Sustainable approach for the immobilization of metals in the saturated zone: In situ bioprecipitation

The challenge faced in this study was to find a cost-efficient technique to remediate groundwater in the vicinity of three non-ferrous industrial sites, which was historically contaminated with metals and sulphate. The groundwater contained high concentrations of metals and high sulphate concentrations (up to 2720 mg/l), and the feasibility of sulphate reduction or sulphidogenesis and subsequent metal immobilization due to metal sulphide precipitation was examined in the laboratory before selecting an appropriate remediation technology. Because of the very high metal concentrations in the groundwater and their potential toxic effects on microbial life chemical reagents were also evaluated for immobilization of the metals in situ.Different characteristics were observed for the three sites and therefore individual feasibility tests were performed. Site 1 was characterized by a contamination of Zn (3 to 500 mg/l) to a depth of 130 m-bg. Microcosm tests were used to screen for inducibility of biological activity at two depths, i.e., 30 m-bg (below ground) and 65 m-bg. Different electron donors were selected including pure chemical agents such as lactate and waste products such as molasses and glycerol. Only glycerol resulted in efficient metal and sulphate removal after about 106 days for the undeep aquifer (30 m-bg) and 2 times more for metal immobilization using deep aquifer/groundwater samples (60 m-bg). Extremely high Zn concentrations (up to 2000 mg/l) were found in the groundwater of the second site. Similar laboratory tests as for site 1 were performed, but in addition chemical agents (NaS2 and CaSx) were used. Whereas the sulphide containing chemical agents immediately resulted in low Zn concentrations in the groundwater, it took > 140 days before biological sulphate removal started. Contrary no sulphate was removed using the chemical agents. Glycerol, lactate and molasses resulted in efficient Zn and sulphate removal. Site 3 was characterized by a relative shallow contamination (< 10 m-bg) of mainly Co (30–300 mg/l), and contained typical sulphate concentrations in the range of 300–1200 mg/l. Rapid microbial sulphate reduction (within 50 days) was induced in the tests containing nutrient-amended lactate, cheese whey and soy oil. About 20× more substrate needed to be supplied for cheese whey compared to the other substrates because of its low active substrate (lactose) content (2%).