To analyze the mechanism of retarded hydrolytic polymerization of Fe(III) and crystallization of the hydrolytic products in the presence of H4SiO4, we measured the changes in the monomeric Fe(III) concentration as a function of pH, H4SiO4 concentration, and aging time, and characterized the hydrolytic reaction products by a combination of methods. The monomeric Fe(III) concentration in solutions with higher H4SiO4 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 FeH3SiO4 2+ 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 H4SiO4 concentration. On the basis of these observations, we concluded that the interfering effect of H4SiO4 on the hydrolytic polymerization involved two mechanisms: 1) the coexistence of H4SiO4 delayed the kinetics of the hydrolytic polymerization of Fe(III) to a certain extent due to the formation of an FeH3SiO4 2+ complex, and 2) H4SiO4 retarded the growth of Fe(III) hydroxides by binding onto the active sites of the hydrolytic products.
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