| Abstract
| - The applicability of in situ metal precipitation (ISMP)based on bacterial sulfate reduction (BSR) with molassesas carbon source was tested for the immobilization of azinc plume in an aquifer with highly unsuitable initial conditions(high Eh, low pH, low organic matter content, and lowsulfate concentrations), using deep wells for substrateinjection. Batch experiments revealed an optimal molassesconcentration range of 1−5 g/L and demonstrated thenecessity of adding a specific growth medium to thegroundwater. Without this growth medium, even sulfate,nitrogen, phosphorus, and potassium addition combined withpH optimization could not trigger biological sulfatereduction. In column experiments, precipitation of ZnS(s)was induced biologically as well as chemically (by addingNa2S). In both systems, zinc concentrations droppedfrom about 30 mg/L to below 0.02 mg/L. After terminationof substrate addition the biological system showedcontinuation of BSR for at least 2 months, suggesting theinsensitivity of the sulfate reducing system for shortstagnations of nutrient supply, whereas in the chemicalsystem an immediate increase of Zn concentrations wasobserved. A pilot experiment conducted in situ at the zinc-contaminated site showed a reduction of zinc concentrationsfrom around 40 mg/L to below 0.01 mg/L. Termination ofsubstrate supply did not result in an immediate stagnationof the BSR process, but continuation of BSR was observedfor at least 5 weeks.
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