Dynamic variation of dissolved As, Sb, Fe and S in paddy soil triggered by nitrate loading to overlaying water

Arsenic (As) and antimony (Sb) are toxic elements that usually co-occur in paddy soils due to their chemical similarity. Those elements are redox-sensitive and shift their species across the soil–water interface (SWI) where redox potentials change in every millimeter. In the real world, the distribution and speciation of As and Sb in soils are often influenced by external redox disturbance, but their temporospatial response remains poorly understood. This study aimed to address this gap by introducing external disturbance by adding nitrate into the overlying water.

Daily changes in the profile of As, Sb, iron (Fe), and sulfur (S) were measured using ICP-MS and the In-situ Porewater Iterative (IPI) sampler array.

The results revealed a rapid formation of a sink for As and Fe at the oxic-anoxic transition zone within one day, persisting for at least 6 days and extending to ~ 30 mm below the SWI. Moreover, Sb was re-mobilized in the same area as the As and Fe sink, but the re-mobilized Sb zone was weaker, lasting only 4 days and extending to ~ 20 mm below the SWI. The presence of aqueous ferrous Fe below the transition zone facilitated the formation of Fe and As sink, while the presence of aqueous sulfide below the transition zone hindered the development of the Sb source.

These findings highlight the importance of carefully evaluating the impact of nitrate-based fertilizers or stabilization reagents when applied in As and Sb contaminated soils.