JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE

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Adsorption of phosphate (PO3 ) from aqueous solution on ac4 tivated red mud (ARM) was studied as a function of time, pH, temperature, concentration of adsorbent and adsorbate in acetic acid–sodium acetate buffer medium. The adsorption of phosphate follows Langmuir as well as Freundlich adsorption isotherms. The process efﬁciency was found to be 80 –90% at room temperature.

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204, 169 –172 (1998) JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO. CS985594 Adsorption of Phosphate from Aqueous Solution Using Activated Red Mud Jyotsnamayee Pradhan, Jasobanta Das, Surendranath Das, and Ravindra Singh Thakur1 Regional Research Laboratory, Council of Scientiﬁc & Industrial Research, Bhubaneswar 751 013, India Received January 7, 1998; accepted April 17, 1998 (2), half-burnt dolomite (3), activated carbon (4, 5), coconut Adsorption of phosphate (PO3 ) from aqueous solution on ac- shell carbon (6), clays (7), bentonite (8), ferrihydrite (9), 4 tivated red mud (ARM) was studied as a function of time, pH, goethite (10), -alumina (11), and hematite (12). The studies temperature, concentration of adsorbent and adsorbate in acetic aimed to understand its ﬁxation in soil and to minimize acid–sodium acetate buffer medium. The adsorption of phosphate eutrophication of lakes, rivers, and ponds (13, 14). The follows Langmuir as well as Freundlich adsorption isotherms. The present paper reports preparation of activated red mud and process efﬁciency was found to be 80 –90% at room temperature. use of the same as an adsorbent for phosphate removal from This can be extended to the treatment of industrial efﬂuents aqueous solution. Adsorption of phosphate was investigated containing phosphates like that from phosphatic fertilizer as a function of pH, time, temperature, and concentration of plants. © 1998 Academic Press adsorbate and adsorbent. Key Words: activated red mud; adsorption; phosphate; efﬂuent treatment. EXPERIMENTAL INTRODUCTION Preparation of Activated Red Mud The problem of solid waste disposal has now attained com- Activated red mud (ARM) was prepared (15) by reﬂuxing plex dimensions. It is essential either to ﬁnd suitable ways to red mud in 20% HCl for 2 h. After the reﬂux solution was safely dispose of these wastes or to suggest novel uses, con- cooled to room temperature, liquor ammonia was added until sidering them as byproducts. Otherwise, this will remain as an complete precipitation. The precipitate was allowed to settle accumulated waste contributing highly to environmental pol- and was then ﬁltered and washed thoroughly with distilled lution (1). Utilization techniques for various industrial wastes water until free from ammonia. The residue was dried at 110°C vary widely depending upon their physico-chemical character- and used for adsorption studies. The speciﬁc surface area of the sample was found to be 249 m2/g. All the chemicals used for istics. Red mud is a waste material formed during the produc- tion of alumina when the bauxite ore is subjected to caustic adsorption experiments were of analytical grade. leaching. It is a brick red colored highly alkaline (pH 10 –12) sludge containing mostly oxides of iron, aluminum, titanium, Adsorption Experiments and silica. The pH of activated red mud, when suspended in distilled Red mud, due to its high aluminum, iron, and calcium water, increases with time even in the absence of an adsor- content, has been suggested as a cheap adsorbent for removal bate metal ion. So the adsorption of phosphate on activated of toxic metals (e.g., As, Cr, Pb, Cd) as well as for water or red mud was carried out in acetic acid–sodium acetate buffer wastewater treatment. The basic advantage of red mud is its medium (16). The experiments were carried out in 100 ml versatility in application. Since it is composed of a mixture of stoppered conical ﬂasks by taking appropriate amounts of useful adsorbents and ﬂocculants, it can be used for treatment synthetic phosphate solution and activated red mud (2 g/L). of several efﬂuents. The ionic strength and pH of the solution were maintained Wastes containing phosphate create eutrophication in the by adding 1 M KCl and acetic acid–sodium acetate buffer receiving bodies of water. In order to eliminate the possible solution, respectively, and the ﬁnal volume was invariably dangers to receiving water sources, it is necessary to treat made up to 50 ml. The ﬂasks were placed in a rotary before discharge. Adsorption of phosphate from aqueous mechanical shaker for a particular period of time and shaken solution has been studied in the past few decades by several gently. After the stipulated contact time, the conical ﬂasks authors using different adsorbents, like activated alumina were taken from shaker and the contents were ﬁltered through G4 crucibles. The ﬁltrates were collected and the 1 To whom correspondence should be addressed. 169 0021-9797/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
170 PRADHAN ET AL. concentration of phosphate in the ﬁltrate was determined spectrophotometrically by measuring the absorbance at 440 nm using the phosphovanadomolybdate method (17). The percentage of phosphate adsorbed was determined from the ratio of the concentration of phosphate present in the solution and particulate phases. PO3 in PO4 eq 3 4 PO4 ads % of PO3 adsorbed 3 1 00 , [1] 4 PO3 in 4 where PO3 in and PO3 eq are the initial and equilibrium con- 4 4 centrations of phosphate, respectively. Each run was made in duplicate. All the spectrophotometric measurements were made with a Chemito-2500 UV–visible FIG. 2. Adsorption of phosphate on activated red mud as a function of recording spectrophotometer using 10 mm matched quartz equilibrium pH. cells. All the pH measurements were made by an Elico digital pH meter (model LI 120) using a combined glass electrode (model CL 51). The pH meter was standardized with NBS tact time constant and varying the pH of the medium from 3.5 to buffers before any measurement. The adsorption experiments 6. Figure 2 represents the effect of pH on the percentage of under varying conditions of time, pH, temperature, and con- removal of phosphate by ARM. It is evident from the ﬁgure that centration of adsorbate and adsorbent were carried out using the percentage of phosphate removal decreases with the increase activated red mud samples. of pH. On the basis of IR and kinetic study (18–24), it has been suggested that adsorption of strongly binding anions such as RESULTS AND DISCUSSION selenite and phosphate on oxide surfaces takes place by a ligand exchange mechanism that involves the exchange of an aqueous Effect of Time ligand for a surface hydroxyl group resulting in the formation of an inner sphere complex. The formation of an inner sphere com- The kinetics of adsorption of phosphate at pH 5.2 showed plex involves coulombic interaction and is referred to as surface that equlibrium was attained in about 6 h (Fig. 1). There was no coordination (25). The reaction is given by further change in equlibrium concentration up to 24 h. Effect of pH K1 PO3 L S–PO2 4|; S ////////OH H H2O, [2] 4 The effect of pH on removal of phosphate was studied by keeping the dosage of ARM, concentration of solution, and con- K2 PO3 L S–HPO4 | ; S ////////OH 2H H2O, [3] 4 K3 PO3 L S–H2PO4 4|; S ////////OH 3H H2O, [4] where S–OH is a surface hydroxyl group and S–PO2 , 4 S–HPO4 , and S–H2PO4 are the adsorbed species. According to the anion adsorption reaction given in the above equations [2– 4], an increase in pH should cause a decrease in the amount of phosphate adsorbed. This trend is indeed observed in phosphate adsorption on activated red mud and is consistent with the previous results of anion adsorption on metal oxides (26 –29). Effect of Temperature The percentage of adsorption of phosphate on activated red mud as a function of temperature was studied in the range 30 – 60°C, and it was found that there is an increase in the FIG. 1. Adsorption of phosphate on activated red mud as a function of percentage of adsorption with increasing temperature. time.
171 PHOSPHATE ADSORPTION BY ACTIVATED RED MUD FIG. 3. Adsorption of phosphate as a function of activated red mud concentration. Effect of Adsorbent and Adsorbate Concentration The percentages of phosphate adsorbed with varying FIG. 5. Langmuir plot of phosphate adsorption on activated red mud at 30°C. amounts of activated red mud and phosphate concentrations are presented in Figs. 3 and 4, respectively. As expected, the amount of phosphate adsorption increases with the increase of Adsorption Isotherm adsorbent concentration whereas it decreases with increase in adsorbate concentration, which indicates that the adsorption The equilibrium adsorption isotherm for phosphate on ARM depends upon the availability of binding sites for phosphate. was drawn for ﬁxed adsorbent dose (2 gm/L) and varying initial phosphate concentrations for 1 h contact time at pH 5.2. The percentage of phosphate adsorption decreases with in- crease in adsorbate concentration, which indicates that the adsorption depends upon the availability of the binding sites for phosphate. In order to determine the adsorption capacity of the sample, the equilibrium data for the adsorption of phos- phate were analyzed in the light of adsorption isotherm models. The experimental data points were ﬁtted to the Langmuir equation C /X 1/ bXm C /Xm , [5] where X indicates the amount of phosphate adsorbed per unit weight of the adsorbent, C represents the phosphate concen- tration in quilibrium solution, b is a constant related to the energy of adsorption, and Xm is the adsorption capacity of the sample. Figure 5 depicts the Langmuir plot of C/X vs C for the experimental data points. The correlation coefﬁcient is found to be 0.98. Xm and b of the Langmuir equation are calculated from the least-squares method applied to the lines of Fig. 5 and found to be 0.75 mmol/g and 8.31, respectively. Ka values (28) FIG. 4. Adsorption of phosphate on activated red mud at different initial which represent the apparent equilibrium constant correspond- phosphate concentration.
172 PRADHAN ET AL. 30 –100 mg/L was observed. This process can be extended to the removal of phosphate from industrial efﬂuents like those of phosphatic fertilizer plants. ACKNOWLEDGMENTS The authors are thankful to Prof. H. S. Ray, Director, Regional Research Laboratory, and Dr. S. B. Rao, Head, Inorganic Chemicals Division, for their encouragement and permission to publish this paper. The ﬁnancial assistance of CSIR, New Delhi, to one of the authors (J.P.) is acknowledged. REFERENCES 1. Goodman, G. T., and Chadwick, M. J., ‘‘Environmental Management of Mineral Wastes.’’ Sijthoff and Noordhoff, USA, 1978. 2. Brattebo, H., and Odegaard, J., Water Res. 20, 977 (1986). 3. Koh, Kyung. Jim., and Chung, G. J., Hwahok Kanghok 23(5), 303 (1989). 4. Roques, H., Jeddy, N., and Libuglf, A., Water Res. 25(8), 959 (1991). 5. Bhargava, D. S., and Sheldarkar, S. B., J. Environ. Pollut. 76(1), 51 (1972). 6. Palanivelu, K., and Elangovan, N., Indian J. Environ. Prot. 14(9), 688 FIG. 6. Freundlich plot of phosphate adsorption on activated red mud. (1994). 7. Fox, I., J. Chem. Technol. Biotechnol. 57, 97 (1993). 8. Gonzalez-Pradas, E., Villafranca Sanchez, M., and Gallego Campo, A., ing to the adsorption process can be calculated as the product J. Chem. Technol. Biochenol. 54, 291 (1992). of Langmuir equation parameters b and Xm. The apparent 9. Hawka, D., Carpenter, P. D., and Hunter, K. A., Environ. Sci. Technol. 23, 187 (1989). equilibrium constant is found to be 6.23 mmol/g, which can be 10. Parﬁtt, R. L., J. Soil Sci. 40, 359 (1989). used as a relative indicator of the red mud afﬁnity for phos- 11. Miller, J. W., Iogan, T. J., and Bigham, J. M., Soil Soc. Am. J. 50, 609 phate ions (30). It is interesting to note that, although the ‘‘b’’ (1986). parameter of the Langmuir equation may be related to the 12. Colombo, C., Barren, V., and Torrent, J., Geochim. Cosmochim. Acta 58, adsorption energy it cannot be taken into account as an indi- 1261 (1994). 13. Malati, M. A., and Fox, I., Int. J. Environ. Stud. 26, 43 (1985). cator of the afﬁnity of activated red mud for phosphate ions. 14. Fox, I., Malati, M. A., and Perry, R., Water Res. 23, 725 (1989). The adsorption values plotted in Fig. 6 were calculated using 15. Pratt, K. C., and Christoverson, V., Fuel 61, 460 (1982). the Freundlich equation 16. Parida, K. M., Gorai, B., and Das, N. N., J. Colloid Interface Sci. 187, 375 (1997). log X / m K 1 / n logC e , [6] 17. ‘‘Standard Methods for Examination of Water and Wastewater.’’ APHA, Washington, DC, 1989. 18. Balistrieri, L. S., and Chao, T. T., Geochim. Cosmochim. Acta 54, 739 where X/m is the amount of phosphate adsorbed per unit weight (1980). of adsorbent (mg/gm), Ce is the concentration of phosphate at 19. Yates, D. E., and Healy, T. W., J. Colloid Interface Sci. 52, 222 (1975). equilibrium (mg/L), and n and K are constants. The straight- 20. Parﬁtt, R. L., and Russel, J. D., J. Soil Sci. 28, 297 (1977). 21. Parﬁtt, R. L., Argon 30, 1 (1978). line nature of the graph indicates that the adsorption conﬁrms 22. Cornell, R. M., and Schindler, P. W., Colloid Polym. Sci. 258, 1171 the Freundlich model. (1980). 23. Mott, C. J. B., in ‘‘Anion and Ligand Exchange in the Chemistry of Soil CONCLUSION Process’’ (D. G. Greenland and M. H. B. Hayes, Eds.), Chap. 5, p. 179. New York, 1981. From the foregoing discussion the following conclusions 24. Harrison, J. B., and Berkneiser, V. B., Clays Clay Miner. 30, 97 (1982). 25. Stumm, W., Kummert, R., and Sigg, L., Croat. Chem. Acta 53, 291 may be drawn. Red mud, a solid waste material from the (1980). aluminum industry, is converted into an adsorbent, and the 26. Balistrieri, L. S., and Chao, T. T., Soil Sci. Soc. Am. J. 51, 1145–1151 suitability of the activated red mud for adsorption of phosphate (1987). from aqueous solution is investigated by batch experiments. 27. Benjamin, M. M., Hayes, H. F., and Leckie, J. O., J. Water Pollut. Control With increasing pH there is a decrease in the percentage of Fed. 54, 1472–1481 (1982). 28. Merrill, D. T., Mantione, M. A., Peterson, J. J., Parks, D. S., Chow, W., adsorption. It was found that Langmuir as well as Freundlich and Hobbs, A. O., J. Water Pollut. Control Fed. 58, 18 –26 (1986). adsorption isotherms were followed. An increase in sorption 29. Mohanty, S., and Parida, K. M., J. Colloid Interface Sci. 199, 22–27 capacity was observed with increasing adsorbent and decreas- (1998). ing phosphate concentration. Almost 80 –90% of phosphate 30. Lopez-Gonzalez, J. D., Valenzuela-Calahorro, C., Jimenez-Lopez, A., and adsorption from aqueous solution at an initial concentration of Ramirez-Saenz, A., An. Quim. 74, 225 (1978).