Metal Ion Partitioning Research Articles

A ternary mechanism for the facilitated transfer of metal ions into room-temperature ionic liquids (RTILs): implications for the “greenness” of RTILs as extraction solvents

Partitioning of sodium ions between aqueous nitrate media and 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imides (Cnmim+Tf2N−) in the presence of dicyclohexano-18-crown-6 is shown to take place via as many as three pathways: conventional nitrato complex extraction and/or either or both of two ion-exchange processes, the relative importance of which is determined by aqueous acidity and the hydrophobicity of the ionic liquid cation. Contrary to expectations, increasing the alkyl chain length of the IL cation (from C5mim+ to C10mim+) is insufficient to eliminate the possibility of ion exchange as a mode of metal ion partitioning between the two phases, an observation with negative implications for the utility of ILs as environmentally benign extraction solvents.

The effects of 1-hydroxyethane-(1,1-diphosphonic acid) on the adsorptive partitioning of metal ions onto γ-AlOOH

1-Hydroxyethane-(1,1-diphosphonic acid) (HEDP) is extensively used in many technical applications. This work, as a first stage, examines the adsorption of aqueous HEDP, Cu(II), and Zn(II) onto boehmite ( γ-AlOOH) as single solutes. These processes can be described by the formation of inner-sphere complexes by the surface complexation model with constant capacitance. Uncomplexed HEDP is adsorbed as a mononuclear complex over the entre pH range. Four mononuclear surface complexes with different protonation levels are required to obtain a good fit of the experimental data for the pH range studied. Cu(II)– and Zn(II)–HEDP complexes at equimolar concentrations are studied at high and low surface density. The results indicate that the presence of HEDP significantly promotes metal adsorption at low pH in defects of surface sites. However, metal adsorption exhibits the same trend in the absence and presence of HEDP when the surface sites are in excess. The constant capacitance model successfully describes the experimental data through a ligand-like adsorption complex at low pH. At high pH, the model predicts separate adsorption of divalent metal and HEDP onto different surface sites as the preferred adsorption form.

超臨界条件下における閃亜鉛鉱およびザクロ石と２Ｍ塩化物水溶液間における２価金属イオンの同時分配実験

Experiments on the simultaneous partitioning of divalent metal ions were carried out in the system K2Mg6Al2Si6O20(OH)4 (phlogopite) − KCl - (Ni2+,Mg2+,Co2+,Zn2+,Fe2+,Mn2+,Ca2+,Sr2+) Cl2 - H2O in the temperature range of 500 to 800 °C at 100 MPa, using the pressure vessel of a cold seal type. The partition coefficient KPN (Phl) for the ion exchange reaction between phlogopite (Phl) and aqueous chloride solution (aq) can be expressed as follows: KPN (Phl) = (X1/6K2Me6Al2Si6O20(OH)4 / X1/6K2Mg6Al2Si6O20(OH)4) Phl / (mMeCl20 / mMgCl20)aq where Me designates divalent metal ion, m: molality, and Xi: mole fraction of end-member i in biotite solid solution. The partition coefficient - ionic radius (PC-IR) curve marks a peak between Mg2+ and Fe2+. Zn2+ shows the same negative anomaly in the partition coefficients as observed in other minerals with 6-fold sites. Co2+ also represents such a negative anomaly as was previously found in tremolite and hedenbergite. Based on the previous and present experimental results, it is considered that the anomaly of Co2+ may be related with polymerization of Si tetrahedra.

LIQUID/LIQUID EXTRACTION OF METAL IONS IN ROOM TEMPERATURE IONIC LIQUIDS

The search for more environmentally-friendly reaction media has prompted the development of a wide array of alternative systems that will sustain biphasic separations with aqueous solutions without the use of volatile organic compounds (VOCs). We have begun to employ Room Temperature Ionic Liquids (RTIL), specifically 1-alkyl-3-methylimidazolium hexafluorophosphate ([Cnmim][PF6]), as VOC replacements in liquid/liquid separations of metal ions from aqueous solutions. Here we show that the partitioning of metal ions in these novel biphasic systems is consistent with traditional liquid/liquid separations: the metal ion affinity for the hydrophobic phase necessitates the presence of an extractant. In this report we explore the application of wellknown organic (1-(pyridylazo)-2-napthol, PAN, and (1-thiazolylazo)- 2-napthol, TAN) and inorganic (CN−, OCN−, SCN−, and halides) extractants for partitioning a variety of metal cations between [C4mim][PF6] or [C6mim][PF6] and an aqueous phase. PAN and TAN show pH dependent extraction of CD2+, Co+ Ni2+ and Fe3+ where their partitioning to the RTIL increases at least 2 orders of magnitude from pH 1 to 13. The effect of the halides on the partitioning of Hg2+ complexes increases F− < Cl− < Br− < I−. Pseudohalides, especially SCN−, had the greatest effect on enhancing the partitioning of Hg2+to the RTIL, whereas CN− and OCN− provided little benefit for the extraction of any of the metal ions examined.

Effects of supercritical fluid extractions on metal ion partitioning as indicated by sequential extractions

We have studied the partitioning of stable elements and uranium to two sediments using sequential extractions (SE). These matrices were then subjected to supercritical fluid extraction (SFE) to attempt to remove the uranium. SEs were repeated on the material remaining after SFE to determine any changes in partitioning. We have confirmed that under suitable chemical conditions, ligand-assisted SFE can be used to extract many metal ions from such matrices. Under the conditions we used, the "easily leachable" metals were extracted, but the refractory metals were not. In addition, SFE appears to redistibute the remaining metal cations among the remaining geochemical after SFE treatment. Our results are presented and discussed in the context of developing decontamination strategies for contaminated soils and sediments.

Traditional Extractants in Nontraditional Solvents: Groups 1 and 2 Extraction by Crown Ethers in Room-Temperature Ionic Liquids

The crown ethers 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DCH18C6), and 4,4‘-(5‘)-di-(tert-butylcyclohexano)-18-crown-6 (Dtb18C6) were dissolved in 1-alkyl-3-methylimidazolium hexafluorophosphate ([Cnmim][PF6], n = 4, 6, 8) room-temperature ionic liquids (RTILs) and studied for the extraction of Na+, Cs+, and Sr2+ from aqueous solutions. In the absence of extractant, the distribution ratios for the metal ions indicate a strong preference for the aqueous phase. With the crown ethers as extractants in RTIL-based liquid/liquid separations, the resulting metal ion partitioning depends on the hydrophobicity of the crown ether and also on the composition of the aqueous phase (e.g., concentration of HNO3 vs Al(NO3)3). Aqueous solutions of HCl, Na3 citrate, NaNO3, and HNO3 (the latter at low concentrations) decrease the metal ion distribution ratios and also decrease the water content of the RTIL phase. High concentrations of HNO3 decompose PF6- and increase both the water content and the water solubility of the RTIL phase. Highly hydrated salts such as Al(NO3)3 and LiNO3 salt out both the RTIL ions and the crown ethers; thus, when the aqueous phase contains Al(NO3)3, the trend more closely resembles traditional solvent extraction behavior where DSr > DCs and the most hydrophobic extracting phase produces the highest partitioning. When [C8mim][PF6] is used as the extracting phase, the metal ions can be loaded from Al(NO3)3 and stripped using water. Dtb18C6 forms 1:1 complexes with Cs+ and Sr2+ and also yields the highest distribution ratios out of the three crowns examined. In comparison to traditional solvent extraction behavior, the metal ion partitioning in these systems exhibits exceptional behavior and, in certain instances, suggests a complicated partitioning mechanism, which necessitates a more thorough understanding of RTILs as solvents before interpretation of the results.

Mn and Zn incorporation into calcite as a function of chloride aqueous concentration

During spiral growth of the calcite rhombohedron {10 1 4}, divalent metals substituting for Ca 2+ are differentially incorporated due to steric differences inherent to the asymmetric kink sites exposed at nonequivalent growth steps. Hence, ions “larger” than Ca 2+ (e.g., Sr 2+ and Ba 2+) exhibit an incorporation trend opposite to that of “smaller” ions (e.g., Mn 2+ and Co 2+). However, Zn 2+ exhibits the same incorporation trend as large ions in coprecipitation experiments conducted from strong NH 4Cl electrolytes. In this study we compared the incorporation trends of Zn and Mn from solutions with various chloride content to test the possibility that the adsorption of “large” ZnCl n 2−n aqueous complexes influences the site preference of Zn. The incorporation trends of Mn and Zn were opposite at the symmetrically nonequivalent growth steps. From a 0.4 M NH 4Cl solution, where Zn aqueous speciation was thermodynamically dominated by the “free” aquo ion, Zn maintained its site preference for the geometrically less constrained surface sites. Thus, Zn exhibits a particular interaction with surface sites and its incorporation trend is not controlled by the prevalence of ZnCl n 2−n complexes. Other factors like the electronic configuration must be considered. The surface microtopography of calcite was found to be sensitive to changes in the aqueous concentrations of NH 4Cl and Zn. Decreases in NH 4Cl concentration resulted in an increase of the density of growth hillocks. The strong adsorbing behaviour of Zn increased the surface roughness, decreased the rate of growth, perturbed the spiral growth mechanism, and triggered the nucleation of discrete surface precipitates (∼0.2 μm) along macrosteps. An increase of Cl incorporation, despite the decrease of its aqueous concentration by dilutions of the parent solution, suggests that surface roughness at the calcite-solution interface is another factor involved in the nonequilibrium process of impurity element incorporation. The significant increase in Mn and Zn incorporation into calcite due to decreases in Cl aqueous concentration demonstrates the role of chloride as an important factor in the partitioning of trace metal ions between calcite and low-temperature aqueous fluids. More importantly, the magnitude of Mn differential incorporation also increased with dilutions which indicates that distribution coefficients commonly calculated from bulk solid analyses may underestimate seriously the surface structural effect imposed on trace element incorporation.

Model for the partition of metal ions in aqueous two-phase systems

A model for the partition of metal ions in aqueous two-phase systems (ATPS) has been developed. The partition coefficient of a metal ion D M, is a function of several variables of the ion (size, charge and electronegativity), characteristics of the ATPS such as type of salt, salt concentration and PEG concentration and additional inorganic salt present in the ATPS. The model has been tested for complex anions of BiX 4 − (BiCl 4 −, BiBr 4 − and BiI 4 −) and cations from groups I and II (Na +, Cs +, Ca 2+, Sr 2+ and Ba 2+) giving a good correlation in both systems. It was found that for these systems partition coefficient increases with ion size and the variables Y which is a characteristic of the ATPS and Z which is a characteristic of the additional salt present in the system. The partition coefficient of BiX 4 − increases with the variable X which is a characteristic of the electrical interactions of the metal ion. The cations from groups II and I exhibit the opposite behavior, which is attributable to the ion charge.

Kinetic and spectroscopic characterization of native and metal-substituted β-lactamase from Aeromonas hydrophila AE036

Two metal ion binding sites are conserved in metallo-β-lactamase from Aeromonas hydrophila. The ligands of a first zinc ion bound with picomolar dissociation constant were identified by EXAFS spectroscopy as one Cys, two His and one additional N/O donor. Sulfur-to-metal charge transfer bands are observed for all mono- and di-metal species substituted with Cu(II) or Co(II) due to ligation of the single conserved cysteine residue. Binding of a second metal ion results in non-competitive inhibition which might be explained by an alternative kinetic mechanism. A possible partition of metal ions between the two binding sites is discussed.

Ion Exchange Basis for Biosorption and Bioremediation of Heavy Metals†

An important aspect of mining operations is the extent of sorption of effluent metals to soil and biomass. Of special interest is the partitioning of metal ions in the aqueous-soil-biomass system. The ion exchange relationship of Zn, Cd, Cu, and Pb was found to apply for the metal in the presence of two sorbents: soil with either peat moss, watercress roots, or rye roots. The uptake of Cd from soil by growing rye grass increases with Cd content in the soil, although the per cent uptake is higher for lower amounts. Exchange constants for displacement of protons from limestone soil by Cd decrease by a factor of 105 as pH changes from 4 to 6.6. A linear relationship for log KH exvs. pH can be interpreted as Cd binding to weaker acid sites in more basic media. Comparable results were found for peat moss and the alga Vaucheria. These findings are relevant to agricultural hazards posted by heavy metals as well as the possible use of certain crops for bioremediation.

Partition of lead in a milk-pectin aqueous biphasic system with complexing agents

Aqueous two-phase systems allow partition of metal ions in nondenaturing conditions. From the fresh skim bovine milk spiked with212Pb and agitated with 4% water solution of apple pectin (degree of esterification-70%), less than 2.3% is recovered to the pectin phase and in the case of pasteurized milk less than 3.0%. Complexation with chelate forming agents was studied. Aminoacids as glycine, alanine and histidine do not exert influence on the yield, but the recovery in the pectin phase was 22% for cysteine, and 80% in the case of EdtacalTM (calcium salt of ethylenediaminetetraacetic acid). The retention of lead in milk phase is extraordinarily high as compared to strontium and corresponds to the behavior of trivalent ions.

Prospects for in vivo NMR methods in xenobiotic research in plants

The application of non-invasive nuclear magnetic resonance (NMR) methods in xenobiotic research is reviewed in relation to: (i) the characterisation of the effects of xenobiotics on the metabolism of plants and plant cell suspensions; (ii) the direct detection of xenobiotics and their degradation products in vivo; and (iii) the spatial localisation of xenobiotics and their derivatives at the subcellular and tissue levels. Novel information has been generated by in vivo NMR studies of both agrochemicals and heavy metals, but a lack of generality in the methods makes it difficult to extrapolate from one successful application to the next. In vivo NMR spectroscopy is shown to be informative when a xenobiotic perturbs metabolic pathways that are accessible to the technique, and it is useful for probing the partitioning of paramagnetic metal ions between the cytoplasm and the vacuole. The successful application of 19F NMR to the analysis of plant tissue extracts also suggests that in vivo 19F NMR spectroscopy may have a role in biotransformation studies of fluorinated xenobiotics. In contrast NMR imaging techniques have been little used for xenobiotic research in plants, and while the method has been shown to be capable of monitoring the uptake and translocation of paramagnetic ions in plants, the potential use of high resolution 1H and 19F NMR imaging for mapping agrochemicals in tissues is still in its infancy.

Affinity Partitioning of Metal Ions in Aqueous Polyethylene Glycol/Salt Two-Phase Systems with PEG-Modified Chelators

Abstract The systematic partitioning of metal ions in the presence of polyethylene glycol (PEG) chelate derivatives in PEG/salt two-phase systems is described. Results show that the partitioning is enhanced significantly, and this effect is apparently based on the characteristics of binding of the chelate and the nature of the metal ion in addition to the effect of the attached PEG molecule. Previous work with PEG-iminodiacetic acid has been extended with the chelating derivatives PEG-TED [tris(carboxymethyl)ethylene diamine], PEG-TREN-Cm [carboxy-methylated-tris(2-aminoethyl)amine], and PEG-ASP (aspartic acid). Their effectiveness to selectively partition the metal ions is demonstrated and discussed. The partition experiments were performed using two-phase systems of PEG (8000 MW), sodium sulfate, the PEG-chelating derivatives, and different concentrations of copper, nickel, and cobalt as model metal ions. Partition coefficients (ratio of top to bottom metal concentrations) greater than 1 were obtained for the metal ions with all the chelating derivatives tested. Selective enhancement of partitioning was observed in all cases. Based on the experimental results and theoretical principles, the method shows potential as an alternative mode to remove efficiently and selectively metal ions from solutions using aqueous two-phase systems and PEG-chelate derivatives.

Metal ion separations in polyethylene glycol-based aqueous biphasic systems: correlation of partitioning behavior with available thermodynamic hydration data

Solvent extraction, utilizing an oil-water mixture (e.g, chloroform-water) and a suitable complexant, is a proven technology for the selective removal and recovery of metal ions from aqueous solutions. Aqueous biphasic systems (ABS), formed by mixing certain inorganic salts and water-soluble polymers, or by mixing two dissimilar water-soluble polymers, have been studied for more than 40 years for the gentle, non-denaturing separation of fragile biomolecules, yet ABS have been virtually ignored as a possible extraction technology for metal ions. In this report we review our metal ion partitioning work and discuss the three major types of partitioning: (1) those rare instances that the metal ion species present in a given solution partitions to the PEG-rich phase without an extractant; (2) the use of halide salts which produce a metal anion complex that partitions to the PEG-rich phase; and (3) the use of a water-soluble extractant which distributes to the PEG-rich phase. In addition, we correlate the partitioning behavior we observed with available thermodynamic data for metal ions and their complexes.

Crown ethers as actinide extractants in acidic aqueous biphasic systems: partitioning behavior in solution and crystallographic analyses of the solid state

The partitioning behavior of UO 2 2+, Pu 4+, Th 4+, and Am 3+ has been investigated in aqueous biphasic systems formed by the addition of polyethylene glycol of average molecular weight 2000 (PEG-2000) and (NH 4) 2SO 4. Water-soluble crown ethers 18-crown-6 and 15-crown-5 have been utilized as extractants. A positive correlation has been observed between crown ether concentration and metal ion partitioning with D An decreasing in the order UO 2 2+ > Pu 4+ > Th 4+ > Am 3+. A distribution ratio above unity, however, has only been observed for UO 2 2+ at very high (1.25 M) concentrations of 18-crown-6. In general the distribution ratios for 15-crown-5 are all lower. Matrix ions introduced as NH 4NO 3, H 2SO 4, or HNO 3 drastically reduce the observed distribution ratios and level the extractant dependencies.

Complexation of carbonate species at the goethite surface: Implications for adsorption of metal ions in natural waters

Headspace P Co 2 was measured with an infrared gas analyzer over an equilibrated goethite suspension to determine adsorption of carbonate species in the pH range 3 to 8. For a 2 g/L goethite suspension in 0.1 N NaClO 4 (~3 10 −4 M surface sites), the fraction of carbonate species adsorbed increased from 0.15 at pH 3 to a maximum of 0.56 at pH 6. In 0.01 N NaClO 4, the fraction of carbonate species adsorbed at pH 6 increased to 0.67. The total concentration of CO 2 in the suspension increased from about 0.4 to 0.6 10 −4 M in the pH range of these experiments. The development of surface charge at the goethite surface was determined in the pH range 4 to 11 by potentiometric titration under controlled low CO 2 conditions. No hysteresis was observed between the acid and base legs of titrations in 0.10, 0.03, and 0.01 N NaClO 4 resulting in a pH pzc of 8.9. The carbonate species adsorption data were modelled using the least squares optimization program FITEQL for the diffuse double-layer model and the triplelayer model using stoichiometries of the type Fe-OCOOH and Fe-OCOO − for surface bound carbonate species. The model results are consistent with separate experiments showing a significant reduction in chromate adsorption on goethite as the partial pressure of CO 2 was increased from <5 to 450 and 40,000 μatm. Our data suggest that mineral oxide surface sites which control solid/solute partitioning of metal ions in natural systems may be largely bound to adsorbed carbonate species.

The crown ether extraction of group 1 and 2 cations in polyethylene glycol-based aqueous biphasic systems at high alkalinity

Abstract

The effects of strongly complexing ligands on the adsorptive partitioning of metal ions

This work investigated the effects of polyphosphate, a model ligand which forms strong complexes with metals, on metal sorption onto ferrihydrite. Adsorption experiments were conducted to study the partitioning of metal ions in systems containing a range of metal and ligand concentrations. The sorption of ligands, the associated sorption of charge and complexation of metals by ligands are important in controlling metal partitioning. In metal-free systems, the apparent maximum adsorption density is about 0.1 mol of polyphosphate per mol of Fe. The sorption of metals and/or metal-ligand complexes increases the maximum sorption density of ligands compared to systems where metal ions are not present. At total polyphosphate concentrations less than that needed to saturate the oxide surface, the presence of polyphosphate enhances the sorption of metals. When the concentration of polyphosphate exceeds that needed to saturate the oxide surface, the sorption of metals diminishes due to both the competition between dissolved and surface polyphosphate for metal ions and the dissolution of iron oxides.

Non-aqueous solvents in anion-exchange separations

Distribution coefficients are measured for the partition of metal ions between anionexchange resin and organic solvent-water mixtures containing hydrochloric acid. The presence of an organic solvent causes metal ions to be taken up at lower hydrochloric acid concentrations. In many cases, distribution coefficients are significantly higher than in water-hydrochloric acid systems. If other conditions are comparable, the order of distribution coefficients in alcohol-water-hydrochloric acid is: isopropyl > ethyl > methyl alcohol. Column separations of metal ion mixtures can be carried out by eluting with alcohol-water-hydrochloric acid mixtures of different compositions. Successful separations of a number of mixtures are reported.