Plastic pollution risks in bioretention systems: a case study

Advancing microplastics remediation in bioretention systems using biochar/kaolin: Optimizing organics removal, plant health, and microbial community dynamics

Following the discovery of microplastics (MPs) in river sediments, the number of MP studies on rivers and other freshwater systems has increased rapidly, revealing that MPs are omnipresent in all freshwater environments. The abundance of MPs in freshwater sediments seems to be affected by population density, urban centers, water flow velocity, water catchment size and position and type of sewage and waste management. However, not all of these relations are consistent. For example, while many studies report good correlations between MP source regions (highly populated and industrialized areas) and MP abundance in river sediments, others do not. This is in contrast to the concentrations in the water itself, for which better links with MP source areas were found. What all these studies have in common, is their large-scale approach, in which sediment samples are obtained over distances of tens to thousands of kilometers along the river; and at each site sediments are than retrieved either from the deepest part of the channel or from the river bank (depending on the study). Here, we study MP distribution in a section of the meandering Leie River, in a rural area, a few kilometers upstream of the city of Ghent (Belgium). Multibeam bathymetry and side-scan sonar images allowed selecting three undisturbed across-channel transects where surface sediments were retrieved. Sediment samples were analyzed for MP content, organic-matter content and grain size of the clastic fraction. Overall the MP concentrations are much (up to an order of magnitude) lower in the thalweg compared to samples near the river banks, resulting in an asymmetric distribution at the bend apex, where the thalweg approaches the outer banks. Furthermore, MP concentrations show strong correlations with the organic matter content and grain-size parameters as expected form hydrodynamic sorting. Exceptions to these correlations are the outer bank samples, where MP concentrations are lower than predicted from sedimentological characteristics. We attribute this to the more erosive character of the current in the thalweg near the outer banks, which inhibits MP deposition, but exposes fine-grained and organic-rich flood plain sediments. We highlight that the different hydrodynamic conditions across a river channel greatly influence MP distribution (with an order of magnitude), but in a sedimentologically predictable manner. Care should thus be taken in environmental studies, as local variability in MP concentration across a river bed may be larger then the large-scale variability.

Microplastic analysis in sediments of the Elbe River by electrostatic separation and differential scanning calorimetry

Bioretention systems are used to treat stormwater. Using coarser filter media than commonly recommended with high saturated hydraulic conductivities may increase annual runoff volume capture, facilitate smaller filters, less overflow and adaptation to cold winters. However, this may affect water quality treatment negatively. Therefore, we investigated total and dissolved metal treatment at three full scale bioretention systems with a coarse filter material and saturated hydraulic conductivities >1500mm/h in Malmö/Sweden. One bioretention system was designed with a coarse sand-based filter medium, another with coarse sand-based filter medium and a submerged zone and the third with a 50:50 mixture of coarse sand and pumice as filter medium. The study included 19 rain events, partly during winter season when road salt was applied. The results suggest that also filter media with high hydraulic conductivity can be an effective option when metal treatment is targeted. The two systems with coarse sand filter media treated total metals effectively with median removals >80% for Cu, Pb and Zn and median removals >35% for Ni and Cr. Dissolved metal treatment was variable reaching from effective treatment for dissolved Cu, Pb and Zn with median removals >60% to overall leaching of dissolved Cd, Ni and Cr. Applying a submerged zone did not showed benefits for total or dissolved metal removal. Further, treatment of total and partly dissolved metals was significantly impaired due to pumice addition of the filter media, discouraging pumice as a filter media amendment. Coarser filter materials could be recommended for sites with space limitations or when frozen ground is expected in winter. Further, they can generally reduce clogging risks and untreated overflows.

Microplastic concentrations and risk assessment in water, sediment and invertebrates from Simon's Town, South Africa

The threat of plastic waste in the environment has evoked rising concern over the past decades. While these versatile and incredibly popular polymer materials undoubtedly fulfil unprecedented services, their extreme durability and alleged toxicity represent major downsides, once in the environment. As plastics break down into smaller microplastics (MP) they evidently pollute almost every thinkable habitat on the globe today. While an estimated 5–12 million tons of plastic end up in the oceans every year, rivers are important pathways, carrying an annual freight of 0.41–4 million tons downstream. The Rhine River, one of Europe’s main streams and one of the World’s busiest waterways, reportedly holds substantial amounts of MP in its near-surface waters and sediments. However, little is known about potential MP sources, seasonal dynamics and the MP pollution of benthic sediments of this major river. In my dissertation I address three pivotal knowledge gaps about MP in the Rhine River and investigate a new type of method to safely and efficiently separate MP from surrounding environmental sample matrices. The surface water MP pollution of the Rhine River was earlier characterised by the distinct appearance of vast amounts (~60%) of rigid polystyrene microbeads of unknown origin and former purpose. In a sustained investigation of a defined river stretch in the Lower Rhine downstream Cologne, we managed to narrow down the entry region of these pollutants and close-to-certainly unveil the particle’s former purpose as ion-exchange resin beads used in diverse fluid purification applications. As more data on MP in rivers emerges, an interesting gap opens: empirical field studies tend to generate temporally restricted snapshot data while modellers, laudably seeking the bigger picture, are confronted with enormous uncertainties in their results. In a bid to reduce this void we embarked on a quadruplicate surface water MP observation in the Rhine catchment during 2016–2017. The investigation included three nival discharge regime tributaries and the Rhine in Switzerland as well as a pluvial Rhine River section towards the German-Dutch border. It became evident that despite a coherent increasing MP concentration gradient downstream, reflecting average discharge and catchment size, overall variability in environmental MP data was large. This hampers clear-cut estimations about concentration fluctuations but at the same time reinforces the theory of high MP seasonal pollution fluxes in the European winter months, when Rhine discharge is highest. Seeking to contribute to the very scarce knowledge of MP and benthic sediment interaction in large dynamic rivers, we took a diving bell and a dredging vessel to the riverbed at two previously identified surface MP hotspots at the German Middle and Lower Rhine. Due to expected water turbulence and flow velocity, substantial settling of MP in such stretches is not necessarily anticipated. Interestingly, it showed that even the Rhine riverbed is not spared from vast MP pollution. The applied research technology allowed for reliable detection of MP down to particle sizes of 11 μm and yielded strongly varying concentrations of 260–11,000 MP kg^–1, with a strongly skewed size class distribution towards the smallest MP particles. Experience showed us that environmental sample preparation for MP research can be a tedious and resource-intensive enterprise – including health and environmental hazards. Instead of attacking the unwanted portions in a sample (e.g. biogenic residue to isolate MP), a more efficient approach is to select the MP specifically. We refined a proposed lipophilicity-based separation technique and broadened its scope by successfully testing it to four different types of environmental matrices. This one-for-all approach may present a promising means for quicker, cheap, safe and efficient MP data compilation. The method was successfully applied for the seasonal surface MP investigation presented in this dissertation.

The accumulation of plastics in the soil ecosystem poses an increasing environmental concern worldwide. However, little is known about the effect of plastic concentrations on soil properties and soil biota. In this study, we investigated the effect of low‐density polyethylene (LDPE) microplastics (MPs) on the chemical and microbial properties of agricultural soil using a set of microcosm experiments. The soil was incubated for 100 days with LDPE at concentrations of 0%, 0.1%, 1%, 3%, 5%, and 7% at 25°C with 70% water‐holding capacity. Along with soil chemical analysis, we conducted an analysis of soil microbial properties on the first day and again after 100 days of incubation. LDPE concentrations of ≥1% significantly ( p < .05) decreased the pH but increased the electrical conductivity of the soil in comparison with the control (0% LDPE at 100 days). Increasing the LDPE concentration did not affect the soil exchangeable cation content or the available Pb concentration. Firmicutes were the most abundant phyla in the soil on the first day, whereas Proteobacteria, Firmicutes and Actinobacteria became dominant in all treatments after 100 days. An increasing LDPE concentration increased the abundance of Actinobacteria and decreased Proteobacteria. Principal component analysis demonstrated that only 7% LDPE was positively correlated with Actinobacteria, indicating that higher concentrations of LDPE contributed to the growth of this phylum. The findings of this study imply that MP contamination could affect soil chemical properties and microbial activity and that these effects primarily depend on MP concentrations in soil.

Microplastics in rainwater/stormwater environments: Influencing factors, sources, transport, fate, and removal techniques

Microplastics (MPs), identified as emerging contaminants, have been detected across diverse environmental media. Their enduring presence and small size facilitate the adsorption of organic pollutants and heavy metals, leading to combined pollution effects. MPs also accumulate in the food chain thus pose risks to animals, plants, and human health, garnering significant scholarly attention in recent years. Aerobic granular sludge (AGS) technology emerges as an innovative approach to wastewater treatment. However, the impacts of MPs on the operational efficiency and microbial characteristics of AGS systems has been insufficiently explored. This study investigated the effects of varying concentration (10, 50, and 100 mg/L) of biodegradable MPs (Polylactic Acid, PLA) and non-biodegradable MPs (Polyethylene Terephthalate, PET) on the properties of AGS and explored the underlying mechanisms. It was discovered that low and medium concentration of MPs (10 and 50 mg/L) showed no significant effects on COD removal by AGS, but high concentration (100 mg/L) of MPs markedly diminished the ability to remove COD of AGS, by blocking most of the nutrient transport channels of AGS. However, both PLA and PE promoted the nitrogen and phosphorus removal ability of AGS, and significantly increased the removal efficiency of total inorganic nitrogen (TIN) and total phosphorus (TP) at stages II and III (P < 0.05). High concentration of MPs inhibited the growth of sludge. PET noticeably deteriorate the sedimentation performance of AGS, while 50 mg/L PLA proved to be beneficial to sludge sedimentation at stage II. The addition of MPs promoted the abundance of Candidatus_Competibacter and Acinetobacter in AGS, thereby promoting the phosphorus removal capacity of AGS. Both 50 mg/L PET and 100 mg/L PLA caused large amount of white Thiothrix filamentous bacteria forming on the surface of AGS, leading to deterioration of the sludge settling performance and affecting the normal operation of the reactor. Comparing with PET, AGS proved to be more resistant to PLA, so more attention should be paid to the effect of non-biodegradable MPs on AGS in the future.

Due to the copious disposal of plastics, marine ecosystems receive a large part of this waste. Microplastics (MPs) are solid particles smaller than 5 millimeters in size. Among the plastic polymers, polystyrene (PS) is one of the most commonly used and discarded. Due to its density being greater than that of water, it accumulates in marine sediments, potentially affecting benthic communities. This study investigated the ingestion of MP and their effect on the meiofauna community of a sandy beach. Meiofauna are an important trophic link between the basal and higher trophic levels of sedimentary food webs and may therefore be substantially involved in trophic transfer of MP and their associated compounds. We incubated microcosms without addition of MP (controls) and treatments contaminated with PS MP (1-µm) in marine sediments at three nominal concentrations (103, 105, 107particles/mL), for nine days, and sampled for meiofauna with collections every three days. At each sampling time, meiofauna were collected, quantified and identified to higher-taxon level, and ingestion of MP was quantified under an epifluorescence microscope. Except for Tardigrada, all meiofauna taxa (Nematoda, turbellarians, Copepoda, Nauplii, Acari and Gastrotricha) ingested MP. Absorption was strongly dose dependent, being highest at 107 particles/mL, very low at 105 particles/mL and non-demonstrable at 103 particles/mL. Nematodes accumulated MP mainly in the intestine; MP abundance in the intestine increased with increasing incubation time. The total meiofauna density and species richness were significantly lower at the lowest MP concentration, while at the highest concentration these parameters were very similar to the control. In contrast, Shannon-Wiener diversity and evenness were greater in treatments with low MP concentration. However, these results should be interpreted with caution because of the low meiofauna abundances at the lower two MP concentrations. At the highest MP concentration, abundance, taxonomic diversity and community structure of a beach meiofauna community were not significantly affected, suggesting that MP effects on meiofauna are at most subtle. However, lower MP concentrations did cause substantial declines in abundance and diversity, in line with previous studies at the population and community level. While we can only speculate on the underlying mechanism(s) of this counterintuitive response, results suggest that further research is needed to better understand MP effects on marine benthic communities.

Microplastic fluxes in a large and a small Mediterranean river catchments: The Têt and the Rhône, Northwestern Mediterranean Sea.

Transfer and transport of microplastics from biosolids to agricultural soils and the wider environment

Emerging threats of microplastics on marine environment: A critical review of toxicity measurement, policy practice gap and future research direction

Plastic waste is a major source of microplastic (MP) pollution, posing adverse environmental and public health risks. This study assessed MP abundance in 24 urban ponds and lakes within the Kolkata Municipal Corporation, India, during the post- and pre-monsoon seasons of 2022-2023. Results showed that MP concentrations were significantly higher in the post-monsoon season (20 ± 3.46 items L⁻1; ANOVA, p < 0.05), with red, white, and black particles being the most prevalent. MPs prevalently ranged between 500 and 1000µm with fibers constituting 58-59% of the total MPs. Fourier-transform infrared (FTIR) analysis identified polyethylene (PE) as the dominant polymer. MP abundance showed negative correlations with dissolved oxygen and turbidity, and positive associations with pH, TDS, BOD₅, and total coliform counts. A Support Vector Machine (SVM) model was developed for morphology-based MP classification, achieving an accuracy of 89%. Additionally, remote sensing and GIS techniques were used to develop index-based models for water-body identification and MP quantification (items L⁻1) using spectral data from Sentinel-2 imagery. These models demonstrated high validation accuracy of 98.01% and 92.60%, respectively. Metal analysis of sediment and MPs of water detected chromium, suggesting possible MP-metal interactions within sediments. Although the Pollution Load Index (PLI) indicated relatively low contamination levels, the Polymer Hazard Index (PHI) exceeded 1000, indicating substantial ecological risk. Future studies should focus on long-term monitoring, socio-economic and health impact assessments and development of effective mitigation strategies particularly addressing plastic-waste derived-MPS.

Bioretention cells are a popular type of green stormwater management infrastructure. In urban catchments, they reduce the 'flashiness' and volume of surface flow resulting from storms by promoting storage, groundwater infiltration, and evapotranspiration. Although they can also remove pollutants from stormwater runoff, their effectiveness in reducing phosphorus (P) loads is highly variable, in part because of the complex biogeochemical cycling of P in bioretention cell systems. Using time series solid-phase P concentrations measured over a 9-year period on the filter medium from a multi-cell bioretention system in Mississauga, Ontario, Canada, we estimated average total P (TP) accumulation rates of 88mg Pkg⁻1y⁻1 in the top 10cm and 33 mg Pkg⁻1y⁻1 at depths of 25-45 cm. These accumulation rates translate into a net sequestration of TP by the filter medium across the entire system of roughly 4 kg per year. The linear TP concentration versus time trends further imply that the filter medium has yet to reach P saturation. The system can, therefore, be expected to continue reducing stormwater runoff P loads in the foreseeable future. On cores collected in 2019 and 2021 (7 and 9 years post-construction), the depth-dependent solid-phase P chemical speciation was analyzed using an established sequential chemical extraction protocol. The more reactive P fractions and organic P compounds were generally enriched in the upper 10 cm of the filter medium. Accumulation of P associated with organic matter contributed most to the net TP accumulation on the filter medium. Non-metric dimensional scaling (NMDS) further indicated that variations in the concentrations of TP and the P fractions among filter media samples were mainly linked to variations in the concentrations of the redox-sensitive metals iron and manganese and that of organic carbon. Overall, our study highlights that the spatial and temporal distributions and chemical speciation of P can help unravel the mechanisms controlling P abatement of urban stormwater by bioretention systems.