Probing the aging process and mechanism of microplastics under reduction conditions

Dynamic process of UV-aging polystyrene microplastics, simultaneous adsorption of drugs, and subsequently coagulative removal together

The aging of plastics and their adsorptive interactions with the residual contaminants in water has attracted increasing attentions. In this study, the dynamic process of UV-aging polystyrene (PS) microplastics (MPs) were semi-quantitatively analyzed using a coulter counter, and the adsorptive interactions between the aged PS MPs and two popular drugs[norfloxacin (NOR) and chloroquine phosphate (CQ)] were investigated simultaneously. The MPs presented a rapid size downtrend, reduced from micrometer to nanometer, and the particle number concentration increased about 2 -3 times after a 36.0 h aging effect. The apparent UV-aging process of PS MPs mainly obeyed the pseudo-first order kinetic model in currently measured MPs' size range. The drug uptakes of the aged MPs were fully consistent with the contents of oxygen-containing groups on MPs surface rather than MPs' size. The involved adsorption mechanisms were investigated in detail mainly including electrostatic attraction, hydrogen bonding, and π-π electron donor-acceptor interaction. The drug adsorbed MPs were subsequently efficiently removed by an enhanced coagulation together owing to the synergistic effects of the two pollutants. This study provides a novel and comprehensive perspective on the fundamental understanding the UV-aging process of MPs and the simultaneous adsorption behaviors, furthermore, a strategy was proposed for their collaborative removal.

In situ formed sulfide-mediated aging of polystyrene microplastics and its impact on the fate of heavy metals in anaerobic digestion

Microplastics (MPs) entering wastewater treatment plants accumulate in sludge and are subsequently introduced into anaerobic digesters, a key sludge treatment process. However, little is known about how MPs undergo transformation during anaerobic digestion (AD). This study investigated the mechanism underlying the aging of polystyrene (PS) MPs in AD and its effect on heavy metal adsorption. In the AD batches containing an initial sulfate concentration of 5.1 mM, significant sulfate reduction (96.1 %) was observed, with reduced sulfur species accounting for 74 % of the total sulfur species deposited on the PS surface, as revealed by X-ray photoelectron spectroscopy. A positive correlation between sulfide formation and the selective proliferation of sulfur-reducing bacteria, particularly Desulfovibrio aminophilus, indicated the involvement of microorganisms in sulfur aging. The decomposition of in situ hydrogen peroxide (H2O2) and formation of hydroxyl radicals (∙OH) under sulfidogenic conditions were more pronounced, suggesting that reactive oxygen species may induce structural changes in PS MPs and potentially facilitate sulfur aging. Finally, isothermal titration calorimetry results showed that sulfur-aged PS had higher binding constants for Pb²⁺ and Cu²⁺ compared to pristine PS, due to the presence of sulfur-containing functional groups and a more negative surface charge. These findings provide valuable insights into the fate of MPs during the sludge treatment and their potential environmental impacts.

Irradiation and DOM mediate lead release from polyvinyl chloride microplastics in natural surface water

Polyvinyl chloride (PVC) is a widely used plastic, but the potential risk of heavy metal additive release from PVC microplastics (MPs) has not been fully explored. This study evaluates the release of lead (Pb) from recycled PVC MPs under natural conditions. The released Pb concentration in the dark was 1079.5-1109.7, 551.4-571.6, and 374.1-433.0 μg/L in agricultural, sea, and river/lake water, respectively. In contrast, the Pb release was markedly inhibited by 34.1-59.1 % under irradiation. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), fluorescence emission-excitation matrix (EEM) spectroscopy, and Mantel test revealed that the dissolved organic matter (DOM), especially lignin/carboxyl-rich acyclic component (41.8-84.8 %), can complex with Pb to facilitate its release from PVC in the dark. However, the Pb released upon irradiation was first promoted and then inhibited. The promotion ascribed to the broken of MPs by reactive intermediates (RIs) including 3DOM* , 1O2, and •OH [(0.5-12.6) × 10-13, (1.2-10.9) × 10-13 and (0.1-8.9) × 10-17 mol/L, respectively]. The inhibition was attributed to two reasons: the photobleaching of DOM reduced Pb dissolution complexed by DOM; the increase of oxygen-containing functional groups enhanced the Pb adsorption on the surface of MPs. In addition, the Pb released from PVC MPs significantly inhibits the growth of Nannochloropsis sp. in seawater. These findings reveal the complicated release mechanism of Pb from MPs under environmental conditions.

Molecular Insights into the Synergistic Inhibition of Microplastics-Derived Dissolved Organic Matter and Anions on the Transformation of Ferrihydrite

Ferrihydrite (Fh), as a ubiquitous iron (oxyhydr)oxide, plays an essential role in nutrient cycling and pollutant transformation due to its high surface area and diversified reaction sites. In the natural environment, Fh transformation could be easily influenced by coexisting components (particularly dissolved organic matter (DOM) and anions). As a new and important carbon source, microplastic-derived DOM (MP-DOM) directly or indirectly affects the morphology and fate of Fh, but limited knowledge exists about the combined effect of MP-DOM and anions on Fh transformation. Herein, this study elucidates the joint effects of polystyrene DOM (PS-DOM) and anions (such as Cl-, SO42-, and PO43-) on Fh transformation. Single anions (especially PO43-) were shown to inhibit the transformation of Fh to hematite (Hm) by hindering the dissolution and recrystallization of Fe(III). However, the inhibitory effect was strongly enhanced when PS-DOM and anions coexisted, which is attributed to their synergetic effects on inhibiting dissolution/recrystallization by occupying more active sites and hindering electron transfer. Furthermore, Fh transformation was predominantly controlled by PS-DOM, especially those containing high-unsaturation, high-oxidation-state, and O-rich phenolic compounds. These findings provide a new perspective on the significance of considering the joint effects of DOM and anions in evaluating the transformation of iron minerals.

Influence of microplastic aging on the adsorption and desorption behavior of Ni(II) under various aging conditions

Plastic products have significantly enhanced convenience in daily life; however, their degradation through weathering and environmental exposure leads to the formation of microplastics. These microplastics can serve as carriers for pollutants, such as heavy metals, through adsorption and desorption processes, posing potential risks to living organisms. This study focuses on the adsorption and desorption characteristics of nickel (Ni) on two representative microplastics-Polystyrene (PS) and Polylactic Acid (PLA)-before and after three aging processes: freeze-thaw cycling, alternating dry-wet conditions, and alkali treatment. Following these aging treatments, both microplastics exhibited increased specific surface area, pore size, and crystallinity, along with the emergence of oxygen-containing functional groups on their surfaces. Adsorption experiments indicated that nickel adsorption kinetics aligned more closely with the proposed second-order model, while adsorption isotherms were best described by the Langmuir model. Aged microplastics demonstrated higher adsorption capacities compared to their unaged counterparts, with adsorption capacity ranking as follows: alkali aging > alternating dry-wet aging > freeze-thaw cycling. Furthermore, PLA exhibited a greater adsorption capacity than PS. Among the aging processes, alkali treatment resulted in the highest nickel desorption rates, whereas freeze-thaw cycling and alternating dry-wet aging produced similar desorption outcomes. These findings contribute to a deeper understanding of microplastic aging mechanisms and their implications for heavy metal adsorption and desorption in environmental systems.

A comprehensive review of microplastic aging: Laboratory simulations, physicochemical properties, adsorption mechanisms, and environmental impacts

As a new type of ecological environment problem, microplastic pollution is a severe challenge faced by the world, and its threat and potential risk to the ecosystem have become a hot research spot in the current environmental field. Microplastics (MPs) in the natural environment will experience aging effect, aging will change the physical and chemical properties of MPs and affect the adsorption behavior. Recently reported characterization techniques of MPs and laboratory simulation of aging are reviewed. The aging mechanism between MPs and different pollutants and the intervention mechanism of environmental factors (MPs, pollutants and water quality environment) were revealed. In addition, to further understand the potential ecological toxicity of MPs after aging, the release and harm of additives during aging, produce the environmentally persistent free radicals, and the mechanism of reactive oxygen species (ROS) removal of pollutants adsorbed on the surface of MPs were summarized. Future research efforts should focus more on bridging the disparity between laboratory aging simulations and natural environmental conditions to enhance the authenticity and ecological relevance of such studies. The ROS production mechanism of MPs provides a reference direction for removing pollutants adsorbed by aged MPs.

Transport of reduced PBAT microplastics in saturated porous media: Synergistic effects of enhanced surface energy and roughness

Microplastic (MP) pollution presents significant global environmental challenges, exacerbated by reduction aging processes in anoxic environments, thereby increasing environmental risks and potential threats to human health. However, the mechanisms underlying the transport of reduced MPs remain poorly understood. In this study, laboratory-scale column experiments were conducted to investigate the transport behavior of polybutylene adipate terephthalate (PBAT), a common biodegradable MPs, and its reduced products obtained through the aging process mediated by two typical reducing agents, NaBH4 and Na2S, under varying conditions (ionic strength (IS), divalent cations, and low molecular weight organic acids (LMWOAs)). The results indicated that reduction aging improved the hydrophilicity of PBAT by increasing the surface roughness (roughness factor increased from 1.300 to 1.642) and surface energy (from 51.80 to 107.03 mN m-1), thereby increasing the mobility of reduced PBAT (with recovery rate increased from 53.77 % to 63.18 %). Increased IS decreased the mobility of reduced PBAT by decreasing the surface negative charge density. Divalent cations inhibited the mobility of both pristine and reduced PBAT in porous media, with pristine PBAT, containing more oxygen functional groups, exhibiting stronger inhibition. Furthermore, LMWOAs promoted the retention of reduced PBAT in porous media, which was dependent on the type of LMWOAs. This study revealed the alterations in MPs properties caused by reduction aging and their effects on transport mechanisms, offering new insights into the transport behavior and environmental risks of reduced MPs.

Single and combined effect of polyethylene microplastics (virgin and naturally aged) and cadmium on pakchoi (Brassica rapa subsp. chinensis) under different growth stages

To protect agro-systems and food security, study on the effect of microplastics and heavy metals on edible plants is of great significance. Existing studies mostly used virgin microplastics to evaluate their effects on plants, effects of naturally aged microplastics and their combined effects with heavy metals are rarely explored. In this study, single and combined effect of polyethylene microplastics (PE, both virgin and naturally aged) and cadmium (Cd) on pakchoi under seedling and mature stages were analyzed from perspectives of growth inhibition, oxidative damage, nutrition content and soil enzyme activities. Results showed that inhibiting effects of naturally aged PE (PEa) on the growth of pakchoi were stronger than virgin PE (PEv), whereas co-contamination of PEa and Cd was less toxic than that of PEv and Cd. The co-contamination of PE and Cd could inhibit pakchoi dry biomass by over 85 %. Both single and combined contamination of PE and Cd promoted soil fluorescein diacetate hydrolase (FDA) activities, which were 1.11 to 2.04 times of that in control group. Soluble sugar contents under co-contamination of PEa and Cd were 14 % to 22 % higher than those in control group. PEa and PEv showed different effects on oxidative damage of pakchoi. Compared with PEv, catalase (CAT) activities were more sensitive with PEa, whereas PEa had lower effect on superoxide dismutase (SOD) activities. The response of pakchoi to PE and Cd changed with growth stage. Chlorophyll contents in pakchoi under seedling stage were generally higher than those under mature stage. For Cd contaminated soils, PE benefited pakchoi growth under seedling stage, i.e. antagonistic effect between Cd and PE but hindered their growth under mature stage, i.e. synergistic effect. The results unraveled here emphasized PE, especially PEa, could trigger negative effects on agro-systems, whereas PE could be beneficial for heavy metal contaminated agro-systems under specific situations.

Thermal aging of polystyrene microplastics within mussels (Mytilus coruscus) under boiling and drying processing

Aged microplastics (MPs) in the environment are a growing concern due to their higher ecological toxicity compared to pristine MPs. While previous studies have explored aging behaviors of MPs under various stress conditions, little is known about their aging during food processing. In this study, we investigated the effects of different thermal food processing methods on the aging of polystyrene (PS) MPs within mussels. We subjected the mussels containing PS MPs to boiling, boiling/solar drying, boiling/hot air drying, and boiling/microwave drying treatments, all of which are common preservation methods used in industry. We analyzed the particle size, surface morphology, yellowing, crystallinity, chemical groups, and hydrophilicity of the PS MPs to understand the aging process. Results show that all processing methods led to aging of PS MPs, with boiling/microwave drying having the most significant impact, followed by boiling/hot air drying, boiling/solar drying, and boiling alone. The aged PS MPs exhibited smaller size, morphological changes, reduced crystallinity, increased yellowness index and carbonyl index, higher presence of O-containing groups, and enhanced hydrophilicity. These findings provide evidence of MPs aging during thermal food processing and emphasize the potential risks associated with this pathway.

Significant contribution of different sources of particulate organic matter to the photoaging of microplastics

Particulate organic matter (POM), as an important component of organic matter, can act as a redox mediator and thus intervene in the environmental behavior of microplastics (MPs). However, quantitative information on the role of POM in the photoaging of MPs under ultraviolet (UV) light is still lacking. To raise the knowledge gap, through environmental simulation experiments and qualitative/quantitative experiments of active substances, we found that POM from peat soil has stronger oxidation capacity than POM from sediment, and the involvement of POM at high water content makes the aging of MPs more obvious. This is because the persistent radicals and electron-absorbing groups on the surface of POM indirectly generate reactive oxygen species (ROS) by promoting electron transfer, and the dissolved organic matter (DOM) released from POM under UV light (POM-DOM) is further excited to generate triplet-state photochemistry of DOM (3DOM*) to promote the aging of MPs. Theoretical calculations revealed that the benzene ring, mainly C = C, and C = O in the main chain in the plastic macromolecule structure are more susceptible to ROS attack, and the differences in the vulnerable sites contained in different plastic structures as well as the differences in the energy band gaps lead to differences in their aging processes. This study firstly elucidates the key role and intrinsic mechanism of POM in the photoaging of MPs, providing a theoretical basis for a comprehensive assessment of the effect of POM on MPs in the environment.

Occurrence of microplastics in agricultural soils in ecologically fragile areas of China

The pollution caused by microplastics (MPs), an emerging pollutant, has been receiving continuous concern. However, the distribution characteristics of MPs in ecologically fragile areas (EFAs), which are sensitive to environmental change and pollution, are still unclear. Here, the abundance and pollution characteristics of MPs in agricultural soils in four typical EFAs in China, namely semiarid farming-pastoral area (SFPA), desert-oasis interlaced area (DOIA), plateau composite erosion area (PCEA) and southwest karst area (SWKA) were investigated. MPs were detected in all agricultural soil samples with a mean abundance of 2685 ± 938 n/kg. DOIA (3193 ± 630 n/kg) had the largest abundance of MPs in agricultural soils, followed by SWKA (2948 ± 819 n/kg), SFPA (2920 ± 935 n/kg), and PCEA (1680 ± 320 n/kg). MPs in four EFAs were mostly small size (0-0.49 mm), accounted for 81.71 %. Fragmented and pelleted MPs were the main shapes, occupying for 51.26 % and 28.53 %, respectively. In addition, Fourier transform infrared (FTIR) was applied to determine the polymer types of MPs and to assess the pollution risk of MPs, which ranged from 157 to 938, indicating a moderate to high risk. The results revealed that EFAs located in remote inland areas were considerably polluted by MPs, close to the developed coastal areas. This study provided systematic data on MPs pollution of EFAs, which is crucial in preventing further environmental degradation and promoting ecological restoration.