Role of root exudates on the sorption of arsenate by ferrihydrite

Iron oxy-hydroxides in soil are known to have a high affinity for As(V) inorganic species. At the soil-root interface such mineral components are embedded by mucilaginous material which is secreted from continuously growing root cap cells. In order to determine the role of plant mucilages on the As(V) sorption by iron oxy-hydroxides, we layered a Ca-polygalacturonate network (CaPGA) onto amorphous iron(III) hydroxides (ferrihydrite, Fer) particles. The scanning electron micrographs of the CaPGA network coating the ferrihydrite (Fer-CaPGA) show a regular structure with a honeycomb-like pattern where interlacing fibrils form a porous system. FT-IR spectra of Fer-CaPGA indicate that CaPGA fibrils are retained by the surficial Fe(III) nuclei of Fer through electrostatic interactions. The sorption experiments carried out at pH 4.3 and 5.8 show a lower amount of As(V) sorbed by Fer-CaPGA with respect to Fer alone, being lower after 3 and 24 hours of reaction by about of 70 and 30%, respectively. The sorption of As(V) by Fer was also studied in the presence of caffeic acid (CAF), an important root exudate. Simultaneous sorption kinetics show that As(V) sorption by Fer is almost independent by CAF concentration indicating a higher affinity of arsenate ions towards the Fer surfaces. However, the amount of As(V) sorbed by the Fer coated by CaPGA, in the presence of 0.25, 0.5 and 1.0 mM CAF, is markedly lower of about 20, 27 and 40 % respectively than that found in the As(V)-CAF-Fer ternary systems. This is due mainly to chemical reactions involving CAF and the surficial Fe(III) nuclei of Fer leading to the formation of CAF oxidation products which prevent As(V) sorption.