Triple-layer Model Research Articles

X-Ray Reflectivity Analysis of SiO2 Nanochannels Filled with Water and Ions: A New Method for the Determination of the Spatial Distribution of Ions Inside Confined Media

Chemical reactions occurring at the material–aqueous solution interface are controlled by an interfacial layer of a few nm where conceptual models such as the Electrical Double or Triple Layer models can be applied. These models describing the spatial distribution of ions in term of perpendicular distance from the planar surface and ignoring topography or structure parallel to the surface are not validated in confined media. In order to investigate the critical dimensions of these models, our first approach was to use a model system consisting of two parallel plane surfaces of SiO2 spaced of 5nm (nanochannels) filled with salt solutions XCl2 (X = Ca2+, Mg2+, Ba2+). These filled nanochannels were characterized using hard X-Ray reflectivity for the determination of electron density profiles perpendicular to the surface. From these results, the surface densities of adsorbed ions at SiO2 surface were calculated and the solution density inside the nanochannels was determined. This method opens new perspectives to a better understanding of water and ion distribution inside nanoconfined media.

Predictive model for Pb(II) adsorption on soil minerals (oxides and low-crystalline aluminum silicate) consistent with spectroscopic evidence

Mobility of Pb(II) in surface condition is governed by adsorption processes on soil minerals such as iron oxides and low-crystalline aluminum silicates. The adsorption effectiveness and the surface complex structures of Pb(II) vary sensitively with solution conditions such as pH, ionic strength, Pb(II) loading, and electrolyte anion type. This study was undertaken to construct a quantitative model for Pb(II) on soil minerals. It can predict the adsorption effectiveness and surface complex structures under any solution conditions using the extended triple layer model (ETLM).The Pb(II) adsorption data for goethite, hydrous ferric oxide (HFO), quartz, and low-crystalline aluminum silicate (LCAS) were analyzed with ETLM to retrieve the surface complexation reactions and these equilibrium constants. The adsorption data on goethite, HFO and quartz were referred from reports of earlier studies. Those data for LCAS were measured under a wide range of pH, ionic strength and Pb(II) loadings in NaNO3 and NaCl solutions. All adsorption data can be reasonably regressed using ETLM with the assumptions of inner sphere bidentate complexation and inner sphere monodentate ternary complexation with electrolyte anions, which are consistent with previously reported spectroscopic evidence.Predictions of surface speciation under widely various solution conditions using ETLM revealed that the inner sphere bidentate complex is the predominant species at neutral to high pH conditions. The inner sphere monodentate ternary complex becomes important at low pH, high surface Pb(II) coverage, and high electrolyte concentrations, of which the behavior is consistent with the spectroscopic observation.Comparisons of the obtained adsorption constants on goethite, HFO and quartz exhibited good linear relations between the reciprocals of dielectric constants of solids and adsorption constants. Those linear relations support predictions of the adsorption constants of all oxides based on Born solvation theory. The adsorption constants of LCAS are comparable to those of goethite. The predicted adsorption constants of soil minerals suggest that the LCAS is an important sorbent for Pb(II).

Design of a hybrid advective-diffusive microfluidic system with ellipsometric detection for studying adsorption

Establishing and maintaining concentration gradients that are stable in space and time is critical for applications that require screening the adsorption behavior of organic or inorganic species onto solid surfaces for wide ranges of fluid compositions. In this work, we present a design of a simple and compact microfluidic device based on steady-state diffusion of the analyte, between two control channels where liquid is pumped through. The device generates a near-linear distribution of concentrations. We demonstrate this via experiments with dye solutions and comparison to finite-element numerical simulations. In a subsequent step, the device is combined with total internal reflection ellipsometry to study the adsorption of (cat)ions on silica surfaces from CsCl solutions at variable pH. Such a combined setup permits a fast determination of an adsorption isotherm. The measured optical thickness is compared to calculations from a triple layer model for the ion distribution, where surface complexation reactions of the silica are taken into account. Our results show a clear enhancement of the ion adsorption with increasing pH, which can be well described with reasonable values for the equilibrium constants of the surface reactions.

The dependence of induced polarization on fluid salinity and pH, studied with an extended model of membrane polarization

The estimation of hydraulic parameters from spectral induced polarization (SIP) measurements is difficult partly because the electrical impedance of sediments depends on several parameters that are not related to the texture. Important parameters that influence the spectral response are fluid salinity and pH. In order to understand the behaviour of SIP spectra from a mechanistic point of view, we carry out simulations with a membrane polarization model. The geometry consists of a sequence of wide and narrow pores with finite radii. The charge distribution at the mineral surface is described by a triple layer model, characterized by the zeta potential and the partition coefficient. We extended an existing model by incorporating known dependencies of the zeta potential and the partition coefficient on fluid salinity and pH.Our simulation results predict a decrease of the maximum phase shift of the complex electrical conductivity with increasing salinity, consistent with experimental observations. For very small pore radii, the phase shift may also show the opposite behaviour and increase with salinity. The imaginary conductivity at 1Hz increases with increasing salinity, followed by a peak and a decrease at high salinities. The fact that our model predicts a decrease of the imaginary conductivity at high salinities is particularly important, because strong experimental evidence was recently found for such a decrease, which was theoretically unexplained so far.Both the maximum phase shift and the imaginary conductivity at 1Hz decrease when pH decreases. The reason is that at low pH, the zeta potential and the partition coefficient both decrease, corresponding to a smaller charge density at the mineral surface, resulting in a weaker impact of the electrical double layer. The few existing experimental studies on pH dependence are qualitatively consistent with our simulation results.

Comparison of the surface ion density of silica gel evaluated via spectral induced polarization versus acid–base titration

Surface complexation models are widely used with batch adsorption experiments to characterize and predict surface geochemical processes in porous media. In contrast, the spectral induced polarization (SIP) method has recently been used to non-invasively monitor in situ subsurface chemical reactions in porous media, such as ion adsorption processes on mineral surfaces. Here we compare these tools for investigating surface site density changes during pH-dependent sodium adsorption on a silica gel. Continuous SIP measurements were conducted using a lab scale column packed with silica gel. A constant inflow of 0.05M NaCl solution was introduced to the column while the influent pH was changed from 7.0 to 10.0 over the course of the experiment. The SIP measurements indicate that the pH change caused a 38.49±0.30μScm−1 increase in the imaginary conductivity of the silica gel. This increase is thought to result from deprotonation of silanol groups on the silica gel surface caused by the rise in pH, followed by sorption of Na+ cations. Fitting the SIP data using the mechanistic model of Leroy et al. (Leroyet al., 2008), which is based on the triple layer model of a mineral surface, we estimated an increase in the silica gel surface site density of 26.9×1016sitesm−2. We independently used a potentiometric acid–base titration data for the silica gel to calibrate the triple layer model using the software FITEQL and observed a total increase in the surface site density for sodium sorption of 11.2×1016sitesm−2, which is approximately 2.4 times smaller than the value estimated using the SIP model. By simulating the SIP response based on the calibrated surface complexation model, we found a moderate association between the measured and estimated imaginary conductivity (R2=0.65). These results suggest that the surface complexation model used here does not capture all mechanisms contributing to polarization of the silica gel captured by the SIP data.

Application of three surface complexation models on U(VI) adsorption onto graphene oxide

The effect of environmental factors (i.e., reaction time, pH, ionic strength, temperature and initial U(VI) concentration) on the adsorption of U(VI) on graphene oxide (GO) had been investigated by batch techniques. The macroscopic experiments indicated that the adsorption kinetics and adsorption isotherms of U(VI) on GO can be satisfactorily fitted by the pseudo-second-order kinetic model and Langmuir model, respectively. No change of ionic strength indicated that the inner-sphere surface complexation dominated the adsorption of U(VI) on GO over the wide pH range. Three surface complexation models (SCMs), including constant capacitance model (CCM), diffuse layer model (DLM) and triple layer model (TLM), had been given an excellent fits for the adsorption of U(VI) on GO with two inner-sphere surface complexes such as SOUO2+ and (SO)2UO2(OH)22− species.

Modeling the Adsorption of Oxalate onto Montmorillonite.

In this work, a multiscale modeling of the interaction of oxalate with clay mineral surfaces from macroscale thermodynamic equilibria simulations to atomistic calculations is presented. Previous results from macroscopic adsorption data of oxalate on montmorillonite in 0.01 M KNO3 media at 25 °C within the pH range from 2.5 to 9 have been used to develop a surface complexation model. The experimental adsorption edge data were fitted using the triple-layer model (TLM) with the aid of the FITEQL 4.0 computer program. Surface complexation of oxalate is described by two reactions: >AlOH + Ox(2-) + 2H(+) = >AlOxH + H2O (log K = 14.39) and >AlOH + Ox(2-) + H(+) = >AlOx(-) + H2O (log K = 10.39). The monodentate complex >AlOxH dominated adsorption below pH 4, and the bidentate complex >AlOx(-) was predominant at higher pH values. Both of the proposed inner-sphere oxalate species are qualitatively consistent with previously published diffuse reflectance FTIR spectroscopic results for oxalate on montmorillonite edge surface (Chem. Geol. 2014, 363, 283-292). Atomistic computational studies have been performed to understand the interactions at the molecular level between adsorbates and mineral surface, showing the atomic structures and IR frequency shifts of the adsorption complexes of oxalate with the edge surface of a periodic montmorillonite model.

Evaluation of Surface Sorption Processes Using Spectral Induced Polarization and a (22)Na Tracer.

We investigate mechanisms controlling the complex electrical conductivity of a porous media using noninvasive spectral induced polarization (SIP) measurements of a silica gel during a pH dependent surface adsorption experiment. Sorption of sodium on silica gel surfaces was monitored as the pH of a column was equilibrated at 5.0 and then successively raised to 6.5 and 8.0, but the composition of the 0.01 M NaCl solution was otherwise unchanged. SIP measurements show an increase in the imaginary conductivity of the sample (17.82 ± 0.07 μS/cm) in response to the pH change, interpreted as deprotonation of silanol groups on the silica gel surface followed by sorption of sodium cations. Independent measurements of Na(+) accumulation on grain surfaces performed using a radioactive (22)Na tracer support the interpretation of pH-dependent sorption as a dominant process controlling the electrical properties of the silica gel (R(2) = 0.99) and confirms the importance of grain polarization (versus membrane polarization) in influencing SIP measurements of silicate minerals. The number of surface sorption sites estimated by fitting a mechanistic, triple-layer model for the complex conductivity to the SIP data (13.22 × 10(16) sites/m(2)) was 2.8 times larger than that estimated directly by a (22)Na mass balance (5.13 × 10(16) sites/m(2)), suggesting additional contributions to polarization exist.

Insights into tetracycline adsorption onto kaolinite and montmorillonite: experiments and modeling.

Adsorption of tetracycline (TC) on kaolinite and montmorillonite was investigated using batch adsorption experiments with different pH, ionic strength, and surface coverage. As a result, pH and ionic strength-dependent adsorption of TC was observed for the two clay minerals. The adsorption of TC decreased with the increase of pH and ionic strength, and high initial TC concentration had high adsorption. In addition, a triple-layer model was used to predict the adsorption and surface speciation of TC on the two minerals. As a result, four complex species on kaolinite (≡X(-)∙H3TC(+), ≡X(-)∙H2TC(±), ≡SOH(0)∙H2TC(±), and ≡SOH(0)∙HTC(-)) and three species on montmorillonite (≡X(-)∙H3TC(+), ≡X(-)∙H2TC(±), and ≡SOH(0)∙HTC(-)) were structurally constrained by spectroscopy, and these species were also successfully fitted to the adsorption edges of TC. Three functional groups of TC were involved in these adsorption reactions, including the positively charged dimethylamino group, the C=O amide I group, and the C=O group at the C ring. Combining adsorption experiments and model in this study, the adsorption of TC on kaolinite and montmorillonite was mainly attributed to cation exchange on the surface sites (≡X(-)) compared to surface complexation on the edge sites (≡SOH) at natural soil pH condition. Moreover, the surface adsorption species, the corresponding adsorption modes, and the binding constants for the surface reactions were also estimated.

Computer Simulations of Quartz (101)–Water Interface over a Range of pH Values

The original force field for clay materials (ClayFF) developed by Cygan et al. (J. Phys. Chem. B 2004, 108, 1255) is modified to describe negative charging of the (101) quartz surface above its point of zero charge (pH ≈ 2.0–4.5). The modified force field adopts the scaled natural bond orbital charges derived by the quantum mechanical calculations which are used to obtain the desired surface charge density and to determine the delocalization of the charge after deprotonation of surface silanol groups. Classical molecular dynamics simulations (CMD) of the (101) surface of α-quartz with different surface charge densities (0, −0.03, −0.06, and −0.12 C m–2) are performed to evaluate the influence of the negative surface charge on interfacial water and adsorption of Na+, Rb+, and Sr2+ ions. The CMD results are compared with ab initio calculations, X-ray experiment, and the triple-layer model. The modified force field can be easily implemented in common molecular dynamics packages and used for simulations of in...

The electrophoretic mobility of montmorillonite. Zeta potential and surface conductivity effects

Clay minerals have remarkable adsorption properties because of their high specific surface area and surface charge density, which give rise to high electrochemical properties. These electrochemical properties cannot be directly measured, and models must be developed to estimate the electrostatic potential at the vicinity of clay mineral surfaces. In this context, an important model prediction is the zeta potential, which is thought to be representative of the electrostatic potential at the plane of shear. The zeta potential is usually deduced from electrophoretic measurements but for clay minerals, high surface conductivity decreases their mobility, thereby impeding straightforward interpretation of these measurements. By combining a surface complexation, conductivity and electrophoretic mobility model, we were able to reconcile zeta potential predictions with electrophoretic measurements on montmorillonite immersed in NaCl aqueous solutions. The electrochemical properties of the Stern and diffuse layers of the basal surfaces were computed by a triple-layer model. Computed zeta potentials have considerably higher amplitudes than measured zeta potentials calculated with the Smoluchowski equation. Our model successfully reproduced measured electrophoretic mobilities. This confirmed our assumptions that surface conductivity may be responsible for montmorillonite’s low electrophoretic mobility and that the zeta potential may be located at the beginning of the diffuse layer.

Interaction between l-aspartate and the brucite [Mg(OH)2]–water interface

The interaction of biomolecules at the mineral–water interface could have played a prominent role in the emergence of more complex organic species in life’s origins. Serpentinite-hosted hydrothermal vents may have acted as a suitable environment for this process to occur, although little is known about biomolecule–mineral interactions in this system. We used batch adsorption experiments and surface complexation modeling to study the interaction of l-aspartate onto a thermodynamically stable product of serpentinization, brucite [Mg(OH)2], over a wide range of initial aspartate concentrations at four ionic strengths governed by [Mg2+] and [Ca2+]. We observed that up to 1.0μmol of aspartate adsorbed per m2 of brucite at pH∼10.2 and low Mg2+ concentrations (0.7×10−3M), but surface adsorption decreased at high Mg2+ concentrations (5.8×10−3M). At high Ca2+ concentrations (4.0×10−3M), aspartate surface adsorption doubled (to 2.0μmolm−2), with Ca2+ adsorption at 29.6μmolm−2. We used the extended triple-layer model (ETLM) to construct a quantitative thermodynamic model of the adsorption data. We proposed three surface reactions involving the adsorption of aspartate (HAsp−) and/or Ca2+ onto brucite:2>SOH+H++HAsp-→SOH2+>SAsp-+H2O,>SOH+HAsp-+Ca2+→SO-_Ca(HAsp)++H+,and>SOH+Ca2++2H2O→SOH2+_Ca(OH)2+H+.We used the ETLM to predict that brucite particle surface charge becomes more negative with increasing [Mg2+], creating an unfavorable electrostatic environment for a negatively-charged aspartate molecule to adsorb. In contrast, our addition of Ca2+ to the system resulted in Ca2+ adsorption and development of positive surface charge. Our prediction of surface speciation of aspartate on brucite with Ca2+ revealed that the calcium–aspartate complex is the predominant surface aspartate species, which suggests that the increase in aspartate adsorption with Ca2+ is primarily driven by calcium adsorption. The cooperative effect of Ca2+ and the inhibitive effect of Mg2+ on aspartate adsorption onto brucite indicate that serpentinite-hosted hydrothermal fluids provide an ideal environment for these interactions to take place.

Mechanisms of antimony adsorption onto soybean stover-derived biochar in aqueous solutions

Limited mechanistic knowledge is available on the interaction of biochar with trace elements (Sb and As) that exist predominantly as oxoanions. Soybean stover biochars were produced at 300 °C (SBC300) and 700 °C (SBC700), and characterized by BET, Boehm titration, FT-IR, NMR and Raman spectroscopy. Bound protons were quantified by potentiometric titration, and two acidic sites were used to model biochar by the surface complexation modeling based on Boehm titration and NMR observations. The zero point of charge was observed at pH 7.20 and 7.75 for SBC300 and SBC700, respectively. Neither antimonate (Sb(V)) nor antimonite (Sb(III)) showed ionic strength dependency (0.1, 0.01 and 0.001 M NaNO3), indicating inner sphere complexation. Greater adsorption of Sb(III) and Sb(V) was observed for SBC300 having higher –OH content than SBC700. Sb(III) removal (85%) was greater than Sb(V) removal (68%). Maximum adsorption density for Sb(III) was calculated as 1.88 × 10−6 mol m−2. The Triple Layer Model (TLM) successfully described surface complexation of Sb onto soybean stover-derived biochar at pH 4–9, and suggested the formation of monodentate mononuclear and binuclear complexes. Spectroscopic investigations by Raman, FT-IR and XPS further confirmed strong chemisorptive binding of Sb to biochar surfaces.

Attachment of ribonucleotides on α-alumina as a function of pH, ionic strength, and surface loading.

The interactions between nucleic acids and mineral surfaces have been the focus of many studies in environmental sciences, in biomedicine, as well as in origin of life studies for the prebiotic formation of biopolymers. However, few studies have focused on a wide range of environmental conditions and the likely modes of attachment. Here we investigated the adsorption of ribonucleotides onto α-alumina surfaces over a wide range of pH, ionic strength, and ligand-to-solid ratio, by both an experimental and a theoretical approach. The adsorption of ribonucleotides is strongly affected by pH, with a maximum adsorption at pH values around 5. Alumina adsorbs high amounts of nucleotides >2 μmol/m(2). We used the extended triple-layer model (ETLM) to predict the speciation of the surface complexes formed as well as the stoichiometry and equilibrium constants. We propose the formation of two surface species: a monodentate inner-sphere complex, dominant at pH <7, and a bidentate outer-sphere complex, dominant at higher pH. Both complexes would involve interactions between the negatively charged phosphate group and the positively charged surface of alumina. Our results provide a better understanding of how nucleic acids attach to mineral surfaces under varying environmental conditions. Moreover, the predicted configuration of nucleotide surface species, bound via the phosphate group, could have implications for the abiotic formation of nucleic acids in the context of the origin of life.

Modeling Selenate Adsorption Behavior on Oxides, Clay Minerals, and Soils Using the Triple Layer Model

Modeling Selenate Adsorption Behavior on Oxides, Clay Minerals, and Soils Using the Triple Layer Model

Ion-exchange equilibrium of cesium/hydrogen ions on zirconium molybdate and zirconium iodomolybdate cation exchangers

The ion-exchange behavior of 134Cs+ onto zirconium molybdate (ZM) and zirconium iodomolybdate (ZIM) under batch conditions was achieved in different media. Acetic acid and EDTA were used as organic ligands; whereas sodium chloride and sodium nitrate were used as inorganic media containing Na+ as competing ion for 134Cs+ adsorption on ZM and ZIM. Based on Davis and Debye–Huckel Equations, the activities and the activity coefficients of the corresponding concentrations were calculated. Different isothermal models were used to express the effect of activity on the amount of 134Cs+ sorbed onto ZM and ZIM. The best-fitted adsorption isotherm models were in the order of BET > Freundlich > Temkin > Sips > Langmuir in case of 134Cs+/ZM, while the order was Temkin > Freundlich > BET > Langmuir > Sips in case of 134Cs+/ZIM. Traditional surface complexation models (SCMs) could not account for the sorption of 134Cs+ onto ZM and ZIM in presence of different ligands or competing ions; the 2-pK basic Stern model and the triple-layer model (TLM) was not satisfactory, due to the high number of adjustable parameters involved in these model variations. Furthermore, a purely diffuse layer model (DLM) generally gave the poorest fit to experimental data when combined with the 1-pK approach and was only slightly better when combined with the 2-pK formalism. Therefore, a new model, surface site competition complexation model (SSCCM) was developed by G. M. Ibrahim and B. El-Gammal, based on 2-pK DLM to test several sets of data, including those containing ligand complexes and competing cations. The theoretical basis, postulates, the model equations, and the calculations were discussed in detail. The new SSCCM succeeded in explanation of the marked sets of sorption data in distinctive ionic strengths giving rise to the different activities, species, and their distributions in existence of both the organic complexing ligands as acetic acid and EDTA and sodium as monovalent competing ion. The SSCCM explained the results of the solubility’s of the inorganic species by calculation of their logarithmic ionic activity products and the corresponding saturation indices. In addition, the surface charges and surface potentials were computed. Since the calculations in the SSCCM are based on the activities, the model could predict the real formation constants, and in turn, it could be precisely used to calculate the different thermodynamic parameters. Negative free energy changes indicate the spontaneous nature of the sorption process, while the positive values for both enthalpy change and entropy change indicates that the sorption process is entropy directed.

Application of surface complexation models to anion adsorption by natural materials

Various chemical models of ion adsorption are presented and discussed. Chemical models, such as surface complexation models, provide a molecular description of anion adsorption reactions using an equilibrium approach. Two such models, the constant capacitance model and the triple layer model, are described in the present study. Characteristics common to all the surface complexation models are equilibrium constant expressions, mass and charge balances, and surface activity coefficient electrostatic potential terms. Methods for determining parameter values for surface site density, capacitances, and surface complexation constants also are discussed. Spectroscopic experimental methods of establishing ion adsorption mechanisms include vibrational spectroscopy, nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity. Experimental determinations of point of zero charge shifts and ionic strength dependence of adsorption results and molecular modeling calculations also can be used to deduce adsorption mechanisms. Applications of the surface complexation models to heterogeneous natural materials, such as soils, using the component additivity and the generalized composite approaches are described. Emphasis is on the generalized composite approach for predicting anion adsorption by soils. Continuing research is needed to develop consistent and realistic protocols for describing ion adsorption reactions on soil minerals and soils. The availability of standardized model parameter databases for use in chemical speciation-transport models is critical.

Structure of water at charged interfaces: a molecular dynamics study.

The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na(+) vs Cs(+)) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na(+) vs Cs(+), the depth (~1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.

Adsorption of lactate and citrate on montmorillonite in aqueous solutions

Lactate and citrate were adsorbed on montmorillonite surface from aqueous solutions from pH values 2 to 12 in the presence of 10mmolL−1 KNO3. The adsorbed molecules were characterized using Attenuated Total Reflectance Fourier transform Infrared (ATR-FTIR) spectroscopy at pH values 4, 6 and 8. Macroscopic adsorption measurements indicate that the adsorption of both ligands is highly pH dependent. ATR-FTIR results indicate that lactate is likely adsorbed by nonspecific electrostatic interactions, since no changes were found in peak position and shape of carboxyl group stretches. Nevertheless, ATR-FTIR results suggest that citrate is adsorbed in the >AlOH groups as an inner-sphere surface complex at low pH. The macroscopic adsorption behavior of the ligands was modeled as a function of pH by using the Diffuse Layer Model (DLM) for lactate and the Triple Layer Model (TLM) for citrate. Surface complexation of lactate and citrate was described with the complex >AlLac (logK=11.25) and the complex >AlCit2− (logK=10.58) respectively.These findings are of relevance for the evaluation of the catalytic effect that takes place during the dissolution of clay minerals in the presence of low-molecular-weight organic ligands.

Modelling spatial variations in dissolved oxygen in fine-grained streams under uncertainty

This paper presents a novel triple-layer model, called VART DO-3L, for simulation of spatial variations in dissolved oxygen (DO) in fine-grained streams, characterized by a fluid mud (fluff or flocculent) layer (an advection-dominated storage zone) as the interface between overlying stream water and relatively consolidated streambed sediment (a diffusion-dominated storage zone). A global sensitivity analysis is conducted to investigate the sensitivity of VART DO-3L model input parameters. Results of the sensitivity analysis indicate that the most sensitive parameter is the relative size of the advection-dominated storage zones (As/A), followed by a lumped reaction term (R) for the flocculent layer, biological reaction rate (μo) in diffusive layer and biochemical oxygen demand concentration (L) in water column. In order to address uncertainty in model input parameters, Monte Carlo simulations are performed to sample parameter values and to produce various parameter combinations or cases. The VART DO-3L model is applied to the Lower Amite River in Louisiana, USA, to simulate vertical and longitudinal variations in DO under the cases. In terms of longitudinal variation, the DO level decreases from 7.9 mg l at the Denham Springs station to about 2.89 mg l−1 at the Port Vincent station. In terms of vertical variation, the DO level drops rapidly from the overlying water column to the advection-dominated storage zone and further to the diffusive layer. The DO level (CF) in the advective layer (flocculent layer) can reach as high as 40% of DO concentration (C) in the water column. The VART DO-3L model may be applied to similar rivers for simulation of spatial variations in DO level. Copyright © 2014 John Wiley & Sons, Ltd.