The removal of chromium(VI) from dilute aqueous solution by activated carbon

Water Research Vo[. [l. pp. 673 to 679. Pergamon Press t977 Printed in Great Britain.

THE REMOVAL OF CHROMIUM(VI) FROM DILUTE AQUEOUS SOLUTION BY ACTIVATED CARBON C. P. HUANG* Department of Civil Engineering, University of Dela~vare. Newark. DE 19711, U.S.A. and M. H. W u f Department of Civil Engineering, Wayne State University. Detroit, MI 48202. U.S.A.

(Received 1 December 1975 ; in revised form 10 February 1977) Abstract--The removal of chromium(VI) by activated carbon, filtrasorb 400. is brought by two major interracial reactions: adsorption and reduction. Chemical factors such as pH and total Cr(VI) that affect the magnitude of Cr(VI) adsorption were investigated. The adsorption of Cr[VI) exhibits a peak value at pH 5 ~ . The particle size of carbon and the presence of cyanide species do not change the magnitude of chromium removal. The reduced Cr(VI), e.g. Cr(lll) is less adsorbable than Cr(VI). The free energy of specific chemical interaction, AG'"'' was computed by the Gouy-Chapman-SternGrahame model. The average values of AG ~"" are -5.57 R T a n d -5.81 RZ respectively, for Cr(Vt) and CN. These values are significant enough to influence the overall magnitude of Cr(VI) and CN adsorption. Results also indicate that HCrO~ and Cr.,O7- are the major CdVI) species involved in surface association.

INTRODUCTION In spite of the great c h r o m i u m removal efficiency exhibited by activated carbon, the mechanistic aspects of c h r o m i u m a t t a c h m e n t is only barely understood. Smithson (1971) has suggested a surface reduction of the hexavalent c h r o m i u m followed by precipitation as part of the removal mechanism. O u r previous experiments with a solid waste material, calcinated coke, revealed that bichromate, HCrO,~, is the major species being removed (Huang & Wu, 1975). This present study reports o u r continuing work on the removal of c h r o m i u m by a commercial activated carbon. The factors that affect the interfacial reactions between c h r o m i u m a n d activated carbon were investigated. EXPERI.MENTAL

Adsorbent A commercial activated carbon, Calgon filtrosorb 400, was used. According to the information provided by the manufacturer, this carbon has an average diameter of 1 mm and a BET surface area of 1050-1200m-'g -1. The activated carbon was used as received.

Chemicals All chemical solutions were prepared from ACS certified grade chemicals without further purification.

Adsorption experiments A continuous mixed batch system was employed. To a series of 145 ml glass bottles, various amounts of distilled water and solutions of sodium chloride (NaCI), sodium

chromate (Na.,CrO,0 or chromium nitrate [Cr(NO3)3] or sodium cyanide (NaCN) were pipetted to a total volume of 50 ml: the components varied in each specific run. After adjusting the initial pH with 1 M HCI or I M NaOH, carbon particles at a dose of 10gl -t were added into each solution. Blanks, in the absence of carbon, were also prepared and used as calibration for initial concentration. The bottles were sealed with parafilm and plastic caps and then shaken for 24h at r.t. (23 + I~C) at a frequency of 200 strokes per min using a gyratory machine. At the end of the reaction period, the equilibrium pH was recorded. Each suspension (40ml) was centrifuged in a Beckman model J-21 centrifuge at 5=C to separate the carbon particles and the supematant. The supernatant was used to determine the equilibrium pH and the concentration of the particular ion. The amount of specific ion adsorbed was determined from the difference between the concentrations before and after the reaction period. Hexavalent chromium was analyzed by the pink color complex developed between diphenylcarbohydrazide and chromium ions in an acidified solution (A.P.H.A. et al.. 1971). Absorbance was then measured at 540nm with a Beckman Acta III spectrophotometer using l cm cuvette. The minimum detectable concentration was I x 10-~M (or 0.005 mgl -~ as Cr). In the acidic solution, both HCrO~ and CrTO~ can be detected by this method. Total chromium was determined by oxidizing the chromium with permanganate followed by analysis of hexavalent chromium. The concentration of cyanide remaining in the supernatant was determined by silver nitrate titration using p-dimetbylamino benzalrhodamine indicator (A.P.H.A. et al., 1971). Chromium does not interfere with the cyanide determination. RESULTS AND DISCUSSION

The effect of pH and total Cr(VI) on the adsorption * To whom correspondence should be addressed. of Cr(VI) 5"Present address: Peco & Associates, Consultants, SumThe effects of p H a n d total Cr(VI) were investimit. Illinois. 673

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gated. Figure 1 shows that the adsorption density of Cr(VI) increases with increasing pH to a maximum value and then declines rather rapidly with further increase in pH. When the pH becomes greater than I0, no appreciable adsorption is observed. The extent of adsorption also increases with total CrfVI). The adsorption data follow the Langmuir isotherm to a certain extent, depending mostly on the pH. Figure 2 gives the adsorption isotherms for Cr(VI); Table I lists the Langmuir constants. The data demonstrate that filtrasorb 400 activated carbon is an effective adsorbent for Cr(VI) removal. Figure 3 demonstrates that Cr(VI) is also removed by reduction into Cr(III) in the presence of activated carbon. In the absence of activated carbon, the Cr(VI) added remained totally at hexavalent state; while in the presence of activated carbon, parts of the originally added Cr(VI) were reduced into Cr(llI). However, reduction reaction only occurred at pH < 6,

since no Cr(III) was found in the supernatant when pH becomes greater than 6. Cr(III) is also adsorbed by activated carbon, but Table

1. Langmuir constants for Cr(VI) and CN adsorption Langmuir constants F m

Adsorbate

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CN

pH

(mmole g- t)

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6.0 6.5 7.0 7.5 8.0 3.0 4.0 8.0 11.0

11.1 8.33 5.00 4.17 0.91 0.54 0.49 0.95 0.42

4.6 2.3 1.9 0.4 1.7 0.5 0.3 1.6 0.5

Removal of chromium(VI) from dilute aqueous solution 60

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The remocal of cyanide Since cyanide is always present in chromium plating wastewater, it is of practical importance to investi-

gate the removal of cyanide by activated carbon. The results shown in Fig. 6 demonstrate that cyanide is removed by activated carbon. Peak adsorption occurred at pH8-10. The minimum adsorption was found at pH 3-5. The results indicate that the adsorption of cyanide at pH 3-5 and pH 8-10 follows a Langmuir isotherm (Fig. 6). The distribution coegficient, D, is defined by the ratio of chromium (or cyanide) adsorbed to chromium (or cyanide) free in solution. The distribution coelficient of chromium (VI) with filtrosorb 400 falls in the same range (10-102) as for Zn(II), Co(II), Fe(III), and Cu(II) with aqua nuclear A under equivalent experimental conditions (Nelson et al., 1974). 0.8

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try of the adsorbent. The nature of original raw materials used, the manulhcturing process, especially the temperature and gases used during the carbonization and activation stage, are important factors in determining the surface chemistry of activated carbon (Mattson & Mark. 1972). The formation of carbon-oxygen complexes at the carbon surface is believed by man~ researchers to be the major thctor in contributing the anionic (base) adsorption capacity of most activated carbons. The nature of carbon-oxygen complexes is a function of activation temperatures and gas used. Frumkin (1930) reported that sugar carbon activated under vacuum and cooled under vacuum adsorbed neither acid nor base. The adsorption capacity was restorable by further exposing the carbon to the atmosphere. Steenberg (1944) divided carbons into two categories by their activation temperature: the carbons activated at low temperature (<500-600'~C) only adsorb bases: they will adsorb only acids if activated at high temperature (<500--600 C). Upon the introduction of activated carbon into distilled water, the "'oxidized" carbon surface becomes positively charged and pH of the solution is increased due to the release of hydroxyl ions: CxO+2H,O~C~OH~++2OH

(1)

This increase in pH was observed throughout the entire laboratory investigation. The surface charge, thereby developed, is pH-dependent. The zero point of charge, pH,,,., of filtrasorb 400 was found to be 7.00 4- 0.1 (Huang & Ostovic, 1977). In principle, there are four major theories being

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The distribution coefficient diminishes as pH approach 9 and thereabove. The distribution coefficients of cyanide and chromium are shown in Fig. 7. The affinity of hydrogen cyanide towards activated carbon is not compatible with that of chromium at pH < 7; while C N - ion can be adsorbed more effectively than chromium at pH greater than 7. The data once again reflect the importance of pH and total adsorbate concentration on the adsorption characteristics of Cr(III) and CN by activated carbon.

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A preliminary model for adsorption onto carbon surface The chromium adsorption capacity of activated carbon is primarily influenced by the surface chemis-

Fig. 7. The distribution coefficients for Cr(VD and CN as functions of pH and total concentrations of Cr(VI) and CN.

Removal of chromium(VI) from dilute aqueous solution developed to describe and interpret the adsorption of solute tmetal ions or anionsl at solid-solution interface: Ill the Gou~=Chapman-Stern-Grahame model which accounts for electrostatic and specific adsorption tBreeuwsma & Lyklema, 1973: Levine & Smith. 1971): the James-Healy model, which considers coulombic, solvation, and specific chemical energy for ion-solvent interaction (James & Healy. 1972: Huang. 1973): (3) the ion-exchange model. according to which ions upon adsorption on the solid surface release protons or hydroxyl ions (Stanton & Maatman. 1963; Dugger et al., 1964); and (4) the surface couplex formation model, in which the hydrous oxide surface groups are treated, similar to amphoteric functional groups in polyelectrolyte as a complex-forming agent (Huang and Stumm, 1971; Holh & Stumm. 1975; Schindler et al., 1975). With the preliminary data available from this investigation, only the G o u y - C h a p m a n - S t e r n - G r a h a m e model was applied. According to this model, the total adsorption density is defined by the following equation: F, = ZF i = Z 2 r i C i exp -[-AGi -l'c' + A G { " ' " ' ) / R T ] ,

(2~ where F, and F~ stand respectively for the total and individual adsorption density of species i with size r~ and equilibrium concentration C;. AG}z~'' and AG~"'m are, individually, the free energy of adsorption contributed by electrostatic and specific chemical interaction. It is given that at pHzp,, AG:, t ' ' ' - O. Therefore, the adsorption density at prize,,, F o, is related to the specific chemical energy, AG~"~'. Fo = ~ 2 r i C i e x p - (AG~h'm/RT).

(3)

By applying mass balance equation and introducing a conversion factor, S, which converts volumetric concentration (mole m -a) into surface concentration (mole m - 2), one obtains 2riC, e x p ( - AG?-h,~/R T) Fo = 1 + ( 2 r i / S ) e x p [ - A G ~ h " m / R T ] "

677

Table 2. Computation of AGi° '~ for CrIVD and CN Species Cr/VI}

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C, (raM)

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AG',"~'iRT)

5.0 2.5 1.0 0.5 9.18 4.63 2.29 0.87 0.51

0.385 0.192 0.087 0.048 0.731 0.400 0.192 0.054 0.039

-5.48 -5.48 - 5.60 -5.71 - 5.84 - 5.92 - 5.89 - 5.59 - 5.79

The ionic size for Cr(Vl) and CN used is 1.6 .-~ for the Cr O bond and 1.16 ~ for the C-N bond tWells. 19621. Cr(VI) concentration used in this investigation only yields HCrO~ as major species, HCrO~- ion obviously plays a more important role in association with activated carbon. The average AG~~.... value of Cr(VI) is -5.57 RT(or -3.23 kcal mole -~ at 20°C), value significant enough to control the adsorption of Cr(VI) by activated carbon, filtrasorb 400. Table 2 also shows the free energy of specific chemical adsorption for CN. The average value (-5.81 R T ) i s compatible with that for Cr(VII. Therefore, specific chemical interaction also applies equally significant to CN. T h e effect o f foreign species on adsorption

Figure 8 shows the effect of mutual inter-influence on the adsorption of chromium and cyanide. Very little effect on the adsorption of chromium in the presence of cyanide was observed and ~ice versa. Only a very slight hindrance in adsorption of either chromium or cyanide was found at the pH region under which the interfering species is favored. In this case, at pH greater than 7, cyanide ion is the favorable species for the carbon let alone that chromium (VII is not adsorbed at pH >_ 9 even in the absence

(4)

During this manipulation, it is also assumed that all species have the same size, r~, and that the specific chemical energy, AG~"era is identical for all species and is a constant. It is further noted that the second term of the denominator is much smaller than unity. Therefore, F0 is expressed as F o = 2riC , exp - ( A G ~ " ~ / R T ) .

(5)

which allows the computation of AGi .... by given Fo at various total adsorbate concentration, C,. The calculated free energy of specific chemical interaction for Cr(III) and CN is shown in Table 2. The AG "h ~ values for Cr(VI) and CN are surprisingly constant with respect to Cr This model also confirms that HCrO2 and Cr_,O-~ions are the major species being adsorbed. Because the distribution of HCrO.~ and Cr,O-~- species is controlled by their ionic equilibria and the total

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of cyanide; therefore, at pH less than 7, chromium is greatly favored, and the adsorption of cyanide (HCN) is inhibited. However, the adverse effect in adsorption of cyanide by chromium is greater than that by cyanide, since the latter is also adsorbed at a slighter extent than chromium at pH less than 7. The effect of particle size The removal of chromium by the activated carbon, at least filtrasorb 400, is not affected by the size of the adsorbent at the concentration of 22.5 mg 1-1 as Croci) (Fig. 9). This is different from the results reported by others. Weber & Morris (1964), for instance, reported that adsorption capacity increases with decreasing particle size. Mattson and Mark (1971) have attributed this effect to this difference in specific surface area. In a separate investigation, the senior author and his associate found that grinding does not change the specific surface area of filtrasorb 400 (Ostovic, 1977). It is obvious that filtrasorb 400 is a highly porous adsorbent and that the specific area is mainly brought by the great number of micropores which upon crushing do not necessarily expose more surface. Figure 9 also indicates that adsorption removal is not influenced by the particle size, at least for a reaction time of 24 h as was used in the study. SUMMARY

The introduction of activated carbon into chromium (VI)-containing solution can result two important chemical reactions relating to the removal of Croci): (I) the removal of CROCI) through reduction into Cr(III), and (2) the adsorption of mostly the ori~nally present Croci) and partly the reduced CrOCi),

e.g. Cr(I[l) species by the activated carbon surface. The reduction-removal step only takes place at pH < 6. and its extent increases with total Cr(VI). The adsorption-removal step has a maximum magnitude at pH between 5 and 6. This implies that for acidic solution, the major residual chromium species are of the trivalent form and that a total removal is only obtainable by further treatment of the Cr(llI) thereby produced. Since the adsorption of Cr(III) onto carbon surface is not as pronounced as that of Cr(VI), therefore, chemical precipitation may be used. In general, filtrasorb 400 is very effective in the removal of Croci), regardless of the removal mechanism. The major Croci) species responsible for adsorption-removal are HCrO,~ and CrzO~,. However, at total Cr(VI) concentrations and pH values generally found in plating industrial wastewater, HCrO,7, ions should play a more important role in adsorption removal than CrzO ~- species. The adsorption characteristics of chromium OCI) can be described by a Langmuir isotherm for pH values between 6 and 8 within which range Cr(VI) removal is mostly achieved through adsorption. According to the Gouy-Chapman-Stern-Grahame model, the specific chemical energy of adsorption, AG "~''", was computed for both the Croci) and the CN. The results indicate that specific chemical interaction plays an important part in the surface association of Croci) and CN. The average values of AG ~.... are -5.57 RT(or -3.23 kcal mole - t ) and -5.81 R T (or -3.37 kcal mole-t) respectively for Croci) and CN. The free energy changes due to electrostatic interaction between a positive surface of + 100mV and a monovalent anion is - 3 . 5 R T (or - 2 . 0 kcal mole-t). This signifies the important of specific chemical interaction. Grinding of activated carbon does not modify the extent of Croci) removal. Apparently, this is due to the unchange of specific surface area of the activated carbon. The presence of cyanide does not interfere with the removal of Cr(VI), nor does the presence of chromium hinder the adsorption of cyanide. Croci) is mostly removed by adsorption at pH less than 7, whereas, at pH greater than 7, cyanide is more removable than Cr(Vt). Cyanide species; namely, C N - and HCN, are also effectively removed by filtrasorb 400. The base ions, C N - , are much favored by the activated carbon than its conjugate acid, HCN. Acknowledgements--This work was supported in part by a research grant from the U.S. Environmental Protection Agency. The financial support (graduate assistantship) to M. H. Wu provided by the department of Civil Engineering, Wayne State University, is also acknowledged.

REFERENCES

A.P.H.A., A.W.W.A. & W.P.C.F. (1971) Standard Methods for the Examination of Water and Wastewater, 13th edition.

Removal of chromium(VI) from dilute aqueous solution Breeuwsma A. and L?klema J. 11973) Adsorption of phosphate by geothite. J. Colloid Interj'. Sci. 43. 437. Dugger D. L.. Stanton J. H.. Irby B. N.. McConnell B. L.. Cummings W. W. and Maatman R. W. The exchange of twenty metal ions with the weakly acidic silanol group of silica gel. J. ph~is. Chem. 68. 757. Frumkin A. (1930) Uber die adsorption yon electrolyten dutch aktivierte kohle. Kolloid Z. 51. 123. Hohl H. and Stumm W. (1976l Interactions of Pb-'" with hydrous 7-AI_,O3. J. Colloid lnterf Sci. 55. 2. 281. Huang C. P. and Stumm W. (1971) Specific adsorption of cations on hydrous 7-A[zO 3. j. Colloid Interj. Sci. 43. 2. 209. Huang C. P. (1975) Adsorption of phosphate at the hydrous 7-Al,O3--electrolyte interface. J. Colloid lnterf Sci. 53, 2. 178. Huang C. P. & Wu M. H. (1975) Chromium removal by carbon adsorption. J. ~I2~t. Pollut. Control Fed. 47, 2437. Huang C. P. & Ostovic F. (1977) The removal of cadmium (ll) from dilute aqueous solution by activated carbon adsorption. Symp. Chemistry of Wastewater Technology. 173rd National Meeting, A.C.S. New Orleans, La. James R. O. and Healy T. W. (1972) The adsorption of hydrolyzable metals at oxide solution interface. J. Colloid Interl~ Sci. 40. 66.

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Levine S. and Smith A. L. (1971) Theory of the differential capacity of the oxide aqueous electrolyte interface. Discuss. Faraday Soc. 52, 290. Mattson J. S. & Mark H. B. (1971) Actirated Carbon. p. 203. Marcel Dekker. New York. Nelson F.. Phillips H. O. & Kraus K. A. [19741 Adsorption of inorganic materials on activated carbon. 29th Ann. Purdue Ind. Waste Conf. Ostovic F. (1977) The removal of cadmium by carbon adsorption. Master thesis, University of Delaware, Newark. Delaware. Schindler P. W.. Furst B., Dick R. & Wolf P. U. (1976) Ligand properties of surface silanol groups. J. Colloid lllterf Sci. 55. 2. 469. Smithson G. R.. Jr. [1971) An investigation of techniques for removal of chromium from etectroplating wastes. Report to U.S. Environ. Protection Agency, 12010 EIE 03/71. Washington. D.C. Stanton J. & Maatman R. W. (1963) The reaction between aqueous uranyl ion and the surface of silica gel. J. Colloid Sci. 13. 132. Steenberg B. (1944) Adsorption and exchange oj" ions on acti~'ated carcoal. Almquist & Wiksell, Uppsala. Wells A. F. (1962) Structural Inorganic Chemistry, 3rd edition. Oxford University Press, Oxford.