Interaction between microplastics and humic acid and its effect on their properties as revealed by molecular dynamics simulations

Influence of landfill leachate microenvironment on the occurrence of microplastics: TOC changes are the main driving factor

Factors such as inorganic salts, heavy metals and organics in landfill leachate can affect the environmental behavior and transport properties of microplastics. However, the influence of the microenvironment on the behavioral effects of microplastics in landfill leachate is still limited. In this study, the abundance characteristics of microplastics in leachate from 15 landfills in the North China Plain were investigated. The results showed that the abundance of microplastics in the leachate in this region was 712.0 items/L, which was mainly composed of small particle size and long fibrous microplastics. The relationships between leachate physicochemical factors and microplastic accumulation patterns were explored using models such as structural equations. Among them, TOC (Total Organic Carbon) had the strongest driving effect on 50-100 μm microplastics. And it had different effects on different microplastics: it promoted the degradation of PET (Polyethylene terephthalate), while it inhibited the degradation of PVC (Polyvinyl chloride), FVMQ (Fluorosilicone rubber) and PSU (Polysulfone). The ridge regression model indicated that the interaction of landfill age with Cr (Chromium) and the interaction of redox potential with Cr were the key factors influencing the behavioral characteristics of microplastics in leachate. These results provide a scientific basis for the treating waste leachate and the controlling the emerging pollutants.

Escherichia coli migration in saturated porous media: Mechanisms of humic acid regulation

The regulatory behavior of humic acid (HA) on the migration of Escherichia coli (E.coli) in saturated porous media has garnered considerable research interest. Although prior studies have confirmed that HA indeed facilitates the migration of E. coli in saturated porous media, investigating the migration process and regulatory mechanisms at the microscale remains challenging. This study compared the differences in the migration behavior of E. coli in saturated porous media under conditions with and without HA, revealing the dynamic mechanism by which HA regulates microbial migration through the "bacterium-medium-solution" triple interface interaction. The results indicated that E. coli achieves the transition of the "run-tumble" movement pattern (run ≈ 1 s, tumble ≈ 0.1 s) through flagellar morphological regulation, thus completing directed migration in a complex pore network. The addition of HA significantly enhanced the migration rate of E. coli, with an increase of at least 5 %. For the bacteria, HA induced the restructuring of lipopolysaccharides on the bacterial surface, altered the surface Zeta potential of the bacteria, and promoted the formation of stable hetero-aggregates between bacteria and HA. At the medium interface, HA modifies the surface charge of the medium, regulates pore structure, and increases hydrophilicity through the adsorption-desorption mechanism. In the solution system, the dissociation characteristics of HA's carboxyl and phenolic hydroxyl groups dynamically regulated the solution's ionic strength and pH value, creating a chemical microenvironment suitable for bacterial migration. This study systematically revealed the multi-dimensional mechanisms by which HA regulates microbial transport through molecular interface engineering. It provides theoretical support for establishing predictive models of pathogen migration in groundwater systems and offers important guidance for optimizing microbial control strategies in water treatment processes.

Release mechanisms of decabromodiphenyl ether from typical e-waste microplastics into water: Insights from molecular dynamics simulations

E-waste-derived microplastics (MPs) serve as a significant source, have been releasing decabromodiphenyl ether (BDE-209) into aquatic environment. Conventional release kinetics experiments fail to effectively distinguish the three-stage release process, which includes internal diffusion, interfacial mass transfer, and diffusion in the environment. Herein, we took typical flame-retardant plastic (polystyrene, PS) as a paradigm to construct diffusion and release models corresponding to the three-stage release process, with large-scale all-atom molecular dynamics (MD) simulations providing insights into the release process. The level of BDE-209's self-diffusion coefficients (D) was calculated at different release stages: 10-14 (PS matrix), 10-12 (PS-water interface), and 10-10 m2 s-1 (bulk water). BDE-209 exhibits a confined diffusion mode within the PS matrix, significantly diminishing its release capability. At the interface, the strength of dispersion attraction between BDE-209 and the PS surface determines the ease of its release and the partition equilibrium between the two phases. Our findings elucidated the molecular-scale dynamic and thermodynamic mechanisms governing BDE-209 release from MPs into water, expanding the understanding of polybrominated diphenyl ether release from e-waste-derived MPs. Moreover, our established MD simulation methods can be adapted to explore the release or adsorption mechanisms of various additives in different kinds of MPs.

Photochemical transformation and interaction of octachlorodibenzofuran (OCDF) with microplastics in suspended particulate matter-water system

Microplastics (MPs) and suspended particulate matter (SPM) are widely present in the aquatic environment, serving as carriers for various pollutants. Understanding the phototransformation behavior of hydrophobic organic pollutants in the presence of coexisting microplastics and SPM is crucial for assessing their environmental fate and potential impacts. In this study, we investigated the photochemical transformation behavior of octachlorodibenzo-p-dioxin (OCDF) in water under simulated solar irradiation, using polypropylene (PP) microplastics and SPM collected from the Pearl River. The results showed that the degradation rate of OCDF increased with the increase of PP content in the system. Experiments using EPR and probe molecules, as well as quenching experiments of reactive species, demonstrated that the presence of PP significantly elevated the concentration of reactive oxygen species (ROS) in the system. Through product analysis, we identified the main degradation pathways of OCDF to involve carbon-oxygen bond breaking, dechlorination and substitution reactions. These pathways were further rationalized and verified through theoretical calculations. In addition, we calculated the reaction energy barriers of OCDF attacked by ROS on the surface of particulate matter. Compared with SPM, the reaction energy barrier for OCDF reacting with •O2- on the PP surface was significantly reduced, suggesting that PP can enhance the photochemical transformation of OCDF by facilitating the reactivity of ROS. This study provides new insights into the photochemical transformation of hydrophobic organic pollutants mediated by microplastics in real aqueous environments, highlighting the role of MPs in altering the fate and behavior of persistent organic pollutants.

Adsorption, Structure, and Dynamics of DNA in Montmorillonite and Montmorillonite-Humic Acid: A Molecular-Level Insight

Extracellular DNA (eDNA) has become a focus in public health, with soil recognized as a key reservoir where eDNA's mobility and stability are primarily controlled by clay minerals and organic matter. Montmorillonite (MMT) often interacts with humic acids (HA), forming MMT-HA complexes, although the molecular mechanisms behind DNA adsorption in these complexes remain unclear. Here, molecular dynamics simulations were used to examine the adsorption, diffusion, interfacial structure, and dynamics of DNA on MMT and MMT-HA complexes, revealing critical molecular interactions. Interaction energy analysis showed that DNA adsorption is energetically more favorable on MMT than on MMT-HA, as HA reduces DNA's adsorption capacity on MMT. Diffusion coefficients indicated that DNA has lower mobility on MMT than on MMT-HA. Ca2+ cations and water molecules bridge MMT surfaces and DNA phosphate groups, enhancing DNA adsorption on MMT, while HA occupies MMT binding sites and forms limited hydrogen bonds with DNA, thereby inhibiting adsorption. These results provide insights into the adsorption, migration, and stability of eDNA on soil clay minerals.

Degradation of microplastics by electrocoagulation technology: Combination oxidation and flocculation effects

Electrocoagulation (EC) technology features a promising prospect for coping with the formidable microplastics (MPs) pollution challenge, albeit the underlying abatement mechanism still needs to be further clarified. Accordingly, in this work, we evaluated the removal performance by EC for four typical MPs, including polyvinyl chloride (PVC), polystyrene (PS), polypropylene (PP), and polyethylene (PE). The Fourier transform infrared spectroscopies of MPs confirmed the presence of electrochemical oxidation during EC process, owing to its hydroxyl radical generation ability as demonstrated by the detected fluorescence spectroscopies and electron paramagnetic resonance results, which has been rarely reported in other works. Specifically, 21.2 ± 0.8 %, 10.8 ± 1.8 %, 15.6 ± 1.6 %, and 7.6 ± 1.4 % abatement efficiency for PVC, PS, PP, and PE, respectively, originated from the oxidation effect, and these values for flocculation effect were 77.2 ± 0.8 %, 74.0 ± 1.6 %, 70.8 ± 1.2 %, and 69.2 ± 1.2 %, successively. Many factors influence these differences, especially the MPs' hydrophilicity, as it facilitates the mass transfer efficiency between MPs (like PVC and PP) and the generated flocs or radicals. To lay a foundation for practical application, we also optimized the operation parameters, demonstrating the wise choice of pH 7 to maintain a balance between the oxidation and flocculation effect. Therefore, we believe our work provides a good reference for promoting MPs abatement efficiency and elucidating the corresponding mechanism, especially the contribution of the oxidation part by EC.

Understanding the structure, distribution, and retention of nanoplastics in montmorillonite nanopore by multi-scale computational simulations

The interfacial adsorption, aggregation and deposition processes of nanoplastics (NPs) on clay mineral surfaces critically regulate their environmental mobility, transformation pathways, and ecotoxicological risks in aquatic ecosystems. A quantitative understanding of the nanoscale interfacial processes is essential. This study employs molecular dynamics (MD) simulations and density functional theory (DFT) calculations to elucidate the aggregation and deposition mechanisms of three types of NPs in their pristine and aged states in the nanopore solution of montmorillonite (Mt). In the wet environment, NPs tend to form aggregates in the nanopore and migrate in solution, increasing environmental risk, while in the dry environment, NPs are more likely to deposit on the basal surface to form larger aggregates, consequently reducing their mobility. Results show hydrophobic interactions play as the primary driving force for the aggregation of pristine NPs, and both hydrophilic and hydrophobic interactions contribute to the aggregation of aged NPs. Aged NPs exhibit stronger binding affinity to Mt through mechanism such as Ca²⁺ bridging and hydrogen bonding, compared to their pristine counterparts. DFT calculations further reveal the formation of hydrogen bonds between the hydroxyl groups of aged NPs and the tetrahedral oxygen atoms in Mt. Through atomic-level characterization of interfacial processes, this work establishes a predictive framework for NP environmental behavior by resolving migration dynamics and retention processes in nanopore water.

Antimony release from e-waste-derived microplastics in aqueous environments: Effect of plastic properties and environmental factors

Antimony (Sb) is an emerging contaminant widely concerned by researchers recently. Sb2O3, the flame-retardant synergist extensively used in plastics for electronic products, is an important source of Sb pollution. It can be released into the environment from e-waste, especially from the formed microplastics (MPs). However, the behavior and mechanisms of Sb release remain unclear. This study investigated the release behavior of Sb from two typical e-waste-derived MPs, acrylonitrile-butadiene-styrene (ABS) and high-impact polystyrene (HIPS). The effects of particle size, plastic aging, and environmental conditions (pH, humic acid, and inorganic ions) on Sb release were explored. It was found that HIPS exhibited higher total Sb (Sbtot) release than ABS, due to differences in their hydrophilicity and crystallinity. When the particle size was reduced from 2 mm to 0.15 mm, Sbtot release from HIPS and ABS increased by 620% and 350%. UV aging increased hydrophilicity and decreased crystallinity of MPs, further enhancing Sbtot release. Notably, there were about 40% Sb(III) in Sbtot released by pristine MPs, whereas in the leachate from the UV-aged MPs, Sbtot was exclusively Sb(V). Sbtot release was greatly enhanced by acidic and alkaline environments, especially at extreme pH levels, while humic acid has an inhibitory effect on the Sbtot release. These results suggest considerable amounts of Sb can be released into the environment from e-waste-derived MPs, and affected by various environmental factors. These findings improve understanding of Sb release from MPs in e-waste areas under various environmental conditions, providing insights into environmental risks tied to additive release from MPs.

Molecular modeling to elucidate the dynamic interaction process and aggregation mechanism between natural organic matters and nanoplastics

The interactions of nanoplastics (NPs) with natural organic matters (NOMs) dominate the environmental fate of both substances and the organic carbon cycle. Their binding and aggregation mechanisms at the molecular level remain elusive due to the high structural complexity of NOMs and aged NPs. Molecular modeling was used to understand the detailed dynamic interaction mechanism between NOMs and NPs. Advanced humic acid models were used, and three types of NPs, i.e., polyethylene (PE), polyvinyl chloride (PVC), and polystyrene (PS), were investigated. Molecular dynamics (MD) simulations revealed the geometrical change of the spontaneous formation of NOMs-NPs supramolecular assemblies. The results showed that pristine NPs initially tend to aggregate homogeneously due to their hydrophobic nature, and then NOM fragments are bound to the formed NP aggregates mainly by vdW interaction. Homo- and hetero-aggregation between NOMs and aged NPs occur simultaneously through various mechanisms, including intermolecular forces and Ca2+ bridging effect, eventually resulting in a mixture of supramolecular structures. Density functional theory calculations were employed to characterize the surface properties and reactivity of the NP monomers. The molecular polarity indices for unaged PE, PS, and PVC were 3.1, 8.5, and 22.2 kcal/mol, respectively, which increased to 43.2, 51.6, and 42.2 kcal/mol for aged NPs, respectively, indicating the increase in polarity after aging. The vdW and electrostatic potentials of NP monomers were visualized. These results clarified the fundamental aggregation processes, and mechanisms between NPs and NOMs, providing a complete molecular picture of the interactions of nanoparticles in the natural aquatic environment.

Factors Influencing the Vertical Migration of Microplastics up and down the Soil Profile

Soil ecosystems are under serious threat from microplastics (MPs), and this is causing worldwide concern. The relationship between soil and MPs has become a popular research topic, and the vertical migration of soil MPs is of increasing interest. This Review summarizes the current status of research into the factors affecting the vertical migration of soil MPs. Published research shows that the characteristics of MPs and the physicochemical properties of the soil affect the infiltration process. Soil organisms play a key role in the vertical migration by acting as vectors or as a result of adsorption. Dissolved organic matter and metal oxides transfer MPs by adsorption-desorption. In addition, rainfall and dry-wet cycles alter the mobility of soil MPs, leading to changes in migration processes. Agricultural activities such as tillage and irrigation may distribute MPs throughout the topsoil. Vertical migration of soil MPs is a process influenced by a combination of factors, and the role of these factors in MP deposition needs to be explored further.

Photoaging-induced variations in heteroaggregation of nanoplastics and suspended sediments in aquatic environments: A case study on nanopolystyrene

Photoaging of nanoplastics (NPs) and heteroaggregate with suspended sediments (SS) determines transport processes and ecological risks of NPs in aquatic environments. This study investigated the disruption of photoaging on the heteroaggregation behavior of polystyrene NPs (PSNPs) and SS in different valence electrolyte solutions and deduced the interaction mechanisms by integrating aggregation kinetics and molecular dynamics (MD) simulation. Increasing the electrolyte concentration significantly enhanced the heteroaggregation between PSNPs and SS, and the divalent electrolytes induced the heteroaggregation more efficiently. MD simulation at the molecular level revealed that PS and SS could spontaneously form clusters, and photoaged PS has a stronger potential to fold into a dense state with SS. Photoaging for 30 d retarded heteroaggregation due to the steric hindrance produced by the leached organic matter in NaCl solutions, and the critical coagulation concentration (CCC) increased by >85.44 %. Contrarily, photoaging caused more oxygen-containing functional groups produced on the surface of PSNPs through Ca2+ bridging promoting heteroaggregation and thus destabilizing in CaCl2 solutions, the CCC decreased by 23.53 % ∼ 35.29 %. These findings provide mechanistic insight into the environmental process of NPs and SS and are crucial for a comprehensive understanding of the environmental fate and transport of NPs in aquatic environments.

Molecular-level insight into the behavior of metal cations and organic matter during the aggregation of polystyrene nanoplastics

In this study, the behavior of metal cations and organic matter during polystyrene nanoplastics (PSNP) aggregation was explored combing experimental measurements and molecular dynamics simulation. The results indicated that coexisting organic matter, including organic pollutants and humic acid (HA), play a complex role in determining PSNP aggregation. The representative organic pollutant, bisphenol A, exhibited competitive behavior with HA during heteroaggregation, and the heteroaggregation between HA and PSNP was impaired by bisphenol A. The bridging effect of metal ions in aggregation is related to their interaction strength with functional groups, binding affinity with water molecules, and concentration. In particular, Mg2+ interacts more strongly with oxygen-containing functional groups on PSNP than Ca2+. However, Mg2+ is more favorable for binding with water and is therefore not as effective as Ca2+ for destabilizing PSNP. Compared with Ca2+ and Mg2+, Na+ showed a weaker association with PSNP; however, it still showed a significant effect in determining the aggregation behavior of PSNP owing to its high concentration in seawater. Overall, we provided a molecular-level understanding of PSNP aggregation and deepened our understanding of the fate of nanoplastics.

Deciphering the mechanisms and interactions of the endocrine disruptor bisphenol A and its analogs with the androgen receptor

Bisphenol A (BPA) and its various forms used as BPA alternatives in industries are recognized toxic compounds and antiandrogenic endocrine disruptors. These chemicals are widespread in the environment and frequently detected in biological samples. Concerns exist about their impact on hormones, disrupting natural biological processes in humans, together with their negative impacts on the environment and biotic life. This study aims to characterize the interaction between BPA analogs and the androgen receptor (AR) and the effect on the receptor's normal activity. To achieve this goal, molecular docking was conducted with BPA and its analogs and dihydrotestosterone (DHT) as a reference ligand. Four BPA analogs exhibited higher affinity (-10.2 to -8.7 kcal/mol) for AR compared to BPA (-8.6 kcal/mol), displaying distinct interaction patterns. Interestingly, DHT (-11.0 kcal/mol) shared a binding pattern with BPA. ADMET analysis of the top 10 compounds, followed by molecular dynamics simulations, revealed toxicity and dynamic behavior. Experimental studies demonstrated that only BPA disrupts DHT-induced AR dimerization, thereby affecting AR's function due to its binding nature. This similarity to DHT was observed during computational analysis. These findings emphasize the importance of targeted strategies to mitigate BPA toxicity, offering crucial insights for interventions in human health and environmental well-being.

Co-impacts of cation type and humic acid on migration of polystyrene microplastics in saturated porous media

The aging process of microplastics (MPs) could significantly change their physical and chemical characteristics and impact their migration behavior in soil. However, the complex effects of different cations and humic acids (HA) on the migration of aged MPs through saturated media are not clear. In this research, the migration and retention of pristine/aged PSMPs (polystyrene microplastics) under combined effects of cations (Na+, Ca2+) (ionic strength = 10 mM) and HA (0, 5, 15 mg/L) were investigated and analyzed in conjunction with the two-site kinetic retention model and DLVO theory. The findings showed that the aging process accelerated PSMPs migration under all tested conditions. Aged PSMPs were less susceptible to Ca2+ than pristine PSMPs. Under Ca2+ conditions, pristine/aged PSMPs showed higher retention than under Na+ conditions in the absence of HA. Furthermore, under Na+ conditions, the migration of aged PSMPs significantly increased at higher concentrations of HA. However, under Ca2+ conditions, the migration of aged PSMPs decreased significantly at higher concentrations of HA. In higher HA conditions, HA, Ca2+, and PSMPs interact to cause larger aggregations, resulting in the sedimentation of aged PSMPs. The DLVO calculations and two-site kinetic retention models' results showed the detention of PSMPs was irreversible under higher HA conditions (15 mg/L) with Ca2+, and aged PSMPs were more susceptible to clogging. These findings may help to understand the potential risk of migration behavior of PSMPs in the soil-groundwater environment.

High salinity promotes the photoaging of polystyrene microplastics with humic acid in seawater

The photoaging of microplastics (MPs) accumulated in the sea can be influenced by humic acid (HA). However, the role of salinity cannot be ignored, as it may potentially disrupt the interaction between MPs and HA, thereby altering the photoaging of MPs. Herein, this study investigated how salinity influences the effect of humic acid (HA, derived from lignite) on the photoaging of polystyrene microplastics (PS MPs) in artificial and natural seawater. The results revealed that HA promoted the photoaging of PS MPs under both low (5 PSU) and high salinity (35 PSU) in light conditions (L), reflected in the formation of fragments, the production of oxygen-containing functional groups (OH, CO, and OCO), and the increase in hydrophilicity of PS MPs. Furthermore, high salinity promoted the photoaging of PS MPs with HA more significantly, as evidenced by the similar indicators and the order of oxygen/carbon atom ratio (O/C): L-HA-High (0.15) > L-HA-Low (0.10) > Unaged (0.02). Interestingly, due to the reduction of electrostatic repulsion, the adsorption of HA on photoaged PS MPs in natural and artificial high salinity seawater was 1.77 mg/g and 0.39 mg/g, respectively, which was significantly higher than those PS MPs photoaged in the low salinity seawater. Furthermore, the electron spin resonance (ESR) results confirmed that more hydroxyl radicals (OH) were generated after adsorbing HA under high salinity conditions, thus promoting the fragmentation and oxidation of PS MPs. Overall, our findings highlight the crucial role of salinity in influencing the photoaging of MPs with HA and help to assess the marine risk of MPs accurately.

Adsorption-desorption behaviors of ciprofloxacin onto aged polystyrene fragments in aquatic environments

As two emerging pollutants of great concern, microplastics (MPs) and antibiotics inevitably cooccur in various aquatic environments and interact with each other, impacting the fate and ecological risks. Aging obviously complicates their interaction and deserves further study. Therefore, the adsorption-desorption behaviors of ciprofloxacin (CIP) onto polystyrene (PS) fragments with various aging extent were investigated, and the key physiochemical properties influencing the interaction and the interaction mechanisms were clarified by redundancy analysis, FTIR and XPS spectra. The physicochemical properties of PS MPs were significantly changed with aging time, and the morphological and chemical changes seemed to occur asynchronously. The adsorption of CIP onto the pristine PS MPs relied on physisorption, especially the ion-involving electrostatic and cation-π interaction. Due to the hydrogen bonding formed by the C-OH, CO, and O-CO groups of PS and CIP, the adsorption capacities of the aged PS MPs were greatly increased. The desorption efficiency of CIP from MPs in the gastric fluid was closely related to the solution ionic strengths, C-OH and CO groups of MPs, while that in the intestinal fluid was associated with O-CO groups of MPs. The different impact factors could be well described by the differences in the chemical components and pHs of the simulated gastric and intestinal fluids. This study gives a comprehensive understanding of the adsorption-desorption behaviors of antibiotics onto MPs at a molecular level and indicates that MPs could act as Trojan horses to transport antibiotics into aquatic organisms.