Fluoride removal efficiencies and mechanism of schwertmannite from KMnO4/MnO2-Fe(II) processes

Fluoride has an adverse effect on both the environment and industrial production. In particular, wastewater from smelting systems containing high concentrations of fluoride is a major cause of fluoride pollution. Based on the characteristics of such wastewater, a targeted and integrated method for removal fluoride using schwertmannite is proposed. The schwertmannite, prepared from the oxidation of Fe(II) by KMnO4 (Ksw) and MnO2(Msw), effectively removed fluoride within 30 min under certain conditions. Under most experimental conditions, the removal efficiency of F ion by Ksw was always 13 % higher than that by Msw, which is attributed to the different concentrations of OH- and SO42- for ion exchange. The calculations showed that the chemical formulas of Ksw and Msw are Fe8O8(OH)(5.42 +/- 0.04)(SO4)(1.29 +/- 0.02) and Fe8O8(OH)(5.28 +/- 0.04)(SO4)(1.36 +/- 0.02), respectively. In the Ksw system, 0.70 mol of OH- and 0.30 mol of SO42- were released per mole of F ions sorbed; those released for the Msw system were 0.69 mol and 0.31, respectively. The results showed that OH- played a primary role in the ion exchange and the schwertmannite showed good practicability for actual industrial wastewater.