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Floating treatment wetlands for stormwater management: Plant species selection and influence of external factors for heavy metal and chloride removal in a cold climate
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.ORCID iD: 0000-0001-5477-1562
2022 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Rening av dagvatten med flytande våtmarker : Val av växter och påverkan av externa faktorer för rening av tungmetaller och klorid i ett kallt klimat (Swedish)
Abstract [en]

Stormwater, which consists of rainwater and snowmelt, often contains pollutants from vehicle traffic, building materials, and industries. These pollutants include chloride and heavy metals, which can cause several environmental issues, such as being toxic to biota at elevated concentrations. A relatively new water treatment method is floating treatment wetlands. These vegetated rafts have given promising results, mainly for nutrient removal in eutrophic watercourses in warmer climates. However, knowledge is lacking about their ability to remove chloride and heavy metals and their performance in a cold climate.

The aim was to identify plant species, intended for floating treatment wetlands, which efficiently can remove chloride and the heavy metals Cd, Cu, Pb, and Zn from water in a cold climate such as Sweden and to understand how changes in the environment affect the removal capacity of the plants. This was studied in various conditions by placing plants in water that contained chloride and heavy metals and measuring the concentration of chloride and heavy metals that remained in the water (plant removal capacity; I, III, IV) and the accumulation of removed chloride and heavy metals in the different plant parts (plant accumulation capacity; III, V, VI). In addition, traits of plants capable of high removal and accumulation were identified by correlating their capacity with their morphological characteristics (II, III, VI).

The results show that there are Swedish wetland plant species with a high ability to treat water containing chloride and heavy metals, even under varying conditions. Many species effectively reduced the levels of heavy metals in water, and the graminoid species Carex pseudocyperus and Carex riparia distinguished themselves by quickly and significantly decreasing the concentrations of heavy metals in the water (I). Hardly any species were effective chloride removers, but a few, including Phalaris arundinacea, removed large amounts of chloride (III). Species with a high removal and accumulation capacity of chloride and heavy metals generally had high total biomass, a large amount of leaf and thin root biomass, and high transpiration (II, III, VI). The absorbed heavy metals mainly accumulated in the roots, while chloride accumulated in the shoot tissue (III, V, VI). External factors affected the removal and accumulation capacities of the plants to varying degrees. Increased salinity in the water led to lower removal of Cd and Pb, and low temperature decreased the removal of all investigated heavy metals, but some species’ removal capacities were less affected by the salt and the cold (IV). The plant's content of the heavy metals usually equilibrated with the surrounding water. This effect led to increases in the plant's uptake of heavy metals when their concentration in the water increased, but a release of some accumulated heavy metals if the concentration in the water sank (V). Under field conditions, uptake patterns differed (VI). The plants on floating treatment wetlands accumulated the most Cu followed by Zn, Pb, and Cd, and P. arundinacea distinguished itself through high growth and high uptake. The plants accumulated more in one of the stormwater ponds with no clear explanation. 

This thesis shows that there is potential in a Swedish climate for floating treatment wetlands for the removal of chloride and heavy metals from polluted water. It will be essential to select species expected to achieve high removal capacity in the intended environment, such as P. arundinacea.

Abstract [sv]

Dagvatten, bestående av regnvatten och smält snö, innehåller ofta föroreningar från fordonstrafik, byggnadsmaterial och industrier. Dessa föroreningar inkluderar tungmetaller och klorid, som kan orsaka flera problem i miljön om halterna är förhöjda, inklusive vara giftiga för levande varelser.  En relativt ny metod för vattenrening är flytande våtmarker som har givit lovande resultat framförallt i varmare klimat och för rening av övergödda vattendrag. Kunskap saknas dock gällande deras förmåga att rena tungmetaller och klorid, samt hur väl de fungerar i kallt klimat.

Syftet med avhandlingen var att identifiera arter, tänkta att placeras i flytande våtmarker, som effektivt kan rena vatten från klorid (Cl) och tungmetallerna kadmium (Cd), koppar (Cu), bly (Pb) och zink (Zn) i ett kallt klimat som i Sverige. Dessutom syftade avhandlingen till att studera hur reningsförmågan hos växter påverkas av olika miljöfaktorer.

Detta undersöktes genom placera växter i vatten som innehöll tungmetaller och klorid i olika miljöförhållanden, och därefter mäta den kvarvarande koncentrationen av tungmetaller och klorid i vattnet (växternas reningsförmåga, I, III, IV) och koncentrationen av tungmetaller och klorid i olika delar av växten (växternas ackumuleringsförmåga, III, V, VI). Dessutom identifierades drag hos de växter som hade hög reningsförmåga genom att korrelera växters reningsförmåga med deras morfologiska egenskaper (II, III, VI).

Resultaten visar att det finns svenska våtmarksväxter med hög förmåga att rena vatten från tungmetaller och klorid även under varierande förhållanden. Många arter effektivt minskade halten tungmetaller i vatten, och halvgräsarterna slokstarr och jättestarr utmärkte sig genom att snabbt och tydligt minska halten tungmetaller i vattnet. Färre arter gav effektiv saltrening, men ett par, däribland rörflen, tog upp stora mängder klorid. Arter med hög reningsförmåga av både tungmetaller och klorid hade generellt sett hög totalbiomassa, stor mängd tunna rötter och blad, samt hög transpiration. De upptagna metallerna lagras framförallt i rötterna, medan klorid mestadels lagras i skottdelarna av växten. Växternas reningsförmåga påverkades i olika grad av olika externa faktorer.  Salt vatten ledde till lägre rening av Cd och Pb, och att låg temperatur gav lägre rening av samtliga undersökta tungmetaller, men att vissa arters reningsförmåga blev mindre påverkade av saltet och kylan. Växtens innehåll av tungmetaller jämviktade sig oftast med det omgivande vattnet, vilket ledde till att växternas upptag av tungmetaller ökade om föroreningshalten ökade, men att delar av det som tagits upp släpptes ut om tungmetallshalten i det omgivande vattnet sjönk. Under fältförhållanden fanns det tydliga skillnader i upptagsmönster mellan metaller, platser och arter. Växterna tog upp mest Cu från vattnet, följt av Zn, Pb och Cd. Rörflen utmärkte sig genom hög tillväxt och högt upptag.  

Avhandlingen visar att det finns potential för att rena förorenat vatten från tungmetaller och klorid med flytande våtmarker i ett svenskt klimat. Det kommer att vara viktigt att välja arter som har hög reningsförmåga i den tilltänkta miljön. Eftersom växterna har olika styrkor rekommenderar vi att en blandning av arter används i flytande våtmarker för att ge en stabil rening under varierande förhållanden. Om bara en art kan väljas, renar rörflen (Phalaris arundinacea) bra under de flesta förhållanden och har hög tillväxt, men renar relativt långsamt enligt studie I, har något sämre Cu- och Pb-rening enligt studie IV samt låg Cl-tolerans under näringsfattiga förhållanden enligt studie III. Även om denna avhandling har haft målet att finna växter till flytande våtmarker för dagvattenrening i Sverige så har vi täckt in många aspekter som antagligen gäller för andra arter och som är relevanta när växter används för rening i andra system. Sammantaget menar vi att detta ger en god bild av växternas reningsförmåga ur många olika aspekter, vilket är nödvändigt för att kunna använda växtbaserad vattenrening på ett optimalt sätt. Fortsatta studier bör titta på långtidseffektiviteten i fält hos de flytande våtmarkerna, skördemetodik samt dessa arters förmåga att rena andra typer av föroreningar.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant sciences, Stockholm University , 2022. , p. 86
Keywords [en]
Floating treatment wetlands, Phytodesalination, Rhizofiltration, Phytoremediation, Stormwater, Heavy metals, Chloride, Plant traits, Carex riparia, Carex pseudocyperus, Phalaris arundinacea
Keywords [sv]
Flytande våtmarker, dagvatten, tungmetaller, klorid, fytoremediering, slokstarr, rörflen, jättestarr
National Category
Botany
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-208147ISBN: 978-91-7911-986-7 (print)ISBN: 978-91-7911-987-4 (electronic)OAI: oai:DiVA.org:su-208147DiVA, id: diva2:1689474
Public defence
2022-10-07, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 13:30 (English)
Opponent
Supervisors
Available from: 2022-09-14 Created: 2022-08-23 Last updated: 2022-09-07Bibliographically approved
List of papers
1. Screening the Capacity of 34 Wetland Plant Species to Remove Heavy Metals from Water
Open this publication in new window or tab >>Screening the Capacity of 34 Wetland Plant Species to Remove Heavy Metals from Water
2020 (English)In: International Journal of Environmental Research and Public Health, ISSN 1661-7827, E-ISSN 1660-4601, Vol. 17, no 13, article id 4623Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
heavy metal removal, hydroponic, phytoremediation, wetland plants, water purification
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-171833 (URN)10.3390/ijerph17134623 (DOI)000550380900001 ()
Funder
Svenska Byggbranschens Utvecklingsfond (SBUF), 13167Svensk Vatten Utveckling (SVU), 15-123
Available from: 2019-08-20 Created: 2019-08-20 Last updated: 2022-08-23Bibliographically approved
2. Plant traits related to the heavy metal removal capacities of wetland plants
Open this publication in new window or tab >>Plant traits related to the heavy metal removal capacities of wetland plants
2019 (English)In: International journal of phytoremediation, ISSN 1522-6514, E-ISSN 1549-7879, Vol. 22, no 4, p. 427-435Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
Floating treatment wetlands, Heavy metal removal, Hydroponic, Phytoremediation, Plant traits, Wetland plants
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-171826 (URN)10.1080/15226514.2019.1669529 (DOI)
Funder
Svenska Byggbranschens Utvecklingsfond (SBUF), 13167Svensk Vatten Utveckling (SVU), 15-123
Available from: 2019-08-20 Created: 2019-08-20 Last updated: 2022-08-23Bibliographically approved
3. Chloride removal capacity and salinity tolerance in wetland plants
Open this publication in new window or tab >>Chloride removal capacity and salinity tolerance in wetland plants
2022 (English)In: Journal of Environmental Management, ISSN 0301-4797, E-ISSN 1095-8630, Vol. 308, article id 114553Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
Phytodesalination, Wetland plants, Chloride Polluted waters, Carex riparia, Phalaris arundinacea
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-202811 (URN)10.1016/j.jenvman.2022.114553 (DOI)000782138000003 ()35121460 (PubMedID)2-s2.0-85123756290 (Scopus ID)
Funder
Svensk Vatten Utveckling (SVU), 15-123Svenska Byggbranschens Utvecklingsfond (SBUF), 13167
Available from: 2022-03-14 Created: 2022-03-14 Last updated: 2022-08-23Bibliographically approved
4. Influence of salinity and temperature on removal of heavy metals and chloride from water by wetland plants
Open this publication in new window or tab >>Influence of salinity and temperature on removal of heavy metals and chloride from water by wetland plants
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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, 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. 

Keywords
Salinity, Temperature, Wetland plants, Heavy metals, Chloride, Phytodesalination
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-208144 (URN)
Available from: 2022-08-20 Created: 2022-08-20 Last updated: 2022-08-23
5. Effect of changes in solution concentration on accumulation and distribution of Cd, Cu, Pb, and Zn in Carex pseudocyperus L.
Open this publication in new window or tab >>Effect of changes in solution concentration on accumulation and distribution of Cd, Cu, Pb, and Zn in Carex pseudocyperus L.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The concentration of pollutants in stormwater varies with the season and amount of precipitation, exposing plants in treatment systems, such as floating wetlands, to different levels of heavy metals. The study aims to investigate if the sedge species Carex pseudocyperus can absorb substantial amounts of heavy metals and continue to accumulate despite changing concentrations. The plants were first exposed to three levels of Cd, Cu, Pb, and Zn. After five days, the plants from each treatment were divided into four groups and exposed to four different levels of the same metals as before for an additional five days, resulting in 12 combinations of metal loads. The results show that the accumulation capacity was affected by a plant's previous accumulation of the same heavy metals as well as its concentration in the surrounding solution. The plants could absorb high levels of heavy metals, which means they can effectively treat, for example, stormwater containing high metal levels. However, the plants released some of the accumulated metals as soon as their concentrations in water decreased. Despite the release, plants exposed to the highest concentrations still showed the highest metal levels in their tissue, indicating a potential for long-term accumulation. Translocation to aboveground tissues was limited for all metals examined, especially for Cu and Pb. The findings of this short-term study suggest that changing pollutant concentration in stormwater systems might limit heavy metal uptake and call for careful timing of plant harvest to ensure efficient removal of pollutants.

Keywords
Phytoremediation, heavy metals, Carex pseudocyperus, accumulation, leakage
National Category
Botany
Research subject
Plant Systematics
Identifiers
urn:nbn:se:su:diva-208145 (URN)
Available from: 2022-08-20 Created: 2022-08-20 Last updated: 2022-08-23
6. Differences in metal accumulation from stormwater by three plant species growing in floating treatment wetlands in a cold climate
Open this publication in new window or tab >>Differences in metal accumulation from stormwater by three plant species growing in floating treatment wetlands in a cold climate
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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 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. The study revealed differences in accumulation between metals, species, plant parts, and sites. 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 suggests that the Zn concentration in the water positively affects plant Zn accumulation. For the other metals, no difference in concentration in the water was detected between the stormwater ponds. 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 metal uptake, meaning that good growing conditions could be essential for metal removal.

Keywords
Floating treatment wetlands, Rhizofiltration, Heavy metals, Stormwater, Carex riparia, Carex pseudocyperus, Phalaris arundinacea
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-208146 (URN)
Available from: 2022-08-23 Created: 2022-08-23 Last updated: 2022-08-23

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