Graphite accelerate dissimilatory iron reduction and vivianite crystal enlargement

Biomineralized vivianite induced by dissimilatory iron reduction bacteria (DIRB) has received increasing attention because it alleviates phosphorus crisis and phosphorus pollution simultaneously. However, the relatively small crystal size and low Fe(III) reduction rate restrict the separation and recovery of vivianite. In this study, graphite was selected as additive to enhance vivianite biomineralization with soluble ferric citrate and insoluble hematite as two representative electron acceptors. As soluble ferric citrate provided abundant accessible electron acceptors, relatively inconspicuous increase (lower than 7%) was observed for graphite on vivianite formation while inoculated with raw sewage or DIRB. In contrast, graphite considerably increased vivianite formation efficiency by 23% in insoluble hematite inoculated with raw sewage. The graphite promotion on vivianite formation in hematite batch was magnified to 70% by DIRB. Dosing hematite inhibited the supply of electron acceptors, while conductive graphite promoted the electrical connection between minerals and DIRB, thus improved the Fe(III) reduction rate and efficiency. In addition, secondary minerals in hematite exhibited a larger aspect ratio and tended to aggregate on graphite. Graphite enlarged the vivianite size in hematite from 10 µm to 90 µm due to aggregation. Enhancing dissimilatory iron reduction (DIR) rate of iron oxides and enlarging crystal size provide new insights for vivianite formation and separation during wastewater treatment.