Pyrrhotite Facilitates Growth and Cr Accumulation in Leersia hexandra Swartz for Effective Cr(VI) Removal in Constructed Wetlands

Hexavalent chromium (Cr(VI)) is a hazardous pollutant frequently found in industrial wastewater. Constructed wetlands (CWs) provide an alternative for Cr(VI) removal, but their effective removal is essentially governed by the extent of Cr accumulation in plants. This study evaluated the effects of pyrrhotite addition on a Cr-hyperaccumulator <i>Leersia hexandra</i> Swartz (<i>L. hexandra</i>) in CW microcosms with different substrates (pyrrhotite and gravel) and influent Cr(VI) concentrations (2 and 10 mg·L^-1). All microcosms achieved substantial Cr(VI) removal, while pyrrhotite significantly facilitated the removal of NO₃^--N, COD, and TP. Pyrrhotite alleviated Cr-induced oxidative stress and thus promoted photosynthesis in <i>L. hexandra</i>, reflected by 27.32-39.09% lower malondialdehyde levels, 1.67-8.37% higher total chlorophyll contents, and 17.36-39.61% higher net photosynthetic rates. Consequently, maximum aboveground Cr standing stock reached 164.50 mg·m^-2 in the P10 group, where <i>L. hexandra</i> contributed 6.63% to the total Cr removal. Microbial analysis showed reduced Cr-stress responses in pyrrhotite groups. Structural equation modeling indicated that pyrrhotite and its dissolution products promote Cr standing stock of <i>L. hexandra</i> through establishing in/ex planta defensive mechanisms. These findings provide new perspectives on phytoremediation coupled with CWs for the treatment of Cr(VI)-containing wastewater.