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

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4 Household sand filtration in the Red River delta Many water treatment facilities worldwide apply aeration and sand filtration by which dissolved iron is removed from the raw water. This treatment also removes As which sorbs to the ferrihydrite formed by the oxidation of Fe(II) after the aeration (Carlson and Schwertmann, 1987; Jessen et al., 2005). The removal process hence utilizes the common co-occurrence of ferrous iron with As in the contaminated groundwater. The most important parameter for the efficiency of the removal process is therefore the ratio of dissolved iron-to-As in the raw water (McNeill and Edwards, 1997; Berg et al., 2006; Hug et al., 2008). The sorption affinity of As for ferrihydrite in natural waters depends on the As oxidation state (III or V), and As(III)-oxidation is therefore an important part of the treatment (Meng et al., 2002; Roberts et al., 2004; Jessen et al., 2005; Hug et al., 2008). In addition, solutes competing for sorption sites on the ferrihydrite surface, especially phosphate, may decrease the removal efficiency (Wilkie and Hering, 1996; Hering et al., 1997; Meng et al., 2000, 2002; Dixit and Hering, 2003; Berg et al., 2006; Hug et al., 2008). Despite a similar geographical and geological setting, the As-affected South East Asian deltas have important differences in the groundwater Fe/As ratio and phosphate concentrations, as shown by Hug et al. (2008). This in turn affects the As removal efficiency of aeration and sand filtration. Generally, the groundwater in the Red River delta has both a high Fe/As molar ratio and a low concentration of phosphate relative to the Bengal and the Mekong deltas (Hug et al., 2008). Hence, generally, a higher As removal efficiency by sand filtration is expected in the Red River delta, relative to the other deltas. Many private tube well owners in rural areas of the Red River delta have built aeration and sand filtration units to remove the iron and its metallic taste (IET and VAST, 2004; Berg et al. 2006; Hug et al., 2008). Interestingly, Berg et al. (2006) showed that As concentrations after treatment were below 50 μg/L in 90% of the studied sand filters, and 40% were below the 10 μg/L WHO guideline. One of the advantages of the treatment method is that no oxidants or other chemical additive is required. In addition, the knowledge of the sand filter construction and maintenance is shared among people. These results indicate a large potential of sand filtration for mitigation of the As problem in rural areas of 21
the Red River delta, which are not supplied by centralized water treatment facilities. Also concentrations of Mn 2+ , which in the Red River delta are often above the 0.4 mg/L WHO guideline value (Agusa et al., 2006; Berg et al., 2006; Jessen et al., 2008) decreased in the sand filters indicating the formation of oxidized Mnprecipitates (Berg et al., 2006). Berg et al. (2006) proposed that a higher As removal efficiency than that predicted from the high proportion of As(III)/As(tot) in the raw water could be due to As(III) oxidation by the Mn. The oxidation of As(III) during the filter passage is likely significant. Complete As(III) oxidation by a surface reaction process was observed in a similar sand filtration unit in Denmark, albeit at a relatively low initial As(III) concentration (Jessen et al., 2005). Moreover, the filter medium in some of the units studied by Berg et al. (2006) contained ‘black sand’ coated by Mn oxides which would be able to rapidly oxidize increased amounts of As(III) (Driehaus et al., 1995; Scott and Morgan, 1995; Manning et al., 2002). However, the composition of the raw water (Fe/As and P), and not the filter medium, was reported to control the As removal efficiency (Berg et al., 2006), suggesting a comparable As(III) oxidation in all of the studied filter media. It should be noted that Mn removal was observed in all filters, suggesting that Mn 2+ release from the ‘black sand’ medium due to the addition of Mn-reducing Fe(II) with the raw water (Postma and Appelo, 2000) is not of concern. Arsenite may also become oxidized by Fe(IV) radicals formed during the Fe(II) oxidation by O2 at neutral pH (Hug and Leupin, 2003; Leupin et al., 2005). Installation of As removal plants (ARPs) to mitigate As in the Bengal delta were initiated in the late 1990s (Hossain et al., 2005, 2006). The installed ARPs each served 200 to 250 users on average (Hossain et al., 2006) and were technically more advanced relative to the Vietnamese household sand filters. However, in a review of the APRs performance, Hossain et al. (2005, 2006) concluded that one out of four ARPs was either not functioning or delivered water with an As concentration above 50 μg/L. In fact, shallow tube wells often contaminated with As continued to be the dominant source of drinking water for the population after an ARP has been installed (Hossain et al., 2005). The overall failure of the ARPs was ascribed in large part to a lack of community participation. In the Red River delta, the socially adopted, low-tech household-based sand filtration unit appears to be more successful (Berg et al., 2006). 22
Page 1 and 2: Groundwater arsenic in the Red Rive
Page 3 and 4: Søren Jessen Groundwater arsenic i
Page 5 and 6: organic matter may prevent As mobil
Page 8 and 9: Resumé Naturligt frigivet arsen (A
Page 10 and 11: efterfølgende forsøgt at opstille
Page 12 and 13: Preface This PhD thesis deals with
Page 14: Table of contents 1 INTRODUCTION: T
Page 17 and 18: The PhD research presented in this
Page 19 and 20: educed form as As(III), which is al
Page 21 and 22: A more complex and intimate relatio
Page 23 and 24: sedimentary As content (Postma et a
Page 25 and 26: (Anawar and Mihaljevi, 2009), and g
Page 28 and 29: 3 Regional As distribution in the R
Page 30 and 31: in large part by changes in the eus
Page 32: In the Red River delta, the Pleisto
Page 38 and 39: 5 The mitigation potential of bank
Page 40: Noteworthy is, that the ratio of Fe
Page 43 and 44: trend in the distribution of the RF
Page 45 and 46: 2008) or the Hach or Merck test kit
Page 47 and 48: Badloe C., Nguyen T. P. T. and Nguy
Page 49 and 50: Driehaus W., Seith R. and Jekel M.
Page 51 and 52: Islam F. S., Boothman C., Gault A.
Page 53 and 54: Mondal D. and Polya D. A. (2008) Ri
Page 55 and 56: A. Ali and Z. Adeel). ITN Centre, B
Page 57 and 58: Ahmed K. M. (2006) A transect of gr
Page 60: Arsenic in groundwater of the Red R
Page 63 and 64: In short, there is a consensus amon
Page 65 and 66: The local geology shows mainly sand
Page 67 and 68: ane concentration and P CO2 values
Page 69 and 70: Fig. 7. The distribution of arsenic
Page 71 and 72: H3AsO3 þ H2O $ H2AsO4 þ 3H þ þ
Page 73 and 74: Fig. 12. The relative composition o
Page 75 and 76: of the Fe-oxides in the upper 3 m c
Page 77 and 78: found that while As(V) coprecipitat
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Page 82 and 83: Controlling geological and hydrogeo
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Here the results of the geological
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sediments. In general, the coarser
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Fig. 5. Thickness of the clay-rich
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a b c The groundwater head distribu
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Table 2 Total As and stable isotope
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delayed yield from the fine grained
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that the situation in Dan Phuong co
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Mathers, S., Zalasiewicz, J., 1999.
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3118 S. Jes sen et al. / Applied Ge
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Postma et al., submitted to Geochim
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The mobility of arsenic in a Red Ri
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Table 1
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