Solubilities of synthetic schwertmannite and ferrihydrite

Schwertmannite, 2-line ferrihydrite, and 6-line ferrihydrite were synthesized at 25°C in the laboratory to determine their solubilities. Chemical and thermal analyses of the synthesized minerals show that schwertmannite has the chemical formula Fe2O3-x(SO4)x·nH2O (0.41 ≤ x ≤ 0.49, 1.51 ≤ n ≤ 2.81) and ferrihydrite Fe2O3-0.5y(OH)y·nH2O (0 ≤ y ≤ 1.96, 0.82 ≤ n ≤ 1.14). The solubility products (K) of the minerals were estimated from the activities of the corresponding species calculated with the computer program PHREEQC. The estimated log K values are 2.01 ± 0.30 for schwertmannite, 8.46 ± 1.40 for 2-line ferrihydrite, and 10.12 ± 0.74 for 6-line ferrihydrite. The solubility of schwertmannite seems to vary depending on the sulfate content, but more investigation is needed to quantify the relationship. The solubility of ferrihydrite does not show any significant correlation with the water content. The stability boundary between schwertmannite and ferrihydrite predicted in this study is biased to higher pH than that observed in nature, which also need further investigation.