The Fate of Arsenic During the Crystallization Process of FeIII Oxyhydroxides: Effect of Reaction Media, Ph Value, and Fe/As Molar Ratio under Relatively Low Arsenic Loading.

Understanding the nature of arsenic (As) adsorbed on FeIII oxyhydroxides, and the subsequent behavior of As during the crystallization process, is critical to predicting its fate in a range of natural and engineered settings. In this work, As adsorbed on FeIII oxyhydroxides formed in the different reaction media at different pH values were characterized with X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), and extended X-ray absorption fine structure spectroscopy (EXAFS) to determine how As is redistributed during the crystallization process. Results showed that at pH 12, a quarter of the added As was still left in the liquid phase with the formation of goethite and hematite as the major and minor product. The concentration of As was found to be the lowest at pH 4 which is independent of the reaction media, indicating the importance of pH value in the crystallization process of the As adsorbed FeIII oxyhydroxides. Under acidic conditions, sulfate and chloride media favored the formation of goethite and hematite, respectively. Arsenic can indeed be incorporated into the structure of the formed goethite at pH 4. The morphology of the formed products changed to rhombus-like particles if both goethite and hematite appeared as the later as the dominant product.