Enhanced removal of manganese in organic-rich surface water by combined sodium hypochlorite and potassium permanganate during drinking water treatment

Abstract

High levels of manganese (Mn) are known to occur in ground waters and some organic-rich surface waters, and are sometimes in a form (e.g. organically-bound) that is difficult to remove during conventional drinking water treatment. In this study the potential benefits of combining permanganate and chlorine prior to coagulation for Mn removal have been investigated, with particular reference to an organic-rich surface water (river Bajiang, China). The respective roles and potential synergy of permanganate and chlorine when applied together were considered by comparing the removal of Mn with the chemicals together and separately, using samples of river water and model organic-Mn solutions (humic acid and EDTA). In addition, the significance of the order of NaClO and KMnO₄ dosing, and the influence of coagulant dose have been evaluated. The results have shown that the combination of the two chemicals is beneficial and synergistic. For river water containing 0.22 mg L⁻¹ Mn, a dose of 1.76 mg L⁻¹ NaClO reduced the half dose of the permanganate required to achieve the drinking water target concentration of 0.05 mg L⁻¹ Mn. The addition of chlorine appears to enhance the release of bound-Mn and the subsequent conversion of Mn(II) to insoluble Mn(IV). The mechanisms responsible are believed to involve chlorine-assisted autocatalytic Mn oxidation and MnO₄⁻ recycling.