Encapsulation of lead in rice phytoliths as a possible pollutant source in paddy soils

Abstract

Due to its serious health risks, lead (Pb) in rice, specifically its uptake, translocation, and accumulation mechanisms and its toxic effects have been studied intensively in recent years. However, it remains unclear about the role of phytolith, a siliceous structure in rice plants, in the storage and release kinetics of Pb in rice. This study aims at elucidating a possible encapsulation of Pb in the phytolith structure (phytPb), and identifying whether or not phytPb provides a source of Pb in soil, when returned to the field with the rice straw or in a related processed product such as ash from on-site burning. To date there has not been any specific work targeted at the determination of phytolith-associated heavy metals in general and phytPb in particular, and therefore this possible source of Pb in soils may have been overlooked. Phytoliths were included in a study of rice paddy soil and rice straw to demonstrate accumulation of phytolith and its associated phytPb in agricultural soils of the Red River Delta (Vietnam). The total content of Pb in rice straw samples was found to be up to 118 mg kg−1, and this Pb sink can be cycled to serve as a new Pb source in soils. The fate of Pb in rice straw might be directly related to open burning activity (a common practice in the Red River Delta), in which volatilization or sub-compartmentation in slagged phytolith appeared as controlled factors. This is supported by the findings from batch experiments for rice straw ash samples, in which release of Pb was low and a portion of Pb in rice straw were found to associate with phytolith structural organic matter. We also observed the presence of phytPb in "aged" phytolith fragments which had accumulated in the paddy field soil. However this Pb pool was relatively low (from 7.8 to 34 kg ha−1) relative to other soil Pb fractions. As the thermal treatments of Pb-tainted rice straw resulted in losses of Pb via volatilization, open-field burning practices for Pb-contaminated rice straw is suggested as an environmental risk.