Pyrite coupled with steel slag to enhance simultaneous nitrogen and phosphorus removal in constructed wetlands

FeS₂-based constructed wetland has garnered increasing interest as an efficient and eco-friendly method for nitrogen and phosphorus removal from carbon limitation wastewater. However, large-scale application is limited due to low nitrogen and phosphorus simultaneous removal performance, slow growth rate of autotrophic denitrifiers and high effluent acidity. This study proposes a novel constructed wetland by coupling with pyrite and steel slag (PSCW) as substrates. The results showed that steel slag addition in PSCW increased total nitrogen removal by 40% ∼ 54%. The higher removal of total phosphorus was mainly attributed to the strong substrate adsorption and metal ions precipitation, which was removed at the pyrite and steel slag layer. The genes involved in denitrification, glycolysis, oxidative phosphorylation and sulfur oxidation were notably enriched in PSCW, indicating the increasing contribution of FeS₂-based autotrophic denitrification coupled with steel slag to NO₃⁻ removal. The XPS analysis identified the formation of SO₄²⁻, S²⁻, S⁰, and Fe(III) on the pyrite surface in PSCW. S⁰ formation and sulfate reducing bacteria enrichment in PSCW were all promoted due to the coupling of pyrite and steel slag, which also maintained higher NO₃⁻ removal efficiency. Thiobacillus was specifically enriched in PSCW due to steel slag addition. The toxicity effect of pyrite on root tip can be alleviated by steel slag, which was conducive to nutrients assimilation by roots. The findings of this study provide new insights into the molecular mechanism of carbon, nitrogen, and sulfur metabolism in the pyrite-steel slag based constructed wetland.