Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca mainly exists on the surface through the outer-sphere complex. Adsorbed Ca entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.