Ageing of synthetic and natural schwertmannites at pH 2–8

The transformation of schwertmannite to geothite was studied by ageing one synthetic and five natural schwertmannites in water at room temperature. Additionally, one synthetic and two natural schwertmannites were kept at variable pH (2, 4, 6 and 8). After one year, only the synthetic sample and one natural schwertmannite had transformed to goethite. However, the oxalate solubility of Fe and trace elements in all the samples decreased, whereas the total Fe/S ratios and specific surface areas of all samples increased. Arsenic and organic matter appeared to suppress the schwertmannite-to-goethite phase transformation. At pH 2, synthetic schwertmannite fully-transformed to goethite, but at pH 4-6 only minor transformation occurred. Depending on pH, many trace elements were released into solution during ageing of the natural schwertmannites. In general, Co, Mn, Zn, and Si were released to solution, whereas As was enriched in the remaining iron oxide fraction. Al was dissolved at pH <4.