Abstract:
Dump groundwaters in the former East-German lignite-mining district are characterized by high amounts of ferrous iron and sulphate. Both the pyrite weathering products endanger the surface water quality when discharged into lakes. Only the precipitation of both contaminants in the subsurface can prevent the further contamination of surface waters. The two-step process of microbial catalyzed sulphate reduction and iron sulphide precipitation is limited by the low availability of natural organic substances as electron donators. Therefore, a new remediation technique is developed based on the injection of a liquid organic electron donator (methanol) into the contaminated aquifer. The saturated aquifer is used as a bioreactor, where iron monosulphides are precipitated in the groundwater-filled pore space. Column experiments were performed under natural pressure and temperature conditions with natural anoxic groundwater and original sediments to test the remediation technology. The test showed that a complete iron removal (4 mmol/l), even under rather acid conditions (pH 3.8), is possible after having established an active sulphate reducer population. The turnover of the added organic substance with sulphate is complete and the amount of the resulting sulphide controls the effluent pH. In addition, intensified microbial activity triggers the turnover of natural organic substances. Also, natural Fe(III) hydroxides react with the sulphide produced. Considering the long natural retention times (decades), artificially enhanced FeS precipitation is spontaneous, although it shows kinetic behaviour in the range of days. In light of the promising results, the development of a field scale application of this technique is considered to be necessary. It will have to focus on the improved precipitation control of the FeS in the subsurface.
