Bridging molecular-scale interfacial science with continuum-scale models.

Solid-water interfaces are crucial for clean water, conventional and renewable energy, and effective nuclear waste management. However, reflecting the complexity of reactive interfaces in continuum-scale models is a challenge, leading to oversimplified representations that often fail to predict real-world behavior. This is because these models use fixed parameters derived by averaging across a wide physicochemical range observed at the molecular scale. Recent studies have revealed the stochastic nature of molecular-level surface sites that define a variety of reaction mechanisms, rates, and products even across a single surface. To bridge the molecular knowledge and predictive continuum-scale models, we propose to represent surface properties with probability distributions rather than with discrete constant values derived by averaging across a heterogeneous surface. This conceptual shift in continuum-scale modeling requires exponentially rising computational power. By incorporating our molecular-scale understanding of solid-water interfaces into continuum-scale models we can pave the way for next generation critical technologies and novel environmental solutions.

Ion solvation as a predictor of lanthanide adsorption structures and energetics in alumina nanopores.

Adsorption reactions at solid-water interfaces define elemental fate and transport and enable contaminant clean-up, water purification, and chemical separations. For nanoparticles and nanopores, nanoconfinement may lead to unexpected and hard-to-predict products and energetics of adsorption, compared to analogous unconfined surfaces. Here we use X-ray absorption fine structure spectroscopy and operando flow microcalorimetry to determine nanoconfinement effects on the energetics and local coordination environment of trivalent lanthanides adsorbed on Al2O3 surfaces. We show that the nanoconfinement effects on adsorption become more pronounced as the hydration free energy, ΔGhydr, of a lanthanide decreases. Neodymium (Nd3+) has the least exothermic ΔGhydr (-3336 kJ·mol-1) and forms mostly outer-sphere complexes on unconfined Al2O3 surfaces but shifts to inner-sphere complexes within the 4 nm Al2O3 pores. Lutetium (Lu3+) has the most exothermic ΔGhydr (-3589 kJ·mol-1) and forms inner-sphere adsorption complexes regardless of whether Al2O3 surfaces are nanoconfined. Importantly, the energetics of adsorption is exothermic in nanopores only, and becomes endothermic with increasing surface coverage. Changes to the energetics and products of adsorption in nanopores are ion-specific, even within chemically similar trivalent lanthanide series, and can be predicted by considering the hydration energies of adsorbing ions.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment.

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Connecting Thermodynamics of Alkali Ion Exchange on the Quartz (101) Surface with Density Functional Theory Calculations.

Periodic plane-wave density functional theory (DFT) calculations were performed on the α-quartz (SiO2) (101) surface to model exchange of adsorbed Li+ and either Na+, K+, or Rb+ in inner- and outer-sphere adsorbed, and aqueous configurations, which are charge-balanced with 2 Cl-. SiO- or SiOH groups represented the adsorption surface sites. The SiO- models included 58 H2O and 2 H3O+ molecules to approximate an aqueous environment, whereas the SiOH models had 59 H2O and 1 H3O+ molecules. The goal of this work is to calculate the heats of exchange for these alkali ions and to compare the results with those measured by flow microcalorimetry to ascertain the most probable mechanisms for these cations exchanging on the α-quartz (101) surface. Energy minimizations of each alkali ion adsorbed as outer-sphere complexes on SiOH surface sites, and as inner- and outer-sphere complexes on SiO- surface sites, were used to determine the energy of exchange (ΔEex) with Li+ for comparison with experimentally determined ΔHex values. Here, we present a novel method for calculating ΔEex using the difference in energies of geometry-optimized end member models. The aqueous and surface structures produced are similar to those observed experimentally. Although the trend for the calculated ΔEex values is consistent with those from the heats of exchange measured experimentally, the magnitude of our modeled ΔEex results is significantly larger than select experimental data from the literature [Peng, L. Zeta-Potentials and Enthalpy Changes in the Process of Electrostatic Self-Assembly of Cations on Silica Surface. Powder Technol. 2009, 193(1), 46-49]; we discuss the reasons for this discrepancy herein. The relative energy differences of the various configurations modeled have implications for the measurements of the surface charge via potentiometric titrations due to the more active role of alkali cations in quartz surface chemistry that have been previously considered as inert background electrolytes.

Interaction of Phosphate with Lithium Cobalt Oxide Nanoparticles: A Combined Spectroscopic and Calorimetric Study.

Adsorption of small ions such as phosphates to the surfaces of metal oxides can significantly alter the behavior of these materials, especially when present in the nanoscale form. Lithium cobalt oxide is a good model system for understanding small-molecule interactions with emerging nanomaterials because of its widespread use in lithium ion batteries and its known activity as a water oxidation catalyst. Here, we present a thermodynamic analysis of phosphate adsorption to LiCoO2 and corroborate the results with additional in situ techniques, including zeta potential measurements and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, at pH values relevant to potential environmental release scenarios. Flow microcalorimetry measurements of phosphate interaction with LiCoO2 at pH 7.4 show that there are two distinct exothermic processes taking place. Time-sequence in situ ATR-FTIR with two-dimensional correlation analysis reveals the spectroscopic signatures of these processes. We interpret the data as an interaction of phosphate with LiCoO2 that occurs through the release of two water molecules and is therefore, best described as a condensation process rather than a simple adsorption, consistent with prior studies, demonstrating that phosphate interaction with LiCoO2 is highly irreversible. Additional measurements for over longer times of 5 months show that phosphate adsorption terminates with one surface layer and that continued transformation over longer periods of time arises from H+/Li+ exchange and slow transformation to a cobalt hydroxide, with phosphate adsorbed to the surface only. To the best of our knowledge, this is the first time that flow microcalorimetry and two-dimensional correlation spectroscopy have been applied in tandem to clarify the specific chemical reactions that occur at the interface of solids and adsorbates.

Calorimetric study of the influence of aluminum substitution in ferrihydrite on sulfate adsorption and reversibility.

Ferrihydrite (Fh) is a nanocrystalline iron (hydr)oxide pervasive in various surface environments. It has high specific surface areas and high density of reactive surface-sites, both of which properties impart a consequential role in determining the fate and transport of environmental nutrients and contaminants. In natural environments, Fh readily reacts with impurities, such as aluminum (Al) and has variable substituted chemical compositions and surface properties. This work examines the effect of aluminum (Al) incorporation (0%, 12% and 24 mol% Al) on the interaction energy of chloride (Cl-) and nitrate (NO3-), and adsorption/desorption of sulfate (SO42-) onto Fh. Microcalorimetry experiments were conducted at pHs 3.0 and 5.6, along with a detailed characterization of all samples. Results showed a significant increase in the energetics of the exothermic peak of NO3- and the endothermic peak of Cl- with increasing Al concentration and decreasing pH values. Furthermore, the exothermic heat of exchange, adsorption, irreversibility and fraction of inner-sphere complexes for sulfate interaction with Fh increased with more Al concentration and acidic pH.

Density functional theory modeling of chromate adsorption onto ferrihydrite nanoparticles.

Density functional theory (DFT) calculations were performed on a model of a ferrihydrite nanoparticle interacting with chromate ([Formula: see text]) in water. Two configurations each of monodentate and bidentate adsorbed chromate as well as an outer-sphere and a dissolved bichromate ([Formula: see text]) were simulated. In addition to the 3-D periodic planewave DFT models, molecular clusters were extracted from the energy-minimized structures. Calculated interatomic distances from the periodic and cluster models compare favorably with Extended X-ray Absorption Fine Structure spectroscopy values, with larger discrepancies seen for the clusters due to over-relaxation of the model substrate. Relative potential energies were derived from the periodic models and Gibbs free energies from the cluster models. A key result is that the bidentate binuclear configuration is the lowest in potential energy in the periodic models followed by the outer-sphere complex. This result is consistent with observations of the predominance of bidentate chromate adsorption on ferrihydrite under conditions of high surface coverage (Johnston Environ Sci Technol 46:5851-5858, 2012). Cluster models were also used to perform frequency analyses for comparison with observed ATR FTIR spectra. Calculated frequencies on monodentate, bidentate binuclear, and outer-sphere complexes each have infrared (IR)-active modes consistent with experiment. Inconsistencies between the thermodynamic predictions and the IR-frequency analysis suggest that the 3-D periodic models are not capturing key components of the system that influence the adsorption equilibria under varying conditions of pH, ionic strength and electrolyte composition. Model equilibration via molecular dynamics (MD) simulations is necessary to escape metastable states created during DFT energy minimizations based on the initial classical force field MD-derived starting configurations.

ATR-FTIR and Flow Microcalorimetry Studies on the Initial Binding Kinetics of Arsenicals at the Organic-Hematite Interface.

The environmental fate of arsenic compounds depends on their surface interactions with geosorbents that include clays, metal oxides, and natural organic matter (NOM). While a number of batch studies reported that NOM inhibits the uptake of arsenicals, it remains unclear how different classes of organic functional groups affect their binding mechanisms. We report herein the adsorption kinetics of arsenate and dimethylarsinic acid (DMA) with hematite nanoparticles pre-exposed to three types of low molecular weight organics: citrate, oxalate, and pyrocatechol as representatives to the majority of reactive organic functional groups in NOM. These studies were conducted using attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) and flow microcalorimetry at pH 7 with an emphasis on the role that electrolytes (KCl, NaCl, and KBr) play in the adsorption process. Results show that (1) negatively charged carboxylate versus hydrophobic phenyl groups influence amounts and initial rates of arsenicals adsorption on hematite nanoparticles to varying degrees depending on the type of complexes they form, (2) the type of electrolytes affects initial adsorption rate of DMA to a greater extent than arsenate when oxalate is present on the surface, and (3) the extent of organics retention by hematite nanoparticles is influenced by the type of the desorbing agent.

Calorimetric study of alkali and alkaline-earth cation adsorption and exchange at the quartz-solution interface.

Cations in natural solutions significantly impact interfacial processes, particularly dissolution and surface charge measurements for quartz and silica, which are amongst the most naturally abundant and technologically important solids. Thermodynamic parameters for cation-specific interfacial reactions have heretofore been mostly derived instead of directly measured experimentally. This work investigates the energetics of adsorption and exchange reactions of alkali metal (M+) and alkaline earth (M2+) cations with the quartz surface by flow adsorption microcalorimetry, in tandem with in-situ pH measurements. The magnitudes of the heats of adsorption and exchange were found to increase along the Hofmeister series i.e., Li+

Ion Exchange Thermodynamics at the Rutile-Water Interface: Flow Microcalorimetric Measurements and Surface Complexation Modeling of Na-K-Rb-Cl-NO3 Adsorption.

Flow microcalorimetry was used to investigate the energetics associated with Rb+, K+, Na+, Cl-, and NO3- exchange at the rutile-water interface. Heats of exchange reflected differences in bulk hydration/dehydration enthalpies (Na+ > K+ > Rb+, and Cl- > NO3-) such that exchanging Na+ or Cl- from the surface was exothermic, reflecting their greater bulk hydration enthalpies. Exchange heats were measured at pH 2, 3.25, 5.8, and 11 and exhibited considerable differences as well as pH dependence. These trends were rationalized with the aid of a molecularly constrained surface complexation model (SCM) that incorporated the inner-sphere binding observed for the cations on the rutile (110) surface. Explicitly accounting for the inner-sphere binding configuration differences between Rb+, K+, and Na+, as well as accompanying differences in negative surface charge development, resulted in much better agreement with measured exchange ratios than by considering bulk hydration enthalpies alone. The observation that calculated exchange ratios agreed with those measured experimentally lends additional credence to the SCM. Consequently, flow microcalorimetry and surface complexation modeling are a useful complement of techniques for probing the energetics associated with ion exchange and adsorption processes and should also serve to help validate molecular simulations of interfacial energetics.

Temperature-dependent infrared and calorimetric studies on arsenicals adsorption from solution to hematite nanoparticles.

To address the lack of systematic and surface sensitive studies on the adsorption energetics of arsenic compounds on metal (oxyhydr)oxides, we conducted temperature-dependent ATR-FTIR studies for the adsorption of arsenate, monomethylarsonic acid, and dimethylarsinic acid on hematite nanoparticles at pH 7. Spectra were collected as a function of concentration and temperature in the range 5-50 °C (278-323 K). Adsorption isotherms were constructed from spectral features assigned to surface arsenic. Values of K(eq), adsorption enthalpy, and entropy were extracted from fitting the Langmuir model to the data and from custom-built triple-layer surface complexation models derived from our understanding of the adsorption mechanism of each arsenical. These spectroscopic and modeling results were complemented with flow-through calorimetric measurements of molar heats of adsorption. Endothermic adsorption processes were predicted from the application of mathematical models with a net positive change in adsorption entropy. However, experimentally measured heats of adsorption were exothermic for all three arsenicals studied herein, with arsenate releasing 1.6-1.9 times more heat than methylated arsenicals. These results highlight the role of hydration thermodynamics on the adsorption of arsenicals, and are consistent with the spectral interpretation of type of surface complexes each arsenical form in that arsenate is mostly dominated by bidentate, MMA by a mixture of mono- and bidentate, and DMA by mostly outer sphere.

Can the soil bacterium Cupriavidus necator sense ZnO nanomaterials and aqueous Zn²âº differentially?

Synthetic metal oxide nanomaterials exert toxicity via two general mechanisms: release of free ions at concentrations which exert toxic effects upon the target cell, or specific surface-mediated physicochemical processes leading to the formation of hydroxyl free radicals and other reactive oxygen species which act to disrupt cell membranes and organelles. From a regulatory standpoint this presents a potential problem since it is not trivial to detect free metal ions in the presence of nanoparticles in biological or natural media. This makes efforts to identify the route of uptake difficult. Although in vitro studies of zinc oxide nanoparticles suggest that toxicity to the soil bacterium Cupriavidus necator is exerted in a similar manner to zinc acetate, we found no free Zn ion is associated with nanoparticle suspensions. The proteome of cells subjected to equal concentrations of either the nanoparticle or zinc acetate suggest that the mode of toxicity is quite different for the two forms of Zn, with a number of membrane-associated proteins up-expressed in response to nanoparticle exposure. Our data suggests that nanoparticles act to interrupt cell membranes thereby causing cell death rather than exerting a strictly toxic effect. We also identify potentially useful genes to serve as biomarkers of membrane disruption in toxicogenomic studies with nanoparticles or to engineer biosensor organisms.

Toxicity of manufactured zinc oxide nanoparticles in the nematode Caenorhabditis elegans.

Information describing the possible impacts of manufactured nanoparticles on human health and ecological receptors is limited. The objective of the present study was to evaluate the potential toxicological effects of manufactured zinc oxide nanoparticles (ZnO-NPs; 1.5 nm) compared to aqueous zinc chloride (ZnCl2) in the free-living nematode Caenorhabditis elegans. Toxicity of both types of Zn was investigated using the ecologically relevant endpoints of lethality, behavior, reproduction, and transgene expression in a mtl-2::GFP (gene encoding green fluorescence protein fused onto the metallothionein-2 gene promoter) transgenic strain of C. elegans. Zinc oxide nanoparticles showed no significant difference from ZnCl2 regarding either lethality or reproduction in C. elegans, as indicated by their median lethal concentrations (LC50s; p = 0.29, n=3) and median effective concentrations (EC50s; Z = 0.835, p = 0.797). Also, no significant difference was found in EC50s for behavioral change between ZnO-NPs (635 mg Zn/L; 95% confidence interval [CI], 477-844 mg Zn/L) and ZnCl2 (546 mg Zn/L; 95% CI, 447-666 mg Zn/L) (Z = 0.907, p = 0.834). Zinc oxide nanoparticles induced transgene expression in the mtl-2::GFP transgenic C. elegans in a manner similar to that of ZnCl2, suggesting that intracellular biotransformation of the nanoparticles might have occurred or the nanoparticles have dissolved to Zn2+ to enact toxicity. These findings demonstrate that manufactured ZnO-NPs have toxicity to the nematode C. elegans similar to that of aqueous ZnCl2.

Energetics of arsenate sorption on amorphous aluminum hydroxides studied using flow adsorption calorimetry.

Flow adsorption calorimetry was used to investigate the energetics of arsenate sorption on amorphous aluminum hydroxide (AHO) and its effect on surface charge and ion exchange. Arsenate sorption at pH 5.7 was exothermic and the molar heats of adsorption were quite variable, ranging from -3.0 to -66 kJ/mol. Repetitive exposure of the same sample to arsenate in the calorimeter showed that the AHO was able to regenerate a considerable amount of reactive surface over time periods as short as 15 to 20 min. The large variability in heats of arsenate adsorption and the ability to regenerate reactive surface is believed to result from the amorphous nature of the AHO used. Heats of Cl/NO3 exchange were much smaller and more consistent, ranging from about 3.0 to 6.0 kJ/mol. The molar ratio of exchangeable Cl:Al was about 6:1 for the AHO, indicating a highly porous material. At pH 5.7, arsenate sorption neutralized surface positive charge as measured by Cl/NO3 exchange. Only at the two highest loadings (>60,000 mg/kg) did arsenate sorption result in any negative surface charge as measured by Na/K exchange. These results showed that most of the arsenate was adsorbed by a mechanism that involved no increase in surface negative charge. The PZNC of the AHO decreased by about 1 pH unit when exposed to arsenate in the flow calorimeter. Exposure to arsenate in a batch system decreased the PZNC about 4 pH units. This difference in behavior between batch and flow systems was related to differences in the amount of arsenate adsorbed.