Permeation Liquid Membrane Research Articles

Diffusion limitation of zinc fluxes into wheat roots, PLM and DGT devices in the presence of organic ligands

Environmental context Zinc is an essential micronutrient for plants and many arid areas of the world have zinc-deficient soils. The bioavailability of Zn to plants is influenced by diffusion limitations and complex lability in the soil solution. To identify the relative importance of these two factors, we investigated the influence of diffusion layer thickness on Zn uptake by wheat and by two bio-mimetic devices in the presence of ethylenediaminetetraacetic acid and two natural ligands found in soil. Abstract Organic ligands can increase metal mobility in soils. The extent to which this can contribute to plant metal uptake depends among others, on complex lability and diffusion limitations in solute transfer from the soil solution to root uptake sites. We investigated the influence of diffusion layer thickness on zinc uptake by wheat seedlings in the presence of ethylenediaminetetraacetic acid (EDTA), citrate and histidine with similar free Zn by measuring 65Zn uptake from stirred, non-stirred and agar-containing solutions. Analogous experiments were performed using permeation liquid membranes (PLM) and ‘diffusive gradients in thin films’ (DGT) probes as bio-mimetic devices. In treatments with low EDTA concentrations (~2µM) or ligand-free Zn solution, increasing diffusion layer thickness reduced Zn fluxes into roots to a similar extent as into PLM and DGT probes, indicating reduced uptake attributable to diffusion limitation. In the citrate treatments root Zn influx was similar to EDTA treatments under stirred conditions, but increasing diffusion layer thickness did not affect Zn uptake. This suggests complex dissociation compensated for reduced Zn2+ diffusion and that the entire complexes were not taken up. The Zn root influxes in the histidine treatments were found to be on average by a factor of 2.5 higher than in the citrate treatments and they also showed no decrease in non-stirred and agar treatments. Dissociation kinetics inferred from PLM measurements explained a large part, although not all, of the increased Zn uptake by the plants in the presence of histidine. The difference may be a result of the uptake of neutral or positive Zn–histidine complexes. The results of this study confirm that labile complexes can contribute to Zn uptake by wheat either through diffusion limitation and complex dissociation or through uptake of entire complexes, depending on the nature of the ligands.

Surfactant mixtures at the oil–water interface

We report the structural study of mixed monolayers of partially deuterated N,N′-di-hexadecyl-(d33)-4,13-diaza-18-crown-6 ether (d-ACE16) and palmitic acid (PA) at the oil–water interface, in order to understand the mechanism of metal ion transport through Permeation Liquid Membrane (PLM) devices. The composition of the mixed monolayers remains constant with increasing spread amount and the saturation of the interface is achieved at a relatively low spread amount. The excess PA material is accommodated in the oil phase, playing an important role in equilibrating the interfacial concentration of ACE-16. The presence of PA increases the surface concentration of ACE-16 at low spread amount and facilitates its dissolution into the oil phase at the high spread amount. The result suggests a dynamic exchange between the bulk phase and the interface ensuring a continuous turnover which reflects their relevance in PLM devices. The conclusions regarding the role of a fatty acid in regulating the surface concentration of the alkylated azacrown ether and its dominant role in the bulk transport of metal ions through the membrane are consistent with the results of macroscopic studies reported earlier.

The use of permeation liquid membranes for free zinc measurements in aqueous solution

Environmental context The free Zn ion concentration in environmental aqueous systems is an important factor in determining Zn deficiency or toxicity to organisms as this species is directly bioavailable. The permeation liquid membrane technique, a tool to measure either free or bioavailable metal concentrations in solution depending on its setup, was evaluated for the first time for Zn speciation in simplified plant nutrient solutions. The technique is low-cost and applicable to a broad range of aqueous samples. Abstract The bioavailability of Zn in environmental water phases strongly depends on its speciation. One important species in studies on Zn deficiency or toxicity to organisms is the free ion. The permeation liquid membrane (PLM) technique is a tool to measure free metal concentrations with a short analysis time of 1 h and at low cost. However, so far it has only been validated for Cd, Cu, Ni and Pb. In this study we tested the effect of carrier concentrations and pH on Zn transport across the organic PLM membrane and the ability of the technique to measure free Zn in synthetic plant nutrient solution. We found that Zn membrane transport is dependent on the concentration of the carrier molecule lauric acid (LA), whereas variations in the concentration of the other carrier molecule, the crown ether Kryptofix 22DD, showed no effect, suggesting that Zn is not transported by the ‘classical’ PLM transport mechanism by binding to the crown ether. Zn preconcentration increased with increasing pH and decreased with increasing ligand concentrations. Using 0.05 M LA, Zn membrane transport is expected to be rate limiting (permeability criterion <<1) and the free Zn concentration can be measured. Under these conditions, PLM measurements agreed well with speciation calculations and with Donnan membrane technique (DMT) measurements in the presence of ligands forming negatively charged Zn complexes (ethylenediaminetetraacetate or citrate). In the presence of L-histidine higher free Zn concentrations than calculated were measured by PLM and DMT, suggesting that positively charged complexes contributed to cross-membrane transport in both methods.

Nickel (II) Preconcentration and Speciation Analysis During Transport from Aqueous Solutions Using a Hollow-fiber Permeation Liquid Membrane (HFPLM) Device.

Nickel (II) preconcentration and speciation analysis using a hollow fiber supported liquid membrane (HFSLM) device was studied. A counterflow of protons coupled to complexation with formate provided the driving force of the process, while Kelex 100 was employed as carrier. The influence of variables related to module configuration (acceptor pH and carrier concentration) and to the sample properties (donor pH) on the preconcentration factor, E, was simultaneously studied and optimized using a 3 factor Doehlert matrix response surface methodology. The effect of metal concentration was studied as well. Preconcentration factors as high as 4240 were observed depending on the values of the different variables. The effects of the presence of inorganic anions (NO2−, SO42−, Cl−, NO3−, CO32−, CN−) and dissolved organic matter (DOM) in the form of humic acids were additionally considered in order to carry out a speciation analysis study. Nickel preconcentration was observed to be independent of both effects, except when cyanide was present in the donor phase. A characterization of the transport regime was performed through the analysis of the dependence of E on the temperature. E increases with the increase in temperature according to the equation E(K) = −8617.3 + 30.5T with an activation energy of 56.7 kJ mol−1 suggesting a kinetic-controlled regime. Sample depletion ranged from 12 to 1.2% depending on the volume of the donor phase (100 to 1000 mL, respectively).

Miniaturised flow-through PLM coupled with spectrophotometric detection for copper(II) speciation in fungicides†

Construction of a novel miniaturised Permeation Liquid Membrane (mPLM) set-up is described. A hydrophobic Celgard 2500 membrane with copper(II) carrier separates the source and receiving solutions flowing in the channels on two sides of the membrane. The set-up consists of two half-cells with channel width of 2 mm and depth variable between 100 and 500 µm made of poly(dimethylsiloxane) (PDMS) using a moulding technology. In contrast to most previous constructions, the new mPLM operates in full flow-through mode, in both source and receiving solutions. This allows for on-line and real-time coupling with the detection system, which significantly reduces the time of analysis. In the present set-up a UV/Vis spectrophotometric detection was applied for Cu(II) transport. For this purpose a composition of the receiving solution was optimised to allow for both free metal ion buffering, which is necessary for maintaining the driving force of the transport, and spectrophtometric detection. The applied UV/Vis detection method is linear in the range 2 × 10−6 M – 4 × 10−5 M (R 2 = 0.996) with a detection limit of 10−6 M in the volume of ca 350 µL (including the receiving channels, cuvette and tubing). The relative standard deviation does not exceed 5%. The transport capabilities of the mPLM set-up are demonstrated using four commercial copper-based fungicides: Nordox 75 WG, Miedzian 50 WP, Curzate 49.5 WP and Mag 50 WP.

Stabilization of Alkylated Azacrown Ether by Fatty Acid at the Air−Water Interface

The adsorbed amount of partially deuterated dihexadecyl-diaza-18-crown-6 ether (d-ACE16) in the presence of different chain length fatty acids as a function of surface pressure was determined by neutron reflectometry technique. The highest adsorbed amount of the azacrown ether was observed for the mixture of ACE16 with hexadecanoic (palmitic) acid, pointing to the importance of chain length matching between the two species for optimum stabilization of the mixed monolayer. The contrast variation technique was used to estimate the contribution to the total adsorbed amount from stearic acid and ACE16. It was found that the mixed Langmuir monolayer is stable against dissolution up to a surface pressure of 20 mN m(-1). Above this pressure, however, the spread and adsorbed amounts start to deviate, indicative of partial dissolution into the aqueous subphase. The consequences of this behavior for the transport of metal ions through the interfaces of permeation liquid membranes (PLMs) are discussed.

Transport of lipophilic metal complexes through permeation liquid membrane, in relation to natural water analysis: Cu(II)–8-hydroxyquinoline complex as a model compound

Lipophilic metal complexes are potentially toxic or bioavailable. These complexes may be present in natural waters but, due to lack of sensitive methods for their detection, their contribution to biouptake is poorly understood. Hence, an analytical technique, based on permeation liquid membrane (or supported liquid membrane) (PLM or SLM) has been investigated for the determination of lipophilic metal complexes. For this purpose, key parameters, such as the nature of solvent, diffusion coefficient in the membrane, D m, partition coefficient between the solution and the membrane, K p, and permeability coefficient, P, governing the transport of Cu(II)–8-hydroxyquinolinate ( Cu Q 2 0 ) through a flat sheet PLM comprising toluene/phenylhexane have been evaluated. Cu Q 2 0 was chosen as model lipophilic complex. Copper–8-hydroxyquinolinate is used as a pesticide and may be present in natural waters. The effects of pH, in the range 6–8, and the membrane thickness on the Cu Q 2 0 transport were also studied. The results showed that the neutral species is transported efficiently across the membrane containing toluene/phenylhexane mixture. Values of D m = 3.0 × 10 −7 cm 2 s −1 and P = 1.45 × 10 −3 cm s −1 were found; P is independent of pH in the studied pH range. The partition coefficient, K p, of Cu Q 2 0 in phenylhexane/toluene mixture was also determined using the classical liquid/liquid extraction method for comparison. The application of PLM method was tested for the trace level determination of lipophilic complexes using hollow fibre permeation liquid membrane to preconcentrate the analyte. High preconcentration factor for Cu Q 2 0 was achieved using the designed HFPLM device. Finally, the applicability of this method to Cu Q 2 0 was tested in a real water sample by spiking it with 8-hydroxyquinoline (10 μmol L −1). The results showed a quantitative recovery. The theoretically estimated LOD is few pmol L −1.

X-ray reflectivity study of the adsorption of azacrown ether at liquid–liquid interface

Adsorption of diaza-18-crown-6 ether substituted with two tetracosane (–C 24H 49) alkyl chains (ACE-24) was investigated at the liquid–liquid interface. X-ray reflectivity measurements determined the structure of a close-packed monolayer at the hexane–water interface, which is consistent with conclusions drawn indirectly from earlier interfacial tension measurements on similar molecules. These data provide further insights into the role of interfacial processes involving azacrown ethers in ion separation techniques such as the permeation liquid membrane.

Influence of inorganic complexes on the transport of trace metals through permeation liquid membrane

Under specific conditions (pH, concentrations), trace metals may form, with environmental inorganic ligands, neutral complexes which, in principle, might diffuse passively through biological membranes or influence the response of (bio)analytical sensors for trace metals based on permeation liquid membrane (PLM). In this study, metal (Cu, Cd, Pb) transport through the planar PLM device was evaluated in the presence of major environmental inorganic ligands such as sulfate, carbonate and chloride under conditions where neutral complexes may be formed (up to 73% of neutral metal complex in the solution). In the presence of sulfate, comparison of predicted and experimental PLM fluxes of Cu, Pb and Cd, suggests that passive transport of neutral sulfate–metal complexes does not occur. This was confirmed by comparing fluxes in the presence and absence of carrier. In the presence of carbonate (for Cd, Cu and Pb) and chloride (for Pb and Cd), however, experimental PLM fluxes were greater than predicted (up to 4 and 25 times in the presence of carbonate and chloride, respectively), but experiments in the absence of carrier in the membrane revealed that no passive transport of neutral complexes (MCl 2 or MCO 3) occurs through PLM. A possible mechanism is discussed. In parallel to the experiments with PLM, the influence of carbonate on the internalization fluxes of Cu(II) and Pb(II) by the freshwater algae, Chlamydomonas reinhardtii, was assessed. Similarly to the results of PLM, the fluxes of these two metals were larger than expected (based on the free metal ion activity model). Thus, even though PLM and bioaccumulation mechanisms are certainly different, similar unexpected behaviours occur for the metal transport through the PLM and biological membrane of C. reinhardtii, in the presence of carbonate.

Trace Metal Speciation and Bioavailability in Surface Waters of the Black Sea Coastal Area Evaluated by HF-PLM and DGT

Trace metal speciation in seawater from the Bulgarian Black Sea coast was studied in situ by hollow fiber permeation liquid membrane (HF-PLM) and by diffusion gradients in thin-film gels (DGT). The concentrations of Cd, Cu, Ni, and Pb determined by HF-PLM were lower than those measured by DGT, in agreement with their analytical windows, e.g., free metal ions provided by the HF-PLM and dynamic (mobile and labile) species by the DGT. The obtained suite of data was further used to evaluate the bioavailability of these metals to the microorganisms, which was then compared with experimental results of metal uptake to green microalga Chlorella salina. Uptake fluxes of the Cd, Cu, Ni, and Pb to C. salina, were predicted from the measured HF-PLM concentrations and laboratory experimentation in artificial seawater, in agreement with theoretical considerations. The HF-PLM and DGT appear to be promising analytical techniques for speciation and bioavailability studies in complex environmental media and allow improved understanding of the role of different chemical species in metal bioavailability (and impact) in seawaters.

EFFECT OF COMPETING IONS AND COMPLEXING ORGANIC SUBSTANCES ON THE CADMIUM UPTAKE BY THE SOIL BACTERIUM SINORHIZOBIUM MELILOTI

In an effort to improve the understanding and prediction of Cd uptake by soil bacteria, adsorbed and intracellular Cd were determined in unpolluted and highly polluted model soil solutions within a concentration range spanning from 10(-9) to 5 x 10(-5) M Cd. In parallel, the free Cd ion concentrations ([Cd(2+)]) were measured by a hollow fiber permeation liquid membrane. Obtained results demonstrated that Cd uptake by bacterium Sinorhizobium meliloti was related to [Cd(2+)] in the solution. Addition of different complexing organic substances reduced [Cd(2+)] in the bacterial medium and decreased both adsorbed and intracellular Cd. The adsorbed Cd was considerably reduced in the presence of 10(-4) to 5 x 10(-2) M [Ca] or [Mg]. No effect on Cd adsorption was observed in the presence of Zn or Mn, even at 100-fold excess. Intracellular Cd decreased in the presence of a high excess of Ca and Zn, while no significant effect was observed in the presence of Mg. An increase of dissolved Mn from 10(-9) to 10(-8) M resulted in a twofold decrease of the intracellular Cd, but no clear trend was observed in the presence of 10(-7) to 10(-6) M Mn. Based on complexation and competition studies, the stability constants, necessary for quantitative description of Cd uptake by S. meliloti and the development of the bacterial biotic ligand model for Cd, were derived and validated in the solutions containing mixtures of Cd, Zn, Ca, and Mg. Further numerical simulations of Cd uptake by S. meliloti exposed to soil pore waters demonstrated the importance of Zn competition and the insignificant influence of Ca and H on Cd uptake.

Mechanism of Cu(II) transport through permeation liquid membranes using azacrown ether and fatty acid as carrier

This paper describes the results of detailed studies on the roles of two components used as carrier for the transport of Cu(II) through the permeation liquid membrane (PLM): lauric acid (LAH) and N, N-didecyl-diaza-18-crown-6 ether (22DD). An overall mechanism of Cu(II) transport is proposed, based on a number of results reported in this and previous papers. The structure of the active forms of Cu(II) species participating in each step of the transport is proposed on the basis of complementary spectroscopic (UV/vis, NMR, FTIR) studies. This, together with the PLM transport experiments at high (5 × 10 −5 M) and low (5 × 10 −7 M) copper(II) concentration allowed us to identify the conditions under which the precipitation of copper(II) laurate on the membrane surface can be avoided in order to optimize the PLM procedure of speciation.

Speciation of Cu(ii) with a flow-through permeation liquid membrane: discrimination between free copper, labile and inert Cu(ii) complexes, under natural water conditions

Metal toxicity is not related to the total metal ion concentration, but to those of some specific Cu(II) species. The Permeation Liquid Membrane technique is based on the carrier-mediated transport of the test metal across a hydrophobic membrane and enables discrimination between various trace metal species in solution. The present work shows how the labile and inert Cu(II) complexes can be determined selectively, by varying the flow-rate of the test solution, in a flow-through cell. A mathematical model of metal flux through the PLM, based on diffusion-limited transport under steady-state conditions, is described. The model and the performance of the technique were studied in well-defined synthetic solutions containing simple organic hydrophilic ligands forming either inert (nitrilotriacetic acid), or labile complexes with Cu(II) (tartaric acid, malonic acid). The results were compared with theoretical predictions of thermodynamic species distribution in solution. Uncertainties on stability constants for copper speciation calculation were taken into account. The detection limits of the device are discussed. This work demonstrates that the flow-through cell is a reliable tool for copper speciation measurements in natural waters.

Transport of lipophilic ligands through permeation liquid membrane in relation to natural water analysis

Trace metal ions can exist in many chemical forms, such as free metal, labile and liposoluble metal complexes in natural waters. Although, free metal ions are well known to be bioavailable, liposoluble species may also be potentially bioavailable and toxic. For understanding the metal uptake and toxicity to microorganisms, analytical tools are required to measure the various metal species under natural water conditions. Permeation liquid membrane (PLM) devices, comprising of 1, 10 dicecyl diaza 18 crown ether 6 and lauric acid in phenylhexane–toluene mixture, were demonstrated to be well suited for the trace metal speciation, in particular, for the free Cu(II), Pb(II) and Cd(II) concentration measurements in natural waters. In the presence of both free metal ions and lipophilic complexes, both these species will be transported across the membrane. In order to evaluate the contribution of the lipophilic species in the measured flux, the concentration of the lipophilic species must be determined. In this paper, transport characteristics of 8-hydroxyquinoline (HQ, also known as oxine) through a flat sheet PLM comprising a mixture of toluene/phenylhexane has been studied to obtain information on the permeability. HQ was chosen as model organic compound because its metal complexations are well known and it forms neutral lipophilic complex with Cu(II). Copper-oxine is used as a pesticide and may be present in natural waters. Important parameters such as diffusion coefficient in the membrane, D m, partition coefficient, K p, between the solution and the membrane, and permeability coefficient, P, have been evaluated by studying the effect of pH and membrane thickness on HQ transport. The results showed that the neutral HQ species is transported efficiently across the membrane and that the charged protonated species are not transported. Values of D m = 3.0 × 10 −7 cm 2 s −1 and P = 2.47 × 10 −3 cm s −1 were found. The partition coefficient, K p, of HQ in phenylhexane/toluene mixture and toluene were also determined using the classical liquid/liquid extraction method for comparison. The results revealed that partition coefficient in toluene was higher than that in phenylhexane/toluene mixtures. However, the two values obtained for K p were comparable as expected.

New liquid membrane technology for simultaneous extraction and stripping of copper(II) from wastewater

In this paper, a new liquid membrane technique, hollow fiber renewal liquid membrane (HFRLM), is presented, which is based on the surface renewal theory, and integrates the advantages of fiber membrane extraction, liquid film permeation and other liquid membrane processes. The results from the system of CuSO 4 + D 2 EHPA in kerosene + HCl show that the HFRLM process is very stable. The liquid membrane is renewed constantly during the process, the direct contact of organic droplets and aqueous phase provides large mass transfer area. These effects can significantly reduce the mass transfer resistance in the lumen side. Then the mixture of feed phase and organic phase flowing through the lumen side gives a higher mass transfer rate than that of stripping phase and organic phase, because the aqueous layer diffusion of feed phase is the rate-controlling step. The overall mass transfer coefficient increases with increasing flow rates and D2EHPA concentration in the organic phase, and with decreasing initial copper concentration in the feed phase. The overall mass transfer coefficient also increases with increasing pH in the feed phase, and reaches a maximum value at pH of 4.44, then decreases. Also, there is a favorable w / o volume ratio of 20:1 to 30:1 for this process. Compared with hollow fiber supported liquid membrane and hollow fiber membrane extraction processes, HFRLM process has a high mass transfer rate. Mathematical model for the HFRLM process based on the surface renewal theory is developed. The calculated results are in good agreement with experimental results under the conditions studied.

Roles of Dynamic Metal Speciation and Membrane Permeability in Metal Flux through Lipophilic Membranes: General Theory and Experimental Validation with Nonlabile Complexes

The study of the role of dynamic metal speciation in lipophilic membrane permeability in aqueous solution requires accurate interpretation of experimental data. To meet this goal, a general theory is derived for describing 1:1 metal complex flux, under steady-state and ligand excess conditions, through a permeation liquid membrane (PLM). The theory is applicable to fluxes through any lipophilic membrane. From this theory, fluxes in the three rate-limiting conditions for metal transport are readily derived, corresponding, namely, to (i) diffusion in the source solution, (ii) diffusion in the membrane, and (iii) the chemical kinetics of formation/dissociation of the metal complex in the interfacial reaction layer. The theory enables discussion of the reaction layer concept in a more general frame and also provides unambiguous criteria for the definition of an inert metal complex. The theoretical flux equations for fully labile complexes were validated in a previous paper. The general theory for semi- or nonlabile complexes is validated here by studying the flux of Pb(II) through PLMs in contact with solutions of Pb(II)-NTA and Pb(II)-TMDTA at different pHs and flow rates.

Application of permeation liquid membrane and scanned stripping chronopotentiometry to metal speciation analysis of colloidal complexes

The potential of permeation liquid membrane (PLM) to obtain dynamic metal speciation information for colloidal complexes is evaluated by measurements of lead(II) and copper(II) complexation by carboxyl modified latex nanospheres of different radii (15, 35, 40 and 65 nm). The results are compared with those obtained by a well characterized technique: stripping chronopotentiometry at scanned deposition potential (SSCP). Under the PLM conditions employed, and for large particles or macromolecular ligands, membrane diffusion is the rate-limiting step. That is, the flux is proportional to the free metal ion concentration with only a small contribution from labile complexes. In the absence of ligand aggregation in the PLM channels, good agreement was obtained between the stability constants determined by PLM and SSCP for both metals.

The permeation liquid membrane as a sensor for free nickel in aqueous samples

There are currently a limited number of techniques to study nickel speciation in aqueous samples. This work reports on the use of the permeation liquid membrane (PLM) for that objective. In this paper, the composition of the organic phase was studied to maximize the Ni flux (thus the sensitivity of the device) over a wide Ni2+ concentration range (50 nM to 100 microM) in order to verify its ability to determine free Ni2+ in the presence of Ni complexes. A mixture containing 1,10-didecyl-1,10-diaza-18-crown-6 ether (22DD) and di(2-ethylhexyl)phosphoric acid (D2EHPA) in toluene/phenylhexane was selected as the optimized organic phase for the PLM. The PLM was shown to be a reliable tool to measure free nickel concentrations down to 10(-7) M. The effect of pH on Ni transport was also studied. Fluxes below pH 6 were reduced significantly, i.e. an order of magnitude smaller than fluxes above pH 7.8. Finally, as part of a broader study examining the ability of trace metals to induce antibiotic resistance in bacteria, we used the PLM to quantify the formation, at pH = 7.2, of a weak complex between Ni and Imipenem (a member of the carbapenem class of beta-lactam antibiotics) or its hydrolysis product(s).

Integrated Microanalytical System for Simultaneous Voltammetric Measurements of Free Metal Ion Concentrations in Natural Waters

AbstractA complexing gel integrated microelectrode (CGIME) for direct measurements of free metal ion concentrations in natural waters has been developed. It is prepared by the successive deposition of microlayers of a chelating resin, an antifouling agarose gel and Hg on a 100‐interconnected Ir‐based microelectrode array. The trace metals of interest are in a first step accumulated on the chelating resin in proportion to their free ion concentration in solution, then released in acidic solution and detected simultaneously by using square wave anodic stripping voltammetry (SWASV). The reliability of this sensor for the simultaneous measurement of copper, lead and cadmium has been studied by a series of replicate laboratory tests. The proportionality between the voltammetric peak current intensity and the free metal ion concentrations in solution has been demonstrated by using malonate as a model ligand. Finally, the CGIME sensor was applied to the Cu and Pb free concentration measurement in sea water samples and the results compared to the free metal ion concentrations measured using hollow fiber based permeation liquid membrane (HF‐PLM) coupled to inductively coupled plasma mass spectrophotometer (ICP‐MS). Comparable concentration values were found for both metals with both techniques allowing to validate the CGIME measurements in complex media.

Roles of Metal Ion Complexation and Membrane Permeability in the Metal Flux through Lipophilic Membranes. Labile Complexes at Permeation Liquid Membranes

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 carrier complex in the membrane, is considered. The respective roles of diffusion layer thickness, ligand concentration, complex stability, carrier 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 carrier 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.