Compaction of peat cover over desulfurized gold mine tailings changes: Arsenic speciation and mobility

Highlights: • Compacted peat cover could decrease As concentration to as low as 11 μg/L with As (V) being the dominant species • Under compacted peat cover, As was bound to crystalline Fe-oxides • Compaction of peat did not decrease dissolved organic carbon • Non-compacted peat cover increased the concentrations of total As with As(III) as the dominant species • As leaching is sensitive to a tight pH-redox potential variation, the stability was found at pH ≤ 6 and Eh ≤ 200 mV Organic covers are increasingly used to control the generation of contaminated mine drainage from tailings impoundment facilities. Their efficiency in controlling the mobility of contaminants in the underlying tailings depends on their biogeochemical and hydrogeological properties. In the present study, fibric peat, an easily accessible material at northern gold mine sites, was evaluated as organic cover to potentially reclaim desulfurized tailings under controlled conditions. Three column testing scenarios were set up: a control of 30 cm tailings (C1), and two of tailings (30 cm) covered with compacted (C2) and non-compacted (C3) peat (100 cm). The columns were rinsed with 2 L of deionized water, in wetting-drying monthly cycles, for 12 months. Results showed that the tailings leached As in the drainage water (0.01–24 mg/L), mainly originating from arsenopyrite (FeAsS) and accessory löllingite (FeAs₂). Non-compacted peat cover increased the concentrations of total As (by 3–88%) and As (III) (by 91–99%) in the leachate despite low dissolved organic carbon concentrations (36–74 mg/L). Compacted peat first increased total As leaching similar to non-compacted peat cover. Then, As concentration decreased to as low as 11 μg/L after 6 months. A slight decrease of pH (≤6) and an increase of redox potential (>200 mV) promoted As stability with the use of a compacted peat cover, As (V) being the dominant species in the leachates. Under compacted peat, mineralogical analysis of the tailings showed that As was associated with Fe-oxides, and parallel extraction confirmed that As was bound to crystalline Fe-oxides. These results highlighted the uncertainty associated with the use of peat covers to reclaim neutral mine tailings bearing As. Biochemical and hydrogeological properties of organic materials are variable and evolve over time. Slight changes proved to have high impacts on mine drainage quality.