Arsenic Removal from Drinking Water Using Iron Oxide-Coated Sand

ARSENIC REMOVAL FROM DRINKING WATER USING IRON OXIDE-COATED SAND O. S. THIRUNAVUKKARASU1, T. VIRARAGHAVAN1∗ and K. S. SUBRAMANIAN2 1 Environmental Systems Engineering, Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada; 2 Product Safety Bureau, Health Canada, Ottawa, Ontario, Canada ∗ ( author for correspondence, e-mail: [email protected], fax: +1 306 585 4855) (Received 22 August 2001; accepted 22 April 2002) Abstract. This article describes experiments in which iron oxide-coated sand (IOCS) was used to − study the removal of both As(V) and As(III) to a level less than 5 µgL 1 in drinking water. Iron oxide-coated sand 2 (IOCS-2) prepared through high temperature coating process was used in batch and column studies to assess the effectiveness and suitability. The isotherm study results showed that the observed data fitted well with the Langmuir model, and the adsorption maximum for IOCS-2 at − pH 7.6 was estimated to be 42.6 and 41.1 µgAsg 1 IOCS-2 for As(V) and As(III), respectively. In the fixed bed column tests to study arsenic removal from the tap water, good performance of IOCS-2 was observed in respect of bed volumes achieved and arsenic removal capacity. Five cycles of column tests were conducted to evaluate the performance of IOCS-2, and arsenic was successfully recovered from the media through regeneration and backwash operations. High bed volumes (860 to 1403) up − to a breakthrough concentration of 5 µgL 1 were achieved in the column studies with tap water, and the bed volumes achieved in the studies with natural water (containing arsenic) were 1520. The results of both the batch and column studies showed that iron oxide-coated sand filtration could be − effectively used to achieve less than 5 µgL 1 As in drinking water. Keywords: adsorption, arsenic removal, batch studies, column tests, drinking water, iron oxide, speciation 1. Introduction The enforcement of stringent standards for arsenic in drinking water by the regulatory agencies calls for pragmatic approach in developing a suitable and cost- effective technology to remove arsenic from drinking water. Arsenic, a cancer causing substance is predominantly present as inorganic species in natural water systems. In oxygen-rich environments where aerobic conditions persist, arsenate − 2− [As(V)] is prevalent and exists as a monovalent (H2AsO4 )ordivalent(HAsO4 ) anion, whereas, arsenite [As(III)] exists as an uncharged molecule (H3AsO3)and − anionic (H3AsO3 ) species in moderately reducing environment where anoxic conditions persist (Ferguson and Gavis, 1972). Though arsenic contamination of drinking water and the associated health risks of drinking arsenic contaminated water were reported in Taiwan three decades ago (Tseng et al., 1968), similar reports of arsenic contamination from many parts of the world (Karim, 2000; Burkel and Stoll, 1999; Koch et al., 1999; Chatterjee et al., Water, Air, and Soil Pollution 142: 95–111, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 96 O. S. THIRUNAVUKKARASU ET AL. 1995; Cebrian et al., 1983) are receiving significant attention at present and they are of major concern to many water utilities and regulatory agencies. The drinking water standard for arsenic has been lowered in many countries (Viraraghavan et al., 1999; Driehaus et al., 1998), and more recently the United States Environmental Protection Agency (USEPA, 2001) adopted a new arsenic standard for drinking water at 10 µgL−1. Several studies have demonstrated that arsenic removal can be achieved by various technologies (Viraraghavan et al., 1994), and particularly coagulation with ferric salts was found to be the most effective method in the case of large-scale water utilities (Scott et al., 1995; Cheng et al., 1994). Fixed bed treatment systems such as adsorption and ion exchange are getting increasingly popular for arsenic removal in small-scale treatment systems because of their simplicity, ease of operation and handling, regeneration capacity and sludge free operation. Iron oxides, oxyhydroxides and hydroxides (all are called ‘iron oxides’) play an important role in a variety of industrial applications, including pigments for paint industry, catalyst for industrial synthesis and raw material for iron and steel industry (Cornell and Schwertmann, 1996). The application of iron oxide has been extended to remove metals from water and wastewater (Benjamin et al., 1996; Edwards and Benjamin, 1989); recently arsenic removal with iron oxides has been investigated (Raven et al., 1998; Driehaus et al., 1998; Joshi and Chaudhuri, 1996; Wilkie and Hering, 1996; Hsia et al., 1994; Pierce and Moore, 1982; Pierce and Moore, 1980). It is generally assumed that arsenate [As(V)] has a strong affinity than arsenite [As(III)] on iron oxide surfaces. However recent studies (Raven et al., 1998) showed that at high initial As concentration, arsenite adsorption on ferrihydrite was higher than arsenate adsorption throughout the pH range of 3– 11. In adsorption studies using hydrous ferric hydroxide (HFO), high adsorption of arsenite on the HFO was observed in the pH range of 4–9, and one of the reasons for the removal was attributed to the partial oxidation of As(III) on HFO surface (Wilkie and Hering, 1996). In column studies with iron oxide-coated sand (Benjamin et al., 1996), a complete removal of arsenite (75 µgL−1)intheinfluent was observed despite the fact that the influent contained 800 mg L−1 sulfate. Though the sorption of arsenic on HFO has been studied in detail (Wilkie and Hering, 1996; Hsia et al., 1994; Pierce and Moore, 1982; Pierce and Moore, 1980), only limited information is available on arsenic adsorption on to iron oxide- coated sand (IOCS). The USEPA had proposed ion exchange, activated alumina, reverse osmosis, modified coagulation/filtration, and modified lime softening as best available technologies (BAT) for arsenic removal from small water facilities, but it considered iron oxide-coated sand filtration as an emerging technology for arsenic removal, for which only limited information is available and more testing is necessary (USEPA, 1999, 2000). There is also a need to understand how different coating procedures affect arsenic removal and retention of the coating. Therefore it was considered necessary to examine the effectiveness of IOCS for arsenate and arsenite removal. In the present study, batch studies were conducted to study the ARSENIC REMOVAL FROM DRINKING WATER 97 extent of adsorption of arsenic on to IOCS-2. Column studies were conducted to study the removal of both As(III) and As(V) that were spiked to the required concentration levels in tap water. A speciation technique was used with the isotherm and column studies on a natural water to speciate arsenite and arsenate present in the effluent. 2. Experimental Section 2.1. WATER AND STANDARD SOLUTIONS Tap water from the City of Regina, Saskatchewan, Canada and natural water from Kelliher Water Treatment Plant, Kelliher, Saskatchewan, Canada were used in the batch and column studies. Kelliher water contains 177.3 µgAsL−1,andthema- jor physicochemical characteristics of the Regina tap water and Kelliher natural water are listed in Table I. Distilled (double) deionized water was used in the preparation of standard solutions and for dilution of the samples. As(V) stock solution (1000 mg L−1) was prepared by dissolving 4.164 g of sodium arsenate (Na2HAsO4·7H2O; Sigma Chemical, Ontario) in 1 L distilled water and was pre- −1 served with 0.5% trace metal grade HNO3 (Fisher Scientific, Ontario). One mg L of As(V) was prepared by pipetting 1 mL of stock solution into a 1 L volumetric flask, and then the solution was made up to 1 L with distilled water. One mg L−1 of As(III) stock solution was prepared by pipetting 1 mL of arsenic oxide (1000 mg L−1 reference solution; Fisher Scientific, Ontario) into a 1 L volumetric flask, and then the solution was made up to 1 L with distilled water. In both the cases [As(V) or As(III)], required working standards were prepared daily from the stock solution. All glassware and sample bottles were washed with a detergent solution, rinsed with tap water, soaked with 10% nitric acid for at least 12 hr, and finally rinsed with distilled water three times. 2.2. PREPARATION OF IRON OXIDE-COATED SAND 2 (IOCS-2) In the preparation of iron oxide-coated sand, coating of iron oxide was achieved on the redflint filter sand purchased from Watergroup Canada Ltd., Regina, Saskat- chewan, Canada. Initially, the sand was sieved to a geometric mean size of 0.6 to 0.8 mm, acid washed (pH 1; 24 hr), rinsed with deionized distilled water three times and dried at 100 ◦C for 20 hr before use. Iron oxide-coated sand prepared and used in the present study was different than that of IOCS-1 (Thirunavukkarasu et al., 2001), and named as IOCS-2; it was prepared similar to that of the procedure by Benjamin et al. (1996), with modifications. Effective coating of iron oxide on the sand was achieved in two steps. In step 1, the solution containing a mixture of 80 mL of 2 M Fe(NO3)3·9H2O and 1 mL of 10 M NaOH was poured over 200 g dried sand placed in a heat resistant dish. After gentle agitation, the mixture was heated for 4 hr at 110 ◦C and then at 550 ◦C for 3 hr. Upon cooling, the coated sand 98 O. S. THIRUNAVUKKARASU ET AL. TABLE I Water quality parameters Parameters Tap watera Kelliher watera pH 7.4–7.6 7.4 Iron 0.07 2.1 Manganese 0.02 1.2 Turbidity (NTU) 0.28 NT Chloride 12 NT Copper 0.001 0.04 Zinc <0.005 0.01 Lead <0.002 0.002 Cadmium <0.001 <0.001 Barium 0.073 0.011 Chromium <0.001 0.001 Chlorine (residual) 0.2 Not analyzed − a All parameters except pH and turbidity are in mg L 1.

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