Ligand Sorption Research Articles

Speciation studies at the Illite - solution interface: Part 2 – Co-sorption of uranyl and phosphate ions

Although it is well known that clays and phosphate ions (hereafter referred to as “P”) can contribute to the long term uptake of uranium (U) in a variety of geochemical systems, work is still needed to document the surface speciation of U(VI) and P sorbed on relevant clay minerals such as illite. Following on from previous work showing strong sorption of phosphate ligands on the surface of a homoionic Na-illite ([1]), we addressed the (competitive/synergistic) mechanisms of (co)sorption of trace levels of uranyl ions (1–10 µM) and phosphate ligands (100 µM) onto this clay, and the identity of the surface species formed, by in situ spectroscopy monitoring of the clay-solution interface along sorption. We also performed complementary batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated a uranyl sorption dependent on pH, aqueous U concentration and clay-to-solution ratio, a signature of P on the U sorption, and a promotion of P sorption with U concentration. Macroscopic and EM data also suggested mostly reversible mechanisms of P and U co-sorption that confer negative charges on the mineral surface and involve several types of uranyl phosphate surface species and/or surface sites present on the clay edges. FTIR interface spectra confirmed that several types of inner sphere uranyl phosphate surface species formed at acidic pH at the illite-solution interface, as a function of U surface coverage and/or reaction time. An inner-sphere uranyl phosphate surface complex, likely with a U-bridging structure, was formed rapidly on high-affinity surface sites existing in limited quantities on the clay edges. Another U-P surface complex (with a possible P-bridging structure) was progressively formed in increasing amounts with U surface coverage and time on low affinity clay edge sites. Finally, a third U-P surface species having an autunite-like structure (probably a U-P polynuclear surface species) was formed on the illite surface at high U concentration (10 µM). The two later species competed with the existence on the illite surface of inner sphere surface complexes and outer-sphere surface complex of phosphate, which were identified in the absence of U. Information on uranyl surface speciation in the presence of phosphate ligands presented here is novel and provides for the first time in situ spectroscopic evidence of synergistic U-P sorption process leading in the formation of several types of uranyl phosphate sorption species on the surface of illite. These results provide a sound basis for better understanding / prediction of U sorption in the environment, including in clayey formations being considered as long term barriers to radionuclide migration in far field of high level radioactive waste repository.

A novel hybrid ferrous sulfide impregnated anion exchanger for trace removal of hexavalent chromium from contaminated water

To effectively remove trace concentration of potential cancer-causing Cr(VI) from impaired drinking water, a novel hybrid material was synthesized through an in-situ synthesis process by dispersing ferrous sulfide nanoparticles within an anion exchanger. Characterization studies revealed that the hybrid material, named hybrid ferrous sulfide impregnated anion exchanger (HISIIX), contained uniformly distributed ferrous sulfide nanoparticles of size 10–40 nm within the anion exchanger host. Apart from FeS2 nanoparticles, it also included nanoparticles of FeO and FeOOH. The incorporation of ferrous sulfide nanoparticles within the anion exchanger contributed to the significant differences in the Cr(VI) uptake capacity of HISIIX. Validation studies using fixed-bed column proved that HISIIX had significantly high Cr(VI) uptake capacity and was able to run for 4200 bed volumes (BVs) before a breakthrough of 50 μg L−1 when subjected to a synthetic aqueous solution containing 200 μg L−1 Cr(VI). Cr(VI) uptake capacity of the parent anion exchanger and HISIIX were determined to be 1.39 mg g−1 and 3.44 mg g−1, respectively, when the columns were allowed to run until exhaustion. Ferrous sulfide nanoparticles acted as a reducing agent transforming Cr(VI) anions into Cr(III) precipitates. It also produced sites for further removal of Cr(VI) anions through ligand sorption upon oxidation. The anion exchanger substrate attracted anions selectively via the Donnan membrane principle, resulting in a synergy of three different processes - ion exchange, redox reaction, and ligand sorption that gave the HISIIX a high capacity for the selective Cr(VI) removal from contaminated water.

Positive Sorption Behaviors in the Ligand Exchanges for Water-Soluble Quantum Dots and a Strategy for Specific Targeting.

N,N,N',N'-Tetramethylethylenediamine (TMEDA) and ethylenediamine (EDA) were investigated in-depth in the ligand exchanges for water-soluble CdSe quantum dots (QDs). TMEDA could assist the phase transfer of QDs from apolar solvents to the aqueous solutions as stabilized by mercaptopropionic acid (MPA). We successfully maintained the stability of a series of MPA-capped QDs of different ligand densities for NMR characterizations in aqueous solutions. The proton NMR spectroscopies of MPA of the binding state were used to analyze the ligand densities on the surface of QDs, which were not explored in the past. The binding thermodynamics of the surface ligands of QDs, as analyzed using the Hill equation, demonstrated a positive promoting effect and possible interactions between ligands. EDA in the purification process underwent a spontaneous adsorption with two-stage thermodynamic behaviors as characterized by isothermal titration calorimetry. Due to the positive role of the already adsorbed ligands, excess EDA would further attach to the surface of QDs in the form of non-bonded physisorption, greatly improving the quantum yield (QY) of QDs, and the ligand of this part would almost not change the stability of QDs. We proposed a strategy for the preparation of aqueous QDs with a high QY, followed by fluorescence quenching-enhancement cycles caused by purification-adsorption operations. The strategy made it possible for the preparation of functional QDs with small molecules after purification operations. Kinetics of the sorption of ligands on the surface of QDs were determined by fluorescence spectroscopy. Modified pseudo-second-order kinetics after consideration of the ligand-ligand interaction effect could well analyze the kinetic data. This kinetic model had advantages over the previous ligand exchange model in terms of accuracy, reproducibility, and physical significance. Finally, we used the above strategy for the design of fluorescent QDs for bioimaging of lysosomes, mitochondria, and cancer cells. This work can simplify the preparation of multifunctional fluorescent QDs and avoid complicated ligand design.

Константы равновесия сорбции пиридинкарбоновых кислот сульфокатионитом полистирольного типа

The processes of sorption of pyridine-3-carboxylic (nicotinic) and pyridine-4-carboxylic (isonicotinic) acids by sulfonic acid cation exchangers of the polystyrene type (CU-2-4 and CU-2-8) from aqueous solutions with different pH values were studied. Analysis of the FTIR spectra of isonicotinic acid, isonicotinic acid sulfate, and CU-2-8 sulfonic cation exchanger filled with isonicotinic acid showed that pyridinecarboxylic acid is in the protonated form in the polymer phase. Experimental data of the equilibrium distribution of acids in the aqueous solution-cation exchange system have been obtained. The values of the equilibrium constants for ion exchange and ligand sorption processes involving various forms of pyridinecarboxylic acid, sulfonic cation exchanger, and protons were calculated. The equilibrium constants for the ion exchange of sulfocationite protons by pyridinecarboxylic acid cations from solution are in the range 3.3-4.4. The selectivity of sulfonic cation exchangers to cations increases in the sequences proton < nicotinic acid cation < isonicotinic acid cation. The values of the equilibrium constant for ligand sorption of molecules are 195-220 dm3/mol for isonicotinic acid and reach 320-330 dm3/mol for nicotinic acid, i.e. the sorption activity of the H-form of the cation exchanger is higher in relation to nicotinic acid molecules. A change in the amount of a crosslinking agent (from 4% to 8% divinylbenzene) in a polystyrene type sulfonic cation exchanger does not significantly affect its sorption activity towards pyridinecarboxylic acids.

From swollen gels to dried films: Relating the IR spectra of ferrihydrite dried as a film on an ATRIR crystal to aqueous suspensions

In aquatic systems ferrihydrite forms as highly porous aggregates that resemble swollen gels. Much insight into the surface chemistry of ferrihydrite has been gleaned using a technique in which ferrihydrite is dried on to the surface of a crystal that acts as an Attenuated Total Reflectance (ATR) medium for in situ infrared (IR) studies of ligand sorption. In this paper BET N2 adsorption, acid-base titration, H4SiO4 isotherms and the relationship between H4SiO4 sorption and H4SiO4 interfacial oligomerization are used to systematically probe the influence of this drying step on ferrihydrite's surface chemistry. This means that insights into sorption reactions revealed from in situ ATR-IR are can be increasingly related to real environmental systems. Comparing the measured acid-base titration curves to the theoretical curve provide a specific surface area (SSA) estimation of ≈600 and 290 m2 g−1 for the fresh and film-dried ferrihydrite respectively, compared to the BET N2 adsorption values of 240 and 140 m2 g−1 where the fresh ferrihydrite was freeze dried prior to BET measurement. The H4SiO4 isotherms were identical on the two ferrihydrite samples when normalized to the titration derived surface areas. The extent of interfacial H4SiO4 oligomerization provides insights into surface structures. Within experimental uncertainty, the process of drying ferrihydrite into a film did not alter either the general trend in the relationship between sorption and oligomerization, or the extent of oligomerization as a function of surface coverage normalized to the titration derived surface areas. Therefore, in comparable laboratory studies where sorption densities for dried films are compared with fresh suspensions the SSA values of 290 and 600 m2 g−1 respectively should be a reasonable estimate.

Effect of Natural Organic Matter on Plutonium Sorption to Goethite.

The effect of citric acid (CA), desferrioxamine B (DFOB), fulvic acid (FA), and humic acid (HA) on plutonium (Pu) sorption to goethite was studied as a function of organic carbon concentration and pH using batch sorption experiments at 5 mgC·L-1 and 50 mgC·L-1 natural organic matter (NOM), 10-9-10-10 M 238Pu, and 0.1 g·L-1 goethite concentrations, at pH 3, 5, 7, and 9. Low sorption of ligands coupled with strong Pu complexation decreased Pu sorption at pH 5 and 7, relative to a ligand-free system. Conversely, CA, FA, and HA increased Pu sorption to goethite at pH 3, suggesting ternary complex formation or, in the case of humic acid, incorporation into HA aggregates. Mechanisms for ternary complex formation were characterized by Fourier transform infrared spectroscopy in the absence of Pu. CA and FA demonstrated clear surface interactions at pH 3, HA appeared unchanged suggesting HA aggregates had formed, and no DFOB interactions were observed. Plutonium sorption decreased in the presence of DFOB (relative to a ligand free system) at all pH values examined. Thus, DFOB does not appear to facilitate formation of ternary Pu-DFOB-goethite complexes. At pH 9, Pu sorption in the presence of all NOM increased relative to pH 5 and 7; speciation models attributed this to Pu(IV) hydrolysis competing with ligand complexation, increasing sorption. The results indicate that in simple Pu-NOM-goethite ternary batch systems, NOM will decrease Pu sorption to goethite at all but particularly low pH conditions.

Manganese(II) and cobalt(II) complexes based on pyridyl diimide: in situ ligand formation, crystal structures, and sorption properties

Two complexes, namely {[Mn(L1)2(H2O)2]·0.5H2O} n (1) and {[Co(L2)(H2O)2]·4H2O} n (2), (L1,1,3-dioxo-2-pyridin-3-ylmethyl-6-[(pyridin-3-ylmethyl)-carbamoyl]-2,3-dihydro-1H-isoindole-5-carboxylic acid; L2,2,5-bis-[(pyridine-3-yl methyl)-carbamoyl]-terephthalic acid), were synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and X-ray single-crystal structure analysis. The ligands HL1 and HL2 were formed in situ by the hydrolysis of N,N′-bis(3-pyridylmethyl)-pyromellitic diimide (L). Apparently, Mn2+ acts as an asymmetric catalyst, such that a new asymmetric pyridinecarboxylate derivative ligand L1 is formed, while Co2+ acts as a symmetric catalyst, giving the pyridinedicarboxylate derivative ligand L2. Complex 1 has a 1D double-chain structure, in which the adjacent chains are arranged in parallel arrays through π···π interactions, resulting in a 2D supramolecular framework. Complex 2 has a grid (4,4)-sql net structure in an edge-to-edge arrangement through hydrogen bonds between adjacent layers, forming a 3D supramolecular framework containing 1D channels to accommodate water guest molecules. The thermal stabilities of both complexes and ethanol adsorption properties of complex 2 were investigated. 1D double-chain Mn(II) complex (1) and 2D (4,4) net Co(II) complex (2) were synthesized. The Mn2+ ions have the asymmetric catalysis property, and a new asymmetric pyridinecarboxylates derivative ligand is formed; the Co2+ ions have symmetric catalysis property, giving the pyridinedicarboxylates derivative ligand. The complex 2 allows for stepwise sorption of ethanol.

Lead Immobilization and Hydroxamate Ligand Promoted Chloropyromorphite Dissolution

The immobilization of lead, a major environmental contaminant, through phosphate amendments to form the sparingly soluble lead phosphate mineral chloropyromorphite [Pb5(PO4)3] (CPY) is an effective in situ strategy for soil remediation. An important question is the effect of microbial processes on this remediation. Here, we investigate the role of the microbial siderophore ligand desferrioxamine-D1 (DFO-D1) and its analog acetohydroxamic acid (aHA) in CPY lability using pH-dependent batch dissolution kinetics and model calculations. Both (0.01) M aHA and (0.00024) M DFO-D1 are similarly effective and enhance lead release from CPY by more than two orders of magnitude at pH > 6 compared to in the absence of ligands. This is consistent with model calculations of pH-dependent (aqueous) complexation of lead with hydroxamate ligands. More importantly, pH-dependent ligand sorption is predictive of its ligand promoted dissolution behavior. Our results suggest that organic ligands can significantly increase CPY lability at alkaline pHs in soils and sediments and that addition of P amendments to immobilize Pb as CPY may only be successful at acid pHs.

The sorption of thiocyanate ions on complex-forming ionites

The ligand sorption of thiocyanate ions on several complex-forming ionites was studied. The ionites were preliminarily transformed into metal forms by saturation with copper(II) ions. ANKB-2 amphoteric ionite in the Cu form had the strongest affinity for thiocyanate ions. The optimum conditions for their extraction were pH ∼ 2 and solution ionic strength 1. IR spectroscopy was used to study the ligand sorption of SCN− ions by ANKB-2 ionite in the Cu form. The stability constants of thiocyanate ionite copper complexes were calculated from formation function \( \bar n \).

Evaluation of phosphorus leaching from contaminated calcareous soils due to the application of sheep manure and ethylenediamine tetraacetic acid

Organic amendment application to heavy metal contaminated soils may contribute to leaching of phosphorus (P). The objectives of this study were to determine the influence of sheep manure and ethylenediamine tetraacetic acid (EDTA) on the P leaching from a wide range of calcareous contaminated soils. Glass tubes, 4.9 cm diameter, and 40 cm long, were packed with contaminated soil. The resulting 20 cm long column of soils had bulk density of 1.3–1.4 g cm−3. The columns were leached with distilled water, 0.01 M EDTA, 0.01 M CaCl2 or sheep manure extract (SME) solutions. The breakthrough curves for P were different and the amounts of P leached varied considerably between different soils and leaching solutions. The amounts leached with SME were less than the amount added through the SME, indicating that some P was retained by the soil, mainly due to preliminary sorption of organic ligands on to the soil with the creation of new sorbing surfaces. The amount leached with EDTA solution varied from 9.9 to 46.3% of the extractable P when 15 pore volumes had passed through the column. Low amounts of P were leached by 0.01 M CaCl2, which is likely to be due to the high concentration of soluble Ca used in the solution. Thus, among leaching solutions the application of EDTA and SME on contaminated calcareous soils might enhance the mobility of P and large amounts of P will be leached, leading to contamination of ground and surface waters.

Kinetics of brucite dissolution at 25°C in the presence of organic and inorganic ligands and divalent metals

Brucite (Mg(OH) 2) dissolution rate was measured at 25°C in a mixed-flow reactor at various pH (5 to 11) and ionic strengths (0.01 to 0.03 M) as a function of the concentration of 15 organic and 5 inorganic ligands and 8 divalent metals. At neutral and weakly alkaline pH, the dissolution is promoted by the addition of the following ligands ranked by decreasing effectiveness: EDTA ≥ H 2PO 4 − > catechol ≥ HCO 3 − > ascorbate > citrate > oxalate > acetate ∼ lactate and it is inhibited by boric acid. At pH >10.5, it decreases in the presence of PO 4 3−, CO 3 2−, F −, oxine, salicylate, lactate, acetate, 4-hydroxybenzoate, SO 4 2− and B(OH) 4 − with orthophosphate and borate being the strongest and the weakest inhibitor, respectively. Xylose (up to 0.1 M), glycine (up to 0.05 M), formate (up to 0.3 M) and fulvic and humic acids (up to 40 mg/L DOC) have no effect on brucite dissolution kinetics. Fluorine inhibits dissolution both in neutral and alkaline solutions. From F sorption experiments in batch and flow-through reactors and the analysis of reacted surfaces using X-ray Photoelectron Spectroscopy (XPS), it is shown that fluorine adsorption is followed by its incorporation in brucite lattice likely via isomorphic substitution with OH. The effect of eight divalent metals (Sr, Ba, Ca, Pb, Mn, Fe, Co and Ni) studied at pH 4.9 and 0.01 M concentration revealed brucite dissolution rates to be correlated with the water molecule exchange rates in the first hydration sphere of the corresponding cation. The effect of investigated ligands on brucite dissolution rate can be modelled within the framework of the surface coordination approach taking into account the adsorption of ligands on dissolution-active sites and the molecular structure of the surface complexes they form. The higher the value of the ligand sorption constant, the stronger will be its catalyzing or inhibiting effect. As for Fe and Al oxides, bi- or multidentate mononuclear surface complexes, that labilize Mg-O bonds and water coordination to Mg atoms at the surface, enhance brucite dissolution whereas bi- or polynuclear surface complexes tend to inhibit dissolution by bridging two or more metal centers and extending the cross-linking at the solid surface. Overall, results of this study demonstrate that very high concentrations of organic ligands (0.01–0.1 M) are necessary to enhance or inhibit brucite dissolution. As a result, the effect of extracellular organic products on the weathering rate of Mg-bearing minerals is expected to be weak.

Expression of andic properties in soils from Galicia (NW Spain) under forest and agricultural use

SummaryThe organic matter (OM) of soils with andic properties has long been considered highly stable because of the presence of Al–humus complexes and sorption of organic ligands onto amorphous compounds. In this study, we characterized soils under different land use regimes located within an amphibolitic massif close to Santiago de Compostela (Spain), where soils with andic properties are present. Slash and burn agriculture was a common practice in the area until the second half of the 20th century. Thereafter, modern agriculture was progressively introduced into the area (AGR soils), and the rest of the land was either reforested or abandoned (FOR soils). We found that the mean organic C content of AGR soils (48.7 g kg−1) was ∼ 50% that of FOR soils (94.2 g kg−1). Mean soil pH was significantly greater (P < 0.05) in the AGR than in the FOR soils (4.95 compared with 4.63), which is attributed to liming and Ca‐phosphate fertilization of the former. Mean concentrations of the Al forms studied (extractable with CuCl2, sodium pyrophosphate, ammonium oxalate, or NaOH) were significantly smaller (P < 0.01) in AGR (1.4, 4.9, 9.3, 11.0 g kg−1, respectively) than in FOR soils (3.9, 10.2, 16.5, 17.9 g kg−1, respectively). The results show the vulnerability of the OM and Al–humus complexes in these soils to modern agricultural practices, which has led to the attenuation – and in some cases even the disappearance – of andic soil properties in a relatively short time (< 30 years) following changes in land use/management. We propose the inclusion of the formative element ‘andic’ in the criteria for the definition of Umbrisol subunits; this would avoid the abrupt discontinuity observed in the current World Reference Base classification.

Determination of binding constants of labile ligands in metal–ligand complexes containing carboxylic groups bound polymer by using breakthrough technique

This study was conducted to confirm the possibilities of determining the binding constants of ligands in metal–ligand complexes containing carboxylic groups bound polymer by using breakthrough technique. Aniline and o -anisidine were selected as model ligands. A mini column apparatus was used to study sorption of ligands onto ligand exchange resin. The experiments were conducted in a constant temperature at 25 °C. The equilibrium sorptions of aniline and o -anisidine were measured in order to determine the part of the sorption on Co(II)-CDAE-sporopollenin resin. The sorption of aniline is seen to be higher than that sorption of o -anisidine. The characteristics of the sorption process were investigated using Scatchard plot analysis ( q / C vs. q ). In the sorption of o -anisidine, deviation from linearity in the plot of q / C versus q was observed, indicating the presence of multi-model interaction and non-Langmuirean behavior. Furthermore, in high concentration of aniline deviation from linearity in the plot of q / C versus q was appeared. Particularly for o -anisidine, divergence from the Scatchard plot was evident, consistent with the participation of secondary equilibrium effects in the adsorption process. However, Scatchard plot analysis of the equilibrium binding data for aniline onto ligand exchanger gave rise to a linear plot. The ligand binding constants of aniline and o -anisidine were determined as 0.071 and 0.156 mol mol −1 Co 2+ , respectively.

Sorption of purin and pyrimidine bases onto chelating resin

Abstract Ligand sorption of nucleic acid bases and nucleosides has been studied by using ligand exchanger sporopollenin. A mini-column apparatus was used to study sorption of nucleic acid bases and nucleosides onto ligand exchanger resin. The experiments were conducted in a constant temperature at 25±0.1 °C. The sorption isotherms of nucleic acid bases and nucleosides from aqueous solution onto the resin were investigated. The ligand concentration was chosen to be 0.0125–0.2 mmol l −1 . The performance of the ligand exchanger in continuous operation was studied by conducting column runs. Ligand sorption constants and correlation coefficients for the adenine, adenosine, uracil and uridine on the chelating resin were calculated from Langmuir as well as Freundlich plots. The sorption binding constant of the ligands increased in the following sequences: uracil

Effect of temperature on separation of purine and pyrimidine bases on novel chelating resin

The novel chelating resin poly- bis [4-(4′-biphenyl)oxamidine]-diaminoglyoxime (PLH 2 ) has been modified as ligand-exchange material with Co(II). The possibility of using PLH 2 as ligand exchanger in the chromatographic separation of nucleic acid bases and nucleosides has been studied at various temperatures. PLH 2 offers the advantage of combining high sorption capacity with high selectivity for the ligands. It has a strong driving force for ligand sorption. Ligand-exchange chromatography on Co(II)-loaded functionalized PLH 2 can be a useful alternative method for separation of nucleic acid bases and nucleosides.

Factors Determining the Sorption Capacity and Selectivity of Metal-Substituted Carbon Sorbents for Ligand Sorption from the Gas Phase

We have established that it is feasible to significantly increase the capacity and selectivity of carbon sorbents by modifying them with metal ions during sorption from gas/air mixtures of sulfur dioxides and nitrogen, hydrogen sulfide, carbon disulfide, diethylamine, acetic acid, etc. We have determined the role of surface complexation and other factors affecting sorption; we have determined the routes and principles for creating and using new high-capacity and selective sorbents for these materials based on various carbonaceous materials (activated carbons, carbonaceous cloths, semicokes).

Sorption of target anions by ligand exchange

The sorption of anionic ligands such as I−, Br−, and SCN− onto diaminoethyl–sporopollenin (DAE–sporopollenin) was investigated in both column and batch experiments at room temperature. The isotherm binding constants suggest that the resin is most efficient at the binding of I− compared with Br− or SCN−. The general ligand sorption affinity series observed was I− > Br− > SCN−. This ligand sorption could be described by considering the hydration of ions in the exchanger. The sorption behavior of the ligand exchanger and the possibilities of selectively removing and recovering ligands are discussed on the basis of their chemical and complexing properties. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 771–774, 1998

Ligands Sorption Studies on Transition Metal Ion Loaded Amberlite IRC-50

Ligands sorption of ammonia, diethanolamine, and triethanolamine on the cation exchanger Amberlite IRC-50 having Cu2+, Ni2+, and Zn2+ ions was studied as a function of temperature 290−318 K and different initial concentrations 1−500 mmol·L-1. Sorption was found to follow the order Cu2+ > Zn2+ > Ni2+ and ammonia > diethanolamine > triethanolamine. Desorption of metal cations from the exchanger to the aqueous phase was also observed when the temperature of the system was increased. The results were explained in terms of ligand sorption and ion exchange, which were found to be dependent upon the stabilities of the complexes inside the resin and in the aqueous solution and the basicity and hydrophobicity of the ligand involved. The ratio of metal released/amine sorbed was determined which indicated two possible mechanisms of ligand sorption. The data were explained with the help of mass action law. IR studies also confirmed the presence of metal complexes and metal−amine complexes inside the resin.

The Influence of Internal Charge on the Sorption Properties of Clays

The surface complexation model (SCM) has been used successfully to describe the adsorption properties of oxides and other solids, such as carbonates and sulphides. This model describes adsorption as a superposition of the electrostatic interactions at the solid-water interface with chemical reactions of solutes with reactive groups at the surface of the solid. The activities of the species at the interface depend on the electrostatic potential of the surface, and are calculated from a so-called 'coulombic term' whose exact form depends on the electrostatic description of the interface (e.g. double layer or triple layer). The surface complexation model has been used to model two kinds of experimental data: acid-base titration curves and metal or ligand sorption edges. It predicts that as the ionic strength, I, decreases, the absolute surface charge density also decreases at a given pH. At low ionic strength, the surface charge is less efficiently shielded and accumulation of charge on the surface is energetically more difficult. In the absence of any strongly binding ion, the SCM also predicts that the titration curves at various I intercept at the same pH, the zero proton condition (ZPC), where the concentrations of adsorbed H + and OHat the surface are equal. Sorption edges of a cation M n+ usually show a strong increase of adsorption with pH, as the competition of H + with the cation for the surface sites decreases. The slope of the [M ~+] adsorbed v s pH curve reflects the net number of H + ions displaced by the adsorption of M n+ which depends on the stoichiometry of the reaction and the variation of the surface potential with pH. In spite of the success of the SCM in describing sorption properties of oxides, its application to clays has not been straigthforward. In particular, some titration data on clays such as kaolinite and montmorillonite show an increase in the ZPC as ionic strength decreases; concomitantly, at low pH, the charge density is higher at low ionic strength, contrary to what is usually observed on oxides. Furthermore, fitting metal sorption on clays with the SCM has proven difficult (Bradbury and Baeyens, 1997). Here, we propose that the different sorption properties of clays compared to oxides may be explained by the porous character of clays, whose interlayers are permeable to water and electrolyte ions, together with their fixed internal charge resulting from isomorphic substitution. The electrostatics of a system consisting of a homogeneous porous solid bearing a fixed internal charge and immersed in a 1:1 electrolyte solution can be described by the Poisson-Boltzmann equation, leading to a generalization of the Gouy-Chapman theory. We thus obtain expressions for the internal and surface potential as a fimction of ionic strength, internal charge density and surface charge density.

Kinetic Study of Ligand Exchange Reaction of Anions as Ligands on DAE-Sporopollenin

The specific rate coefficients and ligand sorption isotherms of Br and I−as ligands on diaminoethyl-sporopollenin were determined as a function of pH and temperature between 20 and 60°C. The ligand exchange rates were measured just after the rapid mixing of the ligand solution with the resin suspension. The data have also been analyzed to obtain activation parameters ΔG, ΔH, and ΔSfor reaction in the corresponding mixture. A single ligand-exchange process was observed; the sorption of ligand increased with decreasing pH and hydrated radius of ligands and increasing temperature. A reaction mechanism which describe the ligand exchange reaction is proposed.