Geochemistry of dissolved aluminum at low pH: Extent and significance of Al-Fe(III) coprecipitation below pH 4.0

This work examines the geochemical behavior of dissolved aluminum in sulfate-rich acidic waters. Our observations were obtained during several years of geochemical and mineralogical research in the San Telmo acidic pit lake and other pit lakes of SW Spain. The work includes scanning and transmission electron microscopy (SEM, TEM) of suspended mineral colloids found in deep lake waters. Energy dispersive spectroscopy (EDS) coupled to scanning and high resolution transmission electron microscopy (STEM, HRTEM) revealed not only the presence and formation of discrete, sub-micron Al solids like alunite, but also the abundance and distribution of Al into Fe(III) phases typical of acid mine drainage, such as schwertmannite and jarosite, at a nanometric resolution. The main conclusion emerging from our work is that the fate and transport of Al at low pH (<4.0) can be largely influenced by adsorption on and/or coprecipitation with both schwertmannite and jarosite. Under the geochemical conditions studied (SO₄^2-=10^-2M, Fe(III)~Al=10^-3M), alunite formation may occur at pH>3.3, as suggested by mineralogical observations and geochemical modelling. Below this pH, and contrary to the extended assumption, Al is not truly conservative, and in the presence of ferric iron, both metals may co-precipitate at a substantial extent to form either particles of Al-rich schwertmannite (containing up to ca. 8at.% Al with [Fe/(Fe+Al)]=0.77) and/or crystals of H₃O⁺- to K⁺-jarosite (containing up to ca. 10at.% Al with [Fe/(Fe+Al)]=0.54). This Al incorporation seems to take place by adsorption on particle surfaces in schwertmannite and by atomic substitution for Fe³⁺ in jarosite. Alunite is also unstable at this low pH range with respect to jarosite, which may lead either to isomorphic transformation and/or to chemically zoned crystals with jarositic rims around previously formed alunite cores. As a whole, the compositional spectrum of the analyzed jarosites and alunites describes a discontinuous, coupled (Al³⁺-Fe³⁺, H₃O⁺-K⁺) solid solution series with an apparent gap at intermediate compositions. However, this gap seems to follow geochemical aspects more than crystallographic factors (i.e., immiscibility). The combination of this macroscopically invisible Al incorporated into Fe(III) solids along with subordinate alunite formation may cause significant Al removal even at very low pH (e.g., 20% decrease in Al concentration in San Telmo at pH<3.1). Furthermore, this Fe(III)-Al co-precipitation may also affect the fate of toxic trace elements like As and Pb.