Molecular dynamics simulations of structural transformation of perfluorooctane sulfonate (PFOS) at water/rutile interfaces

Significance of non-DLVO interactions on the co-transport of levofloxacin and titanium dioxide nanoparticles in porous media

With the application of engineered nanomaterials and antibiotics in the fields of medicine, aerospace, new energy and agriculture, the associated contamination is detected widely in soil-groundwater systems. It is of great scientific and practical significance to deeply explore the environmental interface process between nanoparticles and antibiotics for the scientific assessment of environmental fate and ecological environmental risks, as well as the development of new composite pollution control technologies. In this study, the co-transport behaviors of positively charged titanium dioxide nanoparticles (TiO2-NPs) and negatively charged levofloxacin (LEV) in quartz sand (QS) are investigated in this study. The results show that TiO2-NPs hardly flow out when transported alone in the column because of its positive charge, which creates a strong attraction with the negatively charged quartz sand on the surface. When TiO2-NPs co-migrate with LEV in porous media, the presence of LEV promotes the transport of TiO2-NPs, while the presence of TiO2-NPs inhibits LEV transport. Non-XDLVO interactions based on molecular dynamics (MD) simulations can help explain the observed promotion and inhibition phenomena as well as the correlation between TiO2-NPs and LEV. The results indicate that TiO2-LEV complexes or aggregates can be formed during the co-transportation process of TiO2-NPs and LEV in porous media. As flow velocity increases from 0.204 cm min-1 to 1.630 cm min-1, both the transport capacities of TiO2-NPs and LEV are enhanced significantly. Under the condition of high citric acid (CA) concentration (15 mmol L-1), the transport capacity of TiO2-NPs is slightly inhibited, while the transport capacity of LEV is enhanced. This study provides new insights into the transport of nanometallic oxides and antibiotics in porous media, which suggests that non-XDLVO interactions should be considered together when assessing the environmental risks and fate of nanometallic oxides and antibiotics in soil-groundwater systems.

Insights into PFAS environmental fate through computational chemistry: A review

Per- and polyfluoroalkyl substances (PFAS) are widely used chemicals that exhibit exceptional chemical and thermal stability. However, their resistance to degradation has led to their widespread environmental contamination. PFAS also negatively affect the environment and other organisms, highlighting the need for effective remediation methods to mitigate their presence and prevent further contamination. Computational chemistry methods, such as Density Functional Theory (DFT) and Molecular Dynamics (MD) offer valuable tools for studying PFAS and simulating their interactions with other molecules. This review explores how computational chemistry methods contribute to understanding and tackling PFAS in the environment. PFAS have been extensively studied using DFT and MD, each method offering unique advantages and computational limitations. MD simulates large macromolecules systems however it lacks the ability model chemical reactions, while DFT provides molecular insights however at a high computational cost. The integration of DFT with MD shows promise in predicting PFAS behavior in different environments. This work summarizes reported studies on PFAS compounds, focusing on adsorption, destruction, and bioaccumulation, highlighting contributions of computational methods while discussing the need for continued research. The findings emphasize the importance of computational chemistry in addressing PFAS contamination, guiding risk assessments, and informing future research and innovations in this field.

Efficient Decomposition of Perfluoroalkyl Substances by Low Concentration Indole: New Insights into the Molecular Mechanisms

Perfluoroalkyl substances (PFAS) are a class of persistent organic pollutants known as "forever chemicals". Currently, the hydrated electron-based advanced reduction process (ARP) holds promise for the elimination of PFAS. However, the efficiency of ARP is often challenged by an oxygen-rich environment, resulting in the consumption of hydrated electron source materials in exchange for the high PFAS decomposition efficiency. Herein, we developed a ternary system constructed by indole and isopropyl alcohol (IPA), and the addition of IPA significantly enhanced the PFOA degradation and defluorination efficiency in the presence of low-concentration indole (<0.4 mM). Meanwhile, opposite results were obtained with a higher amount of indole (>0.4 mM). Further exploring the molecular mechanism of the reaction system, the addition of IPA played two roles. On one hand, IPA built an anaerobic reaction atmosphere and improved the yield and utilization efficiency of hydrated electrons with a low concentration of indole. On the other hand, IPA suppressed the attraction between indole and PFOA, thus reducing the hydrated electron transfer efficiency, especially with more indole. In general, the indole/PFAS/IPA system significantly improved the PFAS destruction efficiency with a small amount of hydrated electron donors, which provided new insights for development of simple and efficient techniques for the treatment of PFAS-contaminated wastewater.

Impact of Variable Water Chemistry on PFOS-Goethite Interactions: Experimental Evidence and Surface Complexation Modeling

Perfluorooctanesulfonate (PFOS) has become a major concern due to its widespread occurrence in the environment and severe toxic effects. In this study, we investigate PFOS sorption on goethite surfaces under different water chemistry conditions to understand the impact of variable groundwater chemistry. Our investigation is based on multiple lines of evidence, including (i) a series of sorption experiments with varying pH, ionic strength, and PFOS initial concentration, (ii) IR spectroscopy analysis, and (iii) surface complexation modeling. PFOS was found to bind to goethite through a strong hydrogen-bonded (HB) complex and a weaker outer-sphere complex involving Na+ coadsorption (OS-Na+). The pH and ionic strength of the solution had a nontrivial impact on the speciation and coexistence of these surface complexes. Acidic conditions and low ionic strength promoted hydrogen bonding between the sulfonate headgroup and protonated hydroxo surface sites. Higher electrolyte concentrations and pH values hindered the formation of strong hydrogen bonds upon the formation of a ternary PFOS-Na+-goethite outer-sphere complex. The findings of this study illuminate the key control of variable solution chemistry on PFOS adsorption to mineral surfaces and the importance to develop surface complexation models integrating mechanistic insights for the accurate prediction of PFOS mobility and environmental fate.

A ReaxFF-based molecular dynamics study of the destruction of PFAS due to ultrasound

This work uses a computational approach to provide a mechanistic explanation for the experimentally observed destruction of per- and polyfluoroalkyl substances (PFAS) in water due to ultrasound. The PFAS compounds have caused a strong public and regulatory response due to their ubiquitous presence in the environment and toxicity to humans. In this research, ReaxFF -based Molecular Dynamics simulation under several temperatures ranging from 373 K to 5,000 K and different environments such as water vapor, O2, N2, and air were performed to understand the mechanism of PFAS destruction. The simulation results showed greater than 98% PFAS degradation was observed within 8 ns under a temperature of 5,000 K in a water vapor phase, replicating the observed micro/nano bubbles implosion and PFAS destruction during the application of ultrasound. Additionally, the manuscript discusses the reaction pathways and how PFAS degradation evolves providing a mechanistic basis for the destruction of PFAS in water due to ultrasound. The simulation showed that small chain molecules C1 and C2 fluoro-radical products are the most dominant species over the simulated period and are the impediment to an efficient degradation of PFAS. Furthermore, this research confirms the empirical findings observations that the mineralization of PFAS molecules occurs without the generation of byproducts. These findings highlight the potential of virtual experiments in complementing laboratory experiments and theoretical projections to enhance the understanding of PFAS mineralization during the application of ultrasound.

Modelling of PFAS-surface interactions: Effect of surface charge and solution ions

PER-: and poly-fluoroalkyl substances (PFAS) are a class of substances of increasing concern as environmental contaminants. The interactions between PFAS and surfaces play an important role in PFAS transport and remediation. Previous studies have found PFAS adsorption to be dependent upon properties including pH, organic matter and particle size, along with PFAS functional group and carbon chain length. It is hypothesised that a theoretical examination of PFAS-surface interactions, via Monte Carlo molecular simulation, would show differences resulting from changes in surface charge, H⁺, OH^-, Ca²⁺ concentrations and PFAS carbon chain length. Monte Carlo molecular simulations of perfluorooctane and perfluorobutane sulfonic acids interacting with a graphite surface in an aqueous medium were performed. Variations in surface charge, H⁺, OH^- and Ca²⁺ concentrations were made. The distance-dependent density of molecules from the surface was analysed as a proxy for PFAS adsorption to the surface. Simulation results showed differences in surface behaviour that depended on surface charge, H⁺, OH^- and Ca²⁺ concentrations, along with carbon chain length, with surface charge playing the most prominent role in controlling PFAS adsorption. For negatively charged surfaces, adsorption due to divalent cation bridging was observed in Ca²⁺ solutions. Modelling, such as in this study, of the thermodynamic equilibrium behaviour of low concentrations of molecules, in scenarios where both adsorption and mobility of PFAS occur, can aid in the design and testing of sorptive surfaces for amendment-based PFAS remediation.

Adsorption of perfluoroalkyl acids on granular activated carbon supported chitosan: Role of nanobubbles

The safety threat posed by Perfluoroalkyl acids (PFAAs) in drinking water is a growing concern. In this study, we loaded chitosan (CS) on granular activated carbon (GAC) to adsorb PFAAs, and we explored the role of nanobubbles in the adsorption process through experiments and density functional theory (DFT) calculations. Compared with GAC, we found that the use of the composite adsorbent (CS/GAC) enhanced the removal rate of perfluorooctanoic acid by 136% with the assistance of nanobubbles. PFAAs with different chain lengths have different adsorption mechanisms owing to surface activity differences. PFAAs with longer C-F chains can be directly enriched with amino groups on the CS or air-water interface on composite adsorbents. Additionally, PFAAs can be enriched with nanobubbles in solution to form nanobubble-PFAA colloids, which are adsorbed by protonated amino groups on CS through electrostatic interactions. We found that PFAAs with shorter C-F chains are less affected by nanobubbles, and DFT calculations indicated that the adsorption of short-chain PFAAs is mainly affected by electrostatic interactions. We also proved that the electrostatic interactions between CS and PFAAs are mainly derived from the abundant protonated amino groups.

Contribution of Nanobubbles for PFAS Adsorption on Graphene and OH- and NH2-Functionalized Graphene: Comparing Simulations with Experimental Results

Per- and polyfluoroalkyl substances (PFAS) are ubiquitous in aquatic environments around the world. In recent years, the enrichment of PFAS on the surface of nanobubbles on adsorbents has been proposed, but no direct evidence has been provided to support this new adsorption mechanism. In this study, we used density functional theory (DFT) and molecular dynamics (MD) to simulatively investigate the contribution of nanobubbles for PFAS adsorption on the pristine and functionalized graphene (GR). The adsorption energy of PFAS on GR-NH₂ was higher than that of GR-OH, while GR showed the lowest adsorption energy. When the effect of water molecules was considered, the oleophobic property of the C-F chain made it difficult for hydrophobic interaction to be involved in the adsorption of PFAS on nonpolar GR. With the existence of nanobubbles, both GR and GR-NH₂ can effectively remove PFAS, but their adsorption mechanisms were quite different. For hydrophobic GR, the nanobubbles initially attached to the surface of materials played a major role, while for hydrophilic GR-NH₂, the nanobubbles dispersed in the solution were more important. Moreover, the nanobubbles had a more significant contribution to long-chain PFAS. Our degassing and aeration experiments could support the simulation results. The removal of PFOS decreased by 27.7% at maximum after degassing and increased by 21.0%-29.2% after aeration. The study could provide a theoretical basis for the environmental process and contamination control of PFAS at the solid-liquid interfaces.

Glyphosate adsorption onto kaolinite and kaolinite-humic acid composites: Experimental and molecular dynamics studies

Glyphosate (PMG) has been the most widely used herbicide in the world, and its environmental mobility and fate are mainly controlled by interactions with mineral surfaces. In soil systems, kaolinite is typically associated with humic acids (HAs) in the form of mineral-HA complexes, and hence it is crucial to characterize the molecular-scale interactions that occur between PMG and kaolinite and kaolinite-HA complexes. Batch experiments, Fourier transform infrared spectrum (FTIR) and X-ray photoelectron spectroscopy (XPS), isothermal titration calorimetry (ITC), and molecular dynamics (MD) simulations were performed to decipher the molecular interactions between PMG and kaolinite and kaolinite-HA composites. Our results reveal that kaolinite-HA composites adsorb higher concentrations of PMG than does kaolinite alone, likely due to more adsorption sites existed on kaolinite-HA than on kaolinite. FTIR and XPS analysis reveal that the carboxyl, phosphonyl and amino groups of PMG interacted with kaolinite and kaolinite-humic acid via Hydrogen bonds. The ITC results and interaction energy calculations indicate that the adsorption of PMG onto the kaolinite-HA is more energetically favorable relative to that onto kaolinite. MD simulations suggest that the PMG molecule adsorbs parallel to the surface of kaolinite and the composites through hydrogen bonding. Humic acid increases the adsorption of PMG through the creation of H-bond networks between PMG, the kaolinite surface, and humic acid. The results from this study improve our molecular-level understanding of the interactions between PMG and two important components of soil systems, and hence yield valuable information for characterizing the fate and behavior of PMG in soil environments.

Fluorinated Surfactant Adsorption on Mineral Surfaces: Implications for PFAS Fate and Transport in the Environment

Fluorinated surfactants, which fall under the class of per- and polyfluoroalkyl substances (PFAS), are amphiphilic molecules that comprise hydrophobic fluorocarbon chains and hydrophilic head-groups. Fluorinated surfactants have been utilized in many applications, e.g., fire-fighting foams, paints, household/kitchenware items, product packaging, and fabrics. These compounds then made their way into the environment, and have been detected in soil, fresh water, and seawater. From there, they can enter human bodies. Fluorinated surfactants are persistent in water and soil environments, and their adsorption onto mineral surfaces contributes to this persistence. This review examines how fluorinated surfactants adsorb onto mineral surfaces, by analyzing the thermodynamics and kinetics of adsorption, and the underlying mechanisms. Adsorption of fluorinated surfactants onto mineral surfaces can be explained by electrostatic interactions, hydrophobic interactions, hydrogen bonding, and ligand and ion exchange. The aqueous pH, varying salt or humic acid concentrations, and the surfactant chemistry can influence the adsorption of fluorinated surfactants onto mineral surfaces. Further research is needed on fluorinated surfactant adsorbent materials to treat drinking water, and on strategies that can modulate the fate of these compounds in specific environmental locations.

Removal of perfluorooctanoic acid from water using calcined hydrotalcite - A mechanistic study

Calcined hydrotalcite (CHT) was evaluated for its potential removal of perfluorooctanoic acid (PFOA) from water in this study. The uptake of PFOA by CHT could be as high as 1587 mg/g (ca. 3.8 mmol/g), slightly larger than the anion exchange capacity (AEC) of the hydrotalcite (HT). Such a high removal was fast and pH independent, suggesting the versatile use of CHT. Due to the structural memory effect of HT, the removal involved adsorption of PFOA during HT recovery and intercalation of PFOA into the interlayer of restructured HT at low and high initial concentrations, respectively. Limited by the specific surface area and AEC, the intercalated PFOA would form a vertical bilayer or admicelle conformation. As such, the HT intercalated with PFOA became one-layer stacking with a basal spacing of 2.04 nm in contrast to the 3R polytype of recovered HT having a layer thickness of 0.78 nm, as confirmed by X-ray diffraction, thermogravimetric, and infra-red analyses. Due to its high PFOA removal capacity and large partitioning coefficient, the amount of CHT used, thus, the disposal of PFOA-laden solid could be minimized.

Effect of counterions on anionic fluorocarbon surfactant micelles by dielectric spectroscopy

The effect of counterions on dielectric behaviors of anionic fluorocarbon surfactants solutions was insighted in the frequency of 40–110 MHz. The dielectric increments Δεof all the surfactants can be divided into different groups, the reason was analyzed and the average radiusR̄was calculated according to Grosse's model, which confirmed the reliability of dielectric analysis, and the structure of micelles was proposed as the figure.

Adsorptive removal of emerging polyfluoroalky substances F-53B and PFOS by anion-exchange resin: A comparative study

Chrome plating is an important emission source of perfluorinated compounds (PFCs) industrial uses in China, where two commercial products potassium 2-(6-chloro-1,1,2,2,3,3,4,4,5,5,6,6-dodecafluorohexyloxy) (F-53B) and perfluorooctane sulfonate (PFOS) are applied as mist suppressant, causing non-negligible environmental risk. In this paper, anion-exchange resin IRA67 was evaluated for F-53B and PFOS removal from simulated and actual wastewater. Adsorption kinetics exhibited higher adsorption velocity and capacity of IRA67 for PFOS than F-53B due to their difference in molecular structures. Adsorption isotherms demonstrated the adsorption capacity of F-53B and PFOS on IRA67 was 4.2mmol/g and 5.5mmol/g, respectively. Because of the deprotonating of amine groups, solution pH had significant effect on IRA67 at pH>10. The results indicated that besides anion exchange other interactions including hydrophobic interaction and the formation of micelles or hemi-micelles were all involved in adsorption process. Coexisting sulfate and chromate in wastewater decreased adsorption capacities of F-53B and PFOS. The spent resin could be regenerated by the NaCl/NaOH and methanol mixed solution. In the mixed system and actual wastewater IRA67 can simultaneously remove F-53B and PFOS without obvious preference but the removal percent can be affected by competitive effect.

Single-walled carbon nanotube release affects the microbial enzyme-catalyzed oxidation processes of organic pollutants and lignin model compounds in nature

The question how microbial enzyme-catalyzed oxidation processes of organic pollutants and lignin model compounds (LMCs) are affected by the release of single-walled carbon nanotube (SWCNT) into the environment remains to be addressed at the molecular level. We have, therefore concentrated the effects of SWCNT on some important properties associated with enzyme activity and function during microbial oxidation of polycyclic aromatic hydrocarbons (benzo(a)pyrene, acenaphthene and anthracene), LMCs (2,6-dimethoxyphenol, guaiacol and veratryl alcohol) and β-hexachlorocyclohexane, including the behaviour of water molecules, hydrogen bonds (HBs) and hydrophobic interactions (HYs) between ligand and the enzyme, and conformational dynamics in N- and C-terminus. Our study revealed that SWCNT significantly affected the behaviour of water molecules within 5 Å of both these substrates and their respective enzymes during oxidation (p < 0.01), by increasing or decreasing the water number near them. SWCNT tended to significantly enhance or reduce the stability of atom pairs that formed the HBs and HYs (p < 0.01). N- and C-terminus conformations underwent transitions between positive and negative states or between positive state or between negative state in all analyzed complexes. Significant conformational transitions were found for all C-terminus, but only for a part of N-terminus after the inclusion of the SWCNT. These results showed that SWCNT release would significantly affect the microbial enzyme-catalyzed processes of organic pollutants and LMCs in nature.