The Effects of Pyrite on Nitrogen and Phosphorus in Leersia hexandra Swartz Constructed Wetlands Under Cr(VI) Stress

The discharge of chromium-containing wastewater into natural aquatic systems poses a substantial risk to both ecosystem integrity and human health. In this study, a pyrite-amended Leersia hexandra Swartz (L. hexandra) constructed wetland was established to investigate the mechanisms underlying pyrite-mediated regulation of wetland processes. In normal conditions, the presence of L. hexandra enhanced the removal of NH4+-N, NO3−-N, TN, and TP, with the maximum removal efficiencies increased by up to 35.4%, 17.8%, 9.5%, and 25%, respectively, compared with the constructed wetland without L. hexandra. Although Cr(VI) led to a general decline in removal efficiencies, appreciable removal of NH4+-N, TN, TP, and COD was still sustained. Notably, under Cr(VI) stress, pyrite amendment improved the removal of NH4+-N and COD, with maximum removal efficiencies reaching 99% and 87.5%, respectively, and achieving optimal overall efficiency. Moreover, physiological and biochemical assessments indicated that L. hexandra exhibited superior performance in the pyrite-amended system compared with the unamended control following Cr(VI) exposure, with the peak SOD activity decreased by 27.6%, peak POD activity decreased by 21.9%, peak CAT activity increased by 29.6%, and peak MDA content decreased by 29.4%, suggesting that pyrite mitigated Cr(VI) stress. Given the pivotal role of microbial assemblages in nutrient transformation, shifts in microbial community structure were further analyzed after pyrite addition, revealing discernible alterations. Collectively, these findings demonstrate that pyrite incorporation into L. hexandra constructed wetlands enhances the removal of specific nitrogen species under Cr(VI) stress, while its promotive effect on phosphorus removal remains comparatively limited.