Heavy metal removal by floating treatment wetlands: Plant selection

Abstract: Elevated levels of heavy metals and chloride are commonly found in stormwater, as a consequence of pollution from traffic, building material and industries, and the use of salt for deicing in wintertime. Floating treatment wetlands (FTWs), consisting of vegetated rafts that can be placed in stormwater ponds, may be able to reduce heavy metal and chloride concentrations, but until this date have mainly been used for nutrient removal in warm climates. Plants are essential in FTWs as pollutants are taken up into plant tissues, adsorbed to exposed plant surfaces, precipitated due to chemical interactions with root exudates or bound to plant litter.The aim of the study was to examine: A) which plant species that should be used on FTWs in a cool climate for efficient heavy metal and chloride removal, and B) to identify plant traits that are connected to high pollutant removal capacity as a help for identification of additional suitable species.Thirty-four wetland plant species, all growing in wild in Sweden, were used in the study. These were all grown hydroponically for 5 days in a solution containing 1.2 µg Cd L–1, 68.5 µg Cu L–1 ¸ 78.4 µg Pb L–1, 559 µg Zn L–1 and 55.4 mg Cl L-1. Carex pseudocyperus and Carex riparia were found to quickly reduce the concentration of all added heavy metals, and keep the concentration low for the remainder of the exposure period. In addition, nine species were able to remove all metals except cadmium quickly. High removal capacity of metals was found to be connected to biomass traits, mainly large fine root and leaf biomass, and to transpiration, which is correlated with to leaf biomass. Twenty-three of the tested species have also been evaluated for their chloride uptake, and Phalaris arundinacea and Glyceria maxima were identified as the species with highest chloride removal capacity. Preliminary analysis show that the correlation between biomass and chloride removal capacity is weaker than for heavy metals.In conclusion, the removal capacity of heavy metals and chloride differs between plant species, which can be explained by differences in the traits of the plants. The findings indicate that removal of both heavy metals and chloride can be achieved by FTWs in cold climates using a combination of native plants.