Arsenic Speciation and Distribution in a Paddy Land and its Adjacent Dry Land - Implications for the Origins and Redox Behaviors of Arsenic.

The speciation of arsenic (As) in soils is of critical importance because As-containing species differ largely in their toxicity and mobility. In soils, pH and redox potential are two of the major factors determining the solubility and transformation of As species. To investigate the influence of soil redox condition on the formation and transport of As species, soil samples were collected from different depths (up to 140 cm) of a rice paddy land and its adjacent dry land, both contaminated by As, in Guandu Plain, Taiwan. Since a dry-flooded cycle is characteristic for rice paddy soils and the flooding during rice growing seasons impedes the penetration of air into the soils, the resulting depth-dependent redox gradient is expected to strongly affect the vertical distribution of As species in the rice paddy soil as compared with its counterpart in the dry land. As K-edge X-ray absorption spectroscopy combined with chemical extractions was applied to determine the structures of As species in different particle-size fractions of the soils along the two profiles. The results showed that As(V) predominantly existed along the two soil profiles and As was less recalcitrant in the surface soils of the rice-paddy land.  Nonetheless, the vertical distributions of As showed no indication of enhanced mobility in the rice paddy soil compared with its dry-land counterpart. It was hypothesized that the high contents of Fe and Al oxides in these soils may restrict As mobility during flooding seasons. The As concentrations in the coarse fractions (> 0.5 mm and 0.5 – 0.25 mm) of some of the soil samples were in a range of 1 - 5 g kg^-1. Thus, the high As contents of the soils in Guandu Plain may result from weathering of As-bearing parent materials, although the input of As from the nearby hot spring cannot be completely ruled out. The information acquired in this work will assist in providing strategies of managing these As-containing soils