Surface Complexation Modeling Approach Research Articles

Sorption and mobility of radioactive Ni in Wolsong waste repository site, South Korea

Radioactive nickels are significant environmental concerns because of their long half-lives (59Ni = 76,000 years and 63Ni = 100.1 years) and high proportions in nuclear wastes in the repositories. Therefore, the investigation of sorption and mobility of Ni at the disposal facility sites is important for the prediction and evaluation of radiological risks to the public. Herein, the batch sorption and column experiments were performed under various geochemical conditions using site-specific rock and groundwater samples collected at the Wolsong low and intermediate level waste (LILW) disposal facility site in South Korea. The batch sorption results revealed that the sorption of Ni was strongly affected by the pH. Approximately 20% Ni adsorbed at pH 5, which enhanced over 90% at pH > 8. The batch sorption results were used to develop the surface complexation model (SCM) to predict the Ni sorption and its mobility behavior. The non-electrostatic generalized composite (GC) SCM approach simulated the Ni sorption data well. We believe, our study can interpret and extrapolate the sorption and transport behavior of Ni in the underground repository conditions.

Geochemical Analysis of Hardness on the Adsorption of Surfactants in Carbonates Under Severe Thermodynamic Conditions: Surface Complexation Modeling Approach

Abstract Several core-flooding-based experimental studies demonstrated the effect of calcium and magnesium ions and it is found that these hard ions have detrimental effects on oil recovery during chemical-enhanced oil recovery operations. However, studies regarding the coupled effect of hard ions and surfactant adsorption are very limited. Thus, this study aims to present a novel approach that can capture mineral-brine, brine-oil, and brine-surfactant interactions in the presence of hard ions (Ca2+ and Mg2+). Also, we introduced four oil-surfactant-based surface complexation geochemical reactions (SCGR) in the presence of hard ions for the first time to analyze the oil-surfactant interactions. The developed thermodynamic-based geochemical model is compared and validated with recent core-flooding data. Our results illustrate that the use of oil-surfactant SCGR is important and should be captured for detailed surfactant adsorption. Thus, we observed that in the presence of hard ions, surfactant adsorption increases with the increase in temperature, which is due to the surge in kinetic energy. We also observed that a reduction in hardness reduces the adsorption of surfactants. Additionally, increasing surfactant concentration led to a minor increase in the adsorption of surfactant with a significant increase in its concentration in the discharge/effluent. Therefore, the hard ions (Ca2+ and Mg2+) concentration has a substantial negative effect, as they reduce the solubility of surfactant and increase its adsorption. Furthermore, the lowest level of surfactant adsorption was accomplished by injecting ten times diluted water (<0.070 mg/g).

The impact of aggregation between clay and phytoplanktonic cyanobacteria on trace elemental cycling in coastal environments

Clay minerals and cyanobacteria occur ubiquitously in nature, and both independently influence trace metal cycling due to their high specific surface areas and reactivity. Although the surface reactivity of each is well-studied, the reactivity of clay-cyanobacterial aggregates has received less attention. Here, we performed potentiometric acid-base titrations and Cd adsorption experiments on various concentrations of clay-cyanobacterial aggregates using three clay minerals (kaolinite, illite, and montmorillonite) and two model planktonic cyanobacteria, the freshwater species Synechocystis sp. PCC 6803 and marine species Synechococcus sp. PCC 7002. A component additivity surface complexation modeling (CA-SCM) approach was applied to study the impacts of the aggregation processes on the total surface reactivity. Results demonstrate that the CA-SCM approach effectively predicted the acid-base titration and Cd adsorption behaviors of aggregates at low aggregate concentrations (up to 2.5 g of clay/L for acid-base titration and 0.1 g of clay/L for Cd adsorption), indicating the additivity of the reactivity for the two components. However, at higher aggregate concentrations (25 g of clay/L for acid-base titration and 10 g of clay/L for Cd adsorption), the CA-SCM approach overestimates the acid-base titration and Cd adsorption behaviors for the aggregates, indicating significant surface site blockage during aggregation. Our SEM images show that both cyanobacterial species were covered by layered clay minerals at the surfaces and formed relatively larger aggregates at high aggregate concentrations than at low aggregate concentrations. Based on the combined results, we propose that the interaction between clays and cyanobacteria is relatively weak at low aggregate concentrations (e.g., <10 g of aggregate/L), but that significant obstruction of surface adsorption sites occurs at high aggregate concentrations (e.g., >10 g of aggregate/L), possibly due to the encrustation of bacteria by clay minerals. The implication of this work is that where turbid rivers discharge into estuaries or deltas with high primary phytoplankton productivity, aggregation (or flocculation) processes will cause the depression of surface reactivity of the aggregates and lead to a decrease in metal-binding capacity. Therefore, depending on whether the trace elements are nutrients (e.g., phosphorous) or potentially toxic (e.g., cadmium), this process has the potential to impact the marine biosphere. To ground truth this hypothesis, we further analyzed the correlation between total organic carbon (TOC) content and trace metal compositions in a compiled shale database and the results show clear negative correlation between Cd concentration and the concentration of TOC, Al and Si. Although this offers only an indirect assessment of the role of clay-plankton aggregation, as a first-order test it does indeed show that the aggregation between clay minerals and phytoplanktonic bacteria in estuaries is an essential factor that influences trace-elemental cycling.

Adsorption of chalcophile metals onto thiol-based resins: Modeling adsorption capacity and behavior using surface complexation modeling

Adsorption of chalcophile metals from aqueous solution by thiol resins represents an efficient means of reusing these metals and remediating waste streams, and a predictive modeling approach is needed in order to design and optimize adsorption applications. In this study, potentiometric titration measurements, metal adsorption experiments and surface complexation modeling (SCM) approaches were used to characterize three thiol-based resins (BT40, SP70, and SP300), and to model their proton, Cd, Zn, and Pb adsorption capacities and behaviors. We find that the extent of Cd adsorption onto the three resins correlates strongly to the measured concentration of proton-active thiol sites on each resin, and that the BT40 resin has a significantly higher concentration of proton-active thiol sites than the other two resins tested. Using the BT40 as a model resin, we demonstrate that the measured stability constant (K) of the Cd-thiol surface complex on the resin successfully accounts for the effects of both the metal:resin ratio and pH over a wide range of experimental conditions. We also find that a linear free-energy relationship (LFER) exists between the K values of metal-thiol surface complexes on the BT40 resin and the K values of metal-thiol aqueous complexes for Cd, Zn and Pb. This LFER can be used to estimate the K values of other metal-thiol resin surface complexes that have not been measured experimentally, therefore enabling prediction of the adsorption behavior of a wide range of metals onto thiol-based resins under a wide range of system conditions. This study demonstrates that a SCM approach, coupled with potentiometric titration measurements and metal adsorption experiments, represents a powerful tool for the design and optimization of strategies for separating and recovering chalcophile metals from aqueous media.

Surface complexation modeling of the effects of dissolved inorganic carbon on adsorption of U(VI) onto Fe3O4 nanoparticles coated with lignite humic acid

Abstract Lignite humic acid (LHA) surface-functionalized magnetite nanoparticles (NPs) represent a promising sorbent for removal of U(VI) from industrial waste streams and contaminated environmental waters, but the effects of dissolved inorganic carbon (DIC) on the adsorption mechanisms and efficiency are poorly constrained. We measured the adsorption of U(VI) onto LHA-coated Fe3O4 NPs as a function of pH, ionic strength, adsorbent concentration, DIC concentration and dissolved calcium concentration. The observed U(VI) adsorption onto LHA-coated Fe3O4 NPs is ionic strength independent, and below pH 5, the extent of U(VI) adsorption increases with increasing pH and adsorbent concentration. The extent of U(VI) adsorption decreases dramatically with increasing concentration of added NaHCO3 and CaCl2 above pH 5 and pH 6 respectively. The adsorption data were modeled using a non-electrostatic surface complexation modeling approach to determine the identities and thermodynamic stabilities of possible uranyl complexes on the LHA-coated Fe3O4 NP surfaces. The modeling results indicate that L1-(UO2)+, >L2-UO2(OH), >L2-(UO2)3(OH)5, >L2-UO2(CO3)-, >L3-(UO2)2(CO3)(OH)32-, >L4-UO2(CO3)23- and >L4-(UO2)3(OH)72- are involved in the adsorption under different pH (3.0–9.5) and DIC (0–25 mM) concentration conditions. The log stability constant values (±2σ) of these uranyl surface complexes are calculated as 4.59(±0.12), 6.59(±0.14), 6.33(±0.13), 6.08(±0.18), 6.16(±0.06), 7.07(±0.19) and6.13(±0.15). The model can be applied to predict the distribution of U(VI) in complex LHA-coated Fe3O4 NP-bearing systems under a wide range of pH, ionic strength, DIC, and solute: sorbent conditions.

Cadmium adsorption to clay-microbe aggregates: Implications for marine heavy metals cycling

Interactions between microorganisms and clay minerals influence the transport and cycling of metal contaminants in both marine and terrestrial environments. The present study was conducted to quantify the adsorption of dissolved cadmium, Cd(II), under seawater-like conditions to the marine cyanobacterium Synechococcus sp. PCC 7002, three common clay minerals (kaolinite, montmorillonite and illite), as well as cell-clay aggregates. We show here that the Synechococcus-only experiments removed the most Cd above pH 5.5, followed in decreasing order by aggregates of 50% cells:50% individual clays, aggregates of cells and all 3 clays, and individual clays. Electron microscope imaging showed that clays associated in a tangential edge-on orientation to the cells in Synechococcus-clay mineral aggregates. A non-electrostatic surface complexation modeling approach was used to fit Cd adsorption onto Synechococcus cells and individual clay minerals. The resulting Cd binding constants were then used in consort with surface functional group pKa values and site concentrations to accurately predict the extent of Cd adsorption onto the Synechococcus-clay mineral aggregates using the component additivity (CA) approach. We observed that the addition of cyanobacterial cells to clay mineral suspensions led to significantly larger mean aggregate sizes of clay minerals, enhancing the clay sedimentation rate. Although specifically focused on Cd, our study indicates that the ratio of bacterial plankton to clay minerals is an important determinant in terms of understanding the rate with which metals are transferred from the water column to the seafloor.

Competitive Adsorption of As(III), As(V), and PO4 by an Iron Oxide Impregnated Activated Carbon: Surface Complex Modeling

The objective of this study was to predict the competitive adsorption of As(III), As(V), and PO4 by an iron oxide impregnated carbon (L-Act, 9% Fe(III) amorphous iron oxide) over a range of environmental conditions using the surface complexation modeling (SCM) approach. L-Act surface complexation constants determined from a single pH-adsorption edge were used to predict pH-dependent competitive removal in singular, binary, and tertiary adsorbate systems. As(III), As(V), and PO4 complexes were modeled as bidentate binuclear species at low pH and monodentate species at high pH using the two monoprotic surface site/diffuse electric double layer model (2MDLM). F values determined based on 2MDLM predictions were close to those calculated by FITEQL (a statistical optimization program) demonstrating the effectiveness of the 2MDLM in describing adsorption behavior. F values were generally in the recommended range of 0.1–20 indicating a good fit between the data and the model. The 2MDLM also successfully predicted As(III)/As(V)/PO4 adsorption data of hydrous ferric oxide and goethite adsorbents from the literature.

A Unified Surface Complexation Modeling Approach for Chromate Adsorption on Iron Oxides.

A multistart optimization algorithm for surface complexation equilibrium parameters (MUSE) was applied to a large and diverse data set for chromate adsorption on iron (oxy)hydroxides (ferrihydrite and goethite). Within the Basic Stern and the charge-distribution multisite complexation (CD-MUSIC) framework, chromate binding constants and the Stern Layer capacitance were optimized simultaneously to develop a consistent parameter set for surface complexation models. This analysis resulted in three main conclusions regarding the model parameters: (a) There is no single set of parameter values that describes such diverse data sets when modeled independently. (b) Parameter differences among the data sets are mainly due to different amounts of total sites, i.e., surface area and surface coverages, rather than structural differences between the iron (oxy)hydroxides. (c) Unified equilibrium constants can be extracted if total site dependencies are taken into account. The implementation of the MUSE algorithm automated the process of optimizing the parameters in an objective and consistent manner and facilitated the extraction of predictive relationships for unified equilibrium constants. The extracted unified parameters can be implemented in reactive transport modeling in the field by either adopting the appropriate values for each surface coverage or by estimating error bounds for different conditions. The evaluation of a forward model with unified parameters successfully predicted chromate adsorption for a range of capacitance values.

Mechanisms of the Removal of U(VI) from Aqueous Solution Using Biochar: A Combined Spectroscopic and Modeling Approach.

Biochar has been touted as a promising sorbent for the removal of inorganic contaminants, such as uranium (U), from water. However, the molecular-scale mechanisms of aqueous U(VI) species adsorption to biochar remain poorly understood. In this study, two approaches, grounded in equilibrium thermodynamics, were employed to investigate the U(VI) adsorption mechanisms: (1) batch U(VI) adsorption experiments coupled to surface complexation modeling (SCM) and (2) isothermal titration calorimetry (ITC), supported by synchrotron-based X-ray absorption spectroscopy (XAS) analyses. The biochars tested have considerable proton buffering capacity and most strongly adsorb U(VI) between approximately pH 4 and 6. FT-IR and XPS studies, along with XAS analyses, show that U(VI) adsorption occurs primarily at the proton-active carboxyl (-COOH) and phenolic hydroxyl (-OH) functional groups on the biochar surface. The SCM approach is able to predict U(VI) adsorption behavior across a wide range of pH and at varying initial U(VI) and biochar concentrations, and U adsorption is strongly influenced by aqueous U(VI) speciation. Supporting ITC measurements indicate that the calculated enthalpies of protonation reactions of the studied biochar, as well as the adsorption of U(VI), are consistent with anionic oxygen ligands and are indicative of both inner- and outer-sphere complexation. Our results provide new insights into the modes of U(VI) adsorption by biochar and more generally improve our understanding of its potential to remove radionuclides from contaminated waters.

Change of the point of zero net proton charge (pHPZNPC) of clay minerals with ionic strength

In a system where protons and hydroxide ions are the only aqueous species, the point of zero net proton charge (pHPZNPC) of a mineral defines the pH at which the positively and negatively charged functional groups on its surface are equal (Drever, 1997). Ascertaining the pHPZNPC of clay minerals, a ubiquitous component in soils, sediment and rivers, is useful in predicting its electrostatic interactions with charged aqueous species, colloids, and bacteria. While the pHPZNPC values of most clays have been reported, the variation of pHPZNPC with changing solution ionic strength (IS) and the mathematic relationship between them are not well-understood but critical in assessing the surface reactivity of clays in aqueous solutions ranging from freshwater to brines. To address this gap, we studied the relationship of the pHPZNPC of three clay minerals (kaolinite, illite, and montmorillonite) at seven ionic strengths (from 0.001 to 0.1 M). Titration data for each clay were used to calculate the pHPZNPC by two methods previously documented in literature: (1) using the difference between the blank titration and clay titrations, and (2) by the difference between the number of protons added during titration and the number of protons remaining in solution. The results show that: (1) unlike simple metal oxides (e.g., hematite, gibbsite, quartz), titration curves of clay minerals at different IS do not intersect at a common pHPZNPC value; (2) the pHPZNPC for kaolinite is around 5.6 to 6.6 while the pHPZNPC for illite and montmorillonite is in the range of 9 to10; (3) the pHPZNPC value decreases systematically with increasing IS for all three clay minerals studied; and (4) the change of pHPZNPC is linear with log(IS). A third method using surface complexation modeling (SCM) approach was applied to calculate the pHPZNPC of the clay minerals, and the results match well with Method 2. Our results allow for a more accurate estimation of clay surface charge property in aqueous environment, which, in turn, will improve model predictions of the adsorption of charged species in systems in which ionic strength changes.

Mechanistic Understanding of Uranyl Ion Complexation on Montmorillonite Edges: A Combined First-Principles Molecular Dynamics-Surface Complexation Modeling Approach.

Systematic first-principles molecular dynamics (FPMD) simulations were carried out to study the structures, free energies, and acidity constants of UO22+ surface complexes on montmorillonite in order to elucidate the surface complexation mechanisms of the uranyl ion (UO22+) on clay mineral edges at the atomic scale. Four representative complexing sites were investigated, that is, ≡Al(OH)2 and ≡AlOHSiO on the (010) surface and ≡AlOHOa and ≡SiOOa on the (110) surface. The results show that uranyl ions form bidentate complexes on these sites. All calculated binding free energies for these complexes are very similar. These bidentate complexes can be hydrolyzed, and their corresponding derived p Ka values (around 5.0 and 9.0 for p Ka1 and p Ka2, respectively) indicate that UO2(OH)+ and UO2(OH)2 surface groups are the dominant surface species in the environmental pH range. The OH groups of UO2(OH)2 surface complexes can act as complexing sites for subsequent metals. Additional simulations showed that such multinuclear adsorption is feasible and can be important at high pH. Furthermore, FPMD simulation results served as input parameters for an electrostatic thermodynamic surface complexation model (SCM) that adequately reproduced adsorption data from the literature. Overall, this study provides an improved understanding of UO22+ complexation on clay mineral edge surfaces.

Thermodynamic Analysis of Nickel(II) and Zinc(II) Adsorption to Biochar

While numerous studies have investigated metal uptake from solution by biochar, few of these have developed a mechanistic understanding of the adsorption reactions that occur at the biochar surface. In this study, we explore a combined modeling and spectroscopic approach for the first time to describe the molecular level adsorption of Ni(II) and Zn(II) to five types of biochar. Following thorough characterization, potentiometric titrations were carried out to measure the proton (H+) reactivity of each biochar, and the data was used to develop protonation models. Surface complexation modeling (SCM) supported by synchrotron-based extended X-ray absorption fine structure (EXAFS) was then used to gain insights into the molecular scale metal-biochar surface reactions. The SCM approach was combined with isothermal titration calorimetry (ITC) data to determine the thermodynamic driving forces of metal adsorption. Our results show that the reactivity of biochar toward Ni(II) and Zn(II) directly relates to the site densities of biochar. EXAFS along with FT-IR analyses, suggest that Ni(II) and Zn(II) adsorption occurred primarily through proton-active carboxyl (-COOH) and hydroxyl (-OH) functional groups on the biochar surface. SCM-ITC analyses revealed that the enthalpies of protonation are exothermic and Ni(II) and Zn(II) complexes with biochar surface are slightly exothermic to slightly endothermic. The results obtained from these combined approaches contribute to the better understanding of molecular scale metal adsorption onto the biochar surface, and will facilitate the further development of thermodynamics-based, predictive approaches to biochar removal of metals from contaminated water.

Application of surface complexation modeling to trace metals uptake by biochar-amended agricultural soils

Biochar has emerged as a useful amendment to release nutrients into agricultural soil, to increase crop productivity, and as a sorbent to remediate metals and organics contamination. Since soils have heterogeneous physical properties across a given crop field, and even over a growing season, it is imperative to select the most appropriate biochar for the intended purpose and in defining the amendment level. In this study, we investigate the adsorption of Cd(II) and Se(VI) as model pollutant cations and anions, respectively, to two agricultural soils amended with a wood pin chip biochar (WPC). The proton reactivity of each sorbent was determined by potentiometric titration, and single-metal, single-sorbent experiments were conducted as a function of pH. The resulting data were modeled using a non-electrostatic surface complexation modeling (SCM) approach to determine the proton and metal binding constants and surface functional group concentrations of each soil and WPC. The SCM approach is a considerable advance over empirical modeling approaches because SCM models can account for changes in pH, ionic strength, temperature, and metal-to-sorbent ratio that may happen over the course of a growing season. The constants derived from the single-metal, single-sorbent experiments were then used to predict the extent of metal adsorption in more complex mixtures of Cd, Se, soil and WPC. Overall the SCM approach was successful in predicting metal distribution in multi-component mixtures. In cases where the predictions were poorer than expected, we identify reasons and discuss future experiments needed to further the application of SCM to sorbent mixtures containing biochar.

As(V) and PO4 Removal by an Iron-Impregnated Activated Carbon in a Single and Binary Adsorbate System: Experimental and Surface Complexation Modeling Results

AbstractThe objective of this study was to predict As(V) removal by an iron-oxide impregnated carbon (L-Act, 9% Fe(III) amorphous iron-oxide) over a range of environmental conditions using the surface complexation modeling (SCM) approach. The L-Act surface sites and electric double layer were modeled using the two monoprotic site-diffuse layer model (2MDLM). As(V) and PO4 surface complexes were modeled as bidentate complexes at low hydrogen ion concentration (pH) and monodentate complexes at high pH. L-Act-As/PO4 surface complexation constants were determined from a single pH-adsorbent edge, and these were used to effectively predict pH-dependent removal of As(V) and PO4 across a range of adsorbent/adsorbate ratios in single and binary adsorbate systems and from a National Sanitation Foundation challenge water. The 2MDLM also successfully predicted As(V) removal by three literature-based iron-based adsorbents: two iron-impregnated ion exchange resins (PWX5, 16% Fe; PWX3, 10% Fe) and a hydrous ferric oxide...

Silicic acid competes for dimethylarsinic acid (DMA) immobilization by the iron hydroxide plaque mineral goethite

A surface complexation modeling approach was used to extend the knowledge about processes that affect the availability of dimethylarsinic acid (DMA) in the soil rhizosphere in presence of a strong sorbent, e.g., Fe plaques on rice roots. Published spectroscopic and molecular modeling information suggest for the organoarsenical agent to form bidentate-binuclear inner-sphere surface complexes with Fe hydroxides similar to the inorganic As oxyanions. However, since also the ubiquitous silicic acid oxyanion form the same bidentate binuclear surface complexes, our hypothesis was that it may have an effect on the adsorption of DMA by Fe hydroxides in soil. Our experimental batch equilibrium data show that DMA is strongly adsorbed in the acidic pH range, with a steep adsorption edge in the circumneutral pH region between the DMA acidity constant (pKa=6.3) and the point of zero charge value of the goethite adsorbent (pHpzc=8.6). A 1-pK CD-MUSIC surface complexation model was chosen to fit the experimental adsorption vs. pH data. The same was done for silicic acid batch equilibrium data with our goethite adsorbent. Both model parameters for individual DMA and silicic acid adsorption were then merged into one CD-MUSIC model to predict the binary DMA+Si adsorption behavior. Silicic acid (500μM) was thus predicted by the model to strongly compete for DMA with up to 60% mobilization of the latter at a pH6. This model result could be verified subsequently by experimental batch equilibrium data with zero adjustable parameters. The thus quantified antagonistic relation between DMA and silicic acid is discussed as one of factors to explain the increase of the DMA proportion in rice grains as observed upon silica fertilization of rice fields.

Surface complexation modeling of proton and metal sorption onto graphene oxide

The objective of this investigation was to develop a surface complexation modeling approach to account for proton and metal (Cd, Pb) binding onto many-layered graphene oxide (MLGO) across a range of pH and ionic strength conditions. MLGO particles exhibit large buffering capacities between pH 3 and pH 10 and the buffering behavior is only nominally influenced by ionic strength. In contrast, batch metal sorption experiments illustrate that the striking capacity of MLGO to sorb metals substantially diminishes with increases in ionic strength. X-ray absorption spectroscopy measurements were used to establish reaction stoichiometries and indicate that both Cd and Pb associate with single sites on the MLGO surface. The difference in sorption behaviors for protons and metals is best modeled using a 4-site non-electrostatic surface complexation model that accounts for ionic strength effects as a competition between Na from the background electrolyte and Cd/Pb for available MLGO sorption sites. Using this approach, titration data are used to constrain the site concentrations and pKa values for MLGO binding sites. The pKa values (±1σ) are calculated as 4.55 (±0.91), 6.52 (±0.49), 8.48 (±0.21), and 9.98 (±0.21). We then use these parameters and the metal sorption data to determine thermodynamic stability constants for each important Cd– and Pb–MLGO surface complex. The site concentrations and equilibrium constants provided herein are critical for developing and testing remediation strategies for specific water chemistries.

Equilibrium analysis for heavy metal cation removal using cement kiln dust.

Ion exchange, reverse osmosis, and chemical precipitation have been investigated extensively for heavy metal uptake. However, they are deemed too expensive to meet stringent effluent characteristics. In this study, cement kiln dust (CKD) was examined for the removal of target heavy metals. Adsorption studies in completely mixed batch reactors were used to generate equilibrium pH adsorption edges. Studies showed the ability of CKD to remove the target heavy metals in a pH range below that of precipitation after an equilibrium reaction time of 24 h. A surface titration experiment indicated negative surface charge of the CKD at pH below 10, meaning that electrostatic attraction of the divalent metals can occur below the pH required for precipitation. However, surface complexation was also important due to the substantive metal removal. Accordingly, a surface complexation model approach that utilizes an electrostatic term in the double-layer description was used to estimate equilibrium constants for the protolysis interactions of the CKD surface as well as equilibria between background ions and the sorbent surface. It was concluded that the removal strength of adsorption is in the order: Pb > Cu > Cd. The experiments were also supported by Fourier transform infrared spectroscopy (FTIR).

Surface complexation of antimony on kaolinite

Geochemical fate of antimony (Sb) – a similar oxyanion as arsenic (As) – in a variety of environment is largely unexplored. Kaolinite is an important, naturally occurring clay mineral in soils and aquifers and is known to control the fate of several contaminants via a multitude of geochemical processes, primarily adsorption. Here we report adsorption of antimony on kaolinite as a function of solution chemistry: initial antimony concentration, pH, ionic strength, and a competing anion. A surface complexation modeling (SCM) approach was undertaken to understand the potential mechanistic implications of sorption envelope data. In the SCM, a multicomponent additive approach, in which kaolinite is assumed to be a (1:1) mixture of quartz (SiOH) and gibbsite (AlOH), was tested. Results indicated that ionic strength has a minimal effect on antimony adsorption. For the lower initial antimony concentration (4.11μM), the additive model with binuclear surface complexes on quartz and gibbsite showed a better fit at pH<6, but somewhat under predicted the experimental data above pH 6. At the higher initial antimony concentration (41.1μM), the sorption envelope was of different shape than the lower load. The additive model, which considered binuclear surface complexes for quartz and gibbsite, resulted in over prediction of the adsorption data at pH>3.5. However, the additive model with binuclear surface complex on quartz and mononuclear surface complex on gibbsite showed an excellent fit of the data. Phosphate greatly influenced antimony adsorption on kaolinite at both low and high antimony loadings, indicating competition for available surface sites.

Uranium Reduction by Shewanella oneidensis MR-1 as a Function of NaHCO3 Concentration: Surface Complexation Control of Reduction Kinetics

It is crucial to determine the controls on the kinetics of U(VI) bioreduction in order to understand and model the fate and mobility of U in groundwater systems and also to enhance the effectiveness of U bioremediation strategies. In this study, we measured the rate of U(VI) reduction by Shewanella oneidensis strain MR-1 as function of NaHCO3 concentration. The experiments demonstrate that increasing concentrations of NaHCO3 in the system lead to slower U(VI) reduction kinetics. The NaHCO3 concentration also strongly affects the speciation of U(VI) on the bacterial cell envelope. We used a thermodynamic surface complexation modeling approach to determine the speciation and concentration of U(VI) adsorbed onto the bacteria as a function of the NaHCO3 concentration in the experimental systems. We observed a strong positive correlation between the measured U(VI) reduction rates and the calculated total concentration of U(VI) surface complexes formed on the bacterial cell envelope. This positive correlation indicates that the speciation and concentration of U(VI) adsorbed on the bacterial cell envelope control the kinetics of U(VI) bioreduction under the experimental conditions. The results of this study serve as a basis for developing speciation-based kinetic rate laws for enzymatic reduction of U(VI) by bacteria.

Uranium adsorption by Shewanella oneidensis MR-1 as a function of dissolved inorganic carbon concentration

Although the existence of uranyl-carbonate surface complexes on bacterial cell walls has been documented, the identities and thermodynamic stabilities of those complexes are poorly constrained by previous studies. We measured the adsorption of aqueous uranium [U(VI)] onto Shewanella oneidensis MR-1 with different initial concentrations of NaHCO3. Experiments were conducted in 0.1M NaClO4 to buffer ionic strength, and pH was varied from 3 to 9. The observed extent of U(VI) adsorption onto S. oneidensis was independent of dissolved inorganic carbon (DIC) concentration below pH5. Above pH5, the extent of adsorbed U(VI) decreased with increasing DIC concentration. A thermodynamic surface complexation modeling approach was used to interpret the adsorption data. A series of uranyl-, uranyl-hydroxide-, and uranyl-hydroxide-carbonate-bacterial surface complexes are required in order to account for the observed adsorption behavior, and we used the adsorption measurements to constrain values for the stability constants of these complexes. The modeling results suggest that uranyl-hydroxide-carbonate-bacterial surface complexes form and that competition between these complexes and uranyl-carbonate aqueous complexes controls the U(VI) adsorption behavior under high pH conditions in systems with high DIC concentrations. Hence, our results may be used to predict the extent of U(VI) adsorption onto bacteria in a range of natural and engineered DIC-bearing systems.