Characterization of Arsenate-for-sulfate Substitution in Synthetic Jarosite Using X-ray Diffraction and X-ray Absorption Spectroscopy

Arsenic-bearing synthetic jarosite [KFe3(SO4)2(OH)6] was characterized to elucidate the form of the arsenic. The XANES spectra indicate that the As occurs exclusively as As5+, nominally as AsO4, consistent with the valence state of the arsenic in the reagents used for the syntheses. X-ray-diffraction analyses show that at least 9.9 wt.% AsO4 can be structurally incorporated in the jarosite. Cell refinements show that both a and c increase slightly as the proportion of AsO4 increases. The EXAFS spectra indicate that the As–O interatomic distances are uniform at 1.68 Å, and that the coordination numbers, 4.6 ± 1.7 to 5.4 ± 1.7, are indicative of tetrahedral coordination of the oxygen atoms around the central arsenic atom. The As–Fe interatomic distances are approximately 3.26 Å, with the coordination numbers being around 2, confirming that AsO4 substitution for SO4 occurs in synthetic jarosite. The larger arsenate ions seem to be accommodated in the tetrahedral sites by the expansion of the unit cell and by deficiencies in adjacent Fe–O(OH) octahedral sites. The observed increases in the unit-cell parameter c with increasing arsenate-for-sulfate substitution are similar to those of selenate and chromate substitutions for sulfate in jarosite. We believe that the Fe occupancy affects the amount of arsenate substitution, which is limited to about 17 mole % AsO4/(AsO4 + SO4). We contend that the charge imbalance caused by the substitution of arsenate is compensated by the partial protonation of the AsO4 and SO4, which would be consistent with the ionic species actually present in the synthesis media.