The influence of phospholipids on the structure and composition of ferrihydrite colloids

Organophosphates (OP) are important nutrient components in soils and they readily interact with abundant iron phases. Ferrihydrite (FHY), a ubiquitous iron phase in soils, plays a major role in iron-mediated carbon storage. However, the influence of OP on FHY formation, transformation or stabilization is poorly understood. Here we investigated the effects that glycerol phosphate (GP; model organophosphate compound) has on the structure and properties of synthetic FHY. FHY-GP was synthesized by coprecipitation using variable initial molar P/Fe ratios (0.01 to 0.5). The solids displayed typical 2-line FHY X-ray diffraction pattern and chemical analyses showed that they all had a final P/Fe ratio of no more than 0.2. With increasing initial P/Fe ratio, we observed a sharp decrease in specific surface area from 290 to 3 m2 g-1 with an associated reduction in porosity. Analyses of solid products by small angle X-ray scattering revealed that the at high initial P/Fe ratios the solids formed were cluster with diameters of ~ 1.2 nm. Local structural and bonding environment analysis derived from infrared spectroscopy and Fe K-edge X-ray absorption spectroscopy, showed that with increasing P loading, the bonding configuration gradually transitioned from the initial binary binuclear to monodentate mononuclear geometry, while the average coordination number of edge-shared Fe polyhedral decreased from 3.2 to 1.3 and the corner-shared Fe polyhedral disappeared. These results indicated that during the formation of the FHY-GP, the Fe(III) polymerization was likely impeded by the GP, with an increasing P/Fe ratio strongly affecting the resulting FHY-GP structure.