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

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• Elimination of high acidity and generation of alkalinity (62.8–191 mg L –1 CaCO 3 ). • High sulfate removal efficiency (74%) via sulfidogenesis at COD/ S O 4 2 - of 0.67. • Remobilization of precipitated Fe and Al occurred at low COD/ S O 4 2 - of 0.33. • Metal-stress caused decrease in total chlorophyll from 4.70 to 1.90–2.96 mg g −1 FW. • Dissimilatory sulfate reduction by acidophilic SRB ( Desulfosporosinus meridiei ). 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 S O 4 2 - : 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/ S O 4 2 - 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/ S O 4 2 - , 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/ S O 4 2 - and provides an efficient sustainable solution to mitigate CMD pollution.