Aqueous Activity Coefficients Research Articles

Numerical Modeling of CO2, Water, Sodium Chloride, and Magnesium Carbonates Equilibrium to High Temperature and Pressure

In this work, a thermodynamic model of CO2-H2O-NaCl-MgCO3 systems is developed. The new model is applicable for 0–200 °C, 1–1000 bar and halite concentration up to saturation. The Pitzer model is used to calculate aqueous species activity coefficients and the Peng–Robinson model is used to calculate fugacity coefficients of gaseous phase species. Non-linear equations of chemical potentials, mass conservation, and charge conservation are solved by successive substitution method to achieve phase existence, species molality, pH of water, etc., at equilibrium conditions. From the calculated results of CO2-H2O-NaCl-MgCO3 systems with the new model, it can be concluded that (1) temperature effects are different for different MgCO3 minerals; landfordite solubility increases with temperature; with temperature increasing, nesquehonite solubility decreases first and then increases at given pressure; (2) CO2 dissolution in water can significantly enhance the dissolution of MgCO3 minerals, while MgCO3 influences on CO2 solubility can be ignored; (3) MgCO3 dissolution in water will buffer the pH reduction due to CO2 dissolution.

Extended Deep Earth Water Model for predicting major element mantle metasomatism

Abstract Fluids in the deep crust and upper mantle appear to have played roles in the long-term evolution of the subcratonic lithospheric mantle and the stabilities of the continents, in the geochemical cycles of the elements from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of fluid inclusions in diamonds and xenoliths. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Traditional models have used a C-O-H-type of model, which contains no major rock-forming elements or aqueous ions or metal-complexes. In the present study, we use experimentally measured solubility data for multicomponent K-free eclogite, K-free peridotite and K-bearing peridotite rocks at upper mantle conditions from the literature to construct aqueous speciation solubility models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, and C in an extended Deep Earth Water (DEW) model. New equilibrium constants were retrieved for the aqueous bisilicate anion, a silica trimer, silicate complexes of Ca, Fe, and Al, a silicate complex of Mg and bicarbonate, and formate complexes of Fe and Ca. The aqueous speciation and solubility model also took account of decreases in the activity of water and aqueous activity coefficients of neutral dissolved gases and included consideration of H 2 CO 3 0 . Based on the temperature and pressure dependences of the equilibrium constants, and supporting data covering a wide range of conditions, we then developed aqueous equation of state characterizations of the ions and metal-complex species. Overall, the results form a basis for modeling fluid-rock interactions under upper mantle conditions consistent with experimental solubility measurements.

Thermodynamic modelling of fluids from surficial to mantle conditions

Carbon is subducted to depths where metamorphism liberates water-bearing fluids. The C-bearing fluids facilitate partial melting of the upper mantle, generating magmas that may erupt as arc volcanics. Degassing of the magmas releases CO 2 and other volatile species to the atmosphere. Over geological time, this process contributes to the composition of the atmosphere and planetary habitability. Here I summarize the background needed to carry out theoretical geochemical modelling of fluids and fluid–rock interactions from surficial conditions into the upper mantle. A description of the general criteria for predicting equilibrium and non-equilibrium chemical reactions is followed by a summary of how the thermodynamic activities of species are related to measurable concentrations through standard states and activity coefficients. Specific examples at ambient conditions involving dilute water are detailed. The concept of aqueous speciation and how it can be calculated arises from this discussion. Next, I discuss how to calculate standard Gibbs free energies and aqueous activity coefficients at elevated temperatures and pressures. The revised Helgeson–Kirkham–Flowers equations of state are summarized and the revised predictive correlations for the estimation of equation of state coefficients in the Deep Earth Water (DEW) model are presented. Finally, the DEW model is applied to the solubility and speciation of aqueous aluminium.

Mechanistic Model Describes Wettability Change in Sandstones and Carbonates

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 190236, “Modeling Wettability Change in Sandstones and Carbonates Using a Surface-Complexation-Based Method,” by Aboulghasem K.N. Korrani, SPE, and Gary R. Jerauld, SPE, BP, prepared for the 2018 SPE Improved Oil Recovery Conference, Tulsa, 14–18 April. The paper has not been peer reviewed. Previously proposed models of wettability change have not been tied to the chemistry that controls wettability but instead were driven by simplistic criteria such as salinity level or concentration of an adsorbed species. In this paper, after testing proposed models in the literature on sandstones and carbonates, the authors propose a mechanistic surface-complexation-based model that describes observations quantitatively for ionically treated waterfloods. To the best of the authors’ knowledge, this is the first surface-complexation-based model that describes fully ionic compositional dependence observed in ionically treated waterfloods in both sandstones and carbonates. Introduction While some debate remains about the underlying mechanisms of ionically tuned waterflooding, the geochemical reactions that control the wetting of crude oil on the rock surface are likely to be central to a detailed description of the process. Models of wettability change often have been simplistic and not tied to the chemistry that controls wettability, instead driven by a simplistic criterion such as salinity level or concentration of an adsorbed species. Such models are inadequate for modeling the effect of compositional changes in brine, which is key to optimizing ionic design. One problem has been the lack of reservoir models that included geochemistry. The oilfield reactive-transport simulators are simplistic; in these, either aqueous reactions are assumed to be ideal (i.e., with aqueous activity coefficients of unity) or they lack important geochemical features such as kinetics or surface-complexation reactions. In a previous work, the authors assumed that decrease in the total ionic strength is the driving mechanism for wettability change during low-salinity waterflooding. The reason was that the total ionic strength is the controlling parameter for the double-layer expansion. Double-layer thickness increases as the brine total ionic strength decreases. The thickness of the double layer is an inverse function of the square root of the total ionic strength. However, the proposed model was too simple and was unable to predict many low-salinity or modified-salinity waterflooding observations, such as the detrimental effect of divalents on oil recovery in sandstones, the positive effect of sulfate in chalks, the positive effect of high pH in sand-stones, and the detrimental effect of sodium chloride in chalks. In this study, the authors identified and tested an alternative approach based on a stability analysis of thin water film, which yields a dimensionless group, a ratio of electrostatic-to-Van der Waals (VdW) forces called the stability number. The stability number can be used as a measure of wettability. The authors use simulation software to model surface reactions that can estimate surface charge and potential from surface-complexation models, demonstrating that this process can be used in a model to match a wide variety of data in sandstones and carbonates found in models in the literature. The complete paper provides details of these comparisons.

Lumping Analysis for Sorption of Neutral Organic Compounds in Mixtures to Simulated Aquifer Sorbents

The concept of using lumping analysis to reduce the complexity associated with the sorption of 12 neutral organic compounds (NOCs) comprising complex mixtures to simulated aquifer sorbents was evaluated. The sorbates covered a wide range of octanol-water partitioning coefficients, Kow (i.e., � 0:24 ≤ logKow ≤ 4:23), and the sorbents included different types of mineral surfaces and humic acid- mineral complexes with different fractions of organic carbon, f oc (i.e., 0:006% ≤ f oc ≤ 0:221%). Both a priori lumping criteria (i.e., the aqueous activity coefficient at saturation γ sat , and the organic carbon partitioning coefficient, Koc) and experimentally derived lumping criteria (i.e., Freundlich sorption parameters) were evaluated as potential lumping criteria. The results indicated that the sorption behavior of the 12 NOCs contained in mixtures could be approximated reasonably well by lumping them into a fewer number of pseudocompounds (four to six) with similar sorption behaviors based on Freundlich sorption parameters, and that both γ sat and Koc could be used as viable a priori lumping criteria. However, the number and compositions of the pseudocompounds were a complex function of the hydrophobicity (i.e., γ sat and Koc) of the NOCs and varied as a function of the type of mineral surface (e.g., uncoated, α-FeOOH-coated, and Al2O3-coated sands) and the f oc of the sorbent. DOI: 10.1061/(ASCE)EE.1943-7870.0000498. © 2012 American Society of Civil Engineers. CE Database subject headings: Organic carbon; Organic compounds; Organic matter; Pollutants; Soil pollution; Sorption; Subsurface environment. Author keywords: Lumping analysis; Mixtures; Neutral organic compounds; Pseudocompounds; Simulated aquifer sorbent; Sorption.

Prediction of aqueous solubility from SCRATCH

This study proposes the SCRATCH model for the aqueous solubility estimation of a compound directly from its structure. The algorithm utilizes predicted melting points and predicted aqueous activity coefficients. It uses two additive, constitutive molecular descriptors (enthalpy of melting and aqueous activity coefficient) and two non-additive molecular descriptors (symmetry and flexibility). The latter are used to determine the entropy of melting. The melting point prediction is trained on over 2200 compounds whereas the aqueous activity coefficient is trained on about 1640 compounds, making the model very rigorous and robust. The model is validated using a 10-fold cross-validation on a dataset of 883 compounds for the aqueous solubility prediction. A comparison with the general solubility equation (GSE) suggests that the SCRATCH predicted aqueous solubilities have a slightly greater average absolute error. This could result from the fact that SCRATCH uses two predicted parameters whereas the GSE utilizes one measured property, the melting point. Although the GSE is simpler to use, the drawback of requiring an experimental melting point is overcome in SCRATCH which can predict the aqueous solubility of a compound based solely on its structure and no experimental values.

Prediction or partition coefficients and activity coefficients of two branched compounds using COSMO therm

As our entry for the 5th industrial fluid property simulation challenge, the COSMO-RS method in its COSMO therm implementation has been used to predict the octanol–water partition coefficients and the aqueous activity coefficients of 1-ethylpropylamine, and 3-methyl-1-pentanol. Experimental data for similar compounds have been used in order to fine-tune the predictions.

Performance of a nanofiltration membrane for removal of ethanol from aqueous solutions by pervaporation

In this study, the performance of a hydrophobic nanofiltration membrane (SolSep 3360) for treating alcoholic solutions by pervaporation is investigated and compared to a conventional pervaporation membrane (PV 1070, Sulzer Chemtech and Pervatech PDMS). Both binary ethanol/water mixtures and common multicomponent mixtures (alcoholic beverages) are examined. The experiments were performed at feed ethanol concentrations up to 50 vol% and at temperatures up to 45 °C. The effects of feed ethanol content and temperature were studied in terms of: (1) fluxes and permeances of individual components, and (2) separation factor, enrichment factor and selectivity of ethanol to water. Using permeance and selectivity instead of flux and separation/enrichment factor allows the effects on performance evaluation of operating conditions, such as temperature and swelling, to be decoupled. In this way the contribution by nature of the membrane to separation performance can be clarified and quantified. In addition, previous analyses indicate that the aqueous activity coefficient and the saturated vapour pressure play an important role when evaluating the membrane performance in terms of permeance and selectivity. This is confirmed by this study. Furthermore, it was found that multicomponent alcoholic beverages behave in exact the same manner as binary ethanol/water mixtures. Using a nanofiltration membrane for pervaporation purposes is a suitable possibility, because of the higher fluxes and permeances, while remaining a good separation factor and selectivity. The difference between nanofiltration and pervaporation membranes is explained by the influence of swelling, making the membrane more dense, and the different interactions between permeating molecules and the membrane.

Pervaporation study on the dehydration of aqueous butanol solutions: a comparison of flux vs. permeance, separation factor vs. selectivity

Dehydration of four butanol isomers from their aqueous solutions by pervaporation was studied using a commercial hydrophilic membrane with feed water concentration less 20 wt% at 60 °C to 100 °C. The effects of feed water content on membrane separation performance were studied in terms of: (1) fluxes and permeances of individual components, and (2) separation factor and selectivity of water to butanol. Efforts were devoted to compare and discuss the advantages and disadvantages of using flux or permeance, and separation factor or selectivity as analysis parameters. Using permeance and selectivity instead of flux and separation factor can significantly decouple the effect of operating conditions on performance evaluation, while clarify and quantify the contribution by the nature of the membrane to separation performance. The effects of temperature on polymeric membranes appear to be significantly different when one plots permeance instead of flux versus feed water content at different temperatures, implying that the traditional flux plots might somewhat mislead the analysis. In addition, analyses indicate that aqueous activity coefficient and saturated vapor pressure appear to play important roles when one evaluates the membrane performance in terms of permeance and selectivity. The effects of temperature and feed water concentration on permeance and selectivity can be elucidated nicely with the aid of the known relationship among activity coefficient, saturated vapor pressure, temperature, and feed water concentration.

Testing a surface tension-based model to predict the salting out of polycyclic aromatic hydrocarbons in model environmental solutions

Molar-based Setschenow constants ( K s) for six alkali and alkaline earth metal-based inorganic salts were determined at 20°C to evaluate their influence on the solubilities, and thus the aqueous activity coefficients, of three polycyclic aromatic hydrocarbons (PAHs). The six salts tested exhibited a wide range of K s values, varying from 0.105±0.009 M −1 (for NaClO 4 and pyrene) to 1.29±0.17 M −1 (for K 2SO 4 and perylene). In general, salting out effects with these electrolytes were observed in the order Ca 2+>Na +>K + and SO 4 2−>Cl −>ClO 4 −, consistent with previous reports. However, the expected salting out trend of perylene > pyrene > naphthalene was only observed with K 2SO 4. In CaCl 2 solutions, the K s value of pyrene was significantly lower than that of naphthalene. For NaCl, KCl and NaClO 4, pyrene K s values were found to be lower than, but not significantly different from, those of naphthalene. Setschenow constants for all six salts were predicted using a semi-empirical, thermodynamically-based equation that relates the standard free energy change associated with transferring solutes from water to a salt solution to the difference in surface tensions between the two solutions. With this equation, predicted K s values were in reasonable agreement with observed K s values (generally within±50%). Lack of better agreement between predicted and observed values likely reflects the inability of the simple surface tension model to account for all interactions among the cations, anions, PAH molecules and water molecules in the respective systems.

Estimation of Aqueous Solubility By The General Solubility Equation (GSE) The Easy Way

AbstractThe General Solubility Equation (GSE) provides a simple method of estimating the molar aqueous solubility of an organic non‐electrolyte in water (Sw) as a function of its celsius melting point (MP) and octanol‐water partition coefficient (Kow): log Sw = − 0.01 (MP‐25) − log Kow + 0.5 The melting term of the GSE is based upon the Clausius‐Clapyron equation and Walden's rule. The aqueous activity coefficient is assumed to be the reciprocal of the octanol‐water partition coefficient. The constant is based upon the molarity of pure octanol. There are no fitted parameters in the GSE. Extension of the GSE to weak electrolytes in buffered aqueous solutions is straightforward. The concentration of the ionized species, Si, is accounted for by incorporating one additional term, which contains the pKa of the solute and pH of the solution. For a weak acid, Stotal = Sw + Si = Sw [1 + 10(pH‐pka)] The solubility of a weak electrolyte in unbuffered waster requires further consideration because the solute will determine the pH of the solution. It is shown that in unbuffered media Stotal = Sw + Si = Sw + (SwKa)1/2 Thus, it is not necessary to explicitly know the pH of the saturated solution to estimate the solubility of a weak electrolyte in water. The GSE is validated on data set of over a thousand compounds, covering a wide range of structural categories. The GSE is compared to a number of other solubility estimation techniques using the criteria of accuracy of fit, applicability, parsimony, conveniences, and elegance.

Molecular hologram derived quantitative structure–property relationships to predict physico-chemical properties of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) congeners with various degrees of chlorination and substitution patterns are among the most widespread and persistent man-made organic pollutants. They are toxic, lipophilic and tend to be bioaccumulated. The knowledge of the physico-chemical properties is very useful to explain the environmental behavior of PCBs and to perform an exposure assessment. In this paper, we have used a new molecular representation, the molecular hologram, to generate quantitative structure–property relationship models to predict the physico-chemical properties of biphenyl and all of its chlorinated congeners. The investigated properties include 1-octanol/water partition coefficient (log K ow), aqueous solubility (−log S w), aqueous activity coefficient (−log Y w), Total molecular surface area, Henry’s law constant (log H). The results show that this new quantitative structure–activity relationship approach presents highly predictive models for important physico-chemical properties of PCBs.

Thermodynamic Modeling of the Adsorption of Radionuclides on Selected Minerals. II: Anions

A surface complexation model based on the diffuse-layer theory has been applied to study the adsorption of radionuclide anions (I-, IO3-, SeO32-, SeO42-, and TcO4-) on various minerals. The model is coupled with an aqueous activity coefficient model based on the B-dot equation and takes into account the effect of aqueous speciation on adsorption. Binding constants for the selected radionuclide anions on minerals have been determined. In all examined cases, the model is shown to accurately represent the adsorption data. The selected radionuclide anions have been identified as potentially significant contributors for dosing in high-level nuclear waste because of their high solubilities and reduced interactions with aquifer minerals; the methodology presented in this work can be generalized to the study of ion adsorption effects at water−mineral interfaces in natural systems that implicitly regulate the fate of environmental contaminants.

Aqueous coefficient calculations for chemicals and drugs

Aqueous functional group activity coefficients (AQUAFAC) is a group‐contribution method for estimating the aqueous coefficients. We have written a program for the calculation of these coefficients. The solubility S w of alkanes shows variation of 8 orders of magnitude. The comparison with experiment shows that AQUAFAC gives good S w estimations. For 4'‐substituted acetanilides, I‐, Br‐, nitro‐, Cl‐, F‐ and methoxy‐substituents decrease S w, while formyl‐ and amino‐substituents increase S w. For acetaminophen esters, S w decreases from the acetate to the decanoate. The S w of 29 barbiturates shows typical errors of 0.4 log S w units. For the cyclo‐alkane‐l’,5‐spirobarbituric acids, S w decreases from the cyclopropane to cyclooctane.

GEOSURF: a computer program for modeling adsorption on mineral surfaces from aqueous solution

A new program, GEOSURF, has been developed for calculating aqueous and surface speciation consistent with the triple-layer model of surface complexation. GEOSURF is an extension of the original programs MINEQL, MICROQL and HYDRAQL. We present, here, the basic algorithm of GEOSURF along with a description of the new features implemented. GEOSURF is linked to internally consistent data bases for surface species (SURFK.DAT) and for aqueous species (AQSOL.DAT). SURFK.DAT contains properties of minerals such as site densities, and equilibrium constants for adsorption of aqueous protons and electrolyte ions on a variety of oxides and hydroxides. The Helgeson, Kirkham and Flowers version of the extended Debye-Huckel Equation for 1:1 electrolytes is implemented for calculating aqueous activity coefficients. This permits the calculation of speciation at ionic strengths greater than 0.5 M. The activity of water is computed explicitly from the osmotic coefficient of the solution, and the total amount of electrolyte cation (or anion) is adjusted to satisfy the electroneutrality condition. Finally, the use of standard symbols for chemical species rather than species identification numbers is included to facilitate use of the program. One of the main limitations of GEOSURF is that aqueous and surface speciation can only be calculated at fixed pH and at fixed concentration of total adsorbate. Thus, the program cannot perform reaction-path calculations: it cannot determine whether or not a solution is over- or under-saturated with respect to one or more solid phases. To check the proper running of GEOSURF, we have compared results generated by GEOSURF with those from two other programs, HYDRAQL and EQ3. The Davies equation and the “bdot” equation, respectively, are used in the latter two programs for calculating aqueous activity coefficients. An example of the model fit to experimental data for rutile in 0.001 M–2.0 M NaNO 3 is included.

ZIRCONIUM EXTRACTION INTO OCTYL(PHENYL)-N,N- DIISOBUTYLCARBAMOYLMETHYL PHOSPHINE OXIDE AND TRIBUTYL PHOSPHATE†

ABSTRACT Classical slope analysis techniques were used to determine the octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and nitrate stoichiometrics for the extraction of zirconium by CMPO diluted with diisopropylbenzene (DIPB). The equilibrium constant for the extraction of zirconium by CMPO was also determined using classical slope analysis techniques. The extraction of zirconium by TBP in n-dodecane was used as a control to verify the zirconium species as Zr+4, and to verify the experimental methodology. Equilibrium [CMPO]org and [TBP]org concentrations were determined by accounting for the extraction of HNO3 into both TBP and CMPO solvents. Nitric acid dissociation and aqueous phase activity coefficients were also taken into consideration. Organic activity coefficients, Zr+4 activity coefficients, Zr+4 hydrolysis, and consumption of TBP or CMPO by water were neglected. Nitrate and CMPO dependencies for the extraction of zirconium have been determined from this work to be: An equilibrium constant of 1.13 × 105 ± 1.48 × 104 at 25° C was also determined for this reaction.

3D-modelling and prediction by WHIM descriptors. Part 9. Chromatographic relative retention time and physico-chemical properties of polychlorinated biphenyls (PCBs)

Physico-chemical properties of polychlorinated biphenyls (PCBs) congeners have been extensively studied searching for quantitative structure–property relationships (QSPR). The chemical description of PCBs structure is made in terms of WHIM descriptors, which are 3D molecular descriptors taking into account size, shape, symmetry and atom distribution of the molecules. The regression models have been obtained by optimizing their prediction power and by selecting the best subset of descriptors by genetic algorithm. The results confirm the capability of this approach to give predictive models for important physico-chemical properties, such as relative retention time, log K ow, melting point, total surface area, Henry's law constant, solubility, and aqueous activity coefficients.

Ionic Strength Dependence of Transition and Heavy Metal Adsorption onto Metal Oxides

Transition and heavy metal adsorption onto metal oxides influences the global cycling of metals, plays a role in the formation of enriched metal deposits, and filters natural and waste waters in the subsurface environment. The ionic strength dependence of metal adsorption is important to geochemists because natural waters are composed of different electrolyte types and vary widely in ionic strength. It is generally accepted that transition and heavy metal adsorption onto metal oxides does not exhibit an ionic strength dependence and that this behaviour indicates that metals bind to solid surfaces as inner-sphere complexes. However, Fig. 1 illustrates that while Cd adsorption from NaNO3 solutions onto goethite (Hayes and Leckie, 1987) does not exhibit major variations with ionic strength, Cd adsorption from NaCI solutions onto y-alumina (Kosmulski, 1996) progressively decreases with increasing ionic strength, and Cd adsorption from NaCIO4 solutions onto 7-alumina (Kosmulski, 1996) may increase with increasing ionic strength. An analysis of these datasets and many others reported in the literature indicates that transition and heavy metal adsorption is very strongly dependent on the nature of the electrolyte, and may increase, decrease, or exhibit little variation with increases in the ionic strength of the solution. In this study, experimental data for the adsorption of numerous transition and heavy metals onto quartz, silica, alumina, corundum, goethite, anatase, magnetite and manganese dioxide, from NaC104, NaNO3, KNO3, and NaC1 solutions, over a wide range of ionic strengths, have been fit using the triple layer model of Sahai and Sverjensky (1997), and at most two metal surface complexes. This model accounts for the ionic strength dependence of aqueous ion activity coefficients through the use of the extended Debye-Huckel expression. It is accompanied by a database of surface site densities, capacitances, and equilibrium constants for surface protonation, deprotonation, and electrolyte adsorption, which was modified to accommodate reported surface titration data and experimentally-determined points of zero charge. Recent applications of the triple layer model have suggested that divalent transition and heavy metals (M 2+) form simple surface species such as >SOM + in nitrate solutions (Hayes and Leckie, 1987; Katz and Hayes, 1995). However, using the triple-layer model of Sahai and Sverjensky (1997), surface complexes of the form >SOHM2+_NO3, where M 2+ is bound to a neutral surface site (>SOH) on the 0-plane of the triple layer model and NO3 is on the ]3-plane, are found to best fit data collected over a wide range of ionic strengths for Cd and Pb adsorption onto goethite (Hayes and Leckie, 1987) and Co adsorption onto corundum (Katz and Hayes, 1995). A similar type of surface complex >SOM+_NO3 fits the adsorption data for UO2 onto hydrous ferric oxide at pH values where the system can be assumed to be depleted in CO2. A combination of two complexes, including >SOHM2+_NO3 was found to fit Zn adsorption onto anatase (James and MacNaughton, 1977). In addition to sharing a common electrolyte, the systems described above have several other common characteristics: (a) the solids have intermediate-range dielectric constants between 10 and 22, (2) metal adsorption occurs on positively-charged surfaces, and (3) MNOfis the predominant aqueous complex over the pH range of adsorption and the range of ionic strengths studied. The one exception to (3) is Zn adsorption onto anatase, which required a second surface complex, >SOZnOH, to fit the adsorption data at low ionic strengths, where ZnOH-is the predominant aqueous complex. Adsorption of the predominant aqueous complex rather than the more abundant bare metal ion may be favoured because the free energy of solvation is smaller for a monovalent cationic complex (e.g. MNO3 + or MOH § than for a divalent cation (i.e. M 2+) (James and Healy, 1972). Metal-ligand complexation is also favoured in low dielectric constant environments like the interface. Metal adsorption data over a wide range of ionic strengths in NaC1 and NaC104 solutions are more limited. Calculations for Cd adsorption from NaC1 and NaC104 solutions onto 7-alumina are best matched by calculations using the surface complexes,

Evaluation of internally consistent parameters for the triple-layer model by the systematic analysis of oxide surface titration data

Systematic analysis of surface titration data from the literature has been performed for ten oxides (anatase, hematite, goethite, rutile, amorphous silica, quartz, magnetite, δ-MnO 2, corundum, and γ-alumina) in ten electrolytes (LiNo 3, NaNO 3, KNO 3, CsNO 3, LiCl, NaCl, KCl, CsCl, NaI, and NaClO 4) over a wide range of ionic strengths (0.001 M–2.9 M) to establish adsorption equilibrium constants and capacitances consistent with the triple-layer model of surface complexation. Experimental data for the same mineral in different electrolytes and data for a given mineral/ electrolyte system from various investigators have been compared. In this analysis, the surface protonation constants (K,, and K s,2) were calculated by combining predicted values of Δ pK( log K s,2 − log K s,1) (Sverjensky and Sahai, 1996) with experimental points of zero charge; site-densities were obtained from tritium-exchange experiments reported in the literature, and the outer-layer capacitance (C 2) was set at 0.2 F·m −2. This scheme permitted us to retrieve consistent sets of values for the inner layer capacitance (C 1), and for the electrolyte adsorption constants ( K s,L − and K s,M + ) corresponding, respectively, to the equilibria > SOH 2 + + L aq − = > SOH 2 + − L − and > SO − + M aq + = > SO − − M + Aqueous activity coefficients were calculated using the extended Debye-Huckel equation (Helgeson et al., 1981), which is valid to high ionic strengths (>0.5 M). Systematic analysis of the data reveals important trends and differences between triple-layer model predictions and experimental data and between data for the same mineral/ electrolyte from different investigators. Furthermore, the analysis yields an internally consistent set of triple-layer parameters which will be used in developing a predictive model for electrolyte adsorption based on Born solvation and electrostatic theory (Sahai and Sverjensky, 1997a).

Sorption of Nonionic, Hydrophobic Organic Chemicals to Mineral Surfaces

Equilibrium sorption coefficients (Kd) of a series of polycyclic aromatic hydrocarbons (PAHs) and chlorobenzenes were measured on the pure mineral oxides α-Al2O3 and α-Fe2O3 using column chromatography. Sorbent surface area did not account for differences in the measured distribution coefficients (Kd) for a given compound on α-Fe2O3 and α-Al2O3. This observation indicates that the type of mineral surface is important in the sorption of nonionic, hydrophobic organic chemicals (HOCs). On both minerals, sorption was reversible as sorption isotherms of the PAHs were linear to at least 50% of solute aqueous solubility and the measured enthalpy of sorption similar to the compounds negative enthalpy of dissolution. Solution pH and the sign and magnitude of sorbent surface charge did not affect sorption, nor did a factor 50 change in ionic strength. The relationship between Kd and the aqueous activity coefficient, γsatw , was log Kd = 0.70 log γsatw + −10.68R 2 = 0.94 on α-Al2O3 and log Kd = 0.98 log γsatw + −11....