The Solubility Of Schwertmannite: The Applicability Of A Solid Solution Model

Schwertmannite has been known to be a ferric oxyhydroxysulfate mineral, having the chemical formula of Fe8O8(OH)(8-2x)(SO4)(x)center dot nH(2)O. The solubility of schwertmannite is absolutely necessary for the interpretation and prediction of the behavior of the dissolved species, including the pollutants, in the aqueous geochemical systems near surface. However, there has been dispute on the solubility of schwertmannite, because the reported solubility values of the mineral have shown considerable discrepancies among the researchers. The purpose of this study is to summarize the solubility values of schwertmannite reported by numerous researchers, discuss the plausible reasons for the discrepancies among the reported values, and check the possibility of interpreting the discrepancies with a solid solution model. The solubility discrepancies can be caused by the differences in the saturation degrees of the solution against schwertmannite, the filtration effects during the sampling, the different thermodynamic data set used in the calculation, the metastability and impurities of the sample, and the chemical compositional difference. The most significant factor among these should be the chemical compositional difference among the samples. The continuous chemical compositional variation of schwertmannite within a certain range suggests that the mineral is a solid solution, and thus its solubility should vary as a of the mole fractions of the end members of the solution. This study examined two different possible combinations of the end members of the schwertmannite solid solution. At present, the thermodynamic data for the end members of any combination are insufficient and it is impossible to calculate the solubility as a function of the mole fractions of the end members based on an appropriate model. However, the solid solution model to estimate the solubility of schwertmannite may be still promising and need more investigation, because the plot of the reported solubilities against x values certainly shows a correlation. Meanwhile, the comparison of the mole numbers of Fe and S calculated from the chemical formula of schwertmannite with those from the analytical results of the real samples indicates that schwertmannite is not a single solid solution but a mixture of various phases. It is deduced from a simple comparison of chemistry, while many researchers are still arguing whether the mineral is really a single solution phase, which may suggest that more detailed research on this subject is also required.