Schwertmannite and hydrobasaluminite: A re-evaluation of their solubility and control on the iron and aluminium concentration in acidic pit lakes Javier Sánchez-España a,⇑ , Iñaki Yusta b , Marta Diez-Ercilla a a Unidad de Mineralogía e Hidrogeoquímica Ambiental (UMHA), Instituto Geológico y Minero de España (IGME), Ríos Rosas, 23, 28003 Madrid, Spain b Unidad de Mineralogía e Hidrogeoquímica Ambiental (UMHA), Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV/EHU), Apdo. 644, E-48080 Bilbao, Spain article info Article history: Received 29 July 2010 Accepted 15 June 2011 Available online 26 June 2011 Editorial handling by R. Fuge abstract 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 schwert- mannite constitutes a major control of Fe 3+ 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 Fe 8 O 8 (SO 4 ) x (OH) y nH 2 O, 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 aque- ous concentration of Fe 3+ up to pH  5. Precipitation of Al 3+ also constitutes an important buffering sys- tem at pH 4.2–4.7 and is controlled by the formation of poorly crystalline hydrobasaluminite (with empirical formula Al 4 (SO 4 )(OH) 10 15H 2 O), which is highly unstable in low relative humidity conditions and tends to dehydrate to basaluminite (Al 4 (SO 4 ) 1.2 (OH) 9.6 9–10H 2 O). Removal rates for Fe 3+ and Al 3+ 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 Fe 3+ , Al 3+ , SO 2 4 and H + in solu- tion, allowed the calculation of ionic activity products (log IAP) and solubility product constants (log K sp ) 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 sec- ond one deduces log K sp values from linear regression lines in ion activity-pH plots. Both methods tend to converge and give log K sp = 18.8 ± 1.7 for schwertmannite, and log K sp = 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, respec- tively, although formation of these latter compounds could not be demonstrated. The present work con- firms the results originally obtained for schwertmannite solubility and reports a reliable solubility product constant for freshly precipitated hydrobasaluminite. Integration of the resulting log K sp 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. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Background and scope A careful characterization of the Fe III and Al compounds (oxy- hydroxides, oxy-hydroxysulfates) that commonly form in aqueous systems affected by mine-related pollution (acid mine drainage, acidic mine pit lakes) is fundamental to understanding many bio- geochemical processes. The most typical minerals are schwert- mannite, jarosite and ferrihydrite in Fe III -bearing systems (hereafter, Fe III denotes total ferric iron, whereas Fe 3+ represents the free aqueous ion) and basaluminite/hydrobasaluminite in Al- bearing systems (Nordstrom and Alpers, 1999; Bigham and Nord- strom, 2000). (Note that basaluminite is a microcrystalline variety of felsöbányaite, Farkas and Pertlik, 1997; Jambor et al., 1998; 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.06.020 ⇑ Corresponding author. E-mail address: j.sanchez@igme.es (J. Sánchez-España). Applied Geochemistry 26 (2011) 1752–1774 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem