Fe(II)-catalyzed Transformation of OM-ferrihydrite Complexes: Impacts on C Dynamics and As Oxidation

Abstract:

Aqueous Fe(II) is known to catalyze the abiotic transformation of ferrihydrite to more stable Fe minerals. While ferrihydrite within most natural environments contains high contents of adsorbed or coprecipitated organic matter (OM), little is known regarding the impact of OM on Fe(II)-catalyzed transformation of ferrihydrite and its consequences for C and metal(oid) dynamics. In this study, we investigated the impacts of adsorbed and coprecipitated OM on the extent and the secondary mineral profiles of Fe(II)-induced ferrihydrite transformation and subsequent C dynamics and As(III) oxidation. The effects of OM types (dissolved organic matter (DOM) vs. polysaccharides) were also compared. Regardless of OM types, both adsorbed and coprecipitated OM resulted in a linear decrease in ferrihydrite transformation with increasing C/Fe ratios. At similar C/Fe ratios, a greater degree of ferrihydrite transformation was observed for the presence of EPS than DOM. Regardless of OM types, the difference in the Fe(II)-catalyzed ferrihydrite was small between adsorbed and coprecipitated OM. DOM impeded goethite and magnetite formation and stimulated lepidocrocite formation, whereas EPS did not alter the secondary Fe minerals formed from transformation of ferrihydrite and goethite was the major secondary Fe minerals in the presence of EPS. Adsorption of As(III) impeded goethite formation and increased the formation of lepidocrcote. The solid-phase C content remained unchanged after reaction, suggesting that OM remains associated with Fe minerals following ferrihydrite transformation to more stable Fe minerals. However, C desorbability by H₂PO₄^- from the resulting Fe minerals following reaction was enhanced implying that Fe(II)-catalyzed transformation of ferrihydrite may decrease the stability of OM in natural environments under moderately reducing conditions. In addition, regardless of DOM or EPS, the presence of OM decreased the degree of As(III) oxidation following Fe(II)-catalyzed transformation of ferrihydrite .