Binding Gel Optimization for the Detection of Organomineral Iron Colloids with the Diffusive Gradients in Thin Films Method

Mobile soil colloids are reactive but are difficult to sample from soil in an unbiased way. The use of the diffusive gradients in thin films (DGT) technique is proposed for in situ colloid sampling in undisturbed soil. Here, the first step is presented by the development of a DGT binding layer that enables the accumulation of organomineral iron (Fe) colloids. A high sorption capacity and a high affinity of organic anion binding sites are required to ensure their detection on the gel. The existing 0.1 M ZrO₂ binding gel of in situ precipitated ZrOCl₂ was suggested as a candidate and validated for organomineral Fe colloid accumulation. It was speculated that higher colloid capacity and affinity might be achieved by enhanced concentrations and smaller dimensions of the metal oxide adsorbent particles, which can be obtained by changing the precursor concentration and type. Increasing the ZrOCl₂ concentration had the largest effect on organomineral Fe colloid and phosphate accumulations. Alternative ZrCl₄ and Zr(OC₄H₉)₄ precursors or TiO₂ and Nb₂O₅ adsorbents did neither distinctly enhance the sorption capacity nor the colloid affinity. Incorporating the polyvinylpyrrolidone adsorbent within the hydrogel matrix only marginally enhanced organomineral Fe colloid sorption. Finally, an in situ precipitated, 0.2 M ZrOCl₂-based binding gel with superior sorption properties is put forward as the most promising binding layer for in situ organomineral Fe colloid detection in soil.