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

## TL;DR

Adding pyrrhotite helps a plant called Leersia hexandra grow better and absorb more chromium, improving the cleanup of toxic chromium in wetlands.

## Contribution

Pyrrhotite enhances chromium accumulation in plants and reduces stress, offering a novel approach for phytoremediation in constructed wetlands.

## Key findings

- Pyrrhotite significantly reduced oxidative stress and improved photosynthesis in Leersia hexandra.
- Maximum aboveground chromium standing stock reached 164.50 mg·m−2 with pyrrhotite addition.
- Pyrrhotite and its products promote chromium accumulation through in/ex planta defensive mechanisms.

## Abstract

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 Leersia hexandra Swartz (L. hexandra) 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 L. hexandra, 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 L. hexandra 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 L. hexandra 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.