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

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.