Contribution of redox-active properties of compost-derived humic substances in hematite bioreduction

The compost-derived humic substances(HS)can function as electron mediators for promoting hematite bioreduction because of its redox capacity.Humification process can affect redox capacities of compost-derived HS by changing its intrinsic structure.However,the redox properties of compost-derived HS link-ing with hematite bioreduction during composting still remain unclear.Herein,we investigated the redox capacities of compost-derived HS,and assessed the responses of the redox capacities to the hematite bioreduction.The result showed that compost-derived HS(i.e.,humic acids(HA)and fulvic acids(FA))were able to accept electrons from Shewanella oneidensis MR-1,and the electron accepting capacity was increased during composting.Furthermore,it could be functioned as electron mediators for promoting the hematite bioreduction,achieving 1.19-2.15 times compared with the control experience.Not only the aromatic structures(quinone)but also the non-quinone structures such as nitrogen-and sulfur-containing functional moieties were served as the redox-active functional groups of compost-derived HS.Our work proved that the aromatic functional groups and the heteroatom structures(especially N)were important to the hematite bioreduction.This study highlights the redox-active properties of compost-derived HS and its impact on the microbial reduction of iron mineral.Redox capacity of compost-derived HS might mitigate the environmental risk of contaminants when the composting production was added into the contaminated soils as low-cost repair materials.