Oxidants-assisted sand filter to enhance the simultaneous removals of manganese, iron and ammonia from groundwater: Formation of active MnOx and involved mechanisms.

Oxidants are routinely employed to remove manganese from groundwater deeply, but the conversion pathway of manganese in the process still needed to be explored. In this study, potassium permanganate and sodium hypochlorite were introduced to explore their start-up effect on sand filters in treating high concentration of manganese (1.42-1.94 mg/L). The addition of potassium permanganate would effectively enhance the manganese removal (>97%) and significantly shorten the start-up period (36 days) compared to sodium hypochlorite-added filter (90 days). A good correlation between manganese deposition concentration and manganese removal rate was obtained, which indicated that the removal of manganese in pre-adding oxidants sand filters was dominated by adsorption and auto-catalytic oxidation processes, where δ-MnO played a crucial role and the contribution of bacteria was negligible. The addition of potassium permanganate facilitated the production of MnO and promoted the conversion of Mn(II)-γ-MnO-δ-MnO/todorokite during the 120-day operation. Besides, the residual Mn(II) contributed to converting the freshly generated MnO by pre-adding oxidants into active MnOx. XPS results demonstrated the co-existence system of Mn(II), Mn(III) and Mn(IV) in δ-MnO. The proportion of Mn(III) with high catalytic oxidative activity in potassium permanganate-assisted formed MnOx (57%) was much higher than in sodium hypochlorite-assisted formed MnOx (22%). These findings have practical significance to develop new strategies for rapid, safe and deep removal of manganese.