Low Molecular Weight Carboxylic Acids in Oxidizing Porphyry Copper Tailings

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• 作者：Bernhard Dold ; David W. Blowes ; Ralph Dickhout ; Jorge E. Spangenberg ; and Hans-Rudolf Pfeifer
• 刊名：Environmental Science & Technology
• 出版年：2005
• 出版时间：April 15, 2005
• 年：2005
• 卷：39
• 期：8
• 页码：2515 - 2521
• 全文大小：316K
• 年卷期：v.39,no.8(April 15, 2005)
• ISSN：1520-5851

文摘

The distribution of low molecular weight carboxylic acids(LMWCA) was investigated in pore water profiles fromtwo porphyry copper tailings impoundments in Chile(Piuquenes at La Andina and Cauquenes at El Tenientemine). The objectives of this study were (1) to determinethe distribution of LMWCA, which are interpreted to be themetabolic byproducts of the autotroph microbial communityin this low organic carbon system, and (2) to infer thepotential role of these acids in cycling of Fe and otherelements in the tailings impoundments. The speciation andmobility of iron, and potential for the release of H⁺ viahydrolysis of the ferric iron, are key factors in the formationof acid mine drainage in sulfidic mine wastes. In the low-pH oxidation zone of the Piuquenes tailings, Fe(III) isthe dominant iron species and shows high mobility. LMWCA,which occur mainly between the oxidation front down to300 cm below the tailings surface at both locations (e.g., maxconcentrations of 0.12 mmol/L formate, 0.17 mmol/Lacetate, and 0.01 mmol/L pyruvate at Piuquenes and 0.14mmol/L formate, 0.14 mmol/L acetate, and 0.006 mmol/Lpyruvate at Cauquenes), are observed at the same locationas high Fe concentrations (up to 71.2 mmol/L Fe(II) and16.1 mmol/L Fe(III), respectively). In this zone, secondary Fe(III) hydroxides are depleted. Our data suggest thatLMWCA may influence the mobility of iron in two ways.First, complexation of Fe(III), through formation of bidentateFe(III)-LMWCA complexes (e.g., pyruvate, oxalate), mayenhance the dissolution of Fe(III) (oxy)hydroxides or mayprevent precipitation of Fe(III) (oxy)hydroxides. Soluble Fe(III) chelate complexes which may be mobilized downwardand convert to Fe(II) by Fe(III) reducing bacteria. Second,monodentate LMWCA (e.g., acetate and formate) can beused by iron-reducing bacteria as electron donors(e.g., Acidophilum spp.), with ferric iron as the electronacceptor. These processes may, in part, explain the lowabundances of secondary Fe(III) hydroxide precipitates belowthe oxidation front and the high concentrations of Fe(II)observed in the pore waters of some low-sulfide systems.The reduction of Fe(III) and the subsequent increase ofiron mobility and potential acidity transfer (Fe(II) oxidationcan result in the release of H⁺ in an oxic environment)should be taken in account in mine waste managementstrategies.