions. It may, however, be mentionedthat the oxidation of the carbons also enhances the amountof non-acidic carbon oxygen surface groups, which havebeen postulated as quinones [25–27]. These quinonicgroups can cause reduction of Cr(VI) into Cr(III) ions.Thus in the case of oxidised carbons two processes areoccurring simultaneously: the increase in the removal ofCr(VI) due to its reduction to Cr(III) by the non-acidicquinonic groups and the decrease in adsorption of Cr(VI)ions due to the formation of acidic surface groups. Inactual practice, however, the adsorption of Cr(VI) ions isfound to decrease in all oxidised carbons. It appears thatthe increase in the removal of Cr(VI) ions from thesolution by reduction to Cr(III) is small. This can beattributed to the fact that the optimum pH for the reductionof Cr(VI) to Cr(III) ions is around five. As the oxidisedcarbons generally have pH value of less than five, there islittle or no reduction of Cr(VI) into Cr(III) ions in these Fig. 13. Adsorption isotherms of Cr(VI) on ACF-307 before andcases. after degassing.


granulated activated carbons and to CSIR(India) for thefinancial grant under Project No. 01(1298) /93 /EMR-II.Ms Diksha Aggarwal is thankful to UGC(India) also forthe grant of fellowship.

References

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adsorption of chromium by activated carbon from aqueous solution

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