Structural incorporation of As5+ into rhomboclase ((H5O2)Fe3+(SO4)2·2H2O) and (H3O)Fe(SO4)2

•Rhomboclase can incorporate As5+ by substitution of S6+.•Incorporated As5+ does not impact the dehydration temperature of rhomboclase.•As5+ remains within the (H3O)Fe(SO4)2 structure after rhomboclase dehydration.•Polymerization of AsO4 and FeO6 in (H3O)Fe(SO4)2 leads to AsO4 distortion.

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 As5+ in iron sulfates, however, remains scarce. Here we present evidence for the heterogeneous substitution of S6+ by As5+ in the crystal structure of rhomboclase ((H5O2)Fe3+(SO4)2·2H2O) and its dehydration product (H3O)Fe(SO4)2. Rhomboclase (Rhc) was synthesized in the presence of As5+ 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 As5+ 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 S6+, in the Rhc and (H3O)Fe(SO4)2 structure, is replaced by As5+, 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.