A Laboratory-Scale Investigation of In Situ Treatment of Arsenic-Contaminated Groundwater by Air Sparging
Arsenic contamination of groundwater is a major problem in some areas of the world, particularly in West Bengal (India) and Bangladesh where it is caused by reducing conditions in the aquifer. In situ treatment, if it can be proven as operationally feasible, has the potential to capture some advantages over other treatment methods by being fairly simple, not using chemicals, and not necessitating disposal of arsenic-rich wastes. In situ treatment by injection of aerated water has seen limited application with moderate success. However, another possibility is to employ in situ treatment by injection of compressed air directly into the aquifer (i.e. air sparging). In this investigation, an experimental apparatus simulating conditions of Bangladeshi groundwater was constructed and employed treatment by air sparging. Arsenic was removed to a maximum of 79% in low-flow areas of the apparatus, using a solution with dissolved iron and arsenic only. Jar tests revealed arsenic to be removed by co-precipitation with iron at a molar ratio of approximately 2 (iron/arsenic), and this is encouraging as groundwater with relatively high amounts of dissolved iron then has a large theoretical treatment capacity for arsenic. However, further experiments revealed that phosphate is a significant hindrance to arsenic removal, greatly reducing removal effectiveness. This presents a difficulty, as phosphate is often a significant co-occurring solute with arsenic in groundwater. Additional laboratory research would be valuable to examine more complex solutions better representative of field conditions, to examine treatment capacity under different flow rates, to further investigate competition effects of phosphate, and to examine treatment behaviour under intermittent periods of air sparging. Field testing is also needed to examine the efficacy of air sparging for treatment of arsenic in groundwater in a practical application.