Entrainment and Enrichment of Microplastics in Ice Formation Processes: Implications for the Transport of Microplastics in Cold Regions

Integration of acoustic, optical, and electrical methods in picoliter droplet microfluidics for rare particles enrichment

Rare particle enrichment plays a pivotal role in advancing numerous scientific research areas and industrial processes. Traditional enrichment methods encounter obstacles such as low efficiency, high cost, and complexity. Acoustic focusing, optical fiber detection, and electrical manipulation have shown potential in microfluidics for particle manipulation and analysis. This study pioneers the integration of the acoustic, optical, and electrical units to overcome the traditional limitations. The cooperative dynamics of acoustic and flow focusing are explored. The optical fibers with an enhanced detection algorithm greatly boost optical detection sensitivity. Furthermore, the droplet charging to enhance the tip charging phenomenon is complemented and validated. The detection and sorting accuracy of enriching large-size H22 cells reached 99.8% and 99.3%, respectively, with the target cell concentration increased by nearly 86-fold. Our work significantly enhances detection sensitivity and particle manipulation accuracy, ultimately offering a robust and reliable solution for generating droplets to enrich rare particles.

Nanoscale insight into the interaction mechanism underlying the transport of microplastics by bubbles in aqueous environment

The ecological risk of microplastics (MPs) is raising concern about their transport and fate in aquatic ecosystems. The capture of MPs by bubbles is a ubiquitous natural phenomenon in water-based environment, which plays a critical role in the global cycling of MPs, thereby increasing their environmental threats. However, the nanoscale interaction mechanisms between bubbles and MPs underlying MPs transport by bubbles in complex environmental systems remain elusive. This work for the first time directly measured and evaluated the interactions between bubble and polystyrene microplastic (PSMP) under various environmental factors in aqueous media using atomic force microscope (AFM) combined with a Stokes-Reynold-Young-Laplace (SRYL) model. Since hydrophobic interaction was strong enough to act across the repulsive barrier, bubble-PSMP attachment always occurred at different NaCl concentrations, pH and hydrodynamic conditions, and a decay length D0 of hydrophobic interaction was determined as 0.65 ± 0.05 nm. No bubble attachment was observed during approach for aged PSMP (APSMP) with the weakened hydrophobic interaction (D0 = 0.33 ± 0.02 nm), while in 100 mM NaCl, APSMP-bubble attachment occurred during retraction due to the hydrodynamic suction effect. The decreased D0 arose from the increased oxygen-containing groups on APSMP surfaces that significantly reduced the hydrophobicity of MPs surface as evidenced by X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. It was further evident from transport tests that aging plays a crucial role in MPs transport driven by bubbles. This work provides nanoscale information on the interaction mechanism underlying the MPs transport by bubbles, with implications to evaluate the fate of MPs in aqueous environments.

Amine antioxidants in water, ice, sediment and soil from the Songhua Wetland, Northeast China: Occurrence and fate

Amine antioxidants (ANs) are emerging organic pollutants that are widely used in industrial products. The extensive use of ANs has polluted the environments. At present, there is no report on the pollution situation of ANs in wetland environment. In this study, samples of water, ice, sediment, and soil were collected in the Songhua Wetland to study the behavior of ANs. Total concentrations of ANs ranged from 27.4 to 137 ng L-1 in water, 88.8-127 ng L-1 in ice, 46.0-169 ng g-1 dw in sediment, and 43.7-191 ng g-1 dw in soil. The concentrations of ANs were higher in water during the ice-covered period (108-137 ng L-1) than that in the non-ice-covered (49.0-83.9 ng L-1). The Spearman correlation analysis suggests that most ANs were positively correlated with each other, indicating they were from the similar source. The overall enrichment of ANs in ice relative to water may relate to factors such as salt concentration and turbulence intensity. Since Kd is much <100, it suggests that ANs were more distributed in water compared to sediment. These results provided new perspectives for subsequent related studies.

Microplastics in sea ice drifted to the Shiretoko Peninsula, the southern end of the Sea of Okhotsk

Sea ice is regarded as a temporal sink and carrier of microplastics (MPs). Nevertheless, knowledge and understanding of MPs in sea ice remain sparse. This study investigated the abundance, composition, size (> 30 μm), and shape of MPs in four sea-ice cores retrieved at the southern end of the Sea of Okhotsk. Nine microplastic (MP) types, mostly with fragmentary shapes, were detected among ice-core sections. Most fragmentary MPs were smaller than 120 μm, but all fiber MPs were in the largest size class (> 210 μm). MP concentrations were 0-60 particles/L, with an average of 21 particles/L. Higher occurrences of MPs observed in the lower ice layers are attributable to heavier MP contamination in the southern part of the sea and/or relocation of MPs in the ice matrix. No significant correlation was found between the distributions of MP and inorganic particle (sediment) abundances, implying difference in their kinetics of suspension freezing. Taken together, these findings suggest important implications for elucidating the nature and distribution of MPs in sea ice.

Distribution of microplastics between ice and water in aquatic systems: The influence of particle properties, salinity and freshwater characteristics

Microplastics (MPs) are an anthropogenic emerging pollutant, with global contamination of both marine and freshwater systems extensively documented. The interplay of MP particle properties and environmental conditions needs to be understood in order to assess the environmental fate and evaluate mitigation measures. In cold climate, ice formation has appeared to significantly affect the distribution of MPs, but so far, limited research is available comparing different aquatic systems, especially freshwater. Experiments often rely on artificial water and specific MP model particles. This study used laboratory tests to investigate the ice-water distribution of a variety of environmentally relevant MP particle types (PP, PE, PS and PVC fragments (25-1000 μm), PET fibers (average length 821 μm, diameter 15 μm)) across different water types, including artificial water of high and low salinity, as well as natural water from a lake and a treatment wetland. Overall, ice entrapment of MPs occurred in almost all tests, but the ice-water distribution of MPs differed across the different water types tested. Among the tests with artificial water, salinity clearly increased MP concentrations in the ice, but it cannot be resolved whether this is caused by increased buoyancy, changes in ice structure, or thermohaline convection during freezing. In the natural freshwater tests, the partition of MPs was shifted towards the ice compared to what was seen in the artificial freshwater. The influence of different types of dissolved and particulate substances in the different waters on MPs fate should be considered important and further explored. In this study, the higher content of suspended solids in the lake water might have enhanced MP settling to the bottom and thereby contributed to the absence of MPs in the ice of the lake test, compared to the wetland test with low suspended solids and considerably more MPs in the ice. Furthermore, the higher negative charge in the lake water possibly stabilized the negatively charged MPs in suspension, and reduced ice entrapment. Regarding particle properties, shape had a distinct effect, with fibers being less likely incorporated into ice than fragments. No fibers were found in freshwater ice. However, it became clear that ice entrapment of MPs depends on factors other than the particles' buoyancy based on density differences and particle size and shape alone.

Nanoplastics Distribution during Ice Formation: Insights into Natural Surface Water Freezing Conditions

The migration characteristics of nanoplastics (NPs) in the natural freezing process are complex and have attracted increasing attention in simulating natural freezing in recent years. However, simulated freezing conditions often fall short of replicating natural freezing processes, and studies on the vertical distribution of NPs remain inadequate. This study established a more realistic simulation of the natural freezing process in surface water by controlling both the air temperature (T1) and the water temperature (T2). Additionally, we introduced a new parameter, the local distribution coefficient (Kiw1), to compare with the effective distribution coefficient (Kiw2). The values of Kiw1 and Kiw2 for PS-500 nm were 0.18 and 0.21, respectively, at T1 = -20 °C and T2 = 1 °C. The results revealed the NPs concentration differed in ice, near-ice liquid, and far-ice liquid. Both properties of NPs and environmental factors could regulate the vertical distribution of NPs. The findings underscored the importance of freezing temperature regulated by T1 and T2, elucidating the roles of various influencing factors on the vertical distribution characteristics of NPs and unraveling the mechanisms of NPs distribution in the ice-water system. This study can provide valuable insights for understanding the migration of NPs in surface water in cold regions.

The degradation of polylactic acid face mask components in different environments

The disposal of fossil fuel-based plastics poses a huge environmental challenge, leading to increased interest in biodegradable alternatives such as polylactic acid (PLA). This study focuses on the environmental impact and degradation of PLA face mask components under various conditions (UV (Ultraviolet) radiation, DI water, landfill leachate of various ages, seawater, and enzyme). Under UV exposure, notable changes in physicochemical properties were observed in the PLA masks, including increased oxidation over time. Degradation rates varied across environments, with old landfill leachate and enzyme degradation having a notable impact, especially on meltblown layers. Furthermore, it was found that seawater conditions hampered the degradation of PLA masks, likely due to the inhibitory effect of high salt concentrations. The pathways of chemical group changes during degradation were elucidated using 2D-COS (Two-Dimensional Correlation Spectroscopy) maps. The investigation into the release of microparticles and oligomers further revealed the degradation mechanism. Moreover, PLA masks were found to release fewer microparticles when degraded in studied environments when compared to traditional polypropylene masks. Furthermore, correlation analysis highlighted the influence of factors such as carbonyl index and contact angle on degradation rates, underscoring the complex interplay between environmental conditions and PLA degradation. This comprehensive investigation advances the understanding of PLA degradation pathways, which are crucial for mitigating plastic pollution and promoting the development of sustainable products.

Preliminary assessment of microplastic in rhizosphere and non-rhizosphere region of mangrove at four locations along Karachi coast, Pakistan

Mangrove ecosystem faces significant threats from the various pollutants including microplastic (MPs). The aim of this study was to assess variations in MP distribution in mangrove sediments of rhizosphere (R) and non-rhizosphere (NR) regions. A total of 14,960 MP particles were identified from Sandspit backwater (SS-1 & SS-2) and Creek areas (PQ & KC). Notably, the NR showed higher MP counts (7848) compared to the R region (7112). Analysis revealed variations in MP types, with beads being predominant in both R and NR, followed by film, fiber, and fragments. KC exhibited highest MP contamination, followed by PQ, SS-2, and SS-1. Fourier-transform infrared (FTIR) analysis confirmed the presence of polyethylene terephthalate and polyethylene in sediments samples. This first detailed report on MP in mangrove sediments and other limited studies from Pakistan establishes the widespread distribution of MPs in the coastal area and provide a baseline for further elaboration in future.

Revealing the Freezing-Induced Alteration in Microplastic Behavior and Its Implication for the Microplastics Released from Seasonal Ice

Ice can serve as a significant temporary repository and conveyance mechanism for microplastics (MPs). MPs present in the water column can become entrapped within developing ice formations, subsequently being sequestered and transported by ice floes. With changing temperatures, MPs stored in ice can be released back into the environment, while freezing conditions can alter the properties of MPs, ultimately affecting the fate of MPs in the environment. Freezing of MPs in freshwater ice results in the aggregation of MP particles due to physical compression, leading to an increase in particle size once the MPs are released from the ice. The freezing-induced aggregation enhances buoyancy effects, accelerating the settling/rising velocity of MPs in water. Additionally, freezing can lead to enhanced surface wetting alterations, thus improving the dispersion of hydrophobic MPs. The presence of salt in the water can mitigate the effect of freezing on MPs due to the formation of a brine network within the ice structure, which reduces the pressure on MPs entrapped by ice. In cold regions, numerous MPs undergo freezing and thawing, re-entering the water column.

Spotlight on the vertical migration of aged microplastics in coastal waters

Coastal waters are complex and dynamic areas with unique environmental attributes that complicate the vertical migration of microplastics (MPs). The MPs that enter coastal waters from diverse sources, including terrestrial, riverine, oceanic, and shoreline inputs undergo various aging pathways. In this study, the variations in the physiochemical characteristics of MPs undergoing various aging pathways and their vertical migration under dynamic conditions subjected to the effects of different MP characteristics and coastal environmental features were comprehensively explored. Opposite effects of aging on the vertical migration of hydrophobic and hydrophilic MPs were observed, with aging appearing to promote the dispersion of hydrophobic MPs but enhance the vertical migration of hydrophilic ones. The positive role of salinity and the negative role of humic acid (HA) concentrations on MP vertical migration were identified, and the mechanisms driving these effects were analyzed. Notably, intense turbulence not only promoted the floating of positively buoyant MPs but also reversed the migration direction of negatively buoyant MPs from downward to upward. Aging-induced changes in MP characteristics had a limited effect on MP vertical migration. The inherent characteristics of MPs and the surrounding environmental features, however, played major roles in their vertical migration dynamics. ENVIRONMENTAL IMPLICATION: Microplastics (MPs) have emerged as a significant global environmental concern and the coastal zones are the hotspots for MP pollution due to their high population density. This study comprehensively investigated the variations in the physiochemical characteristics of MPs undergoing various aging pathways. Their vertical migration patterns under dynamic conditions subjected to the effects of different MP characteristics and coastal environmental features were revealed. The roles of turbulence and MP density in their migration were identified. The findings of this study have important implications for understanding the transport and determining the ecological risks of MPs in coastal waters.

Prolonged drying impedes the detachment of microplastics in unsaturated substrate: Role of flow regimes

The detachment of microplastics (MPs) from porous media under different moisture conditions and flow regimes has garnered limited attention within the research community. The present study investigates the detachment of MPs from porous media under wet and dry conditions combined with steady and transient flow. For both the wet and dry conditions, the increase in flow rates is found to decrease the detachment of hydrophobic polyethylene of two sizes and of hydrophilic polymethylmethacrylate. Intermittent flow is found to result in effluent peaks and a higher rate of MP detachment compared to steady flow. The ionic strength of inflow drops in a stepwise manner, leading to abrupt peaks followed by a tail corresponding to the arrival of each ionic strength front. Each step increase in flow rate leads to a steep peak followed by slow release over several pore volumes. Although transient flow facilitates the detachment of MPs, drying significantly impedes the detachment of MPs irrespective of flow regime. Ultraviolet weathering of MPs for 60 days weakens the inhibition effect of drying on hydrophilic polymethylmethacrylate, facilitating their detachment. Furthermore, the release of MPs decreases markedly with an increase in air-drying duration from 0 h to 72 h. Hydrus-1D two-site kinetic models are used to successfully simulate time-dependent processes, implying that drying heightens the energy barrier for MPs to detach. Our analysis confirms the significance of moisture in determining the remobilization of MPs, providing valuable insights concerning the fate of MPs in unsaturated substrate under prolonged drought conditions.

Aging of plastics in aquatic environments: Pathways, environmental behavior, ecological impacts, analyses and quantifications

The ubiquity of plastics in our environment has brought about pressing concerns, with their aging processes, photo-oxidation, mechanical abrasion, and biodegradation, being at the forefront. Microplastics (MPs), whether originating from plastic degradation or direct anthropogenic sources, further complicate this landscape. This review delves into the intricate aging dynamics of plastics in aquatic environments under various influential factors. We discuss the physicochemical changes that occur in aged plastics and the release of oxidation products during their degradation. Particular attention is given to their evolving environmental interactions and the resulting ecotoxicological implications. A rigorous evaluation is also conducted for methodologies in the analysis and quantification of plastics aging, identifying their merits and limitations and suggesting potential avenues for future research. This comprehensive review is able to illuminate the complexities of plastics aging, charting a path for future research and aiding in the formulation of informed policy decisions.

Spatiotemporal variability and environmental effects of greenhouse gases, nutrients, and dissolved carbons in an ice-covered reservoir

Ice cover restructures the distribution of substances in ice and underlying water and poses non-negligible environmental effects. This study aimed to clarify the spatiotemporal variability and environmental effects of methane (CH4), nitrous oxide (N2O), total nitrogen (TN), total phosphorus (TP), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) in ice and water columns during different ice-covered periods. We surveyed the ice-growth, ice-stability, and ice-melt periods in an ice-covered reservoir located in Northeast China. The results showed that underlying water (CH4: 1218.9 ± 2678.9 nmol L-1 and N2O: 19.3 ± 7.3 nmol L-1) and ice (CH4: 535.2 ± 2373.1 nmol L-1 and N2O: 9.9 ± 1.5 nmol L-1) were sources of atmospheric greenhouse gases. N2O concentrations were the highest in the bottom water of the reservoir while CH4 accumulated the most below the ice in the riverine zone. These can be attributed to differences in the solubilities and relative molecular masses of the two gases. Higher concentrations of N2O, TN, TP, DOC, and DIC were recorded in the underlying water than those in the ice due to the preferential redistribution of these substances in the aqueous phase during ice formation. Additionally, we distinguished between bubble and no-bubble areas in the riverine zone and found that the higher CH4 concentrations in the underlying water than those in the ice were due to CH4 bubbles. In addition, we reviewed various substances in ice-water systems and found that the substances in ice-water systems can be divided into solute exclusion and particle entrapment, which are attributed to differences between dissolved and particulate states. These findings are important for a comprehensive understanding of substances dynamics during ice-covered periods.