Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

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Postma et al., submitted to Geochim. Cosmochim. Acta is turned over in incubation experiments and extrapolation towards residence times relevant for field conditions remains problematic. 4.2 Mobilization of arsenic from sediments The identification of goethite and hematite as the only Fe-oxides present in the sediments also places some thermodynamic constraints on the mobilization of arsenic as compared to iron. Figure 11 shows a redox diagram for aqueous arsenic speciation with superimposed lines for Fe(II)/Fe-oxide equilibria drawn for an aqueous Fe 2+ concentration of 0.1 mM which is close to the concentration commonly found in the reduced Holocene aquifers contaminated with arsenic. The aquifers have a near neutral pH and, as shown in Fig. 11, under these conditions As(V) can only be a stable aqueous phase in the presence of ferrihydrite. Since our sediments exclusively contain goethite and hematite, As(III) must be the stable aqueous phase in the groundwater. The observed presence of As(V) in the groundwater at Dan Phuong (Postma et al., 2007) is therefore a distinct non-equilibrium feature as was previously deduced from reactive transport modelling. It also follows from Fig. 11 that during the sequential reduction of As and Fe species, the reduction of As(V) to As(III) thermodynamically should proceed before the reduction of either goethite or hematite to Fe(II), while in the presence of ferrihydrite, the two processes may take place more or less simultaneously. With goethite and hematite there is a kinetic complication when As(V) is contained in the Fe- oxides because iron reduction has to precede the energetically more favourable step of reducing As(V) to As(III). Once As(V) is liberated from the Fe-oxide crystal lattice it will be reduced and as the result As(III) and Fe 2+ will be released simultaneously during the reductive dissolution of goethite or hematite containing As(V). However, if the Fe 21
Postma et al., submitted to Geochim. Cosmochim. Acta and As reduction processes are uncoupled, i.e. As(V) is adsorbed on the surface of the iron oxide, then in the presence of goethite or hematite, As(III) should be produced before the Fe-oxides are reduced. A further complication is the possible adsorption of part of the produced As(III) or Fe(II) as well as the desorption of As(III) and As(V) as the Fe oxide surface area diminishes during reduction (Tufano et al., 2008; Pedersen et al., 2006). The incubated river sediments and oxidized aquifer sediment show a concomitant release of As and Fe (Figs. 8-9), corresponding to the situation where a goethite or a hematite containing As is being reduced in a coupled process. A simultaneous appearance of As and Fe in solution during aquifer sediment incubation was also reported by van Geen et al. (2004), Gault et al. (2005) and Anawar et al. (2006). In contrast, Radloff et al. (2007) reported results from incubation of Holocene aquifer sediments where the release of As started before the release of Fe, corresponding to the situation where As(V) reduction is uncoupled and starts before the reduction of a Fe-oxide phase with a stability in the range goethite to hematite. Reversely, Islam et al. (2004) reported results from aquifer sediment incubation experiments showing the release of Fe(II) before that of As(III) and proposed a decoupling of the processes of reduction for Fe(III) and As(V). This release sequence during reduction only is possible in the presence of highly unstable iron oxides like ferrihydrite. Clearly different sediments will during reduction yield dissimilar As/Fe release patterns depending on both the Fe-oxide mineralogy and the arsenic speciation in the solid phase. Leaching of the sediment with ascorbic acid is expected to preserve the As(V)/As(III) speciation in the iron oxides (Pedersen et al., 2006; Jung and Zheng et al., 22
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Groundwater arsenic in the Red Rive
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Søren Jessen Groundwater arsenic i
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organic matter may prevent As mobil
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Resumé Naturligt frigivet arsen (A
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efterfølgende forsøgt at opstille
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Preface This PhD thesis deals with
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Table of contents 1 INTRODUCTION: T
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The PhD research presented in this
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educed form as As(III), which is al
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A more complex and intimate relatio
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sedimentary As content (Postma et a
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(Anawar and Mihaljevi, 2009), and g
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3 Regional As distribution in the R
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in large part by changes in the eus
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In the Red River delta, the Pleisto
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the Red River delta, which are not
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5 The mitigation potential of bank
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Noteworthy is, that the ratio of Fe
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trend in the distribution of the RF
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2008) or the Hach or Merck test kit
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Badloe C., Nguyen T. P. T. and Nguy
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Driehaus W., Seith R. and Jekel M.
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Islam F. S., Boothman C., Gault A.
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Mondal D. and Polya D. A. (2008) Ri
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A. Ali and Z. Adeel). ITN Centre, B
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Ahmed K. M. (2006) A transect of gr
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Arsenic in groundwater of the Red R
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In short, there is a consensus amon
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The local geology shows mainly sand
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ane concentration and P CO2 values
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Fig. 7. The distribution of arsenic
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H3AsO3 þ H2O $ H2AsO4 þ 3H þ þ
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Fig. 12. The relative composition o
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of the Fe-oxides in the upper 3 m c
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found that while As(V) coprecipitat
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desh: further evidence of decouplin
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Controlling geological and hydrogeo
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Here the results of the geological
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Jessen et al., manuscript to be sub
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Table 1
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Fig. 1
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Fig. 3
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Fig. 5
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Fig. 7
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Fig. 9
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Fig. 11
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