Acid mine drainage (AMD), characterized by extreme acidity, high ionic strength, and elevated concentrations of heavy metals, poses significant environmental risks. While soil-bentonites are widely used as backfills in slurry trench cutoff walls, their long-term performance exposed to aggressive AMD remains poorly understood. This study comprehensively evaluates the containment performance of sand-bentonite (S_dB) and silt-bentonite (S_tB) using synthetic AMD (pH=2.4-2.8) through long-term flexible-wall permeameter tests (lasting up to 614 days) coupled with real-time geochemical testing. The results demonstrate that S_tB exhibits: (1) superior chemical compatibility, with only 1.8-3 times increase in hydraulic conductivity versus 7.4-30 times for S_dB; (2) remarkable acid-buffering capacity, maintaining effluent pH within the neutral range for a long test period; and (3) enhanced metal retention, significantly delaying Al³⁺/Fe³⁺ breakthrough compared to S_dB. These performance advantages of silt-bentonite arise from dissolution of calcite in the silt base soil by the acidic solution, which effectively neutralizes H⁺ ions in AMD and promotes precipitation of pore-filling secondary minerals (i.e. gypsum and schwertmannite) and hydrolysis of trivalent cations. The findings highlight the potential of calcite-bearing soil-bentonites as cutoff wall backfills for acid mine drainage containment.

 

chemical compatibility; hydraulic conductivity, acid buffering, breakthrough, precipitation