Efterbehandling av anrikningssanden i Adak gruvområde – Dispergerat alkaliskt substrat (DAS) som möjlig behandlingsmetod

Adak, a mining area located in Malå municipal in northern Sweden, was abandoned in 1979 and has since been leaching polluted mine drainage from its tailings storage facility (TSF) to the surrounding environment. The tailings consists of sulfidic minerals resulting in acid mine drainage (AMD), which is characterized by its low pH and metal-rich contents. Previous remediation techniques utilized for the TSF has included liming and covering with moraine. As water quality of the drainage remained poor although measures taken, the Swedish Geological Survey (SGU) installed limestone walls in attempt to improve the quality of the outflowing water from the magazine. The limestone walls did however not function as intended and the water quality remained unsatisfactory. Since early 2000s, a method called dispersed alkaline substrate (DAS) has been developed and tested in lab-, pilot- and full scale. The method is based on mixing fine grained reagent with wood shavings, resulting in high contact surface while reducing clogging of the system. The DAS-technology consists of a multi-step treatment typically including natural Fe-oxidizing lagoons (NFOL), limestone-DAS reactors and/or MgO- or BaCO3-DAS reactors. In cooperation with SGU, Geosyntec and Luleå University of Technology, this study was done to explore why the limestone walls at Adak did not function as intended and implement a lab-scale experiment using the DAS method to explore if it could be used to treat the drainage from Adak TSF, in purpose to aid in reducing future environmental effects around the mine site. The goal set by SGU was to at least reduce As and Zn concentrations in the outflowing AMD by 50 %. m-XRF analysis was done to evaluate the material used for the limestone walls. Results showed that material consisted of mostly calcium, most likely in the form of calcite. Two setups, P6in and P6ut, were constructed for the DAS-system. After going through the system, the pH of the AMD increased from an average of 2.65 and 2.74, to an average of 8.01 and 8.17 for P6in and P6ut respectively. Average As concentrations decreased from 150 µg/l and 170 µg/l to 0.87 µg/l and 1.38 µg/l, a reduction by 99.4 % and 99.2 % respectively. Average Zn concentrations decreased from 940 µg/l and 570 µg/l to 113 µg/l and 82 µg/l, a reduction by 88.0 % and 85.6 % respectively. Additionally, average concentrations of Al, Co, Cu, Fe and Mn were decreased by between approximately 94.9 – 100 % and 95.8 – 100 %, while average Ni concentrations increased by 66.7 % and 149.4 %. The material used for the current limestone walls was deemed not to be the cause of the poor efficiency of treatment. Several factors could contribute, although from site inspection, the most likely source of poor treatment is due to surface inhibition by Fe-precipitates. The DAS-system showed high efficiency in respect to treating the As and Zn from the AMD drainage of the TSF at Adak. The additional reduction in concentration for Al, Co, Cu, Fe and Mn proves useful in an ecotoxicological context. Although the increase in Ni concentrations, end concentrations remain within Havs- och vattenmyndigheten legislations regarding classification of environmental quality standards regarding surface waters. Although the promising results, an in-situ pilot study would be vital in order to determine how the system would function in regard to the challenging subarctic climate that northern Sweden provides.