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Although PO 4 3- is commonly found in association with iron (oxyhydr)oxide, the effect of PO 4 3- on ferrihydrite reduction, mineralogical transformation, and associated As behavior in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this study, batch experiments, together with geochemical, mineralogical, and biological analyses, were conducted to elucidate these processes. The results showed that SRB can reduce ferrihydrite via direct and indirect processes, and PO 4 3- promoted ferrihydrite reduction by supporting SRB growth at low and medium PO 4 3- loadings. However, at high loadings, PO 4 3- stabilized the ferrihydrite. PO 4 3- shifted the transformation of ferrihydrite from magnetite and mackinawite to vivianite, which scavenges As effectively by incorporating As into its particle. In systems with 0.5 mM SO 4 2-, PO 4 3- exerted a weak effect on As mobilization. However, in systems with 10 mM SO 4 2-, substantial amounts of As were released into the solution, and PO 4 3- impacted As behavior strongly. Low PO 4 3- loadings increased the mobilization of As because of the competitive adsorption of PO 4 3- on mackinawite. Medium and high PO 4 3- loadings were beneficial for As immobilization because of the substitution of mackinawite by vivianite. These findings have important implications for understanding the biogeochemistry of iron (oxyhydr)oxide and As behavior in SRB-containing sediments. [Display omitted] • High SO 4 2- (10 mM) resulted in a substantial release of As into the solution. • PO 4 3- complexly affected ferrihydrite reduction and the associated As behavior. • High PO 4 3- loading benefited As immobilization, whereas low PO 4 3- loading did not. • Vivianite scavenged As effectively via incorporating As into its particles. [ABSTRACT FROM AUTHOR]