Analytical Sciences

Determination of the As(III)/As(V) Ratio in Soil by X-ray Absorption Near-edge Structure (XANES) and Its Application to the Arsenic Distribution between Soil and Water

Yoshio TAKAHASHI,*  Naoya OHTAKU,* Satoshi MITSUNOBU,* Kouichi YUITA,** and Masaharu NOMURA***

*Department of Earth and Planetary Systems Sci., Graduate School of Sci., Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
**National Institute for Agro-Environmental Sciences (NIAES), Tsukuba, Ibaraki 305-8604, Japan
***Photon Factory, Institute of Materials Structure Sci., KEK, Oho, Tsukuba, Ibaraki 305-0801, Japan

The details of a method used to determine the As(III)/As(V) ratio in soil by arsenic K-edge XANES spectroscopy are described. The spectra of mixtures of NaAs(III)O₂ and NaH₂As(V)O₄, conducted for an As(III)/As(V) calibration, were well-fitted by combining normalized spectra of NaAsO₂ and Na₂HAsO₄, where the coefficients multiplied by the normalized spectra were identical to the molar ratio of As(III) and As(V) in the mixtures. XANES spectra of arsenic in soil samples could also be fitted by a linear combination of the spectra of NaAsO₂ and Na₂HAsO₄, which enabled us to estimate the As(III)/As(V) in a soil containing 10.2 mg/kg arsenic. The As(III)/As(V) ratio in the soil was compared with that of a soil solution contacted with the soil determined by HPLC-ICP-MS, showing that As(III) is distributed to water more readily than As(V). The application of the XANES method is important for a better understanding of the behavior of As(III) and As(V) independently in a natural aquifer.

J-STAGE: View this article in J-STAGE