Enhance Fe(III) reduction in sludge fermentation for vivianite formation using exogenous H assisted by activated carbon.

Vivianite, a notable secondary mineral formed through dissimilatory iron reduction (DIR), demonstrates great potential in addressing both eutrophication and phosphorus deficiency. However, the presence of competition for electrons from the methanogenic pathway and the low rates of Fe(III) reduction limit the creation of vivianite. In this research, H was utilized as electron donor assisted by activated carbon (AC) to promote Fe(Ⅲ) reduction with FePO as electron acceptors. The introduction of H and H/AC increased the Fe(III) reduction by 23.8 % and 34.3 %, respectively. Both also increase the rate of vivianite formation. H acted as an electron donor to promote Fe(III) reduction by both direct Fe(III) reduction and homoacetogenesis-acetate reduction pathways. It also suppressed the process of methanogenesis to avoid extra consumption of electrons. AC increased the rate of electron transfer, increased hydrogenase and homoacetogenesis-related enzyme activities, and enriched more Dissimilatory iron reduction bacteria (DIRB). H promoted the up-regulation of Wood-Ljungdahl pathway, TCA cycle and electron transport chain related genes. AC enhanced H capture and functioned as an electron shuttle. These results offer fresh perspectives on promoting Fe(III) reduction to facilitate vivianite formation.