Role of caffeic acid on the As(V) sorption by a ferrihydrite coated with a Ca‐polygalacturonate network

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) on amorphous iron(III) hydroxides (ferrihydrite) particles. The scanning electron micrographs (SEM) 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 in the presence of CaPGA, show a lower amount of As(V) sorbed with respect to Fer alone, being lower after 3 and 24 hours of reaction by about 70 and 30%, respectively. This was mainly attributed to a barrier effect exerted by the PGA gel rather than the occlusion of Fer micropores by the polysaccharidic chains. The simultaneous sorption kinetics of As(V) and caffeic acid (CAF) by Fer show that As(V) sorption is almost independent by CAF concentration indicating a higher affinity of arsenate ions towards Fer surfaces. However, the amount of As(V) sorbed by the Fer‐CaPGA, in the presence of 0.25, 0.5 and 1.0 mM CAF, is markedly lower of about 20, 25 and 40 % respectively than that found in As(V)‐CAF‐Fer ternary systems. This suggests a different ability of CAF and As(V) to cross the CaPGA network, although chemical reactions involving CAF and the surficial Fe(III) nuclei of Fer could have a role, as suggested by FT‐IR spectra, in restricting As(V) sorption.