title: Structural incorporation of As5+ into rhomboclase ((H5O2)Fe3+(SO4)2 · 2H2O) and (H3O)Fe(SO4)2.
abstract: Iron sulfates represent an essential sink for the toxic element arsenic in arid and semi-arid mining areas with high evaporation rates. Information about the structural incorporation of As(5+) in iron sulfates, however, remains scarce. Here we present evidence for the heterogeneous substitution of S(6+) by As(5+) in the crystal structure of rhomboclase ((H5O2)Fe(3+)(SO4)2 · 2H2O) and its dehydration product (H3O)Fe(SO4)2. Rhomboclase (Rhc) was synthesized in the presence of As(5+) with molar As/Fe ratios of 0, 0.25, 0.5, 0.75 and 1.0, resulting in As loads of 0.0, 0.93, 1.44, 1.69 and 1.87 wt.%, respectively. The unit cell parameters of Rhc increase from 9.729(6), 18.303(2), and 5.432(1) Å for a, b, and c, to 9.745(9), 18.332(5), and 5.436(8) Å when Rhc is crystallized at a molar As/Fe ratio of 1. Simultaneously, the crystallite size decreased from 304 to 176 nm. In situ dehydration of Rhc to (H3O)Fe(SO4)2, investigated by powder X-ray diffraction, shows that Rhc starts to dehydrate at 76 °C, which is completed at 86 °C. The presence of As(5+) does not impact the start or end temperatures of Rhc dehydration but does accelerate the dehydration. X-ray absorption fine structure spectroscopy (EXAFS) reveals that S(6+), in the Rhc and (H3O)Fe(SO4)2 structure, is replaced by As(5+), while the polymerization of AsO4-tetrahedra and FeO6-octahedra during the formation of (H3O)Fe(SO4)2 results in a strong distortion of the AsO4-tetrahedron.
authors.affiliation: Friedrich-Schiller-University Jena, Institute of Geosciences, Carl-Zeiss-Promenade 10, Jena, 07745, Germany. Electronic address: ralph.bolanz@uni-jena.de.
authors.forename: Ralph M
authors.initials: RM
authors.lastname: Bolanz
authors.affiliation: Karlsruhe Institute of Technology, ANKA Synchrotron Radiation Facility, Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Göttlicher
authors.affiliation: Karlsruhe Institute of Technology, ANKA Synchrotron Radiation Facility, Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Ralph
authors.initials: R
authors.lastname: Steininger
authors.affiliation: Friedrich-Schiller-University Jena, Institute of Geosciences, Carl-Zeiss-Promenade 10, Jena, 07745, Germany.
authors.forename: Arkadiusz
authors.initials: A
authors.lastname: Wieczorek
mesh_terms: D001151:Arsenic / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry*; D008906:Mining; D014874:Water Pollutants, Chemical / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D058085:Iron Compounds; D014874:Water Pollutants, Chemical; D001151:Arsenic
keywords: (H(3)O)Fe(SO(4))(2); Arsenic; Rhomboclase; Structural incorporation