Mass action expressions for bidentate adsorption in surface complexation modeling: theory and practice

Generic phosphate affinity constants of the CD-MUSIC-eSGC model to predict phosphate adsorption and dominant speciation on iron (hydr)oxides

Estimating the availability of phosphorus in soils and sediments is complicated by the diverse mineralogical properties of iron (hydr)oxides that control the environmental fate of phosphorus. Despite various surface complexation models have been developed, lack of generic phosphate affinity constants (logKPO4s) for iron (hydr)oxides hinders the prediction of phosphate adsorption to iron (hydr)oxides in nature. The aim of this work is to derive generic logKPO4s for the Charge Distribution-Multisite Complexation extended-Stern-Gouy-Chapman (CD-MUSIC-eSGC) model using a large phosphate adsorption database and previously derived generic protonation parameters. The optimized logKPO4s of goethite, hematite and ferrihydrite are located in a much narrower range than those in the RES3T database. Specifically, the logKPO4 ranges of FeOPO3, FeOPO2OH, FeOPO(OH)2, (FeO)2PO2, and (FeO)2POOH complexes were 17.40-18.00, 24.20-27.40, 27.90-29.80, 26.50-29.60, and 30.70-33.40, respectively. A simplified CD-MUSIC-eSGC model with species FeOPO2OH and (FeO)2PO2 and generic logKPO4 values 26.0 ± 0.9 and 27.9 ± 0.8, respectively, provides an accurate prediction of phosphate adsorption and dominant speciation to the iron (hydr)oxides at environmental pH and phosphate levels. For ferrihydrite at low pH and high phosphate levels the species FeOPO(OH)2 and (FeO)2POOH cannot be neglected. The simplified model expands the application boundaries of CD-MUSIC-eSGC model in predicting the phosphate adsorption on natural iron (hydr)oxides without laborious characterization.

Predicting the binding configuration and release potential of heavy metals on iron (oxyhydr)oxides: A machine learning study on EXAFS

Heavy metals raise a global concern and can be easily retained by ubiquitous iron (oxyhydr)oxides in natural and engineered systems. The complex interaction between iron (oxyhydr)oxides and heavy metals results in various mineral-metal binding configurations, such as outer-sphere complexes and edge-sharing inner-sphere complexes, which determine the accumulation and release of heavy metals in the environment. However, traditional experimental approaches are time-consuming and inadequate to elucidate the complex binding relationships and configurations between iron (oxyhydr)oxides and heavy metals. Herein, a workflow that integrates the binding configuration data of 11 heavy metals on 7 iron (oxyhydr)oxides and then trains machine learning models to predict unknown binding configurations was proposed. The well-trained multi-grained cascade forest models exhibited high accuracy (> 90%) and predictive performance (R2 ∼ 0.75). The underlying effects of mineral properties, metal ion species, and environmental conditions on mineral-metal binding configurations were fully interpreted with data mining. Moreover, the metal release rate was further successfully predicted based on mineral-metal binding configurations. This work provides a method to accurately and quickly predict the binding configuration of heavy metals on iron (oxyhydr)oxides, which would provide guidance for estimating the potential release behavior of heavy metals and remediating heavy metal pollution in natural and engineered environments.

Nanoconfinement boosts affinity of hydrated zirconium oxides to arsenate: Surface complexation modeling study

Nanoscale hydrated zirconium oxide (HZO) holds great potential in groundwater purification due to its ability to form inner-sphere coordination with arsenate. Despite being frequently used, especially as encapsulations in host materials for practical application in water treatment, the adsorption mechanisms of solutes on HZO are not appropriately explored, in particular for arsenate adsorption. In this study, we investigated the Zr-As coordination configuration and identified the most credible Zr-As configuration using surface complexation modeling (SCM), XPS and FT-IR analysis. The corresponding intrinsic coordination constants (Kintr) values was calculated by SCM, and the nanoconfinement effects were distinguished by comparing bare HZO with the HZO nanoparticles (NPs) encapsulated inside the strongly basic anion exchanger D201. Potentiometric titration suggests that the surface Zirconium hydroxyl groups (≡ZrOH) mainly exist in protonated form (≡ZrOH2+). Batch adsorption experiments demonstrate that the D201 hosts could adsorb As(V) through ion exchange by the quaternary ammonium groups under the low ionic strength (≤0.01 M NaNO3) and at pH > 6. The nanocomposite (HZO@D201) exhibits a higher adsorption capacity in a wide range of pH (3-10) and ionic strength (0.001-0.1 M NaNO3) than bare HZO. SCM simulations reveal that the coordination configuration of diprotonated monodentate mononuclear (MM-H2) dominates at pH 3-6, while deprotonated bidentate binuclear (BB-H0) dominates at pH > 7. For each configuration, the intrinsic coordination constants (Kintr) of HZO@D201 (10-0.66 and 10-16.10, respectively) are significantly higher than those of bare HZO (10-12.24 and 10-44.42, respectively), indicating a superior chemical bonding affinity caused by nanoconfinement. The obtained Kintr values are used to predict arsenate adsorption isotherms in pH 3 and 9, and the results align with the SCM simulation outcomes. This study may offer a feasible method for investigating the nanoconfinement effect of emerging nanocomposite adsorbents from a thermodynamic perspective, and provide reference coordination equilibrium constants of HZO for research and practical application.

Unraveling the Np(V) sorption on ZrO2: A batch, spectroscopic and modeling combined approach

The interactions of the long-lived actinide neptunium with the corrosion product zirconia (ZrO2) have to be considered in the safety assessment of a repository for radioactive waste. The sorption of Np(V) on ZrO2 was investigated in the absence of carbonate at the macroscopic and molecular scale. At the macroscopic level, the Np(V) uptake was independent of ionic strength and the isoelectric point of the pristine zirconia was increased, both suggesting the presence of inner-sphere Np(V) surface complexes. The Np(V) sorption isotherms indicated the presence of strong and weak sorption sites. Molecular level information were derived from in situ attenuated total reflection Fourier-transform infrared spectroscopy and extended X-ray absorption fine structure spectroscopy (EXAFS), which confirmed the presence of Np(V) inner-sphere complexes. EXAFS experiments revealed the formation of a bidentate inner-sphere surface complex in the weak sorption site regime. The derived information at the macroscopic and molecular levels were used to parametrize a charge distribution multi-site complexation (CD-MUSIC) model. The derived thermodynamic constants can help to better predict the environmental fate of Np(V) in the context of nuclear waste repository assessments and can also support the appraisal of safety-relevant scenarios for the extended interim storage of spent nuclear fuel.

Eu(III) and Am(III) adsorption on aluminum (hydr)oxide minerals: surface complexation modeling

Americium is a highly radioactive actinide element found in used nuclear fuel. Its adsorption on aluminum (hydr)oxide minerals is important to study for at least two reasons: (i) aluminum (hydr)oxide minerals are ubiquitous in the subsurface environment and (ii) bentonite clays, which are proposed engineered barriers for the geologic disposal of used nuclear fuel, have the same ≡AlOH sites as aluminum (hydr)oxide minerals. Surface complexation modeling is widely used to interpret the adsorption behavior of heavy metals on mineral surfaces. While americium sorption is understudied, multiple adsorption studies for europium, a chemical analog, are available. In this study we compiled data describing Eu(III) adsorption on three aluminum (hydr)oxide minerals-corundum (α-Al2O3), γ-alumina (γ-Al2O3) and gibbsite (γ-Al(OH)3)-and developed surface complexation models for Eu(III) adsorption on these minerals by employing diffuse double layer (DDL) and charge distribution multisite complexation (CD-MUSIC) electrostatic frameworks. We also developed surface complexation models for Am(III) adsorption on corundum (α-Al2O3) and γ-alumina (γ-Al2O3) by employing a limited number of Am(III) adsorption data sourced from literature. For corundum and γ-alumina, two different adsorbed Eu(III) species, one each for strong and weak sites, were found to be important regardless of which electrostatic framework was used. The formation constant of the weak site species was almost 10,000 times weaker than the formation constant for the corresponding strong site species. For gibbsite, two different adsorbed Eu(III) species formed on the single available site type and were important for the DDL model, whereas the best-fit CD-MUSIC model for Eu(III)-gibbsite system required only one Eu(III) surface species. The Am(III)-corundum model based on the CD-MUSIC framework had the same set of surface species as the Eu(III)-corundum model. However, the log K values of the surface reactions were different. The best-fit Am(III)-corundum model based on the DDL framework had only one site type. Both the CD-MUSIC and the DDL model developed for Am(III)-γ-alumina system only comprised of one site type and the formation constant of the corresponding surface species was ~ 500 times stronger and ~ 700 times weaker than the corresponding Eu(III) species on the weak and the strong sites, respectively. The CD-MUSIC model for corundum and both the DDL and the CD-MUSIC models for γ-alumina predicted the Am(III) adsorption data very well, whereas the DDL model for corundum overpredicted the Am(III) adsorption data. The root mean square of errors of the DDL and CD-MUSIC models developed in this study were smaller than those of two previously-published models describing Am(III)-γ-alumina system, indicating the better predictive capacity of our models. Overall, our results suggest that using Eu(III) as an analog for Am(III) is practical approach for predicting Am(III) adsorption onto well-characterized minerals.

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.

Selective biosorption of lanthanides onto Galdieria sulphuraria

The recovering of trivalent Lanthanides from aqueous solutions, by biosorption process onto Galdieria sulphuraria lifeless cells, was investigated. Potentiometry, UV-Vis, FTIR-ATR spectroscopy and SEM-EDS analysis were used. All the experiments were performed at 25 °C, in 0.5 M NaCl. Ln³⁺ biosorption is greater in the 5-6 pH range with values ranging from 80 μmol/g to 130 μmol/g (dry weight). The adsorbed Ln³⁺ ions can be recovered at higher acidity (pH<1) and the biosorbent can be reused. Specific molecular interactions between Ln³⁺ ions and the functional groups on G. sulphuraria surface were highlighted. Particularly, proteins are involved if Ln³⁺=Pr³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Tm³⁺, while Ce³⁺, Ho³⁺, Er³⁺ form bonds with carbohydrates. Finally, both proteins and carbohydrates are involved if Gd³⁺ and Yb³⁺. A Surface Complexation approach, with a good graphical fitting to potentiometric experimental collected data, was used to describe the biosorption mechanism. This study could be of great applicative utility for removing of trivalent actinides, from waste aqueous solutions, by biosorption. As well known the lanthanides were used as model to simulate the chemical behaviour of actinides in the same oxidation state.

Generic CD-MUSIC-eSGC model parameters to predict the surface reactivity of iron (hydr)oxides

The surface reactivity of iron (hydr)oxides plays a crucial role in controlling their interfacial reactions, for which various surface complexation models have been developed. The diversity of mineralogical properties of iron (hydr)oxides has resulted in a redundancy of model parameters, which hampers the modeling of iron (hydr)oxides in soils and sediments, where goethite, hematite and ferrihydrite dominate the iron (hydr)oxide mass fraction. To capture their combined surface reactivity, optimized generic protonation parameters of the Charge Distribution-Multisite Complexation (CD-MUSIC) extended-Stern-Gouy-Chapman (eSGC) model were derived by reanalyzing literature datasets and tested with some newly synthesized iron (hydr)oxides. It was observed that the proton and monovalent ion affinity constants of the different iron (hydr)oxides were located in a narrow range. For the singly- and triply-coordinated hydroxyl sites the obtained generic log(affinity constants) were 8.3 and 11.7 for the protonation reaction and -0.5 for the reaction with the monovalent background ions. Their combination with fixed site densities of singly-/triply-coordinated hydroxyl sites of 3.45/2.70, 5.00/2.50, and 5.80/1.40 sites/nm² for goethite, hematite, and ferrihydrite, respectively, provided good results. The Stern layer capacitances of the inner and outer Stern layers were set equal and could be acquired by an empirical correlation with the sample specific surface area (SSA). The CD-MUSIC-eSGC model with the generic model parameters enables good quality predictions of the proton reactivity of iron (hydr)oxides in 1:1 electrolyte solutions regardless of the sample heterogeneity. The advantages of the generic CD-MUSIC-eSGC model are twofold: (1) protonation of iron (hydr)oxides can be described without making use of spectroscopic measurements and proton titrations, and (2) the model calculations are greatly simplified.

Y(III) Sorption at the Orthoclase (001) Surface Measured by X-ray Reflectivity

Interactions of heavy metals with charged mineral surfaces control their mobility in the environment. Here, we investigate the adsorption of Y(III) onto the orthoclase (001) basal plane, the former as a representative of rare earth elements and an analogue of trivalent actinides and the latter as a representative of naturally abundant K-feldspar minerals. We apply in situ high-resolution X-ray reflectivity to determine the sorption capacity and molecular distribution of adsorbed Y species as a function of the Y³⁺ concentration, [Y³⁺], at pH 7 and 5. With [Y³⁺] ≥ 1 mM at pH 7, we observe an inner-sphere (IS) sorption complex at a distance of ∼1.5 Å from the surface and an outer-sphere (OS) complex at 3-4 Å. Based on the adsorption height of the IS complex, a bidentate, binuclear binding mode, in which Y³⁺ binds to two terminal oxygens, is proposed. In contrast, mostly OS sorption is observed at pH 5. The observed maximum Y coverage is ∼1.3 Y³⁺/A_UC (A_UC: area of the unit cell = 111.4 Ų) for all the investigated pH values and Y concentrations, which is in the expected range based on the estimated surface charge of orthoclase (001).

Combining batch experiments and spectroscopy for realistic surface complexation modelling of the sorption of americium, curium, and europium onto muscovite

For a safe enclosure of contaminants, for instance in deep geological repositories of radioactive waste, any processes retarding metal migration are of paramount importance. This study focusses on the sorption of trivalent actinides (Am, Cm) and lanthanides (Eu) to the surface of muscovite, a mica and main component of most crystalline rocks (granites, granodiorites). Batch sorption experiments quantified the retention regarding parameters like pH (varied between 3 and 9), metal concentration (from 0.5 µM Cm to 10 µM Eu), or solid-to-liquid ratio (0.13 and 5.25 g·L^-1). In addition, time-resolved laser fluorescence spectroscopy (TRLFS) using the actinide Cm(III) identified two distinct inner-sphere surface species. Combining both approaches allowed the development of a robust surface complexation model and the determination of stability constants of the spectroscopically identified species of (S-OH)₂M³⁺ (logK^o -8.89), (S-O)₂M⁺ (logK^o -4.11), and (S-O)₂MOH (logK^o -10.6), with all values extrapolated to infinite dilution. The inclusion of these stability constants into thermodynamic databases will improve the prognostic accuracy of lanthanide and actinide transport through groundwater channels in soils and crystalline rock systems.

Chromium removal from tannery wastewaters with a strong cation exchange resin and species analysis of chromium by MINEQL+

Chromium (III) salts are highly applied for tanning purpose in tannery industries. The purpose of this study was removal and recovery of chromium(III) from tannery wastewater with a strong cation exchange resin. For this purpose, Amberlite 252 ZU was chosen as a strong cation exchange resin. In the first part of this study, The MINEQL+ computer program was applied depending on the optimum concentration and pH for determining Cr species in aqueous solutions. The second part of the work consists of measuring the exchange equilibrium of H+ ions and Cr(III) ions. Therefore, solutions containing fixed amounts of chromium were brought into contact with different amounts of resins. The evaluation of the obtained equilibrium parameters was done by surface complexing theory. Retention and regeneration steps were successfully performed in the column without any significant change up to 10 cycles. Efficiency was between 90 and 98% in removal studies, and between 81 and 92% in recovery studies. The results showed that a strong cation exchange resin Amberlite 252 ZU can be successfully used for chromium removal and recovery.

Kinetics-informed global assessment of mine tailings for CO2 removal

Chemically reactive mine tailings are a potential resource for drawing down carbon dioxide out of the atmosphere in mineral weathering schemes. Such carbon dioxide removal (CDR) systems, applied on a large scale, could help to meet internationally agreed targets for minimising climate change, but crucially we need to identify what materials could react fast enough to provide CDR at relevant climate change mitigation timescales. This study focuses on a range of silicate-dominated tailings, calculating their CDR potential from their chemical composition (specific capacity), estimated global production rates, and the speed of weathering under different reaction conditions. Tailings containing high abundances of olivine, serpentine and diopside show the highest CDR potential due to their favourable kinetics. We conclude that the most suitable tailings for CDR purposes are those associated with olivine dunites, diamond kimberlites, asbestos and talc serpentinites, Ni sulphides, and PGM layered mafic intrusions. We estimate the average annual global CDR potential of tailings weathered over the 70-year period 2030-2100 to be ~93 (unimproved conditions) to 465 (improved conditions) Mt/year. Results indicate that at least 30 countries possess tailings materials that, under improved conditions, may offer a route for CDR which is not currently utilised within the mining industry. By 2100, the total cumulative CDR could reach some 33 GtCO₂, of which more than 60% is contributed by PGM tailings produced in Southern Africa, Russia, and North America. The global CDR potential could be increased by utilization of historic tailings and implementing measures to further enhance chemical reaction rates. If practical considerations can be addressed and enhanced weathering rates can be achieved, then CDR from suitable tailings could contribute significantly to national offset goals and global targets. More research is needed to establish the potential and practicality of this technology, including measurements of the mineral weathering kinetics under various conditions.

Adsorption of Strontium onto Synthetic Iron(III) Oxide up to High Ionic Strength Systems

In this work, the adsorption behavior of Sr onto a synthetic iron(III) oxide (hematite with traces of goethite) has been studied. This solid, which might be considered a representative of Fe3+ solid phases (iron corrosion products), was characterized by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), and its specific surface area was determined. Both XRD and XPS data are consistent with a mixed solid containing more than 90% hematite and 10% goethite. The solid was further characterized by fast acid-base titrations at different NaCl concentrations (from 0.1 to 5 M). Subsequently, for each background NaCl concentration used for the acid-base titrations, Sr-uptake experiments were carried out involving two different levels of Sr concentration (1 × 10−5 and 5 × 10−5 M, respectively) at constant solid concentration (7.3 g/L) as a function of −log([H+]/M). A Surface Complexation Model (SCM) was fitted to the experimental data, following a coupled Pitzer/surface complexation approach. The Pitzer model was applied to aqueous species. A Basic Stern Model was used for interfacial electrostatics of the system, which includes ion-specific effects via ion-specific pair-formation constants, whereas the Pitzer-approach involves ion-interaction parameters that enter the model through activity coefficients for aqueous species. A simple 1-pK model was applied (generic surface species, denoted as >XOH−1/2). Parameter fitting was carried out using the general parameter estimation software UCODE, coupled to a modified version of FITEQL2. The combined approach describes the full set of data reasonably well and involves two Sr-surface complexes, one of them including chloride. Monodentate and bidentate models were tested and were found to perform equally well. The SCM is particularly able to account for the incomplete uptake of Sr at higher salt levels, supporting the idea that adsorption models conventionally used in salt concentrations below 1 M are applicable to high salt concentrations if the correct activity corrections for the aqueous species are applied. This generates a self-consistent model framework involving a practical approach for semi-mechanistic SCMs. The model framework of coupling conventional electrostatic double layer models for the surface with a Pitzer approach for the bulk solution earlier tested with strongly adsorbing solutes is here shown to be successful for more weakly adsorbing solutes.

Charge regulated solid-liquid interfaces interacting on the nanoscale: Benchmarking of a generalized speciation code (SINFONIA)

Surface chemistry of mineral phases in aqueous environments generates the electrostatic forces involved in particle-particle interactions. However, few models directly take into account the influence of surface speciation and changes in solution speciation when the diffuse layer potential profiles of approaching particles overlap and affect each other. These electrostatic interactions can be quantified, ideally, through charge regulation, considering solution and surface speciation changes upon particle approach by coupling state-of-the-art surface complexation models for the two particle surfaces with a Poisson-Boltzmann type distribution of electrostatic potential and ions in the inter-particle space. These models greatly improve the accuracy of inter-particle force calculations at small inter-particle separations compared to constant charge and constant potential approaches. This work aims at advancing charge regulation calculations by including full chemical speciation and advanced surface complexation models (Basic Stern-, three-, or four plane models and charge distribution concepts), for cases of similar and dissimilar surfaces involving the numerical solution of the Poisson-Boltzmann equation for arbitrary electrolytes. The concept was implemented as a Python-based code and in COMSOL. The flexibility and precision of both, concept and implementations are demonstrated in several benchmark calculations testing the new codes against published results or simulations using established speciation codes, including aqueous speciation, surface complexation and various interaction force examples. Due to the flexibility in terms of aqueous chemistry and surface complexation models for various geometries, a large variety of potential applications can be tackled with the developed codes including industrial, biological, and environmental systems, from colloidal suspensions to gas bubbles, emulsions, slurries like cement paste, as well as new possibilities to assess the chemistry in nano-confined systems.

Intercomparison and Refinement of Surface Complexation Models for U(VI) Adsorption onto Goethite Based on a Metadata Analysis

Adsorption of uranium onto goethite is an important partitioning process that controls uranium mobility in subsurface environments, for which many different surface complexation models (SCMs) have been developed. While individual models can fit the data for which they are parameterized, many perform poorly when compared with experimental data covering a broader range of conditions. There is an imperative need to quantitatively evaluate the variations in the models and to develop a more robust model that can be used with more confidence across the wide range of conditions. We conducted an intercomparison and refinement of the SCMs based on a metadata analysis. By seeking the globally best fit to a composite dataset with wide ranges of pH, solid/sorbate ratios, and carbonate concentrations, we developed a series of models with different levels of complexity following a systematic roadmap. The goethite-uranyl-carbonate ternary surface complexes were required in every model. For the spectroscopically informed models, a triple-plane model was found to provide the best fit, but the performance of the double-layer model with bidentate goethite-uranyl and goethite-uranyl-carbonate complexes was also comparable. Nevertheless, the models that ignore the bidentate feature of uranyl surface complexation consistently performed poorly. The goodness of fitting for the models that ignore adsorption of carbonate and the charge distributions was not significantly compromised compared with that of their counterparts that considered those. This approach of model development for a large and varied dataset improved our understanding of U(VI)-goethite surface reactions and can lead to a path for generating a single set of reactions and equilibrium constants for including U(VI) adsorption onto goethite in reactive transport models.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Application of surface complexation modeling on adsorption of uranium at water-solid interface: A review

Precise prediction of uranium adsorption at water-mineral interface is of great significance for the safe disposal of radionuclides in geologic environments. Surface complexation modeling (SCM) as a very useful tool has been extensively investigated for simulating adsorption behavior of metals/metalloids at water-mineral interface. Numerous studies concerning the fitting of uranium adsorption on various adsorbents using SCM are well documented, but the systematic and comprehensive review of uranium adsorption using various SCM is not available. In this review, we briefly summarized the rationale of SCM, including constant-capacitance-model (CCM), diffuse-layer-model (DLM), triple-layer-model (TLM); The recent progress in the application of SCM on the fitting of uranium adsorption towards metal (hydr)oxides, clay minerals and soil/sediments was reviewed in details. This review hopefully provides the beneficial guidelines for predicting the transport and fate of uranium in geologic environments beyond laboratory timescales.

Natural attenuation of lead by microbial manganese oxides in a karst aquifer

Lead is a toxic environmental contaminant associated with current and historic mine sites. Here we studied the natural attenuation of Pb in a limestone cave system that receives drainage from the ancient Priddy Mineries, UK. Extensive deposits of manganese oxides were observed to be forming on the cave walls and as coatings in the stream beds. Analysis of these deposits identified them as birnessite (δ-MnO₂), with some extremely high concentrations of sorbed Pb (up to 56 wt%) also present. We hypothesised that these cave crusts were actively being formed by microbial Mn(II)-oxidation, and to investigate this the microbial communities were characterised by DNA sequencing, enrichment and isolation experiments. The birnessite deposits contained abundant and diverse prokaryotes and fungi, with ~5% of prokaryotes and ~ 10% of fungi closely related to known heterotrophic Mn(II)-oxidisers. A substantial proportion (up to 17%) of prokaryote sequences were assigned to groups known as autotrophic ammonia and nitrite oxidisers, suggesting that nitrogen cycling may play an important role in contributing energy and carbon to the cave crust microbial communities and consequently the formation of Mn(IV) oxides and Pb attenuation. Enrichment and isolation experiments showed that the birnessite deposits contained Mn(II)-oxidising microorganisms, and two isolates (Streptomyces sp. and Phyllobacterium sp.) could oxidise Mn(II) in the presence of 0.1 mM Pb. Supplying the enrichment cultures with acetate as a source of energy and carbon stimulated Mn(II)-oxidation, but excess organics in the form of glucose generated aqueous Mn(II), likely via microbial Mn(IV)-reduction. In this karst cave, microbial Mn(II)-oxidation contributes to the active sequestration and natural attenuation of Pb from contaminated waters, and therefore may be considered a natural analogue for the design of wastewater remediation systems and for understanding the geochemical controls on karst groundwater quality, a resource relied upon by billions of people across the globe.

Adsorption of p-Arsanilic Acid on Iron (Hydr)oxides and Its Implications for Contamination in Soils

Because of the diversification of industries in developing cities, the phenomenon of the simultaneous contamination of various kinds of pollutants is becoming common, and the environmental process of pollutants in multi-contaminated environmental mediums has attracted attention in recent years. In this study, p-arsanilic acid (ASA), a kind of organic arsenic feed additive that contains the arsenic group in a chemical structure, is used as a typical contaminant to investigate its adsorption on iron oxides and its implication for contaminated soils. The adsorption kinetics on all solids can be fitted to the pseudo-second-order kinetic model well. At the same mass dosage conditions, the adsorption amount per unit surface area on iron oxides follows the order α-FeOOH > γ-Fe2O3 > α-Fe2O3, which is significantly higher than that for actual soil, because of the lower content of iron oxides in actual soil. Lower pH conditions favor ASA adsorption, while higher pH conditions inhibit its adsorption as a result of the electrostatic repulsion and weakened hydrophobic interaction. The presence of phosphate also inhibits ASA adsorption because of the competitive effect. Correlations between the amount of ASA adsorption in actual soil and the Fe2O3 content, total phosphorus content, arsenic content, and organic matter content of actual soil are also investigated in this work, and a moderate positive correlation (R2 = 0.630), strong negative correlation (R2 = 0.734), insignificant positive correlation (R2 = 0.099), and no correlation (R2 = 0.006) are found, respectively. These findings would help evaluate the potential hazard of the usage of organic arsenic feed additives, as well as further the understanding of the geochemical processes of contaminants in complicated mediums.

Immobilizing a π-Conjugated Catecholato Framework on Surfaces of SiO2 Insulator Films via a One-Atom Anchor of a Platinum Metal Center to Modulate Organic Transistor Performance

Chemical modification of insulating material surfaces is an important methodology to improve the performance of organic field-effect transistors (OFETs). However, few redox-active self-assembled monolayers (SAMs) have been constructed on gate insulator film surfaces, in contrast to the numerous SAMs formed on many types of conducting electrodes. In this study, we report a new approach to introduce a π-conjugated organic fragment in close proximity to an insulating material surface via a transition metal center acting as a one-atom anchor. On the basis of the reported coordination chemistry of a catecholato complex of Pt(II) in solution, we demonstrate that ligand exchange can occur on an insulating material surface, affording SAMs on the SiO₂ surface derived from a newly synthesized Pt(II) complex containing a benzothienobenzothiophene (BTBT) framework in the catecholato ligand. The resultant SAMs were characterized in detail by water contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and cyclic voltammetry. The SAMs served as good scaffolds of π-conjugated pillars for forming thin films of a well-known organic semiconductor C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), accompanied by the engagements of the C8-BTBT molecules with the SAMs containing the common BTBT framework at the first layer on SiO₂. OFETs containing the SAMs displayed improved performance in terms of hole mobility and onset voltage, presumably because of the unique interfacial structure between the organic semiconducting and inorganic insulating layers. These findings provide important insight into creating new elaborate interfaces through installing coordination chemistry in solution to solid surfaces, as well as OFET design by considering the compatibility between SAMs and organic semiconductors.

Enthalpy of Uranium Adsorption onto Hematite

The influence of temperature on the adsorption of metal ions at the solid-water interface is often overlooked, despite the important role that adsorption plays in metal-ion fate and transport in the natural environment where temperatures vary widely. Herein, we examine the temperature-dependent adsorption of uranium, a widespread radioactive contaminant, onto the ubiquitous iron oxide, hematite. The multitemperature batch adsorption data and surface complexation models indicate that the adsorption of uranium, as the hexavalent uranyl (UO₂²⁺) ion, increases significantly with increasing temperature, with an adsorption enthalpy (ΔH_ads) of +71 kJ mol^-1. We suggest that this endothermic, entropically driven adsorption behavior is linked to reorganization of the uranyl-ion hydration and interfacial water structures upon U^VI adsorption at the hematite surface. Overall, this work provides fundamental insight into the thermodynamics driving metal-ion adsorption reactions and provides the specific enthalpy value necessary for improved predictive geochemical modeling of U^VI adsorption in the environment.

A comprehensive study of the sorption mechanism and thermodynamics of f-element sorption onto K-feldspar

The mobility of heavy metal contaminants and radionuclides in the environment is directly controlled by their interactions with charged mineral surfaces, hence an assessment of their potential toxicity, e.g. in the context of radioactive waste disposal sites, requires understanding of sorption processes on the molecular level. Here, we investigate the sorption of a variety of rare earth elements (REE) and trivalent actinides (Am, Cm) on K-feldspar using batch sorption, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and a surface complexation model. Initially, a reliable pK_a for K-feldspar's surface deprotonation reaction was determined as 2.5 ± 0.02 by column titration experiments, in excellent agreement with a measured pH_IEP of 2.8. Batch sorption experiments over a broad range of experimental conditions in terms of mineral grain size, pH, [M³⁺], ionic radius, solid/liquid ratio, ionic strength, and equilibration procedures were carried out to quantify macroscopic retention. The trivalent d-block element Y, early, mid, and late lanthanides (La, Eu, Nd, Lu), as well as two minor actinides (Am, Cm) were used for batch sorption experiments and showed similar pH dependent uptake behavior, underlining their chemical analogy. In parallel, spectroscopic investigations provided insight into surface speciation. Cm TRLFS spectra indicate the formation of three inner-sphere sorption complexes with increasing hydrolysis. Additionally, a ternary K-feldspar/Cm/silicate complex was found for pH > 10, and batch and spectroscopic data at low pH (<4) point to small amounts of outer sphere sorption complexes. Based on TRLFS data, batch sorption, and titration data, a generic geochemical sorption model was developed, that describes sorption edges for all investigated M³⁺/K-feldspar systems satisfactorily. The derived stability constants for the binary sorption complexes (logK_1-4 = -3.6, -7.7, -11.5, and -17.4, respectively) could successfully be used to reproduce literature data. The stability constants obtained for the surface complexes were included into the database for the Smart K_d-concept, which will further improve the safety assessment of potential repositories for radioactive waste.

Process-based modeling of arsenic(III) oxidation by manganese oxides under circumneutral pH conditions

Numerous experimental studies have identified a multi-step reaction mechanism to control arsenite (As(III)) oxidation by manganese (Mn) oxides. The studies highlighted the importance of edge sites and intermediate processes, e.g., surface passivation by reaction products. However, the identified reaction mechanism and controlling factors have rarely been evaluated in a quantitative context. In this study, a process-based modeling framework was developed to delineate and quantify the relative contributions and rates of the different processes affecting As(III) oxidation by Mn oxides. The model development and parameterization were constrained by experimental observations from literature studies involving environmentally relevant Mn oxides at circumneutral pH using both batch and stirred-flow reactors. Our modeling results highlight the importance of a transitional phase, solely evident in the stirred-flow experiments, where As(III) oxidation gradually shifts from fast reacting Mn(IV) to slowly reacting Mn(III) edge sites. The relative abundance of these edge sites was the most important factor controlling the oxidation rate, whereas surface passivation restricted oxidation only in the stirred-flow experiment. The Mn(III) edge sites were demonstrated to play a crucial role in the oxidation and therefore in controlling the long-term fate of As. This study provided an improved understanding of Mn oxide reactivity and the significance in the cycling of redox-sensitive metal(loid)s in the environment.

Thermodynamic Impact of Mineral Surfaces on Amino Acid Polymerization: Aspartate Dimerization on Goethite

This article presents a thermodynamic predictive scheme for amino acid polymerization in the presence of minerals as a function of various environmental parameters (pH, ionic strength, amino acid concentration, and the solid/water ratio) using l-aspartate (Asp) and goethite as a model combination. This prediction is enabled by the combination of the surface adsorption constants of amino acid and its polymer, determined from the extended triple layer model characterization of the corresponding experimental results, with the thermodynamic data of these organic compounds in water reported in the literature. Calculations for the Asp-goethite system showed that the goethite surface drastically shifts the Asp monomer-dipeptide equilibrium toward the dipeptide side; when the dimerization of 0.1 mM Asp was considered in the presence of 10 m² L^-1 of goethite, an Asp dipeptide concentration around 10⁵ times larger was computed to be thermodynamically attainable compared with that in the absence of goethite at acidic pH (4-5) and low ionic strength (0.1 mM NaCl). Under this condition, the dipeptide-to-monomer molecular ratio in the adsorbed state reached 20%. In contrast, no significant enhancement by goethite was predicted at alkaline pH (>8), where the electrostatic interactions of the goethite surface with Asp and Asp dipeptide are weak. Thus, mineral surfaces should have had a significant impact on the thermodynamics of prebiotic peptide bond formation on the early Earth, although the influences likely depended largely on the environmental conditions. Future experimental studies for various amino acid-mineral interactions using our proposed methodology will provide a quantitative constraint on favorable geochemical settings for the chemical evolution on Earth. This approach can also offer important clues for future exploration of extraterrestrial life.

Interactions and Reductive Reactivity in Ternary Mixtures of Fe(II), Goethite, and Phthalic Acid Based on a Combined Experimental and Modeling Approach

The interactions between organic ligands, Fe(II), and iron oxides are important in biogeochemical redox processes. The effect of phthalic acid (PHA) on the reductive reactivity of Fe(II) associated with goethite was examined using batch adsorption and kinetic studies, attenuated total reflectance?Fourier transform infrared spectroscopy (ATR?FTIR), and surface complexation modeling (SCM). PHA significantly inhibited the reductive reactivity of Fe(II)/goethite, as quantified by the pseudo-first-order reduction rate constants ( k) of p-cyanonitrobenzene. The k value decreased from 1.68 ? 0.03 to 0.338 ? 0.14 h^?1 at pH 6.0 as the PHA concentration increased from 0 to 1000 ?M. The effects of the co-adsorption of Fe(II) and PHA onto goethite were then investigated to study the inhibition mechanism. The adsorption experiments showed that Fe(II) slightly enhanced PHA adsorption, whereas PHA did not affect Fe(II) adsorption, suggesting that the inhibition was not due to different amounts of Fe(II) adsorbed. The ATR?FTIR spectra of the adsorbed PHA in the ternary mixtures demonstrated that the major surface species was outer-sphere species, with minor inner-sphere complexes formed. SCM results showed that the presence of PHA (L) led to the formation of a type A ternary species ((?FeOFe⁺)₂???L^2?) on the goethite surface, decreasing the abundance of the reactive species (?FeOFeOH). Moreover, the adsorption of PHA on the surface of goethite might block the reactive sites and inhibit the electron transfer between Fe(II) and goethite, thus decreasing the reactivity. Overall, these findings provided new insights into the reaction mechanisms of surface-adsorbed Fe(II), which will facilitate the development of new technologies for site remediation and more accurate risk assessment.

Removal Mechanisms of Phosphate by Lanthanum Hydroxide Nanorods: Investigations using EXAFS, ATR-FTIR, DFT, and Surface Complexation Modeling Approaches

Lanthanum-based materials are effective for sequestering phosphate in water, however, their removal mechanisms remain unclear, and the effects of environmentally relevant factors have not yet been studied. Hereby, this study explored the mechanisms of phosphate removal using La(OH)₃ by employing extended X-ray absorption spectroscopy (EXAFS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), density functional theory (DFT) and chemical equilibrium modeling. The results showed that surface complexation was the primary mechanism for phosphate removal and in binary phosphate configurations, namely diprotonated bidentate mononuclear (BM-H2) and bidentate binuclear (BB-H2), coexisting on La(OH)₃ in acidic conditions. By increasing the pH to 7, BM-H1 and BB-H2 were the two major configurations governing phosphate adsorption on La(OH)₃, whereas BB-H1 was the dominant configuration of phosphate adsorption at pH 9. With increasing phosphate loading, the phosphate configuration of on La(OH)₃ transforms from binary BM-H1 and BB-H2 to BB-H1. Amorphous Ca₃(PO₄)₂ forms in the presence of Ca, leading to enhanced phosphate removal at alkaline conditions. The contributions of different mechanisms to the overall phosphate removal were successfully simulated by a chemical equilibrium model that was consistent with the spectroscopic results. This study provides new insights into the molecular-level mechanism of phosphate removal by La(OH)₃.

Cobinding of Pharmaceutical Compounds at Mineral Surfaces: Mechanistic Modeling of Binding and Cobinding of Nalidixic Acid and Niflumic Acid at Goethite Surfaces

Although emerging contaminants rarely exist individually in environmental contaminated systems, only limited information on their adsorption mechanisms in multicomponent solutions is currently available. To address this shortcoming, this work examines for the first time the accuracy of a surface complexation model in predicting the cooperative adsorption of nalidixic acid (NA) and niflumic acid (NFA) at goethite (α-FeOOH) surfaces. Our model adequately predicts cobinding of an outer-sphere (OS) complex of NFA onto NA bound to goethite through metal-bonded (MB), hydrogen-bonded (HB), or OS complexes. More positive charge is introduced in the system via sodium interactions in order to describe the NFA adsorption at high NaCl concentrations in both single and binary systems. Our model confidently predicts multilayers of NA on goethite as well as NFA binding on goethite-bound NA over a large range of pH and salinity values as well as NA and NFA loadings. These findings have strong implications in the assessment and prediction of contaminant fate in multicomponent contaminated systems by invoking a nontraditional form of ligand-ligand interaction in this field of study.

Kinetics of Cation and Oxyanion Adsorption and Desorption on Ferrihydrite: Roles of Ferrihydrite Binding Sites and a Unified Model

Quantitative understanding the kinetics of toxic ion reactions with various heterogeneous ferrihydrite binding sites is crucial for accurately predicting the dynamic behavior of contaminants in environment. In this study, kinetics of As(V), Cr(VI), Cu(II), and Pb(II) adsorption and desorption on ferrihydrite was studied using a stirred-flow method, which showed that metal adsorption/desorption kinetics was highly dependent on the reaction conditions and varied significantly among four metals. High resolution scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy showed that all four metals were distributed within the ferrihydrite aggregates homogeneously after adsorption reactions. Based on the equilibrium model CD-MUSIC, we developed a novel unified kinetics model applicable for both cation and oxyanion adsorption and desorption on ferrihydrite, which is able to account for the heterogeneity of ferrihydrite binding sites, different binding properties of cations and oxyanions, and variations of solution chemistry. The model described the kinetic results well. We quantitatively elucidated how the equilibrium properties of the cation and oxyanion binding to various ferrihydrite sites and the formation of various surface complexes controlled the adsorption and desorption kinetics at different reaction conditions and time scales. Our study provided a unified modeling method for the kinetics of ion adsorption/desorption on ferrihydrite.

Measurement and Surface Complexation Modeling of U(VI) Adsorption to Engineered Iron Oxide Nanoparticles

Surface-functionalized magnetite nanoparticles have high capacity for U(VI) adsorption and can be easily separated from the aqueous phase by applying a magnetic field. A surface-engineered bilayer structure enables the stabilization of nanoparticles in aqueous solution. Functional groups in stearic acid (SA), oleic acid (OA), and octadecylphosphonic acid (ODP) coatings led to different adsorption extents (SA≈ OA > ODP) under the same conditions. The impact of water chemistry (initial loading of U(VI), pH, and the presence of carbonate) has been systematically examined for U(VI) adsorption to OA-coated nanoparticles. A diffuse double layer surface complexation model was developed for surface-functionalized magnetite nanoparticles that could simulate both the measured surface charge and the U(VI) adsorption behavior at the same time. With a small set of adsorption reactions for uranyl hydroxide and uranyl carbonate complexes to surface sites, the model can successfully simulate the entire adsorption data set over all uranium loadings, pH values, and dissolved inorganic carbon concentrations. The results show that the adsorption behavior was related to the changing U(VI) species and properties of surface coatings on nanoparticles. The model could also fit pH-dependent surface potential values that are consistent with measured zeta potentials.

MINFIT: A Spreadsheet-Based Tool for Parameter Estimation in an Equilibrium Speciation Software Program

Determination of equilibrium constants describing chemical reactions in the aqueous phase and at solid-water interface relies on inverse modeling and parameter estimation. Although there are existing tools available, the steep learning curve prevents the wider community of environmental engineers and chemists to adopt those tools. Stemming from classical chemical equilibrium codes, MINEQL+ has been one of the most widely used chemical equilibrium software programs. We developed a spreadsheet-based tool, which we are calling MINFIT, that interacts with MINEQL+ to perform parameter estimations that optimize model fits to experimental data sets. MINFIT enables automatic and convenient screening of a large number of parameter sets toward the optimal solutions by calling MINEQL+ to perform iterative forward calculations following either exhaustive equidistant grid search or randomized search algorithms. The combined use of the two algorithms can securely guide the searches for the global optima. We developed interactive interfaces so that the optimization processes are transparent. Benchmark examples including both aqueous and surface complexation problems illustrate the parameter estimation and associated sensitivity analysis. MINFIT is accessible at http://minfit.strikingly.com .

X-ray Absorption Spectroscopic Quantification and Speciation Modeling of Sulfate Adsorption on Ferrihydrite Surfaces

Sulfate adsorption on mineral surfaces is an important environmental chemical process, but the structures and respective contribution of different adsorption complexes under various environmental conditions are unclear. By combining sulfur K-edge XANES and EXAFS spectroscopy, quantum chemical calculations, and surface complexation modeling (SCM), we have shown that sulfate forms both outer-sphere complexes and bidentate-binuclear inner-sphere complexes on ferrihydrite surfaces. The relative fractions of the complexes vary with pH, ionic strength (I), and sample hydration degree (wet versus air-dried), but their structures remained the same. The inner-sphere complex adsorption loading decreases with increasing pH while remaining unchanged with I. At both I = 0.02 and 0.1 M, the outer-sphere complex loading reaches maximum at pH ∼5 and then decreases with pH, whereas it monotonically decreases with pH at I = 0.5 M. These observations result from a combination of the ionic-strength effect, the pH dependence of anion adsorption, and the competition between inner- and outer-sphere complexation. Air-drying drastically converts the outer-sphere complexes to the inner-sphere complexes. The respective contributions to the overall adsorption loading of the two complexes were directly modeled with the extended triple layer SCM by implementing the bidentate-binuclear inner-sphere complexation identified in the present study. These findings improve our understanding of sulfate adsorption and its effects on other environmental chemical processes and have important implications for generalizing the adsorption behavior of anions forming both inner- and outer-sphere complexes on mineral surfaces.

In situ spectroscopic identification of neptunium(V) inner-sphere complexes on the hematite-water interface

Hematite plays a decisive role in regulating the mobility of contaminants in rocks and soils. The Np(V) reactions at the hematite-water interface were comprehensively investigated by a combined approach of in situ vibrational spectroscopy, X-ray absorption spectroscopy and surface complexation modeling. A variety of sorption parameters such as Np(V) concentration, pH, ionic strength, and the presence of bicarbonate was considered. Time-resolved IR spectroscopic sorption experiments at the iron oxide-water interface evidenced the formation of a single monomer Np(V) inner-sphere sorption complex. EXAFS provided complementary information on bidentate edge-sharing coordination. In the presence of atmospherically derived bicarbonate the formation of the bis-carbonato inner-sphere complex was confirmed supporting previous EXAFS findings.1 The obtained molecular structure allows more reliable surface complexation modeling of recent and future macroscopic data. Such confident modeling is mandatory for evaluating water contamination and for predicting the fate and migration of radioactive contaminants in the subsurface environment as it might occur in the vicinity of a radioactive waste repository or a reprocessing plant.

Sorption of radionuclides from aqueous systems onto graphene oxide-based materials: a review

Graphene oxide-based nanomaterials are suitable materials for the preconcentration of radionuclides and heavy metal ions from aqueous solutions in environmental pollution cleanup.

Additive surface complexation modeling of uranium(VI) adsorption onto quartz-sand dominated sediments

Many aquifers contaminated by U(VI)-containing acidic plumes are composed predominantly of quartz-sand sediments. The F-Area of the Savannah River Site (SRS) in South Carolina (USA) is an example. To predict U(VI) mobility and natural attenuation, we conducted U(VI) adsorption experiments using the F-Area plume sediments and reference quartz, goethite, and kaolinite. The sediments are composed of ∼96% quartz-sand and 3-4% fine fractions of kaolinite and goethite. We developed a new humic acid adsorption method for determining the relative surface area abundances of goethite and kaolinite in the fine fractions. This method is expected to be applicable to many other binary mineral pairs, and allows successful application of the component additivity (CA) approach based surface complexation modeling (SCM) at the SRS F-Area and other similar aquifers. Our experimental results indicate that quartz has stronger U(VI) adsorption ability per unit surface area than goethite and kaolinite at pH ≤ 4.0. Our modeling results indicate that the binary (goethite/kaolinite) CA-SCM under-predicts U(VI) adsorption to the quartz-sand dominated sediments at pH ≤ 4.0. The new ternary (quartz/goethite/kaolinite) CA-SCM provides excellent predictions. The contributions of quartz-sand, kaolinite, and goethite to U(VI) adsorption and the potential influences of dissolved Al, Si, and Fe are also discussed.

The variable capacitance model: a strategy for treating contrasting charge-neutralizing capabilities of counterions at the mineral/water interface

Thermodynamic models predicting ion adsorption at mineral/water interfaces can have limitations from the simplifying assumptions that compact plane thicknesses and capacitance values are constant, and that charge densities of electrolyte counterions of different charge-to-size ratios lie at the same planes of adsorption, or split between different planes. To address these limitations a thermodynamic adsorption modeling framework was developed to account for coexisting compact planes for each type of counterion complexes formed on a single mineral surface. This framework was developed to predict charge development at lepidocrocite (γ-FeOOH) particle surfaces suspended in aqueous solutions of NaCl and NaClO4. The model incorporates properties of Cl(-), ClO4(-), and Na(+) complexes formed at the (001) and (010) faces of this mineral obtained by molecular dynamics (MD) simulations. This concept was incorporated in a thermodynamic adsorption model that predicts an overall variable compact plane capacitance in terms of a linear combination of the capacitances of ion-specific EDL structures scaled for their relative surface loadings. These capacitance values are in turn constrained by compact plane thicknesses of every Cl(-), ClO4(-), and Na(+) complex, based on their MD-derived structures and atomic densities. The model predicts experimental potential-determining (H+, OH-) data for submicrometer-sized synthetic lepidocrocite particles exhibiting both (001) and (010) faces. It also isolates electrostatic contributions from these faces. A computer code solving for this Variable Capacitance Model--VCM--is provided in the Supporting Information section of this article, and can be readily modified to predict molecular-level details of any other mineral/water interface systems using this methodology.