Subsurface Iron and Arsenic Removal

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2 Subsurface iron removal in the Netherlands there is still a concern about what happens to the accumulated deposits when operation is stopped. The aquifer will return to its anoxic or anaerobic state and reducing conditions will prevail. Reductive dissolution of the deposits may promote the mobilization of iron, manganese or even trace compounds, such as arsenic (Appelo and Postma, 2005): Equation 2.5 [Fe], [Mn] in mmol.L -1 0.1 0.08 0.06 0.04 0.02 0 0 100,000 200,000 300,000 400,000 2 CH O 4 2 + − + H 2O → CO2 + 4H + e Equation 2.6 + − 2+ Fe ( OH ) As + 3H + e → Fe + H O + As 3 3 Although this is a valid concern, especially at sites with high organic carbon content in the soil and/or groundwater, the mobilization of iron has not been observed to exceed its background value after stopping with subsurface iron removal (Figure 2.12). At WTP Lekkerkerk, the iron and manganese concentrations have been measured at the SIR well before and after the well was exposed to periodic injection. Dissolved organic carbon (DOC) concentrations in the abstracted groundwater fluctuated around 3 mg.L -1 (±0.5). This particular SIR well had been in operation for 18 injection-abstraction cycles, and Figure 2.12 illustrates the results from the start of cycle 18. The measurements show an initial increase in Fe concentration, but after 140,000 m 3 (approx. 10 months) the concentration stabilized at 0.08 mmol.L -1 . This is lower than the iron concentrations observed during cycle 1 and 7 (Figure 2 Pumped volume (m 3 ) Iron Manganese Figure 2.12 Iron and manganese measurements once subsurface iron removal had stopped after 18 injectionabstraction cycles (WTP Lekkerkerk) 2.7), which supports the theory that pyrite oxidation may have occurred during the initial cycles. Manganese concentrations also increased initially, but stabilized at 0.015 mmol.L -1 , which corresponds closely to the background concentration in Figure 2.9. Based on these findings it can be concluded that accumulated iron and manganese oxides during subsurface iron removal were not remobilized to result in elevated concentrations to the groundwater at WTP Lekkerkerk once SIR operation had stopped. Iron or manganese remobilization has not been observed in the >2 years period that subsurface iron removal was stopped at WTP Lekkerkerk. Nevertheless, trace (heavy) metals incorporated or adsorbed to the Fe/Mn oxide matrix could be released through desorption, exchange or re-crystalization (Jessen et al., 2005). Desorption may occur if groundwater conditions 33
Subsurface iron and arsenic removal for drinking water treatment in Bangladesh 2 [PO 4 -3 ] in mmol.L-1 0.05 0.04 0.03 0.02 0.01 0 0 0 100,000 200,000 300,000 400,000 0.1 0.08 0.06 0.04 0.02 [As] in µmol.L -1 did not exceed 0.01 mmol.L -1 , which is far below the concentrations measured during cycle 1 (Figure 2.10). Arsenic concentrations remained below 0.1 µmol.L -1 and were thus also not found to remobilize after SIR was stopped at WTP Lekkerkerk. Pumped volume (m 3 ) Conclusions Phosphate Arsenic Figure 2.13 Phosphate and arsenic measurements once subsurface iron removal had stopped after 18 injectionabstraction cycles (WTP Lekkerkerk) suddenly change (e.g., pH), which is not likely in a normal well field. Exchange of the adsorbed anion with other passing anions with higher affinity could promote the release of retained arsenic or phosphate from the soil grain surface. However, once groundwater conditions stabilize around the stopped SIR well no large water quality variations are expected. Re-crystallization of (amorphous) iron or manganese oxides to oxides of higher crystallinity is a slow process, but could release small amounts of trace elements. Although Figure 2.13 shows some variations in the arsenic and phosphate concentrations, it is obvious that high peak concentrations are not observed. It may be concluded that the retained phosphate was not mobilized to elevated solute concentrations once injection was no longer applied. Phosphate concentrations generally During subsurface iron removal at WTP Lekkerkerk, iron, manganese, phosphate and arsenic were measured to be retained in the aquifer at varying efficacies. SIR technology was found to improve in efficacy with every successive cycle, which correlates well with findings in other literature. No peak concentrations of the studied constituents were observed in the abstracted groundwater once SIR was stopped and therefore not threatening the sustainability of the groundwater production well. pH was shown to be the most pronounced water quality parameter determining the efficacy of SIR, the operational V/Vi ratio more than doubled between pH 7.0 and 7.4 at WTP Corle. The experiences in the Netherlands have shown that subsurface iron removal is an effective, robust and sustainable iron removal technology with great potential for worldwide application. 34
Page 1 and 2: Doris van Halem Subsurface Iron and
Page 3 and 4: Dit proefschrift is goedgekeurd doo
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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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