Molecular level insight into the different interaction intensity between microplastics and aromatic hydrocarbon in pure water and seawater

Unveiling the adsorption mechanism of perfluorooctane sulfonate onto polypropylene nanoplastics: A combined theoretical and experimental investigation

Polypropylene (PP) is a key component of nanoplastics detected globally in water, which can carry pollutants through co-transport. In this regard, the co-transport of perfluoroalkyl substances (PFAS) by nanoplastics (NPs) raises significant concern, as NPs can act as vectors that enhance PFAS uptake and bioaccumulation in organisms during co-exposure. In this context, research has shown interactions between NPs and PFAS, but the adsorption mechanism remains still unclear. In this work, a powerful synergic approach combining several computational and experimental techniques has been used to unveil the adsorption mechanism of perfluorooctanesulfonate (PFOS), which is one of the most widespread contaminants of emerging concerns (CECs) on PP nanoparticles. According to our DFT results, PFOS adsorbs onto the outer and inner surface of the nanoparticle, with a maximum adsorption energy of ≈ 18 kcal/mol and an adsorption mechanism mainly governed by dispersion forces between the two fragments. Batch experiments have confirmed that PFOS rapidly adsorbs on PP nanoparticle, showing that pH can reduce the adsorption capacity thus affecting the co-transport. Moreover, the dipole moment of the PFOS-nanoparticle complex has been found to be significantly larger as compared to the bare nanoparticle, resulting in a more pronounced transport in aqueous environment and making the PFOS-PP nanoparticle complex much more dangerous than the bare PP nanoparticle. Altogether, our results allowed us to disentangle the adsorption mechanism of PFAS on PP nanoparticles, which is a fundamental step to understand the co-occurrence of such dangerous pollutants in environmental matrices, as well as to obtain new information for toxicity and risk-models development.

Adsorption behavior of crude oil hydrocarbons on polyethylene microplastics in batch experiments

This study examines the impact of microplastics on the fate of spilled crude oil in water. Batch adsorption experiments were conducted using polyethylene microplastics ranging in size between 300 and 600 μm. Environmentally relevant concentrations of crude oil and microplastics were tested. Samples processing involved liquid-liquid extraction (LLE) followed by quantitative analysis using Gas-Chromatography coupled to Mass Spectrometry. Kinetic analyses employed the most commonly used models in microplastic adsorption studies, including the pseudo-first order, pseudo second-order, Elovich, and intra-particle diffusion models. Results mainly conformed to the Elovich model, followed by the pseudo-second order model, suggesting chemisorption. Isotherm evaluations involved the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models, selected for their effectiveness in describing the behavior of microplastics in adsorption studies. These models revealed diverse behaviors: alkanes from nC11-nC21 conformed to the Freundlich isotherm, suggesting multilayer adsorption. While nC10, nC27-nC29, nC33, and nC34 were best described by the Langmuir model, and nC22-nC26 and nC30-nC32 adhered to the Temkin model, both indicative of monolayer adsorption. Notably, nC35 adsorption was best described by the Dubinin-Radushkevich model. The different PAHs exhibited preferences for either the Freundlich or the Langmuir model. The maximum adsorption capacities of the contaminants onto polyethylene were 263.12 and 101.57 mg.g-1 for the targeted alkanes and PAHs, respectively, corresponding to a maximum adsorption of 5.75 mg of targeted hydrocarbons per m2 of polyethylene. The study highlighted the potential role of microplastics in influencing the environmental fate of selected crude oil hydrocarbons and provided insights into their interaction and partitioning behavior in water.

Effect of pH and salinity on the release of polystyrene microplastics derived dissolved organic matter as revealed by experimental studies and molecular dynamic simulations

Microplastics-derived dissolved organic matter (MPs-DOM) poses a significant risk to aquatic systems. This study characterized MPs-DOM from polystyrene microplastics (PSMPs) upon photoaging in freshwater and seawater. For pristine PSMPs, plastic additives are the predominant substances in MPs-DOM. As the degree of aging increases, intermediates emerge as the new predominant substances in MPs-DOM. Both higher pH and salinity accelerate the aging of PSMPs and MPs-DOM release. Molecular dynamics simulations align with experiments showing that increased pH and salinity levels enhance the release of MPs-DOM. Interaction energy calculations revealed a link between MPs-DOM release amount and the interaction intensity between PSMPs and MPs-DOM. Generally, MPs-DOM having lower interaction energy with PSMPs is more liable to release, and aging of PSMPs leads to a decrease in their interaction energy with MPs-DOM. For example, the interaction energies in the pH 10 seawater system were slightly lower than those in the pH 7 seawater system. In the pH 7 seawater system, the interaction energy between butyl acetate and PSMPs was -41.97 kJ/mol, while in the pH 10 seawater system, this value was -26.86 kJ/mol. These insights are crucial for assessing the environmental behavior of MPs and MPs-DOM in aqueous environments.

Exploring the molecular mechanisms of herbicide adsorption on microplastics: A quantum chemical approach

The widespread presence of microplastics in the environment has raised significant concerns, particularly regarding their potential interactions with herbicides and the combined pollution effects on ecosystems. In this study, quantum chemical calculations were employed to investigate the interaction mechanisms between polyethylene (PE) and polyvinyl chloride (PVC) microplastics and phenoxyacetic herbicides. The results revealed that PVC exhibits a stronger adsorption capacity compared to PE, and that low ionic strength conditions weaken the interactions between microplastics and herbicides. The energy decomposition analysis indicates that dispersion and electrostatic interactions are the predominant components contributing to the interaction energy, thus positioning the herbicide adsorption sites on microplastics near the minima of van der Waals and electrostatic potentials. The presence of hydrogen bond acceptors in microplastics influences the formation of intramolecular or intermolecular hydrogen bonds with the carboxylic groups of herbicides, resulting in significant changes in vibrational modes and infrared spectral absorption peaks, which offers a potential method for in situ monitoring of herbicide adsorption on microplastics. Additionally, different charge transfer phenomena are observed during the adsorption process, with PVC tending to lose electrons and PE to gain electrons. These insights provide a theoretical foundation for a deeper understanding of the adsorption behavior of phenoxyacetic herbicides on microplastics and hold significant implications for the optimization of environmental remediation strategies.

Investigating the adsorption of organic compounds onto microplastics via experimental, simulation, and prediction methods

Exploring the adsorption of organic compounds onto microplastics (MPs) is of great significance for understanding their environmental fate and evaluating their ecological risks. To date, various techniques, e.g., experiments, simulations, and prediction models, have been utilized for exploring the adsorption of different organic compounds onto MPs. In this review, we systematically introduce the sources of MPs, the interactions between MPs and organic compounds, the factors influencing the adsorption of organic compounds onto MPs, and research advances in investigating the adsorption of organic compounds by microplastics with different techniques. We also point out that the structures of MPs and environmental factors can have distinct effects on the adsorption mechanisms, and the adsorption mechanisms for numerous organic compounds onto MPs are still unclear. Besides, there is a paucity of multi-dimensional models for predicting the adsorption of organic compounds by MPs under different environmental conditions with a single click. We hope that our review can provide insights into the environmental behavior and fate of organic compounds and microplastics, as well as also guiding future research on the adsorption of organic compounds onto microplastics.

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.

Study on low-rank coal flotation collector screening and performance based on molecular polarity index

CONTEXT

Extensive studies using a trial-and-error approach have been conducted on low-rank coal flotation collectors. However, screening efficient collectors remains a considerable challenge due to the lack of suitable screening principles. It has proven that polar compounds such as carboxylic acids and esters are effective collectors for low-rank coal flotation. In this work, the effects of carboxylic acid, alcohol, and methyl ester on the floatability of low-rank coal were compared, the flotation performance of the polar collector was evaluated with theoretical calculations, a suitable evaluation parameter was determined and a screening principle based on this parameter was determined. The results show that the enhancement effects of polar collectors on low-rank coal floatability follow the order of methyl decanoate > methyl laurate > methyl octanoate > sec-octanol > methyl oleate (or methyl oleate > sec-octanol) > n-octanoic acid. Compared with the molecular polarity index, the hydrophobicity indices log P and dipole moment cannot be used to accurately evaluate different types of collectors and the same type of collectors, respectively. At room temperature, liquid polar compounds with molecular polarity indices in the range of 6.0 ~ 8.0 kcal/mol effectively enhance the floatability of low-rank coal. The molecular polarity index of the collector is used for the first time to screen effective collectors of low-rank coal in this work. This parameter is anticipated to be highly important for the development and research of low-rank coal and other mineral collectors.

METHODS

To obtain reasonable and accurate molecular structure, geometry optimization and frequency calculations of the studied collectors were conducted via the Gaussian 09 software package based on density functional theory at the B3LYP/6-311 + G (d, p) level. The integral equation formalism for the polarizable continuum model (IEF-PCM) was utilized with water as the solvent (dielectric constant = 78.36, T = 298 K) for all the calculations. Then, the atomic charge distributions (MPA and NPA) and electrostatic potential maps, the dipole moment and molecular polarity index, and the log P and water solubilities of studied collectors were shown or calculated by Gauss View 5.0, Mutiwfn program and website ( https://www.molsoft.com/mprop/mprop.cgi ), respectively.

Environmental condition-dependent effects of aquatic humic substances on the distribution of phenanthrene in microplastic-contaminated aquatic systems

Numerous studies have focused on the interaction between microplastics (MPs) and phenanthrene (PHE) in aquatic environments. However, the intricate roles of aquatic humic substances (HS), which vary with environmental conditions, in influencing PHE-MP interactions are not yet fully understood. This study investigates the variable and environmentally sensitive roles of HS in modifying the interactions between PHE and polyethylene (PE) MPs under laboratory-simulated aquatic conditions with varying solution chemistry, including pH, HS types, HS concentrations, and ionic strength. Our findings show that the presence of HS significantly reduces the adsorption of PHE onto both pristine and aged PE MPs, with a more pronounced reduction observed in aged PEs. This effect is highlighted by a notable decrease in the partitioning coefficient (Kd) of PHE, which falls from 2.60 × 104 to 1.30 × 104 L/kg on MPs in the presence of HS. The study also demonstrates that alterations in the net charge of HS solutions are crucial in modifying PHE distribution onto PEs. An initial decrease in Kd values at higher pH levels is reversed when HS is introduced. Furthermore, an increase in HS concentrations is associated with lower Kd values. In conditions of higher ionic strength, the retention of PHE by HS is intensified, likely due to an enhanced salting-out effect. This research highlights the significant role of aquatic HS in modulating the distribution of PHE in MP-polluted waters, which is highly influenced by various solution chemistry factors. The findings are vital for understanding the fate of PHE in MP-contaminated aquatic environments and can contribute to refining predictive models that consider diverse solution chemistry scenarios.

An Atomic-Level Perspective on the interactions between Organic Pollutants and PET particles: A Comprehensive Computational Investigation

Microplastics (MPs) have recently attracted a lot of attention worldwide due to their abundance and potentially harmful effects on the environment and on human health. One of the factors of concern is their ability to adsorb and disperse other harmful organic pollutants in the environment. To properly assess the adsorption capacity of MP for organic pollutants in different environments, it is pivotal to understand the mechanisms of their interactions in detail at the atomic level. In this work, we studied interactions between polyethylene terephthalate (PET) MP and small organic pollutants containing different functional groups within the framework of density functional theory (DFT). Our computational outcomes show that organic pollutants mainly bind to the surface of a PET model via weak non-bonding interactions, mostly hydrogen bonds. The binding strength between pollutant molecules and PET particles strongly depends on the adsorption site while we have found that the particle size is of lesser importance. Specifically, carboxylic sites are able to form strong hydrogen bonds with pollutants containing hydrogen bond donor or acceptor groups. On the other hand, it is found that in such kind of systems π-π interactions play a minor role in adsorption on PET particles.

Effect of microplastics on the adherence of coexisting background organic contaminants to natural organic matter in water

Microplastics (MPs) may interact with background organic substances (including natural organic matter and organic pollutants) after entering the aquatic environment and affect their original binding. Thus, the interaction of MPs with background organic substances (i.e., humic acid (HA), polychlorinated biphenyls (PCBs), and hydroxy PCBs) were elucidated. According to the results, PCB and hydroxy PCB displayed a strong propensity to adhere to HAs in the absence of MPs. However, the PCBs and hydroxy PCBs that were initially bound to HAs shifted from HAs to MPs in the presence of MPs. Further analysis demonstrated that this transfer was dominated by van der Waals interactions, with hydrogen bond interactions as an additional driving force. Upon the interaction, large MPs-HA-PCB/ hydroxy PCB aggregates with MPs as the core and HAs as the outermost layer were formed. Significant changes in the properties of background organic matter, including the distribution of PCB/hydroxy PCB around HA, diffusion coefficient, and hydrogen bond networks in the HA-PCB/ hydroxy PCB domains, occurred during the MP-HA-PCB/hydroxy PCB interaction. These results provide molecular-level evidence that the intrusion of MPs changes the binding preference of background organic pollutants and can lead to a redistribution of background organic pollutants.

Effect of aggregation behavior on microplastic removal by magnetic Fe3O4 nanoparticles

Magnetic nanotechnologies have been shown to be an efficient approach to the reduction of microplastic (MP) pollution in aquatic environments. However, uncertainties remain regarding the relationship between particle stability and MP removal under varying water conditions, hindering the practical application of magnetic nanotechnologies for MP removal. Herein, the influence of particle aggregation behavior on nano-scale MP removal by Fe3O4 nanoparticles (FNPs) was investigated, by monitoring dynamic light scattering parameters and analyzing the microstructures of particle aggregates. Results showed that 83.1 %-92.9 % of MPs could be removed by FNPs within 1 h, and MP removal exhibited a high degree of Pearson correlation (R = 0.95; P = 0.04) with particle aggregation behavior mediated by the FNPs dosage. Furthermore, pH-dependent electrostatic interactions significantly influenced particle aggregation behavior and the removal of MPs. Under pH <6.7 conditions, electrostatic attraction between electropositive FNPs and electronegative MPs led to charge neutralization-induced aggregation and efficient removal MP performance. Under increasingly saline conditions, compression of the electrical double layer enhanced the self-aggregation behavior of MPs, weakening the electrostatic repulsion between FNPs and MPs under alkaline conditions. Therefore, salinity improved the MP removal efficiency, especially under alkaline conditions, with MP removal increasing from 4.47 % to 55.1 % when the mass fraction of NaCl was increased from 0 % to 1 %. These findings further our understanding of the effect of aggregation behavior on MP removal by FNPs and highlight the potential for magnetic nanotechnology application in the removal of nano-scale MPs from aquatic environments, while also providing valuable insights for the design of FNP-based materials.

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

The characteristics and fates of microplastics (MPs) and humic acid (HA) in the environment are significantly influenced by their interactions. Thus, the influence of the MP-HA interaction on their dynamic characteristics was explored. Upon MP-HA interaction, the number of hydrogen bonds established in the HA domains decreased significantly, and the water molecules bridging the hydrogen bonds shifted to the exterior regions of the MP-HA aggregates. The distribution intensity of Ca²⁺ located at ∼0.21 nm around HA deceased, indicating that the coordination of Ca²⁺ with the carboxyl on HA was impaired in the presence of MPs. Additionally, the Ca²⁺-HA electrostatic interaction was suppressed because of the steric hindrance of the MPs. However, the MP-HA interaction improved the distribution of water molecules and metal cations around the MPs. The diffusion coefficient of HA decreased from 0.34 × 10^-5 cm²/s to 0.20-0.28 × 10^-5 cm²/s in the presence of MPs, implying that the diffusion of HA was retarded. The diffusion coefficients of polyethylene and polystyrene increased from 0.29 × 10^-5 cm²/s and 0.18 × 10^-5 cm²/s to 0.32 × 10^-5 cm²/s and 0.22 × 10^-5 cm²/s, respectively, indicating that the interaction with HA accelerated the migration of polyethylene and polystyrene. These findings highlight the potential environmental hazards posed by MPs in aquatic environments.