Surface Complexation Model Research Articles

Elucidating Phosphate and Cadmium Cosorption Mechanisms on Mineral Surfaces with Direct Spectroscopic and Modeling Evidence.

The simultaneous sorption of cations and anions at the mineral-water interface can substantially alter their individual sorption characteristics; however, this phenomenon lacks a mechanistic understanding. Our study provides direct spectroscopic and modeling evidence of the molecular cosorption mechanisms of the cadmium ion (Cd2+) and phosphate (P) on goethite and layered manganese (Mn) oxide of birnessite, through in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR), P K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and surface complexation modeling. Phosphate synergistically cosorbed with Cd on goethite predominantly through P-bridged ternary complexes (≡Fe-P-Cd) and electrostatic interactions at wide pH conditions. Likewise, P and Cd exhibited synergistic cosorption on birnessite by forming P-bridged ternary complexes (≡Mn-P-Cd) and weak competitive sorption at the layer edge sites. As pH and Cd loading increased, the surface P species transitioned from a binary complex to a ternary complex and/or Cd3(PO4)2 precipitate for both goethite and birnessite. Compared to that in solution at pH 8, the formation of Cd3(PO4)2 was inhibited by the presence of goethite and birnessite, ascribed to the specific adsorption of P and Cd, more pronounced in birnessite due to the stronger sorption of Cd at its vacant sites. The discovered cosorption mechanisms of P and Cd have important implications for understanding and predicting their mobility and availability in Cd-contaminated settings.

Heavy-Metal Ions Control on PFAS Adsorption on Goethite in Aquatic Systems.

Per- and polyfluoroalkyl substances (PFAS) are ubiquitous environmental contaminants that often co-occur with heavy metals. Despite their prevalence, the mobility of PFAS in complex, multicomponent systems, particularly at the molecular scale, remains poorly understood. The vast diversity of PFAS and their low concentrations alongside anthropogenic and natural substances underscore the need for integrating mechanistic insights into the sorption models. This study explores the influence of metal cations (Cu(II), Cd(II), and Fe(II)) on the adsorption of four common PFAS (PFOA, PFOS, PFDA, and GenX) onto goethite (α-FeOOH), a common iron (oxyhydr)oxide in both aquatic and terrestrial environments. PFAS adsorption was highly dependent on the PFAS type, pH, and metal ion concentration, with a surface complexation model effectively predicting these interactions. Cu(II) and Cd(II) enhanced PFOS and PFDA adsorption via ternary complexation while slightly reducing PFOA and GenX adsorption. Under anoxic conditions, Fe(II) significantly increased the adsorption of all PFAS, showing reactivity greater than those of Cu(II) and Cd(II). Additionally, natural organic matter increased PFAS mobility, although metal cations in groundwater may counteract this by enhancing PFAS retention. These findings highlight the key role of metal cations in PFAS transport and offer critical insights for predicting PFAS behavior at oxic-anoxic environmental interfaces.

Probing atomic-scale processes at the ferrihydrite-water interface with reactive molecular dynamics

Interfacial processes involving metal (oxyhydr)oxide phases are important for the mobility and bioavailability of nutrients and contaminants in soils, sediments, and water. Consequently, these processes influence ecosystem health and functioning, and have shaped the biological and environmental co-evolution of Earth over geologic time. Here we employ reactive molecular dynamics simulations, supported by synchrotron X-ray spectroscopy to study the molecular-scale interfacial processes that influence surface complexation in ferrihydrite-water systems containing aqueous MoO42-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {MoO}_4}^{2-}$$\\end{document}. We validate the utility of this approach by calculating surface complexation models directly from simulations. The reactive force-field captures the realistic dynamics of surface restructuring, surface charge equilibration, and the evolution of the interfacial water hydrogen bond network in response to adsorption and proton transfer. We find that upon hydration and adsorption, ferrihydrite restructures into a more disordered phase through surface charge equilibration, as revealed by simulations and high-resolution X-ray diffraction. We observed how this restructuring leads to a different interfacial hydrogen bond network compared to bulk water by monitoring water dynamics. Using umbrella sampling, we constructed the free energy landscape of aqueous MoO42-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {MoO}_4}^{2-}$$\\end{document} adsorption at various concentrations and the deprotonation of the ferrihydrite surface. The results demonstrate excellent agreement with the values reported by experimental surface complexation models. These findings are important as reactive molecular dynamics opens new avenues to study mineral-water interfaces, enriching and refining surface complexation models beyond their foundational assumptions.Graphic

Uranium removal from contaminated groundwater using goethite-loaded composite microporous membrane

In this study, we have coupled adsorption and membrane separation for the removal of uranium from contaminated groundwater in environmentally relevant conditions at low energy requirements. This study mainly focuses on elucidating uranium, U(VI), adsorption mechanisms using surface complexation modeling approach in a novel goethite-loaded composite microfiltration membrane (GLM). This experiment involved immobilizing goethite nanorods in a microporous (0.22 μm pore size) poly (vinylidene fluoride) (PVDF) membrane. The effect of varying goethite loading and hydraulic residence time on U(VI) removal was investigated at field-relevant pH (i.e. pH 8.5). U(VI) adsorption (i.e. 4.95 μg·mg−1) was optimum at a goethite loading 1.20 mg·cm−2. The effect of varying hydraulic residence time had no impact on U(VI) removal which was also confirmed via performing batch adsorption kinetic experiments. GLM membrane loaded at 1.2 mg·cm−2 could treat 275 L of U(VI) contaminated water having 200 μg of U L−1 below WHO drinking water limit (i.e. 30 μg of U L−1) with 1 m2 of membrane surface area with a adsorption capacity of 6.12 μg·mg−1. Varying the pH of aqueous solution, containing U(VI) from pH 4.0 to pH 10.0, showed a significant impact on uranium uptake ranging from 0.7 μg·mg−1 to 2.63 μg·mg−1 by the composite membrane. The adsorption mechanism of uranium onto goethite was explained via the formation of bidentate surface complexes using the Surface Complexation Model (SCM). The results of batch pH edge experiments and SCM have been compared with pH experiments performed using GLM. The results of SCM predicted the batch pH edge experiment within a RMSE of 0.055. The trend of U(VI) removal in membrane experiments was observed to be similar to that of batch pH edge experiments and was well predicted SCM model. Our results showed that the novel goethite-loaded membrane has the potential for the effective removal of uranium with a lower specific energy consumption.

A new semi-analytic model for Stern-layer polarization in pore throats

SUMMARY To explain induced polarization, membrane polarization is often referred to as a relevant process taking place in granular media – particularly, when narrow pore throats are present. This polarization effect is based on the membrane-like behaviour of pore throats caused by the presence of an usually negative charge on the pore surface, that influences charge transport in the pore fluid. Existing analytical, 1D models describe the pore system as a series of cylindrical pores with different radii and lengths. The polarization response is calculated by solving the Poisson–Nernst–Planck system for the current densities of one single anion and one single cation species representing the charge transport in the electrolyte and the diffuse layer at the pore surface. To include charge transport in the Stern layer, cations in the Stern layer have so far simply been considered by increasing the concentration of the diffuse layer cations. As we know from numerical modelling, this approach fails to predict the polarization response when the Stern layer is significantly charged. Here, we present a new semi-analytical model that treats the Stern-layer cations as a separate ion species and allows the Stern layer to polarize individually. To validate our new model, we compare it to the previously used analytical model and numerical simulations for different relative charges in Stern- and diffuse layer. We also use electrostatic surface-complexation models for two mineral surfaces (quartz and montmorillonite) to simulate the response of real geologic material under varying chemical conditions. This work is a step forward for considering realistic pore properties in induced-polarization modelling.

Adsorption behavior and surface complexation models of plumbum and cadmium in basalt yellow soil

Adsorption behavior and surface complexation models of plumbum and cadmium in basalt yellow soil

Mechanistic insights into selenite and selenate immobilization using brucite-rich magnesium precipitate derived from seawater electrochlorination facility

This study explored the application of magnesium precipitate (MP) obtained from seawater electrochlorination storage tanks as an innovative and sustainable adsorbent for Se(IV) and Se(VI) in water. Initially, the collected pristine MP mainly consisted of the low crystalline Mg(OH)2 (brucite) phase. Subsequently, the MP underwent calcination (CMP), and analyses via scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed an improved crystallinity, displaying a typical hexagonal plate structure in the aqueous phase. Notably, the CMP exhibited a significant adsorption capacity (qm: 85.0 mg/g) for Se(IV), while the adsorption capacity for Se(VI) was lower, determined as 3.0 mg/g using the Langmuir adsorption model. Se(IV) adsorption remained relatively consistent across a wide pH range and ionic strength in the presence of competing anions. In contrast, Se(VI) adsorption varied under the same conditions. This differing adsorption behavior of Se(IV) and Se(VI) on CMP could be ascribed to different mechanisms: inner-sphere predominant surface complexation for Se(IV) and outer-sphere complexation for Se(VI). These mechanisms were confirmed via X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), XRD, and SEM analyses, and surface complexation modeling (PHREEQC–PEST) of the batch experimental results. In conclusion, these findings underscored the potential utility of MP as an effective adsorbent for selenium, positioning it as an environmentally sustainable and significant novel material. Furthermore, the study offered mechanistic insights into the interaction between CMP, Se (IV), and Se(VI) in the aqueous phase.

Understanding the wettability alterations in carbonate reservoirs: Integrating experimental data and a surface complexation model for enhanced predictions

In this study, we developed a laboratory-based model to analyze wettability alteration in the oil–water-rock system. The selected Surface Complexation Model (SCM), based on experimental data, is refined through sensitivity analysis and comparison with experimental measurements, including zeta potential and contact angle. Our analysis indicates that, in addition to magnesium, calcium, and sulfate ions, the adsorption of sodium and chlorine on calcite surfaces, as well as sodium adsorption on oil surfaces, significantly affects electrostatic interactions and surface potential alterations. Consequently, the complexing reactions of these ions were incorporated into the model and adjusted based on sensitivity analysis. Moreover, surface complex reactions exhibit distinct behaviors in neutral, acidic, and alkaline environments. Therefore, different reactions were considered and sensitivity analyzed for oil and rock surfaces in neutral conditions. The model’s reliability was further validated by comparing predicted and experimental zeta potentials. Additionally, disjoining pressures were computed, and contact angles were simulated, demonstrating the model’s accuracy. Key findings include the successful modification of reaction parameters to achieve accurate results across different pH levels and ion concentrations. This confirms the applicability and reliability of the developed SCM for wettability alteration analysis, further supported by stability maps that consider varying ion concentrations and pH levels.

Insights on the Contradiction between the Affinity and Capacity of Ferrihydrite toward As(III) and As(V): Surface Reaction Revisited.

Ferrihydrite is omnipresent in nature, and its adsorption of As(III/V) decides the migration of arsenic. Although As(III) is commonly recognized as the more mobile species of inorganic arsenic, it sometimes exhibits less mobility in ferrihydrite systems, which calls for further insights. In this study, we elucidated the adsorption behavior and mechanisms of As(III/V) on ferrihydrite under different loading levels (molar ratio As/Fe = 0-0.38), solution pH (3-10), and coexisting ions [P(V) and Ca(II)] based on batch adsorption experiments, surface complexation modeling, density functional theory calculations, and X-ray photoelectron spectroscopy. Our results show that As(III) exhibits weaker adsorption affinity but a larger capacity compared with that of As(V). On ferrihydrite, As(III) and As(V) are adsorbed mainly as bidentate mononuclear complexes at type-a sites [≡Fe(OH-0.5)2] and bidentate binuclear complexes at type-b sites (2≡FeOH-0.5), respectively. As the dosage increases, As(III) further forms mononuclear monodentate complexes at both surface sites, resulting in a higher site utilization efficiency, while As(V) does not due to repulsive electrostatic interaction. The difference in surface species of As(III/V) also leads to complex responses when coexisting with high concentrations of P(V) and Ca(II). This study helps us to understand environmental behavior of As(III/V) and develop remediation strategy in As(III/V) contaminated systems.

Integrated experiments and numerical simulations for chromium (VI) surface complexation in natural unconsolidated sediments

This study focuses on the adsorption and transport of Cr (VI) on natural sediments from Qiqihar, China. Through batch and column experiments, we assessed the adsorption capacities influenced by factors such as contact time, initial concentration, pH, ionic strength, solid-to-solution ratio, coexisting ions in groundwater, sediment characteristics and flow rate. The adsorption kinetics of Cr (VI) follow the pseudo-first-order model well, while the isotherms of all three sediments are accurately represented by the Freundlich model. The adsorption edges reveal a strong pH dependence in Cr (VI) adsorption: the stronger the acidity, the more favorable it is for adsorption. The adsorption capacity decreases with an increasing solid-to-solution ratio, stabilizing at higher ratios. Coexisting ions in groundwater reduce Cr (VI) adsorption in loam under neutral pH. Additionally, Fourier transform infrared spectroscopy (FTIR) results indicate that the hydroxyl group is the primary reactive functional group in all three sediments. X-ray photoelectron spectroscopy (XPS) results further show partial adsorbed Cr (VI) was reduced to Cr (III) by organic matters. However, surface complexation reactions dominate the removal of Cr (VI). On the base above, we introduced a surface complexation model, optimizing equilibrium complexation constants by fitting adsorption edges. Subsequently, reactive transport models incorporating both surface complexation and reduction processes for Cr (VI) were established to simulate column experiments. As the flow rate decreases, the adsorption capacity and the amount of reduction reaction for Cr (VI) increase, while the reduction rate decreases. Specifically, the reduction for Cr (VI) was found to be more significant in loam compared to sand, correlating with the organic matter content. The results emphasize the existence of surface complexation reactions and the role of organic matters in electron transfer. Our study provides significant information for understanding Cr (VI) adsorption and transport behavior in natural aquifer sediments.

Ultrasonic‐assisted activation of clay for efficient removal of U(VI) and Cr(VI)

AbstractThe natural clay mineral was activated using the acid treatment under ultrasonic conditions to improve its adsorption performance. The acid‐activated clay (AAC) was investigated to remove Cr(VI) from wasted lubricant and U(VI) from aqueous solution by batch, spectroscopy, and modeling. Batch experiments showed that the isotherms and adsorption kinetics of Cr(VI)/U(VI) on AAC were satisfactorily followed by Frenudlich and the pseudo‐second‐order model, respectively. The decreased adsorption of U(VI) on AAC with the increase of ion strength indicated the outer surface complexation of U(VI) on AAC. The interaction mechanism of Cr(VI)/U(VI) on AAC was demonstrated to be surface complexation by oxygenated functional groups (e.g., hydroxyl) according to X‐ray photoelectron spectroscopy (XPS) analysis. According to fitting of surface complexation models using three approaches, the adsorption of U(VI) on AAC at different pH can be better fitted by the double‐layer model compared to the constant‐capacitance model and triple‐layer model. These findings are great important for the application of acid activated clay in the removal of heavy metals and radionuclides from the environment.

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.

Quantifying the impact of damming on phosphorus reallocation: Finer particles offset the reduction in soluble reactive phosphorus (SRP) by decreasing suspended sediment concentration

In this research, an innovative approach to quantify the impact of damming on phosphorus (P) reallocation between suspended sediments (SS) and water was proposed. P allocation can be described by the surface complexation model, with the impact of damming quantified by four variables: P load, suspended sediment concentration (SSC), particle size, and pH. Iron/aluminium (Fe/Al) oxide-adsorbed P (Fe/Alo-P) was identified as the exchangeable P during adsorption/desorption equilibrium with a series of heterogeneous sediment samples from two large Asian rivers, the Mekong River and the Yellow River. In both rivers, the Fe/Alo-P concentration increased from the tail towards the dam of the reservoirs, primarily attributed to the decrease in particle size from the tail towards the dam of the reservoirs. The Fe/Alo-P concentration in the Lancang River was higher than that in the Yellow River, ranging from 14.5 to 119.9 mg kg−1 and from 14.5 to 22.1 mg kg−1, respectively. The soluble reactive P (SRP) concentration decreased with decreasing SSC, while finer suspended sediment particles containing more Fe/Alo-P greatly offset the reduction in SRP concentration. When the maximum Fe/Alo-P concentration in the finest particles of SS was assumed to be 100 mg kg−1, the P equilibrium concentration (ce) decreased from 0.028 mg L−1 to 0.008 mg L−1 when the SSC decreased from 64 g L−1 to 1 g L−1 for SS with a median grain size (D50) of 32 μm and an Fe/Alo-P concentration of 11 mg kg−1. However, ce increased from 0.008 mg L−1 to 0.021 mg L−1 when the D50 of SS decreased from 32 μm to 4 μm with an SSC of 1 g L−1 and an Fe/Alo-P concentration of 76 mg kg−1 for 4-μm SS. The SRP concentration is sensitive to the Fe/Alo-P concentration in SS, and the P allocation ratio between sediments and water is comparable.

Spectroscopic and modeling approaches of arsenic (III/V) adsorption onto Illite

Illite plays an essential role in arsenic (As) transportation in the subsurface. Despite extensive investigations into As adsorption onto illite, debates persist due to the absence of direct evidence revealing the underlying processes. In this research, we conducted batch experiments and employed spherical aberration-corrected scanning transmission electron microscope, X-ray absorption spectroscopy, and density functional theory-based calculations to elucidate the mechanisms for the adsorption of two major inorganic As species (As(III) and As(V)) onto illite. Experimental results indicate adsorption capacities of 0.251 and 0.667 μmol/g for As(III) and As(V) onto illite, respectively. As(III) adsorption occurs within 300 min, whereas As(V) is rapidly adsorbed within 500 min, after which it tends to stabilize. Both As species can adsorbed onto the basal surface via electrostatic forces, where cations act as a bridge, leading to specific-cation effects. Conversely, As adsorption onto the edge surface can be ascribed to inner-sphere complexes via As-O-Al bonds, causing a negatively shifted isoelectric point of illite. These mechanisms collectively account for the partially reversible adsorption and two-stage kinetics pattern. Finally, a process-based surface complexation model was developed to predict As adsorption onto illite, which includes the inner/outer-sphere complexation and monodentate/bidentate complexes.

Cadmium Isotope Fractionation during Adsorption onto Edge Sites and Vacancies in Phyllomanganate.

Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd2+ adsorption on edge sites (Δ114/110Cdedge-solution = -1.54 ± 0.11‰) is smaller than that on vacancies (Δ114/110Cdvacancy-solution = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

Adsorption Behavior of Cadmium in Argillaceous Limestone Yellow Soils Simulated by The Surface Complexation Model

Adsorption Behavior of Cadmium in Argillaceous Limestone Yellow Soils Simulated by The Surface Complexation Model

Solid–liquid partitioning and speciation of Pb(II) and Cd(II) on goethite under high pH conditions, as examined by subnanomolar heavy metal analysis, X-ray absorption spectroscopy, and surface complexation modeling

The adsorption of heavy metals on iron oxides generally increases with pH and is almost complete at neutral to slightly alkaline pH. However, almost complete adsorption on a linear scale does not imply sufficient removal of the heavy metals in terms of their toxicity. Here, we elucidated the chemical reactions that determine the solid–liquid partitioning of Pb(II) and Cd(II) on goethite at high pH. While the removal of both heavy metals was almost complete on a linear scale above pH 7 for Pb(II) and pH 9 for Cd(II), the dissolved metal concentrations decreased on a logarithmic scale with pH, reaching minima at around pH 10 for Pb(II) and pH 10–11 for Cd(II), and then they increased with pH thereafter. The XAFS spectra of Pb(II)- or Cd(II)-adsorbed goethite prepared at pH > 11 were almost the same as those at neutral pH, suggesting that removal of the heavy metals from solution was achieved by a single adsorption reaction over the entire pH range. Based on the observed macroscopic and microscopic adsorption behaviors at high pH, a robust surface complexation model was developed to predict the solid–liquid partitioning of divalent heavy metals over the entire pH range.

Influence of Fe(II), Fe(III), and Al(III) isomorphic substitutions on acid-base properties of edge surfaces of cis-vacant montmorillonite: Insights from first-principles molecular dynamics simulations and surface complexation modeling

Abstract Knowing the influence of isomorphic substitutions on the acid-base properties of smectite edge surfaces is an important aspect of the detailed understanding of clay minerals’ interfacial properties with implications in the modeling of adsorption processes. We investigated the intrinsic acidity constants of Fe(II)/Fe(III) and Al(III) substituted edge surface sites of montmorillonite with a cis-vacant structure, which includes four crystallographic orientations perpendicular to [010], [010], [110], and [110], using the first-principles molecular dynamics (FPMD) based vertical energy gap method. Fe(II) and Fe(III) substitutions resulted, respectively, in a significant increase and decrease in pKa values of amphoteric groups directly associated with Fe octahedra. In addition, Fe(II) substitution increased the pKa values of the neighboring silanol sites, while Fe(III) substitution had a weak influence on these sites. The Al-substituted tetrahedra had amphoteric sites with higher pKa values than the non-substituted Si tetrahedra, and they increased significantly the pKa values of the sites bridging the tetrahedral and octahedral sheets on surfaces perpendicular to [010] and [110]. The acid-base properties of substituted and non-substituted surface sites of cis-vacant montmorillonite were used to build a state-of-the-art surface complexation model, which successfully reproduced the best available experimental acid-base titration data. This model was further used to predict acid-base properties of dioctahedral smectites (montmorillonite, beidellite, and nontronite) according to their cis- or trans-vacant structures and their layer chemistry. According to these predictions, these smectites exhibit very similar overall pH buffering properties despite significant differences in structure and chemistry. A detailed analysis of the acid-base properties as a function of crystallographic directions demonstrated, however, that these differences should have a large influence on the adsorption of ionic species.

Sorption of U(VI) on MX-80 bentonite, illite, shale and limestone in Na–Ca–Cl saline solutions

Uranium (U) has been identified as an element of interest for the safety assessment of a deep geological repository for used nuclear fuel. In this study, the sorption of U(VI) was studied in Na–Ca–Cl solutions at ionic strengths = 0.1–6 mol/kgw (m) in a CO2 free environment at pHm (molal H+ concentration where pHm = −log mH+) = 4–9 for MX-80 bentonite, illite and Queenston shale and at pHm = 5–9 for limestone. U(VI) sorption on MX-80 bentonite increased with pHm from pHm = 4 to 6, then decreased with pHm until pHm = 7, and then increased again with pHm to pHm = 9. U(VI) sorption on illite increased with pHm reaching a maximum at pHm = 7, and then decreased with further increases in pHm. The sorption behavior of U(VI) on shale was similar to that of illite, but the extent of decrease in the sorption coefficient (Rd) value with pHm was slightly more pronounced for the shale than observed for sorption on illite at pHm > 7. U(VI) sorption on limestone increased with pHm up to pHm = 8 and then seemed to be constant at pHm = 8–9. U(VI) sorption on all four solids was independent of ionic strength (0.1–6.0 m). The 2 site protolysis non-electrostatic surface complexation and cation exchange model successfully simulated the sorption of U(VI) onto MX-80 and illite, and the optimized values of surface complexation constants were estimated.

IORSim: A Mathematical Workflow for Field-Scale Geochemistry Simulations in Porous Media

Reservoir modeling consists of two key components: the reproduction of the historical performance and the prediction of the future reservoir performance. Industry-standard reservoir simulators must run fast on enormous and possibly unstructured grids while yet guaranteeing a reasonable representation of physical and chemical processes. However, computational demands limit simulators in capturing involved physical and geochemical mechanisms, especially when chemical reactions interfere with reservoir flow. This paper presents a mathematical workflow, implemented in IORSim, that makes it possible to add geochemical calculations to porous media flow simulators without access to the source code of the original host simulator. An industry-standard reservoir simulator calculates velocity fields of the fluid phases (e.g., water, oil, and gas), while IORSim calculates the transport and reaction of geochemical components. Depending on the simulation mode, the geochemical solver estimates updated relative and/or capillary pressure curves to modify the global fluid flow. As one of the key innovations of the coupling mechanism, IORSim uses a sorting algorithm to permute the grid cells along flow directions. Instead of solving an over-dimensionalized global matrix calling a Newton–Raphson solver, the geochemical software tool treats the species balance as a set of local nonlinear problems. Moreover, IORSim applies basis swapping and splay tree techniques to accelerate geochemical computations in complex full-field reservoir models. The presented work introduces the mathematical IORSim concept, verifies the chemical species advection, and demonstrates the IORSim computation efficiency. After validating the geochemical solver against reference software, IORSim is used to investigate the impact of seawater injection on the NCS Ekofisk reservoir chemistry.Article HighlightsThe IORSim sorting algorithm decouples the nonlinear geochemical reaction calculations into recurring one-dimensional problems to assure numerical stability and computation efficiency. To the best of our knowledge, this work presents the mathematical concept, implementation, and application of topological sorting for the first time on (industry) field-scale problems.IORSim combines topological sorting with basis swapping and splay trees to significantly reduce computation times. Moreover, a high-speed forward simulation mode was developed to allow the post-advection of chemical components to visualize species distribution, water chemistry, and mineral interactions. If the geochemical reactions interfere with the fluid flow, the IORSim backward mode uses relative permeability curves to update the global fluid flow at each time step.We validate the implemented topological scheme on a reservoir grid, show the computation efficiency, and compare the impact of explicit, implicit, and grid refinement on numerical dispersion.The decoupled flow simulator and geochemical reaction calculations allow seamless integration of full-field reservoir models that contain complex geological structures, a large number of wells, and long production histories.The computation capabilities of IORSim are demonstrated by simulating and reproducing the impact of seawater injection in the southern segment of the giant Ekofisk field (more than 50 years of injection and production history). IORSim shows that seawater injection changed the Ekofisk mineralogy and impacted the produced water chemistry. In the investigated Ekofisk case, seawater promoted calcite dissolution and led to the precipitation of magnesite and anhydrite. Moreover, surface complexation modeling revealed that sulfate is adsorbed on the calcite surface.