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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]

Iron (Fe) oxides and fulvic acid (FA) are the key components affecting the fate of cadmium (Cd) in soil. The presence of FA influences Fe mineral transformation, and FA may complicate phase transformation and dynamic behavior of Cd. How varying Fe minerals and FA affect Cd immobilization during the ferrihydrite transformation induced by various Fe(II) concentrations, however, is still lack of quantitative understanding. In this study, we built a model for Cd species quantification during phase transformation based on mechanistic insights obtained from batch experiments. Spectroscopic analysis showed that Fe(II) concentrations affected secondary Fe minerals formation under the condition of co-existence of Cd and FA, and ultimately changed the distribution of Cd and FA. Microscopic analysis revealed that besides surface adsorption, part of Cd was sequestrated by magnetite, whereas FA was able to diffuse into lepidocrocite defects. The model revealed that adsorbed Cd was mainly controlled by FA and ferrihydrite, and direct complexation of Cd by FA had a strong impact on the continuous change in Cd at lower Fe(II) concentration. The results contribute to an in-depth understanding of the mobility of Cd in the environment and provide a method for quantifying the dynamic behavior of heavy metals in multi-reactant systems. [Display omitted] • High concentration of Fe(II) is beneficial to the Cd incorporation. • Cd distribution was quantitatively assessed by an integrated surface complexation model. • Adsorbed Cd was mainly controlled by fulvic acid and ferrihydrite. • Quantitative mode can assess the roles of each reactant for Cd adsorption. [ABSTRACT FROM AUTHOR]