A new method for direct observation of microscale multielemental behavior in waterlogged soil: μXRF-μXAFS combined live soil imaging chamber (LOACH)

Abstract

In this study, we developed a novel non-destructive method, a combined μXRF–μXAFS live soil imaging chamber method, to directly observe the local behavior of metal(loid)s in the oxidative–reductive layer typically found in waterlogged soil such as flooded paddy soil. The newly-designed soil chamber was made of titanium plate and glass with low gas permeability and high corrosion resistance, and thus specializes in the experiment of the waterlogged soil with anoxic environment. The soil chamber can be mounted directly in the synchrotron μXRF–μXAFS beamline. Hence, by using this method, we can observe the intact paddy soil without physical and chemical disturbances. The application of μXRF-μXAFS analysis allows us to analyze the elemental abundances and chemical speciation of soil elements at a microscale. In addition, since the μXRF–μXAFS has both and high elemental selectivity and high applicability for various elements, we can investigate the behavior of target element as well as its relationships with related elements. In this study, we applied the new method to examine the behavior of arsenic (As) in the oxidative–reductive layer formed at the surface of flooded paddy soil. The μXRF imaging of As-contaminated paddy soil newly showed that As in the soil was strongly accumulated in the topsoil layer in a millimeter scale by 1 month incubation under flooded condition. Moreover, the μXAFS analysis of As and related metals indicated that the As accumulation was closely attributed to the change in the oxidation state of As with soil depth and the coexistence of the most favored sorbent (Fe(III) hydroxides) in the oxidative–reductive layer of the paddy soil.