JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
204, 169 –172 (1998)
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
Adsorption of phosphate (PO32 4 ) from aqueous solution on activated 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. This can be extended to the treatment of industrial effluents containing phosphates like that from phosphatic fertilizer plants. © 1998 Academic Press Key Words: activated red mud; adsorption; phosphate; effluent treatment.
(2), half-burnt dolomite (3), activated carbon (4, 5), coconut shell carbon (6), clays (7), bentonite (8), ferrihydrite (9), goethite (10), t-alumina (11), and hematite (12). The studies aimed to understand its fixation in soil and to minimize eutrophication of lakes, rivers, and ponds (13, 14). The present paper reports preparation of activated red mud and use of the same as an adsorbent for phosphate removal from aqueous solution. Adsorption of phosphate was investigated as a function of pH, time, temperature, and concentration of adsorbate and adsorbent. EXPERIMENTAL
Preparation of Activated Red Mud
The problem of solid waste disposal has now attained complex dimensions. It is essential either to find suitable ways to safely dispose of these wastes or to suggest novel uses, considering them as byproducts. Otherwise, this will remain as an accumulated waste contributing highly to environmental pollution (1). Utilization techniques for various industrial wastes vary widely depending upon their physico-chemical characteristics. Red mud is a waste material formed during the production of alumina when the bauxite ore is subjected to caustic leaching. It is a brick red colored highly alkaline (pH 10 –12) sludge containing mostly oxides of iron, aluminum, titanium, and silica. Red mud, due to its high aluminum, iron, and calcium content, has been suggested as a cheap adsorbent for removal of toxic metals (e.g., As, Cr, Pb, Cd) as well as for water or wastewater treatment. The basic advantage of red mud is its versatility in application. Since it is composed of a mixture of useful adsorbents and flocculants, it can be used for treatment of several effluents. Wastes containing phosphate create eutrophication in the receiving bodies of water. In order to eliminate the possible dangers to receiving water sources, it is necessary to treat before discharge. Adsorption of phosphate from aqueous solution has been studied in the past few decades by several authors using different adsorbents, like activated alumina 1
Activated red mud (ARM) was prepared (15) by refluxing red mud in 20% HCl for 2 h. After the reflux solution was cooled to room temperature, liquor ammonia was added until complete precipitation. The precipitate was allowed to settle and was then filtered and washed thoroughly with distilled water until free from ammonia. The residue was dried at 110°C and used for adsorption studies. The specific surface area of the sample was found to be 249 m2/g. All the chemicals used for adsorption experiments were of analytical grade.
To whom correspondence should be addressed. 169
Adsorption Experiments The pH of activated red mud, when suspended in distilled water, increases with time even in the absence of an adsorbate metal ion. So the adsorption of phosphate on activated red mud was carried out in acetic acid–sodium acetate buffer medium (16). The experiments were carried out in 100 ml stoppered conical flasks by taking appropriate amounts of synthetic phosphate solution and activated red mud (2 g/L). The ionic strength and pH of the solution were maintained by adding 1 M KCl and acetic acid–sodium acetate buffer solution, respectively, and the final volume was invariably made up to 50 ml. The flasks were placed in a rotary mechanical shaker for a particular period of time and shaken gently. After the stipulated contact time, the conical flasks were taken from shaker and the contents were filtered through G4 crucibles. The filtrates were collected and the 0021-9797/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
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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. PO432ads ~% of PO32 4 adsorbed! 5
32 PO32 4 in 2 PO4 eq 3 100 , PO32 4 in
32 where PO32 4 in and PO4 eq are the initial and equilibrium concentrations of phosphate, respectively. Each run was made in duplicate. All the spectrophotometric measurements were made with a Chemito-2500 UV–visible recording spectrophotometer using 10 mm matched quartz 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 buffers before any measurement. The adsorption experiments under varying conditions of time, pH, temperature, and concentration of adsorbate and adsorbent were carried out using activated red mud samples.
RESULTS AND DISCUSSION
Effect of Time The kinetics of adsorption of phosphate at pH 5.2 showed that equlibrium was attained in about 6 h (Fig. 1). There was no further change in equlibrium concentration up to 24 h. Effect of pH The effect of pH on removal of phosphate was studied by keeping the dosage of ARM, concentration of solution, and con-
FIG. 2. Adsorption of phosphate on activated red mud as a function of equilibrium pH.
tact time constant and varying the pH of the medium from 3.5 to 6. Figure 2 represents the effect of pH on the percentage of removal of phosphate by ARM. It is evident from the figure that the percentage of phosphate removal decreases with the increase of pH. On the basis of IR and kinetic study (18–24), it has been suggested that adsorption of strongly binding anions such as selenite and phosphate on oxide surfaces takes place by a ligand exchange mechanism that involves the exchange of an aqueous ligand for a surface hydroxyl group resulting in the formation of an inner sphere complex. The formation of an inner sphere complex involves coulombic interaction and is referred to as surface coordination (25). The reaction is given by K1 L ; S–PO22 S ////////OH 1 H1 1 PO32 4 | 4 1 H2O,
K2 S ////////OH 1 2H1 1 PO32 L | ; S–HPO2 4 4 1 H2O,
K3 S ////////OH 1 3H1 1 PO32 L ; S–H2PO4 1 H2O, 4 |
where S–OH is a surface hydroxyl group and S–PO22 4 , S–HPO2 4 , 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
FIG. 1. Adsorption of phosphate on activated red mud as a function of time.
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 percentage of adsorption with increasing temperature.
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 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 adsorbent concentration whereas it decreases with increase in adsorbate concentration, which indicates that the adsorption depends upon the availability of binding sites for phosphate.
FIG. 5. 30°C.
Langmuir plot of phosphate adsorption on activated red mud at
Adsorption Isotherm The equilibrium adsorption isotherm for phosphate on ARM 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 increase 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 phosphate were analyzed in the light of adsorption isotherm models. The experimental data points were fitted to the Langmuir equation C /X 5 1/~bX m! 1 C /X m ,
FIG. 4. Adsorption of phosphate on activated red mud at different initial phosphate concentration.
where X indicates the amount of phosphate adsorbed per unit weight of the adsorbent, C represents the phosphate concentration 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) which represent the apparent equilibrium constant correspond-
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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.
Freundlich plot of phosphate adsorption on activated red mud.
ing to the adsorption process can be calculated as the product of Langmuir equation parameters b and Xm. The apparent equilibrium constant is found to be 6.23 mmol/g, which can be used as a relative indicator of the red mud affinity for phosphate ions (30). It is interesting to note that, although the ‘‘b’’ parameter of the Langmuir equation may be related to the adsorption energy it cannot be taken into account as an indicator of the affinity of activated red mud for phosphate ions. The adsorption values plotted in Fig. 6 were calculated using the Freundlich equation log X /m 5 K 1 1/n logC e ,
where X/m is the amount of phosphate adsorbed per unit weight of adsorbent (mg/gm), Ce is the concentration of phosphate at equilibrium (mg/L), and n and K are constants. The straightline nature of the graph indicates that the adsorption confirms the Freundlich model. CONCLUSION
From the foregoing discussion the following conclusions may be drawn. Red mud, a solid waste material from the aluminum industry, is converted into an adsorbent, and the suitability of the activated red mud for adsorption of phosphate from aqueous solution is investigated by batch experiments. With increasing pH there is a decrease in the percentage of adsorption. It was found that Langmuir as well as Freundlich adsorption isotherms were followed. An increase in sorption capacity was observed with increasing adsorbent and decreasing phosphate concentration. Almost 80 –90% of phosphate adsorption from aqueous solution at an initial concentration of
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