Hand-pump subsurface arsenic removal: the effect of groundwater conditions and intermittent operation

Abstract

Hand-pump subsurface arsenic removal (SAR) has been investigated in rural Bangladesh with different groundwater conditions and intermittent operation modes. Multiple injection-abstraction cycles were performed after injection of 1 m3 of aerated water. From these experiments it can be concluded that hand-pump SAR, in the traditional injection-abstraction design, does not provide drinking water below the WHO arsenic guideline of 10 μg/L. Results show that arsenic removal was not enhanced by: (i) injection of O2-rich water, (ii) higher Fe:As ratios in the groundwater, or by (iii) multiple injection-abstraction cycles, i.e. at location 1, the breakthrough occurred at abstraction-injection ratios of Va/Vi = 2, for cycle 23. It is proposed that dissolved organic carbon (DOC), bicarbonate and phosphate have a significant effect on the arsenic adsorption process. However, iron removal was very efficient and abstraction-injection ratios increased within successive cycles, with Va/Vi > 8 for cycle 23. Furthermore, intermittent operation reduced arsenic concentrations after stop and restart, suggesting insufficient contact time between soluble arsenic and oxidized iron surfaces around the tube well.

Arsenic Contamination of Groundwater Is Determined by Complex Interactions between Various Chemical and Biological Processes

“…The magnetite may be re-oxidized by oxygen to become hematite and the arsenite may be oxidized to arsenate, which may adsorb onto the hematite (Equation (10)). SAR technology may be extended by setting the levels of other abiotic factors, such as pH, SiO 4 , HPO 4 2− , and HCO 3 − , which may affect the success of SAR [ 89 , 92 , 93 ].…”

“…Van Halem et al (2010) [ 89 ] found that the removal of arsenic was not as tightly coupled to iron oxidation as suggested (but not really proven) by Equations (6)–(10). In a study by Freitas et al [ 92 ], the arsenic level of groundwater was reduced by SAR, but could not be brought below the WHO guideline of 10 µg/L. Bicarbonate and phosphate appeared to compromise SAR.…”

“…In parallel, there may have been arsenite oxidation to arsenate, which then adsorbed onto the ferrihydrites. After all, SAR does remove some arsenic from the water; the issue is that it does not reduce the arsenic concentration sufficiently (Freitas et al [ 92 ]). When the oxygen and nitrate run out and the well thereby achieves reducing conditions, iron- and arsenic-reducing microorganisms may benefit and re-reduce the iron and arsenate, causing the dissolution of some of the ferrihydrite precipitates and the adsorbed arsenate.…”