Schwertmannite and hydrobasaluminite: A re-evaluation of their solubility and control on the iron and aluminium concentration in acidic pit lakes

The solubility of schwertmannite and hydrobasaluminite and their control on the concentration of Fe(III) and Al in acid-sulfate aqueous systems has been re-examined through a series of titration experiments with waters from two acidic pit lakes (Cueva de la Mora and San Telmo) from the Iberian Pyrite Belt in SW Spain. The work also includes a microscopic study (SEM–EDS) of natural Fe(III) precipitates found in the water column of both lakes. The microscopic study of natural precipitates confirms that schwertmannite constitutes a major control of Fe3+ concentration in the lakes at pH ∼ 3.0, whereas jarosite may also be abundant at near-surface conditions and is typical of low pH (<2.5). Goethite is eventually found, although it basically results from ageing of less stable precursor phases. The titration study comprised chemical analysis of waters sampled at increments of 0.2 pH units in the pH range 2.6–10.0, along with chemical (ICP-AES, XRF) and mineralogical (XRD, SEM, EDS, TEM) analysis of the resulting precipitates at different pH values (3.5, 4.5, 4.8, 5.2). The experimental results indicate that the precipitation of a highly hydrated schwertmannite (with empirical formula Fe8O8(SO4)x(OH)y·nH2O, where x = 1.4–1.5, y = 5.0–5.2, and n = 13–17) strongly buffers the solutions in the pH range of 3.0–3.5 and appears to control the aqueous concentration of Fe3+ up to pH ∼ 5. Precipitation of Al3+ also constitutes an important buffering system at pH 4.2–4.7 and is controlled by the formation of poorly crystalline hydrobasaluminite (with empirical formula Al4(SO4)(OH)10·15H2O), which is highly unstable in low relative humidity conditions and tends to dehydrate to basaluminite (Al4(SO4)1.2(OH)9.6·9–10H2O). Removal rates for Fe3+ and Al3+ by precipitation of these two minerals approached 96–98% at pH 3.5 and 5.2, respectively. The obtained compositional stoichiometries, along with computation of the activities of Fe3+, Al3+, SO 4 2 - and H+ in solution, allowed the calculation of ionic activity products (log IAP) and solubility product constants (log Ksp) for the precipitating phases. The solubility product constants have been deduced by two independent approaches. The first one averages ionic activity products obtained for a given pH range, whereas the second one deduces log Ksp values from linear regression lines in ion activity-pH plots. Both methods tend to converge and give log Ksp = 18.8 ± 1.7 for schwertmannite, and log Ksp = 23.9 ± 0.7 for hydrobasaluminite. In the pH range 5–9, the results are compatible with additional solubility controls from ferrihydrite and an amorphous to nano-crystalline Al(OH)3 phase on the aqueous concentrations of Fe and Al, respectively, although formation of these latter compounds could not be demonstrated. The present work confirms the results originally obtained for schwertmannite solubility and reports a reliable solubility product constant for freshly precipitated hydrobasaluminite. Integration of the resulting log Ksp values in the MINTEQ.V4 database and the PHREEQCI geochemical modeling program provides a precise description of the geochemical behaviour of Fe and Al in these acidic pit lakes.