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• 4.3.2.1 Process Chemistry <br />Sulfate reduction is a redox process performed by microorganisms which use a wide variety of <br />organic carbon compounds as reductants, creating carbon dioxide and sulfide by the following <br />reaction: <br />SO,'-+2(CH,O)+H' -NHS-+2C0~+2H,O <br />The chemical formula for organic carbon is written generically as CHZO. The sulfide produced <br />in this reaction will react with and precipitate a wide range of metals. The metal sulfides that <br />precipitate aze often highly insoluble. When iron is abundant, co-precipitation of trace metals <br />with iron sulfides is also a mechanism for removal of dissolved metals (Huerta-Diaz and Morse, <br />1992). Co-precipitation of manganese with iron sulfide (as pyrite) requires significant (> 1.0 <br />µM) levels of sulfide. This reaction is slower than for many other metals (Morse, 1999). <br />4.3.3 Passive Anaerobic-Aerobic Treatment <br />In a passive anaerobic treatment design, groundwater extracted from the backfil] would be <br />introduced into an anaerobic upflow cell constructed to optimize sulfate reduction. This cell <br />would be filled with iron-containing materials from on-site (to precipitate sulfide), and <br />inoculated with sulfate reducing bacteria. The organic carbon for the sulfate reduction reaction <br />would be supplied by a mixture of alcohols and sugars. These compounds would be added at a <br />rate sufficient to reduce all of the sulfate in the recovery water. Water from the sulfate reduction <br />cell would then be recovered in an overflow basin, and gravity fed into manganese oxidation <br />cells. Oxygen would be added through passive aeration by cascading the water down rock-lined <br />channels into holding cells. These rocks would be inoculated with manganese oxidizing bacteria. <br />As the water is oxygenated, residual sulfide and organic carbon not removed in the anaerobic cell <br />will first be oxidized, followed by residual iron and manganese that have not precipitated in the <br />anaerobic cell. <br />• <br />Ba7rfe Moumain Resources. !rtc <br />p.l/001671reponslmarchrpllrr26xxrmng(march.doc 46 March 2?,1999 <br />