Accumulation and mobilization of arsenate by Fe(III) polyions trapped in a Ca-polygalacturonate network

The role of Fe(III) stored at the soil–root interface in the accumulation of arsenate and the influence of citric acid on the As(V) mobility were investigated by using Ca-polygalacturonate networks (PGA). The results indicate that in the 2.5–6.2 pH range Fe(III) interacts with As(V) leading to the sorption of As(V) on Fe(III) precipitates or Fe–As coprecipitates. The FT-IR analysis of these precipitates evidenced that the interaction produces Fe(III)–As(V) inner-sphere complexes with either monodentate or bidentate binuclear attachment of As(V) depending on pH. In the 3.0–6.0 pH range, As(V) diffuses freely through the polysaccharidic matrix that was found to exert a negligible reducing action towards As(V). At pH 6.0 citric acid is able to mobilize arsenate from the As–Fe–PGA network through the complexation of the Fe(III) polyions that leads to the release of As(V).