Arsenic removal by household sand filters

Abstract

Household sand filters have been widely used for removing groundwater contaminants such as As(III), Fe(II), Mn(II), and NH4+. This method has been particularly popular in the Red River Delta region, Vietnam, where millions must rely on filtered water for drinking and cleaning. The mechanisms involved in arsenic removal in these filters are complex, involving a combination of abiotic and biotic processes. However, there is insufficient research on the stability, effectiveness, and environmental implications of using household sand filters to remove As and co-contaminants like Fe, Mn, and NH4+. Therefore, this study aims to enhance our understanding of the various mechanisms that regulate the retention of As in household sand filters and to provide initial insights into the potential environmental risks associated with the open disposal of filter materials. The findings of this project have led to several key conclusions: i) The high removal efficiency of Fe, Mn, and As can be attributed to the complementary action of microbial processes. Core microbial communities in the filters played a key role in the oxidation of Fe(II), Mn(II), NH4+, and NO2-, which correlated with the functional processes in sand filters. ii) Household sand filters typically operate through a cycle of unsaturated to saturated flow, which starts with the fully oxic (unsaturated) flow and following by saturated conditions in which semi-oxic to anoxic zones are formed within the sand layers. Under unsaturated conditions, the removal rates for Fe(II), As(III), and Mn(II) exceeded 91%, and solid-associated Fe(III), As(V), and Mn(III)/(IV) dominated in the sand materials. Under saturated conditions, Fe and As removal rates remained high (99% and 95%, respectively), while a significant amount of dissolved Mn(II) was leached from the sand column. Up to 46% of Fe(III) and 15% of As(V) were reduced in the anoxic zone, with the presence of reduced Mn(II) confirmed. The observations indicated that Mn oxides formed during the unsaturated conditions acted as a secondary hosting phase and oxidant for both Fe(II) and As(III)/(V) under saturated conditions. iii) Disposing sand filter materials into soils was found to mobilize As from the sand materials into the pore water under reducing conditions. Microcosm experiments demonstrated that microbially-mediated Fe(III)/As(V) reduction resulted in the mobilization of 210 μg/L of As in the mixture of sand materials with soil. In the solid phase, up to 10% Fe(III) and 32% As(V) were reduced in the mixture microcosm, mimicking the interaction of filter materials with the soil. Furthermore, 77-100% of Fe and As concentrations in the colloidal fraction rapidly decreased upon the onset of reducing conditions. Results suggest that As mobilization as colloids is significant, particularly in the early stages of reducing conditions, but becomes less important over time. In summary, this Ph.D. thesis has advanced our understanding of the role of microbial processes and manganese cycling in removing As, Fe, Mn, and NH4+ in household sand filters. It has also underscored the potential risks associated with the open disposal of filter materials in the environment. The findings of this thesis can serve as valuable guidelines for the safe and sustainable use of household sand filters.