Groundwater As mobilization in the Bengal Delta Plain, the use of ferralite as a possible remedial measure—a case study

Abstract

High As groundwater (50–1600 μg l −1) poses the greatest threat to human health in the Holocene alluvial aquifers of the Bengal Delta Plain (BDP) with increasing global concern in recent years. This study deals with groundwater quality and As mobilization vis-à-vis employing ferralite as a remedial option for removal of As from groundwater. The investigation suggests that Fe-rich As traps undergo degeneration to produce Fe oxyhydroxide (HFO) as coating/precipitation on the fine-grained sediment surface and release redox sensitive species (As, Fe and Mn) as well as PO 4 3− into the groundwater under local reducing conditions. Sediment analysis reveals the presence of As T (average 17.2 mg/kg), Fe T (average 0.93 g/kg) and organic matter (average 7.6 g/kg). Sediment As T and Fe T content cannot validate the presence of high groundwater As/Fe. Fe II catalysed Fe III reduction, induced by dissimilatory Fe reducing bacteria liberate the more toxic As III than As V. The release of redox sensitive species (As, Fe and Mn) are the functions of bio-available forms of Fe oxides, concentration and distribution of fresh organic matter and availability of electron donors within the sediment. Further attempt is made to establish the role of ferralite, enriched with natural HFO as an As scavenger. Batch studies demonstrate the competency of the material over the natural/commonly used chemical coagulants generally used for water treatment. The high pHpzc value, 8.5 of ferralite along with the adsorption studies over a wide range of pH elucidate the effectiveness of the material in adsorbing both As III and As V from the well-buffered groundwater. The presence of Fe II in the system enhances the As removal process. The Langmuir adsorption isotherm further confirms the merit of ferralite as an efficient As scavenger. The material has been shaped for a fixed bed filter medium to remove As from groundwater (both laboratory and field scale). Ferralite is also cost effective (US$ 8/metric ton of ferralite with a density 1.17 kg/dm 3). Transportation cost for ferralite (from ferralitic bed to the affected area) is US$ 16/ton/1000 km whereas US$ 0.6/100 l is required for treatment of contaminated water.