Field and Laboratory Evidence for Manganese Redox Cycling Controlling Iron and Arsenic Retention in Household Sand Filters

Abstract

Application of household sand filters (SFs) is widespread in lowincome regions, such as West Bengal, Bangladesh, and Vietnam, for the removal of groundwater contaminants including Fe, Mn, and As. SF operation typically transitions from (oxic) unsaturated conditions to saturated conditions, creating oxygen-limited zones within sand layers. To ensure the safe and effective use of SFs, understanding filter performance and spatiotemporal changes of solidassociated Fe, As, and Mn in different saturation conditions is crucial but remains unknown. Therefore, column experiments were conducted to follow Fe, As, and Mn removal and their distribution and speciation on sand grain surfaces under unsaturated and saturated conditions. On average, 99 ± 0.2, 93 ± 0.7, and 91 ± 8% of Fe(II) aq , As(III) aq , and Mn(II) aq were removed under unsaturated conditions. Under saturated conditions, Fe and As removal remained constant, whereas up to 5 mg/L Mn(II) aq was leached from columns. μXANES analysis showed that solid-associated Fe(III), As(V), and Mn(III)/(IV) dominated in unsaturated sand. However, under saturated conditions, up to 46 and 15% of Fe(III) and As(V) were reduced, and the presence of Mn(II) was confirmed in the anoxic zones. The results suggest that Mn(IV) oxides formed during unsaturated conditions, serving as hosts and oxidants for Fe and As in SFs under reducing conditions.