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NITRATE REDUCTION BY ZEROVALENT IRON: EFFECTS OF FORMATE, OXALATE, CITRATE, CHLORIDE, SULFATE, BORATE, AND PHOSPHATE
Citation:
Su, C. AND R W. Puls*. NITRATE REDUCTION BY ZEROVALENT IRON: EFFECTS OF FORMATE, OXALATE, CITRATE, CHLORIDE, SULFATE, BORATE, AND PHOSPHATE. ENVIRONMENTAL SCIENCE AND TECHNOLOGY. American Chemical Society, Washington, DC, 38(9):2715-2720, (2004).
Impact/Purpose:
Published journal article to inform public
Description:
Recent studies have shown that zerovalent iron (Fe0) may potentially be used as a chemical medium in permeable reactive barriers (PRBs) for nitrate remediation in groundwater; however, the effects of commonly found organic and inorganic ligands in soil and sediments on nitrate reduction by Fe0 have not been well understood. A 25.0 mL nitrate solution of 20.0 mg of N L-1 (1.43 mM nitrate) was reacted with 1.00 g of Peerless Fe0 at 200 rpm at 23 0C for up to 120 hours in the presence of each of the organic acids (formic, oxalic, and citric, with one, two, and three carboxylic groups in the molecular structures, respectively), and inorganic acids (HCl, H2SO4, H3BO3, and H3PO4), at a free H+ concentration of 3.0 mM under conditions of pH < 9.3. Consequently, the initial concentrations were 3.0 mM formic, 1.5 mM oxalic, 1.0 mM citric, 3.0 mM chloric, 1.5 mM sulfuric, 3.0 mM boric, and 1.5 mM phosphorous acids. Nitrate reduction rates (pseudo-first order) increased in the order: H3PO4 < citric acid < H3BO3 < oxalic acid < H2SO4 < formic acid < HCl, ranging from 0.00278 to 0.0913 h-1, corresponding to surface area normalized rates ranging from 0.126 to 4.15 h-1 m-2 mL. This sequence of reactivity corresponds to the strength of soluble Fe2+/Fe3+-organic ligand complexes in aqueous solution, and also to surface adsorption and complexation of the three organic ligands to iron oxides, which decreases in the order: formate < oxalate < citrate. The results are also consistent with the sequence of strength of surface complexation of the inorganic ligands to iron oxides, which decreases in the order: chloride < sulfate < borate < phosphate. The blockage of reactive sites on the surface of Fe0 and its corrosion products by specific adsorption of the inner-sphere complex forming ligands (sulfate, oxalate, borate, citrate, and phosphate) may be responsible for the decreased nitrate reduction by Fe0 relative to the chloride system.
