Solid-phase characterisation of an effective household sand filter for As, Fe and Mn removal from groundwater in Vietnam

Abstract

Environmental context Household sand filters are widely used in Vietnam to remove As, Fe and Mn from groundwater used as drinking water. From the solid-phase characterisation of a sand filter that has been used for 8 years, we conclude that As and Fe are retained by a combination of fast sorption reactions, surface-catalysed Fe oxidation and mediated As co-oxidation and that microbial processes are probably involved in effective Mn retention. This study contributes to a better mechanistic understanding of filter functioning as a basis for further improvements in filter design and operation. Abstract Household sand filters are widely used in Vietnam to remove As, Fe and Mn from anoxic groundwater used as a drinking water resource. To expand the mechanistic knowledge of the filter functioning, we investigated the bulk and micrometre-scale distribution of Fe, As, P and Mn and the speciation of Fe, Mn and As in a sand filter after 8 years of operation using bulk and micro-focussed X-ray fluorescence spectrometry (XRF) and X-ray absorption spectroscopy (XAS) and scanning electron microscopy coupled with energy dispersive X-ray detection (SEM-EDX). Effective oxygenation of the anoxic groundwater enables the oxidative removal of Fe, As and Mn in the filter sand. Our results show that Fe is retained in the filter as a 2-line ferrihydrite-like FeIII-precipitate that coats sand grains, and that As accumulates dominantly as pentavalent arsenate. The very close spatial correlation of accumulated As and P with Fe throughout the filter sand and down to the micrometre-scale and the effective Fe, P and As retention at an estimated average water residence time of only 30min suggest that their uptake is governed by a combination of fast sorption reactions, surface-catalysed FeII oxidation and mediated AsIII co-oxidation. In contrast, Mn is retained in separate MnIV/III-(oxyhydr)oxide coatings and concretions, probably as a result of coupled surface-catalysed and microbial MnII oxidation. Silicate sorbed to the ferrihydrite-like FeIII-coatings inhibits their crystallisation and associated remobilisation of P and As. The periodic drainage and aeration of the filter favours the oxidation of any residual FeII and AsIII and the formation of dense Fe precipitates and may thereby contribute to effective filter operation over several years.