Search

The impact of ligand protonation on metal complexation kinetics is determined on the basis of the Eigen mechanism for complex formation in aqueous system. The results have shown that the conventional approach, with consideration of the free ligand L only, increasingly overestimates the lability of the system with decreasing pH in the range where the protonated outer-sphere complex is significant.

The various physicochemical factors that influence the flux of carrier-transported metal ions through permeation liquid membranes (PLM) are studied systematically. Understanding PLM behavior is important (i) to optimize the application of PLM as metal speciation sensors in environmental media and (ii) because PLM may serve as bioanalogical devices that help to elucidate the environmental physicochemical processes occurring at the surface of biological membranes. Diffusion of free and complexed metal ions in solution, as well as diffusion of the metal carder complex in the membrane, is considered. The respective roles of diffusion layer thickness, ligand concentration, complex stability, carder concentration, and membrane thickness are studied experimentally in detail and compared with theory, using various labile complexes, namely, Pb(II)--diglycolate, Cu(II)--diglycolate, and Cu(II)--N-(2-carboxyphenyl)glycine. Conditions where either membrane diffusion or solution diffusion is rate limiting are clearly discriminated. It is shown in particular, that, by tuning the carder concentration or membrane thickness, either the free metal ion concentration or the total labile metal species are measured. PLM can thus be used to determine whether models based on the free ion activity in solution (such as BLM or FIAM models) are applicable to metal uptake by microorganisms in a real natural medium.

The consequences for the electrokinetic properties of metallic surfaces in the presence of a quasi-reversible redox couples are analyzed. The corresponding formalism completes the rigorous basis for a quantitative interpretation of electrokinetic features of electron conducting surfaces depolarized by irreversible, quasi-reversible, or reversible bipolar faradaic processes.

The diffusion and binding properties of an agarose gel is investigated and the data is interpreted with the models describing solute diffusion in the gel, which results for the electrical, steric, and chemical interactions.

The adsorption and diffusion of linear alkanes in ERI-, CHA-, and LTA-type zeolites is studied. ERI-, CHA-, and LTA-type zeolites consist of cages separated by 0.4 nm wide windows and these windows form large free energy barriers to diffusion.

Speciation measurements with the permeation liquid membrane (PLM) technology require necessarily a good control of the flux of the analyte. In this perspective, a PLM-based multichannel flow-through cell has been designed. The first objective of this study has been to adapt the classical Levich model commonly used for electrochemical flow devices to the characteristic geometry of the PLM cell. In the latter case and contrary to the Levich model, the effects of the channel lateral walls on the flux of active species through the membrane have to be taken into account. The problem was solved by considering the existence of two parabolic Poiseuille profiles perpendicular to each other and developing along the fluid motion. The theoretical results obtained match satisfactorily with experimental data. The analytical study of this PLM system has been performed with copper(II) ions as test species and has shown that the preconcentration factor is (1) linear at least for preconcentration times of [less than or equal to] 120 min, (2) reproducible on the same membrane as well as on different membranes, and (3) independent of the initial test metal ion concentration in the sample solution. The capabilities of this cell to determine metal speciation by considering lability of complexes and the flux of metal at variable flow rates of the test solution is also discussed by means of Cu(II)/sulfosalicylic complexes.

We present the first sensing system for metal ions based on the combination of separation/preconcentration by a permeation liquid membrane (PLM) and fluorescence detection with an optical fiber. As a model, a system for the detection of Cu(II) ions was developed. The wall of a polypropylene hollow fiber serves as support for the permeable liquid membrane. The lumen of the fiber contains the strip solution in which Cu(II) is accumulated. Calcein, a fluorochromic dye, acts as stripping agent and at the same time as metal indicator. The quenching of the calcein fluorescence upon metal accumulation in the strip phase is detected with a multimode optical fiber, which is incorporated into the lumen. Fluorescence is excited with a blue LED and detected with a photon counter. Taking advantage of the high selectivity and sensitivity of PLM preconcentration, a detection limit for Cu(II) of ~50 nM was achieved. Among five tested heavy metal ions, Pb(II) was the only major interfering species. The incorporation of small silica optical fibers into the polypropylene capillary allows for real-time monitoring of the Cu(II) accumulation process.

Researchers from the University of Geneva and the Swiss Federal Institute for Environmental Science and Technology studied the diffusion coefficients determination of humic and fulvic acids in the Suwannee River by using fluorescence correlation spectroscopy.

The role of lauric acid (LAH) in the transport of copper(II) through a permeation liquid membrane (PLM) comprising 1,10-didecyldiaza-18-crown-6 (22DD) and lauric acid (ratio 1:1) in 1:1 v/v toluene/phenylhexane has been investigated by determining the stoichiometry of metal extraction and of the metal complex formed in the organic phase by performing (super 1)H NMR and liquid/liquid and liquid/membrane extraction measurements. In the absence of copper(II), the (super 1)H NMR data suggest that there is a strong interaction between the proton of IAH and the nitrogen of the 22DD macrocycle but no interaction between the aliphatic long chains of LAH and 22DD. Thus, in the organic solution, the two compounds are associated as (22DD-H)(super +)-LA(super -), the laurate being away from (22DD-H)(super +). The signal intensity of the acidic proton was found to decrease when the metal Pb(II) was incorporated by the carrier after its extraction from the aqueous phase. Additionally, liquid/liquid as well as liquid/ membrane extraction results reveal that Cu(II) extraction proceeds via the loss of two protons from the organic phase. The Cu(II) is found to be located in the 22DD cavity and the stoichiometry of the complex in the organic phase is (22DD-Cu)(super 2+)-2LA(super -). Metal extraction is governed by 22DD and laurate acts only as counteranion. An unexpected feature was observed in the liquid/liquid extraction which was that, at low 22DD and LAH concentrations, the slope for log(K(sub p)) = f(pH) was 2 whereas it was much lower at high carrier concentration. This unexpected result seems to stem from impurities present in 22DD: only 0.1 mol % of impurity can indeed influence the exchange ratio of Cu(II) and H(super +). This type of anomaly, however, is not found in the normal procedure of liquid/ membrane extraction possibly due to the lower carrier/ metal molar ratio which is used in the classical PLM conditions.

The paper reports a new approach based on a gel-integrated Hg-plated-Ir-based microelectrode array (GIME), for measuring Cu, Pb, Cd, and Zn speciation in natural waters. This paper focuses on the quantitative discrimination between mobile and colloidal metal species (size limit of a few nanometers), for which most classical separation techniques present severe drawbacks. Previous papers have shown qualitatively that GIME combined with square wave anodic stripping voltammetry (SWASV) has the basic characteristics required to discriminate between these two fractions directly on the unperturbed sample. In addition, because of the large sensitivity provided by GIME, complexation parameters (equilibrium constants and site concentrations) can be determined in little-perturbed samples, particularly without metal addition or with small addition compared with natural concentrations. The advantages of this procedure are exemplified and the possible artifacts occurring when titrating the sample with metals, in particular intermetallic compound formation and other problems, are discussed in detail. The present paper shows that the characteristics of GIME make it a unique tool to get quantitative information on metal speciation at nanomolar or even subnanomolar levels, with only minor sample handling.

The mechanism of macrocycle-mediated Cu(II) transport through a supported liquid membrane containing 1,10-didecyl-1,10-diaza-18-crown-6 ether-fatty acid in toluene/phenylhexane solvent mixture has been investigated. The mechanism was elucidated by studying the effects of parameters such as stirring speed, membrane thickness, and temperature on Cu(II) flux. The results showed that a counteraction transport is controlled by a sodium gradient. The rate-limiting step of Cu(II) transport in the system has been found to be diffusion of metal-carrier complex in the membrane. But, depending on the hydrodynamic conditions and cell geometry, diffusion of metal in the stagnant aqueous Nernst layer is found to be rate limiting. In addition, a linear Cu(II) gradient in the membrane was found by performing Cu(II) transport experiments in a stack of eight membranes and analyzing the copper(II) concentration in each of the membranes separately, supporting diffusion-limited steady-state transport. Comparative studies at optimum operational conditions have been done with two types of supports, Celgard 2500 and Accurel, differing in their pore size and porosity. From lag time measurements, the effective diffusion coefficient, [Mathematical Expression Omitted], of the metal-carrier complex in the membrane was evaluated and the values were found to be 5.2 x [10.sup.-8] and 2.55 x [10.sup.-7] [cm.sup.2] [s-1] for Celgard and Accurel membranes, respectively. These differences in [Mathematical Expression Omitted] between the two membranes has been interpreted in terms of molecular diffusion of the complex in the solvent, [D.sub.m], and of the equilibrium constant for its adsorption on the pore wall of the membrane.

Colloidal interactions in aqueous solutions are found to vary and that the concentration of stable colloids are dependent on the relative proportions of fulvic compounds, compact inorganic colloids and rigid biopolymers. Fulvic compounds are found to stabilize organic colloids while rigid biopolymers are found to have a destabilizing effect with the formation of aggregates dependent on the amount of each in an aquatic system.

A novel voltammetric in situ profiling system (VIP) was utilized as a new method for determining Mn(II) levels in aquatic systems such as lakes. The system, which uses a gel-integrated microsensor and a group of microelectrodes to prevent surface fouling from organic matter, permits direct measurements in complex media. The new technique, which allows for in situ surveying of natural systems, compares with traditional techniques such as chemical laboratory analysis.

The pyrolysis/gas chromatography/mass spectrometry was utilized to determine the characteristics of natural organic matter (NOM) and their role in colloid transport. The levels of polysaccharide, proteins and aliphatic compounds in samples collected from Lake Bret, Switzerland increased during the summer season. Furthermore, the high levels of polysaccharide during the summer season are caused by microbial cell wall degradation during summer.