Inorganic and organic P retention by coprecipitation during ferrous iron oxidation

Phosphorous (P) cycling is often closely coupled to iron (Fe), particularly following mineral weathering or in hydromorphic soils where Fe redox reactions can control the equilibrium between P retention and release. While surface adsorption of P on Fe (hydr)oxides is a well-known and widely studied process, less attention has been devoted to the understanding of P (especially organic P) coprecipitation following Fe(II) oxidative precipitation. In this work we synthesized and followed the kinetics of a series of Fe-P systems with increasing P/Fe ratio, prepared by either surface adsorption on ferrihydrite (Fh) or oxidative coprecipitation of Fe(II) with inorganic phosphate (Pi), inositol hexaphosphate (myoInsP6) or phosphatidylcholine (PC). The obtained materials were characterized for P and Fe contents, specific surface area (SSA), porosity and surface charge. XRD, TEM, XPS and IR techniques were used to investigate their properties. P retention by coprecipitation was generally greater with respect to adsorption, especially at the highest initial P/Fe ratio. Inorganic phosphate caused interference and poisoning of the crystallization process, slowing down Fe(II) oxidation and precipitation rates at low P/Fe ratios, with the formation of nanometric particles and phosphate concentrated on their surface. With increasing P loadings, more aggregated particles with a lower SSA and greater porosity were obtained. Aside from precipitation, the retention mechanisms also involved adsorption and/or inclusion of Pi within the particles. myoInsP6, on the other hand, contributed to accelerating the precipitation of Fe, leading to coprecipitates bearing the organic P compound within the structure, and was retained by precipitation and adsorption mechanisms. Conversely, irrespective of the P/Fe ratio, PC did not influence the rate of Fe(II) oxidation and precipitation due to its hydrophobic character. The prevailing mechanism involved in its retention during coprecipitation was physical retention on the surface, leading to a drastic decrease of SSA and pore volume. Coprecipitation is thus a complex process, involving several mechanisms as a function of the P species and initial P/Fe ratio, and further contributing to the stabilization and selective accumulation of myoInsP6 in soil with respect to other organic P forms.