Formation of iron(III) hydroxides from homogeneous solutions

The technique of precipitation from homogeneous solutions was used in this study to simulate the formation of iron(III) hydroxides under conditions similar to those at the redox boundary of natural waters. The technique allowed the precipitation of iron(III) hydroxides to occur, while the precipitant iron(III) was generated very slowly by the oxygenation of iron(II) and, subsequently, hydrolyzed. During the precipitation, pH heterogeneity did not occur locally as was usually observed in the conventional preparation of iron(III) hydroxides in the laboratory (e.g. addition of a base to an acidic solution of iron(III). Thus, the rapid hydrolysis and the extensive polymerization of iron(III) hydroxides were effectively prevented, particularly at the early stage of the precipitation. The iron(III) precipitates formed in this way are more representative of iron(III) hydroxides occurring particularly at the redox boundary of natural waters than those formed by the conventional procedure in the laboratory. The effects of various solutes, such as H4SiO4, H2PO−4 and oxalate which commonly occur in natural waters, on the composition, size and shape of the iron(III) hydroxides formed have also been studied.