Role of root exudates on the sorption of arsenate by ferrihydrite

Iron oxy-hydroxides in soil are known to have a large affinity for arsenate (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 As(V) sorption by iron oxy-hydroxides, we layered a calcium (Ca)-polygalacturonate network (CaPGA) onto amorphous iron (Fe) (III) hydroxides (ferrihydrite, Fh) particles. The scanning electron micrographs of the CaPGA network coating the ferrihydrite (Fh-CaPGA) show a regular structure with a honeycomb-like pattern where interlacing fibrils form a porous system. The FT-IR spectra of Fh-CaPGA suggest that CaPGA fibrils are retained by the surface Fe(III) nuclei of Fh through electrostatic interactions. The sorption experiments carried out at pH 4.3 and 5.8 indicated a smaller amount of As(V) sorbed by Fh-CaPGA with respect to Fh alone, being less after three and 24 hours of reaction by about of 70 and 30%, respectively. The sorption of As(V) by Fh was also studied in the presence of caffeic acid (CAF), an important root exudate. Simultaneous sorption kinetics show that As(V) sorption by Fh is almost independent by CAF concentration indicating a greater affinity of arsenate ions towards the Fh surfaces. However, the amount of As(V) sorbed by the Fh coated by CaPGA, in the presence of 0.25, 0.5 and 1.0 mM CAF, is markedly smaller by about 20, 27 and 40%, respectively, than that found in the As(V)-CAF-Fh ternary systems. This is caused mainly by redox reactions involving CAF and the surface Fe(III) nuclei of Fh leading to the formation of CAF oxidation products which prevent As(V) sorption.