Properties of synthetic monosulfate as a novel material for arsenic removal

Monosulfate was examined as a novel material for As(V) removal since its layered double hydroxide structure was expected to possess a high capacity for anion exchange. Phase-pure monosulfate was synthesized by hydration at 80-90°C for 36 h using a stoichiometric mixture of tricalcium aluminate (calcined at 1300°C) and gypsum. The analyses of PXRD, WDXRF, and FE-SEM confirmed the successful synthesis of highly pure monosulfate with a negligible impurity of ettringite. Batch experiments were carried out to investigate the kinetics of As(V) removal by monosulfate. A close relationship between As(V) uptake and sulfate release was observed. The intercalation of arsenate in the interlayer of monosulfate was confirmed by PXRD and FT-IR analyses. From a series of equilibrium batch experiments, it is seen that initial sorption of As(V) on monosulfate follows Langmuir isotherm, whereas further injection of As(V) caused transformation of monosulfate to ettringite, which was confirmed by FE-SEM micrographs. However, after the transformation, the solid phases in the equilibrium experiments were found to significantly lose their ability to take up As(V) in exchange for sulfate. A possible explanation for this result was hypothesized and discussed in the context of the literature.