Diffuse Layer Model Research Articles

A robust prediction of U(VI) sorption on Fe3O4/activated carbon composites with surface complexation model

A robust prediction of U(VI) on Fe3O4/activated carbon (Fe3O4/AC, fabricated by co-precipitation method under N2 conditions) under different pH was developed via diffuse layer model, in accordance with FI-IR, XRD and XPS analysis. No influence of ionic strength onto U(VI) adsorption by Fe3O4/AC under ambient conditions suggested the inner-sphere surface adsorption, which was attributed to abundant surficial functional groups according to FT-IR and XPS analysis. The batch experiments indicated Fe3O4/AC with fast adsorption rate (equilibrium within 60 min), high adsorption capacity (56 mg/g at pH 4.0) towards U(VI). The adsorbed U(VI) was partly reduced by Fe2+ of Fe3O4/AC by XPS analysis. Surface complexation modeling showed that a single set of monodentate and mononuclear species (SOUO2+) cannot predict U(VI) adsorption at high pH, whereas the robust prediction of U(VI) adsorption over wide pH range was observed by adding the other binuclear and tridentate species ((SO)2UO2(CO3)6-). These findings revealed that magnetic AC as a candidate for immobilization and/or preconcentration of radioactive wastewater in environment management.

Ultrafast and highly capture of U(VI) by hierarchical mesoporous carbon

Abstract In this study, the hierarchical mesoporous carbon (HMC) was synthesized by the hydrothermal method. The batch adsorption experiments showed that HMC exhibited the ultrafast equilibrium fate (80 % U(VI) capture efficiency within 5 min), high UO2 2+ capture capacity (210 mg/g, pH = 4.5) and well recyclability. The investigations of XPS techniques indicated the oxygen-containing functional groups were responsible for high efficient UO2 2+ adsorption. The pH-dependent adsorption was simulated by three surface complexation modellings, revealing that UO2 2+ adsorption on HMC was excellently fitted by triple layer model using two inner-sphere complexes (i. e. SOUO2 + and SOUO2(CO3)3 5− species) compared to constant capacitance model and diffuse layer model. These findings are crucial for expanding actual applications of HMC towards the removal of radionuclides under environmental cleanup.

Molecular Mechanisms of Lead Binding to Ferrihydrite-Bacteria Composites: ITC, XAFS, and μ-XRF Investigations.

Binding of Pb(II) to ferrihydrite-Bacillus subtilis composites formed in the presence of bacterial cells were investigated through macroscopic and microscopic techniques. Diffuse layer model (DLM) fitting and isothermal titration calorimetry (ITC) analysis indicated that the hydroxyl group played a key role in Pb(II) sorption onto composites by masking reactive sites, such as carboxyl and phosphoryl groups of bacterial cells. Negative enthalpy (from -39.29 to -57.87 kJ mol-1) and positive entropy (from 135.61 to 193.47 kJ mol-1) of Pb(II) sorption onto composites revealed that inner-sphere complexes formed through exothermic reactions and was driven by both entropy and enthalpy. Spatial distribution of these inner-sphere species at varied Pb(II) loading demonstrated that interactions between Pb(II) and bacterial cells preceded that of mineral components in composites, using microfocus X-ray fluorescence spectroscopy (μ-XRF) maps and microfocus X-ray absorption near edge structure (μ-XANES) spectra. Combined with bulk Pb LIII-edge X-ray absorption fine structure (XAFS) spectrum, we inferred that mononuclear bidentate edge-sharing hydroxyl-Pb complexes, monodentate mononuclear carboxyl-Pb and phosphoryl-Pb complexes predominantly contributed to Pb(II) inner-sphere binding with mineral and bacterial fractions in composites, respectively. The molecular binding mechanisms obtained in this study provide further insight into the sequestration and migration of toxic metals in natural environments.

Modifications of the turbulent diffusion layer model for the condensation heat transfer under the presence of noncondensable gases

The condensation heat transfer model of the MARS-KS1.5 code tends to under-predict the condensation in the presence of noncondensable gases. In our previous study, we adopted the diffusion layer model and modified it in order to consider the effect on turbulence on diffusive mass transfer in the diffusion layer. In this work, further modifications were made by introducing the boundary layer theory for natural convection flow.The new condensation model has been validated against 442 experimental data collected from eight different facilities. It is shown that the results of the new model for natural convection flow are more accurate result than those of the previous model. It is confirmed that the new model works well for the condensation in forced convection flow. The relative root mean square error of the new model is reduced to 0.193 from 0.242 of the previous model and 0.401 of the original one.

Diffusion layer model for condensation of vapor with the presence of noncondensable gas under natural convective condensation

A diffusion layer model was developed for condensation heat transfer of a vapor with noncondensable gases. The model predicts the heat transfer coefficient for free convection condensation over a vertical surface in the presence of air with consideration of the effect of the variation of the mixture molecular weight and density across the diffusion layer on the vapor diffusion. The model also takes into account the effect of the suction due to the condensation on the sensible heat transfer and fog formation. The model predictions compare well with previous experimental data and correlations. This model more accurately predicts the convection and condensation heat transfer of steam and air along a vertical surface with an average error of 14.74% and with almost all the points within ±20% error bands. The results show that the suction and fog formation significantly affect the heat transfer. For high noncondensable mass fractions, the fog formation should be considered, especially at high pressures.

Spatial-temporal Characteristics and Driving Factors of Greenhouse Gas Emissions from Rivers in a Rapidly Urbanizing Area

Rivers play an important role in greenhouse gas emissions. Over the past decade, because of global urbanization trends, rapid land use changes have led to changes in river ecosystems that have had a stimulating effect on the greenhouse gas production and emissions. Presently, there is an urgent need for assessments of the greenhouse gas concentrations and emissions in watersheds. Therefore, this study was designed to evaluate river-based greenhouse gas emissions and their spatial-temporal features as well as possible impact factors in a rapidly urbanizing area. The specific objectives were to investigate how river greenhouse gas concentrations and emission fluxes are responding to urbanization in the Liangtan River, which is not only the largest sub-basin but also the most polluted one in Chongqing City. The thin layer diffusion model method was used to monitor year-round concentrations of pCO2, CH4, and N2O in September and December 2014, and March and June 2015. The pCO2 range was (23.38±34.89)-(1395.33±55.45) Pa, and the concentration ranges of CH4 and N2O were (65.09±28.09)-(6021.36±94.36) nmol·L-1 and (29.47±5.16)-(510.28±18.34) nmol·L-1, respectively. The emission fluxes of CO2, CH4, and N2O, which were calculated based on the method of wind speed model estimations, were -6.1-786.9, 0.31-27.62, and 0.06-1.08 mmol·(m2·d)-1, respectively. Moreover, the CO2 and CH4 emissions displayed significant spatial differences, and these were roughly consistent with the pollution load gradient. The greenhouse gas concentrations and fluxes of trunk streams increased and then decreased from upstream to downstream, and the highest value was detected at the middle reaches where the urbanization rate is higher than in other areas and the river is seriously polluted. As for branches, the greenhouse gas concentrations and fluxes increased significantly from the upstream agricultural areas to the downstream urban areas. The CO2 fluxes followed a seasonal pattern, with the highest CO2 emission values observed in autumn, then successively winter, summer, and spring. The CH4 fluxes were the highest in spring and the lowest in summer, while N2O flux seasonal patterns were not significant. Because of the high carbon and nitrogen loads in the basin, the CO2 products and emissions were not restricted by biogenic elements, but levels were found to be related to important biological metabolic factors such as the water temperature, pH, DO, and chlorophyll a. The carbon, nitrogen, and phosphorus content of the water combined with sewage input influenced the CH4 products and emissions. Meanwhile, N2O production and emissions were mainly found to be driven by urban sewage discharge with high N2O concentrations. Rapid urbanization accelerated greenhouse gas emissions from the urban rivers, so that in the urban reaches, CO2/CH4 fluxes were twice those of the non-urban reaches, and all over the basin N2O fluxes were at a high level. These findings illustrate how river basin urbanization can change aquatic environments and aggravate allochthonous pollution inputs such as carbon, nitrogen, and phosphorus, which in turn can dramatically stimulate river-based greenhouse gas production and emissions; meanwhile, spatial and temporal differences in greenhouse gas emissions in rivers can lead to the formation of emission hotspots.

Prediction of swelling characteristics of compacted GMZ bentonite in salt solution incorporating ion-exchange reactions

AbstractSalt solutions have complex effects on the swelling characteristics of compacted bentonite; these effects are caused by the inhibitory action of salinity and the ion-exchange reaction between the solution and bentonite. In order to characterize the swelling properties of compacted bentonite in a salt solution, swelling deformation tests were carried out for Gao-Miao-Zi (GMZ) bentonite specimens in NaCl and CaCl2 solutions. Swelling characteristics decreased with increasing salt concentration. Swelling strains in NaCl solution were larger than those in CaCl2 solution, even though the ionic concentration of 1.0 mol/L (M) NaCl solution is larger than that of 0.5 M CaCl2. According to the exchangeable cations tests, cation exchange was different for specimens immersed in different salt solutions. The swelling fractal model was used to predict the swelling strains of compacted bentonite in a concentrated salt solution. In this model, the effective stress incorporating osmotic suction was applied to take the effect of salinity into consideration, and the swelling coefficient, K, was employed to describe the swelling properties affected by the variation in exchangeable cations. In the model, fractal dimension was measured by nitrogen adsorption, and the salt solution had little effect on fractal dimension. K was estimated by the diffuse double layer (DDL) model for osmotic swelling in distilled water. Comparison of fractal model estimations with experimental data demonstrated that the new model performed well in predicting swelling characteristics affected by a salt solution.

Understanding Li-Al-CO3 layered double hydroxides. (II) Interface electrochemical properties

In the current work, the interface electrochemical (IEC) properties of Li-Al-CO3 layered double hydroxides (LDHs) with structural positive charges were investigated by experiments and model fitting. The surface charge densities of the LDHs at different electrolyte (NaNO3) concentrations and pH were determined using acid-base titration. The effective structural charge density and the point of zero net charge are obtained to be ∼5.27 × 10−3 C/m2 and 9.22, respectively. On the basis of the general surface-charging (1-pK and 2-pK) models, the intrinsic surface reaction equilibrium constants pK, pKa1int, and pKa2int are obtained to be −8.50, 7.49, and 9.51, respectively. The general 2-pK model is combined with the diffuse layer model (DLM, an electrostatic model) to fit the surface-charging data. Model predictions are in good agreement with the experimental data, indicating that the general surface-charging models are reasonable and that the surface-charging behavior of Li-Al-CO3 LDHs can be described using the 2-pK/DLM model. To the best of our knowledge, this is the first report on the IEC properties of Li-Al LDHs, providing a better understanding for the features of Li-Al LDHs.

Recovery of potash values from feldspar

ABSTRACTIndia does not possess any soluble potassium (K) sources but an abundant resource of alumino-silicate rock such as feldspar is available. In the present work, an attempt has been made to unlock the potassium values from feldspar sample by mechanical milling, roasting and leaching and delineating the underlying mineral phase changes. CaCl2 was found to be effective flux for K release with valuable by-products such as quartz and anorthite. The roasting kinetics follows the product layer diffusion model with an activation energy of 49.50 kJ/mol. It is found that the planetary ball milling has a significant effect on enhancing K-dissolution. With the combination of different treatments, it is possible to recover more than 95% K from feldspar.

Kinetic Modeling of pH‐Dependent Molybdenum(VI) Adsorption and Desorption on Iron Oxide‐Coated Sand

Core Ideas Molybdenum(VI) sorption and kinetic rate are significantly reduced by increasing pH. A Diffuse Layer Model simulated protonated monodentate complex was dominant at low pH. A kinetic two‐site model successfully described Mo(VI) adsorption–desorption behavior. Quantitative understanding of retention kinetics controlling molybdenum [Mo(VI)] on soil constituents such as iron oxide binding sites is crucial for accurately evaluating environmental risk of ground and surface water contamination. In this study, kinetic batch experiments were performed to investigate Mo(VI) adsorption–desorption behavior on iron oxide‐coated sand (IOCS) at different reaction conditions and time scale. Batch results indicated that adsorption of Mo(VI) on IOCS was highly nonlinear and kinetic, where both sorption and the extent of kinetics decreased with increasing solution pH. A two‐site model based on the heterogeneity of binding sites and variations of solution chemistry was developed to describe Mo(VI) adsorption and desorption on IOCS. The approach integrates kinetic nonlinear sorption into the surface complexation Diffuse Layer Model (DLM). Our results indicated that kinetic rate of non‐protonated monodentate complexes was much higher than that of protonated monodentate complexes. The kinetic model was capable of describing the experimental data with limited number model fitting parameters.

Surface complexation modeling of arsenic mobilization from goethite: Interpretation of an in-situ experiment

Abstract Sorption has been recognized as a predominant process for the mobility and transport of arsenic (As) in groundwater. However, the model-based description of the chemical and physical mechanisms controlling As interaction with mineral surfaces under natural hydrochemical conditions remains a formidable challenge. In this study we assess and compare the ability of existing surface complexation models (SCMs), the diffuse double layer model (DDL) and the charge distribution multisite complexation model (CD-MUSIC), to simulate As desorption from goethite in groundwater. We consider the outcomes of an in-situ experiment recently performed in an arsenic-contaminated aquifer of Northern China where As-loaded goethite-coated sand was deployed in 7 monitoring wells. Determination and measurements of As-surface species were carried out over a time period of 80 days in all monitoring wells. Simultaneous calibration of the models with the measurements in the 7 wells was performed to obtain single sets of surface complexation parameters for the DDL and the CD-MUSIC models, respectively. Although the general dynamic pattern of the As release at the site is captured by both models, the approach based on the CD-MUSIC agrees best with the field experimental data without modifications of the surface complexation database compiled from previous studies on goethite. Conversely, calibration of the DDL affinity constants led to substantial improvement in the agreement between model simulations and the considered field dataset. The model outcomes and the exploration of the sensitivity of the goethite surface composition to changes in hydrochemical conditions provide insights into the mechanisms controlling arsenic sorption and their different description in the DDL and the CD-MUSIC models. Both SCMs indicate that PO43− acts as the main competitor for As(V) sorption sites, and that Fe(II) does not have a significant effect on the As(V) release despite its strong affinity for the goethite surface. Neither SCM suggests direct binding of As(III) to goethite. However, the CD-MUSIC model predicts significant formation of a goethite-Fe(II)-As(III) complex under the mildly alkaline conditions of the groundwater at the field site and that such complex is insensitive to phosphate competition. The CD-MUSIC implementation also allows capturing the non-linear charge effects of the major ions, including C a 2+ and M g 2+ , which appear to have important implication in the mechanisms of As sorption at the field site.

Wettability Estimation Using Surface-Complexation Simulations

Summary Wettability controls the fluid-phase distribution and flow properties in the reservoir. The ionic compositions of brine, the oil chemistry, and the reservoir-rock mineralogy have profound effects on wettability. Wettability measurement can be obtained from special core analysis (SCAL), but those data are not readily available, and the cost and time of analyzing different possible injection waters can be excessive. There is thus a need for early evaluation of wettability because it is crucial for selecting optimal field-development options. Information about wettability can be indirectly obtained from logging of other rock properties, but the uncertainty in the estimated wettability range is often high. In addition, wettability alteration by injection brines cannot be analyzed by logging. This study seeks to estimate the wettability by assessing the electrostatic interactions existing between the mineral/brine and the oil/brine interfaces using a surface-complexation model (SCM) supported with relatively simple and fast flotation experiments. The SCM is a chemical equilibrium technique of characterizing surface adsorption phenomenon. The SCM provides a cost-effective technique of characterizing the wettability of minerals at reservoir conditions. Ionic composition of the brine and the properties of the minerals were used as input to the model. In addition, the polar oil components in the crude oil were converted into their equivalent organic acid and base concentrations to be incorporated into the model. The electric-double-layer model that was used in the SCM was the diffuse-layer model. The SCM simulation is a fast and inexpensive wettability-characterization tool if reservoir cores and crude oil required in conventional wettability measurements are not readily available. From the flotation and SCM results, it could be concluded that the latter could capture the oil-adhesion tendencies of the former. Not only does the SCM predict the wetting tendencies of the minerals, but also it has the capacity to evaluate the mechanisms that led to their wetting preferences. For instance, the SCM results reveal that for negatively charged mineral/brine and oil/brine interfaces, divalent cations such as Ca2+ and Mg2+ can serve as a bridge between the two interfaces, thereby leading to oil adhesion. On the other hand, for positively charged mineral/brine interfaces such as calcite, direct adsorption of the carboxylic oil component was the dominant mechanism for oil adhesion. The SCM technique of characterizing wettability can be used to screen possible injection-water compositions to assess their potential to alter the wettability to more water-wet. Finally, the SCM technique could capture the trend of ζ-potential measurements from literature.

Continuum models of the electrochemical diffuse layer in electronic-structure calculations.

Continuum electrolyte models represent a practical tool to account for the presence of the diffuse layer at electrochemical interfaces. However, despite the increasing popularity of these in the field of materials science, it remains unclear which features are necessary in order to accurately describe interface-related observables such as the differential capacitance (DC) of metal electrode surfaces. We present here a critical comparison of continuum diffuse-layer models that can be coupled to an atomistic first-principles description of the charged metal surface in order to account for the electrolyte screening at electrified interfaces. By comparing computed DC values for the prototypical Ag(100) surface in an aqueous solution to experimental data, we validate the accuracy of the models considered. Results suggest that a size-modified Poisson-Boltzmann description of the electrolyte solution is sufficient to qualitatively reproduce the main experimental trends. Our findings also highlight the large effect that the dielectric cavity parameterization has on the computed DC values.

Treatment of temperature dependence of interfacial speciation by speciation codes and temperature congruence of oxide surface charge

In several published articles dealing with the temperature dependence of surface charging of and solute adsorption at mineral/water interfaces that involve surface complexation modelling via the purely diffuse layer model or the triple layer model, speciation codes were employed that do not allow specification of temperature in the input and therefore no explicit temperature variation. Omitting temperature variation should cause errors in the calculations. A priori it is expected that the temperature dependence of the dielectric constant of water, changes in volume (and thus molar concentration), and the appearance of absolute temperature in the electrostatic term of surface complexation models cannot be simply neglected. Calculations involving the models without temperature correction and with temperature correction, however, show that in the case of surface charging, no noticeable difference arises. In the present work we show that this arises from compensation of the respective temperature dependencies of the different variables. Our considerations and calculations explain why previous omission of temperature dependence has gone unnoticed. We also discuss temperature congruence of surface charging, i.e. master curves that do not depend on temperature, when the charge is normalized with respect to the point of zero charge at the respective temperatures. We conclude that temperature congruence is probably not a general phenomenon. Consequently, the validity of assuming temperature congruence has to be verified. We suggest a simple way to do so.

Transformation of cadmium-associated schwertmannite and subsequent element repartitioning behaviors.

Schwertmannite is an important sink for cadmium (Cd) in acid mine drainage (AMD) environments and is unstable when environmental conditions change. However, the release and redistribution of Cd during schwertmannite transformation with respect to pre-bound Cd are poorly understood. In this work, the transformation of cadmium-associated schwertmannite and subsequent Cd repartitioning behaviors were investigated. The way of schwertmannite associated with Cd was predominant by absorption, and the diffuse layer model (DLM) showed that Cd2+ existed as monodentate complexes ≡Fe(1)OCd+ and ≡Fe(2)OCd+ on schwertmannite surfaces. Kinetics of SO42- release and mineralogical characterization both showed that the mineral transformation rates decreased and more lepidocrocite aggregated with increasing adsorbed Cd levels. The shrinking core model revealed that Fe(II)-induced process would affect mineral dissolution by changing surface reaction-controlled step to internal diffusion-controlled step, and significantly promote the dissolution rate of Cd-adsorbed schwertmannite. Adsorbed Cd blocked the surface sites for later Fe(II) adsorption and the Fe(II)-Fe(III) electron transfer, then resulted in the decelerated transformation and the accumulation of intermediate phase lepidocrocite. The maximum release of aqueous Cd occurred after 1mM Fe2+ addition, then over 69% of initial added Cd(aq) re-bound to solid-phase accompanying with mineral transformation, and finally, Cd was mainly associated with the secondary minerals by complexation with surficial OH groups. These findings are useful for developing the strategies for treating Cd contamination in AMD affected areas.

Defluoridation by magnesia–pullulan: Surface complexation modeling and pH neutralization of treated fluoride water by aluminum

Abstract The magnesia–pullulan composite (MgOP) achieved effective fluoride removal in previous research. In the present study, an acid-base titration experiment was conducted to investigate the properties of MgOP surface and further explore the mechanism of fluoride adsorption on MgOP. Results showed that the presence of chloride ions could improve fluoride adsorption on MgOP; however, additional nitrate ions had negligible impacts. A diffuse layer model and chemical equilibrium software (Visual MINTEQ 3.1) were used to simulate the acid-base titration data. The effects of initial pH values on the rate of fluoride uptake by MgOP were also studied. Moreover, aluminum salts were added to the fluoride solution with MgOP for the pH neutralization of treated water, in which aluminum chloride was preferred.

Sensitivity and Selectivity of Ion-Selective Electrodes Interpreted Using the Nernst-Planck-Poisson Model.

The Nernst-Planck-Poisson model is used for modeling the sensitivity and selectivity of ion-selective electrodes (ISEs) with plastic membranes. Two pivotal parameters characterizing ISE response are in focus: sensitivity and selectivity. An interpretation of sensitivity, which considers the concurrent influence of anions and cations on the ISE slope, is presented. The interpretation of selectivity shows the validity and limits of approaches hitherto taken to measure the true (unbiased) selectivity coefficient. The validity of more idealized interpretations by the diffusion-layer model is conceived.

Improvement of the condensation heat transfer model of the MARS-KS1.3 code using a modified diffusion layer model

The thermal-hydraulic system code, MARS-KS1.3, tends to underestimate the condensation heat transfer under the presence of non-condensable gases. To improve its condensation heat transfer model, we adopted the Herranz's diffusion layer model and further developed it, mainly focusing on thermal-hydraulic conditions in a nuclear containment during a hypothetical accident. Two key modifications include (i) the correlations to calculate heat and mass transfer on a vertical external surface for various flow conditions are adopted and (ii) a turbulent diffusion coefficient is applied to consider the effect of turbulence on mass diffusion.The modified diffusion layer model was implemented into MARS-KS1.3 and it has been validated using 157 condensation experiments from six different facilities. By using the turbulent diffusion coefficient in the modified diffusion layer model, the effect of turbulence on mass diffusion and then condensation was captured very well. For most cases, the results of the modified model are in a better agreement with the experimental data, resulting in a root-mean-square error of 21.3%. It is also shown that the modified diffusion layer model can predict local condensation heat transfer change along the condensation surface well.

The dissolution kinetics of industrial brine sludge wastes from a chlor-alkali industry as a sorbent for wet flue gas desulfurization (FGD)

ABSTRACTThe disposal of industrial brine sludge waste (IBSW) in chlor-alkali plants can be avoided by utilization of IBSW as a sorbent in wet flue gas desulfurization (FGD). The shrinking core model was used to determine the dissolution kinetics of IBSW, which is a vital step in wet FGD. The effects of solid-to-liquid ratio (m/v), temperature, pH, particle size, and stirring speed on the conversion and dissolution rate constant are determined. The conversion and dissolution rate constant decreases as the pH, particle size, and solid-to-liquid ratio are increased and increases as the temperature, concentration of acid, and stirring speed are increased. The sorbents before and after dissolution were characterized using x-ray fluorescence (XRF), x-ray diffraction (XRD), and scanning electron microscopy (SEM). An activation energy of 7.195 kJ/mol was obtained and the product layer diffusion model was found to be the rate-controlling step.Implications: The use of industrial brine sludge waste as an alternative sorbent in wet flue gas desulfurization can reduce the amounts of industrial wastes disposed of in landfills. This study has proved that the sorbent can contain up to 91% calcium carbonate and trace amounts of sulfate, magnesium, and so on. This can be used as new sorbent to reduce the amount of sulfur dioxide in the atmosphere and the by-product gypsum can be used in construction, as a plaster ingredient, as a fertilizer, and for soil conditioning. Therefore, the sorbent has both economic and environmental benefits.

Effect of pH on adsorption of anionic surfactants on limestone: Experimental study and surface complexation modeling

We investigate surfactant adsorption on a model carbonate rock over a wide range of pH and surfactant-to-solid ratios, by both an experimental and a theoretical approach, to obtain a quantitative understanding of how mineral constituents affect the adsorption equilibrium and dynamics. A combination of bulk mineralogy, elemental composition analysis, dissolution behavior, and water ionic composition data were used to characterize the samples’ surface heterogeneity. The adsorption of anionic surfactants is 2.4mg/m2 at pH of ∼8 and decreases approximately linearly with pH values above 9. To constrain and compare the relative adsorption affinity and the likely modes of attachment on mineral constituents as pH changes, we performed surface complexation calculations using a two-surface multisite diffuse layer model. We propose the formation of two surface species on both the major (i.e., calcite) and trace (i.e., the oxide-like sites on the edges of clay platelets) minerals: a monodentate inner-sphere complex and a weak or hydrogen bonding complex. Our modeling results suggest that charge-regulated inner-sphere complexation is the dominant adsorption mechanism on the calcite and oxide-like sites at low pH values regardless of the surface loading. We found weak or hydrogen bond adsorption to be significant on the calcite surface, and this became the dominant adsorption mode at pH ∼10. While the adsorption on calcite increases with surface loading, adsorption on the oxide-like sites remains independent of surface loading. These results suggest that surfactant adsorption can be comparable on the abundant low-surface-area calcite and trace high-surface-area oxide-like sites.