Iron mineral transformations and their impact on As (im)mobilization at redox interfaces in As-contaminated aquifers

• Published in: Geochimica et cosmochimica acta Vol. 296; pp. 189 - 209
• Main Authors: Kontny, Agnes, Schneider, Magnus, Eiche, Elisabeth, Stopelli, Emiliano, Glodowska, Martyna, Rathi, Bhasker, Göttlicher, Jörg, Byrne, James M., Kappler, Andreas, Berg, Michael, Thi, Duyen Vu, Trang, Pham T.K., Viet, Pham H., Neumann, Thomas
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2021
• Subjects: As contaminated groundwater; Couples redox reactions; Dissolution and precipitation; Fe minerals in aquifer; Red River (Vietnam); As contaminated groundwater; Dissolution and precipitation; Couples redox reactions; Red River (Vietnam); Fe minerals in aquifer
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2020.12.029

Iron minerals are the most important arsenic host in As-contaminated deltaic sediments. Arsenic release from Fe minerals to groundwater exposes millions of people worldwide to a severe health threat. To understand the coupling of Fe mineralogy with As (im)mobilization dynamics, we analyzed the geochemistry and mineralogy of a 46 m long sediment core drilled into the redox transition zone where a high As Holocene aquifer is juxtaposed to a low As Pleistocene aquifer in the Red River delta, Vietnam. We specifically concentrated on mm- to cm-scale redox interfaces within the sandy aquifer. Various Fe phases, such as Fe- and Mn- bearing carbonates, pyrite, magnetite, hematite and Fe-hydroxides (goethite, lepidocrocite) with distinct As concentrations were identified by a combination of high-resolution microscopic, magnetic and spectroscopic methods. The concentration of As and its redox species in the different Fe-minerals were quantified by microprobe analysis and synchrotron X-ray absorption. We developed a conceptual model integrating Fe-mineral transformations and related As (im)mobilization across the redox interfaces. Accordingly, As is first mobilized via the methanogenic dissolution of Fe(III) (oxyhydr)oxide mineral coatings on sand grains when reducing groundwater from the Holocene aquifer intruded into the Pleistocene sands. This stage is followed by the formation of secondary Fe(II)-containing precipitates (mainly Fe- and Mn-bearing carbonates with relatively low As < 70 μg/g), and minor pyrite (with high As up to 5800 μg/g). Due to small-scale changing redox conditions these Fe(II) minerals dissolve again and the oxidative behavior of residual Fe(III)-phases in contact with the reducing water leads to the formation of abundant Fe(III)/Fe(II) (oxyhydr)oxides especially at the studied redox interfaces. Microcrystalline coatings and cementations of goethite, magnetite and hematite have intermediate to high As sorption capacity (As up to 270 μg/g) creating a key sorbent responsible for As (im)mobilization at interfingering redox fronts. Our observations suggest a dynamic system at the redox interfaces with coupled redox reactions of abiotic and biotic origin on a mm to cm-scale. In a final stage, further reduction creates magnetite with low As sorption capacity as important secondary Fe-mineral remaining in reduced gray Pleistocene aquifer sands while considerable Fe and As is released into the groundwater. The presented redox-dependent sequence of Fe phases at redox interfaces provides new insights of their role in As (im)mobilization in reducing aquifers of south and southeast Asia.