On the distribution and speciation of arsenic in the soil-plant-system of a rice field in West-Bengal, India: A μ-synchrotron techniques based case study

Abstract

Worldwide, West-Bengal is one of the areas most affected by elevated levels of arsenic in groundwater (50–3000 μg/l). This groundwater does not only endanger humans owing to its use as drinking water. More and above that, irrigation of rice paddies consumes huge quantities of arsenic contaminated groundwater. Consequently, arsenic accumulates in soil and endangers the nutrition chain via arsenic uptake by plants. Rice is one of the staple foods in this region. Lately, there is a considerable intensification of research on the fate of arsenic in affected agricultural systems with most of them resorting to bulk analytical methods. However, so far, knowledge on the μ-scale distribution of arsenic in soil and plants in such agricultural systems is rather limited.This case study combined μ-synchrotron studies on soil, rice root and rice grain from a rice paddy irrigated with groundwater containing about 519 μg/L As. The investigation of a soil aggregate has shown that As is mainly associated with Fe and is not equally distributed over the whole aggregate but occurs in local enrichments of few tens μm in size.In soil, As was mainly associated with Fe-(oxy)hydroxides. Rice root coatings consisted of a similar assemblage of arsenic bearing minerals. Furthermore the incorporation of soil matter in the coating could be shown. On μm-scale, As concentrations in rice root coatings showed an inhomogeneous, patchy distribution (100–2400 mg/kg; median 500 mg/kg) and correlated with Fe concentrations. Some small amounts of arsenic could also be detected in the interior of the root (3–60 mg/kg; median 21 mg/kg). In the rice grain, trace elements such as Zn and Cu were mainly enriched along the grain coating, while As in contrast showed the highest concentrations in the germ and some hot spots in the coating (up to 13 mg/kg). Thus, peeling of rice grain would remove some, but not all of the arsenic.