Control of Metals Contamination in Groundwater Using Iron-oxide-coated Sand and Magnetite-coated Sand

摘要

Many hazardous waste sites have more than a single contaminant. Groundwater contamination at wood preservative sites, in particular, often contains metals mixtures. Among common metals formulations used as wood preservatives are copper, chromium, and arsenic (CCA), and ammoniacal copper zinc arsenate (ACZA). Copper, zinc, chromate, and arsenate are toxic elements and potentially hazardous. The chemical properties of these metals in aqueous solution are quite different. Cu(Ⅱ) and Zn(Ⅱ) are cations and strongly adsorb on oxide surfaces in soils, while Cr(Ⅵ) and As(Ⅴ) are oxyanions and redox sensitive under environmental conditions. Cr(Ⅵ) species (as HCrO_4~- and CrO_4~(2-)) are the greatest environmental concern due to their toxicity and mobility, while Cr(Ⅲ) is less toxic and is removed from solution as the Cr(OH)_3~0 precipitated phase. Contrasting with chromium, the reduced forms of arsenic (As(Ⅲ), as HAsO_2 and AsO_2~-) are more toxic and mobile than the oxidized forms (As(Ⅴ), as HAsO_4~(2-) and H_2AsO_4~-). Metals can be removed from contaminated groundwater by adsorption and/or reduction onto iron-oxide minerals (e.g. hematite and biotite). Iron-oxide minerals are promising adsorbent materials for removal of low concentrations of heavy metals from contaminated groundwater (Benjamin and Leckie 1981). Iron-oxide-coated sand (IOCS) has been successfully used in our lab in both batch and column reactor experiments to remove Pb, Cr, Cu, and As from aqueous solution (Azizian and Nelson, 1997; Khaodhiar et al., 1998). The Fe(Ⅱ) component of selected iron oxides reacts with electroactive metals such as Cr(Ⅵ) to effect their reduction. Magnetite is a mixed oxidation state (ferrous-ferric) oxide, providing both adsorption arid reduction capabilities. Prepared as magnetite-coated-sand (MCS), it provides simultaneous removal of adsorbable and electroactive metals from aqueous solution, for example, Pb(Ⅱ) and Cr(Ⅵ). In mixed-metals contaminated ground waters (e.g. Cu(Ⅱ), Cr(Ⅵ), and As(Ⅴ)), MCS can be used to for simultaneous removal of Cu(Ⅱ) by adsorption, Cr(Ⅴ) by reduction to Cr(Ⅲ), and As(Ⅴ) by adsorption without reduction to As(Ⅲ). One possible advantage of MCS is that it adsorb As(V) and prevents it from reducing to the more toxic and mobile As(Ⅲ) species, compared to the metallic iron (Fe°), which is thermodynamically favorable for reduction of both Cr(Ⅵ) and As(Ⅴ). MCS and IOCS could be used for metals removal in a filter, a packed-bed reactor, or in a passive permeable barrier for wastewater and groundwater treatment.