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Metal and microplastic treatment in stormwater bioretention

Published: 20 December 2021

On December 10, 2021, Katharina Lange defended her PhD in Urban Water Engineering at Luleå University of Technology presenting her thesis "Metals and microplastic treatment in stormwater bioretention". Bioretention cells are increasingly being used for stormwater treatment. The overall aim of the doctoral thesis was to contribute to a better understanding of the treatment process of metals and their various fractions as well as microplastics in stormwater bioretention.

The opponent at the dissertation was Elodie Passeport, Associate Professor of environmental engineering (University of Toronto, Canada). The grading board consisted of Maria Greger (Stockholm University), Allen P. Davis (University of Maryland, USA) and Jens Tränckner (University of Rostock, Germany).

Both metals and microplastic particles are common pollutants in stormwater. Regarding metals, the thesis focused on the treatment of different metal fractions. Previous studies have shown that bioretention has good ability to reduce the total content of metals, but few studies have been carried out on the treatment of dissolved and truly dissolved metals. The thesis was able to show that dissolved and truly dissolved metals (which are more bioavailable than particulate metals) are not treated as well in biofilters as particulate metals. Biofilter outflow concentrations of dissolved and truly dissolved metals in some cases exceeded the Swedish environmental quality standards. The studies carried out in the framework of this thesis showed particularly high outflow levels of metals after a dry period or after application of road salt, but it could also be shown that different plant species could partially reduce these negative effects. This means that in addition to the filter material, the choice of suitable vegetation is important for the functioning of biofilters. Furthermore, the effect of different plant properties such as photosynthesis reaction, the ability to form mycorrhiza, hyperaccumulation and root biomass, on metal treatment was investigated. Of these, only the root biomass had a negative impact. However, it has been shown that hyperaccumulators can absorb a higher quantity of metals than other plants and thus have the potential to be used for metal treatment in bioretention.

The studies also showed that bioretention facilities are suitable for microplastic treatment of particles bigger than 20 μm. By combining stereomicroscopy and ATR-FTIR analytical methods, it was possible, for the first time, to integrate data of black particles such as bitumen coming from asphalt or rubber from car tires into the analysis of microplastic treatment in biofilters. A comparison of microplastic treatment in a vegetated bioretention cell and a sand filter without vegetation has shown that vegetation has a positive effect on microplastic treatment of particles in the size of 20-100 μm.

Katharina Lange

Katharina Lange, Postdoctoral position

Phone: +46 (0)920 493264
Organisation: Urban Water Engineering, Architecture and Water, Department of Civil, Environmental and Natural Resources Engineering