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 Adsorption by activated red mud (ARM) is investigated as a possible alternative to the conventional methods of Cr(VI) re- moval 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. The applicability of the Langmuir as well as Freundlich adsorption isotherms for the present system is tested. The loading factor (i.e., milligrams of Cr(VI) adsorbed per gram of ARM) increased with initial Cr(VI) concentration, whereas a negative trend was observed with increasing tempera- ture. The influence of the addition of anions on the adsorption of Cr(VI) depends on the relative affinity of the anions for the surface and the relative concentrations of the anions. © 1999 Academic Press Key Words: activated red mud; adsorption; hexavalent chro- mium removal. INTRODUCTION Rapid industrialization and usage of heavy metals in indus- trial processes have resulted in an unprecedented increase in the heavy metal flux into groundwater and industrial effluents. Like many heavy metals, chromium in traces is necessary for life processes. However, with higher concentration of this element in environment and the consequent increase in human intake, chromium concentrations have reached toxic levels and manifested in a variety of ailments such as dermatitis, conges- tion of respiratory tracts, and perforation of the nasal septum. It is also a proven mutagen which may lead to cancer (1). Chromium can exist in several valence states of which the trivalent and hexavalent forms are common and the hexavalent form is highly toxic. In view of the pollution hazard caused by hexavalent chro- mium, several methods of removal have been reported, includ- ing chemical precipitation, reverse osmosis, ion exchange, foam flotation, electrolysis, and adsorption. Among all the above mentioned methods, adsorption is an economically fea- sible alternative. A variety of materials are used as adsorbents for Cr(VI), and various studies have been published document- ing its adsorption on activated carbon (2), starch xanthate (3), alumina (4), low-grade manganese ore (5), crushed coconut shell (6), fly ash (7), sawdust (8, 9), rice husk carbon (10), wood charcoal (11), bituminous coal (12), and lignite (13). Removal of chromium by different physical and chemical methods has been reviewed (14). In the present study the material used is an industrial waste/byproduct of the aluminum industry. Here, an attempt is made to prepare activated red mud and to study its feasibility as an adsorbent for removal of hexavalent chromium from aqueous solution/industrial efflu- ents. The process is investigated as a function of pH, time, concentration of adsorbate, amount of adsorbent, and temper- ature. The effects of other extraneous anions on the adsorption of Cr(VI) is also investigated. EXPERIMENTAL Activated red mud is prepared (15) by simple acid dissolu- tion followed by ammonia precipitation and drying at 110°C. Procedural details are given in our earlier paper (16). Analyt- ical grade reagents are used to prepare KCl solution and buffer solution to maintain the ionic strength and pH of the medium. The solutions of Cr(VI) are prepared from AR quality K 2 Cr 2 O 7 . Adsorption experiments for Cr(VI) were carried out in 100-ml stoppered conical flasks by taking appropriate amounts of potassium dichromate solution and activated red mud. The ionic strength of the medium was maintained by adding 1 M KCl. Acetic acid–sodium acetate buffer was used to maintain the pH of the solution in the range 3.0–5.9 and KH 2 PO 4 – NaOH buffer in the range 6.0–8.0, and the final volume was made up to 50 ml. After gentle shaking for a stipulated contact time in a mechanical shaker, the contents were filtered through G 4 crucibles. Concentrations of Cr(VI) in the filtrate were determined spectrophotometrically (17) using diphenylcarba- zide solution at ␭ ϭ 540 nm. The percentage of Cr(VI) ad- sorbed was determined from the ratio of chromium (VI) in the solution and particulate phases: Cr͑VI͒ ads ͑% of chromium ͑VI͒ adsorbed͒ ϭ Cr͑VI͒ in Ϫ Cr͑VI͒ eq Cr͑VI͒ in ϫ 100, [1] 1 To whom correspondence should be addressed. Journal of Colloid and Interface Science 217, 137–141 (1999) Article ID jcis.1999.6288, available online at http://www.idealibrary.com on 137 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved. 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 ϩ and OH Ϫ 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 attained in about 2 h. There is no significant change in equi- librium concentration after2hupto24h.Itisfurther observed that the removal curve is smooth and continuous indicating the possibility of the formation of monolayer coverage of Cr(VI) ion at the interface of the ARM. Effect of pH The initial pH is varied between 3.5 and 7.0. The adsorption of hexavalent chromium at a fixed Cr(VI) concentration (20 mg/L) as a function of equilibrium pH is shown in Fig. 2. It is evident from the figure that the adsorption is higher at lower pH, it is maximum at pH 5.2, and it suddenly decreases becoming almost negligible at pH 7.06. A similar type of behavior is also reported for the adsorption of anionic species on metal oxides/oxyhydroxides (19–23), fly ash (24, 25), and coal (26). The effect of pH on the adsorption capacity of 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- mates (HCrO 4 Ϫ ), chromates (CrO 4 2Ϫ ), dichromates (Cr 2 O 7 2Ϫ ), and other oxyanions. From the stability diagram for the Cr(VI)–H 2 O system, it is evident that at low pH, acid chromate ions (HCrO 4 Ϫ ) are the dominant species. As the pH increases, there is little increase in the percentage of adsorption, and it is maximum at pH ϳ5.2. 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 Ϫ and chromate ions FIG. 1. Adsorption of Cr(VI) on activated red mud as a function of time. FIG. 2. Adsorption of Cr(VI) on activated red mud as a function of pH. 138 PRADHAN, DAS, AND THAKUR (Cr 2 O 7 2Ϫ ), 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 Ͼ7.0. 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. The uniformity or heterogeneity of the surface sites of an 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 adsorption are calculated from adsorption isotherms at two different temperatures: ⌬H r ϭ R ln͓C 2 /C 1 ͔/͑1/T 2 Ϫ 1/T 1 ͒, [2] where ⌬H r is the isosteric heat of adsorption in kJ/mole at a given adsorption density, R is the gas constant, and C 1 and C 2 are the equilibrium concentrations of the ion at temperatures T 1 and T 2 . If the isosteric heat of adsorption is independent of 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 ϭ 1/͑bX m ͒ ϩ C/X m , [3] where C/X is the amount of Cr(VI) adsorbed per unit weight of FIG. 3. Surface charge of ARM as a function of pH in the presence of KNO 3 . FIG. 4. Adsorption of Cr(VI) on activated red mud as a function of temperature. 139HEXAVALENT CHROMIUM ADSORPTION BY ACTIVATED RED MUD 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 Langmuir plot of C/X vs C for the experimental data points. The correlation coefficient is found to be 0.99. X m and b are calculated from the Langmuir equation by applying the least squares method to the lines of Fig. 7 and found to be 30.74 mmol/g and 0.2691, respectively. K a values (31), which represent the apparent equilibrium 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 Ϫ ), sulfate (SO 4 2Ϫ ), and phosphate (PO 4 3Ϫ ) 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 sequence for adsorption of such anions on ARM is PO 4 3Ϫ Ͼ SO 4 2Ϫ Ͼ NO 3 Ϫ . 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- bilization point of view was studied. It was found that the FIG. 5. Adsorption of Cr(VI) as a function of the amount of adsorbent. FIG. 6. Adsorption of Cr(VI) on activated red mud as a function of initial concentration. FIG. 7. Langmuir plot of Cr(VI) adsorption on activated red mud at room temperature. 140 PRADHAN, DAS, AND THAKUR Cr(VI) release from the sludge in aqueous medium after con- tact time of Ͼ72 hours was negligible. However, partial de- sorption of Cr(VI) is observed in a strongly alkaline medium (pH Ͼ 8). The solution obtained after Cr(VI) adsorption was subjected to AAS and none of the trace metals was found to be in the detectable range. Thus the release of harmful trace metals into the environment after adsorption of Cr(VI) is ruled out. Applicability to Industrial Effluents Adosrption studies were extended to the effluents from the sodium dichromate and basic chromium sulphate industries containing very high values of Cr(VI) using ARM under sim- ilar conditions. In all cases the percentage of adsorption was found to be more than 90%. Details of the results obtained will be communicated in a separate paper. CONCLUSION From the previous discussions, it may be concluded that hexavalent chromium adsorption on activated red mud is a surface phenomenon and the Langmuir and Freundlich iso- therm curves show linearity. The best conditions for adsorption were found to be pH 5.2 and a temperature of 303 K in the concentration range 2–30 mg/L with a solid:liquid ratio of 1:500. The presence of other ions in solution influenced ad- sorption. Although the nitrates had very little effect, ions like sulphate and phosphate had noticeable effects due to the higher 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. 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Sujana, M. G., Thakur, R. S., and Rao, S. B., J. Colloid Interface Sci. 206, 94 (1998). FIG. 8. Freundlich plot of Cr(VI) adsorption on activated red mud at room temperature. 141HEXAVALENT CHROMIUM ADSORPTION BY ACTIVATED RED MUD . the solution and particulate phases: Cr͑VI͒ ads ͑% of chromium ͑VI͒ adsorbed͒ ϭ Cr͑VI͒ in Ϫ Cr͑VI͒ eq Cr͑VI͒ in ϫ 100, [1] 1 To whom correspondence should be addressed. Journal of Colloid and Interface Science. is smooth and continuous indicating the possibility of the formation of monolayer coverage of Cr(VI) ion at the interface of the ARM. Effect of pH The initial pH is varied between 3.5 and 7.0 amount of Cr(VI) adsorbed per unit weight of FIG. 3. Surface charge of ARM as a function of pH in the presence of KNO 3 . FIG. 4. Adsorption of Cr(VI) on activated red mud as a function of temperature. 139HEXAVALENT