Ferrihydrite Containing Gel for Chemical Imaging of Labile Phosphate Species in Sediments and Soils Using Diffusive Gradients in Thin Films

We report on a novel binding gel for phosphate, based on ferrihydrite, and its use in diffusive gradients in thin films (DGT) for measuring labile phosphate species in waters, sediments, and soils. An existing method of binding layer preparation was modified to overcome potential problems with deterioration of ferrihydrite due to conversion to goethite. The gel was characterized regarding its suitability for conventional DGT measurements as well as for measuring two-dimensional distributions of P with high spatial resolution using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). The effects of pH, ionic strength and storage time of gels on phosphate binding were investigated and the kinetics of binding and the maximum binding capacity were determined. The gel is shown to have a considerably higher P capacity than the conventional ferrihydrite DGT binding layers. LA-ICPMS analysis of DGT standards with P concentrations ranging from 0.088 +/- 0.005 to 4.47 +/- 0.16 mu g cm(-2) resulted in reproducible calibration curves which could be described using a simple power function. We demonstrate that the new gel is well suited for analyzing smallscale changes of P concentrations in soils. Moreover, the gel can be used as an alternative to conventional DGT gels that incorporate powdered ferrihydrite, with improved characteristics for the determination of labile phosphate.