摘要：

EPA adopted the current maximum contaminant level (MCL) for arsenic (As) in drinking water of 50 mg/L in 1975. More recently, the National Academy of Sciences concluded there is sufficient evidence from human epidemiological studies in Taiwan, Chile, and Argentina to suggest that chronic ingestion of inorganic As causes bladder and lung cancer as well as skin cancer, and has recommended lowering the current MCL [1]. By June 2000 EPA is expected to respond with a new proposed MCL for As closer to, or even lower than the World Health Organization (WHO) guideline value for arsenic in drinking water of 10 mg/L. As occurs both naturally and as the result of anthropogenic contamination. In the United States, surface water sources rarely have As concentrations > 5 mg/L, whereas 16-46 percent of groundwater sources exceed this value, depending on the region of the country [2]. In other parts of the world such as Taiwan, Bangladesh, India, Argentina, and Chile, naturally occurring arsenic in groundwater wells used for drinking water has been identified as serious threats to public health [1] [3]. The Baird & McGuire Superfund Site. Baird & McGuire, Inc. operated a chemical mixing and batching company in Holbrook, Massachusetts beginning in 1912, manufacturing a range of products including wood preservatives, pesticides, herbicides, and insecticides梡roducts whose indiscriminate use prompted Rachel Carson to publish her landmark 1962 book, Silent Spring, sparking the environmental movement in the United States [4]. Improper waste disposal practices resulted in widespread contamination of soils, river sediments, and groundwater surrounding the Baird & McGuire Site, including Holbrook's drinking water supply wells. Besides As, a number of inorganic and organic compounds including lead, solvents, pesticides, herbicides, and dioxin were identified [5]. Baird & McGuire's operations were eventually shut down in 1983 and the site was placed on EPA's National Priority List (NPL). Comprehensive clean up efforts at the site have involved the on-site incineration of 250,000 tons of contaminated soils and the installation of a 200 gpm groundwater treatment facility (GWTF) [6]. Long term, full-scale groundwater pump and treatment combined with Light Non-aqueous Phase Liquid (LNAPL) collection has been ongoing since 1993. The GWTF includes flow equalization and oil skimming, a 2-stage metals removal system, flow neutralization, aeration with vapor phase carbon adsorption, pressure filtration, and granular activated carbon adsorption. The operator must meet drinking water MCLs for contaminants prior to discharge of treated water to on-site infiltration basins. Metals Removal System. An iron coagulation process is used for the removal of metals at the Baird & McGuire Site (Figure 1). A 2-stage process involves rapid mixing with addition of lime slurry, Ca(OH)_2, and ferric chloride, FeCl_3, followed by flocculation and sedimentation. Agglomerated floc, or sludge generated from both stages is combined, thickened, and dewatered prior to disposal at an off-site landfill. Arsenic Chemistry. Arsenic can exist in four oxidation states in water: +5, +3, 0, and -3 but is generally found in the trivalent As (III) arsenite or the pentavalent As (V) arsenate states [7]. Results of recent As speciation testing at the Baird & McGuire Site indicate that approximately 30 % of the As in the plant influent exists as As (III), and 70% as As (V). Arsenic Removal Techniques. The most commonly used technologies for arsenic removal include coprecipitation and adsorption onto coagulated floc, lime softening, sulfide precipitation, adsorption onto activated carbon, activated alumina, ion exchange, and reverse osmosis [8]. Arsenic removal has also been demonstrated by manganese greensand (glauconite) filtration with potassium permanganate, KMnO_4 as a preoxidant [9] [10].

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