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 Just Accepted

This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.

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

Andreas Voegelin, Ralf Kaegi, Michael Berg, Katja Nitzsche, Andreas Kappler, Vi Lan, Pham Trang, Joerg Goettlicher, Ralph Steininger


Household sand filters are widely used in Vietnam to remove As, Fe and Mn from anoxic groundwater used as drinking water resource. To expand the mechanistic knowledge on filter functioning, we investigated the bulk and micrometer-scale distribution of Fe, As, P, and Mn and the speciation of Fe, Mn and As in a sand filter after eight years of operation using bulk and micro-focused X-ray fluorescence spectrometry (XRF) and X-ray absorption spectroscopy (XAS) and scanning electron microscopy coupled with an energy dispersive X-ray detection system (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(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 micrometer-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-catalyzed Fe(II) oxidation, and mediated As(III) co-oxidation. In contrast, Mn is retained in separate Mn(IV/III)-(oxyhydr)oxide coatings and concretions, probably as a result of coupled surface-catalyzed and microbial Mn(II) oxidation. Silicate sorbed to the ferrihydrite-like Fe(III)-coatings inhibits their crystallisation and associated remobilization of P and As. The periodic drainage and aeration of the filter promote the oxidation of any residual Fe(II) and As(III) and the formation of dense Fe precipitates and thereby contribute to effective filter operation over several years.

EN14011  Accepted 05 May 2014
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