Arsenic fractionation and mobility in sulfidic wetland soils during experimental drying

Abstract

In this study, two Czech wetland soils enriched in authigenic sulfide minerals (especially realgar) were collected from the saturated zone (60-100 cm), flooded with local groundwater and allowed to dry for up to 98 days. The objective was to examine the mobility of As, Fe, S and trace metals using selective chemical extractions, S isotopes and X-ray diffraction through the drying process. During the initial stage of incubation (∼20 days), the re-flooding of the soils triggered a microbially-mediated SO₄^2- reduction, which immobilized the Co, Cu and Ni. The reductive dissolution of As-bearing Fe (oxyhydr)oxides and the release of As were documented only in the Fe-rich/organic-low soil. Over the next stage of incubation (∼75 days), the exposure and drying of the soils led to the oxidation of the Fe and As sulfides. The arsenic and trace metals released via oxidation of the sulfide phases (particularly Fe sulfides) were almost entirely sequestered by the Fe(III) (oxyhydr)oxides, but acidification during the oxidation stage of the incubation resulted in the pH-dependent release of the As and trace metals (Co, Cu, Ni) (especially in the Fe-rich/organic-low soil). These findings suggest that sulfidic soils in wetlands can be considered as long-term sources of As during major drought events.

Keywords: Arsenic; Mobility; Oxidation; Realgar; Wetland soil.