Proton-promoted dissolution of α-FeOOH nanorods and microrods: Size dependence, anion effects (carbonate and phosphate), aggregation and surface adsorption

Electrical-Driven Directed-Evolution of Copper Nanowires Catalysts for Efficient Nitrate Reduction to Ammonia

The electrocatalytic conversion of nitrate (NO3 - ) to NH3 (NO3 RR) at ambient conditions offers a promising alternative to the Haber-Bosch process. The pivotal factors in optimizing the proficient conversion of NO3 - into NH3 include enhancing the adsorption capabilities of the intermediates on the catalyst surface and expediting the hydrogenation steps. Herein, the Cu/Cu2 O/Pi NWs catalyst is designed based on the directed-evolution strategy to achieve an efficient reduction of NO3 ‾. Benefiting from the synergistic effect of the OV -enriched Cu2 O phase developed during the directed-evolution process and the pristine Cu phase, the catalyst exhibits improved adsorption performance for diverse NO3 RR intermediates. Additionally, the phosphate group anchored on the catalyst's surface during the directed-evolution process facilitates water electrolysis, thereby generating Hads on the catalyst surface and promoting the hydrogenation step of NO3 RR. As a result, the Cu/Cu2 O/Pi NWs catalyst shows an excellent FE for NH3 (96.6%) and super-high NH3 yield rate of 1.2 mol h-1  gcat. -1 in 1 m KOH and 0.1 m KNO3 solution at -0.5 V versus RHE. Moreover, the catalyst's stability is enhanced by the stabilizing influence of the phosphate group on the Cu2 O phase. This work highlights the promise of a directed-evolution approach in designing catalysts for NO3 RR.

Analysis of micro- and nanoscale heterogeneities within environmentally relevant thin films containing biological components, oxyanions and minerals using AFM-PTIR spectroscopy

Minerals in groundwater interact with various chemical and biological species including organic matter, proteins, and prevalent oxyanions, resulting in surface coatings and thin films of these different components. Surface interactions and the surface adsorption of these components on both oxide and oxyhydroxide iron surfaces have been widely investigated using a variety of spectroscopic methods. Despite these numerous studies, there still remains uncertainty with respect to interactions between these individual components, as well as heterogeneities and phase segregations within these thin films. In this study, we investigate mixtures containing Fe-containing minerals, proteins, and oxyanions to better understand surface interactions and phase segregation using Atomic Force Microscopy PhotoThermal Infrared (AFM-PTIR) spectroscopy. The results of this study show that AFM-PTIR spectroscopy can identify both nano- and microscale heterogeneities present within these thin films that are difficult to discern with other more conventional techniques such as ATR-FTIR spectroscopy due to phase segregation and mineral surface interactions. Overall, AFM-PTIR spectroscopy provides insights into multi-component environmental films that are difficult to uncover using other methodologies. This method has the potential to differentiate between bound and unbound toxic species as well as biological components, including environmental DNA, which can be used to assess the fate and transport of these species in the environment.

Surface Site Density of Synthetic Goethites and Its Relationship to Atomic Surface Roughness and Crystal Size

The capacity for crystals to adsorb elements and molecules is a function of the structures of their crystal faces and the relative proportions of those faces. More importantly, this study shows that the surface structure of crystal faces is affected by their surface roughness and is the dominant factor controlling the absorption site density. In a continuation of the study of synthetic goethites with varying single crystal size distributions, two more synthetic goethites with intermediate sizes were analyzed by Brunauer-Emmett-Teller (BET) and atomic-resolution scanning transmission electron microscopy (STEM) to determine the effects of crystal size on their shape, atomic-scale surface roughness, and ultimately on their total surface site density. Results show that surface roughness scales directly with the size [or inversely with the specific surface area (SSA)] of synthetic goethites in the SSA range of 40-75 m²/g. This surface roughness, in turn, increases the total site density over ideal atomically smooth crystals. The total site density of synthetic goethite increases from a combination of decreasing crystal length/width ratio and increasing surface roughness.

Environmental Aspects of Oxide Nanoparticles: Probing Oxide Nanoparticle Surface Processes Under Different Environmental Conditions

Surface chemistry affects the physiochemical properties of nanoparticles in a variety of ways. Therefore, there is great interest in understanding how nanoparticle surfaces evolve under different environmental conditions of pH and temperature. Here, we discuss the use of vibrational spectroscopy as a tool that allows for in situ observations of oxide nanoparticle surfaces and their evolution due to different surface processes. We highlight oxide nanoparticle surface chemistry, either engineered anthropogenic or naturally occurring geochemical nanoparticles, in complex media, with a focus on the impact of (a) pH on adsorption, intermolecular interactions, and conformational changes; (b) surface coatings and coadsorbates on protein adsorption kinetics and protein conformation; (c) surface adsorption on the temperature dependence of protein structure phase changes; and (d) the use of two-dimensional correlation spectroscopy to analyze spectroscopic results for complex systems. An outlook of the field and remaining challenges is also presented.

Nucleotide Adsorption on Iron(III) Oxide Nanoparticle Surfaces: Insights into Nano-Geo-Bio Interactions Through Vibrational Spectroscopy

Molecular processes at geochemical interfaces impact many environmental processes that are critical to the fate and transport of contaminants in water systems. Often these interfaces are coated with natural organic matter, oxyanions, or biological components, yet little is understood about these coatings. Herein, we are interested in better understanding the interaction of biological components with nanoscale iron oxide minerals. In particular, we use attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to investigate the adsorption behavior of deoxyadenosine monophosphate (dAMP) on hematite nanoparticle surfaces as a function of pH and in the presence and absence of adsorbed phosphate. These results show that fewer nucleotides adsorb at higher pH. Additionally, when phosphate anions are preadsorbed, nucleotide adsorption is significantly limited due to site-blocking by adsorbed inorganic phosphate. The pH dependence provides insights into the adsorption process and the importance of electrostatic interactions. Preadsorbed phosphate affects the binding mode of dAMP, suggesting synergistic interactions between the coadsorbates. Two-dimensional correlation spectroscopy was used to further analyze the infrared spectra. Based on this analysis, a dAMP adsorption pathway onto a preadsorbed phosphate-hematite surface was proposed, suggesting the displacement of adsorbed phosphate by dAMP. Overall, this study provides some insights into geochemical-biological interactions on nanoscale iron oxide surfaces using vibrational spectroscopy.

Phosphate adsorption performance and mechanisms by nanoporous biochar-iron oxides from aqueous solutions

To evaluate the adsorption mechanism and performance of phosphate onto the composite of low-cost biochar and iron oxide, four biochar-iron oxides, namely biochar-magnetite (BC-M), biochar-ferrihydrite (BC-F), biochar-goethite (BC-G), and biochar-hematite (BC-H), were prepared by fabricating iron oxide to porous biochar. The biochar-iron oxides had huge surface areas of 691-864 m²/g and average pore diameters of 3.4-4.0 nm. Based on the characterization analysis of FTIR, XRD, XPS, and zeta potential, the interactions of electrostatic attraction, ligand exchange, and deposition dominated the phosphate adsorption onto biochar-iron oxides. The maximum adsorption capacity of phosphate followed the order of BC-G > BC-F > BC-H > BC-M. The isotherm data of BC-M and BC-H were well fitted by the Langmuir and Freundlich models, while those of BC-G and BC-F followed the Langmuir model. In addition, BC-M, BC-F, BC-G, and BC-H owned excellent regeneration ability and adsorption performance in practical (simulated) wastewater environment. Then the biochar-iron oxides exerted extensive and satisfactory prospect in wastewater remediation and recycling application in soil.

Crystal Face Distributions and Surface Site Densities of Two Synthetic Goethites: Implications for Adsorption Capacities as a Function of Particle Size

Two synthetic goethites of varying crystal size distributions were analyzed by BET, conventional TEM, cryo-TEM, atomic resolution STEM and HRTEM, and electron tomography in order to determine the effects of crystal size, shape, and atomic scale surface roughness on their adsorption capacities. The two samples were determined by BET to have very different site densities based on Cr^VI adsorption experiments. Model specific surface areas generated from TEM observations showed that, based on size and shape, there should be little difference in their adsorption capacities. Electron tomography revealed that both samples crystallized with an asymmetric {101} tablet habit. STEM and HRTEM images showed a significant increase in atomic-scale surface roughness of the larger goethite. This difference in roughness was quantified based on measurements of relative abundances of crystal faces {101} and {201} for the two goethites, and a reactive surface site density was calculated for each goethite. Singly coordinated sites on face {210} are 2.5 more dense than on face {101}, and the larger goethite showed an average total of 36% {210} as compared to 14% for the smaller goethite. This difference explains the considerably larger adsorption capacitiy of the larger goethite vs the smaller sample and points toward the necessity of knowing the atomic scale surface structure in predicting mineral adsorption processes.

Freezing-Enhanced Dissolution of Iron Oxides: Effects of Inorganic Acid Anions

Dissolution of iron from mineral dust particles greatly depends upon the type and amount of copresent inorganic anions. In this study, we investigated the roles of sulfate, chloride, nitrate, and perchlorate on the dissolution of maghemite and lepidocrocite in ice under both dark and UV irradiation and compared the results with those of their aqueous counterparts. After 96 h of reaction, the total dissolved iron in ice (pH 3 before freezing) was higher than that in the aqueous phase (pH 3) by 6-28 times and 10-20 times under dark and UV irradiation, respectively. Sulfuric acid was the most efficient in producing labile iron under dark condition, whereas hydrochloric acid induced the most dissolution of the total and ferrous iron in the presence of light. This ice-induced dissolution result was also confirmed with Arizona Test Dust (AZTD). In the freeze-thaw cycling test, the iron oxide samples containing chloride, nitrate, or perchlorate showed a similar extent of total dissolved iron after each cycling while the sulfate-containing sample rapidly lost its dissolution activity with repeating the cycle. This unique phenomenon observed in ice might be related to the freeze concentration of protons, iron oxides, and inorganic anions in the liquid-like ice grain boundary region. These results suggest that the ice-enhanced dissolution of iron oxides can be a potential source of bioavailable iron, and the acid anions critically influence this process.

Effects of pH and phosphate on CeO2 nanoparticle dissolution

As the result of rapidly grown nanotechnology industries, release of engineered nanoparticles (ENPs) to environment has increased, posing in a serious risk to environmental and human health. To better understand the chemical fate of ENPs in aquatic environments, solubility of CeO2 NPs was investigated using batch dissolution experiments as a function of pH (1.65-12.5), [phosphate] and particle size (33 and 78 nm). It was found that CeO2 dissolution was only significant at pH<5 and inversely proportional to surface area. After 120 h, the release of Ce was ∼3 times greater in large NPs than that in small NPs that is likely contributed by the difference in exchangeable Ce(III) impurity (small: 0.3 mM kg(-1), large: 1.56 mM kg(-1)). When 100 μM of phosphate was added, the dissolution rate of CeO2 NPs was decreased in small NPs by 15% at pH 1.65 and 75% at pH 4.5 and in large NPs by 56% at pH 1.65 and 63% at pH 4.5. The inner-sphere surface complexation of P that is revealed by the zeta potential measurements is effectively suppressing the CeO2 NP dissolution. Predicting the fate and transport of CeO2 NPs in aquatic environment, pH and P ligands might play important roles in controlling the solubility of CeO2 NPs.

Electrolyte ion binding at iron oxyhydroxide mineral surfaces

Electrolyte ion loadings at the surfaces of synthetic goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) particles that were pre-equilibrated in aqueous solutions of 10 mM NaCl and NaClO4 at 25 °C were investigated by cryogenic X-ray photoelectron spectroscopy (XPS). Atomic concentrations of Cl(-), ClO4(-), and Na(+) were correlated to potential determining ion (pdi; H(+), OH(-)) loadings obtained by potentiometric titrations. While Cl(-) promoted more pdi adsorption than ClO4(-), due to its greater charge-to-size ratio, both ions followed the same loading dependence on pdi adsorption, in contrast to previous studies supporting the concept for negligible perchlorate adorption. Lepidocrocite particles exhibited a stronger response of electrolyte adsorption to pdi loadings due electrolyte ion adsorption on the proton inactive (010) plane. These particles also acquired greater sodium loadings than goethite. These loadings were moreover considerably enhanced by perchlorate adsorption, possibly due to a thickening of the interfacial region in NaClO4 on the (010) plane. Finally, goethite particles with rougher surfaces acquired greater pdi and ion loadings than on those with smoother surfaces. No strong differences could be discerned between Cl(-) and ClO4(-) loadings on these materials. This work thus identified key aspects underpinning the relationship between pdi and electrolyte loadings at FeOOH mineral surfaces of environmental and technological importance.

Dissolution of hematite nanoparticle aggregates: influence of primary particle size, dissolution mechanism, and solution pH

The size-dependent dissolution of nanoscale hematite (8 and 40 nm α-Fe(2)O(3)) was examined across a broad range of pH (pH 1-7) and mechanisms including proton- and ligand- (oxalate-) promoted dissolution and dark (ascorbic acid) and photochemical (oxalate) reductive dissolution. Empirical relationships between dissolution rate and pH revealed that suspensions of 8 nm hematite exhibit between 3.3- and 10-fold greater reactivity per unit mass than suspensions of 40 nm particles across all dissolution modes and pH, including circumneutral. Complementary suspension characterization (i.e., sedimentation studies and dynamic light scattering) indicated extensive aggregation, with steady-state aggregate sizes increasing with pH but being roughly equivalent for both primary particles. Thus, while the reactivity difference between 8 and 40 nm suspensions is generally greater than expected from specific surface areas measured via N(2)-BET or estimated from primary particle geometry, loss of reactive surface area during aggregation limits the certainty of such comparisons. We propose that the relative reactivity of 8 and 40 nm hematite suspensions is best explained by differences in the fraction of aggregate surface area that is reactive. This scenario is consistent with TEM images revealing uniform dissolution of aggregated 8 nm particles, whereas 40 nm particles within aggregates undergo preferential etching at edges and structural defects. Ultimately, we show that comparably sized hematite aggregates can exhibit vastly different dissolution activity depending on the nature of the primary nanoparticles from which they are constructed, a result with wide-ranging implications for iron redox cycling.