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

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Postma et al., submitted to Geochim. Cosmochim. Acta Here J is the reduction rate (mol s -1 ), k’ the apparent rate constant (s -1 ), mo the initial sum of reactive iron oxides, m the remaining mass at a given time (both in mol). For synthetic Fe-oxides, the exponent varies with the crystal structure and the distribution of the crystal population (Larsen and Postma, 2001). In natural sediments characterizes a reactive continuum of dissolving Fe(III) phases (Postma, 1993). The rate law (Eq. (1) is based on the fraction of undissolved iron oxide (m/mo) but the experimental data yields the dissolved amount, mt that equals mo-m. The difference in the amount of Fe extracted by respectively HCl and ascorbic acid, both at pH 3, was attributed to the reductive dissolution of Fe-oxides and is therefore equal to mt. The data on mt versus time data obtained in this way (Fig. 7) was fitted to the integrated form of Eq. (1) after substitution of mt = mo-m: 1 for 1 : m = m [ 1- (1- k' ( 1- ) t ) 1- ] (2) t 0 Eq. (2) is the same as Eq. (9) of Larsen et al. (2006) but corrected for a typo (the last square parenthesis should be placed at the end of the equation, not before the exponential term). Eq. (2) contains three fit parameters; mo, k’ and . The data for mt versus time and the data fit of Eq. (2) are displayed in Fig. 7. Naturally this procedure could not be followed for the reduced aquifer sediment where the difference between ascorbic acid and HCl extractable Fe was zero. Fitting the data measured for the Red River sand and the oxic aquifer sediment yield more or less the same values for mo and k’ but the in the oxic aquifer sediment is twice as high as in river sand indicating a significant heterogeneity in the pool of iron oxides. The rate constant k’ for the river mud is much higher than for the other samples. 15
Postma et al., submitted to Geochim. Cosmochim. Acta 3.3 Incubation experiments Sediment incubation experiments were carried out in parallel for unamended sediments and sediments with added acetate. The objective of acetate addition was to check whether the availability of reactive organic carbon did limit the rate of the redox processes in the system. The unamended river mud sample (Fig. 8) showed after an initial lag period of a few days an almost linear increase in both Fe 2+ and total As over the entire incubation period of 30 days. A low concentration of As(V) was identified after 13 days, but generally nearly all arsenic is present in solution as As(III). Addition of acetate did not have a great influence on the As and Fe release rates although the Fe release seem to be enhanced somewhat. In the case of the incubated river sand, (Fig. 8) low Fe and As contents appear in solution after five days but there is no further increase in concentration over time. Addition of acetate resulted in the stimulation of both the release of Fe and As. Again most arsenic is present in solution as As(III). The unamended oxidized aquifer sediment (Fig. 9) did not show a significant release of As or Fe over the 50 day incubation period. However, addition of acetate resulted, after an initial lag period of a few days, in a rapid increase in both Fe 2+ and arsenic. About 70% of the arsenic in solution is present as As(III) while the remainder is in the As(V) oxidation state. In the case of the unamended reduced aquifer sediment there is a small but steady increase in As and Fe over the incubation period. Again the addition of acetate resulted in an enhanced release of As and Fe, quite similar to the pattern observed in the oxidized zone, and again with a significant part of the As being present as As(V). 16
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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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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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Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

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