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 SO₄^2- 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 SO₄^2- 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.