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

   https://doi.org/10.3390/surfaces3040037  

   Alves, Anthony V. ; Tsianou, Marina ; Alexandridis, Paschalis ( December 2020 , Surfaces)

   null (Ed.)

   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. 

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2. Effects of Ionic Strength on Arsenate Adsorption at Aluminum Hydroxide–Water Interfaces

   https://doi.org/10.3390/soils2010001  

   Xu, Tingying ; Catalano, Jeffrey G ( March 2018 , Soil Systems)

   Adsorption processes at mineral–water interfaces control the fate and transport of arsenic in soils and aquatic systems. Mechanistic and thermodynamic models to describe this phenomenon only consider inner-sphere complexes but recent observation of the simultaneous adsorption of inner- and outer-sphere arsenate on single crystal surfaces complicates this picture. In this study, we investigate the ionic strength-dependence of the macroscopic adsorption behavior and molecular-scale surface speciation of arsenate bound to gibbsite and bayerite. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge at pH 7, as indicated by zeta-potential measurements. Extended X-ray absorption fine structure (EXAFS) spectroscopy finds that the number of second shell Al neighbors around arsenate is lower than that required for arsenate to occur solely as an inner-sphere surface complex. Together, these observations demonstrate that arsenate displays macroscopic and molecular-scale behavior consistent with the co-occurrence of inner- and outer-sphere surface complexes. This demonstrated that outer-sphere species can be responsible for strong adsorption of ions and suggests that environments experiencing an increase in salt content may induce arsenic release to water, especially under weakly acidic conditions. 

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3. In situ investigation of water on MXene interfaces

   https://doi.org/10.1073/pnas.2108325118  

   Zaman, Wahid ; Matsumoto, Ray A. ; Thompson, Matthew W. ; Liu, Yu-Hsuan ; Bootwala, Yousuf ; Dixit, Marm B. ; Nemsak, Slavomir ; Crumlin, Ethan ; Hatzell, Marta C. ; Cummings, Peter T. ; et al ( November 2021 , Proceedings of the National Academy of Sciences)

   A continuum of water populations can exist in nanoscale layered materials, which impacts transport phenomena relevant for separation, adsorption, and charge storage processes. Quantification and direct interrogation of water structure and organization are important in order to design materials with molecular-level control for emerging energy and water applications. Through combining molecular simulations with ambient-pressure X-ray photoelectron spectroscopy, X-ray diffraction, and diffuse reflectance infrared Fourier transform spectroscopy, we directly probe hydration mechanisms at confined and nonconfined regions in nanolayered transition-metal carbide materials. Hydrophobic (K + ) cations decrease water mobility within the confined interlayer and accelerate water removal at nonconfined surfaces. Hydrophilic cations (Li + ) increase water mobility within the confined interlayer and decrease water-removal rates at nonconfined surfaces. Solutes, rather than the surface terminating groups, are shown to be more impactful on the kinetics of water adsorption and desorption. Calculations from grand canonical molecular dynamics demonstrate that hydrophilic cations (Li + ) actively aid in water adsorption at MXene interfaces. In contrast, hydrophobic cations (K + ) weakly interact with water, leading to higher degrees of water ordering (orientation) and faster removal at elevated temperatures. 

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4. Interactions of emerging contaminants with model colloidal microplastics, C 60 fullerene, and natural organic matter – effect of surface functional group and adsorbate properties

   https://doi.org/10.1039/D0EM00026D  

   Williams, Tyler ; Walsh, Clare ; Murray, Keith ; Subir, Mahamud ( January 2020 , Environmental Science: Processes & Impacts)

   Surface adsorption of two commonly detected emerging contaminants, amlodipine (AMP) and carbamazepine (CBZ), onto model colloidal microplastics, natural organic matter (NOM), and fullerene nanomaterials have been investigated. It is found that AMP accumulation at these colloidal–aqueous interfaces is markedly higher than that of CBZ. Measurements of surface excess and particle zeta potential, along with pH-dependent adsorption studies, reveal a distinct influence of colloidal functional group on the adsorption properties of these pharmaceuticals. AMP shows a clear preference for a surface containing carboxylic group compared to an amine modified surface. CBZ, in contrast, exhibit a pH-dependent surface proclivity for both of these microparticles. The type of interactions and molecular differences with respect to structural rigidity and charge properties explain these observed behaviors. In this work, we also demonstrate a facile approach in fabricating uniform microspheres coated with NOM and C 60 nanoclusters. Subsequent binding studies on these surfaces show considerable adsorption on the NOM surface but a minimal uptake of CBZ by C 60 . Adsorption induced colloidal aggregation was not observed. These findings map out the extent of contaminant removal by colloids of different surface properties available in the aquatic environment. The methodology developed for the adsorption study also opens up the possibility for further investigations into colloidal–contaminant interactions. 

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5. Investigations of water/oxide interfaces by molecular dynamics simulations

   https://doi.org/10.1002/wcms.1537  

   Wang, Ruiyu ; Klein, Michael L. ; Carnevale, Vincenzo ; Borguet, Eric ( June 2021 , WIREs Computational Molecular Science)

   Water/oxide interfaces are ubiquitous on earth and show significant influence on many chemical processes. For example, understanding water and solute adsorption as well as catalytic water splitting can help build better fuel cells and solar cells to overcome our looming energy crisis; the interaction between biomolecules and water/oxide interfaces is one hypothesis to explain the origin of life. However, knowledge in this area is still limited due to the difficulty of studying water/solid interfaces. As a result, research using increasingly sophisticated experimental techniques and computational simulations has been carried out in recent years. Although it is difficult for experimental techniques to provide detailed microscopic structural information, molecular dynamics (MD) simulations have satisfactory performance. In this review, we discuss classical and ab initio MD simulations of water/oxide interfaces. Generally, we are interested in the following questions: How do solid surfaces perturb interfacial water structure? How do interfacial water molecules and adsorbed solutes affect solid surfaces and how do interfacial environments affect solvent and solute behavior? Finally, we discuss progress in the application of neural network potential based MD simulations, which offer a promising future because this approach has already enabled ab initio level accuracy for very large systems and long trajectories.

   This article is categorized under:

   Theoretical and Physical Chemistry > Spectroscopy

   Molecular and Statistical Mechanics > Molecular Interactions

   Structure and Mechanism > Molecular Structures

    

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