Variable effects of soil organic matter on arsenic behavior in the vadose zone under different bulk densities

• Published in: Journal of hazardous materials 2023-04, Vol.447, p.130826-130826, Article 130826
• Main Authors: Tran, Tho Huu Huynh, Kim, Sang Hyun, Lee, Hosub, Jo, Ho Young, Chung, Jaeshik, Lee, Seunghak
• Format: Article
• Language: English
• Subjects: Arsenic; Groundwater vulnerability; Soil organic matter; Vadose zone; Wet-dry cycle
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2023.130826

The nonstationary nature of water and oxygen content in the vadose zone determines various biogeochemical reactions regarding arsenic (As) therein, which affects the groundwater vulnerability to As contamination at a site. In the present study, we evaluated the effect of soil organic matter (OM) on the behavior of As using specifically designed soil columns that simulated the vadose zone. Three wet-dry cycles were applied to each of the four columns with different OM contents and bulk densities. OM was found to exhibit variable effects, either inhibiting or accelerating the mobilization of As, depending on bulk density. At a moderate bulk density (< 1.27 g/cm3), OM slightly lowered the pH of pore water, which enhanced the sorption of As onto the iron (Fe) oxides, promoting the retention of As in soil. In the soil column with a relatively higher bulk density (1.36 g/cm3), however, the dissimilatory reduction of iron oxides was triggered by rich OM under oxygen-limited conditions. X-ray absorption spectroscopy analysis revealed that alternate wetting and drying transformed the Fe oxides in the soil by reductive dissolution and subsequent re-precipitation. Consequently, As was not stably retained in the soil, and its mobilization downwards was further accelerated. [Display omitted] •Soil organic matter(OM) affects the groundwater vulnerability to As contamination.•Under moderate soil density, OM enhances the As retention in a vadose zone.•Under high soil density, OM accelerates the As mobilization through a vadose zone.•Redox dynamics from nonstationarity altered mineral composition in soil water.