Arsenic distribution and speciation in an arsenic hyperaccumulator fern by X-ray spectrometry utilizing a synchrotron radiation source

Abstract

The arsenic distribution in the arsenic hyper-accumulating fern (Pteris vittata L.) was investigated using a synchrotron X-ray fluorescence microprobe. Fronds of various ages were subjected to XRF imaging analysis, and it was found that the arsenic distribution in the pinnae of fronds changed according to their stage of growth. The focused microbeam (3.5 × 5.5 μm²) produced by Kirkpatrick–Baez optics was applied to the fern in order to determine the elemental distribution in plant tissue and cell levels. The results indicated that high levels of arsenic accumulate at the base of sporangium with lamina of pinnae. Also, the fern was subjected to X-ray absorption near edge structure (XANES) analysis without any sample treatment to directly elucidate the arsenic oxidation state in fern. It was found that arsenic exists as the As(III) form in pinnae, and as a mixture of As(III) and As(V) in rachis, while As(V) is present in cultivated soil. These findings indicate that the fern uptakes arsenic as As(V) from soil and that the As(V) is then partially reduced to As(III) within the plant, with the arsenic finally accumulating as As(III) in a specific area of the pinna.