Fate of adsorbed arsenate during phase transformation of ferrihydrite in the presence of gypsum and alkaline conditions

•We investigated the fate of adsorbed arsenate during phase transformation of ferrihydrite in the presence of Ca2 + and SO42 −.•XRD and RS analyses show that ferrihydrite transforms to hematite after 168 h of aging.•Hematite generated in the presence of As is spindle shaped.•Despite the loss of SA, the aqueous concentrations of As decreased from 1.4 to 0.1 mg/L.•XAS analyses suggest that As is incorporated into the newly formed hematite.

We investigated the fate of adsorbed arsenate during phase transformation of ferrihydrite in the presence of calcium (Ca2 +) and sulfate (SO42 −), a condition frequently encountered in oxic mine tailings, waste rocks, and metallurgical operations. Ferrihydrite transformation under conditions that mimic those present during milling at the Key Lake uranium mine, Canada (i.e., pH, porewater chemistry, Fe/As molar ratio) was examined using batch experiments. Solid samples were analyzed using X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller surface area analyses (BET), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray absorption spectroscopic analyses (XAS). Aqueous samples were analyzed for arsenic (As) and iron (Fe) concentrations using ICP-MS. X-ray diffraction and Raman spectroscopy analyses show that ferrihydrite completely transforms to hematite (with traces of goethite) after 168 h of aging (75 °C and pH ~ 10). Hematite generated in the presence of As is spindle shaped, in contrast to the hexagonal or rhomb shaped morphology in the absence of As. The rate of ferrihydrite transformation is enhanced in the presence of partially dissolved gypsum (possibly due to an ionic strength effect) and the kinetics of transformation follow a logistic decay model. BET analyses suggest that the reactive surface area is reduced by ~ 94% by the end of the batch experiment. Despite the loss of the reactive surface area, the aqueous concentrations of As decreased from 1.4 to 0.1 mg/L during 168 h of aging. XAS analyses suggest the As is incorporated into the newly formed hematite via both bidentate-mononuclear and binuclear corner sharing complexes. Although this transformation is a slow process and could take years to complete, this structural incorporation mechanism could nevertheless be a relevant pathway of As sequestration in the environment.