A sand tank experimental study of distribution, migration and transformation mechanism of iron and phosphorus species under redox fluctuation in a simulated riparian zone

The coupling transformation of iron (Fe) and phosphorous (P) in riparian zones is significant to their biogeochemical cycle in the interaction between surface water and groundwater systems. However, the spatiotemporal distribution and biochemical transformation within the river bank media are not well elucidated in the literature. In this study, we used a sand tank experiment to investigate the migration and transformation of iron and phosphorus under redox fluctuation forced by the variability in river stage in the riparian zone. Results show that there is a significant positive correlation between the trend of iron/aluminum coupled phosphorus (Fe/Al coupled P) and amorphous total iron (TFe). Additionally, more attention should be paid to the effect of dissolved organic carbon (DOC) rather than dissolved oxygen on the redox condition in the underground environment. For example, with the organic carbon decreasing, the inverse crystalline transformation process of Fe species (i.e. conversion of high crystalline Fe species into the low ones) gradually changes to the crystallization process, resulting in the immobilization and release of the aqueous phosphorus (PO43-). Furthermore, due to the adsorption and oxidation process, the migration of aqueous iron (Fe2+) was seriously inhibited, while the distribution and transformation of solid Fe species was similar at different locations. On the other hand, PO43- was observed to accumulate in the transition area of the riparian zone, and was hard to migrate through the river regardless of the recharge or discharge relationship between the river and the groundwater. Overall, this study elucidates the distribution, migration and transformation mechanism of Fe and P in a typical fine sandy riparian zone, and first provides theoretical support for tracing and controlling the source of phosphorus pollution in riparian aquifers.

Automated summary

This study investigated the migration and transformation of iron and phosphorus in riparian zones using a sand tank experiment. The results showed a significant positive correlation between iron/aluminum coupled phosphorus and amorphous total iron. Dissolved organic carbon was found to have a greater effect on the redox condition than dissolved oxygen. Phosphate was observed to accumulate in the transition area of the riparian zone and was difficult to migrate through the river.