TY - JOUR
T1 - Geochemistry of dissolved aluminum at low pH
T2 - Extent and significance of Al-Fe(III) coprecipitation below pH 4.0
AU - Sánchez-España, Javier
AU - Yusta, Iñaki
AU - Gray, Jennifer
AU - Burgos, William D.
N1 - Funding Information:
This study was funded by different institutions, including the former Spanish Ministry of Science and Innovation–MICINN through research project number CGL2009-09070, the Spanish Ministry of Education, Culture and Sports (Subprograma Estatal de Movilidad, Plan Estatal de Investigación Científica y Técnica y de Innovación 2013–2016), the American Council for International Exchange of Scholars (CIES, US Department of State) through a Fulbright visiting scholar fellowship to JSE, the Pennsylvania State University (PSIEE, Department of Civil and Environmental Engineering, and Materials Research Institute) and Gobierno Vasco (Grupo Invest. IT-762-13). We thank Jesús Reyes for handling the chemical analyses of waters at IGME laboratories, and personnel at the SGIker facilities of the Basque Country University (Ana Martínez and Sergio Fernández) and at the Materials Characterization Laboratory (MCL) of the Materials Research Institute (MRI) in PSU (Key Wang), for their technical support during scanning and transmission electron microscopy studies. We are also grateful to Sarah Cronk and Chris Gorski from the Department of Civil and Environmental Engineering at PSU for the Mossbauer spectra on Fe(III)–Al solids. This manuscript has substantially benefitted from constructive comments by three anonymous reviewers on an earlier version.
Publisher Copyright:
© 2015 Elsevier Ltd.
Copyright:
Copyright 2020 Elsevier B.V., All rights reserved.
PY - 2016/2/15
Y1 - 2016/2/15
N2 - 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 (SO42-=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.
AB - 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 (SO42-=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.
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U2 - 10.1016/j.gca.2015.10.035
DO - 10.1016/j.gca.2015.10.035
M3 - Article
AN - SCOPUS:84951000099
VL - 175
SP - 128
EP - 149
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
ER -