Performance of field-pilot passive biofilters using peat and Fe-rich sludge to remove arsenic from neutral mine drainage under a subarctic climate.

Effective mitigation of As-rich neutral mine drainage (As-NMD) is critical for responsible mine site rehabilitation. This study evaluated the performance of pilot-scale field biofilters for As removal from As-NMD. Two duplicate biofilters were filled with a mixture of organic material (peat) and Fe-rich sludge from acid mine drainage treatment, and set-up at an active gold mine in northern Québec, Canada. The biofilters, designed with an upward vertical flow and a volume of 1 m, were operated for three months with a 1-day hydraulic retention time. An As removal efficiency of 96 ± 2.9 % was maintained for 3 months, with final As concentrations of 0.04 ± 0.02 mg/L, in compliance with regulations. At the onset of the test, a first flush effect was noticed, resulting in the leaching of As, Fe, and dissolved organic carbon. However, the performance of biofilters stabilized within 5-10 days, with As removal primarily driven by sorption to Fe-(oxy)hydroxides and organic matter. The presence of neutrophilic prokaryotes that catalyze transformations of S and As at circumneutral pH indicated the occurrence of complex biogeochemical cycling. The mean abundance of As-metabolizers was 4.6 % of total reads in the biofilter samples, while sulfur-oxidizers (highly dominated by Thiobacillus spp.) and sulfate reducers accounted for 5.6 %, and 4.3 %, respectively. While the design of the pilot-scale field biofilter showed promising results for As removal, further studies are necessary to assess its long-term performance including the influence of fluctuations in temperature, hydraulic conductivity, and mineral phase stability.