Speciation Reactions Research Articles

Modelling the phase and chemical equilibria of aqueous solutions of alkanolamines and carbon dioxide using the SAFT-γ SW group contribution approach

The speciation reactions that take place in mixtures of water (H2O), carbon dioxide (CO2), and alkanolamines make the modelling of the chemical and fluid-phase equilibria of these systems challenging. We demonstrate for the first time that the statistical associating fluid theory (SAFT), formulated within a group-contribution (GC) framework based on transferable intermolecular square-well (SW) potentials (SAFT-γ SW), can be used to model successfully such complex reacting systems. The chemical reactions in these mixtures are described via a physical association model. The concept of second-order groups is introduced in the SAFT-γ SW approach in order to deal with the multifunctional nature of the alkanolamines. In developing the models, several compounds including ethylamine, propylamine, ethanol, propanol, 2-aminoethanol, and 3-amino-1-propanol are considered. We present calculations and predictions of the fluid-phase behaviour of these compounds and a number of their aqueous mixtures with and without CO2. The group-contribution nature of the models is used to predict the absorption of CO2 in aqueous solutions of 5-amino-1-pentanol and 6-amino-1-hexanol. The proposed predictive approach offers a robust platform for the identification of new solvents and mixtures that are viable candidates for CO2 absorption, thereby guiding experimental studies.

Comment on “Spectroscopic studies of oxygen speciation in potassium silicate glasses and melts”

Recent O1s XPS studies suggest that significant free oxide (O2– or K–O–K) is present in potassium silicate glasses, in contrast with what was concluded from 29Si and 17O solid-state NMR data. An alternative peak assignment of the Qn peaks in the 29Si NMR spectra was proposed to bring the 29Si NMR data in line with the O1s data, but this reassignment (i) is not supported by any evidence other than the resulting agreement between the NMR and XPS data, (ii) is at odds with the spectral properties of the bands (peak position and width), (iii) ignores the strong evidence for the original peak assignment, and (iv) results in highly implausible equilibrium constants for the Qn speciation reactions. More likely causes for the apparent discrepancy between the XPS and NMR data are the incorrect estimation of the precision of the bridging oxygen content from the O1s XPS data, the systematic overestimation of the bridging oxygen content from the O1s XPS data, and (or) the incorrect estimation of the accuracy of the compositional analysis, which was based on the precision rather than the accuracy of the electron microprobe analysis. Thus, the available evidence strongly suggests that the original assignment of the 29Si NMR spectra is correct. Neither the 29Si NMR nor the O1s XPS data support the presence of significant amounts of free oxide in potassium silicate glasses with K2O/SiO2 ≪ 2.

PH control in biological systems using calcium carbonate.

Due to its abundance, calcium carbonate (CaCO3) has high potentials as a source of alkalinity for biotechnological applications. The application of CaCO3 in biological systems as neutralizing agent is, however, limited due to potential difficulties in controlling the pH. The objective of the present study was to determine the dominant processes that control the pH in an acid-forming microbial process in the presence of CaCO3. To achieve that, a mathematical model was made with a minimum set of kinetically controlled and equilibrium reactions that was able to reproduce the experimental data of a batch fermentation experiment using finely powdered CaCO3. In the model, thermodynamic equilibrium was assumed for all speciation, complexation and precipitation reactions whereas, rate limited reactions were included for the biological fatty acid production, the mass transfer of CO2 from the liquid phase to the gas phase and the convective transport of CO2 out of the gas phase. The estimated pH-pattern strongly resembled the measured pH, suggesting that the chosen set of kinetically controlled and equilibrium reactions were establishing the experimental pH. A detailed analysis of the reaction system with the aid of the model revealed that the pH establishment was most sensitive to four factors: the mass transfer rate of CO2 to the gas phase, the biological acid production rate, the partial pressure of CO2 and the Ca(+2) concentration in the solution. Individual influences of these factors on the pH were investigated by extrapolating the model to a continuously stirred-tank reactor (CSTR) case. This case study indicates how the pH of a commonly used continuous biotechnological process could be manipulated and adjusted by altering these four factors. Achieving a better insight of the processes controlling the pH of a biological system using CaCO3 as its neutralizing agent can result in broader applications of CaCO3 in biotechnological industries.

Extracting local surface charges and charge regulation behavior from atomic force microscopy measurements at heterogeneous solid-electrolyte interfaces.

We present a method to determine the local surface charge of solid-liquid interfaces from Atomic Force Microscopy (AFM) measurements that takes into account shifts of the adsorption/desorption equilibria of protons and ions as the cantilever tip approaches the sample. We recorded AFM force distance curves in dynamic mode with sharp tips on heterogeneous silica surfaces partially covered by gibbsite nano-particles immersed in an aqueous electrolyte with variable concentrations of dissolved NaCl and KCl at pH 5.8. Forces are analyzed in the framework of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in combination with a charge regulation boundary that describes adsorption and desorption reactions of protons and ions. A systematic method to extract the equilibrium constants of these reactions by simultaneous least-squared fitting to experimental data for various salt concentrations is developed and is shown to yield highly consistent results for silica-electrolyte interfaces. For gibbsite-electrolyte interfaces, the surface charge can be determined, yet, an unambiguous identification of the relevant surface speciation reactions is not possible, presumably due to a combination of intrinsic chemical complexity and heterogeneity of the nano-particle surfaces.

The nearly complete dissociation of water in glasses with strong aluminum avoidance

Water is dissolved in silicate glasses and melts as hydroxyl groups and molecular water, with mostly hydroxyl groups at low water contents and mostly molecular water at high water contents. However, we recently predicted that water will be dissociated nearly completely in potassium aluminosilicate glasses with more alumina than silica because of the strong aluminum avoidance and the strong tendency for Al-O-Al linkages to hydrolyze in such glasses. In the present study, I test this prediction on hydrous K 2 Al 2 SiO 6 glasses: the Raman and infrared absorption spectra show that water is indeed predominantly present as hydroxyl groups, even for glasses with more than 7 wt% water. This observation validates the previously proposed speciation reactions, demonstrates that variations in water speciation are related to the nature of the cations to which the hydroxyl groups are bonded, and indicates that the classical picture of water dissolution, with predominantly molecular water at high water contents, may not apply near compositional extremes.

Thermodynamic Modeling of Multi-Staged Extraction Systems for Chiral Separations through Coupled Analysis of Species Equilibria and Mass Transfer

A flexible and comprehensive model for predicting and optimizing separations of racemates of amino acids and other chiral metabolites in liquid-liquid multi-staged extraction systems is presented. Enantiomer partition coefficients are computed along the extraction path using multiple chemical equilibria theory and measured equilibrium formation constants for every complex formed in the two phases. The large number of speciation reactions typically occurring in ligand-exchange extraction systems requires the development of a robust numerical algorithm, and we present a method to rapidly and accurately solve the large nonlinear set of governing equations. Model performance is assessed through comparison to data for continuous extraction of various racemates within a series of hollow-fiber membrane modules. For each extraction, a chiral-ligand exchange selector molecule is solubilized in the organic phase flowing countercurrent to the aqueous phase into which the racemate is loaded. Enantiomer eluent profiles predicted at different conditions are in very good agreement with experiment. Through its predictive power, the model provides a useful in silico platform for optimizing these complex separations, and model results demonstrating this capability are presented.

Interactions between network cation coordination and non-bridging oxygen abundance in oxide glasses and melts: Insights from NMR spectroscopy

In aluminosilicate glasses, recent experimental work has greatly increased our knowledge of structural details that go beyond conventional models, such as high-coordinated Al and non-bridging oxygens (NBO) in metaluminous and even peraluminous compositions. Particularly as network cation coordination increases at high pressure, the interplay of such species (how their concentrations depend on each other through reaction equilibria) becomes especially important to understand and to relate to physical properties: for example, in NBO-rich compositions, this species is systematically reduced as Al or Si coordination increases. These interactions can often be seen more clearly in ambient-pressure borosilicate and germanate systems, where composition and temperature can have dramatic effects on network and oxygen speciation. In particular, the field strengths of the network modifier cations have strong effects on structure, as higher field strength often promotes the concentration of negative network charge as either NBO or highly-charged BO such as Al–O–Al groups; accompanying changes in network cation coordination often occur but are linked in a complex fashion. Here we compare such speciation reactions in silicate, aluminosilicate, borosilicate and germanate melt systems, primarily as sampled by high-resolution NMR spectroscopy, note commonality of behavior, and point out important gaps in our knowledge for future exploration.

Reactive transport modelling of acid mine drainage within discretely fractured porous media: Plume evolution from a surface source zone

A numerical model is developed for investigating the transport behaviour and geochemical evolution of acidic mine drainage (AMD) in discretely fractured porous media. The simulation approach is tested using a conceptual model of a reactive mine waste system in which an active source of AMD overlies a fractured silicate-rich porous host rock with a low but non-zero matrix permeability. Source composition is based on measured data from an existing mine tailings site. The numerical model includes groundwater flow, AMD infiltration, multi-component advective-dispersive transport, equilibrium geochemical speciation and water-rock pH-buffering reactions within a discrete fracture network (DFN). An analytical solution for parallel-fractures is used to verify the model, which is then applied to simulate the evolution of pH, the major aqueous species from the AMD source, as well as selected mineral buffers. As the acidic drainage water infiltrates into the initially uncontaminated fracture networks, high concentration gradients develop within the matrix along fracture interfaces inducing diffusion-limited pH buffering and precipitation of secondary minerals within the rock matrix. A comparison of AMD evolution in three fracture networks shows that even within a densely fractured network, AMD plume evolution can be significantly different from that obtained from assuming an equivalent porous medium (EPM). The paper also addresses issues of time scales and matrix diffusion. The results have implications for predicting environmental impacts of acid mine drainage in complex mining environments and for coupling of hydro-geochemical and geotechnical models. The model can also be applied to other hydrogeological systems including fractured clays and tills, to other contaminants including hydrocarbons or organic solvents, and to simulate geochemical evolution in natural flow systems.

Simulating adsorption of U(VI) under transient groundwater flow and hydrochemistry: Physical versus chemical nonequilibrium model

Coupled intragrain diffusional mass transfer and nonlinear surface complexation processes play an important role in the transport behavior of U(VI) in contaminated aquifers. Two alternative model approaches for simulating these coupled processes were analyzed and compared: (1) the physical nonequilibrium approach that explicitly accounts for aqueous speciation and instantaneous surface complexation reactions in the intragrain regions and approximates the diffusive mass exchange between the immobile intragrain pore water and the advective pore water as multirate first‐order mass transfer and (2) the chemical nonequilibrium approach that approximates the diffusion‐limited intragrain surface complexation reactions by a set of multiple first‐order surface complexation reaction kinetics, thereby eliminating the explicit treatment of aqueous speciation in the intragrain pore water. A model comparison has been carried out for column and field scale scenarios, representing the highly transient hydrological and geochemical conditions in the U(VI)‐contaminated aquifer at the Hanford 300A site, Washington, USA. It was found that the response of U(VI) mass transfer behavior to hydrogeochemically induced changes in U(VI) adsorption strength was more pronounced in the physical than in the chemical nonequilibrium model. The magnitude of the differences in model behavior depended particularly on the degree of disequilibrium between the advective and immobile phase U(VI) concentrations. While a clear difference in U(VI) transport behavior between the two models was noticeable for the column‐scale scenarios, only minor differences were found for the Hanford 300A field scale scenarios, where the model‐generated disequilibrium conditions were less pronounced as a result of frequent groundwater flow reversals.

Adsorption of single-ringed N- and S-heterocyclic aromatics on carbon nanotubes

Adsorption of single-ringed N- and S-heterocyclic aromatics on single-walled carbon nanotubes (SWCNTs) was examined to explore the potential of using carbon nanotubes (CNTs) as drug carriers and environmental adsorbents. Adsorbates included pyrimidine, 2-aminopyrimidine, 4,6-diaminopyrimidine, thiophene, benzene and aniline. Adsorbents included pristine SWCNTs, oxidized SWCNTs, and nonporous graphite. Adsorption of N- and S-heterocyclic aromatics was significantly enhanced by non-hydrophobic interactions. Particularly, the –NH 2-substituted compounds exhibited much stronger (up to 2 orders of magnitude) adsorption affinities to oxidized SWCNTs than benzene, even though they are much less hydrophobic. The π–π coupling or electron donor–acceptor (EDA) interactions are likely adsorption-enhancement mechanisms for all six compounds. The lone-pair electrons of the N heteroatoms and the –NH 2 group can enable n–π EDA interactions with SWCNT surfaces. Lewis acid–base interactions are another significant adsorption-enhancement mechanism for the –NH 2-substituted compounds (and possibly for pyrimidine) on SWCNTs. For the N-heterocyclic aromatics, adsorption affinity is highly dependent on the O-functionality of the SWCNT surface and on solution pH, due to the speciation reactions of both adsorbates and SWCNT surface O-functional groups, indicating that selective adsorption of N-heterocyclic aromatics is possible by combining the surface functionality of CNTs and solution chemistry.

Implementation of variably saturated flow into PHREEQC for the simulation of biogeochemical reactions in the vadose zone

A software tool for the simulation of one-dimensional unsaturated flow and solute transport together with biogeochemical reactions in the vadose zone was developed by integrating a numerical solution of Richards' equation into the geochemical modelling framework PHREEQC. Unlike existing software, the new simulation tool is fully based on existing capabilities of PHREEQC without source code modifications or coupling to external software packages. Because of the direct integration, the full set of PHREEQC's geochemical reactions with connected databases for reaction constants is immediately accessible. Thus, unsaturated flow and solute transport can be simulated together with complex solution speciation, equilibrium and kinetic mineral reactions, redox reactions, ion exchange reactions and surface adsorption including diffuse double layer calculations. Liquid phase flow is solved as a result of element advection, where the Darcy flux is calculated according to Richards' equation. For accurate representation of advection-dominated transport, a total-variation-diminishing scheme was implemented. Dispersion was simulated with a standard approach; however, PHREEQC's multicomponent transport capabilities can be used to simulate species-dependent diffusion. Since liquid phase saturation is recalculated after every reaction step, biogeochemical processes that modify liquid phase saturation, such as dissolution or precipitation of hydrated minerals are considered a priori. Implementation of the numerical schemes for flow and solute transport have been described, along with examples of the extensive code verification, before illustrating more advanced application examples. These include (i) the simulation of infiltration with saturation-dependent cation exchange capacity, (ii) changes in hydraulic properties due to mineral reactions and (iii) transport of mobile organic substances with complexation of heavy metals in varying geochemical conditions.

A properties model of the HI–I 2–H 2O–H 2 system in the sulfur–iodine cycle for hydrogen manufacture

A model for the phase equilibria and energetics of the HI–I 2–H 2O–H 2 system has been developed for use in process simulations of reactive distillation to form H 2 from HI in the Sulfur–Iodine cycle for thermochemical decomposition of water. Comparisons with data and other models demonstrate that this approach is uniquely successful in describing the phase equilibria and properties of this complex system. In particular, it is able to follow the pressure minimum of the HI–H 2O binary and HI–I 2–H 2O ternary when x HI ≈ 0.15; the dramatic increase of pressure and HI vapor composition, as well as formation of two liquid phases with greater amounts of HI; and the emergence of a single liquid phase in the ternary, even though binaries of HI and of I 2 with H 2O are only partially miscible. The model incorporates two novel speciation reactions and the Electrolyte-NRTL excess Gibbs energy model with parameters that cover wide ranges of temperature, pressure and composition. The model is formulated so that process simulation, design and optimization may be done reliably to explore feasibility and innovations of the S–I process.

A new numerical model for pore scale dissolution of calcite due to CO 2 saturated water flow in 3D realistic geometry: Principles and first results

A fully coupled reactive transport model at pore-scale has been developed using finite volumes in order to improve the comprehension of reactive flow-through experiments by CO 2-saturated water. Six constituents (H +, OH −, HCO 3 −, Ca 2+, CO 2⁎ and CO 3 2−) are considered for reactive transport through the 3D pore network geometry of a limestone sample assumed to be of pure calcite. Three speciation reactions at equilibrium (giving three mass action relations) are involved in the bulk of the fluid phase, the electro-neutrality of the solution is imposed (giving one relation), and two transport equations are solved to compute the concentrations of the six constituents with space and time. Fick's law models diffusion and different diffusion coefficients are used for the different constituents. Calcite dissolution rate at the fluid–mineral interface is written as a function of the activities of all the constituents appearing in the dissolution reactions. The pressure and velocity fields of the one-phase solution circulating through the sample are computed solving Stokes equations. For negative times the circulating solution is in equilibrium with the rock sample, and at t = 0 a disequilibrium is introduced (increase of CO 2 pressure and/or decrease of Ca 2+ concentration). Then, the non-linear system of equations representing the reactive transport is solved until steady state. Applications on realistic 3D geometry (defined from real media images obtained by X-ray computed micro-tomography) illustrate the possibilities offered by this model. The behaviour of an effective reaction rate has been examined for samples having different geometry, showing that, at the pore scale, calcite dissolution is mainly influenced by the mean pore fluid velocity.

Osmotic Coefficients of the Li2B4O7+LiCl + H2O System at T=273.15 K

Osmotic coefficients and water activities for the Li2B4O7+LiCl+H2O system have been measured at T=273.15 K by the isopiestic method, using an improved apparatus. Two types of osmotic coefficients, φS and φE, were determined. φS is based on the stoichiometric molalities of the solute Li2B4O7(aq), and φE is based on equilibrium molalities from consideration of the equilibrium speciation into H3BO3,B(OH)4− and B3O3(OH)4−. The stoichiometric equilibrium constants Km for the aqueous speciation reactions were estimated. Two types of representations of the osmotic coefficients for the Li2B4O7+LiCl+H2O system are presented with ion-interaction models based on Pitzer’s equations with minor modifications: model (I) represents the φS data with six parameters based on considering the ion-interactions between three ionic species of Li+, Cl−, and B4O72−, and model (II) for represents the φE data based on considering the equilibrium speciation. The parameters of models (I) and (II) are presented. The standard deviations for the two models are 0.0152 and 0.0298, respectively. Model (I) was more satisfactory than model (II) for representing the isopiestic data.

Fe 2+ sorption onto nontronite (NAu-2)

The sorption of ferrous iron to a clay mineral, nontronite (NAu-2, a ferruginous smectite), was investigated under strictly anoxic conditions as a function of pH (3–10), Fe 2+ concentration (0.01–50 mM), equilibration time (1–35 days), and ionic strength (0.01–0.5 M NaClO 4). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site density in order to interpret the Fe 2+ sorption data. Fe 2+ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe 2+ sorption through ion exchange and surface complexation reactions. Fe 2+ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, remained constant from pH 4.5 to 7.0, increased again with further increase of pH from pH 7.0 to 8.5, and reached a maximum above pH 8.5. The Fe 2+ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe 2+ sorption at different ionic strengths, however, diminished with increasing equilibration time. The Fe 2+ sorption from pH 4.5 to 7.0 increased with increasing equilibration time up to 35 days and showed stronger kinetic behavior in higher ionic strength solutions. The kinetic uptake of Fe 2+ onto NAu-2 is consistent with a surface precipitation mechanism although our measurements were not able to identify secondary precipitates. An equilibrium model that integrates ion exchange, surface complexation and aqueous speciation reactions reasonably well describes the Fe 2+ sorption data as a function of pH, ionic strength, and Fe 2+ concentration measured at 24 h of equilibration. Model calculations show that the species Fe(OH) + was required to describe Fe 2+ sorption above pH 8.0 satisfactorily. Overall, this study demonstrated that Fe 2+ sorption to NAu-2 is affected by complex equilibrium and kinetic processes, likely caused by surface precipitation reactions.

Surface complexation effects on phosphate adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers

Non-conservative behavior of dissolved inorganic phosphate (DIP) in estuaries is generally ascribed to desorption from iron and aluminum (hydr)oxides with increasing salinity. Here, we assess this hypothesis by simulating the reversible adsorption of phosphate onto a model oxide (goethite) along physico-chemical gradients representative of surface and subsurface estuaries. The simulations are carried out using a surface complexation model (SCM), which represents the main aqueous speciation and adsorption reactions of DIP, plus the ionic strength-dependent coulombic interactions in solution and at the mineral–solution interface. According to the model calculations, variations in pH and salinity alone are unlikely to explain the often reported production of DIP in surface estuaries. In particular, increased aqueous complexation of phosphate by Mg 2+ and Ca 2+ ions with increasing salinity is offset by the formation of ternary Mg-phosphate surface complexes and the drop in electrical potential at the mineral–water interface. However, when taking into account the downstream decrease in the abundance of sorption sites, the model correctly simulates the observed release of DIP in the Scheldt estuary. The sharp increase in pH accompanying the admixing of seawater to fresh groundwater should also cause desorption of phosphate from iron oxyhydroxides during seawater intrusion in coastal aquifers. As for surface estuaries, the model calculations indicate that significant DIP release additionally requires a reduction in the phosphate sorption site density. In anoxic aquifers, this can result from the supply of seawater sulfate and the subsequent reductive dissolution of iron oxyhydroxides coupled to microbial sulfate reduction.

Inverse microbial and geochemical reactive transport models in porous media

Microbial processes play a major role in controlling geochemical conditions in subsurface systems. Various laboratory and in situ experiments have been performed to evaluate the relevance of microbial processes and derive key microbial parameters. Such experiments are often interpreted in a subjective manner by trial-and-error curve fitting. A numerical model for the inverse problem of coupled flow, reactive solute transport, geochemical and microbial processes is presented here which overcomes the limitations of trial-and-error methods. It extends the capabilities of existing inverse models which deal mostly with flow and chemically-reactive solute transport. Our inverse model relies on the microbial reactive transport model of Zhang (2001) and Samper et al. (2006a) and improves the inverse reactive transport model of Dai and Samper (2004) by allowing the simultaneous estimation of geochemical and microbial parameters. The inverse model has been implemented in a finite element code, INVERSE-BIOCORE 2D and its capabilities have been verified and tested with a synthetic experiment involving equilibrium speciation, kinetic sorption/desorption and kinetic biodegradation reactions. Model results indicate that both chemical and microbial parameters can be estimated accurately for error-free data. Estimation errors of microbial parameters are larger than those of kinetic sorption parameters and generally increase with increasing standard deviation of data noise. Estimation error of yield coefficient is the smallest of all microbial parameters and does not depend on data noise. The inverse model has been used also to estimate microbial parameters of DOC aerobic respiration responsible for oxygen consumption at the REX in situ experiment. Estimates of microbial parameters are found to be within the range of reported values and have small estimation errors.

High temperature calorimetric studies of heat of solution of NiO, CuO, La2O3, TiO2, HfO2 in sodium silicate liquids

The enthalpies of solution of La2O3, TiO2, HfO2, NiO and CuO were measured in sodium silicate melts at high temperature. When the heat of fusion was available, we derived the corresponding liquid–liquid enthalpies of mixing. These data, combined with previously published work, provide insight into the speciation reactions in sodium silicate melts. The heat of solution of La2O3 in these silicate solvents is strongly exothermic and varies little with La2O3 concentration. The variation of heat of solution with composition of the liquid reflects the ability of La(III) to perturb the transient silicate framework and compete with other cations for oxygen. The enthalpy of solution of TiO2 is temperature-dependent and indicates that the formation of Na–O–Si species is favored over Na–O–Ti at low temperature. The speciation reactions can be interpreted in terms of recent spectroscopic studies of titanium-bearing melts which identify a dual role of Ti4+ as both a network-former end network-modifier. The heats of solution of oxides of transition elements (Ni and Cu) are endothermic, concentration-dependent and reach a maximum with concentration. These indicate a charge balanced substitution which diminishes the network modifying role of Na+ by addition of Ni2+ or Cu2+. The transition metal is believed to be in tetrahedral coordination, charge balanced by the sodium cation in the melts.

A Kinetic Model for the Aqueous Degradation of MTBE by Fe0/H2O2

Methyl tertiary-butyl ether (MTBE) is a possible human carcinogen that has been used as a gasoline oxygenate at concentrations of up to 15% by volume for about 45 years in the US. However, its high water solubility has exacerbated spills at gasoline stations, sometimes resulting in local groundwater MTBE contamination levels of over 100 mg/L. Advanced oxidation using Fe0 and H2O2 is a promising technique for mineralizing organic contaminants, but current understanding of the remediation chemistry needs to be improved to facilitate design of subsurface or engineered systems. A kinetic model for the degradation of MTBE in batch systems applying zero-valent iron (Fe0) andhydrogen peroxide (H2O2) oxidation in aqueous solution was developed. The model includes: H2O2 and water chemistry, iron speciation, and MTBE oxidation reactions. H2O2/water and MTBE degradation equilibrium and reaction rate parameters were taken from the literature. Reaction rate and equilibrium parameters for iron speciation were taken from the literature, or from our prior work. The rate constant for the dissolution of Fe0 was found from this work. The model was compared to experimental data from the literature for MTBE degradation using Fe0/H2O2

Isopiestic determination of the osmotic coefficients and Pitzer model representation for Li 2B 4O 7(aq) at T = 298.15 K

Isopiestic molalities and water activities have been measured for aqueous Li 2B 4O 7 solutions from (0.0275 to 2.9397) mol · kg −1 at T = 298.15 K by isopiestic method, using an improved apparatus. These measurements have extended into the very dilute and the supersaturated molality region. Two types of osmotic coefficients of ϕ S and ϕ E were determined. ϕ S is based on the stoichiometric molalities of the solute Li 2B 4O 7(aq), and ϕ E is based on equilibrium molalities based on consideration of the equilibrium speciation of H 3 BO 3 , B ( OH ) 4 - , B 3 O 3 ( OH ) 4 - , and B 4 O 5 ( OH ) 4 2 - in the Li 2B 4O 7(aq) solutions. The stoichiometric equilibrium constants K mi for the aqueous speciation reactions were estimated. Three types of representations of the osmotic coefficients for the (Li 2B 4O 7 + H 2O) system with the ion-interaction models based on Pitzer’s equations of osmotic coefficients with minor modifications were presented: the model (I) for representation of the data of ϕ S with four parameters based on considering the ion-interactions between two ionic species of Li + and B 4 O 7 2 - , the model (II) for representation of the data of ϕ E based on considering the equilibrium speciation with 18 parameters, and the model (III) simplified from model (II) with six ion-interaction parameters estimated. The reasonable agreements between the experimental osmotic coefficient data and those calculated using the models above were obtained with the standard deviations of 0.0215, 0.0055, and 0.0150 for the three models, respectively. The thermodynamic osmotic coefficient properties for the complex system containing polymetric boron anions and lithium may be explained by use of the Pitzer ion-interaction model with minor modifications in combination with speciation reaction equilibria.