Stormwater is a source of pollutants in urban areas and should be treated to prevent negative environmental effects. A newer technique uses floating rafts with plants, called floating treatment wetlands (FTWs), which are placed in the polluted water. Few earlier studies have examined heavy metal removal by FTWs, and none has examined stormwater in cold climates. This study therefore aimed to determine whether plants growing in FTWs could accumulate heavy metals from stormwater ponds in a cold climate. This study examined the abilities of three native wetland species (i.e., Carex riparia, C. pseudocyperus, and Phalaris arundinacea) to accumulate Cd, Cu, Pb, and Zn. The plants were planted on FTWs, which were placed in two stormwater ponds in Stockholm, Sweden, for 12 weeks. Phalaris arundinacea accumulated more Cd, Cu, and Zn than did the Carex species, and C. pseudocyperus accumulated less Pb than did the other species during the experimental period. In most cases, the roots had higher metal concentrations than did the shoots. Carex pseudocyperus had smaller differences between shoot and root metal contents, whereas P. arundinacea had higher Cd and Cu contents and lower Zn contents in its roots than in its shoots. The metal content in the plants increased with higher biomass. The plants that grew in the stormwater pond with a higher Zn concentration had a higher Zn tissue concentration and total Zn content per plant after treatment. This study shows that wetland plants growing on FTWs can accumulate metals from stormwater ponds in a cold climate. Phalaris arundinacea appears to be a good candidate for metal removal use in FTWs. Furthermore, high plant biomass positively affects the metal uptake, meaning that good growing conditions could be essential for metal removal.

Förhöjda halter av tungmetaller och klorid är vanligt förekommande i dagvatten. Detta är orsakat av föroreningar från trafik, byggnadsmaterial och industriell verksamhet samt från användandet av vägsalt under vintrar. Flytande våtmarker som består av bevuxna flottar som kan placeras i dagvattendammar skulle potentiellt kunna minska koncentrationen av tungmetaller och klorid. Hittills har dock denna typ av våtmarker mestadels använts för att minska halterna av kväve och fosfor i vatten i länder med varmt klimat.

Växterna i den flytande våtmarken är nödvändiga för att reningen ska bli effektiv. De tar upp en del av föroreningarna genom rötterna och ackumulerar i växten. Föroreningarna fäster också på ytan på rötter och döda växtdelar samt fäller ut och sedimenterar genom kemiska förändringar i vattnet runt rötterna. Växternas rötter som hänger ned i vattnet minskar vattenhastigheten, vilket ger ökad sedimentation av föroreningar som är bundna till partiklar. Därefter kan växtdelarna och sedimentet transporteras från platsen och tas om hand på ett säkert sätt.

Syftet med denna studie var att: A) Undersöka vilka växtarter som kan användas i flytande våtmarker i kallt klimat för effektiv rening av tungmetaller och vatten, samt B) identifiera vilka egenskaper hos växterna som är kopplade till hög reningsförmåga hos växterna, för att på så sätt lättare kunna identifiera fler lämpliga arter.

Studien baseras på 34 arter av svenska våtmarksväxter. De odlades hydroponiskt, och exponerades i fem dagar för en lösning innehållande 1,2 µg Cd L^–1, 68,5 µg Cu L^–1 ¸ 78,4 µg Pb L^–1, 559 µg Zn L^–1 och 55,4 mg Cl L^-1, en föroreningsmängd baserat på halterna i dagvatten från högtrafikerad väg.

Slokstarr (Carex pseudocyperus) och jättestarr (Carex riparia) visade sig snabbast minska koncentrationen av samtliga tungmetaller i lösningen. Ytterligare nio arter minskade snabbt koncentrationen av koppar, bly och zink, men inte kadmium. Växternas reningsförmåga visade sig korrelerat med mängden biomassa, framförallt mängden blad och tunna rötter, samt med växtens transpiration som i sin tur beror på mängden blad. Förmågan att rena klorid har analyserats hos 23 av arterna och rörflen (Phalaris arundinacea) och jättegröe (Glyceria maxima) visade sig vara de mest effektiva arterna. Preliminära analyser visar att korrelationen mellan saltupptag och biomassa är svagare än för tungmetaller.

Slutsatsen av denna studie är att växters förmåga att rena vatten från tungmetaller och klorid uppvisar stora skillnader, som kan förklaras genom skillnader i växternas egenskaper. Detta indikerar att rening av dagvatten från både tungmetaller och klorid i kallt klimat kan uppnås med flytande vårmarker med inhemska växter.

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.

Plants are the crucial component of floating treatment wetlands (FTWs). However, heavy metal removal capacity varies between plant species, and the relationships between plant traits and differences in removal capacity remain unclear. This study sought to determine: (1) the relationships between plant traits and removal of Cd, Cu, Pb, and Zn from water, and (2) the relationships between the removal patterns of these metals. Plants of 34 wetland plant species were exposed to heavy metal concentrations common in stormwater for five days, and 20 traits were measured on each plant. Results indicate that the most important plant traits for heavy metal removal from water are transpiration and high total biomass, especially large amounts of fine roots and leaves. The same traits were generally related to removal both initially and after longer exposure, with stronger correlations found after longer exposure. Plant removal of one metal was likely correlated with removal of the other metals, and the plant removal capacity after 30 min of exposure was correlated with the removal capacity five days later. The present results can be used in selecting plants for enhanced heavy metal removal by FTWs and in identifying additional useful plant species, allowing adaptation to local conditions.

Stormwater with low temperatures and elevated salinity, common in areas where deicing salt is used, might affect the removal of heavy metals by plants in stormwater treatment systems such as floating treatment wetlands. This short-term study evaluated the effects of combinations of temperature (5, 15, and 25 °C) and salinity (0, 100, and 1000 mg NaCl L⁻¹) on the removal of Cd, Cu, Pb, and Zn (1.2, 68.5, 78.4, and 559 μg L⁻¹) and Cl⁻ (0, 60, and 600 mg Cl⁻ L⁻¹) by Carex pseudocyperus, C. riparia, and Phalaris arundinacea. These species had previously been identified as suitable candidates for floating treatment wetland applications. The study found high removal capacity in all treatment combinations, especially for Pb and Cu. However, low temperatures decreased the removal of all heavy metals, and increased salinity decreased the removal of Cd and Pb but had no effect on the removal of Zn or Cu. No interactions were found between the effects of salinity and of temperature. Carex pseudocyperus best removed Cu and Pb, whereas P. arundinacea best removed Cd, Zu, and Cl⁻. The removal efficacy for metals was generally high, with elevated salinity and low temperatures having small impacts. The findings indicate that efficient heavy metal removal can also be expected in cold saline waters if the right plant species are used.

Floating treatment wetlands (FTWs), consisting of vegetated rafts, may reduce heavy metal levels in polluted water, but the choice of plant species for efficient metal removal needs to be further investigated. We screened the capacity of 34 wetland plant species to remove metals dissolved in water to identify suitable species for FTWs. The plants were 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, and 559 µg Zn L⁻¹. Results show large variation in metal removal rate and capacity between the investigated species. The species with highest removal capacity could remove up to 52–94% of the metals already after 0.5 h of exposure and up to 98–100% of the metals after 5 days of exposure. Plant size contributed more to high removal capacity than did removal per unit of fine roots. Carex pseudocyperus and C. riparia were the most efficient and versatile species. The findings of this study should be considered as a starting point for further investigation of plant selection for improved water purification by FTWs.