Geochemical stability of As-rich residues from a 20-year-old passive field biofilter used for neutral mine drainage treatment

Abstract

Arsenic-rich contaminated neutral mine drainage (As-CND) is a major environmental problem frequently faced during gold and silver mining of As-bearing ores. Passive treatment systems, particularly passive biofilters, have proven to be a promising technique for As-CND treatment. However, these systems generate significant quantities of residues at the end of their operational lifespan with variable geochemical stabilities and environmental impacts that involve several parameters (e.g., water quality, treatment mechanism, leaching test, and regulatory criteria). This study evaluated the environmental behaviour of residues from a 20-year-old biofilter installed for the treatment of As-CND at the rehabilitated Wood-Cadillac mine site in northwest Quebec, Canada. Thirty samples of post-treatment residues were collected from five different locations within the system, including two within the inlets, the outlet, and at various depths (0–30 cm, 30–60 cm, and 60–90 cm). Physicochemical and mineralogical characterizations were undertaken to understand the As removal mechanisms in the biofilter. The potential mobility of metals was assessed via static leaching tests including TCLP, SPLP, and FLT_m. The results showed that the residues were characterized by high metal contents (up to 2.3 g/kg of As, 41.1 g/kg of Fe, and 19.5 g/kg of Al), mainly concentrated in the upper layers of the biofilter. Metals were precipitated in the form of oxyhydroxides in the upper and mid layers and as secondary sulfides at the bottom. Arsenic was predominantly removed through sorption onto Fe and Al oxyhydroxides in the upper layers; in contrast, reducing conditions at the bottom layers facilitated the precipitation of amorphous As sulfides (FeAsS and AsS). None of the TCLP and SPLP release results exceeded USEPA guidelines, and the residues could therefore be classified as non-hazardous with no potential risk of groundwater contamination from leachates. However, 66 % of TCLP and 20 % of SPLP concentrations exceeded D019 guidelines for As (0.2 mg/L). Additionally, FLT_m results indicated a high leaching potential for As (>1 mg/L). Consequently, these residues should not be dried or crushed during ultimate disposal. These findings will contribute to improving the management strategy for As-rich residues.