Effects of oxide layers in sediments on migration and transformation of iron, phosphorus and sulfur in a large-scale surface flow constructed wetland

The Yanlonghu constructed wetland, a typical surface flow constructed wetland (SFCW), was selected as a case study to investigate the effect of oxide layers on the migration and release of iron, phosphorus, and sulfur at the sediment -water interface (SWI) using diffusive gradient in thin film technology (DGT). Results showed that an identifiable oxide layer with thickness ranging from 1 to 6 cm is formed on the surface sediment. Generally, there is a significant decrease in the concentrations of P(V)-DGT, Fe(II)-DGT and S(-II)-DGT in the oxide layer compared to the reduction layer. And the release fluxes of Fe(II), P(V) and S( -II) in the oxide layers decreased by 91 % - 102 %, 79 % - 84 %, and 153 % - 269 %, respectively, suggesting that the oxide layer can inhibit release of biogenic elements from the sediment. The concentration of Fe(II)-DGT has significant positive correlation with concentrations of S(-II)-DGT, and coupled release of Fe(II) and P(V) happens in the oxide layers except for that with dense plants. Plant density influences the thickness and properties of the oxide layer, and thus affects the migration and transformation behaviors as well as the coupling mechanisms of Fe(II), P(V) and S( -II). Moreover, the coupled release of Fe(II), P(V) and S( -II) occurs in the reduction layer. Fe - P release, is however, also an important mechanism for release of Fe(II) and P(V) from the sediments. Consequently, the formation of the oxide layer plays a pivotal role in impeding the release fluxes of biogenic elements at the SWI, thereby crucially maintaining the steady -state redox environment and ensuring long -term water purification efficacy in wetland ecosystems.