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 m³, 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.