EFFECTIVE PASSIVE TREATMENT OF MINE EFFLUENT USING A PHOSPHATE REACTIVE MEDIUM 1

The Nevada Stewart Mine is an abandoned lead-zinc mine located within the Coeur d'Alene Mining District, Idaho. In 2002, a passive Apatite II TM Treatment System (ATS) was installed by MSE Technology Applications, Inc. (MSE) to treat metal-laden adit discharge (pH values are circum-neutral). The treatment medium consisted of a mixture of fish bone (Apatite II TM ) and gravel. Monthly performance monitoring of the ATS over a 21-month period showed attenuation of cadmium, lead, zinc, iron and manganese. In comparison to the ATS inflow, redox conditions in the outflow were more reduced, with higher calcium, sulfide, nitrogen and phosphorus concentrations. Geochemical modeling using the United States Geological Survey code PHREEQC was conducted to identify the reaction mechanisms responsible for the observed changes in water quality. Modeling results were reviewed in the context of available solid-phase data (i.e., chemical composition and detailed mineralogy) and experience at other sites to identify the most likely attenuation mechanisms. Zinc attenuation was attributed to precipitation as zinc sulfide, subsequently confirmed by mineralogical analysis. Collagen was identified as the most likely source of nitrogen release from the ATS, and was likely responsible for reducing conditions in the effluent. The increase in phosphorus concentrations was attributed to dissolution of the Apatite II TM and subsequent formation of insoluble metal phosphates. Manganese phosphate (MnHPO4) and chloropyromorphite (Pb10(PO4)6Cl2) were identified as possible controls on manganese and lead concentrations, respectively.