Phosphorus sorption during iron(II) oxidation in the presence of dissolved silica

This study investigated the interactions of P and Si with ferrous-derived Fe oxides under conditions simulating natural systems. Iron oxides were formed from the oxidation of Fe(II) at pH 7.0 under a CO2/air mixture. Phosphate and/or silicate were either present during the oxidation of Fe(II) or added 24 h after the Fe(II) had oxidized and the solid had formed. Oxidation of Fe(II) in the presence of P resulted in very high initial P sorption densities; however, much of this sorbed P was subsequently released into solution, probably as a result of coagulation and crystallite growth of the Fe oxide. If Si was present as well during Fe(II) oxidation, initial P sorption densities were very high and almost all of the sorbed P remained in the solid phase. The formation of Fe(II)-derived Fe oxide in the presence of Si and P resulted in a very effective sink for P. This solid may have applications in wastewater treatment and important implications for P chemistry in the natural environment. The presence of Si during Fe(II) oxidation at molar ratios of 0.36 Si/Fe or higher promoted the formation of ferrihydrite, an Fe oxide with a high specific surface area and a high concentration of reactive surface sites. Coagulation and crystallite growth of the ferrihydrite were inhibited by the Si, thereby limiting the release of P over time. When P was added 24 h after solid formation, the sorption capacity of the mixed Fe–Si oxide was greater than the pure Fe oxide but, in both cases, sorption densities were much less than when P and Si were both present during solid formation. Competition of Si and P for Fe oxide sorption sites appeared to be less important to P sorption than the influence of Si on Fe oxide mineralogy, at least under the experimental conditions studied.