Biochemical treatment of coal mine drainage in constructed wetlands: Influence of electron donor, biotic–abiotic pathways and microbial diversity

Abstract

The management and treatment of extremely acidic coal mine drainage (CMD) from Northeastern Coalfield (NEC), Assam, India, remains a perpetual environmental challenge. The present work proposes an effective passive bioremediation strategy for highly acidic (pH of 2.2) synthetic CMD simulated to represent mine discharge from the NEC (in mg L–1) (Fe: 100, Al: 25, Mn: 6, Zn: 5, Co: 1, Ni: 1, Cr: 1 and SO42-: 1000–1200), using a horizontal sub-surface flow constructed wetland (HSSF-CW). Gravel was used as the media in HSSF-CW and operated continuously for 218 days, including acclimatization (I–IV) and treatment (V–VI) phases at a hydraulic retention time of 7 days. COD/SO42- ratio was varied as 0.67 in phase I–V and 0.33 in phase VI. High average metal removal efficiency was achieved for Fe (73%), Al (79%), Zn (98%), Co (95%), Ni (99%) and Cr (100%), but Mn (21%) in the treatment phase. In phase V, high sulfate removal efficiency (74%) conforming sulfidogenesis pathway was observed. The prominence of acidophilic sulfate-reducing bacteria (SRB) (Desulfosporosinus meridiei) was revealed. Sulfate reduction by SRB generated alkalinity and increased pH to 5.1–6.8, which assisted in metal retention (as oxides, hydroxides and sulfides precipitates). At lower COD/SO42-, alkalinity decreased due to incomplete sulfate reduction (45%) as COD became the limiting factor, evident from the lower pH and subsequent remobilization of iron and other adsorbed metals in phase VI. This exploratory study recommends the optimization of COD/SO42- and provides an efficient sustainable solution to mitigate CMD pollution.