Effect of temperature and mixture composition on arsenic removal from neutral mine drainage in passive biofilters using iron-rich sludge and peat

In passive biofilters, sorption is the main mechanism of As removal from As-rich neutral mine drainage (As-NMD), but biochemical processes contributing to their performance needs to be better documented. This study aims to evaluate the suitability of two locally available materials (peat and Fe-rich acid mine drainage treatment sludge (AMD-S)) for As-NMD treatment at an operating gold mine located in northern Québec, Canada. First, the efficiency of different mixtures of peat and AMD-S was evaluated. Batch tests were conducted at 22 and 5 °C using synthetic (pH 7.16; 2.0 mg/L As) and real (pH 7.63; 0.91 mg/L As) As-NMD. Results revealed that the P50/AMD-S50 mixture (50 wt% peat +50 wt% AMDS) was the most efficient for As immobilization. Further tests with P50/AMD-S50 showed that As removal was higher for synthetic As-NMD (88 % at 22 °C, 82 % at 5 °C) than for real As-NMD (77 % at 22 °C, 48 % at 5 °C), which was ascribed to residual salinity. P50/AMD-S50 was then used in a lab-scale biofilter system operated in a continuous mode for 3 months (22 °C; hydraulic retention time 1 day). A maximum of As removal efficiency of 98 % was obtained within the first week while As leaching occurred after 27 days. A mixture of aerobic and anaerobic bacteria was observed in the biofilter samples, including Fe-reducers (11 % of total reads), S-oxidizers (6.1 %), SO₄^2--reducers (6.0 %), and As-metabolizing prokaryotes (3.5 %). The remobilization of As from the sorbent remains to be addressed to ensure long-term stability of contaminant treatment systems under real mine site conditions.