Ind. Eng. Chem. Res. 2006, 45, 8105-8110
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SEPARATIONS Removal of Arsenic in Aqueous Solutions by Adsorption onto Waste Rice Husk Md. Nurul Amin,*,† Satoshi Kaneco,† Taichi Kitagawa,† Aleya Begum,‡ Hideyuki Katsumata,† Tohru Suzuki,§ and Kiyohisa Ohta†
Ind. Eng. Chem. Res. 2006.45:8105-8110. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/23/19. For personal use only.
Department of Chemistry for Materials, Faculty of Engineering, and EnVironmental PreserVation Center, Mie UniVersity, Tsu, Mie 514-8507, Japan, and Department of Zoology, UniVersity of Dhaka, Dhaka 1000, Dhaka, Bangladesh
Rice husk has tremendous potential as a remediation material for the removal of arsenic from groundwater. The present work investigates the possibility of the use of rice husk adsorption technology without any pretreatment in the removal of arsenic from aqueous media. Various conditions that affect the adsorption/ desorption of arsenic are investigated. Adsorption column methods show the complete removal of both As(III) and As(V) under the following conditions: initial As concentration, 100 µg/L; rice husk amount, 6 g; average particle size, 780 and 510 µm; treatment flow rate, 6.7 and 1.7 mL/min; and pH, 6.5 and 6.0, respectively. The desorption efficiencies with 1 M of KOH after the treatment of groundwater were in the range of 71-96%. The present study might provide new avenues to achieve the arsenic concentrations required for drinking water recommended by Bangladesh and the World Health Organization (WHO). Introduction Arsenic contamination is one of the most challenging environmental problems today. Millions of people worldwide are exposed to naturally occurring As-contaminated groundwater, which they use as their only source of drinking water.1 Elevated concentrations of arsenic in groundwater are found in many countries such as Bangladesh, India, Vietnam, and Chile.2 Increased usage of groundwater for drinking has caused serious health problems,3,4 because arsenic is known to be highly detrimental to human beings and animals. They include several neurological,5 dermatological,6 gastrointestinal,7 and cardiorenal diseases;8 arsenic also is a suspected carcinogenic.9 Furthermore, recent research has suggested that As acts as an endocrine disruptor at extremely low concentrations.10 Recently, because of its high nuisance value, various regulatory agencies have revised the maximum contaminant level of arsenic in drinking water from 50 to 10 µg/L.11,12 This situation will require the development of simple, low-cost methods for the removal of As from groundwater for drinking water. The predominant forms of As in groundwater and surface water are the inorganic species arsenate [As(V)] and arsenite [As(III)]. The As(V) species exists as oxyanions (H2AsO4- and HAsO42-) at neutral pH, whereas the predominant As(III) species is neutral H3AsO3.13,14 As(III) is more toxic and more difficult to remove with the conventionally applied physicochemical treatment methods than As(V). Because most of the technologies are inadequate for the removal of As(III) on an industrial scale, they are mainly applied for the removal of * To whom correspondence should be addressed. Tel.: 81-59-2319427. Fax: 81-59-231-9427. E-mail:
[email protected]. † Department of Chemistry for Materials, Faculty of Engineering, Mie University. ‡ University of Dhaka. § Environmental Preservation Center, Mie University.
As(V).14-19 Therefore, a preoxidation step is usually required to transform the trivalent to the pentavalent form. The oxidation procedures are performed by the addition of chemical reagents, such as potassium permanganate, chlorine, ozone, hydrogen peroxide, or manganese oxide.20,21 Although these reagents are effective for the oxidation of trivalent arsenic, they can also cause several secondary problems, arising from the presence of residual reagents and the formation of side products. This process can result in a significant increase in operating costs. Therefore, the simultaneous removal of As(III) and As(V) is needed for the treatment of groundwater. Many technologies, including coprecipitation with iron or alum, adsorption onto coagulated floc, ion-exchange resin, reverse osmosis, and membrane techniques, have been used to remove As from aqueous solution.15-19 In these methods, however, the adsorption techniques are simple and convenient and have the potential for regeneration and sludge-free operation. So far, various adsorbents for arsenic removal have been developed that include such materials as metal-loaded coral limestone,22,23 hematite and feldspar,24 activated carbon,25,26 activated alumina,27,28 and hydrous zirconium oxide.29 However, most of these adsorbents entail several problems in terms of efficiency and cost. Because rice husk (RH) is a byproduct of the rice milling industry, treatment with RH will become a promising simple method for the removal of arsenic. The present work was performed to evaluate the use of waste rice husk without any pretreatment as an alternate adsorbent for removing arsenite and arsenate from aqueous mediums. Moreover, the RH was applied to the removal of arsenic from Bangladeshi As-contaminated drinking water samples in a single-step column operation. Materials and Methods Reagent. All reagents used throughout this work were of analytical-grade purity. Potassium arsenite (KAsO2) and potas-
10.1021/ie060344j CCC: $33.50 © 2006 American Chemical Society Published on Web 11/01/2006
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sium arsenate (KH2AsO4) were procured from Wako Pure Chemical Industries Ltd., Osaka, Japan, and NaOH, KOH, HCl, HNO3, and H2SO4 were obtained from Nacalai Tesque Inc., Kyoto, Japan. Stock solutions (1000 mg/L) of As(III) and As(V) were prepared in distilled water from solid KAsO2 and KH2AsO4, respectively. Dilute standard solutions were prepared daily before use. Pure water was purified with an ultrapure water system (Advantec MFS, Inc., Tokyo, Japan) resulting in a resistivity of >18 MΩ cm. Preparation of Adsorbent. The RH used in the present work was obtained from a local rice mill in Bangladesh. The chemical composition of RH has been reported as 49.3 wt % oxygen, 44.6 wt % carbon, and 5.6 wt % hydrogen (by elemental analysis); 34.4 wt % cellulose, 29.3 wt % hemicellulose, 19.2 wt % lignin, and 17.1 wt % ash (by component analysis); and 59.5 wt % volatiles, 17.1 wt % ash, and 7.9 wt % moisture (by proximate analysis).30 The collected materials were washed with pure water several times to remove dust and fines. The washing process was repeated until the color of the wash water was transparent. The washed materials were then dried in a hot-air oven at 60 °C for 24 h. The dried material was sieved into the following five size fractions: 710-850 µm (avg 780 µm), 600710 µm (avg 655 µm), 425-600 µm (avg 510 µm), 355-425 µm (avg 390 µm), and 150-355 µm (avg 250 µm). The materials were used for the removal of arsenic without further physical or chemical treatment. Adsorption and Analytical Procedures. RH was added to the treatment glass columns (2 × 30 cm). The adsorption experiments were carried out in columns that were equipped with a stopper for controlling the column eluate flow rate (treatment rate). Adsorption factors including the amount of rice husk (0.5-7 g), average particle size (250-780 µm), treatment flow rate (0.6-33 mL/min), initial sample concentration (50500 µg/L), and pH (2-14) were evaluated. After the pH had been adjusted to the desired value with HCl and NaOH solutions, the sample solution (100 mL) was passed through the adsorption column at a given flow rate. The treatment flow rates of 0.6, 0.8, 1.7, 3.3, 6.7, 20, and 33 mL/min correspond to retention times of 5.3 min, 4 min, ∼109 s, ∼58 s, ∼29 s, ∼10 s, and ∼6 s, respectively, for 1 g of rice husk. The packing density of the treatment column was 0.32 g/cm3. A small piece of tissue paper was inserted into the bottom of the column to prevent the loss of rice husk during the treatment. The flow rate was kept constant by controlling the stopper valve. The removal treatment was performed at ambient temperature. The number of experiments for the removal of As was greater than five. A graphite furnace atomic absorption spectrometer (GFS97, Thermo Electron Corp.) was used for the determination of arsenic under the following conditions: resonance line, 193.7 nm (As hollow cathode lamp); pyrolysis temperature, 1200 °C (20 s); atomization temperature, 2600 °C (3 s); and matrix modifier, nickel nitrate (10 mg/L). The removal (adsorption) efficiency was calculated using the equation
removal (adsorption) efficiency ) (C0 - Ce)/C0 × 100 (1) where C0 and Ce are the concentration of As in the sample solution before and after treatment, respectively. Results and Discussion First, the performances of four adsorbents (rice husk, rice straw, tea leaves, and newspaper) were evaluated for the removal of As(III) and As(V) from aqueous solutions. Preliminary studies showed that the complete removal of As(III) and As(V) was
Figure 1. Effect of adsorbent amount on the removal of As(III) and As(V) by adsorption onto rice husk (initial As concentration, 100 µg/L). (a) As(V); avg RH particle size, 510 µm. Treatment flow rate: b, 1.7; O, 6.7 mL/min. (b) As(III); avg RH particle size, 780 µm. Treatment flow rate: 0, 1.7; 4, 6.7 mL/min.
achieved under the following conditions: initial concentration, 100 µg/L; amount of rice husk, 6 g; treatment flow rates, 6.7 and 1.7 mL/min; and average particle sizes, 780 and 510 µm, respectively. The removal efficiencies with rice straw, tea leaves, and newspaper were 65%, 57%, and 18%, respectively, for As(III) and 75%, 58% and 55%, respectively, for As(V). These three adsorbents had lower removal efficiencies than rice husk. Therefore, they were not considered for further investigations. The rice husk used was an available material in the rice mill, and its size distribution was as follows: 40% >850 µm, 18% 710-850 µm, 10% 600-710 µm, 13% 425-600 µm, 5% 355425 µm, 8% 150-355 µm, and 6% 12. For the removal of As(III), the efficiency curve was essentially a plateau in the pH range of 4-10, and then, the efficiency tended to decrease with increasing pH. The poor As removal efficiencies at high pH can be attributed to the following factors: First, both chemical species for As(III) and As(V) in this pH region are oxyanions. Next, hydroxyl groups are more plentiful on the surface of RH with increasing pH. The maximum removal efficiencies for As(III) and As(V) were
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Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 Table 1. Influence of the Eluent on the Desorption of As(III) and As(V)a desorption (%)b desorption agent KOH (2 M)
KOH (1 M)
Figure 6. Breakthrough curves for the removal of As(III) and As(V) by adsorption onto rice husk (initial As concentration, 100 µg/L). b: As(V); RH, 6 g; avg particle size, 510 µm; treatment flow rate, 1.7 mL/min. 0: As(III); RH, 6 g; avg particle size, 780 µm; treatment flow rate, 6.7 mL/ min.
observed in comparatively neutral regions (at pH 6.5 and pH 8.0, respectively). These results should be of great advantage for the practical implementation of arsenic removal from groundwater. Breakthrough Curves. Breakthrough curves were investigated for the removal of As(III) and As(V) at an initial concentration of 100 µg/mL by adsorption onto rice husk using the optimized conditions, as illustrated in Figure 6. In the experimental process, the As sample solution was continuously fed into the RH-packed column, and at a given treatment time (volume), 3 mL of treated solution was sampled and subjected to GFAAS analysis. The breakthrough volumes for As(III) and As(V) can be relatively small (approximately 100 mL). Because the exhaustion volumes were comparatively large, the total amount of each solute removed from the sample upon saturation of the adsorbent, which is equal to the amount of solute that the adsorbent phase contains, was calculated by the integration of the area above the breakthrough curve. The amounts adsorbed under the specified removal conditions were 20 and 7 µg of As/g of RH for As(III) and As(V), respectively. Adsorption Mechanism. Arsenic removal by adsorption onto RH can be supposed to occur mainly through two routes: (i) affinity adsorption and (ii) anion exchange between the arsenic in the water and the carbon surface of the rice husk. Affinity adsorption is related to the surface behavior of RH, whereas anion exchange relates to the existing forms of the arsenic species. OH groups are created on the carbon surface during the activation process.39,40 The mechanism of adsorption of metal anions onto activated carbon is generally well explained by electrochemical theory: Carbon in contact with water reduces oxygen to a hydroxyl group41
O2 + 2H2O + 2e- f H2O2 + 2OH-
(2)
and thus, the carbon loses electrons and become positively charged. Electrical neutrality is maintained with hydroxyl ions, resulting in their adsorption. Although the rice husk used in the present work was not subjected to any chemical or physical activation treatment, a large number of OH groups will remain on the surface of the RH after the drying process. When a metalbearing solution contains anions that exhibit a higher affinity toward carbon than the hydroxyl groups, the latter are exchanged, as shown in the case of As(V)
carbon-(OH)2 + HAsO42- f carbon-HAsO4 + 2OH(3) In the present study, the removal of uncharged As(III) species from the solution by adsorption onto RH took place more readily
NaOH (1 M) HCl (1 M) HNO3 (1 M) H2SO4 (1 M)
standing time (h)
As(III)
8 12 18 24 36 8 16 20 8 8 8 8
65 65 67 69 71 65 65 68 51 3 3 2
As(V) 81 80 80 82 83 77 75 0.6 1.4 0.5
a Adsorption process: initial As concentration, 100 µg/L; RH, 4 g; average particle size, 780 µm for As(III) and 510 µm for As(V); treatment flow rate, 6.7 mL/min for As(III) and 1.7 mL/min for As(V); efficiency, 89% for As(III) and 87% for As(V). b Volume of desorption agent, 50 mL.
than the removal of charged As(V) species. At pH values of less than 9, neutral H3AsO3 is predominantly present as As(III) species. The As(III) removal efficiency from solution by adsorption onto rice husk was found to be much better than that of As(V) in the neutral pH range. These results can be attributed to the lack of electrostatic repulsion between the surface and the neutral As(III) species. The physical adsorption and interaction between the H3AsO3 species and the rice husk surface might be partly responsible for the removal of As(III). From the information in the literature, it is not clear which process is predominant for the adsorption of As onto rice husk. These mechanisms are still being examined. Desorption. Recovery of the adsorbed material and regeneration of the adsorbent are also important aspects of wastewater treatment. Attempts were made to desorb both As(III) and As(V) from the RH surface with various eluents, such as hydrochloric, sulfuric, and nitric acid solutions and base solutions containing sodium hydroxide and potassium hydroxide. This desorption process was performed using the batch method. For each experiment, 50 mL of desorption solution was added to the column and held there for a fixed period of time. After the standing time, the solution was passed through the column. The results are presented in Table 1. Although the achievement of arsenic elution using strong acidic or alkaline solutions has been reported in the literature,42 the present work showed that effective desorption was obtained with alkaline solutions. These phenomena are consistent with the results observed for the effect of pH. In general, the desorption efficiency of arsenic tended to increase with increasing desorption time. Consequently, potassium hydroxide solution was useful for the desorption of arsenic from the surface of RH. Application of the Developed Treatment System. The utility of the waste RH was evaluated for the treatment of Ascontaminated Bangladeshi groundwater samples. The concentrations of total arsenic in the samples were 270 and 595 µg/L. It has been reported that the total arsenic concentration in the tubewell water is in the range 0.25-1 mg/L, with 60-90% of the arsenic present as As(III) species.43 Because the pH of these groundwater samples was around 7, the arsenic species might be HAsO42- for As(V) and H3AsO3 for As(III).13,14,44 The treatment results are presented in Table 2. Although 12 g of adsorbent was applied in the treatment, the concentrations of arsenic in the treated sample water could be lowered to 10 and 26 µg/L. The desorption efficiencies with 100 mL of 1 M KOH were 71% and 96%. From the present results, the arsenic was
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 8109 Table 2. Removal and Desorption of As from the Contaminated Groundwater of Bangladesh
pH initial As concentration (µg/L) final concentration (µg/L) removala (%) desorptionb (%)
sample 1
sample 2
7.8 270 10 96 96
7.6 595 26 96 71
a Removal: RH, 12 g; average particle size, 780 µm; flow rate, 0.8 mL/ min. b Desorption: 1 M KOH, 100 mL; standing time, 8 h.
successfully removed from practical As-contaminated groundwater, and the adsorbed As could be recovered from the surface of RH. On the basis of the present treatment system, purification of 10 L of As-contaminated groundwater will be achievable with 1.2 kg of rice husk in a treatment time of 125 min. Conclusions The proposed column treatment systems are appropriate and suitable homemade approaches to arsenic removal in local areas, because of their simplicity and easy operation and handling. The present method is effective for a wide range of concentrations (i.e., 50-500 µg/L), which were quite similar to those observed in contaminated Bangladeshi groundwater. No secondary-pollution problem will occur, because desorption of the arsenic is possible. Direct removal of both arsenite and arsenate can be achieved without first oxidizing arsenite to arsenate, whereas the traditional methods require the oxidation process. Because rice husks are abundant agricultural wastes, they are readily available. Moreover, after the desorption process, RH can be used as a fuel source because it does not contain any harmful substances. Acknowledgment This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Grantin-Aid for the Japan Society for the Promotion of Science (JSPS) Fellows 16‚04403. Literature Cited (1) Nordstrom, D. K. Public healthsWorldwide occurrences of arsenic in ground water. Science 2002, 296, 2143. (2) Berg, M.; Tran, H. C.; Nguyen, T. C.; Pham, H. V.; Schertenleib, R.; Giger, W. Arsenic contamination of groundwater and drinking water in Vietnam: A human health threat. EnViron. Sci. Technol. 2001, 35, 2621. (3) Bagla, P.; Kaiser, J. India’s spreading health crisis draws global arsenic experts. Science 1996, 274, 174. (4) Nickson, R.; McArthur, J.; Burgess, W.; Ahmed, K. M.; Ravenscroft, P.; Rahman, M. Arsenic poisoning of Bangladesh groundwater. Nature 1998, 395, 338. (5) Shugi, K.; Singh, T. S.; Pant, K. K. Equilibrium and kinetics studies on removal of arsenite by iron oxide coated activated alumina. Ind. J. EnViron. Health 2003, 45, 151. (6) Ayotte, J. D.; Montgomery, D. L.; Flanagan, S. M.; Robinson, K. W. Arsenic in groundwater in eastern New England: Occurrence, controls and human health implications. EnViron. Sci. Technol. 2003, 37, 2075. (7) Hughes, M. F. Arsenic toxicity and potential mechanisms of action. Toxicol. Lett. 2002, 133, 1. (8) Chakravarty, S.; Dureja, V.; Bhattacharyya, G.; Maity, S.; Bhattacharjee, S. Removal of arsenic from ground water using low cost ferruginous manganese ore. Water Res. 2002, 36, 625. (9) Thomas, D. J.; Styblo, M.; Lin, S. The cellular metabolism and systemic toxicity of arsenic. Toxicol. Appl. Pharmacol. 2001, 176, 127. (10) Stoica, A.; Pentecost, E.; Martin, M. B. Effects of arsenite on estrogen receptor-R expression and activity in MCF-7 breast cancer cells. Endocrinology 2000, 141, 3595. (11) Special Report on Ingested Inorganic Arsenic Skin Cancer: Nutritional Essentiality; Report EPA/625 3-87-13; U.S. Environmental Protection Agency; Washington, DC, 1999.
(12) Arsenic in Drinking Water 2001 Update; National Research Council, National Academy Press: Washington, DC, 2001. (13) Meng, X.; Bang, S.; Korfiatis, G. P. Effect of silicate, sulfate and carbonate on arsenic removal by ferric chloride. Water Res. 2000, 34, 1255. (14) Ghimire, K. N.; Inoue, K.; Yamaguchi, H.; Makino, K.; Miyajima, T. Adsorptive separation of arsenate and arsenite anions from aqueous medium by using orange waste. Water Res. 2003, 37, 4945. (15) Katsoyiannis, I. A.; Zouboulis, A. I. Application of biological processes for the removal of arsenic from groundwaters. Water Res. 2004, 38, 17. (16) Kim, J.; Benjamin, M. M. Modeling a novel ion exchange process for arsenic and nitrate removal. Water Res. 2004, 38, 2053. (17) Jay, J. A.; Blute, N. K.; Hemond, H. F.; Durant, J. L. Arsenicsulfides confound anion exchange resin speciation of aqueous arsenic. Water Res. 2004, 38, 1155. (18) Hege, K. V.; Verhaege, M.; Verstraete, W. Electro-oxidative abatement of low-salinity reverse osmosis membrane concentrates. Water Res. 2004, 38, 1550. (19) Balasubramanian, N.; Madhavan, K. Arsenic removal from industrial effluent through electrocoagulation. Chem. Eng. Technol. 2001, 24, 519. (20) Jekel, M. R. Removal of arsenic in drinking water treatment. In Arsenic in the EnVironment. Part 1: Cycling and Characterization; Nriangu, J. O., Ed.; Wiley: New York, 1994; p 119. (21) Kim, M. J.; Nriangu, J. Oxidation of arsenite in groundwater using ozone and oxygen. Sci. Total EnViron. 2000, 247, 71. (22) Maeda, S.; Ohki, A.; Saikoji, S.; Naka, K. Iron(III) hydroxideloaded coral limestone as an adsorbent for arsenic(III) and arsenic(V). Sep. Sci. Technol. 1992, 27, 681. (23) Ohki, A.; Nakayachigo, K.; Naka, K.; Maeda, S. Adsorption of inorganic and organic arsenic compounds by aluminium-loaded coral limestone. Appl. Organomet. Chem. 1996, 10, 747. (24) Singh, D. B.; Prasad, G.; Rupainwar, D. C. Adsorption technique for the treatment of As(V)-rich effluents. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 111, 49. (25) Manjare, S. D.; Sadique, M. H.; Ghoshal, A. K. Equilibrium and kinetics studies for As(III) adsorption on activated alumina and activated carbon. EnViron. Technol. 2005, 26, 1403. (26) Huang, C. P.; Fu, P. L. K. Treatment of arsenic(V)-containing water by the activated carbon process. J. Water Pollut. Control Fed. 1984, 56, 233. (27) Ghosh, M. M.; Yuan, J. R. Adsorption of inorganic arsenic and organoarsenicals on hydrous oxides. EnViron. Prog. 1987, 6, 150. (28) Hathaway, S. W.; Rubel, F. J. Removing arsenic from drinking water. J. Am. Water Works Assoc. 1987, 79, 61. (29) Suzuki, T. M.; Bomani, J. O.; Matsunaga, H.; Yokoyama, T. Removal of As(III) and As(V) by a porous spherical resin loaded with monoclinic hydrous zirconium oxide. Chem. Lett. 1997, 11, 1119. (30) Williams, P. T.; Nugranad, N. Comparison of products from the pyrolysis and catalytic pyrolysis of rice husks. Energy 2000, 25, 493. (31) Daifullah, A. A. M.; Girgis, B. S.; Gad, H. M. H. Utilization of agro-residues (rice husk) in small waste water treatment plans. Mater. Lett. 2003, 57, 1723. (32) Gupta, V. K.; Saini, V. K.; Jain, N. Adsorption of As(III) from aqueous solutions by iron oxide-coated sand. J. Colloid Interface Sci. 2005, 288, 55. (33) Pokhrel, D.; Viraraghavan, T. Arsenic removal from an aqueous solution by a modified fungal biomass. Water Res. 2006, 40, 549. (34) Nakajima, T.; Xu, Y. H.; Mori, Y.; Kishita, M.; Takanashi, H.; Maeda, S.; Ohki, A. Combined use of photocatalyst and adsorbent for the removal of inorganic arsenic(III) and organoarsenic compounds from aqueous media. J. Hazard. Mater. 2005, 120, 75. (35) Munaf, E.; Zein, R. The use of rice husk for removal of toxic metals from wastewater. EnViron. Technol. 1997, 18, 359. (36) Wong, K. K.; Lee, C. K.; Low, K. S.; Haron, M. J. Removal of Cu and Pb by tartaric acid modified rice husk from aqueous solutions. Chemosphere 2003, 50, 23. (37) Seco, A.; Gabaldon, C.; Marzal, P.; Aucejo, A. Effect of pH, cation concentration and sorbent concentration on cadmium and copper removal by a granular activated carbon. J. Chem. Technol. Biotechnol. 1999, 74, 911. (38) Seco, A., Marzal, P., Gabaldon, C.; Ferrer, J. Adsorption of heavy metals from aqueous solutions onto activated carbon in single Cu and Ni systems and in binary Cu-Ni, Cu-Cd, and Cu-Zn systems. J. Chem. Technol. Biotechnol. 1997, 68, 23. (39) Corapcioglu, M. O.; Huang, C. P. The adsorption of heavy metals onto hydrous activated carbon. Water Res. 1987, 21, 1031.
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(40) Manju, G. N.; Raji, C.; Anirudhan, T. S. Evaluation of coconut husk carbon for the removal of arsenic from water. Water Res. 1998, 32, 3062. (41) Navarro, P.; Alguacil, F. J. Adsorption of antimony and arsenic from a copper electrorefining solution onto activated carbon. Hydrometallurgy 2002, 66, 101. (42) Lorenzen, L.; Deventer, J. S. J. V.; Landi, W. M. Factors affecting the mechanism of the adsorption of arsenic species on activated carbon. Miner. Eng. 1995, 8, 557. (43) Khoe, G. H.; Emett, M. T.; Robins, R. G. Photoassisted Oxidation of Species in Solution; U.S. Patent 5,688,378, 1997.
(44) Wickramasinghe, S. R.; Han, B.; Zimbron, J.; Shen, Z.; Karim, M. N. Arsenic removal by coagulation and filtration: comparison of groundwaters from the United States and Bangladesh. Desalination 2004, 169, 231.
ReceiVed for reView March 22, 2006 ReVised manuscript receiVed September 25, 2006 Accepted September 25, 2006 IE060344J
