Effect of Monosilicic Acid on Hydrolytic Polymerization of Fe(III) and Structure of Hydrolytic Products

概要

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To analyze the mechanism of retarded hydrolytic polymerization of Fe(III) and crystallization of the hydrolytic products in the presence of H_4SiO_4,we measured the changes in the monomeric Fe(III) concentration as a function of pH, H_4SiO_4 concentration, and aging time, and characterized the hydrolytic reaction products by a combination of methods. The monomeric Fe(III) concentration in solutions with higher H_4SiO_4 concentrations was significantly higher than that in an Si-free solution but the differences gradually narrowed with aging time. Chemical equilibrium calculation showed that the concentration of the FeH_3SiO^<2+>_4 complex was not high enough to account for the reduced rate of hydrolytic polymerization. Electron microscopy and infrared spectroscopy showed that the hydrolytic products were ferrihydrite or Si-containing ferrihydrite and their particle size decreased with increasing H_4SiO_4 concentration. On the basis of these observations, we concluded that the interfering effect of H_4SiO_4 on the hydrolytic polymerization involved two mechanisms : 1) the coexistence of H_4SiO_4 delayed the kinetics of the hydrolytic polymerization of Fe(III) to a certain extent due to the formation of an FeH_3SiO^<2+>_4 complex, and 2) H_4SiO_4 retarded the growth of Fe(III) hydroxides by binding onto the active sites of the hydrolytic products.
社団法人日本土壌肥料学会の論文