Investigating subsurface iron and arsenic removal: anoxic column experiments to explore efficiency parameters A laboratory study to optimize a low cost technology for small scale drinking water treatment in rural Bangladesh

A laboratory study to optimize a low cost technology for small scale drinking water treatment in rural Bangladesh Harmen van der Laan Abstract High concentrations of arsenic in drinking water form a major threat for public health in more than 70 countries worldwide. The situation is Bangladesh is seen as the worst incidence of arsenic poisoning: it is believed that 30 million people in Bangladesh are exposed to an arsenic concentration exceeding 50 μg/L. Recently subsurface treatment as a possibility for arsenic removal for drinking water supply on community scale in rural Bangladesh has been recognized. The application of this subsurface treatment technique for arsenic removal is seen as very promising, since it has many advantages: it is a residue free, low-cost technology and no hazardous chemicals or expensive filter media are required. The technique, already applied for decades in Europe for subsurface iron removal, involves the injection of aerated water into an anoxic aquifer, where consequently ferrous iron is oxidized by the injected oxygen. Subsequently, the flow is reversed and a multiple of the injected volume can be extracted with lowered iron concentrations, which makes the technology successful. Efficiency ratios (V/Vi) of extracted water (V) over injected water (Vi) are typically increasing over the successive cycles and may range from 3 to 30. The freshly formed ferric oxides are potentially effective adsorbents for trace elements like arsenic. Yet, there is a lack of insight in both (i) the fundamental knowledge of the responsible mechanisms causing the (increasing) sorption of iron and arsenic, and (ii) the operational factors through which the removal efficiency can be optimized. The primary goal of this study is to gain a better understanding of these dominant mechanisms and the observed increasing efficiency, in order to optimize the operation of this technology in the field. Several successive injection/abstraction cycles were simulated with anoxic column experiments. As a starting point, the following conditions were applied: [Fe 2+ ] 73 μM (4 mg/L), [As(III)] 2.7 μM (200 μg/L), pH 6.9 and ionic strength 2·10-2. Various experiments were performed, adjusting and comparing the influence of different parameters: two types of soil material (virgin sand and sand sampled from an aquifer where subsurface iron removal has been applied for several years), pH variation, a different volume of injection water and the influence of a background electrolyte (0.1M NaNO 3). The results of this study are promising and provide new insights …