Non-electrostatic Surface Complexation Model Research Articles

Sorption of Eu3+ on Na-montmorillonite studied by time-resolved laser fluorescence spectroscopy and surface complexation modeling

The influence of pH and concentrations of Eu3+ and NaNO3 on the sorption of Eu3+ to Na-montmorillonite were investigated through batch sorption measurements and time-resolved laser fluorescence spectroscopy. The pH had little effect on the distribution coefficients (Kd) in the range of pH 4–7 at 0.01 M NaNO3, which indicates that the cation exchange reaction is a dominant sorption process. Meanwhile, the Kd strongly depended on pH at 1 M NaNO3, suggesting the formation of inner-sphere surface complexes. A cation exchange model combined with a one-site non-electrostatic surface complexation model was successfully applied to the measured Kd data. Linear free-energy relationship was used to estimate the formation constants of the surface species from those of the corresponding aqueous hydrolyzed species. The TRLFS spectra of Eu3+ sorbed on Na-montmorillonite were processed by parallel factor analysis, which provided the fluorescence spectra, decay lifetimes, and relative intensity profiles of three Eu3+ surface species. These species corresponded to one outer-sphere (factor A) and two inner-sphere (factors B and C) complexes. It turned out that factors A and B correspond to Eu3+ sorbed by ion exchange to permanent charge sites of Na-montmorillonite and inner-sphere complexation with surface hydroxyl groups of the edge faces. Factor C became dominant at relatively high pH and ionic strength and likely corresponded to the precipitation of Eu(OH)3 on the surface of Na-montmorillonite.

Selenium sorption on clays in synthetic groundwaters representing crystalline bedrock conditions

The sorption behaviour of selenium on kaolinite and illite minerals in synthetic groundwaters was evaluated using the mass distribution coefficient, R d, obtained in batch sorption experiments. The applied solutions were model solutions of fresh water, brackish water, saline water and glacial meltwater. The distribution coefficient values for illite varied between (0.4–60) × 10−3 and (11–720) × 10−3 m3 kg−1 for kaolinite. No clear effect of ionic strength or calcium concentration on the sorption was observed. The aqueous speciation calculations and the nonelectrostatic surface complexation model applied were in good accordance with the measured R d values at a neutral pH range.

Adsorption of metals onto graphene oxide: Surface complexation modeling and linear free energy relationships

Abstract In this study, the adsorption behaviors of a range of metals onto multi-layered graphene oxide (GO) as a function of pH and metal:GO ratio were measured. The GO exhibited an affinity for the metals in the following order: Pb(II) > Cu(II) > > Cd(II) ≈ Ni(II) ≈ Zn(II) > Sr(II) ≈ Ca(II). A four discrete-site non-electrostatic surface complexation model was used to account for the observed adsorption behaviors. For most of the metals studied, we modeled the adsorption by invoking divalent cation binding onto two or more of the deprotonated GO surface sites. The stability constant for each metal-site complex was calculated from the modeling, and the resulting models in general yield good fits to the experimental data. The calculated stability constants exhibit a correlation to stability constants for the corresponding aqueous metal-acetate, or the corresponding metal-hydroxide complexes. The calculated stability constants and the derived relationships from this study enable preliminary but useful predictions of the extent of metal adsorption onto GO for metals which have yet to be studied experimentally, and we tested the accuracy of these predictions by estimating and measuring the extent of Nd(III) adsorption onto GO as a function of EDTA concentration. The predicted extent of Nd adsorption onto GO is within 5% of the measured extent of Nd adsorption in an EDTA-free solution, and the predicted Nd adsorption behavior with increasing EDTA concentration is also in reasonable agreement with the measurements. Our results demonstrate the high affinity that GO has for binding a wide range of aqueous metals, and the results can be used to estimate with a reasonable degree of certainty the distribution of metals in complex GO-bearing environmental or engineered systems.

Neptunium redox speciation at the illite surface

Neptunium (Np(V)) sorption onto a purified illite is investigated as a function of pH (3–10) and [NpVO2+]tot(3×10−8–3×10−4M) in 0.1M NaCl under Ar atmosphere. After about one week reaction time, only insignificant variation of Np sorption is observed and the establishment of reaction equilibrium can be assumed. Surprisingly, solid–liquid distribution ratios (Rd) are clearly higher than those measured for Np(V) sorption onto illite under aerobic conditions. The observation that Rd increases with decreasing pe (pe=−logae−) suggests partial reduction to Np(IV), although measured redox potentials (pe values) at a first glance suggest the predominance of Np(V). Reduction to Np(IV) at the illite surface could indeed be confirmed by X-ray absorption near-edge spectroscopy (XANES). Np speciation in presence of the purified Na-illite under given conditions is consistently described by applying the 2 sites protolysis non-electrostatic surface complexation and cation exchange model. Measured pe data are taken to calculate Np redox state and surface complexation constants for Np(IV) are derived by applying a data fitting procedure. Constants are very consistent with results obtained by applying an existing linear free energy relationship (LFER). Taking Np(IV) surface complexation constants into account shifts the calculated Np(V)/Np(IV) redox borderline in presence of illite surfaces by 3–5 pe units (0.2–0.3V) towards redox neutral conditions. Our study suggests that Np(V) reduction in presence of a sorbing mineral phase is thermodynamically favored.

Sorption of Cm(III) and Eu(III) onto clay minerals under saline conditions: Batch adsorption, laser-fluorescence spectroscopy and modeling

The present work reports experimental data for trivalent metal cation (Cm/Eu) sorption onto illite (Illite du Puy) and montmorillonite (Na-SWy-2) in NaCl solutions up to 4.37molal (m) in the absence of carbonate. Batch sorption experiments were carried out for a given ionic strength at fixed metal concentration (mEu=2×10−7m, labeled with 152Eu for γ-counting) and at a constant solid to liquid ratio (S:L=2g/L) for 3<pHm<12 (pHm=−logmH+). The amount of clay sorbed Eu approaches almost 100% (with logKD>5) for pHm>8, irrespective of the NaCl concentration. Variations in Eu uptake are minor at elevated NaCl concentrations. Time-resolved laser fluorescence spectroscopy (TRLFS) studies on Cm sorption covering a wide range of NaCl concentrations reveal nearly identical fluorescence emission spectra after peak deconvolution, i.e. no significant variation of Cm surface speciation with salinity. Beyond the three surface complexes already found in previous studies an additional inner-sphere surface species with a fluorescence peak maximum at higher wavelength (λ∼610nm) could be resolved. This new surface species appears in the high pH range and is assumed to correspond to a clay/curium/silicate complex as already postulated in the literature for kaolinite. The 2 site protolysis non-electrostatic surface complexation and cation exchange sorption model (2SPNE SC/CE) was applied to describe Eu sorption data by involving the Pitzer and SIT (specific ion interaction) formalism in the calculation of the activities of dissolved aqueous species. Good agreement of model and experiment is achieved for sorption data at pHm<6 without the need of adjusting surface complexation constants. For pHm>6 in case of illite and pHm>8 in case of montmorillonite calculated sorption data systematically fall below experimental data with increasing ionic strength. Under those conditions sorption is almost quantitative and deviations must be discussed considering uncertainties of measured Eu concentrations in the range of analytical detection limits.

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.

Fe(II) uptake on natural montmorillonites. II. Surface complexation modeling.

Fe(II) sorption edges and isotherms have been measured on low structural Fe-content montmorillonite (STx) and high structural Fe-content montmorillonite (SWy) under anoxic (O2 < 0.1 ppm) and strongly reducing conditions (Eh = -0.64 V). Under anoxic conditions Fe(II) sorption on SWy was significantly higher than on STx, whereas the sorption under reducing conditions was essentially the same. The uptake behavior of Fe(II) on STx under all redox conditions (Eh = +0.28 to -0.64 V) and SWy under reducing conditions (Eh = -0.64 V) was consistent with previous measurements made on other divalent transition metals. All of the sorption data could be modeled with the two-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model including an additional surface complexation reaction for Fe(II) which involved the surface oxidation of ferrous iron surface complexes (≡S(S,W)OFe(+)) to ferric iron surface complexes (≡S(S,W)OFe(2+)) on both the strong and weak sites. The electron acceptor sites on the montmorillonite are postulated to be the structural Fe(III).

Integrated sorption and diffusion model for bentonite. Part 1: clay–water interaction and sorption modeling in dispersed systems

To predict the long-term migration of radionuclides (RNs) under variable conditions within the framework of safety analyses for geological disposal, thermodynamic sorption models are very powerful tools. The integrated sorption and diffusion (ISD) model for compacted bentonite was developed to achieve a consistent combination of clay–water interaction, sorption, and diffusion models. The basic premise considered in the ISD model was to consistently use the same simple surface model design and parameters for describing RNs sorption/diffusion as well as clay surface and porewater chemistry. A simple 1-site non-electrostatic surface complexation model in combination with a 1-site ion exchange model was selected to keep sorption model characteristics relatively robust for compacted systems. Fundamental parameters for the proposed model were evaluated from surface titration data for purified montmorillonite. The resulting basic model was then parameterized on the basis of selected published sorption data-sets for Np(V), Am(III), and U(VI) in dispersed systems, which cover a range of key geochemical conditions such as pH, ionic strength, and carbonate concentration. The sorption trends for these RNs can be quantitatively described by the model considering a full suite of surface species including hydrolytic and carbonate species. The application of these models to the description of diffusive-sorptive transport in compacted bentonites is presented in Part 2.

Thermodynamic analysis of Bacillus subtilis endospore protonation using isothermal titration calorimetry

Bacterial proton and metal adsorption reactions have the capacity to affect metal speciation and transport in aqueous environments. We coupled potentiometric titration and isothermal titration calorimetry (ITC) analyses to study Bacillus subtilis spore-proton adsorption. We modeled the potentiometric data using a four and five-site non-electrostatic surface complexation model (NE-SCM). Heats of spore surface protonation from coupled ITC analyses were used to determine site specific enthalpies of protonation based on NE-SCMs. The five-site model resulted in a substantially better model fit for the heats of protonation but did not significantly improve the potentiometric titration model fit. The improvement observed in the five-site protonation heat model suggests the presence of a highly exothermic protonation reaction circa pH 7 that cannot be resolved in the less sensitive potentiometric data. From the log Ks and enthalpies we calculated corresponding site specific entropies. Log Ks and site concentrations describing spore surface protonation are statistically equivalent to B. subtilis cell surface protonation constants. Spore surface protonation enthalpies, however, are more exothermic relative to cell based adsorption suggesting a different bonding environment. The thermodynamic parameters defined in this study provide insight on molecular scale spore-surface protonation reactions. Coupled ITC and potentiometric titrations can reveal highly exothermic, and possibly endothermic, adsorption reactions that are overshadowed in potentiometric models alone. Spore-proton adsorption NE-SCMs derived in this study provide a framework for future metal adsorption studies.

Fe(II) Sorption on a Synthetic Montmorillonite. A Combined Macroscopic and Spectroscopic Study

Extended X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopy combined with macroscopic sorption experiments were employed to investigate the sorption mechanism of Fe(II) on an iron-free synthetic montmorillonite (Na-IFM). Batch sorption experiments were performed to measure the Fe(II) uptake on Na-IFM at trace concentrations as a function of pH and as a function of sorbate concentration at pH 6.2 and 6.7 under anoxic conditions (O2 < 0.1 ppm). A two-site protolysis nonelectrostatic surface complexation and cation exchange sorption model was used to quantitatively describe the uptake of Fe(II) on Na-IFM. Two types of clay surface binding sites were required to model the Fe(II) sorption, the so-called strong (≡S(S)OH) and weak (≡S(W)OH) sites. EXAFS data show spectroscopic differences between Fe sorbed at low and medium absorber concentrations that were chosen to be characteristic for sorption on strong and weak sites, respectively. Data analysis indicates that Fe is located in the continuity of the octahedral sheet at trans-symmetric sites. Mössbauer spectroscopy measurements confirmed that iron sorbed on the weak edge sites is predominantly present as Fe(II), whereas a significant part of surface-bound Fe(III) was produced on the strong sites (∼12% vs ∼37% Fe(III) species to total sorbed Fe).

A surface complexation model of YREE sorption on Ulva lactuca in 0.05–5.0 M NaCl solutions

We present distribution coefficients, logiKS, for the sorption of yttrium and the rare earth elements (YREEs) on BCR-279, a dehydrated tissue homogenate of a marine macroalga, Ulva lactuca, resembling materials featured in chemical engineering studies aimed at designing renewable biosorbents. Sorption experiments were conducted in NaCl solutions of different ionic strength (0.05, 0.5, and 5.0M) at T=25°C over the pH range 2.7–8.5. Distribution coefficients based on separation of the dissolved and particulate phase by conventional filtration (<0.22μm) were corrected for the effect of colloid-bound YREEs (>3kDa) using an existing pH-dependent model. Colloid-corrected values were renormalized to free-cation concentrations by accounting for YREE hydrolysis and chloride complexation. At each ionic strength, the pH dependence of the renormalized values is accurately described with a non-electrostatic surface complexation model (SCM) that incorporates YREE binding to three monoprotic functional groups, previously characterized by alkalimetric titration, as well as binding of YREE–hydroxide complexes (MOH2+) to the least acidic one (pKa∼9.5).In non-linear regressions of the distribution coefficients as a function of pH, each pKa was fixed at its reported value, while stability constants of the four YREE surface complexes were used as adjustable parameters. Data for a single fresh U. lactuca specimen in 0.5M NaCl show generally the same pH-dependent behavior but a lower degree of sorption and were excluded from the regressions. Good linear free-energy relations (LFERs) between stability constants of the YREE–acetate and YREE–hydroxide solution complex and surface complexes with the first and third functional group, respectively, support their prior tentative identifications as carboxyl and phenol. A similar confirmation for the second group is precluded by insufficient knowledge of the stability of YREE–phosphate complexes and a perceived lack of YREE binding in 0.05M NaCl; this issue awaits further study. The results indicate that SCMs can be successfully applied to sorbents as daunting as marine organic matter. Despite remnant challenges, for instance resolving the contributions of individual groups to the aggregate sorption signal, our approach helps formalize seaweed’s avowed promise as an ideal biomonitor or biofilter of metal pollution in environments ranging from freshwaters to brines by uncovering what chemical mechanisms underlie its pronounced affinity for YREEs and other surface-reactive elements.

Ni(II) sorption on natural chlorite

Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01–0.5 M) and Ni concentration (10−8–10−6 M) in an Ar atmosphere using batch sorption with radioactive 63Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH ∼ 8 (Kd = ∼10−3 cm3/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit to the sorption results using FITEQL. The high Kd values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces.

Predictive sorption modelling of Ni(II), Co(II), Eu(IIII), Th(IV) and U(VI) on MX-80 bentonite and Opalinus Clay: A “bottom-up” approach

A considerable amount of work has been carried out over the past several years to develop mechanistic sorption models capable of describing and predicting the sorption of radionuclides on clay minerals over a wide range of conditions. The focus of these studies has been on montmorillonite and illite. Sorption edges and isotherms were measured for a large number of radionuclides with valences from II to VI on both systems and could be very well described by a relatively simple model, the 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model. The aim of this work is to test the capabilities of the model and associated model parameters to predict sorption isotherms of radionuclides measured in complex mineral/groundwater systems using a blind modelling procedure. The approach lying at the basis of the modelling methodology applied here is that by understanding the mechanistic sorption processes on single minerals, and developing models to describe them, this knowledge can be used to understand and quantitatively predict the uptake of radionuclides in complex mineral/groundwater systems, the so called "bottom-up" approach. The work here describes the results from such an approach tested on sorption isotherms measured on MX-80 bentonite (for Ni(II), Eu(III), Th(IV) and U(VI)) and on Opalinus Clay (for Ni(II), Co(II), Eu(III), Th(IV) and U(VI)).

Enthalpies of proton adsorption onto Bacillus licheniformis at 25, 37, 50, and 75 °C

Understanding bacterial surface reactivity requires many different lines of investigation. Toward this end, we used isothermal titration calorimetry to measure heats of proton adsorption onto a Gram positive thermophile Bacillus licheniformis at 25, 37, 50, and 75 °C. Proton adsorption under all conditions exhibited exothermic heat production. Below pH 4.5, exothermic heats decreased as temperature increased above 37 °C; above pH 4.5, there was no significant difference in heats evolved at the temperatures investigated. Total proton uptake did not vary significantly with temperature. Site-specific enthalpies and entropies were calculated by applying a 4-site, non-electrostatic surface complexation model to the calorimetric data. Interpretation of site-specific enthalpies and entropies of proton adsorption for site L 1, L 2, and L 4 are consistent with previous interpretations of phosphoryl, carboxyl, and hydroxyl/amine site-identities, respectively, and with previous calorimetric measurements of proton adsorption onto mesophilic species. Enthalpies and entropies for surface site L 3 are not consistent with the commonly inferred phosphoryl site-identity and are more consistent with sulfhydryl functional groups. These results reveal intricacies of surface reactivity that are not detectable by other methods.

Abstract: P1228 THE SIGNIFICANCE OF THE METABOLIC SYNDROME IN THE DEVELOPMENT OF CHRONIC RENAL DISEASE (CKD) IN NONDIABETIC OBESE SUBJECTS

Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01–0.5 M) and Ni concentration (10−8–10−6 M) in an Ar atmosphere using batch sorption with radioactive 63Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH ∼ 8 (Kd = ∼10−3 cm3/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit to the sorption results using FITEQL. The high Kd values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces.

Sorption modelling on illite. Part II: Actinide sorption and linear free energy relationships

Sorption edge data for Ni(II), Co(II), Eu(III) and Sn(IV) [Bradbury M. H. and Baeyens B. (2009) Sorption modelling on illite. Part I: titration measurements and sorption of Ni(II), Co(II), Eu(III) and Sn(IV), Part I] on purified Na-Illite du Puy are available from some previous work, and some new measurements for Am(III), Th(IV), Pa(V) and U(VI) are presented here. All of these sorption edge measurements have been modelled with a 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model for which the site types, site capacities and protolysis constants were fixed [Bradbury M. H. and Baeyens B. (2009), Part I]. In addition, two further data sets for the sorption of Am(III) and Np(V) on Illite du Puy, obtained from the literature, were also modelled in this work. Thus, surface complexation constants for the strong sites in the 2SPNE SC/CE sorption model for nine metals with valence states from II to VI have been obtained. A linear relationship between the logarithm of strong site metal binding constants, S K x −1, and the logarithm of the corresponding aqueous hydrolysis stability constant, OH K x , extending over nearly 35 orders of magnitude is established here for illite for these nine metals. Such correlations are often termed linear free energy relationships (LFER), and although they are quite common in aqueous phase chemistry, they are much less so in surface chemistry, especially over this large range. The LFER for illite could be described by the equation: log S K x - 1 = 7.9 ± 0.4 + ( 0.83 ± 0.02 ) log OH K x where, “ x” is an integer. A similar relationship has been previously obtained for montmorillonite, thus LFERs relating to the sorption on two of the most important clay minerals present in natural systems have been established. Such an LFER approach is an extremely useful tool for estimating surface complexation constants for metals in a chemically consistent manner. It provides a means of obtaining sorption values for radionuclides for which there are no measured values and thus allows gaps in missing sorption data to be filled. An ultimate goal of this approach is to develop a thermodynamic sorption database. This could then be used in radioactive waste management performance assessment studies to calculate sorption in natural systems, and thereby replace the current usage of single solid liquid distribution coefficients ( K d values) to describe radionuclide uptake. Finally, with the data now available, the 2SPNE SC/CE sorption model can be ported into reactive transport models allowing radionuclide migration to be calculated under spatially and temporally changing conditions.

The influence of carbonate complexation on lanthanide/actinide sorption on montmorillonite

AbstractThe safety case for radioactive waste repositories is, to a large extend based on the physical and chemical retention of radionuclides on clay minerals which are important constituents in both the man-made engineered barriers and in argillaceous host rock formations. The presence of carbonate, one of the most important inorganic ligands for lanthanide and actinide ions in groundwaters and clay porewaters, can have a significant influence on their sorption behaviour on clay minerals. The sorption of Eu(III) on Na-montmorillonite in the absence and the presence of carbonate was investigated in batch sorption experiments. In the presence of carbonate two sets of experimental conditions were investigated. Eu(III) sorption measurements were performed as a function of pH in equilibrium with atmosphericpCO2(=10−3.5 bar) and at a variablepCO2(=10−1.4to 10−3.4 bar). The sorption data were modelled using the 2 site protolysis non-electrostatic surface complexation and cation exchange model. Both experimental data sets in the presence of carbon could be quantitatively modelled by including in the sorption model two additional surface complexation reactions on the strong sites forming ≡SSOEuCO30and ≡SSOEuOHCO3−surface complexes.

Modelling sorption data for the actinides Am(III), Np(V) and Pa(V) on montmorillonite

The retention characteristics of the bentonite near-field engineered barrier proposed in most of the concepts for the deep geological disposal of high-level waste and spent fuel are an important component in repository performance assessment studies. Montmorillonite generally constitutes 65 to 90 wt.% of the bentonite. Sorption edge measurements have been performed at trace concentrations for the actinides Am(III), Np(V) and Pa(V) on purified and conditioned SWy-1 montmorillonite under anoxic, carbonate free conditions. To the best of the author´s knowledge, this is the first time a sorption data set has been measured for 233Pa(V) uptake on montmorillonite. In some earlier work, the 2 site protolysis non-electrostatic surface complexation and cation exchange quasi mechanistic sorption model has been used to model the sorption edges measured for 11 metals with valencies between II and VI. From this study a linear correlation between strong site surface complexation constants and the corresponding hydrolysis constants was established: a Linear Free Energy Relation (LFER). The new measurements for 241Am(III), 239Np(V) and 233Pa(V) fit very well on this previously established LFER. Through the LFER, a chemically consistent picture emerges which explains quantitatively why some metals tend to exhibit very high sorption and others only moderate to weak sorption.

Experimental Measurement of Proton, Cd, Pb, Sr, and Zn Adsorption Onto the Fungal Species Saccharomyces Cerevisiae

Proton, Cd, Pb, Sr, and Zn adsorption onto the fungal species Saccharomyces cerevisiae were measured in bulk adsorption experiments as a function of time, pH, surface: metal ratio, and ionic strength, and we measured the electrophoretic mobility of the cells as a function of pH. We modeled the acid/base properties of the fungal cell wall by invoking a nonelectrostatic surface complexation model with four discrete surface organic acid functional group types, with average pKa values (with 1 sigma uncertainties) of 3.4 +/- 0.4, 5.0 +/- 0.2, 6.8 +/- 0.4, and 8.9 +/- 0.6. The affinity of the fungal cells for the metal ions follows the following trend: Pb > Zn > Cd > Sr. We used the metal adsorption data to determine site-specific stability constants for the important metal fungal surface complexes. Our results suggest that S. cerevisiae may represent a novel biosorbent for the removal of heavy metal cations from aqueous waste streams.

Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-illite. Part 1: Batch sorption and time-resolved laser fluorescence spectroscopy experiments

Sorption of Cm(III) and Eu(III) at trace concentrations onto Ca-montmorillonite (SWy-1) and Na-illite (Illite du Puy) has been studied under anaerobic conditions by batch sorption experiments and time-resolved laser fluorescence spectroscopy (TRLFS). Comparison of the results from spectroscopic and batch sorption experiments with Cm and Eu indicates the existence of outer-sphere complexes at pH <4 in the experiments with Na-illite (0.25 g/L solid; 2.5 × 10 −7 mol/L Cm; 0.1 mol/L NaClO 4). In the case of Ca-montmorillonite, (0.25 g/L solid, 2.5 × 10 −7 mol/L Cm or 10 −6 mol/L Eu, 0.066 mol/L Ca(ClO 4) 2), Cm/Eu outer-sphere complexes do not form at significant levels due to the Ca 2+ competition for the clay mineral cation-exchange sites. TRLFS spectra indicate the formation of inner-sphere surface complexes at pH >5 for both clay minerals. Five H 2O/OH − molecules remain in the first metal ion coordination sphere of the sorbed Eu/Cm. Measured fluorescence lifetimes of sorbed Eu/Cm and peak deconvolution of Cm-spectra are consistent with the formation of surface complexes of the form ≡S-O-Eu/Cm(OH) x (2−x)(H 2O) 5−x. At pH ≥ 12 Cm becomes incorporated into a surface precipitate at the Ca-montmorillonite surface presumably composed of Ca(OH) 2 or calcium silicate hydrate. A dramatic shift of the fluorescence emission band by more than 20 nm and a clear increase in the fluorescence lifetime suggests the almost complete displacement of coordinated H 2O and OH −. The pH dependent Eu sorption data obtained in batch experiments are consistent with spectroscopic data on Eu and Cm within experimental uncertainties thus demonstrating the validity of Eu as a homologue for trivalent actinides. Parameterization of a two-site protolysis nonelectrostatic surface complexation and cation exchange model using the batch sorption data and spectroscopic results is discussed in Part 2 of this work.