INTRODUCTIONOne of the most common groups of contaminants in landfill leachate is heavy metals such as mercury (Hg), chromium (Cr), nickel (Ni), zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb), and iron (Fe). Temporal variations of heavy metal characteristics of typical leachate have been analyzed in several investigations (Ludvigsen et al., 1998;Syed & Walter, 1994;Christensen et al., 1994). Typical concentrations in municipal landfill leachate include (Cd 2 to 20 µg/l, Ni 100 to 400 µg/l, Zn 500 to 2,000 µg/l, Cu 20 to 100 µg/l, Pb 50 to 200 µg/l, and Fe 10 to 500 mg/l, (Christensen & Jensen, 1999;Ludvigsen et al., 1998). However, concentrations of heavy metals may vary depending on the differences in waste composition and landfill technology (Ehrig, 1983).Heavy metals can be removed in situ by co-precipitation with the end product of bacterial metabolism (Gadd & White, 1993;White et al., 1988). If sufficient amounts of electron acceptors are available, the microorganism can use organic material in the leachate to produce sulphide and carbonates. Metals then react chemically with these compounds, producing precipitates, which are often too stable to dissolve once formed. Several researchers have investigated the effect of sulphate-reducing bacteria (SRB) to remove heavy metals as a sulphide precipitate (Gundry et al., 1989;Dovark et al., 1992;Harada et al., 1994;Bayoumy et al., 1999 Kinetics of Biochemically Driven Metal Precipitation in Synthetic Landfill LeachateThe research described in this article examines the effect of organic carbon and sulphate concentration on SRB and methanogen activity, which mediate metal precipitation.an up-flow anaerobic fixed film reactor is reported as Cu>Zn>Ni>Pb (Harada et al., 1994). Sulphate removal rates in these studies were 0.14 to 4.23 mg/l/day/g of media. Gundry et al. (1989) found that SRB removed 96 to 99 percent of nickel in an anaerobic filter. Dovark et al. (1992) found that Fe, Zn, Mn, and Cd concentrations were lowered by more than 95 percent when these metals were precipitated as insoluble sulphide along with the hydrogen sulphide produced by SRB. Bayoumy et al. (1999) investigated the relationship between metal level and total organic carbon (TOC).This study found that the TOC removal rate in an anaerobic up-flow reactor including SRB was 50 percent. But when the metal loading was increased, the TOC removal rate was reduced. Metal precipitation may also occur as a carbonate compound and is mediated by methanogenic bacteria (Rittmann & VanBriesen, 1996).These studies provide broadbased information on the chemical processes of metal precipitation under equilibrium conditions. However, no studies have specifically described the influence of active microbial metabolism on the fate, kinetic behavior, and reaction pathways of heavy metals in landfill leachates.The research described in this article examines the effect of organic carbon and sulphate concentration on SRB and methanogen activity, which mediate metal precipitation. Microbial activity was stimulated usin...