Hotspots and dynamics of dissolved thallium species at oxic-anoxic interfaces in flooded soils

Thallium (Tl) is highly toxic, predominantly existing in its monovalent Tl(I) state in the environment. However, the redox "niche" of both dissolved trivalent and monovalent Tl(III/I) remain under-studied in typical oxic-anoxic interfaces. Here, we hypothesize that the behavior of dissolved Tl is constrained by manganese (Mn), iron (Fe), and sulfur (S) species. Dissolved Tl(III/I) profiles and associated redox species Fe(III/II), Mn(II), and S(VI/II) were quantified spatially (a resolution of 1.8 mm) and temporally (diurnal and seasonal) at the rice rhizosphere and the soil-water interface (SWI). The results showed that hotspots of both dissolved Tl(III) and Tl(I) were identified at the rhizosphere and SWI, with redox niche closely following Mn-reducing conditions and also supporting an oxidation mechanism from Tl(I) to Tl(III). Supporting evidence were that newly formed ferromanganese plaque strongly retained the dissolved Tl, while dissolved Tl(III/I) hotspots coincided with moderate Mn-reducing conditions in both SWI and rhizosphere. Dissolved Tl(III) prevailed at higher ratios of dissolved Fe(III)-to-Fe(II). Additionally, porewater Tl(III/I) and copper (Cu(II)) showed similar behaviors at different temporospatial scales, likely due to shared redox constraints. Collectively, the evidence suggests that Mn oxides serve as both sources and sinks of dissolved Tl(III/I) at redox fluctuated areas. Tl mobility was greatly constrained in both more reduced (Fe and S) and more oxidized conditions, highlighting implications for developing effective remediation strategies.