Chloride removal capacity and salinity tolerance in wetland plants

Deicing with sodium chloride maintains safe roads in the winter, but results in stormwater runoff with high chloride (Cl^-) content that causes various downstream problems. Chloride-rich water risks contaminating groundwater, shortening the lifespan of concrete and metal constructions, and being toxic to aquatic organisms. Current stormwater treatment methods are unable to remove Cl^-, but wetland plants with high chloride uptake capacity have potential to decrease Cl^- concentrations in water. The aim was to identify suitable plant species for removing Cl^- from water for future studies on phytodesalination of water, by comparing 34 wetland plant species native to Sweden in a short-term screening. Additionally, Carex pseudocyperus, C. riparia, and Phalaris arundinacea was further compared as to their salinity tolerance and tissue Cl^- concentration properties. Results show that Cl^- removal capacity, tissue accumulation, and tolerance varied between the investigated species. Removal capacity correlated with biomass, dry:fresh biomass ratio, water uptake, and transpiration. The three tested species tolerated Cl^- levels of up to 50-350 mg Cl^- L^-1 and accumulated up to 10 mg Cl^- g^-1 biomass. Carex riparia was the most Cl-tolerant species, able to maintain growth and transpiration at 500 mg Cl^- L^-1 during 4 weeks of exposure and with a medium removal capacity. Due to a large shoot:plant biomass ratio and high transpiration, C. riparia also had high shoot accumulation of Cl^-, which may facilitate harvesting. Phalaris arundinacea had the highest removal capacity of the investigated species, but displayed decreased growth above 50 mg Cl^- L^-1. From this study we estimate that wetland plants can remove up to 7 kg Cl^- m^-2 from water if grown hydroponically, and conclude that C. riparia and P. arundinacea, which have high tolerance, large biomass, and high accumulation, are suitable candidates for further phytodesalination studies.