Enhancement of nitrogen removal and phosphorus immobilization in sediments by zero-valent iron: Linking iron speciation and microbial community

Eutrophication caused by internal nutrient loading from sediments remains a major challenge in lake ecosystem restoration. In this study, to address the endogenous pollution problem caused by the release of nitrogen (N) and phosphorus (P) from lake sediments, zero-valent iron (ZVI) was used as a remediation material to study its effect on the transport and transformation of N and P in sediments. It was found that the performance of ZVI-remediated sediments was enhanced, with a 41.26-72.62% increase in phosphorus adsorption capacity, an 84.91-99.15% decrease in potential phosphorus release, and an increase in the content of phosphorus bound to Fe/Al oxides and hydroxides (Fe/Al-P). Meanwhile, the denitrification rate (D_R) presented a 16.51-39.76% increase, and the microbial functional community changed significantly, with increasing relative abundance of denitrifying and iron-cycling functional microbes, including Hyphomicrobium and Trichococcus. And ZVI remediation increased the content of amorphous and nanoparticulate iron (oxyhydr)oxide phases (Fe_ox1), carbonate-bound iron (Fe_carb), crystalline iron (oxyhydr)oxides (Fe_ox2), and magnetite (Fe_mag) in the sediments. The process of ZVI enhancing N removal and P immobilization in sediments was positively associated with Fe_ox1, Fe_carb, and Fe_ox2, as the Fe-N cycle was promoted and phosphate was adsorbed and immobilized. The findings highlighted the potential of ZVI as an effective amendment and provided a scientific basis for in-situ ZVI remediation to control endogenous pollution in lake sediments.