Systematic CFD-based evaluation of physical factors influencing the spatiotemporal distribution patterns of microplastic particles in lakes.

Spatiotemporal distribution patterns of microplastic (MP) particles in lakes hinge on both the physical conditions in the lake and particle properties. Using numerical simulations, we systematically investigated the influence of lake depth and bathymetry, wind and temperature conditions, MP particle release location and timing, as well as particle diameter (10, 20, and 50 µm). Our results indicate that maximum lake depth had the greatest effect on the residence time in the water column, as it determines the settling timescale and occurrence of hydrodynamic complexity such as density-driven flows in the lake. Increasing particle size from 10 to 20 and 50 µm also significantly reduced the residence time making particle size the factor with the second strongest effect on the residence time and, in turn, on the availability of MP particles for uptake by organisms. Changing bathymetry from a uniform to a non-uniform had a less pronounced effect on particle residence time compared to maximum depth and particle size. Release location, wind conditions, and release time had comparably little effect on particle behavior but became more important as MP particle size decreased. The release of the 10 µm MP particles in the deeper lakes with uniform bathymetry during summer with stable thermal stratification, resulted in a nearly month-long turnover phase in the fall in which both settling and rising of particles occurred simultaneously. This was caused by convective heat and water transport during this period. In these scenarios about 2.6 to 5.4 % of the released MP particles were held in or returned to the water layers near the lake surface. While acknowledging the dominant role of lake depth and MP particle size on the particle residence time, this study further emphasizes that it is ultimately a particular combination of different factors and their interactions that shape MP distribution patterns in lakes.

Quantifying microplastic residence times in lakes using mesocosm experiments and transport modelling.

The microplastic residence time in lakes is a key factor controlling its uptake by lake organisms. In this work we have, for the first time, conducted a series of microplastic addition experiments in a 12 × 3 m lake mesocosm and traced its transport through the lake water column. This was combined with a 1D physically based random walk model of microplastic transport. Four experiments were conducted using three microplastic size ranges (1-5, 28-48, and 53-63 µm) over one year during thermal stratification and lake turnover. The results showed that the residence time in the water column largely depended on particle size and lake hydrodynamics, although the smallest particles were poorly represented by the model. Residence times in the mesocosm ranged between ∼1 day for the largest particles to 24 days for the small particles during summer. The modeled residence time were similar to the measured values of the 28-48 µm and 53-63 µm particles, but for the smallest particles residence times were calculated to be >200 d. The discrepancy is likely due to aggregation between the small microplastic particles and natural lake particles, which increases the microplastic settling velocity. Aggregation is favored for the small particles due their large surface area to volume ratio. In contrast, density instabilities in the water column during autumn likely led to turbulent convective mixing and rapid microplastic transport within the water column. This work shows that microplastic transport within lakes is complex and not fully understood, especially for the smallest sizes, and involves interactions between physical, physicochemical and biological processes.

Enhanced periphyton biodegradation of endocrine disrupting hormones and microplastic: Intrinsic reaction mechanism, influential humic acid and microbial community structure elucidation.

Endocrine-disrupting compounds (EDCs), as well as microplastics, have drawn global attention due to their presence in the aquatic ecosystem and persistence in wastewater treatment plants (WWTPs). In the present study, for simultaneous bio-removal of two EDCs, 17α-ethinylestradiol (EE2), bisphenol A (BPA), and a microplastic, polypropylene (PP) four kinds of periphytic biofilms were employed. Additionally, the effect of humic acid (HA) on the removal efficacy of these biofilms was evaluated. It was observed that EE2 and BPA (0.2 mg L-1 each) were completely (∼100%) removed within 36 days of treatment; and the biodegradation of EE2, BPA, and PP was significantly enhanced in the presence of HA. Biodegradation of EE2 and BPA was evaluated through Ultra-high performance liquid chromatography (UHPLC), and Gas chromatography coupled with tandem mass spectrometry (GC-MS/MS) was used to determine the mechanism of degradation. Gel permeation chromatography (GPC) and SEM had validated the biodegradation of PP (5.2-14.7%). MiSeqsequencing showed that the community structure of natural biofilm changed after the addition of HA, as well as after the addition of EDCs and PP. This change in community structure might be a key factor regarding variable biodegradation percentages. The present study revealed the potential of periphytic biofilms for the simultaneous removal of pollutants of different chemical natures, thus provides a promising new method for wastewater treatment applications.

Effects of clay minerals on the transport of nanoplastics through water-saturated porous media.

Clay minerals are important constituents of porous media. To date, only little is known about the transport and retention behavior of nanoplastics in clay-containing soil. To investigate the effects of clay minerals on the mobility of nanoplastics in saturated porous media, polystyrene nanoplastics (PS-NPs) were pumped through columns packed with sand and clay minerals (kaolinite and illite) at different pH and ionic strengths (IS). Mobility of PS-NPs decreased with increasing clay content attributed to physical straining effects (smaller pore throats and more complex flow pathways). Variations in pH and IS altered the surface charges of both PS-NPs and porous media and thus affecting the interaction energy. An increase of IS from 10 mM to 50 mM NaCl decreased the maximum energy barrier and secondary minimum from 142 KBT to 84 KBT and from -0.1 KBT to -0.72 KBT, respectively. Thus, the maximum C/C0 ratio decreased from ~51% to ~0% (pH 5.9, 3% kaolinite). Among the two clay minerals, kaolinite showed a stronger inhibitory effect on PS-NPs transport compared to illite. For instance, at the same condition (3% clay content, pH 5.9, 10 mM NaCl), the (C/C0)max of PS-NPs in kaolinite was ~51%, while for illite, it was ~77%. The difference in transport inhibition was mainly attributed to amphoteric sites on the edges of kaolinite which served as favorable deposition sites at pH 5.9 (pHpzc-edge is ~2.5 for illite and ~6.5 for kaolinite). Besides, the morphology of kaolinite was more complex than illite, which may retain more PS-NPs in kaolinite. Results and conclusions from the study will provide some valuable insights to better understand the fate of NPs in the soil-aquifer system.

A novel method using a silicone diffusion membrane for continuous ²²²Rn measurements for the quantification of groundwater discharge to streams and rivers.

²²²Rn is a natural radionuclide that is commonly used as tracer to quantify groundwater discharge to streams, rivers, lakes, and coastal environments. The use of sporadic point measurements provides little information about short- to medium-term processes (hours to weeks) at the groundwater-surface water interface. Here we present a novel method for high-resolution autonomous, and continuous, measurement of ²²²Rn in rivers and streams using a silicone diffusion membrane system coupled to a solid-state radon-in-air detector (RAD7). In this system water is pumped through a silicone diffusion tube placed inside an outer air circuit tube that is connected to the detector. ²²²Rn diffuses from the water into the air loop, and the ²²²Rn activity in the air is measured. By optimizing the membrane tube length, wall thickness, and water flow rates through the membrane, it was possible to quantify radon variations over times scales of about 3 h. The detection limit for the entire system with 20 min counting was 18 Bq m⁻³ at the 3σ level. Deployment of the system on a small urban stream showed that groundwater discharge is dynamic, with changes in ²²²Rn activity doubling on the scale of hours in response to increased stream flow.