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Removal of Arsenic(III) from Aqueous Solution by Activated Carbons Prepared from Solvent Extracted Olive Pulp and Olive Stones T. Budinova,*,† N. Petrov,† M. Razvigorova,† J. Parra,‡ and P. Galiatsatou§ Bulgarian Academy of Sciences, Laboratory of Chemistry of Solid Fuels, Institute of Organic Chemistry, Academy G. BoncheV 9 Street, Sofia 1113, Bulgaria, Instituto Nacional del Carbon, Consejo Superior de InVestigaciones Cientificas, Apartado de Correos 73, 33080 OViedo, Spain, and Institute of Technology of Agricultural Producta, Nagref, 1 S. Venizelou, 14123 LykoVrissi, Greece
The removal of As(III) from aquatic solutions at different concentrations and pH by using four types of activated carbons obtained from solvent extracted olive pulp and olive stone waste materials was studied. The adsorbents are obtained by chemical (K2CO3 and HNO3) activation and physical (water vapor) activation. The results show that carbons produced by chemical and physical activation of solvent extracted olive pulp and olive stones are efficient adsorbents for arsenic removal. Arsenic adsorption follows a Langmuir isotherm. The best results were obtained with the adsorbent, obtained from extracted olive pulp by pyrolysis in the presence of water vapor. The maximum removal was found to be 18.60 µmol/g. The adsorption capacity for adsorbents, obtained from extracted olive pulp and olive stones, is 11.42 and 9.85 µmol/g, respectively. The test showed that alkaline aqueous medium favored the removal of As(III). Some experiments were carried out to study the oxidation of arsenite to arsenate in the presence of activated carbons. Introduction Modernization and increasing industrialization have brought problems of pollution. Increased concern about the environment and tighter national and international regulations on water pollution and the discharge of heavy metals make it necessary to develop efficient and cost-effective technologies for their removal. Sorption onto solid substrate materials is considered to be the most suitable process for the removal of heavy-metal ions from solution at low and high concentrations. Numerous approaches have been studied for the development of cheaper metal adsorbents, such as microbial biomass,1 biosorbents,2 different agricultural byproducts,3-6 and resins.7,8 Arsenic is one of the most toxic pollutants introduced into natural waters by geochemical reactions, industrial waste discharges, agricultural use of arsenic pesticides, discharges from coal fired thermal power plants, herbicides, fertilizers, petroleum refining, and ceramic industries, etc.9 Recent epidemiological evidence of arsenic carcinogenecity suggests that the standard of 50 µg dm3 water may not be sufficient to reduce the risk of cancer.10 Therefore, the USEPA is planning to enforce a standard in the range of 2-20 µg dm9 for arsenic.11 In the past, several workers have successfully tried to remove As(III) from drinking water by using precipitation methods.12 Adsorption by low cost adsorbents from inorganic resources is considered to be less expensive than membrane separation and more versatile than ion exchange processes. Innovative technology such as the coating of Fe oxides onto the surface of sand to effectively remove trace metals has been used by many researchers.13-16 The results from their studies confirm that the utilization of iron oxide-coated sand is worth developing for the removal of metal ions from water. The solid fraction of red mud, a waste product from bauxite processing, has been used effectively for the removal of arsenic.17 * To whom correspondence should be addressed. Tel: (00359)-29606145; fax: (00359)-2-8700225; e-mail:
[email protected]. † Bulgarian Academy of Sciences. ‡ Consejo Superior de Investigaciones Cientificas. § Institute of Technology of Agricultural Producta.
Activated carbons are used in a broad range of applications that are concerned with air and water purification, molecular separation, and chemical recovery because they offer a high surface area, fast adsorption kinetics, and a mesoporous structure. Also, they contain functional groups: carboxyl, phenols, esters, and ketons, as a result of the oxidation of the activated carbon precursor under air or steam at high temperatures or chemical activation. The presence of these structures on the surface of carbons gives them cationic exchange properties. The objective of the present study is to test activated carbons, obtained from olive waste materials by physical and chemical activation, as alternate suitable arsenic adsorbents. Materials and Methods Activated Carbon Production. Olive stones and solvent extracted olive pulp were used as raw materials for the preparation of activated carbons. Raw Material Carbonization. A total of 6 g of the raw materials was heated in laboratory installation under vacuum (20 kN/m2) and atmospheric pressure with a heating rate of 60 °C/min to a final carbonization temperature of 800 °C. The duration of this treatment at the final temperature was 10 min. After treatment, the samples were left to cool. Carbon Modification. Activation with Water Vapor. After carbonization, the obtained carbon from extracted olive pulp was activated in a stream at 800 °C. The duration of this treatment at the final temperature was 2 h (carbon A). Chemical Activation. The precursors (solvent extracted olive pulp and olive stones) were ground and sieved to obtain particles with sizes less than 0.5 mm. The sample was mixed with the activating reagent K2CO3 (in the proportion 1:1) and water, held for 12 h, and kneaded. This mixture was then dried at 110 °C to prepare the impregnated sample. The sample was heated to the carbonization temperature (950 °C) under N2 flow at the rate of 10 °C/min and was held at this temperature for 10 min. After carbonization, the sample was cooled under N2 flow, and the carbonized sample was washed sequentially several times with hot water and finally with cold distilled water to remove
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residual chemicals. The washed sample was dried at 110 °C to prepare the activated carbon (carbon B from extracted olive pulp and carbon C from olive stones). Oxidation Treatment with HNO3. For obtaining carbon with a great number of oxides, carbon A was oxidized with HNO3. The oxidation treatment with HNO3 was performed according to the following procedure: about 100 g of the samples was treated with 10% HNO3 (1:3) and boiled for 1 h. After that, the sample was washed and dried at 105 °C (carbon D). Pore Structure Analysis. The porous structure of carbon adsorbents was studied by N2 adsorption at 77 K. The total pore volume (Vtotal) was derived from the amount of N2 adsorbed at a relative pressure 0.95, assuming that the pores were then filled with liquid adsorbate. The micropore volume was calculated from the amount adsorbed at a relative pressure of 0.1 in the N2 isotherm, and the mesopore volume was calculated by subtracting the amount adsorbed at a relative pressure of 0.1 from that at a relative pressure of 0.95.18 Oxygen Functional Group Content. The content of oxygencontaining functional groups with acidic character on the carbon surface was determined by applying the Boehm method of titration with basic solutions with different base strengths (NaHCO3, Na2CO3, NaOH, and EtONa).19 The basic group contents of the activated carbons were determined with 0.05 M HCl.20 pH Measurements. The pH of the carbons was measured according to the following procedure: exactly 4.0 g of carbon was weighed into a 250 mL beaker, and 100 mL of water was added. The beaker was covered with a watch glass, and the mixture was boiled for 5 min. The suspension was set aside, and the supernatant liquid was poured off as hot as possible but not below 60 °C. The decanted portion was cooled to ambient temperature, and its pH was measured to the nearest 0.1 pH unit. Adsorption Measurements. Adsorption of As(III). All adsorption experiments were carried out by using an adsorbent fraction with particles 0.2 mm in size. The adsorption capacity of the adsorbents toward As(III) ions was investigated by using an aqueous solution of the metal. The As(III) standard (1000 mg/L) was prepared by dissolving 1.3200 g of AS2O3 in 25 mL of 1.0 M NaOH. The solution was diluted to about 100 mL with water, and two drops of 0.2% phenolphthalein were added. It was then neutralized with 1.0 M HCl and further diluted to 1 L.21 This stock solution was diluted to obtained a standard solution containing 5-20 mg/L As(III). Batch adsorption studies were carried out with 250 mg of adsorbents and 25 mL of the As solution with the desired concentration in 100 mL conical flasks. Stopper flasks containing the adsorbent and the adsorbate were agitated for predetermined time intervals at room temperature on the mechanical shaker. At the end of agitation, the suspension was filtered through microporous filter paper (hydrochloric acid filter and hydrofluoric extratcion, FILTRAK 390). The amount of the As(III) in the final volume was determined by titration with iodine.22 The interaction between iodine and As2O3 is shown by the following reaction:
As2O3 + 2I2 + 2H2O ) As2O5 + 4 HI
(1)
The solution containing As(III) was titrated with the solution of iodine as the indicator starch. The titration stopped when it appeared as a pale blue color, which was retained after mixing the solution. A Langmuir isotherm study was carried out with four initial concentrations of As(III): 5, 10, 15, and 20 mg/L with an adsorbent dose of 250 mg/25 mL.
Table 1. Chemical Composition of the Raw Materials proximate analysis (wt %) sample
W
ash
volatile matter
olive stones 7.4 0.61 olive pulp 8.7 1.34
80.6 78.2
ultimate analysis (wt %, maf) C
H
N
S
Odif
C/H C/O
51.5 6.3 0.2 0.1 41.9 0.68 1.64 56.7 5.5 0.3 0.3 37.2 0.86 2.03
Table 2. Chemical and Physicochemical Characteristics of the Activated Carbons, Obtained from Solvent Extracted Olive Pulp parameter, value
carbon A
carbon B
carbon C
carbon D
moisture (wt %) ash (wt %) volatile (wt %, daf) carbon (wt %, daf) hydrogen (wt %, daf) sulfur (wt %, daf) nitrogen (wt %, daf) oxygen (diff,wt %) pH surface area (m2/g) iodine number (mg/g) micropore volume (m3/g) mesopore volume (m3/g) macropore volume (m3/g) total pore volume (m3/g)
1.8 2.8 3.4 88.90 2.10 0.50 1.10 7.40 8.8 1030 850 0.355 0.108 0.202 0.665
3.5 7.9 6.9 92.50 1.70 0.27 1.10 4.43 8.1 1850 1100 0.461 0.180 0.257 0.898
3.8 3.2 5.9 91.50 2.10 0.24 1.10 5.06 8.1 1610 980 0.437 0.159 0.247 0.843
1.43 1.27 3.90 80.65 2.13 0.40 1.57 15.25 4.4 732 488 0.315 0.100 0.230 0.645
The effect of pH of the solution on the equilibrium adsorption of As(III) on the four different carbon adsorbents was investigated using 250 mg of adsorbent and a 5 mg/L As(III) concentration at a 1 h time of treatment. Hydrochloric acid and sodium hydroxide were used for adjusting the pH of the system. Determination of As(V). The As(V) standard solution (1000 mg/L) was prepared by mixing 25 mL of 1000 mg/L As(III) standard with 25 mL of freshly prepared aqua regia (3:1 HCL/ HNO3 mixture) in a 150 mL beaker.21 After evaporation to dryness in a hot water bath, the As(V) product was dissolved and diluted with water to 25 mL. Intermediate standard solutions were prepared from this solution by a series of dilutions using water. The initial and equilibrium concentrations of As(V) ion in the solution were determined spectrophotometrically at 845 nm on a Specord UV-vis. Colorimetry measurements were done by the following procedure: an aliquot of test solution was placed in a 25 mL volumetric flask, and 2.5 mL of ammonium molybdate reagent, 2.5 mL of freshly prepared 0.5% hydrazine sulfate, and 10 mL of 1 M HCL were added. A calibration curve for 5-50 µg of As(V) in a 25 mL flask by the previous procedure was prepared. A suitable aliquot (up to 10 mL) of solution containing As(V) was transferred to a 25 mL volumetric flask. Results of Discussion The proximate analysis of raw materials (Table 1) shows that olive stones and olive pulp have relatively low ash and sulfur contents, which are desirable features for activated carbon production. The elmental analysis date shows that the olive pulp posesses a higher carbon and low hydrogen and oxygen content than olive stones. C/H and C/O atomic ratios of the olive pulp are considerably higher than those of the olive stones. The results indicate differences in the chemical composition of the raw materials. These differences could affect the properties of obtained carbons. The results from proximate and ultimate analyses of carbons A-D are present in Table 2. Both activated carbons obtained by steam activation and HNO3 oxidation of the carbon from solvent extracted olive pulp possess a lower carbon content and higher oxygen content than
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Table 3. Oxygen Surface Groups on the Surface of Activated Carbons acidic groups (meq g-1)
sample
carboxylic
carboxyl in lactone-like binding structures
carbon A carbon B carbon C carbon D
0.318
0.172
1.450
0.040
basic groups (meq g-1) phenolic hydroxyl
carbonyl
0.890 0.205 0.175 0.290
2.080 1.898 2.208 1.780
0.820 1.190 1.070
those of the carbons, obtained by chemical activation with K2CO3. Data also show the higher carbon and lower hydrogen content of the adsorbent obtained by chemical activation of solvent extracted olive pulp in comparison with adsorbent obtained from olive stones. These data indicate that both the method of treatment and the raw material composition influence the composition of obtained carbon. The porous structure data indicate that activated carbons with a prevailing content of micropores and high surface area were prepared from solvent extracted olive pulp and olive stones by chemical activation with K2CO3. The chemical activation leads to the formation of a larger surface area and better adsorption characteristics in comparison with adsorbents obtained by steam activation and steam-HNO3 oxidation. The content of the oxygen-containing functional groups is a very important characteristic for the chemical properties of the carbon surface. Table 3 compares the amounts of different oxygen groups on the surface of carbons obtained by steam activation and chemical activation with K2CO3 and HNO3. Different oxygen-containing groups of acidic character (carboxylic groups, carboxylic groups involved in lactone-like binding structures, and phenolic hydroxyl and carbonyl groups) are determined. The carboxylic groups and carboxylic in lactonelike binding structures were not found on the surface of carbons obtained by chemical activation. A considerably higher content of carboxylic groups on the surface of the carbon oxidized with HNO3 in comparison with other carbons is established. The oxygen groups with alkaline character were not found on the surface of oxidized carbon. This determines the acidic character of its surface (pH ) 4.4). Effect of Contact Time. The distribution of metal ion between the activated carbon and the metal ion solution for the system at equilibrium is of importance for determining the maximum adsorption capacity of the activated carbon for the arsenic ion. The probability for catalytic influence of all carbons on the oxidation of As(III) to As(V) was investigated in more detail to obtain a real assessment of the adsorptive performance of the different materials toward As(III). Activated carbons were tested following the procedures using 5, 10, 15, and 20 mg/L As(III) for the determination of arsenite and arsenate in a neutral aqueous solution containing 0.250 g of each carbon. The kinetics of accumulation of As(V) in aqueous solution in concentrations from 5 to 20 mg/L shows the rapid increase of the content of As(V) in the beginning of the adsorption process and attains equilibrium in 15 min (Figure 1). The amount of the obtained arsenate As(V) by the result of the catalytic influence of the activated carbons is from 30 to 10% for the carbons with alkaline character of the surface and from 50 to 12% for the carbon with acidic character of the surface in the range of concentrations of 5-20 mg/L (As III). Figure 1 shows the results for the % accumulation of As(V) with respect to the initial amount of As(III) during the adsorption process at 10 mg/L As(III) concentration.
Figure 1. Effect of treatment time on the accumulation of As(V) (%) in the result of the oxidation of As(III). Conditions: As(III) concentration, 10 mg/L and pH, 7.0.
Figure 2. Effect of treatment time and initial concentration of As(III) on the adsorption of As(III) on carbon A. Conditions: carbon concentration, 250 mg/25 mL and pH 7.0; As(III) concentration: (9) 5 mg/L, (b) 10 mg/L, (2) 15 mg/L, and (1) 20 mg/L.
The amount of adsorbed As(III) was calculated taking into account the amount of As(III) oxidized to As(V). Figure 2 shows the effect of the time of treatment on the removal of As(III) by activated carbon A from solutions with metal concentrations of 5-20 mg/L. Data indicate that the removal of As(III) increases with time and attains equilibrium in 60 min for all initial concentrations of the As(III) solution. Metal ion adsorption increases sharply for a short time and slows gradually when the equilibrium is approached. The behavior can be attributed to the relative decrease in the number of available sites on the carbon surface as the process proceeds. The plots show that the amounts of As(III) adsorbed on the adsorbents (mg/g) vary in single smooth and continuous curves, leading to saturation and suggesting the possibility of the formation of monolayer coverage of As(III) on the surface of the adsorbent. The kinetics curves of the removal of As(III) versus time of treatment for other carbons B-D show the same dependence. The amounts of the adsorbed As(III) of these activated carbons at equilibrium time for the initial concentrations of 5, 10, 15, and 20 mg/L (As III) are the following: carbon B: 0.116, 0.233, 0.302, and 0.372 mg/g carbon; carbon C: 0.114, 0.212, 0.288, and 0.353 mg/g carbon; and carbon D: 0.07, 0.110, 0.14, and 0.16 mg/g carbon, respectively.
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Figure 3. As(III) isotherm on activated carbon A. Conditions: As(III) concentration 5-20 mg/L; treatment time, 120 min; carbon concentration, 250 mg/25 mL; and pH, 7.0.
Figure 4. Langmuir plots for adsorption of As(III) on carbons A-D. Conditions: As(III) concentration, 5-20 mg/L; treatment time, 120 min; carbon concentration, 250 mg/25 mL; and pH, 7.0. Table 4. Data for Arsenic Ion Adsorption Obtained from Langmuir Plots sample
Qo (mg/g)
Qo (µmol/g)
b (L/mg)
carbon A carbon B carbon C carbon D
1.393 0.855 0.738 0.210
18.60 11.42 9.85 2.47
0.282 0.052 0.603 0.161
Adsorption Isotherms. The adsorption isotherm of As(III) on carbon A is presented in Figure 3. The amount of metal ions adsorbed at equilibrium per carbon mass unit is presented as a function of the equilibrium metal ion concentration. The adsorption isotherm in Figure 3 can be assigned to the L(2) class according to the Giles classification.23 L-type isotherms reflect adsorption at higher contaminant concentrations, corresponding to the completion of monolayers in experimental concentration ranges. The linear form of the Langmuir equation24
Ce/qe ) 1/Qob + Ce/Qo
(2)
where Ce (mg/L) and qe (mg/g) are the concentration and amount of As(III) adsorbed at equilibrium used to calculate the Langmuir constants Qo and b, which are related to the adsorption capacity and energy of adsorption, respectively. The linear plot of Ce/qe versus Ce shows that adsorption obeys the Langmuir isotherm model (Figure 4). The Qo and b values determined from the slope and intercept of the plot are present in Table 4. All these isotherms were fitted to the adsorption data obtained. Calculated correlation coefficients for these isotherms by using
linear regression procedures are the following: R ) 0.99917 for carbon A, R ) 0.99119 for carbon B, R ) 0.99983 for carbon C, and R ) 0.99142 for carbon D. As seen, the Langmuir isotherms yielded the best fit to the experimental data. To determine whether the arsenic adsorption process by activated carbons is favorable or unfavorable for the Langmuir type adsorption process, the isotherm shape has to be classified by the term r, a dimensionless constant separation factor, which is defined as r ) 1/(1 + bCo), where r is a dimensionless separation factor, Co is the initial concentration (mg/L), and b is the Langmuir constant (L/g).25-27 The parameter r indicatives the shape of the isotherm accordingly: r > 1 unfavorable; r ) 1 linear; 0 < r < 1 favorable; and r ) 0 irreversible. The calculated r values indicate that adsorption of As(III) on activated carbons is favorable at all concentrations. All four adsorbents documented comparable good levels of adsorption. On the basis of the data from Table 4 for adsorption capability, the adsorbents can be arranged in the following order: A > B > C > D. The adsorption capacity (18.60 µmol/g) for arsenic ions of the activated carbon prepared after pyrolysis in a vacuum by water vapor activation was higher than the ones obtained by chemical activation and HNO3 oxidation. Further values of Qo of carbons B and C, obtained by chemical activation of extracted olive pulp and olive stones with K2CO3, are 11.42 and 9.85 µmol/g. On the oxidized carbon D (obtained by additional treatment with HNO3 of the carbon obtained by water vapor activation) surface was determined that acidic oxygen groups adsorbed arsenic ions to a lower degree than other carbons. Carbon A, obtained by pyrolysis in water vapor, possesses considerable amounts of different oxygen groups and lower BET surface areas, higher values of pH (8.8) of the surface, and slightly lower values of the micro- and mesopores than other carbons with alkaline character. The oxygen-containing groups usually play an important role in the adsorption process as they form different complex compounds with As(III). The results of Table 3 show that carbon A, activated with water vapor, is characterized by the presence of carboxylic, carbonyl in lactone-binding structures, phenolic, carbonyl, and basic groups on the surface. At slightly lower values of the micro- and mesopores of carbon A than other carbons with alkaline character on the surface, the arsenic removal efficiency is increased with the enhacement of the content of oxygen groups. When As(III) is present in the solution, the surface complexes may be formed.28 The results of this study showed that the BET surface area was not the one and only effective criteria for relating As(III) adsorption on adsorbents with alkaline character of their surface. It has been reported that the maximum As(III) adsorption capacity of hematite is 2.63 µmol/g,13 sulfate-modified iron oxide-coated sand (SMIOCS) is 1.91 µmol/g,16 activated bauxite is 16 µmol/g,29 activated alumina is 14 µmol/g,29,30 iron hydroxide loaded coral lime stone (Fe-coral)31 is 0.17 µmol/ g,31 and red mud is 8.86 µmol/g,17 respectively. In the present study, the As(III) adsorption capacity of activated carbon A shows the highest adsorption capacity toward the previously mentioned adsorbents. Other carbons, obtained by chemical activation, can compete against the adsorbents such as hematite, SMIOCS, red mud, and iron-coral. Influence of the Solution pH. The effect of pH on As(III) adsorption and the accumulation of As(V) in the result of the oxidation of As(III) by adsorbent, obtained in the presence of water vapor, was studied in the initial pH range between 2 and 10 at the contact time of 60 min.
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5-20 mg/L. The results are found to be highly promising. The removal of arsenic follows the order A (water vapor activation) > B (K2CO3 activation of extracted olive pulp) > C (K2CO3 activation of olive stones) > D (HNO3 activation). Both factors (content of different oxygen groups and surface characteristics) influence the adsorption of arsenic on the carbons. The adsorption of As(III) was pH dependent, and the pH adsorption edge data showed that As(III) removal was optimum in the pH range of 7-9. Acknowledgment The authors acknowledge financial support for these investigations from MES-NFSI-Bulgaria. Literature Cited Figure 5. Effect of pH on As(III) removal on carbon A. Conditions: treatment time, 120 min; carbon concentration, 250 mg/25 mL; and As(III) concentration, 5 mg/L.
Figure 5 depicts the effect of pH on the removal of As(III) by carbon A. It was chosen as the representative carbon for this study from other carbons with alkaline character on the surface. The removal of As(III) increases as the pH of the system increases and reaches a maximum at about pH 7.9, followed by a sharp decrease in the extent of adsorption up to 10. These results show that in highly acidic medium, where the adsorbent surfaces are highly protoned and As(III) mostly exists in the form of neutral H3AsO3 species, As(III) removal is not favorable. It is noteworthy that at pH 9.2, arsenite (H3AsO3) begins dissociating (pKa ) 9.2). Therefore, only physical adsorption driving forces between H3AsO3 and adsorbent are present, resulting in less adsorption. At near neutral pH values (7-9), slow dissociation of weak arsenic (H3AsO3) produces the arsenite ion. This partially neutral and partially negatively charged arsenite ion is attracted to the positively charged (below 8.2) surface, resulting in high As(III) removal in this range to form H2AsO3. When the pH was greater than 8.2, the surface of carbon becomes negatively charged. The decrease in AS(III) removal by adsorbent in the pH range of 9.0-10 may be a result of the net negative charge of adsorbent. The amount of As(V) obtained by the oxidation of As(III) in the result of catalytic interaction of activated carbon in strongly acidic solution (pH = 2) is lower by nearly 23% than that in neutral pH (6.5-8) since the amount of As(V) in alkaline solution above pH 9.5 is 12% higher. The arsenic acid H3AsO3, which is a strong reductor in alkaline solution, is oxidized more difficultly in acidic solution. This is a probable explanation of the lower content of As(V) in highly acidic medium. The role of the oxygen functional groups during the process of the adsorption of As(III) is evident because carbon A with a lower surface area than carbons B and C shows a higher adsorptive capacity toward As(III). The presence of different oxygen groups creates a possibility of forming complexes with the As(III) ion.28 Conclusions The present investigation showed that wastes of extracted olive pulp and olive stones can be effectively used as a raw materials for the preparation of activated carbons using different activation procedures. Solvent extracted olive pulp and olive stones were converted by water vapor activation, oxidation with HNO3, and chemical activation with K2CO3 into activated carbons. These carbons were characterized and utilized for the removal of As(III) from water in the range of concentrations of
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ReceiVed for reView November 2, 2005 ReVised manuscript receiVed January 17, 2006 Accepted January 20, 2006 IE051217A
