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- Title
Solid-phase characterisation of an effective household sand filter for As, Fe and Mn removal from groundwater in Vietnam.
- Authors
Voegelin, Andreas; Kaegi, Ralf; Berg, Michael; Nitzsche, Katja Sonja; Kappler, Andreas; Vi Mai Ian; Pham Thi Kim Trang; Cöttlicher, Jörg; Steininger, Ralph
- 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 Fe[sup III]-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 30 min suggest that their uptake is governed by a combination of fast sorption reactions, surface-catalysed Fe[sup II] oxidation and mediated As[sup III] co-oxidation. In contrast, Mn is retained in separate Mn[sup IV/III]-(oxyhydr)oxide coatings and concretions, probably as a result of coupled surface-catalysed and microbial Mn[sup II] oxidation. Silicate sorbed to the ferrihydrite-like Fe[sup III]-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 Fe[sup II] and As[sup III] and the formation of dense Fe precipitates and may thereby contribute to effective filter operation over several years.
- Publication
Environmental Chemistry (14482517), 2014, Vol 11, Issue 5, p566
- ISSN
1448-2517
- Publication type
Article
- DOI
10.1071/EN14011