Enhanced bioreduction of iron and arsenic in sediment by biochar amendment influencing microbial community composition and dissolved organic matter content and composition.

Biochar derived from the pyrolysis at 500 °C with fresh biogas slurry and residue, was conducted to investigate its potential role in mediating the speciation and mobilization of As(V) and Fe(III) from arsenic-contaminated tailing mine sediment, with consideration of the changes in microbial populations and dissolved organic matter (DOM). The reduction of As(V) (10-13%) and Fe(III) (12-17%) were partly in response to biochar abiotically causing desorption and reduction effect, but were predominantly (87-90% and 83-88% for As(V) and Fe(III)) attributed to biochar stimulating biological reduction. The level of As(III) released from sediment upon biochar amendment (656.35±89.25 μg L(-1)) was significantly higher than the level released without biochar amendment (98.06±19.38 μg L(-1)) after 49 days incubation. Although a low level of Fe(II) (0.81±0.07 mg L(-1)) was determined in the solution when amending with biochar, most of released Fe(II) (166.25±40.25 mg L(-1)) was formed as biochar-Fe(II)minerals composite. More importantly, biochar stimulated the DOM bioavailability in association with bacterial activities mediating As(V) and Fe(III) reduction. High-throughput sequencing results indicated biochar application shifted the soil microbial community and increased the relative abundance of As(V)-/Fe(III)-reducing bacteria, mostly Geobacter, Anaeromyxobacter, Desulfosporosinus and Pedobacter. The discovery of biochar-bacteria-DOM consortium may broaden new understanding into speciation and mobilization of metals, which arouses attention to exploit feasible bioremediation for metal-contaminated sediment.