Zoom link for the dissertation, you can find here.

Abstract

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.