<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 />
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