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<br /> <br />(Lapakko and Eger, 1981; Dvorak and <br />Mclntine, 1992; Maree and Strydom, 1985): <br />SOQ Z + 2 C3H~O3' (lactate) _ <br />2 C,H,O; t (acetate) + 2 CO3 z + S~2 <br />[l1 <br />The carbonate or bicarbonate produced buffers <br />the solution pH and neutralizes acidity if <br />present. The short chain compounds needed as <br />substrate and a source of carbon will be <br />provided by the organic material. In addition to <br />sulfate reduction, the same organisms can <br />through a similar biochemical process reduce <br />the oxidized forms of selenium (i.e„ selenite <br />and selenate) to the elemental metal (Baldwin, <br />et al, 1985; Levine, 1924}. The removal of <br />selenium through the use of anaerobic bacterial <br />reduction has been investigated by Times <br />Limited and more recently by the U.S. Bureau <br />of Mines in Salt Lake City (Adams, et al, 1993; <br />Altringer, et al, 1991). The sulfide and <br />carbonate produced can combine with free and <br />complexed monovalent and divalen[ catiotts, <br />such as cadmium, lead, nickel, mercury, silver, <br />and zinc, to form very insoluble compounds <br />according to the following reactions: <br />[2] <br />~3J <br /> <br />The genera of organisms principally responsible <br />for dertitrification include Psettdomonas, <br />Microcorcus, Achromobacter, and Bacillus. <br />Although methanol is the preferred cazbon <br />source for the engineered processes used in <br />municipal wastewater treatment facilities, other <br />organic compounds found in composted <br />manure, wastewater sludge, brewery wastes, <br />and food processing wastes can also be utilized <br />by the microorganisms. <br />The other bacterially mediated reaction of <br />impor[ance is the anaerobic degradation of <br />residual cyartide. Although the aerobic <br />oxidation of cyanide is well documented and <br />forms the basis of full scale heap leach <br />decotttmissioning and mine water treatment <br />facilities, for example at the Homestake Mine in <br />Lead, South Dakota, the degradation of cyanide <br />under anoxic or anaerobic conditions is less <br />common (Canby, 1993; Altringer, et al, 1992). <br />However, anaerobic degradation of free and <br />complexed cyanide does occur in solutions and <br />in tailings, although at a slower rate, according <br />to one of the following reactions: <br />HCN + 3H, = CHa (methane) + NH3 <br />HCN + 2H,O = NHyCOOH (ammonia <br />formate) <br />[5] <br />[6] <br />With the organic layer within the Biopass <br />System being in an anoxic and chemically <br />reducing environment, the metal sulfides will <br />remain stable and insoluble. In conjunction with <br />the bacterial reduction of sulfate, nitrate is <br />converted to nitrogen gas or is taken up as a <br />nutrient through the microbially mediated <br />process of denitrifcation according to the basic <br />following reaction (Water Pollution Control <br />Federation, 1983; Metcalf and Eddy, 1979; <br />U.S. Environmental Protection Agency, 1975): <br />NO3' + 1.08 CH,O (methano0 + H~ _ <br />0.065 CSH~O~N (biomass) + 0.47 N, + 0.76 <br />CO, + 2.44 H,O <br />[4] <br />-9- <br />The genera of bacteria responsible for <br />anaerobic degradation of cyanide include <br />Pseudomonas, and other strictly anaerobic and <br />facultative microorganisms. Cyanide can be <br />used as a sole carbon source or can be degraded <br />in the presence of a co-metabolite supplied by <br />substrates contained in the organic material <br />present in the Biopass System. <br />nti loafed Procecc Performa_~ce and <br />Limitations <br />As discussed in the previous section, the <br />primary biochemical processes at work in the <br />Biopass System include bacterial sulfate <br />reduction, denitrifica[ion, and cyanide <br />