Rigidity and Flexibility: Unraveling the Role of Fulvic Acid in Uranyl Sorption on Graphene Oxide Using Molecular Dynamics Simulations

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.

Reverse osmosis membrane fouling caused by typical surfactants in the integrated circuit industry: Fouling mechanism and control strategies

In the integrated circuit manufacturing process, reverse osmosis (RO) membranes are widely used for wastewater reclamation. However, fouling by typical surfactants significantly reduces membrane efficiency and lifespan. This study investigates the fouling mechanisms of typical surfactants-cetyl trimethyl ammonium bromide (CTAB, cationic), sodium dodecyl sulfate (SDS, anionic), and polyoxyethylene octyl phenyl ether (TX, nonionic)-on RO membranes. Quartz crystal microbalance analysis results show that CTAB and TX exhibit significantly stronger adhesion to RO membranes than SDS. The order of adsorption mass on the membrane surface is CTAB > TX > SDS, with CTAB causing the most severe fouling. Molecular dynamics (MD) simulations indicate that unclustered CTAB molecules contribute to severe fouling by inserting into the membrane surface. As surfactant concentration increases, clustered CTAB is less likely to enter the membrane's surface layer. A comparison of oxidative technologies-continuous dual-wavelength ultraviolet (VUV/UV), intermittent VUV/UV, and intermittent VUV/UV with chlorine, ozone alone, chlorine alone, and ozone combined with chlorine (ozone/chlorine)- reveals that pre-treating surfactants with ozone/chlorine (simultaneous dosing at 10 mg/L each) before membrane filtration effectively controls fouling. After 30 min of treatment, 29 % of CTAB and 86 % of TX were degraded, respectively. Ozone/chlorine oxidation significantly alleviates membrane fouling, increasing the normalized steady-state permeate flux (Jpss) of CTAB and TX by 245 % and 151 %, respectively. The extended Derjaguin-Landau-Verwey-Overbeek theory calculations and MD simulations show that oxidation weakens the adhesion of CTAB and TX to RO membranes, reducing fouling. Ozone/chlorine treatment also effectively mitigates membrane fouling in actual wastewater from the electronics industry. Post-oxidation, the flux ratio (J/J0) increased from 0.28 to 0.52, resulting in a 116.7 % improvement in the Jpss. This study combines experimental data, theoretical calculations, and MD simulations, highlighting the significance of molecular clustering in surfactant-induced fouling before and after oxidation.

Insight into sequestration and release characteristics of uranium(VI) on phlogopite in the presence of humic acid

Knowledge of the sorption speciation and surface configuration of uranium(VI) at the mineral/water interface is essential to construct reliable retention and migration models. However, the ubiquitously existing natural organic substances at U(VI)-contaminated sites readily interact with U(VI) and interfere with the environmental fate of U(VI). In this work, the adsorption behavior and mechanism of U(VI) on phlogopite in the presence of humic acid (HA) were investigated by combining batch experiments, cryogenic time-resolved laser fluorescence spectroscopy (TRLFS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The batch sorption experiments at different HA concentrations suggested that HA had little effect at pH < 4 but suppressed U(VI) sorption on phlogopite from pH 4 to 12. Fluorescence spectral characteristics indicated the formation of multiple surfaces and aqueous U(VI)-humate species, whose abundances varied with pH. The TRLFS coupled with EXAFS spectra suggested that the HA-U(VI) hybrids preferentially bind to surface sites via U(VI) rather than HA. The humate uranium species increased uranium release and migration risk in the natural environment. These findings elucidate the species characteristics and environmental behavior of U(VI) in the presence of natural humic acid and provide guidance for remediation treatments and safety assessment of uranium-contaminated sites.

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.

Molecular Mechanisms of Humic Acid in Inhibiting Silica Scaling during Membrane Distillation

Membrane distillation (MD) efficiently desalinizes and treats high-salinity water as well as addresses the challenges in handling concentrated brines and wastewater. However, silica scaling impeded the effectiveness of MD for treating hypersaline water and wastewater. Herein, the effects of humic acid (HA) on silica scaling behavior during MD are systematically investigated. The interaction mechanism between typical components of HA and active silica was evaluated by molecular dynamics simulations. We find that the addition of HA alleviated the significant decrease in water flux, with recoveries surpassing 60% and 80% at 10 and 20 ppm of HA, respectively. Quantum chemical calculations indicate that the presence of HA greatly raised the free-energy barriers of silica polymerization compared to the system without HA (489.7 vs 45.1 kJ mol-1). Moreover, the interaction between HA molecules and silica significantly weakened the diffusion capacity of silica scale in water (diffusion coefficient from 1.04 × 10-5 to 0.08 × 10-5 cm2 s-1), consequently decreasing the likelihood of contact between silica scale and the hydrophobic membrane. Finally, a neural network analysis model for the HA and silica interaction was developed to design effective inhibitors for silica polymerization. Overall, this study develops nanoscale modeling and simulations to understand how HA inhibits silica scaling in membrane processes, guiding the formation of new approaches to enhance MD performance.

U(VI) sorption on illite in the Co-existence of carbonates and humic substances

The presence of carbonates or humic substances (HS) will significantly affect the species and chemical behavior of U(VI) in solution, but lacking systematic exploration of the coupling effect of carbonates and HS under near real environmental conditions at present. Herein, the sorption behavior of U(VI) on illite was systematically studied in the co-existence of carbonates and HS including both humic acid (HA) and fulvic acid (FA) by batch technique. The distribution coefficients (Kd) increased as function of time and temperature but decreased with increasing concentrations of initial U(VI), Ca2+, and Mg2+, as well as ion strength. At pH 2.0-10.5, the Kd values first increased rapidly and then decreased visibly, with its maximum value appearing at pH 5.0, owning to the changes in the interaction between illite and the dominant species of U(VI) from electrostatic attraction to electrostatic repulsion. The sorption was a heterogeneous, spontaneous, and endothermic chemical process, which could be well described by pseudo-second-order kinetic and Flory-Huggins isotherm models. When carbonates and HA/FA coexisted, the Kd values always increased first and then decreased as a function of pH, with the only difference for HA and FA being the key pH (pHkey) at which the promoting and inhibiting effects on the sorption of U(VI) onto illite undergo a transition. The carbonates and HS have a synergistic inhibitory effect on the U(VI) sorption onto illite at pH 7.8. FTIR and XPS spectra demonstrated that the hydroxyl groups on the illite surface and in the HS were involved in U(VI) sorption on illite in the presence of carbonates. These results provide valuable data for a deeper understanding of U(VI) migration in geological media.

Enhanced Photocatalytic Removal of U(VI) from Real Radioactive Wastewater by Modulating the Surface Charge Microenvironment in Porphyrin-Based Hydrogen-Bonded Organic Framework

Reduction of soluble U(VI) to insoluble U(IV) based on photocatalysts is a simple, environmentally friendly, and efficient method for treating radioactive wastewater. The present study involved the systematic comparison of the photoelectric properties of three metalloporphyrins with different metal centers and the synthesis of a novel porphyrin-based hydrogen-bonded organic framework (Ni-pHOF) photocatalyst by modulating the surface charge microenvironment in porphyrin for enhanced photocatalytic removal of U(VI) from wastewater. Compared to the metal-free HOF, the surface charge microenvironment around the Ni atom in Ni-pHOF accelerated the reduction kinetics of U(VI) under visible light illumination at the initial moment, showing a high removal rate, even in air. The removal rate of U(VI) from aqueous solution by Ni-pHOF can achieve over 98% in the presence of coexisting nonoxidizing cations and only decreased by less than 8% after five cycles, exhibiting high selectivity and good reusability. Furthermore, Ni-pHOF can remove 86.74% of U(VI) from real low-level radioactive wastewater after 120 min of illumination, showcasing practical application potential. Density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectra indicated that modulating the surface charge microenvironment in Ni-pHOF through porphyrin metallization is conducive to improving the charge separation efficiency, prompting more e- and •O2- to participate in the reduction reaction of U(VI). This work provides new insights into the metallization of porphyrin-based HOFs and paves a new way for the tailoring of porphyrin-based HOFs/COFs by modulating the surface charge microenvironment to achieve efficient recovery of U(VI) from real radioactive wastewater.

Molecular insights into the adsorption and penetration of oil droplets on hydrophobic membrane in membrane distillation

Membrane fouling induced by oily substances significantly constrains membrane distillation performance in treating hypersaline oily wastewater. Overcoming this challenge necessitates a heightened fundamental understanding of the oil fouling phenomenon. Herein, the adsorption and penetration mechanism of oil droplets on hydrophobic membranes in membrane distillation process was investigated at the molecular level. Our results demonstrated that the adsorption and penetration of oil droplets were divided into four stages, including the free stage, contact stage, spreading stage, and equilibrium stage. Due to the extensive non-polar surface distribution of the polytetrafluoroethylene (PTFE) membrane (comprising 95.41 %), the interaction between oil molecules and PTFE was primarily governed by van der Waals interaction. Continuous oil droplet membrane fouling model revealed that the new oil droplet molecules preferred to penetrate into membrane pores where oil droplets already existed. The penetration of resin (a component of medium-quality oil droplets) onto PTFE membrane pores required the "pre-paving" of light crude oil. Finally, the ΔE quantitative structure-activity relationships (QSAR) models were developed to evaluate the penetration mechanism of pollutant molecules on the PTFE membrane. This research provides new insights for improving sustainable membrane distillation technologies in treating saline oily wastewater.

Atomic Insights into the Relationship between Molecular Structure and Dispersion Performance of Phenyl Polymer on Graphene Oxide

The adsorption of organic polymers onto the surface of graphene oxide is known to improve its dispersibility in cement-based materials. However, the mechanism of this improvement at the atomic level is not yet fully understood. In this study, we employ a combination of DFT static calculation and umbrella sampling to explore the reactivity of polymers and investigate the effects of varying amounts of phenyl groups on their adsorption capacity on the surface of graphene oxide. Quantitative analysis is utilized to study the structural reconstruction and charge transfer caused by polymers from multiple perspectives. The interfacial reaction between the polymer and graphene oxide surface is further clarified, indicating that the adsorption process is promoted by hydrogen bond interactions and π-π stacking effects. This study sheds light on the adsorption mechanism of polymer-graphene oxide systems and has important implications for the design of more effective graphene oxide dispersants at the atomic level.