Optimisation of the operational parameters for a comprehensive bioelectrochemical treatment of acid mine drainage

Bioelectrochemical systems provide a promising tool for the treatment of acid mine drainage (AMD). Biological sulphate reduction powered with electrical energy consumes acidity and produces sulphide, which can precipitate metals. However, the produced sulphide and the changes in pH resulting from the biological processes affect the efficiency and the environmental impacts of this treatment significantly. In this work, the effects of pH and sulphur speciation on the sulphate reduction rate (SRR) and comprehensive AMD treatment were evaluated in two-chamber microbial electrolysis cells at a cathode potential of -0.8 V vs. NHE. The increase of initial sulphate concentration from below 1000 mg to above 1500 mg S-SO₄^2-/L increased SRR from 121 ± 25 to 177 ± 19 mg S-SO₄^2-/L/d. SRR further increased to 347 mg S-SO₄^2-/L/d when the operation mode was changed from batch to periodical addition of sulphate and acidity (363 mg S-SO₄^2-/L/d and 22.6 mmol H⁺/L/d, respectively). The average SRR remained above 150 mg S-SO₄^2-/L/d even at pH above 8.5 and with the total dissolved sulphide concentration increasing above 1300 mg S-TDSu/L. Operation at pH above 8 enabled the recovery of over 90% of the sulphur as dissolved sulphide and thus assisted in minimising the formation and release of toxic H₂S.