Function of Leersia hexandra Swartz in constructed wetlands for Cr(VI) decontamination: A comparative study of planted and unplanted mesocosms

Abstract

Constructed wetlands planted with Leersia hexandra Swartz can effectively remove Cr(VI) from water. This work aims to assess the function of L. hexandra in constructed wetlands that treat Cr(VI)-contaminated water. Cr(VI) removal performance of planted and unplanted mesocosms were compared under two different temperatures (25°C and 5°C). Results indicated that the Cr(VI) removal performance of the planted mesocosm was more efficient and stable than that of the unplanted mesocosm. The planted mesocosm showed enhanced Cr(VI) removal even in cold season (5°C) when the shoots were wilted. Therefore, L. hexandra played important roles in enhancing Cr(VI) removal. However, direct Cr uptake by plants only accounted for 9.3% of the total amount of Cr loading in the wetlands. This finding suggested that L. hexandra indirectly improved Cr(VI) removal by influencing other removal processes. Organic content and microbial abundance in the sediments of the planted mesocosm were significantly higher than those in the sediments of the unplanted mesocosm (p<0.05). The major functions of L. hexandra as an ecosystem engineer include producing organic matter and facilitating microbial growth, which are the key factors that influence Cr(VI) reduction and sequestration. A small proportion of Cr accumulated in biomass (4.7–8.6%) was released during plant litter decomposition, indicating that the effect of litter decomposition on water quality was negligible.