Subsurface Iron and Arsenic Removal

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2 Subsurface iron removal in the Netherlands [SO4] in mmol.L -1 0.7 0.6 0.5 0.4 stop injection 0 20 40 60 80 abstracted volume (x10,000m 3 ) Figure 2.8 Sulphate measurements during and after the operation of subsurface iron removal stopped (Figure 2.8). The measurements show a slight increase in sulphate during SIR, from 0.5 mmol.L -1 at the end of a cycle to 0.6 mmol.L -1 , indicating the occurrence of sulphate releasing processes, such as pyrite oxidation. Once SIR had stopped, sulphate concentrations slowly reduced to their original value. When combining this information with the observed mobilization of arsenic, it may be assumed that also some pyrite-bound arsenic or arsenopyrite (FeAs S ) was present in the targeted aquifer. It should be noted that arsenic concentrations did not exceed 0.2 µmol.L -1 and after dry biosand filtration the concentrations were always below the provisional guideline of the World Health Organization (10 µg.L -1 ; 2006). Although iron is released from the pyrite as well, it is immediately oxidized and immobilized and will therefore not be found in elevated concentrations in the abstracted water. The pyrite oxidation process, however, limits the iron removal efficacy, as part of the oxygen is consumed for oxidation of disulfide (Equation 2.4). Co-removal of manganese, phosphate and arsenic Apart from subsurface iron removal subsurface manganese removal has also been operated in the Netherlands and other European countries (Braester and Martinell, 1988; van Beek 1983) and, more recently, in Egypt (Olsthoorn, 2000). Van Beek (1983) reported V/Vi efficiencies for manganese removal of 3.5 and 12, and indicated that operation for manganese removal is more sensitive to operational variations than iron. Abiotic Mn 2+ oxidation is a slower reaction than Fe 2+ oxidation and will start once Fe 2+ oxidation is more [Mn] mmol.L -1 0.025 0.020 0.015 0.010 0.005 0.000 0 5 10 15 V/Vi Cycle 1 Cycle 7 Cycle 22 Figure 2.9 Manganese measurements during cycle 1, 7 and 22 at WTP Lekkerkerk 31 2
Subsurface iron and arsenic removal for drinking water treatment in Bangladesh 2 [PO4 3- ] mmol.L -1 0.05 0.04 0.03 0.02 0.01 0.00 Cycle 1 Cycle 7 Cycle 22 0 5 10 15 V/Vi Figure 2.10 Phosphate measurements during cycle 1, 7 and 22 at WTP Lekkerkerk [As] µmol.L -1 0.20 0.15 0.10 0.05 0.00 0 5 10 15 V/Vi Cycle 1 Cycle 7 Cycle 22 Figure 2.11 Arsenic measurements during cycle 1, 7 and 22 at WTP Lekkerkerk or less complete. Mn 4+ precipitates are found closer to the subsurface treatment well than Fe 3+ precipitates (Hallberg and Martinell, 1976; Rott, 1985; van Beek 1985). The separation between iron and manganese deposits has also been observed in the dry filters at WTP Lekkerkerk (de Vet, 2011). In a completely different setting, namely in oxygen-diffusion around rice plants, the same Fe-Mn separation was found (Frommer et al., 2011). The manganese measurements at WTP Lekkerkerk during cycle 1, 7 and 22 are shown in Figure 2.9. Removal efficiencies increased slightly after multiple cycles, but not as distinctive as for iron. Anion co-removal during SIR has been reported in the literature before (Rott et al., 2002, Appelo and de Vet, 2003); the measurements for phosphate and arsenic are depicted in Figure 2.10 and Figure 2.11, respectively. Phosphate behaviour correlates well to the removal efficacy of iron, showing significant lower concentrations during cycle 22 than cycle 1 and 7. For arsenic the breakthrough curves are not as uniform, and during the first cycles arsenic concentrations do not seem to be very stable. It even seems as if arsenic is mobilized during cycle 1 and 7, whereas that process does not seem relevant anymore during cycle 22. An explanation for the release of arsenic during early cycles may be that the oxidized pyrite was arsenic-bearing, resulting in elevated arsenic concentrations in solution. This has also been observed during an experiment with injection of aerated water into a deep tube well at the nearby located Langerak (Wallis et al., 2010). Contaminant remobilization Fe 3+ oxides will accumulate in the aquifer around a tube well when subsurface iron removal is operated for several years (Mettler et al., 2001; van Halem et al., 2011). Although clogging of the aquifer has not been reported, 32
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6 Characterization of accumulated d
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7 Catalysis by accumulated deposits
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8 Influence of groundwater composit
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Concluding remarks9
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9 Concluding remarks outlook is giv
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9 Concluding remarks with SIR in th
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9 Concluding remarks have the tende
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9 Concluding remarks Sharma S.K. (2
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Summary Subsurface iron and arsenic
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Summary Apart from adsorptive-catal
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Samenvatting Ondergrondse ijzer-en
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Samenvatting Fe 3+ oxiden. Niettemi
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Samenvatting drinkwaterkwaliteit, m
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List of publications List of public
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List of publications Disaster Relie
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Subsurface Iron and Arsenic Removal

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