Metal and microplastic treatment in stormwater bioretention

Metals and microplastic particles are common stormwater pollutants, which can be harmful when released to the environment. The aim of this thesis is to increase the understanding of the treatment of metals and their different fractions, as well as microplastic, in bioretention systems. For investigating total, dissolved and truly dissolved metal treatment two laboratory (paper I and II) and two field studies (paper III and IV) were carried out. For investigating microplastic treatment two field studies were carried out (paper V and VI). The resulting papers I to VI are the basis for this thesis, which presents new knowledge about dissolved (<0.45 μm) and truly dissolved (<3 kDa) metal removal as well as microplastic removal in bioretention systems. The results of the laboratory and field studies, which are described in the appended papers I to VI, were obtained by using sampling, contaminant fractionation or analysis methods which have been rarely, or never, used previously in bioretention research. Further, the effects of the factors “vegetation”, “wetting regime” and “salt” on the removal of these pollutants was also investigated. It was found that dissolved, but specifically truly dissolved, metals were generally less efficiently removed by bioretention systems than total metals. This led to a change of metal speciation from more particulate metals in the bioretention system influent to higher colloidal and/or truly dissolved metals in the effluent. In respect of microplastic treatment (including typical highway runoff components such as rubber and bitumen particles), it was found that particles bigger than 20 μm and smaller than 300 μm were, in general, efficiently removed by the bioretention system. An investigation of intra-event variations of metal concentrations revealed that total, dissolved and truly dissolved metal concentrations may peak at the beginning of some effluent events, while metal concentrations remained stable throughout the whole event for others. Salt application has been shown to affect the removal of metals in bioretention systems negatively. Similarly, antecedent drying mostly increased total and dissolved metal effluent concentrations and, in some cases, caused metal leaching, but it was also observed that some of the investigated plant species could mitigate the negative effect of the drying period. Consequently, vegetation selection based on plant traits had significant effects on total and dissolved metal treatment. For hyperaccumulating plants, the shoot/soil ratios were generally greater than one for Cd, Cu and Zn concentrations, indicating that such plants could be used for metal phytoextraction. However, from the plant characteristics studied (photosynthesis pathway, mycorrhizal association, hyperaccumulation, root/shoot biomass), only root biomass seemed to affect metal treatment negatively. While treatment of microplastic particles in the fraction 100–300 μm were generally not affected by the presence of vegetation, particles in the fraction 20–100 μm were significantly better removed by a vegetated bioretention cell compared to an unvegetated system. Despite efficient total metal removal, specifically dissolved and truly dissolved Zn and Cu concentrations were often higher in the bioretention effluents than environmental threshold values, not only when concentration peaks occurred at the beginning of effluent events but often throughout the effluent events. Similarly, although bioretention systems treated 20–300 μm microplastic particles efficiently, microplastic concentrations were still high in the effluent of bioretention systems compared to concentrations in receiving water bodies. Even so, the evaluated bioretention systems were able to mitigate the impact of pollutant discharges from stormwater on the receiving water body. This thesis describes how the measurement of dissolved and truly dissolved metals, as well as of microplastic particles, is important for understanding the environmental impact of bioretention systems and that further research is needed to understand the treatment variability of these contaminants better. In terms of microplastic treatment, the inclusion of the investigation of particles smaller than 20 μm in future studies is specifically recommended.