Journal of Colloid and Interface Science 217, 137–141 (1999)

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Adsorption by activated red mud (ARM) is investigated as a possible alternative to the conventional methods of Cr(VI) removal from aqueous synthetic solutions and industrial efﬂuents. Adsorption characteristics suggest the heterogenous nature of the adsorbent surface sites with respect to the energy of adsorption. Various factors such as pH, contact time, Cr(VI) concentration, amount of adsorbent, and temperature are taken into account, and promising results are obtained.

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Journal of Colloid and Interface Science 217, 137–141 (1999) Article ID jcis.1999.6288, available online at http://www.idealibrary.com on Adsorption of Hexavalent Chromium from Aqueous Solution by Using Activated Red Mud Jyotsnamayee Pradhan, Surendra Nath Das, and Ravindra Singh Thakur 1 Regional Research Laboratory (CSIR), Bhubaneswar 751013, India Received January 7, 1999; accepted April 14, 1999 ing its adsorption on activated carbon (2), starch xanthate (3), Adsorption by activated red mud (ARM) is investigated as a alumina (4), low-grade manganese ore (5), crushed coconut possible alternative to the conventional methods of Cr(VI) re- shell (6), ﬂy ash (7), sawdust (8, 9), rice husk carbon (10), moval from aqueous synthetic solutions and industrial efﬂuents. wood charcoal (11), bituminous coal (12), and lignite (13). Adsorption characteristics suggest the heterogenous nature of the Removal of chromium by different physical and chemical adsorbent surface sites with respect to the energy of adsorption. methods has been reviewed (14). In the present study the Various factors such as pH, contact time, Cr(VI) concentration, material used is an industrial waste/byproduct of the aluminum amount of adsorbent, and temperature are taken into account, and industry. Here, an attempt is made to prepare activated red mud promising results are obtained. The applicability of the Langmuir and to study its feasibility as an adsorbent for removal of as well as Freundlich adsorption isotherms for the present system hexavalent chromium from aqueous solution/industrial efﬂu- is tested. The loading factor (i.e., milligrams of Cr(VI) adsorbed per gram of ARM) increased with initial Cr(VI) concentration, ents. The process is investigated as a function of pH, time, whereas a negative trend was observed with increasing tempera- concentration of adsorbate, amount of adsorbent, and temper- ture. The inﬂuence of the addition of anions on the adsorption of ature. The effects of other extraneous anions on the adsorption Cr(VI) depends on the relative afﬁnity of the anions for the surface of Cr(VI) is also investigated. and the relative concentrations of the anions. © 1999 Academic Press Key Words: activated red mud; adsorption; hexavalent chro- EXPERIMENTAL mium removal. Activated red mud is prepared (15) by simple acid dissolu- tion followed by ammonia precipitation and drying at 110°C. INTRODUCTION Procedural details are given in our earlier paper (16). Analyt- ical grade reagents are used to prepare KCl solution and buffer Rapid industrialization and usage of heavy metals in indus- solution to maintain the ionic strength and pH of the medium. trial processes have resulted in an unprecedented increase in The solutions of Cr(VI) are prepared from AR quality the heavy metal ﬂux into groundwater and industrial efﬂuents. K 2Cr 2O 7. Like many heavy metals, chromium in traces is necessary for Adsorption experiments for Cr(VI) were carried out in life processes. However, with higher concentration of this 100-ml stoppered conical ﬂasks by taking appropriate amounts element in environment and the consequent increase in human of potassium dichromate solution and activated red mud. The intake, chromium concentrations have reached toxic levels and ionic strength of the medium was maintained by adding 1 M manifested in a variety of ailments such as dermatitis, conges- KCl. Acetic acid–sodium acetate buffer was used to maintain tion of respiratory tracts, and perforation of the nasal septum. the pH of the solution in the range 3.0 –5.9 and KH 2PO 4– It is also a proven mutagen which may lead to cancer (1). NaOH buffer in the range 6.0 – 8.0, and the ﬁnal volume was Chromium can exist in several valence states of which the made up to 50 ml. After gentle shaking for a stipulated contact trivalent and hexavalent forms are common and the hexavalent time in a mechanical shaker, the contents were ﬁltered through form is highly toxic. G 4 crucibles. Concentrations of Cr(VI) in the ﬁltrate were In view of the pollution hazard caused by hexavalent chro- determined spectrophotometrically (17) using diphenylcarba- mium, several methods of removal have been reported, includ- zide solution at 540 nm. The percentage of Cr(VI) ad- ing chemical precipitation, reverse osmosis, ion exchange, sorbed was determined from the ratio of chromium (VI) in the foam ﬂotation, electrolysis, and adsorption. Among all the solution and particulate phases: above mentioned methods, adsorption is an economically fea- sible alternative. A variety of materials are used as adsorbents Cr VI % of chromium VI adsorbed ads for Cr(VI), and various studies have been published document- Cr VI in Cr VI eq 1 00, [1] Cr VI in 1 To whom correspondence should be addressed. 137 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
138 PRADHAN, DAS, AND THAKUR where Cr(VI) in and Cr(VI) eq are the initial and equilibrium concentrations. Each run was made in duplicate. All of the pH measure- ments were made with an Elico-Digital pH meter (model LI-120) using a combined glass electrode (Model CL-51). All of the spectrophotometric measurements were made with a Chemito-2500 recording UV-Vis spectrophotometer using 10-mm matched quartz cells. The pH pzc value of ARM was determined by potentiometric acid-base titration following the method of Parks and Bruyn (18) where 0.2 g of the sample was suspended in KNO 3 solutions at concentrations of 0.1, 0.01, and 0.001 moles/L as a supporting electrolyte. The surface charge ( °) in coulombs/g was calculated by taking the difference between H and OH ions added to attain a particular pH, as reported earlier (19). RESULTS AND DISCUSSION Effect of Contact Time Adsorption experiments were carried out for 24 h to ﬁnd the optimum contact time. The kinetics of adsorption of hexavalent chromium at pH 5.2 (Fig. 1) show that the equilibrium is FIG. 2. Adsorption of Cr(VI) on activated red mud as a function of pH. attained in about 2 h. There is no signiﬁcant change in equi- librium concentration after 2 h up to 24 h. It is further observed mg/L) as a function of equilibrium pH is shown in Fig. 2. It is that the removal curve is smooth and continuous indicating the evident from the ﬁgure that the adsorption is higher at lower possibility of the formation of monolayer coverage of Cr(VI) pH, it is maximum at pH 5.2, and it suddenly decreases ion at the interface of the ARM. becoming almost negligible at pH 7.06. A similar type of behavior is also reported for the adsorption of anionic species Effect of pH on metal oxides/oxyhydroxides (19 –23), ﬂy ash (24, 25), and The initial pH is varied between 3.5 and 7.0. The adsorption coal (26). The effect of pH on the adsorption capacity of of hexavalent chromium at a ﬁxed Cr(VI) concentration (20 activated red mud may be attributed to the combined effect of pH on the nature of activated red mud surfaces, adsorbed Cr(VI) species, and the presence of acid and base used to adjust the pH of the solution. To explain the observed behavior of Cr(VI) adsorption with varying pH, it is necessary to examine various mechanisms such as electrostatic attraction/repulsion, chemical interaction, and ion exchange which are responsible for adsorption on sorbent surfaces. From the stability diagram (27), it is evident that the most prevalent forms of Cr(VI) in aqueous systems are acid chro- 2 2 mates (HCrO 4 ), chromates (CrO 4 ), dichromates (Cr 2O 7 ), and other oxyanions. From the stability diagram for the Cr(VI)–H 2O system, it is evident that at low pH, acid chromate ions (HCrO 4 ) are the dominant species. As the pH increases, there is little increase in the percentage of adsorption, and it is maximum at pH 5.2. When the pH increases to 6, a sharp decrease in the percentage of adsorption is observed. This may be caused by a decrease in net positive centers on the surface of the adsorbent due to adsorbed Cr(VI) species which results in weakening of electrostatic forces between the adsorbate and adsorbent and ultimately leads to a reduction in the sorption capacity. When the pH increases beyond 6.0, a gradual de- crease in the percentage of adsorption is observed which may be due to the competition between OH and chromate ions FIG. 1. Adsorption of Cr(VI) on activated red mud as a function of time.
139 HEXAVALENT CHROMIUM ADSORPTION BY ACTIVATED RED MUD 2 (Cr 2O 7 ), the former being the dominant species at higher pH values. The net positive surface potential of sorbent decreases resulting in weakening of electrostatic forces between sorbate and sorbent which ultimately lead to the lowering of the sorption capacity. Similar results are also observed with ﬂy ash–wollastonite (25) and coal (26). Determination of pH pzc The pH at point of zero charge (pzc) was found to be approx. 8.5 (Fig. 3) which is comparable to the values reported earlier for untreated red mud (28) as well as pure systems of alumina (4) and goethite (20). This is in agreement with our experimental obser- vation showing almost zero adsorption at pH 7.0. Effect of Temperature The percentage of adsorption of Cr(VI) on activated red mud was studied as a function of temperature in the range 298 –323 K. The results are presented in Fig. 4. There is a decrease in the percentage of adsorption with a rise in temperature which may be due to higher desorption caused by an increase in the thermal energy of the adsorbate. FIG. 4. Adsorption of Cr(VI) on activated red mud as a function of The uniformity or heterogeneity of the surface sites of an temperature. activated red mud sample has been deduced from the isosteric heats of adsorption as a function of adsorption density using the Clausius–Clapeyron equation (29). The isosteric heats of where H r is the isosteric heat of adsorption in kJ/mole at a adsorption are calculated from adsorption isotherms at two given adsorption density, R is the gas constant, and C 1 and C 2 different temperatures: are the equilibrium concentrations of the ion at temperatures T 1 and T 2 . If the isosteric heat of adsorption is independent of Hr R ln C 2 / C 1 / 1 / T 2 1/T1 , [2] adsorption density, then the surface is homogenous, and if it decreases with increasing adsorption density, then the surface is heterogenous (30). A decrease in the percentage of adsorp- tion with a rise in temperature and variation in the isosteric heats of adsorption support the heterogenous nature of the activated red mud sample. Similar types of observations are also made in selenite adsorption on iron oxyhydroxides (19) and manganese nodules (20). This may be due to different types of adsorption sites or the interaction of adsorbing ions. Effect of Adsorbent and Adsorbate Concentration The percentage of Cr(VI) adsorption with varying amounts of activated red mud and Cr(VI) concentration is presented in Figs. 5 and 6. An increase in percentage of adsorption with higher amounts of adsorbent and a decrease with higher con- centration of adsorbate indicate that the adsorption is depen- dent upon the availability of the binding sites. To determine the adsorption capacity of the sample, the equilibrium data for the adsorption of Cr(VI) are analyzed in the light of the Langmuir adsorption isotherm model. Experimental data points are ﬁtted into the Langmuir equation: C/X 1/ bXm C / X m, [3] FIG. 3. Surface charge of ARM as a function of pH in the presence of where C / X is the amount of Cr(VI) adsorbed per unit weight of KNO 3.
140 PRADHAN, DAS, AND THAKUR FIG. 5. Adsorption of Cr(VI) as a function of the amount of adsorbent. FIG. 7. Langmuir plot of Cr(VI) adsorption on activated red mud at room temperature. the sample, C is the Cr(VI) concentration in equilibrium solu- tion, b is a constant related to the energy of adsorption, and X m is the adsorption capacity of the sample. Figure 7 depicts the squares method to the lines of Fig. 7 and found to be 30.74 Langmuir plot of C / X vs C for the experimental data points. mmol/g and 0.2691, respectively. The correlation coefﬁcient is found to be 0.99. X m and b are K a values (31), which represent the apparent equilibrium calculated from the Langmuir equation by applying the least constant corresponding to the adsorption process, can be cal- culated as the product of Langmuir equation parameters b and X m. The apparent equilibrium constant was found to be 8.2737 mmol/g, which can be used as a relative indicator of the red mud’s afﬁnity for chromate ions (32). The adsorption values were ﬁtted to the Freundlich isotherm model and were found to be in order (Fig. 8). The linearity of the Langmuir and Freund- lich isotherms indicate that the adsorption is a surface phenom- enon (4). Effect of Competitive Ions The effect of different competitive ions like nitrate (NO 3 ), 2 3 sulfate (SO 4 ), and phosphate (PO 4 ) on the adsorption of Cr(VI) was studied at various concentrations. It was observed that the percentage of adsorption of Cr(VI) decreased with increasing concentrations of externally added ions. The afﬁnity 3 sequence for adsorption of such anions on ARM is PO 4 2 SO 4 NO 3 . Increasing dosages of these ions from 5 to 20 mg/L had little effect. Similar observations were made for ﬂuoride removal using treated alum sludge (33). Desorption Studies After adsorption, the resulting Cr(VI) containing ARM is safe for disposal. The stability of this sludge from the resolu- FIG. 6. Adsorption of Cr(VI) on activated red mud as a function of initial bilization point of view was studied. It was found that the concentration.
141 HEXAVALENT CHROMIUM ADSORPTION BY ACTIVATED RED MUD selectivity of the activated red mud surface for these ions. Thus, the activated samples of red mud serve as an excellent alternate adsorbent for removal of hexavalent chromium from aqueous medium. ACKNOWLEDGMENT The authors are grateful to the Director, Regional Research Laboratory at Bhubaneswar for kindly permitting the publication of this paper. REFERENCES 1. Bianchi, V., and Levis, A. G., Toxic Environ. Chem. 9, 1 (1984). 2. Haung, C. P., and Wu, M. H., Water Res. 11, 673 (1975). 3. Chaudhari, S., M. Tech. Thesis, Indian Institute of Technology, Kanpur (1985). 4. Gupta, D. C., and Tiware, U. C., Indian J. Environ. Health 27, 205 (1985). 5. Prasad, S. C., and Venkobachar, C., Asian Environ. 10, 11 (1988). 6. Prasad, S. C., and Venkobachar, C., Asian Environ. 10, 3 (1988). 7. Grover, M., and Narayanaswamy, M. S., J. Environ. Eng. 63, 36 (1982). 8. Poots, V. J. R., McKay, G., and Healy J. J., J. Water Pollution Control Fed. 50, 926 (1978). 9. Poots, V. J. R., McKay, G., and Healy, J. J., Water Res. 10, 1061 (1976). 10. Srinivasan, K., Balasubramanian, N., and Ramakrishna, T. V., Indian J. Environ. Health 30, 376 (1988). 11. Das, D., and Gupta, A. K., Indian J. Environ. Health 33, 297 (1991). FIG. 8. Freundlich plot of Cr(VI) adsorption on activated red mud at room 12. Nagesh, N., and Krishnaiah, A., Indian J. Environ. Health 31, 304 (1989). temperature. 13. Kannan, N., and Vanangamudi, A., Indian J. Environ. Health 11, 241 (1991). 14. Maruyama, T., Sidney, A. H., and Cohen, J. M., J. Water Pollution Control Fed. 47, 962 (1975). Cr(VI) release from the sludge in aqueous medium after con- 15. Pratt, K. C., and Christoverson, V., Fuel 61, 460 (1982). tact time of 72 hours was negligible. However, partial de- 16. Pradhan, J., Das, J., Das, S. N., and Thakur R. S., J. Colloid Interface Sci. sorption of Cr(VI) is observed in a strongly alkaline medium 203, 169 (1998). (pH 8). The solution obtained after Cr(VI) adsorption was 17. APHA, “Standard Methods of Examination of Water and Wastewater,” 16th ed., American Public Health Association, New York, 1985. subjected to AAS and none of the trace metals was found to be 18. Parks, G. A., and De Bruyn P. L., J. Phys. Chem. 66, 967 (1962). in the detectable range. Thus the release of harmful trace 19. Parida, K. M., Satapathy, P. K., and Das, N. N., J. Colloid Interface Sci. metals into the environment after adsorption of Cr(VI) is ruled 181, 456 (1996). out. 20. Parida, K. M., Gorai, B., and Das, N. N., J. Colloid Interface Sci. 187, 375 (1997). Applicability to Industrial Efﬂuents 21. Balistrieri, L. S., and Chao, T. T., Geochim. Cosmochim. Acta 54, 739 (1980). Adosrption studies were extended to the efﬂuents from the 22. Balistrieri, L. S., and Chao, T. T., Soil. Sci. Soc. Am. J. 51, 1145 (1987). sodium dichromate and basic chromium sulphate industries 23. Yates, D. E., and Healy, T. W., J. Colloid Interface Sci. 52, 222 (1975). 24. Gupta, G. S., Prasad, G., and Singh, V. N., IAWPC Tech. Annu. 15, 98 containing very high values of Cr(VI) using ARM under sim- (1988). ilar conditions. In all cases the percentage of adsorption was 25. Pandey, K. K., Prasad, H., and Singh, V. N., Chem. Tech. Bio. Tech. 34, found to be more than 90%. Details of the results obtained will 367 (1984). be communicated in a separate paper. 26. Agrawal, S. C., Rai, S. S., and Mathur K. C., J. Indian Water Works Assoc. 2, 133 (1989). 27. Beneﬁeld, L. D., Judkins, J. P., and Wend, B. L., “Process Chemistry for CONCLUSION Water and Wastewater Treatment,” Prentice Hall, Englewood Cliffs, NJ, 1982. From the previous discussions, it may be concluded that 28. Apak, R., Guelu, K., and Turgut, M. H., J. Colloid Interface Sci. 203, 122 hexavalent chromium adsorption on activated red mud is a (1998). surface phenomenon and the Langmuir and Freundlich iso- 29. Chaiu, C. T., Shoup, T. D., and Porter, P. E., Org. Geo–Chem. 8, 9 (1985). therm curves show linearity. The best conditions for adsorption 30. Balistrieri, L. S., and Chao, T. T., Soil Soc. Am. J. 51(5) (1987). 31. Merril, D. T., Mantione, M. A., Peterson, J. J., Parks, O. S., Chow, W., and were found to be pH 5.2 and a temperature of 303 K in the Hobbs, A. O., J. Water Pollution Control Fed. 58, 18 (1986). concentration range 2–30 mg/L with a solid:liquid ratio of 32. Lopez-Gonzalez, J. D., Valenzuela-Calalorro, C., Jimenez-Lopez, A., and 1:500. The presence of other ions in solution inﬂuenced ad- Ramirez-Saenz, A., An. Quim. 74 (1978). sorption. Although the nitrates had very little effect, ions like 33. Sujana, M. G., Thakur, R. S., and Rao, S. B., J. Colloid Interface Sci. 206, sulphate and phosphate had noticeable effects due to the higher 94 (1998).