Triple Layer Surface Complexation Model Research Articles

Salt-Responsive Phenol Formaldehyde Resin: Changes of Interface Energy on the Aggregation Process.

Phenol formaldehyde resins (PFRs) as a colloidal oil displacement agent were commonly used to plug pores in crude oil reservoirs for enhanced oil recovery (EOR). The aggregation-dispersion and charging behavior of PFR may affect the rheology and plugging performance of the suspension. To understand the aggregation-dispersion and charge of PFR, turbidity, dynamic light scattering, and electrophoretic light scattering experiments were carried out at pH = 10 with different concentrations of salt solutions (NaCl, MgCl2, CaCl2, NaCl/MgCl2, and NaCl/CaCl2). The aggregation rate and ζ-potential were measured, and the critical coagulation concentration (CCC) and critical coagulation ionic strength (CCIS) were further obtained. Based on the triple-layer surface complexation (TL) model, the adsorption ability of cations and the surface characteristics of the PFR particles were studied, and these differences were explained by interface energy. Thus, Derjaguin-Landau and Verwey-Overbeek (DLVO) theory modified by interface energy was applied to explain the aggregation behavior of PFR particles in different types of ion systems. We concluded that, in the presence of multiple ions, DLVO theory modified by interface energy has good applicability to the aggregation-dispersion of PFR particles.

Removal of heavy metals from stormwater runoff using granulated drinking water treatment residuals

Stormwater runoff is a significant source of heavy metals, including cadmium (Cd), chromium (Cr), and nickel (Ni), which pose ecological and human health problems. Various filter media have been evaluated for heavy metal removal from stormwater via adsorption, most involving chemical- or energy-intensive processes. Aluminum-based drinking water treatment residuals (WTR), a non-hazardous byproduct of drinking water treatment, are an inexpensive sorbent for heavy metals. However, the low permeability of WTR is a problem and requires mixing with sand and carbon materials to improve flow; but such amendments also reduce its sorption capacity. To overcome this problem, a granulated WTR sorbent was generated using a green technique involving organic materials and a low-energy process. Batch studies showed that WTR granules remove Cd, Cr, and Ni simultaneously. Metal removal was adequately described by pseudo-second-order kinetic models and Langmuir and Freundlich isotherm models. The overall removal performance was Cr > Cd > Ni. The presence of divalent cations in solution negatively affected metal removal; anions had a strong effect on Cd removal. A triple-layer surface complexation model adequately described metal removal. Results demonstrated the strong potential of the WTR granules to emerge as green filter media for the removal of heavy metals from stormwater runoff. • WTR granules are sustainable sorbents that can simultaneously remove Cd, Cr, and Ni. • WTR granules were made from a waste byproduct and organic material with low energy. • The environmental parameters affected the removal of Cd, Cr, and Ni. • Surface complexation model could adequately describe the removal of Cd, Cr, and Ni.

Evaluation of sour gas-low salinity waterflooding in carbonate reservoirs - A numerical simulation approach

Although significant amount of H2S (sour gas) rich natural gas is estimated globally, but not much attention has been given to the application of H2S in the oil recovery process. Recent studies on the use of H2S in oil recovery processes showed that H2S has the potential of improving the oil recovery, and it can be even more effective than using CO2 in some processes. H2S can equally dissolve in the water, react with the reservoir rock to change its surface charge, porosity, and permeability. However, previous investigations on H2S oil recovery attributed the improved oil recoveries to the higher miscibility of H2S in the oil, and the reduction in the oil viscosity. Therefore, there is limited understanding on the H2S-oil-brine-rock geochemical interactions, and how they impact the oil recovery process. This study aims to investigate the interactions between H2S, oil, and carbonate formations, and to assess how the combination of H2S and low salinity water can impact the wettability and porosity of the reservoirs. A triple layer surface complexation model was used to understand the influence of key parameters (e.g., pressure, brine salinity, and composition) on the H2S-brine-oil-rock interactions. Moreover, the effects of mineral content of the carbonate rock on H2S interactions were studied. Thereafter, the results of the H2S-oil-brine-rock interactions were compared with a study where CO2 was used as the injected gas. Results of the study showed that the seawater and its diluted forms yielded identical ζ-potential values of about 3.31 mV at a pH of 3.24. This indicates that at very low pH condition, pH controls the ζ-potential of the oil-brine interface regardless of the brine's ionic strength. The study further demonstrated that the presence of other minerals in the carbonate rock greatly reduced the calcite dissolution. For instance, the calcite dissolution was reduced by 4.5% when anhydrite mineral was present in the carbonate rock. Findings from the simulation also indicated that CO2 produced negative ζ-potential values for the carbonate rocks, and these values were reduced by 18.4%–20% when H2S was used as the gas phase. This implies that the H2S shifted the carbonate rock ζ-potentials towards positive. The outcomes of this study can be applied when designing CO2 flooding and CO2 storage where the gas stream contains H2S gas since H2S greatly influences the dissolution of the carbonate mineral.

Aggregation kinetics and morphological changes of PFR and establishment of triple-layer surface complexation model to explain ion difference

Aggregation kinetics and morphological changes of PFR and establishment of triple-layer surface complexation model to explain ion difference

A calibrated surface complexation model for carbonate-oil-brine interactions coupled with reservoir simulation - Application to controlled salinity water flooding

Vast majority of past studies that have been conducted on controlled salinity water flooding (CSWF) use diffuse layer model (DLM) that calculates only the surface potential, which can be significantly different from the ζ-potential of the interface. Importantly, stability of the water-film (between the oil and the rock surface) which dictates ultimate oil recovery is related to ζ-potential. As such, using DLM calculated surface potential (directly) for CSWF can be misleading. Additionally, most of the existing DLMs use integer charges instead of fractional charges to model carbonate-brine interactions, which may not represent the actual carbonate crystallographic. We present a triple layer surface complexation model (TLM) which offers the option to locate and distribute charge of the adsorbing ion(s) at three locations. TLM can therefore calculate potentials at three different locations within the Stern layer. To the best of our knowledge, only few authors have used TLM to simulate CSWF. However, some surface reactions were ignored in these models. For instance, adsorption of monovalent ions such as Na + and Cl − ions was ignored in some of the few available TLMs. Also, the effect of basic-oil components was not considered in some TLMs, and lastly, some TLMs used integer charges for the carbonate surface group. This study introduced a comprehensive TLM that includes all these complexities. Moreover, we introduced a new wettability indicator (WI) that is related to the electrostatic forces between the oil-brine and the rock-brine interfaces. That is, WI was calculated from ζ-potentials at the oil-brine and rock-brine interfaces. The TLM, built in a geochemical simulator, PHREEQC, was then coupled with UTCHEM, a multiphase reservoir simulator. The developed simulator based on the TLM-CSWF model was tested against several experimentally measured oil recovery data sets. Results of the model suggest that Oil basic component significantly impacts oil-brine interface ζ-potential irrespective of temperature, brine composition, and ionic strength. On the otherhand, Na + ions in brine may influence oil-brine ζ-potential, and this relates to temperature, brine composition, and ionic strength. The results further suggest that injection of (sulfate-free) diluted brine in chalk resulted to increased oil adhesion, shifting the reservoir to oil-wet condition. Hence, no additional oil is recovered with this brine injected at tertiary oil recovery stage. Finally, the results showed that wettability is related to ζ-potentials at the oil-brine and rock-brine interfaces, and the carbonate rock would be strongly oil-wet or water-wet only at significantly large magnitude of ζ-potentials at the interfaces. • A new triple-layer surface complexation model developed to characterize carbonate-brine-oil interactions. • The model is tested against several experimentally measured zeta potential and oil recovery data sets. • The model suggests that SO 4 −2 ion adsorption tends to decrease CO 3 −2 adsorption at the rock surface and vice-versa. • The model showed that only a small fraction of the carbonate-oil-brine reactive sites are involved in the geochemical reactions.

Triple-layer surface complexation modelling: Characterization of oil-brine interfacial zeta potential under varying conditions of temperature, pH, oil properties and potential determining ions

Low salinity water flooding (LSWF) is a popular enhanced oil recovery technique. Among factors that affect the performance of LSWF, geochemical interaction at the oil-brine interface and the associated enterokinetic properties plays a prominent role. This work presents a triple-layer surface complexation model for predicting the zeta potential of the oil-brine interface. We improve upon previous modelling studies by incorporating the effects of (1) temperature variation, (2) basic (-NH) oil surface group interactions (3) adsorption of sodium ions on outer and inner Helmholtz planes and (4) the presence of sulphate ions in brine. Model validation against published experimental data shows model accuracy between 66% and 99%. The model displays higher accuracies at lower salinities (making it particularly suited for LSWF applications), intermediate pH and higher total acid number. In addition, a correlation between (-NH) site density and total acid/base numbers is proposed. A sensitivity study performed utilising the developed model showed that higher sulphate concentration in the brine and elevated temperature makes the zeta potential at the oil-brine interface more negative. In addition, the sensitivity study indicates that a higher concentration of basic polar oil compounds is less favourable as it may result in less water-wet conditions in the reservoir.

Electrostatic properties of C–S–H and C-A-S-H for predicting calcium and chloride adsorption

The adsorption capacity of cement hydrates considerably affects the ionic ingress into cementitious materials. In this study, the surface electrostatic properties of calcium silicate hydrate (C–S–H) and calcium aluminosilicate hydrate (C-A-S-H) were determined to understand the effects of the properties on calcium and chloride adsorption. The density of the surface functional groups was determined by analysing the structure of C–S–H and C-A-S-H through 27Al and 29Si MAS NMR. The surface sites of ≡SiOH and ≡AlOH are available in C-A-S-H whereas C–S–H has ≡SiOH groups for ionic adsorption. We found that the incorporation of aluminium decreases the number of total adsorption sites in C-A-S-H. Furthermore, the site density increased with Ca/(Si + Al). To understand the C-A-S-H/solution interface, a triple-layer surface complexation model was developed and the associated equilibrium constants for deprotonation, calcium, and chloride adsorption were determined by fitting the experimental data of potentiometric titration and zeta potential measurement results. The estimated surface complexation modelling parameters were verified by predicting the experimental data of calcium and chloride adsorption on C–S–H and C-A-S-H.

Effect of Electrokinetics and Thermodynamic Equilibrium on Low-Salinity Water Flooding for Enhanced Oil Recovery in Sandstone Reservoirs.

Wettability alteration (from oil-wet to mixed- or water-wet condition) is the most prominent mechanism in low-salinity water flooding (LSWF) for enhanced oil recovery (EOR) in sandstone reservoirs. Although several factors influence the wettability alteration, many efforts have been made to find the main controlling factor. In this study, the influence of interface properties of sandstone/brine and thermodynamic equilibrium of sandstone minerals were evaluated to understand the wettability alteration during LSWF. A triple-layer surface complexation model built-in PHREEQC was applied to a quartz/brine interface, and the modeling results were verified with zeta potential experimental data. This model was combined with that of kaolinite/brine to predict sandstone/brine interface properties. The measured and predicted sandstone zeta potentials were between those obtained for quartz and kaolinite in the diluted seawater. The predicted surface potential of sandstone together with that of crude oil was used in extended Derjaguin–Landau–Verwey–Overbeek theory to estimate the attractive or repulsive force. Consideration of thermodynamic equilibrium between minerals and solution significantly increased the pH and hence resulted in an increase in negative surface potential in the surface complexation. This provided a strong repulsive force between crude oil and sandstone, thus resulting in a more water-wet condition.

Adsorption of nitrogen heterocyclic compounds (NHC) on soil minerals: Quinoline as an example

Understanding the adsorption behavior of nitrogen heterocyclic compounds (NHCs) on soil minerals underlies more effective design of wastewater treatment and soil/groundwater remediation. We investigated adsorption of quinoline, a representative NHC, on quartz sand (using Berea sandstone as a model), at 3 < pH < 9, in low (0.05 M) and high (0.7 M) ionic strength solutions where NaCl was the background electrolyte. Minor clay (kaolinite) in the sandstone contributed significantly to quinoline uptake. Adsorption peaked at pH∼6. It decreased from 2 equivalent monolayers at low ionic strength to a monolayer at NaCl activity approaching that of seawater. A triple layer surface complexation model fits the data well, where quartz and kaolinite contributed sites for three types of quinoline (Q) complexes: 1) innersphere ≡SiOHQ; 2) outersphere ≡SiO-QH+ and 3) innersphere ≡AlOHQ2. Aluminol kaolinite sites promote multilayer quinoline adsorption, whereas only monolayers form on silanol sites. Site density calculations and molecular dynamics (MD) confirmed that quinoline adsorbs upright, on edge, and multilayer adsorption follows formation of the initial monolayer. Our results confirm the effectiveness of sand(stones) and clays for removing NHCs from waste and groundwaters.

Crude oil/brine/rock interface in low salinity waterflooding: Experiments, triple-layer surface complexation model, and DLVO theory

The determination of the electrokinetic properties of crude oil/brine and rock/brine interfaces is necessary to understand and evaluate the low salinity waterflooding (LSWF) effect on wettability alteration and enhanced oil recovery (EOR). The interface properties and characteristics of oil and rock minerals significantly affect LSWF and must be elucidated in detail. Herein, the interaction of crude oil, brine, and rock was studied to elucidate the LSWF effect in carbonate and sandstone reservoirs. A triple-layer surface complexation model was proposed to describe the calcite/brine and kaolinite/brine interfaces and was verified with zeta potential experiments at various pH and calcium, magnesium, and sulphate concentrations. Furthermore, the surface complexation model prediction of zeta potential agreed well with the measured data for the suspension in seawater, formation water, and associated dilutions. The predicted surface potential of crude oil, calcite, and kaolinite was used in Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to estimate the total disjoining pressure as attractive/repulsive forces. The interaction between crude oil and calcite/kaolinite on seawater dilution and in the presence of sulphate in the diluted solution for calcite was evaluated via disjoining pressure. Moreover, the importance of electrokinetic properties on attractive/repulsive forces and the mechanisms for wettability alteration in crude oil-brine-kaolinite due to LSWF as well as in oil-brine-calcite because of sulphate addition are discussed.

Sorption of americium / europium onto magnetite under saline conditions: Batch experiments, surface complexation modelling and X-ray absorption spectroscopy study

HypothesisThis study investigates the adsorption of americium and its chemical analogue europium on magnetite, which is expected to form as a major long-term steel canister corrosion product under anoxic and highly saline conditions. ExperimentsThe sorption of europium on magnetite (solid/liquid ratio = 0.5 g/L) was investigated batch wise in NaCl brines with ionic strength I = 1 m, 3.5 m, and 6.67 m, as a function of pHm for two europium concentrations (6 × 10−10m, 1.2 × 10−5m). Information on the chemical nature of the surface species was obtained by X-ray absorption spectroscopy (XAS) at the americium L3-edge. FindingsRetention of europium by magnetite of >99.5% was found above pHm 6.4 for all ionic strengths for europium concentration of 6 × 10−10m. No ionic strength effect was observed in this pHm range. At 1.2 × 10−5m europium concentration, 95 ± 4% sorption was found above pHm 7.5 for I = 1 m and above pHm 8.0 for I = 3.5 m and 6.67 m. A small ionic strength effect was observed in this case. X-ray absorption spectroscopy (XAS) results are consistent with the batch sorption experiment outcomes, showing an insignificant effect of ionic strength on the pHm dependent sorption. Results from potentiometric titrations of the solid phase, batch sorption experiments and spectroscopy were interpreted consistently with a charge distribution multi-site (CD-MUSIC) triple layer surface complexation model assuming surface coordination of the metal ion via a tridentate binding mode.

Predicting the electrokinetic properties of the crude oil/brine interface for enhanced oil recovery in low salinity water flooding

The low-salinity waterflooding (LSWF) technique during enhanced oil recovery has received increasing attention over the last decade. Several studies have attempted to understand the effects of LSWF through both experiments and modelling, but their results are inconsistent due to a lack of understanding of the crude oil/brine and brine/rock interfaces. In this paper, the crude oil/brine interface was studied by developing a triple-layer surface complexation model. The carboxyl groups (–COOH) were attributed to the surface charge and electrical triple-layer development of the crude oil in LSWF. The zeta potentials of the emulsion at various pH levels and the calcium and magnesium concentrations were measured to examine the interface. These data were then directly fitted to the simulated zeta potentials to determine the surface site density of –COOH and the associated equilibrium constants for the dissociation and adsorption of calcium and magnesium. The –COOH site density was determined by fitting the pH-independent zeta potential, while the equilibrium constant values were estimated from the variations in the zeta potential with the changes in pH and the concentrations of calcium and magnesium. The determined surface complexation parameters were validated by comparing the experimental zeta potential data from different ionic solutions. The developed surface complexation model was used along with the estimated parameters to predict the interface of crude oil in seawater, formation water, and their dilutions. The simulated zeta potential results agreed well with the experimental data, demonstrating that the model is applicable to understand the crude oil/brine interface in LSWF. Finally, the importance of the prediction of the surface and zeta potentials in the evaluation of the interface and the estimation of electrostatic forces, and thus the wettability alteration, was discussed.

Effect of ionic strength on barium transport in porous media

Hydraulic fracturing (or fracking) is a well stimulation technique used to extract resources from a low permeability formation. Currently, the most common application of fracking is for the extraction of oil and gas from shale. During the operation, a large volume of brine, rich in hazardous chemicals, is produced. Spills of brine from wells or pits might negatively impact underground water resources and, in particular, one of the major concerns is the migration of radionuclides, such as radium (Ra2+), into the shallow subsurface. However, the transport behaviour of Ra2+ through a reactive porous medium under conditions typical of a brine, i.e., high salinity, is not well understood, yet. Here, a study on the transport behaviour of barium (Ba2+, congener of radium) through a porous medium containing a common mineral such as goethite (FeO(OH)) is presented. Batch and column flood tests were carried out at conditions resembling the produced brine, i.e., large values of ionic strength (I), namely, 1 to 3mol/kg. The measurements were described with the triple layer surface complexation model coupled with the Pitzer activity coefficient method and a reactive transport model, in the case of the transport tests. The experimental results show that the adsorption of Ba2+ onto FeO(OH) increases with pH but decreases with I and it becomes negligible at the brine conditions. Moreover, even if isotherms show adsorption at large I, at the same conditions during transport, Ba2+ travels without retardation through the FeO(OH) porous medium. The triple layer model agrees very well with all batch data but it does not describe well the transport tests in all cases. In particular, the model cannot match the pH measurements at large I values. This suggests that the chemical reactions at the solid-liquid interface do not capture the mechanism of Ba2+ adsorption onto FeO(OH) at large salinity. Finally, this study suggests that barium, and potentially its congeners, namely, radium, calcium, magnesium, and strontium, may travel at the average flow velocity through a soil where the dominant reactive mineral is goethite.

Adsorption of Antimonate by Kaolinite

Core Ideas Sb(V) adsorption by kaolinite is pH and ionic strength dependent, with adsorption decreasing with increasing pH and ionic strength. Sb(V) adsorption is hysteretic in acidic systems, but reversible in alkaline. Adsorbed Sb(V) generates a downward shift in the isoelectric point and an upward shift in the point of zero net proton charge. Phosphate decreases Sb(V) adsorption, but the influence of sulfate is minimal. The macroscopic findings indicate that Sb(V) is retained by innersphere in acidic and outersphere in neutral to alkaline systems. The environmental fate and behavior of antimony (Sb) is controlled by adsorption processes at the solid–solution interface of surface‐reactive solids. One such solid is kaolinite, which bears the aluminol (≡AlOH) surface functional group. The objectives of this study are to characterize antimonate [Sb(V) in the Sb(OH)6– species] adsorption by kaolinite as a function of pH, ionic environment, and competing ligands (PO4 and SO4); to describe the reversibility of the adsorption and the influence of adsorbed Sb(V) on the surficial properties of kaolinite; and to develop predictive surface complexation models. The results of this study show adsorbed Sb(V) is at a maximum in strongly acidic suspensions, and retention decreases with increasing pH and ionic strength. Adsorption is hysteretic in pH &lt; 5 suspensions, becoming reversible with increasing pH. The point of zero net proton charge (PZNPC) increases from 4.5 in KNO3 to 5.6 in KSb(OH)6. Correspondingly, the isoelectric point decreases from 4.5 in KNO3 to &lt;3 in KSb(OH)6. The macroscopic findings indicate that Sb(V) is retained by a combination of inner and outersphere mechanisms, with the former dominating in acidic suspensions. The inclusion of SO4 had little impact on the retention of Sb(V); however, PO4 reduced Sb(V) retention. The inclusion of both inner‐ and outersphere complexes in the triple layer surface complexation model (TLM) provided an adequate description of the Sb(V) adsorption envelopes. Further, the TLM successfully predicted ligand retention in competitive systems without re‐optimization.

The impact of particle size on the adsorption of citrate to hematite

We investigated the adsorption of citric acid on the surface of two different sized hematite nanoparticles using batch adsorption experiments, Fourier-transform infrared spectroscopy, surface complexation modeling and computational molecular modeling. Citrate adsorption reached a maximum between pH approximately 2.5 and 5.5 and declined as the pH was increased or decreased from that range. At high surface loading conditions, the dominant adsorbed citrate structure was outer-sphere in nature with a protonation state that varied with pH. At low pH, there was also evidence of an inner-sphere complex consistent with a binuclear, bidentate structure where the hydroxyl group was deprotonated and played an active role in the adsorption. An inner-sphere complex was also detected at low citrate surface loading conditions. Surface-area normalized surface coverages were similar for both sizes of hematite, however, the inner sphere complex appeared to be slightly more prevalent on the smaller hematite. Based on these structures, a triple layer surface complexation model comprised of two outer-sphere complexes and one inner-sphere complex was used to describe the adsorption data for both hematite sizes across a range of solution conditions with a single set of surface area dependent equilibrium constants.

Detection of ion adsorption at solid–liquid interfaces using internal reflection ellipsometry

We use imaging internal reflection ellipsometry (IRE) in combination with a microfluidic device to study the adsorption of inorganic salt ions to silica–water interfaces. In our data analysis, the measured polarization-dependent reflectivity is compared to calculations from a layer stack model, where the electric double layer is modeled as a separate layer. Due to the high resolution of our technique, we are able to quantify the adsorption of Na+ and Ca2+ ions from aqueous solutions of their chloride salts as a function of their bulk concentrations at pH 3 and 10. Our measurements demonstrate a preferential adsorption of Ca2+ counterions. The experimental results are well described by calculations using a triple layer surface complexation model for the electric double layer with published equilibrium constants.

Sorption behavior of hydroselenide (HSe−) onto iron-containing minerals

The sorption behavior of selenium onto iron-containing minerals such as goethite, ferrous oxide, magnetite, and biotite under reducing conditions were investigated by batch sorption experiments. Selenium was spiked as HSe− and Se42− in the experimental solutions and reducing conditions were maintained throughout the sorption periods. The sorption behaviors of HSe− were analyzed by the triple-layer surface complexation model with the Visual Minteq computer program. The sorption data and the model calculations suggested that the dominant sorption mechanisms of HSe− were inner-sphere surface complexation for goethite and ferrous oxide but outer-sphere surface complexation for magnetite and biotite. The intrinsic equilibrium constant of the inner-sphere surface complexation reaction of HSe− with goethite, logKintHSe = 6.0, was determined by the fitting to the experimental results. The value of the equilibrium constant for outer-sphere complexation for magnetite and biotite was estimated to be around 12.

Arsenic(III, V) adsorption on a goethite-based adsorbent in the presence of major co-existing ions: Modeling competitive adsorption consistent with spectroscopic and molecular evidence

Adsorption of the two oxyanions, arsenate (As(V)) and arsenite (As(III)), on a common goethite-based granular porous adsorbent is studied in the presence of major co-existing ions in groundwater (i.e., phosphate, silicic acid, sulfate, carbonate, magnesium, and calcium) and predicted using the extended triple layer model (ETLM), a dipole modified single-site triple layer surface complexation model consistent with spectroscopic and molecular evidence. Surface species of all ions were selected according to the previous ETLM studies and published experimental spectroscopic/theoretical molecular information. The adsorption equilibrium constants for all ions were determined using adsorption data obtained in single-solute systems. The adsorption equilibrium constants referenced to the site-occupancy standard state (indicated by Kθ) were compared with those for goethite in the literature if available. The values of these constants for the goethite-based adsorbent are found to be close to the values for goethite previously studied. These “constrained” adsorption equilibrium constants determined in single-solute systems were used in the ETLM to predict the competitive interactions of As(III, V) with the co-existing ions in binary-solute systems. The ETLM is capable of predicting As(III, V) adsorption in the presence of oxyanions (phosphate, silicic acid, sulfate, and carbonate). This study presents the first successful and systematic prediction of the competitive interactions of As(III, V) with these oxyanions using the ETLM. The ETLM prediction of surface (and aqueous) speciation also provides insights into the distinct adsorption behavior of As(III, V) in the presence of the oxyanions. Magnesium and calcium significantly enhanced As(V) adsorption at higher pH values, while they had little effect on As(III) adsorption. The enhanced adsorption of As(V), however, could not be predicted by the ETLM using the surface species proposed in previous ETLM studies. Further studies are necessary to identify ternary complexes, especially at high pH. Adsorption isotherms of As(V), a dominant form of arsenic in adsorptive water treatment systems, in the presence of the co-existing ions under relevant conditions of water treatment systems are also obtained and predicted by the ETLM to study and compare the effect of the co-existing ions on As(V) removal.

In situ ATR–FTIR and surface complexation modeling studies on the adsorption of dimethylarsinic acid and p-arsanilic acid on iron-(oxyhydr)oxides

Arsenic is an element that exists naturally in many rocks and minerals around the world. It also accumulates in petroleum, shale, oil sands, and coal deposits as a result of biogeochemical processes, and it has been found in fly ash from the combustion of solid biofuels. Arsenic compounds in their organic and inorganic forms pose both a health and an environmental risk, and continue to be a challenge to the energy industry. The environmental fate and removal technologies of arsenic compounds are controlled to a large extent by their surface interactions with inorganic and organic adsorbents. We report thermodynamic binding constants, K binding, from applying the triple-layer surface complexation model to adsorption isotherm and pH envelope data for dimethylarsinic acid (DMA) and p-arsanilic acid (p-AsA) on hematite and goethite. Ligand exchange reactions were constructed based on the interpretation of ATR–FTIR spectra of DMA and p-AsA surface complexes. Surface coverage of adsorbates was quantified in situ from the spectral component at 840 cm −1. The best fit to the DMA adsorption data was obtained using outer-sphere complex formation, whereas for p-AsA, the best fit was obtained using two monodentate inner-sphere surface complexes. The significance of the results is discussed in relation to improving modeling tools used by environmental regulators and the energy sector for optimum control of arsenic content in fuels.

Interactions of Al(III), La(III), Gd(III), and Lu(III) with the Fused Silica/Water Interface Studied by Second Harmonic Generation

The interactions of the trivalent metal cations Al(III), La(III), Gd(III), and Lu(III) with the silica/water interface were studied using the nonlinear optical technique of second harmonic generation (SHG). Specifically, the Eisenthal chi(3) technique was used to quantify the thermodynamics of trivalent ion adsorption to the bare fused silica surface. SHG adsorption isotherms were measured and fit with the triple layer surface complexation model to obtain adsorption free energies, binding constants, and interfacial charge densities. The adsorption free energy for Al(III) was found to be -37.2(5) kJ/mol, while the adsorption free energies for the three trivalent lanthanide cations ranged from -29.9(9) to -32.2(7) kJ/mol. Despite identical ionic charges, the metals under investigation displayed different affinities for the fused silica/water interface, and this finding is analyzed and interpreted in the context of size-dependent metal cation properties and metal ion speciation. The thermodynamic results from this work are valuable benchmarks for computer simulations of trivalent metal transport in the environment.