Fe(II)-Induced Mineral Transformation of Ferrihydrite–Organic Matter Adsorption and Co-precipitation Complexes in the Absence and Presence of As(III)

The poorly crystalline ferrihydrite (Fh) is often associated with organic matter (OM) and metal(loid)s, such as arsenic (As). The transformation of Fh to more stable and, hence, less reactive phases is catalyzed by the surface reaction with Fe(II). However, little is known regarding the impact of various specific OM types and the co-existence of OM and As on the secondary mineralization of Fh. Accordingly, we explored the extent and the resulting secondary minerals of Fe(II)-induced transformation of [As(III)-adsorbed] OM–Fh adsorption and co-precipitation complexes, which were synthesized using two types of OM: dissolved organic matter (DOM) extracted from the O horizon of an ultisol and polygalacturonic acid (PGA) as a proxy for polysaccharides. Regardless of OM type, increased contents of the co-precipitated or adsorbed OM led to a decrease in Fh transformation in both the absence and presence of As(III). Adsorbed As(III) caused a decrease in Fh conversion for both pure Fh and OM–Fh complexes. We observed only small differences in Fe(II)-induced transformation of OM–Fh co-precipitates versus adsorptive complexes. However, without adsorbed As(III), OM types strongly influenced the secondary mineral products: DOM impeded goethite (Gt) and stimulated lepidocrocite (Lp) formation, while only Gt was formed for PGA. When As(III) and OM coexists, As(III) favored Lp over Gt formation for both DOM–Fh and PGA–Fh complexes. These findings provide evidence that the mineral evolution of Fh largely depends upon the potential additive/competing influence of coexisting constituents.