Simple Surface Complexation Model Research Articles

Modeling plutonium sorption to kaolinite: Accounting for redox equilibria and the stability of surface species

Plutonium with its particularly complex redox chemistry may be thermodynamically stable in the states +III to +VI depending on the redox conditions in the environment. Mineral surfaces can also affect Pu redox speciation. Therefore, the interpretation of Pu sorption data becomes particularly challenging, even for simplified laboratory experiments. The present study focuses on Pu sorption to kaolinite. Am(III), Th(IV), Np(V) and U(VI) literature sorption data are used as analogues for the corresponding Pu redox states to calibrate a simple surface complexation model, and the Nernst formalism is applied. Two independent pH–pe diagrams, one for the kaolinite surface and another for the aqueous phase, are constructed and superimposed. This allows visualization of the prevalent Pu redox state in both phases. The model suggests that the stability field of the most strongly adsorbing redox state is larger at the surface than in solution. Because Pu(V) weakly sorbs to kaolinite, it never prevails at the surface. Within the stability field of Pu(V) in 0.1M NaClO4 solution, Pu(VI) and Pu(IV) prevail at the kaolinite surface under oxidizing and slightly reducing conditions, respectively. By contrast, the Pu(IV)/Pu(III) boundary is hardly affected because both redox states strongly sorb to kaolinite, especially for pH>6. The present method is applied to literature data for Pu sorption to kaolinite. By estimating the pe from a Pu redox state analysis in solution, overall Pu uptake could be predicted. Generic equations are derived that are applicable to minerals and actinides other than kaolinite and Pu. The present study provides important progress in understanding Pu geochemistry, especially in the context of nuclear waste disposal where thermodynamic models are particularly necessary to predict Pu mobility.

Analysis of Sorption Behavior of Cesium and Iodide Ions on Pumice Tuff

In the reliable safety assessment of radioactive waste disposal, the understanding and modeling of the sorption behavior onto the host rock under various groundwater conditions are indispensable. The influence of pH, ionic strength, and the initial concentrations of Cs and I on the sorption of Cs and I on tuff and pumice isolated from the original pumice tuff rock was investigated. For both rocks, the proton concentration has little effect on the Kd values forCs in the pH range 3–10. As the ionic strength of the solution increases in the presence of sodium perchlorate as a matrix ion, the Kd value of cesium apparently decreases, reflecting the competition of the electrolyte Na+ with thespecific sorption of Cs+ on the negatively charged sites. In contrast, no significant dependence of ionic strength on the Kd of anionic iodine was found. A simple surface complexation model without considering electrostatic works was applied to simulate the sorption of ions on rocks, and the model parameters were determined. The Kd values at the given chemical conditions were estimated using the parameters and compared with the ones in the literature.

Environmental mobility of cobalt—Influence of solid phase characteristics and groundwater chemistry

The adsorption of cobalt on samples from a potential waste repository site in an arid region was investigated in batch experiments, as a function of various solution phase parameters including the pH and ionic strength. The samples were characterized using a range of techniques, including BET surface area measurements, total clay content and quantitative X-ray diffraction. The statistical relationships between the measured cobalt distribution coefficients ( K d values) and the solid and liquid phase characteristics were assessed. The sorption of cobalt increased with the pH of the aqueous phase. In experiments with a fixed pH value, the measured K d values were strongly correlated to the BET surface area, but not to the amount of individual clay minerals (illite, kaolinite or smectite). A further set of sorption experiments was undertaken with two samples of distinctive mineralogy and surface area, and consequently different sorption properties. A simple surface complexation model (SCM) that conceptualized the surface sites as having equivalent sorption properties to amorphous Fe-oxide was moderately successful in explaining the pH dependence of the sorption data on these samples. Two different methods of quantifying the input parameters for the SCM were assessed. While a full SCM for cobalt sorption on these complex environmental substrates is not yet possible, the basic applicability and predictive capability of this type of modeling is demonstrated. A principal requirement to further develop the modeling approach is adequate models for cobalt sorption on component mineral phases of complex environmental sorbents.

Measurement and modeling of the surface potential evolution of hydrated cement pastes as a function of degradation

Hydrated cement pastes (HCP) have a high affinity with a lot of (radio)toxic products and can be used as waste confining materials. In cementitious media, elements are removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms as surface complexation equilibria. In this study, to improve the knowledge of the surface charge evolution vs the degradation of the HCP particles, two cements have been studied: CEM-I (ordinary Portland cement, OPC) and CEM-V (blast furnace slag and fly ash added to OPC). Zeta potential measurements showed that two isoelectric points exist vs HCP leaching, i.e., pH. Zeta potential increases from −17 to +20 mV for pH 13.3 to pH 12.65 (fresh HCP states) and decreases from 20 to −8 mV for pH 12.65 to 11 (degraded HCP states). The use of a simple surface complexation model of C-S-H, limited in comparison with the structural modeling of C-S-H in literature, allows a good prediction of the surface potential evolution of both HCP. Using this operational modeling, the surface charge is controlled by the deprotonation of surface sites (>SO −) and by the sorption of calcium (>SOCa +), which brings in addition a positive charge. The calcium concentration is controlled by portlandite or calcium silicate hydrate (C-S-H) solubilities.

Surface properties of various powdered hydroxyapatites

Electrophoretic mobilities of various synthetic and semisynthetic hydroxyapatites (Ca 10(PO 4) 6(OH) 2, HAP) suspended in aqueous solutions have been measured as a function of pH and calcium concentration. The studied powders differ in particle size, crystallinity degree and surface contamination (carbonate). When equilibrated in mineral acids or bases, a large plateau of negative mobility is observed in the pH range 5–8, with increasing negative values at higher pH. Only in the case of the sample composed of nanoparticles, positive mobility obtains at pH < 8.9. When Ca 2+ is added, positive mobility values are observed for all samples, and a bell-shaped profile results as a function of pH. Two possible models are explored to describe the results: the Nernstian approach, which assumes solubility equilibrium and surface potentials determined by the three potential-determining ions (Ca 2+, PO 3− 4, and OH −), and the surface complexation approach, based on the idea of negligible phase transfer of structural phosphate. The Nernstian model is inadequate, whereas a very simple surface complexation model based on the equations Ca 5(PO 4) + 3 = Ca 4(PO 4) − 3 + Ca 2+, Ca 4(PO 4) − 3 + H + = Ca 4(PO 4) 2(PO 4H), Ca 5(PO 4) + 3 + OH − = Ca 5(PO 4) 3(OH), coupled with a very simple electrical double layer, model suffices to reproduce the bell-shaped profile of the mobility as a function of pH in the presence of added calcium salts. The results also show that the sample composed of nanoparticles exchanges ions more easily with the solution, without reaching the solubility equilibrium in the explored timespans. In the presence of soluble phosphate salts, it is postulated that the same surface ensembles define the surface charge, with participation of phosphate as described by the equation 4 5 Ca 5(PO 4) + 3 + 3 5 PO 3− 4 = Ca 4(PO 4) − 3. HAP is just one member of a family of calcium phosphates with different (Ca)/(P) ratios. Electrophoretic mobilities of another member, tricalcium diphosphate, Ca 3(PO 4) 2, were also measured and shown to be described by the same basic model. Comparison with previous literature data shows that the negative plateau in the mobility is a general feature of many HAP samples at low Ca 2+, again in agreement with the surface complexation model. FTIR data demonstrates that surface phosphate indeed undergoes protonation, as postulated in the model.

Modeling of single and competitive metal adsorption onto a natural polysaccharide.

Sugar beet pulp, a common agricultural waste, was studied in the removal of metal ions from aqueous solutions. Potentiometric titrations were used to characterize the surface acidity of the polysaccharide. The acid properties of the material can be described by invoking three distinct types of surface functional groups with the intrinsic acidity constants (pKa(int)) values 3.43+/-0.1, 6.05+/-0.05, and 7.89+/-0.1, respectively. The contents of each functional group (i.e., the carboxyl and phenol moieties) were also determined. Then, a simple surface complexation model with the diffuse layer model successfully described the sorption of several metal ions (Cu2+, Zn2+, Cd2+, and Ni2+) onto the polysaccharide under various experimental conditions: pH ranging from 2 to 5.5, ionic strength from 0.01 to 0.1 M, metal concentration between 10(-4) and 10(-3) M, for a constant sorbent concentration equal to 2.5 g x L(-1). It was observed experimentally that the affinity of the polysaccharide was in the sequence of Cu2+ > Zn2+ > Cd2+ > Ni2+. Predictions of sorption in binary-metal systems based on single-metal data fits represented competitive sorption data reasonably well.

Metal Ions Binding to Natural Organic Matter Extracted from Wheat Bran: Application of the Surface Complexation Model

Ions binding to solid organic matter was investigated in this study. A simple surface complexation model, the diffuse double-layer model, was used to describe Pb(II), Cd(II), Zn(II), and Ni(II) binding to a lignocellulosic substrate extracted from wheat bran. The lignocellulosic substrate was represented by two acid sites: a low-pH (“carboxylic”) site and a high-pH (“phenolic”) site, the phenolic-type sites contributing significantly to the binding behavior, even at relatively low pH. By using the previously determined concentrations of sites and acidity constants, the surface complexation model was applied to Pb, Cd, Zn, and Ni binding as a function of pH in a 0.1 M NaNO3 medium. The model fits were good over a wide range of pH (2<pH<10). The parameters deduced from these single-metal data were then used to predict metals binding under different conditions. The model accounted well for the change in solid concentration ([solid]=0.2–1 g/L), metal concentration ([M]=2×10−5 to 1.4×10−3 M), and ionic strength ([NaNO3]=0.01–0.1 M). It also allowed reasonably good prediction of the competition between up to five metals for the available sites.

Acid/base and Cu(II) binding properties of natural organic matter extracted from wheat bran: modeling by the surface complexation model

A simple surface complexation model, the double-layer model (DLM), was used to describe the sorption of Cu(II) on a lignocellulosic substrate extracted from wheat bran. The solid was first characterized by IR and 13C-NMR and the binding of protons was studied by potentiometric titrations. The organic solid was represented by two acid sites. From the acid–base titration data, total concentrations of these sites and H + binding constants (S i OH⇔S i O −+H +) were obtained by using the computer program FITEQL 3.2. The binding of Cu(II) was then investigated. The thermodynamic approach was completed by two spectroscopic studies. A laser microspectrofluorimetry study was used to specify the adsorption phenomena. The square planar geometry of the solid/Cu complexes and the formation of inner-sphere complexes were suggested by an ESR study. After identifying the stoichiometry of the surface complexes by potentiometric titrations of the Cu 2+/solid system, the surface complexation model was applied to describe Cu(II) binding by the lignocellulosic substrate. A single set of data was used; the model fit was good and covered a wide range of pH. The surface complexation constants derived from this single metal data set were then used to predict the sorption for increasing metal loading, different solid to solution ratios or different ionic strength. The predictions agreed well with the experimental data showing the capability for the DLM surface complexation model to describe Cu binding to natural organic matter with a minimum number of adjustable parameters (only five parameters were necessary to describe the data).

Sorption mechanisms of three actinides on a clayey mineral

The retention of three actinides (U VI, Np V and Am III) on a purified clayey mineral (kaolinite/smectite) has been studied performing batch experiments. The influence of pH and electrolyte concentration on the distribution coefficients has been examined: increasing the pH induces an enhancement of the sorption of actinides. The concentration of the bulk electrolyte (NaClO 4) seems to play an important role. The sorption is reversible for uranium and americium. The affinity of the metallic element for the clay increases from neptunium to americium. Two different models have been tested: a simple surface complexation model and a cation exchange one. The latter permitted to propose sorption mechanisms for each actinide and to point out the presence of two kinds of sites. The acid/base behaviour of these reactive groups has been estimated by the concentration constants. For each actinide, values for the selectivity coefficients were proposed.

Modeling of strontium sorption and speciation in a natural sediment-groundwater system

The sorption of Sr in 21 natural sediment-groundwater systems from Gorleben, F.R.G., has been modeled using a simple surface complexation model. An apparent constant of Sr sorption has been computed taking into account bulk cation-exchange capacity of the sediment, the analytical composition of groundwater and experimental sorption data obtained from related systems. The method of determining the prevailing surface complex has been described in detail, using changing properties of the natural systems. Apparent sorption constants of natural groundwater components — Ca 2+, Mg 2+, K + and Na + — have been estimated. Competition from natural groundwater components for sorption sites has been taken into account. The main parameters governing Sr 2+ sorption are cation-exchange capacity of the sediment and ionic strength of the groundwater. The model describes the influence of experimental volume to mass ratio and of Sr 2+ concentration on sorption and allows one to compare the varying sorption data of similar systems. Computations were performed using the geochemical code MINEQL.

Modeling of Nickel Sorption and Speciation in a Natural Sediment-Groundwater System

ABSTRACTThe sorption of nickel in 15 natural sediment-groundwater systems from Gor leben has been modeled using a simple surface complexation model. An apparent constant of nickel sorption has been computed taking into account the bulk cation exchange capacity of the sediment, the analytical composition of the groundwater and experimental sorption data obtained from related systems. The method of determining the prevailing surface complex has been described in detail, using changing properties of the natural systems. Competition from natural groundwater components for sorption sites has been taken into account.The main parameters governing nickel sorption are cation exchange capacity of the sediment, together with ionic strength and pH of the groundwater. The model describes the influence of citrate on sorption and allows one to compare the varying sorption data of similar systems.Computations were performed using the geochemical code MINEQL.

Adsorption of Cd, Pb and Cu during the precipitation of hydrous ferric oxide in a natural water

Adsorption of Cd, Pb and Cu (pH 7) has been studied as a function of time during the slow oxidation (24 hr.) of ferrous iron naturally present in samples of anoxic lake water. A simple surface complexation model fits the Cd and Pb behaviour. Initial uptake of these metals is fairly evenly distributed across all size fractions of the precipitate. There appears, though, to be a transfer of Pb, and to a lesser extent Cd, from the smaller to the larger-size fractions during the course of the precipitation. This is explained in terms of slow desorption kinetics coupled with the slow aggregation of the precipitate. Cu behaves unexpectedly when spiked at a low concentration at the beginning of the experiment. An initial high uptake is focussed on the small-size fraction (0.1–0.4 μm). This is followed by a steady desorption from this size fraction. Three hypotheses to account for this behaviour are explored, but none provides a wholly satisfactory answer.