Evaluation of As(III) and Sb(III) Adsorption to Paddy Soils from Irrigated Rice Fields in Bangladesh

Arsenic (As) levels in drinking water and in irrigation water used for rice paddies can exceed the World Health Organisation (WHO) drinking water quality limit of 0.05 mg L across many regions in South East Asia. This is due to the dissolution of As-rich minerals in the groundwater hosting bedrock. In polluted groundwater, concentrations can exceed water quality limits to a factor of 900, reaching μg mL concentration levels. A key process that controls the As geochemistry in terrestrial ecosystems is the adsorption on soils and bedrocks. Antimony (Sb) has been recognised to co-occur with As in iron and sulphide minerals associated with sedimentary and igneous mineral formations. Due to the potentially similar aqueous chemistry to As as oxyanion and metalloid, Sb could well be involved in the As crisis, for example by affecting As adsorption in groundwater systems and leading to higher As concentration in well waters. McCarthy et al. tested recently this hypothesis and analysed two hundred forty five tube well water samples from various regions in Pabna City, Bangladesh. They found that Sb concentration were ≤1 ng mL, thus significant adsorption on the soils and subsequent removal from the aqueous phase was suggested. Paddy soil (Hydragric Anthrosol) is an environmentally engineered soil type formed and modified by human activity of irrigation and added organic materials. It constitutes rice fields across Bangladesh and adsorbs significant amounts of As from irrigation waters. This process has been invoked as a possible explanation leading to elevated As content in rice, however, the As dynamics in paddy soils are little understood. Given these recent findings, it is of great interest to assess the adsorption capacity of As and other potentially toxic elements on irrigated paddy fields and to understand potential processes affecting the adsorption and release of As from the soils. To this end, it was conducted preliminary adsorption studies determining the As(III) and Sb(III) adsorption on irrigated paddy soil and their potentially competitive behaviour. It is showed that As(III) sorption capacity is low, ca. 0.4 mg g, and that Sb(III) has a stronger adsorption, potentially explaining the low Sb concentration in wells in Bangladesh. Antimony(III) does only affect slightly the As(III) adsorption.