Cation Exchange Model Research Articles

Sorption of U(VI) on MX-80 bentonite, illite, shale and limestone in Na–Ca–Cl saline solutions

Uranium (U) has been identified as an element of interest for the safety assessment of a deep geological repository for used nuclear fuel. In this study, the sorption of U(VI) was studied in Na–Ca–Cl solutions at ionic strengths = 0.1–6 mol/kgw (m) in a CO2 free environment at pHm (molal H+ concentration where pHm = −log mH+) = 4–9 for MX-80 bentonite, illite and Queenston shale and at pHm = 5–9 for limestone. U(VI) sorption on MX-80 bentonite increased with pHm from pHm = 4 to 6, then decreased with pHm until pHm = 7, and then increased again with pHm to pHm = 9. U(VI) sorption on illite increased with pHm reaching a maximum at pHm = 7, and then decreased with further increases in pHm. The sorption behavior of U(VI) on shale was similar to that of illite, but the extent of decrease in the sorption coefficient (Rd) value with pHm was slightly more pronounced for the shale than observed for sorption on illite at pHm > 7. U(VI) sorption on limestone increased with pHm up to pHm = 8 and then seemed to be constant at pHm = 8–9. U(VI) sorption on all four solids was independent of ionic strength (0.1–6.0 m). The 2 site protolysis non-electrostatic surface complexation and cation exchange model successfully simulated the sorption of U(VI) onto MX-80 and illite, and the optimized values of surface complexation constants were estimated.

Effect of grain size variation on strontium sorption to heterogeneous aquifer sediments

Strontium-90 (90Sr) is a major contaminant at nuclear legacy sites. The mobility of 90Sr is primarily governed by sorption reactions with sediments controlled by high surface area phases such as clay and iron oxides. Sr2+ adsorption was investigated in heterogeneous unconsolidated aquifer sediments, analogous to those underlying the UK Sellafield nuclear site, with grainsizes ranging from gravels to clays. Batch sorption tests showed that a linear Kd adsorption model was applicable to all grainsize fractions up to equilibrium [Sr] of 0.28 mmol L−1. Sr2+ sorption values (Kd; Langmuir qmax) correlated well with bulk sediment properties such as cation exchange capacity and surface area. Electron microscopy showed that heterogeneous sediments contained porous sandstone clasts with clay minerals (i.e. chlorite) providing an additional adsorption capacity. Therefore, gravel corrections that assumed that the > 2 mm fractions are inert were not appropriate and underestimated Kd(bulk) adsorption coefficients. However, Kd (<2 mm) values measured from sieved sediment fractions, were effectively adjusted to within error of Kd (bulk) using a surface area dependant gravel correction based on particle size distribution data. Amphoteric pH dependent Sr2+ sorption behaviour observed in batch experiments was consistent with cation exchange modelling between pH 2–7 derived from the measured cation exchange capacities. Above pH 7 model fits were improved by invoking a coupled cation exchange/surface complexation which allowed for addition sorption to iron oxide phases. The overall trends in Sr2+ sorption (at pH 6.5–7) produced by increasing solution ionic strength was also reproduced in cation exchange models. Overall, the results showed that Sr2+ sorption to heterogeneous sediment units could be estimated from Kd (<2 mm) data using appropriate gravel corrections, and effectively modelled using coupled cation exchange and surface complexation processes.

Cation exchange parameters for Opalinus Clay and its confining units

The knowledge of cation exchange parameters is important to understand and model the porewater chemistry in clayrocks. The Opalinus Clay (OPA) is a well-studied formation in view of its importance as host rock for radioactive waste repositories, but uncertainties exist regarding its cation exchange parameters, in particular its selectivity coefficients (Kc). In fact, porewater chemistry models for OPA have so far been based on generic selectivity coefficients.Thanks to a recent extensive siting programme, involving eight vertical deep boreholes in northern Switzerland, a large number of porewater and cation exchange data from the OPA and its clay-rich confining units could be analysed. Thus, from the combination of porewater compositions (obtained from high-pressure squeezing and advective displacement), and exchangeable cation data (obtained from Ni ethylenediamine extraction), reliable selectivity coefficients could be derived. The values for KcNa/K, KcNa/Ca, KcNa/Mg are consistent with those obtained in parallel from the Mont Terri underground rock laboratory although the scatter is somewhat larger in the latter dataset. The derived selectivity coefficients indicate no dependency on ionic strength or in-situ temperature. Minor differences in Kc values between OPA and confining units can be attributed to differences in clay mineralogy. The validity of a two-site cation exchange model including illite and smectite for estimating exchangeable Na, Ca and Mg from measured porewater compositions could be shown, while this simple model underpredicts the share of exchangeable K. The exchangeable cation extraction procedure induces the dissolution of Sr-bearing minerals whose amount is in the same range as exchangeable Sr, thus not enabling the derivation of selectivity coefficients for Sr.Finally, derived selectivity coefficients confirm results of previous studies, thus providing confidence in the applied experimental and modelling protocols on the one hand and in the outcome of those studies on the other hand.

Empirical and Mechanistic Modeling of Release Kinetics of Heavy Metals and Their Chemical Distribution in the Rhizosphere and Non-rhizosphere Soils Under Vegetable Cultivation.

Biochemical processes in the rhizosphere affect the availability and distribution of heavy metals (HMs) in various forms. Rhizosphere soil (RS) and non-rhizosphere soil (NRS) samples were collected from 10 fields under tarragon(Artemisia dracunculus L.) cultivation to investigate the release kinetics and distribution of HMs including cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), and zinc (Zn) in five fractions. The cumulative amounts of Cu and Fe released after 88h were in the following ranges, respectively: 1.31-2.76 and 3.24-6.35mgkg-1 in RS and 1.41-2.72 and 3.15-5.27mgkg-1 in NRS. The parabolic diffusion and pseudo-second-order equations provided the best fit to the release kinetics data of Cu and Fe, respectively. The cation exchange model (CEM) based on Gaines-Thomas selectivity coefficients implemented in the PHREEQC program could well simulate the release of Cu and Fe suggesting that cation exchange was the dominant mechanism in the release of Fe and Cu from soils by 0.01 MCaCl2. Cadmium was predominantly found in fraction F2, while other HMs were mainly present in fraction F5. According to the risk assessment code, there was a very high risk for Cd, a medium risk for Co and Cu, a very low risk for Fe, and a low risk for Zn. Correlation analysis showed that soil physicochemical properties were effective in the distribution and transformation of HMs. Significant positive correlations between five fractions indicated that different forms of HMs can potentially transform into each other.

Diverse sorption capacities and contribution of multiple sorptive sites on illitic clays to assess the immobilization of dissolved cesium in subsurface environments

The multiple sorptive sites on illitic clays (e.g., frayed edge [FES], type II [TS], and planar sites [PS]) are related to variable 137 Cs immobilization in subsurface environments. This study investigated the diverse Cs + sorption using 10 illitic clays under various competing K + (distilled water−10 −1 mol/L) and Cs + concentrations (10 −7 −10 −3 mol/L). In addition, the multisite cation exchange model was simulated the best-fit sorption models compared to experimental datasets, and subsequently, optimized the sorption capacities of multiple sorptive sites on the illitic clays. The best-fit sorption model exhibited that diverse Cs + sorption of 10 illitic clays was closely linked to the individual sorption capacities at the FES (1.76 × 10 −5 to 1.12 × 10 −4 eq/kg), TS (1.59 × 10 −3 to 9.76 × 10 −3 eq/kg), and PS (2.14 × 10 −2 to 1.51 × 10 −1 eq/kg). The FES predominantly sorbed Cs + at low aqueous-phase concentrations, whereas the TS and PS contributed to Cs + sorption at relatively high concentrations. These sorption capabilities were correlated to illite contents and crystallinity of 10 illitic clays so that such parameters could be significant factors to predict the Cs + sorption for illitic clays. Finally, the 1-D transport simulations showed significantly diverse Cs + retardation ( R d ≈ 20 to 100) at low dissolved Cs + , implying that the various FES capacities of illitic clays could play an important role on Cs + distribution in actual radioactive contamination sites. • 10 illitic clays had diverse Cs + sorption capacities at multiple sorptive sites. • Multisite cation exchange model was optimized to evaluate Cs + sorption to illitic clays. • Statistical relationship between Cs + sorption and mineralogical data was evaluated. • Selectivity coefficients of Cs + were affected by competing cation concentrations.

Mass Transport Limitations in Electrochemical Conversion of CO2 to Formic Acid at High Pressure

Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental data.

Uranium (VI) sorption on illite under varying carbonate concentrations: Batch experiments, modeling, and cryogenic time-resolved laser fluorescence spectroscopy study

Dissolved inorganic carbon (DIC) present in groundwaters can affect both the aqueous and surface species of hexavalent uranium (U(VI)) owing to its strong complexation ability with U(VI). However, the effect of DIC on U(VI) sorption on illite, which is a critical component of argillaceous rocks, is still unclear. In this study, we investigated the sorption of U(VI) on conditioned illite du Puy with varying DIC concentrations (up to 250 mM DIC) as a function of pH through batch sorption experiments, surface complexation modeling, and cryogenic time-resolved laser fluorescence spectroscopy (TRLFS). The distribution coefficients of U(VI) decreased with an increase in the DIC concentration. At relatively high DIC concentrations with respect to experimentally investigated range (≥100 mM DIC), no U(VI) sorption was observed. The U(VI) sorption on illite was modeled using the two-site protolysis non-electrostatic surface complexation and cation exchange model. Two ternary surface complexation reactions with carbonate were needed to depict the experimental sorption data in addition to binary and ternary hydroxo surface complexation reactions used to describe the U(VI) sorption to illite without carbonate. Cryogenic TRLFS data revealed that U(VI) did sorb to illite at high DIC concentrations (up to 10 mM DIC). The spectra were unchanged with varying DIC concentrations (2.0, 5.0, and 10 mM DIC) at pH ∼8.5, indicating that the surface speciation of U(VI) remained the same. The decay curves were biexponential, which further indicates that at least two surface species were responsible for the sorption. These results could improve the performance assessment of a deep geological disposal system in a sedimentary host rock.

Sorption of Np(IV) on MX-80 in Ca-Na-Cl Type Reference Water of Crystalline Rock

The pH dependence of sorption distribution coefficient (Kd) of Np(IV) on MX-80 in Ca-Na-Cl type solution with the ionic strength of 0.3 M, which was similar to one of the reference groundwaters in crystalline rock, was experimentally investigated under the reducing conditions. The overall trend of Kd on MX-80 was independent of pH at 5 ≤ pH ≤ 10 but increased as pH increased at pH ≤ 5. The 2-site protolysis non-electrostatic surface complexation and cation exchange model was applied to the experimentally measured pH dependence of Kd and the optimized surface complexation constants of Np(IV) sorption on MX-80 were estimated. The values of surface complexation constants in this work agreed relatively well with those in the Na-Ca-Cl solution previously evaluated, suggesting that compared to Na+, the competition of Ca2+ with Np(IV) for surface complexation on MX-80 was not much strong in Ca-Na-Cl solution. The sorption model well predicted the pH dependence of Kd values but slightly overestimated the sorption at the low pH region.

Surface complexation of Ca and competitive sorption of divalent cations on montmorillonite under alkaline conditions

In the geological disposal system of high-level radioactive and transuranic wastes, an increase in Ca concentration (arising from the alteration of cementitious materials) could affect the retention of other radionuclides due to competitive sorption. Batch sorption experiments were performed to investigate the sorption behavior of Ca and its competition with other divalent cations (Sr and Ni) on the edge sites of Kunipia F montmorillonite under alkaline conditions. Ca sorption increased with pH (when pH > 8) and it was more pronounced under high ionic strength conditions, indicating that Ca formed a surface complexation with the edge sites. The sorption behavior of Sr was similar to that of Ca. Meanwhile, the Ni sorption increased with pH, when this was >6 (for lower pH values than in the case of Ca and Sr), Ni tends to form surface complexes more readily than Ca and Sr. The results of a fitting analysis conducted based on the 2-site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model indicate that Ca and Sr sorb onto the ≡SW2OH sites and Ni sorbs onto the ≡SSOH sites. Additionally, the sorption of the CaOH+ and SrOH+ ionic pair species by cation exchange was implied at pH ~ 12. Competitive sorption experiments were also carried out to evaluate the effect of Ca on the sorption of Sr and Ni. Sr sorption decreased with Ca concentration in the alkaline pH region, whereas Ni sorption was not affected by Ca concentration. Overall, these results indicate that Ca and Sr sorb onto the same sites, while Ca and Ni sorb onto different sites; additionally, the competitive sorption seemed to depend on chemical similarities (e.g., hydrolysis behavior). The sorption model parameters obtained from the single-element batch sorption experiments successfully reproduced the results of the competitive sorption experiments.

A simple cation exchange model to assess the competitive adsorption between the herbicide paraquat and the biocide benzalkonium chloride on montmorillonite

Since adsorption on clay minerals is one of the most popular water-pollutant removal method, it is important to consider the coexistence of several compounds that can affect pollutant retention. In this work, the competitive behaviour between cationic herbicide paraquat (PQ) and a benzalkonium chloride compound (BAC) on montmorillonite (MMT) was performed by adsorption experiments and analysed by a theoretical cation exchange model. Experimental studies included adsorption isotherms, XRD, FTIR and zeta potential (ζ). Binary systems (BAC + PQ) were performed at different initial concentrations ratio BAC/PQ = 0.1; 0.5; 1.0; 2.0 and 3.0. Results showed that at higher BAC initial concentration, the maximum adsorbed PQ amount decreases. X-Ray diffractograms showed that as the BAC/PQ ratio increases, the MMT basal spacing peak shifts to lower angles suggesting that the basal spacing is mainly controlled by the presence of BAC. Besides, the total adsorbed amount (adsorbed PQ plus adsorbed BAC) is closer to clay CEC value (0.91 mEq g−1), at this value ζ ≅ 0 mV suggesting that the adsorption occurs through a cation exchange mechanism. The theoretical model applied was originally developed to describe the exchange of simple cations on clay minerals, however it was effective in predicting the behaviour of larger cations such as PQ and BAC. In this scenario, thermodynamic parameters calculated through the model indicate that cation exchange process is spontaneous towards adsorbed BAC, suggesting that an adsorbed PQ cation is replaced by an equivalent amount of BAC cation (expressed in terms of electrical charge).

Sorption of Pd on illite, MX-80 bentonite and shale in Na–Ca–Cl solutions

This paper examines sorption of Pd(II) onto illite, MX-80 bentonite, and Queenston shale in Na–Ca–Cl solutions of varying ionic strength (IS) from 0.01 to 6.0 mol/L (M) and pHc ranging from 3 to 9 under atmospheric conditions. A 2-site protolysis non-electrostatic surface complexation and cation exchange model was applied to the Pd sorption onto illite and MX-80 using PHREEQC, and the model results were compared to the experimental ones obtained in this work. Surface complexation and cation exchange constants were estimated for both illite and MX-80 through the optimization process to bring the predicted distribution coefficients from the model into alignment with the experimentally derived values. These optimized surface complexation constants were compared to existing linear free energy relationships (LFER).

Thallium sorption and speciation in soils: Role of micaceous clay minerals and manganese oxides

Sorption processes control the solubility of toxic thallium (Tl) in soils and thereby its potential leaching into groundwater or uptake by plants. Micaceous clay minerals and Mn oxides are considered to be key sorbents for Tl in soils. We studied the sorption and speciation of Tl in 36 geogenically Tl-rich topsoil materials from the Swiss Jura Mountains by combining chemical extractions, isotope exchange experiments, adsorption experiments, X-ray absorption spectroscopy (XAS), and sorption modelling. We demonstrate that the relation between exchangeable and soluble Tl determined in batch extractions of soils with only geogenic and with freshly spiked Tl matches adsorption isotherms of freshly spiked Tl, and that this relation can be described with a published 3-site cation exchange model for Tl adsorption onto illite. Complemented with XAS data, the results show that micaceous clay minerals control the short-term solubility of Tl via cation exchange, but also the long-term sequestration of most geogenic soil Tl (>90%) via structural fixation. Adsorption competition with K+ and NH4+ at the frayed edges of micaceous clay minerals greatly affects Tl solubility. Increases in the dissolved concentrations of K and NH4 in soil pore water may therefore lead to the release of Tl into solution. The fractions of geogenic Tl associated with Mn oxides were about half as high as the fractions of exchangeable Tl. This Mn-associated Tl is not readily mobilized by cation exchange, but could be released during periodic water logging and soil reduction. In (periodically) reducing environments, the potential of Mn oxides for long-term Tl sequestration is therefore limited. In conclusion, the results from this study highlight the importance of micaceous clay minerals for Tl cycling in soils and sediments, and suggest that concepts developed to assess the sorption of (radio)caesium onto micaceous clay minerals in soils and sediments are transferable to Tl.

A coherent approach for cation surface diffusion in clay minerals and cation sorption models: Diffusion of Cs+ and Eu3+ in compacted illite as case examples

The conceptual setup and the parametrisation of surface diffusion models for different types of cations in negatively charged clay minerals has still not been fully developed. In particular, the contribution of different types of surface-associated cationic species to the overall mass transfer rates is an open question. Further, the transferability of sorption data gained on dispersed clay suspensions to the compacted clay minerals or consolidated clay rocks remains also unanswered. This contribution presents experimental results for the diffusion of 134Cs+ and 152Eu3+ in and sorption on compacted illite (bulk-dry densities of 1900 and 1700 kg m−3, respectively) and their interpretation using thermodynamic and transport modelling. Different solution parameters, such as pH, ionic strength and the concentration of stable isotope background were varied. The results give information on both diffusional processes of the two radionuclides and their sorption behaviour at relevant solid-to-liquid ratios. The existing two site protolysis non electrostatic surface complexation and cation exchange (2SPNE-SC/CE) model turned out to be fully valid for the description of the equilibrium distribution of the test cations between the solution and the clay phase in the compacted state. The model had to be extended by an electrical double layer description of cationic species bound to the planar surfaces, involving mobile species in the diffuse layer, in order to successfully model the observed diffusion profiles. Despite the notable differences in sorption behaviour of Cs+ and Eu3+ on illite, their diffusional behaviour could thus be satisfyingly described using the same model approach.

Extraction and complexation of americium(III) and europium(III) by a N-donor ligand of 3,5-bis(2-pyridyl)pyrazole

The synthesis, Eu3+ complexation, and selective solvent extraction of trivalent americium (Am3+) over trivalent europium (Eu3+) in nitric acid solution by N-donor ligand of 3,5-bis(2-pyridyl)pyrazole (BisPypzH) were described. By using tert-butyl benzene as diluent, BisPypzH ligand in combination with 2-bromohexanoic acid demonstrated the extraction selectivity for Am3+ over trivalent lanthanides (Ln3+) with the separation factor (SFAm/Ln) value ranging from 14.3 to 59.4. Slope analysis showed the formation of 1:1 and 1:2 metal/ligand extracted species corresponding to the BisPypzH concentration below and above 0.025 mol/L, respectively, which was also confirmed by electrospray ionization mass spectrometry (ESI-MS) and dynamic light scattering (DLS) analyses. The extraction conformed to a cation-exchange model. Additionally, the 1:1 and 1:2 metal/ligand complexes were also observed in methanol solution and in solid state, severally, by means of UV–vis spectroscopy, luminescence spectroscopy and ESI-MS. Furthermore, the analyses of Fourier transform infrared spectra (FTIR), Raman spectra, and time-resolved laser-induced fluorescence spectra (TRLFS) indicated the presence of three bidentate NO3− and the absence of H2O molecules in the inner coordination sphere of the central Eu cation in the complex. Meanwhile, the stability constant for BisPypzH-Eu complex was also given.

Contribution of Ternary Reaction to Pd Sorption on MX-80 in Na-Ca-Cl Solution at High Ionic Strength

In this study, we first examined the sorption of Pd on MX-80 in Na-Ca-ClO4 solution as a function of pHc (3–9) and ionic strength (0.1 M–4 M) and confirmed that the experimentally derived Kd values could be fitted by a 2-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) model using three binary surface complexation constants previously estimated. Then, we investigated the sorption of Pd on MX-80 in Na-Ca-Cl-ClO4 solution as a function of pHc (3–9) and Cl-/ClO4- molar concentration ratio (0–∞) at the ionic strength = 4 M. We found that the sorption of Pd on MX-80 in Na-Ca-Cl-ClO4 solution could be simulated only by the three binary and one ternary surface complexations (S-OH+Pd2++4Cl-↔S-OPdCl43-+H+). This suggests that the contribution of other ternary surface complexations such as ≡S-OH + Pd2++xCl-↔ ≡S-OPdClxx-1-+H+ (x = 1, 2 and 3) to Pd sorption in Na-Ca-Cl-ClO4 solution with ionic strength = 4 M was negligibly small.

Determination of complex formation constants of neptunium(V) with propionate and lactate in 0.5–2.6 m NaCl solutions at 22–60°C using a solvent extraction technique

Abstract Natural clay rocks like Opalinus (OPA) and Callovo-Oxfordian (COx) clay rock are considered as potential host rocks for deep geological disposal of nuclear waste. However, small organic molecules such as propionate and lactate exist in clay rock pore water and might enhance Np mobility through a complexation process. Therefore, reliable complex formation data are required in the frame of the Safety Case for a nuclear waste repository. A solvent extraction technique was applied for the determination of NpO 2 + ${\rm{NpO}}_2^ + $ complexation with propionate and lactate. Extraction was conducted from isoamyl alcohol solution containing 10−3 M TTA and 5 · 10−4 M 1,10-phenanthroline. Experiments were performed in 0.5–2.6 m NaCl solutions at temperatures ranging from 22 to 60 °C. Formation of 1:1 Np(V) complexes for propionate and lactate was found under the studied conditions. The SIT approach was applied to calculate equilibrium constants β°(T) at zero ionic strength from the experimental data. Log β°(T) is found linearly correlated to 1/T for propionate and lactate, evidencing that heat capacity change is near 0. Molal reaction enthalpy and entropy ( Δ r H m ∘ ${\Delta _{\rm{r}}}H_{\rm{m}}^ \circ $ and Δ r S m ∘ ${\Delta _{\rm{r}}}S_{\rm{m}}^ \circ $ ) could therefore be derived from the integrated van’t Hoff equation. Data for log β° (298.15 K) are in agreement with literature values for propionate and lactate. Np(V) speciation was calculated for concentrations of acetate, propionate and lactate measured in clay pore waters of COx. In addition, the two site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model was applied to quantitatively describe the influence of Np(V) complexation on its uptake on Na-illite, a relevant clay mineral of OPA and COx.

Measuring and Simulating Co(II) Sorption on Waste Calcite, Zeolite and Kaolinite

Sorption of cobalt (CoII) ions from aqueous solution by waste calcite, kaolinite and zeolite was investigated in a series of batch experiments. The sorption capacity of the sorbents was a function of the initial solution pH, contact time and sorbent/sorbate ratio. For these three sorbents, the kinetic and isotherm experimental data were well fitted to pseudo-second-order and Langmuir equations, respectively. The maximum sorption capacity (mg g−1) of Co(II) was 4.67, 3.76 and 2.23 for waste calcite, zeolite and kaolinite, respectively. Desorption experiments showed that the desorption capacities were in the order of zeolite > kaolinite > waste calcite. The equilibrium and kinetic results indicated that waste calcite had the best performance for the removal of Co(II) compared to zeolite and kaolinite. To simulate and predict Co(II) sorption mechanisms, the surface complexation and cation exchange models in PHREEQC program were used. The model results suggested that the main mechanisms of Co(II) sorption on waste calcite and zeolite were surface complexation and cation exchange, respectively. In the case of kaolinite, the model predicted that both mechanisms were involved in the sorption of Co(II), but the surface complexation was the predominant mechanism.

Cation exchange and surface complexation of lead on montmorillonite and illite including competitive adsorption effects

The adsorption of divalent lead on the homoionic Na forms of illite (Na-IdP) and montmorillonite (Na-SWy) was investigated by batch type adsorption experiments. Ion equilibrium experiments between Pb2+ and Na+ yielded selectivity coefficients for cation exchange on the planar sites of both clay minerals. Pb adsorption edges (pH 2–11) and isotherms (pH 3, 6 and 7 and at 10−12 M < [Pbeql] < 10−3 M) were measured and modelled with the two site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model. The surface complexation constants derived from an iterative fitting of the edges and isotherms could describe all of the Pb adsorption data sets satisfactorily. The anomalous adsorption behaviour of trace Pb on Na-IdP in the acidic region required the consideration of an additional site with a low capacity and high affinity and an associated pH independent surface complexation reaction. Adsorption experiments with Pb/Ni, Pb/Co, Pb/Zn and Pb/Eu on Na-IdP indicated, depending on the experimental set-up, partially and/or non-competitive behaviour. The competitive adsorption experiments could be quantitatively described with a modified 2SPNE SC/CE model. In the case of non-competitive adsorption additional metal specific sets of strong sites are required. In the case of partial competitiveness, a certain fraction of the strong site capacity is assumed to be competitive whereas the remaining fraction is non-competitive.

Long-term simulation of some soil chemical properties under continuous wheat cultivation irrigated with waters of different qualities

The use of low-quality waters for irrigation in arid and semiarid regions is increasing due to the shortage of good-quality water supplies. Geochemical simulations are cost-effective ways to determine the suitable strategies for sustainable water and soil management. In the first step of this study, the combination of one-dimensional solute transport equation and cation exchange model in PHREEQC was used to predict breakthrough curves of the exchangeable sodium percentage (ESP) of nine soil columns during the leaching with 20 pore volumes of a solution with sodium adsorption ratio (SAR) of 10. In the second step, the trend of changes in solution and exchangeable phases of a non-sodic agricultural soil was simulated over 20 years of continuous wheat cultivation irrigated with waters of different qualities. The graphical and statistical comparison (RMSE values ranged from 0.36 to 0.95) between experimental and predicted results showed that the geochemical simulations in the first step were successful. Simulation results of the second step indicated that the soil ESP decreased from 10.1 to 5.6% after a 20-year consecutive application of water with SAR 2.5, while the long-term use of irrigation waters with SAR 10 and SAR 40 increased the final soil ESP to 27.8 and 88.5%, respectively. In the case of application of irrigation water with SAR 10, the final ESP values of wheat residues-, gypsum-, and calcite-amended soils were 18.5, 16.9, and 20.2% which were less than that obtained for control soil.

Highly Efficient Trivalent Americium/Europium Separation by Phenanthroline-Derived Bis(pyrazole) Ligands.

The synthesis, Eu3+ complexation, and solvent extraction of Am3+ and Eu3+ from nitric acid solutions by tetradentate phenanthroline-derived bis(pyrazole) (BPPhen) ligands were described. By using meta-nitrobenzotrifluoride as diluent, BPPhen ligands in combination with 2-bromohexanoic acid extracted Am3+ and Eu3+ with remarkably high efficiency, excellent selectivity, and fast extraction kinetics. Stripping posed no issues. The ligands also showed excellent hydrolytic stability and acid tolerance. 2-Bromohexanoic anion neutralized the charge and increased the lipophilicity of the extracted ion pair. The extraction conformed to a cation exchange model. Slope analysis demonstrated the extraction of 1:2 metal/ligand complexes. Analyses by electrospray ionization mass spectrometry, time-resolved laser-induced fluorescence spectroscopy, Raman, and Fourier transform infrared techniques indicated that the composition of the extracted species is [Eu(nOct-BPPhen)2(H2O)]3+. The formation of 1:2 complexes was also confirmed by UV-vis spectroscopic titration and microcalorimetric titration methods. Meanwhile, the stability constants ( K) and the thermodynamic parameters (Δ H, Δ S, Δ G) for the complexation of Eu3+ with nOct-BPPhen were presented too.