Stripping chronopotentiometry at scanned deposition potential (SSCP)

Electroanalytical Trace Metal Cations Quantification and Speciation in Freshwaters: Historical Overview, Critical Review of the Last Five Years and Road Map for Developing Dynamic Speciation Field Measurements

An historical overview covering the field of electroanalytical metal cations speciation in freshwaters is presented here, detailing both the notable experimental and theoretical developments. Then, a critical review of the progress in the last five years is given, underlining in particular the improvements in electrochemical setups and methodologies dedicated to field surveys. Given these recent achievements, a road map to carry out on-site dynamic metal speciation measurements is then proposed, and the key future developments are discussed. This review shows that electroanalytical stripping techniques provide a unique framework for quantitatively assessing metals at trace levels while offering access to both thermodynamic and dynamic features of metal complexation with natural colloidal and particulate ligands.

Electroanalytical metal sensor with built-in oxygen filter

A rapid and reliable oxygen elimination system was evaluated here for the electroanalytical study of metals. Dissolved oxygen was removed locally in the vicinity of a sensor by the means of electrochemical oxygen filter constructed from platinum grids. Three metals (Cd, Pb, and Zn) were determined by stripping chronopotentiometry (SCP) at a mercury film screen-printed electrode. Limits of detection of metals were in the nanomolar range under optimized experimental conditions. The electrochemical device was also tested for metal quantification in simple electrolyte solutions and in a natural water matrix. The proposed combination of oxygen elimination system with the metal sensor completely removes the need to purge the sample before SCP measurement. This makes the determination of metals by SCP faster, portable and more suited for on-field applications.

Investigating the Binding Heterogeneity of Trace Metal Cations With SiO2 Nanoparticles Using Full Wave Analysis of Stripping Chronopotentiometry at Scanned Deposition Potential

Silica oxides nano- and microparticles, as well as silica-based materials, are very abundant in nature and industrial processes. Trace metal cation binding with these bulk materials is generally not considered significant in speciation studies in environmental systems. Nonetheless, this might change for nanoparticulate systems as observed in a previous study of Pb(II) with a very small SiO₂ particle (7.5 nm diameter). Besides, metal binding by those nanoparticles is surprisingly characterized by a heterogeneity that increases with the decrease of metal-to-particle ratio. Therefore, it is interesting to extend this study to investigate different trace metals and the influence of the nanoparticle size on the cation binding heterogeneity. Consequently, the Cd(II), Pb(II), and Zn(II) binding by two different sized SiO₂ nanoparticles (Ludox LS30 and TM40) in aqueous dispersion was studied for a range of pH and ionic strength conditions, using the combination of the electroanalytical techniques Scanned Stripping ChronoPotentiometry and Absence of Gradients and Nernstian Equilibrium Stripping. The coupling of these techniques provides the free metal concentration in the bulk (AGNES) and information of the free and complex concentration at the electrode surface for each Stripping Chronopotentiometry at Scanned deposition Potential (SSCP). A recent mathematical treatment allows the reconstruction of a portion of the metal to ligand binding isotherm with the included heterogeneity information using the full SSCP wave analysis. In this work, we observed that the Zn(II) binding is homogeneous, Cd(II) is slightly heterogeneous, and Pb(II) is moderately heterogeneous, whereas the results obtained with the 7.5 nm diameter nanoparticle are slightly more heterogeneous than those obtained with the one of 17 nm. These findings suggest that the Zn(II) binding is electrostatic in nature, and for both Cd(II) and Pb(II), there should be a significant chemical binding contribution.

Structural effects of soft nanoparticulate ligands on trace metal complexation thermodynamics

Metal binding to natural soft colloids is difficult to address due to the inherent heterogeneity of their reactive polyelectrolytic volume and the modifications of their shell structure following changes in e.g. solution pH, salinity or temperature. In this work, we investigate the impacts of temperature- and salinity-mediated modifications of the shell structure of polymeric ligand nanoparticles on the thermodynamics of divalent metal ions Cd(ii)-complexation. The adopted particles consist of a glassy core decorated by a fine-tunable poly(N-isopropylacrylamide) anionic corona. According to synthesis, the charges originating from the metal binding carboxylic moieties supported by the corona chains are located preferentially either in the vicinity of the core or at the outer shell periphery (p(MA-N) and p(N-AA) particles, respectively). Stability constants (K_ML) of cadmium-nanoparticle complexes are measured under different temperature and salinity conditions using electroanalytical techniques. The obtained K_ML is clearly impacted by the location of the carboxylic functional groups within the shell as p(MA-N) leads to stronger nanoparticulate Cd complexes than p(N-AA). The dependence of K_ML on solution salinity for p(N-AA) is shown to be consistent with a binding of Cd to peripheral carboxylic groups driven by Coulombic interactions (Eigen-Fuoss mechanism for ions-pairing) or with particle electrostatic features operating at the edge of the shell Donnan volume. For p(MA-N) particulate ligands, a scenario where metal binding occurs within the intraparticulate Donnan phase correctly reproduces the experimental findings. Careful analysis of electroanalytical data further evidences that complexation of metal ions by core-shell particles significantly differ according to the location and distribution of the metal-binding sites throughout the reactive shell. This complexation heterogeneity is basically enhanced with increasing temperature i.e. upon significant increase of particle shell shrinking, which suggests that the contraction of the reactive phase volume of the particulate ligands promotes cooperative metal binding effects.

Protonation effects on dynamic flux properties of aqueous metal complexes

The degree of (de)protonation of aqueous metal species has significant consequences for the kinetics of complex formation/dissociation. All protonated forms of both the ligand and the hydrated central metal ion contribute to the rate of complex formation to an extent weighted by the pertaining outer-sphere stabilities. Likewise, the lifetime of the uncomplexed metal is determined by all the various protonated ligand species. Therefore, the interfacial reaction layer thickness, μ, and the ensuing kinetic flux, Jkin, are more involved than in the conventional case. All inner-sphere complexes contribute to the overall rate of dissociation, as weighted by their respective rate constants for dissociation, kd. The presence of inner-sphere deprotonated H2O, or of outer-sphere protonated ligand, generally has a great impact on kd of the inner-sphere complex. Consequently, the overall flux can be dominated by a species that is a minor component of the bulk speciation. The concepts are shown to provide a good description of experimental stripping chronopotentiometric data for several protonated metal–ligand systems.

Dynamic speciation analysis of atrazine in aqueous latex nanoparticle dispersions using solid phase microextraction (SPME)

Solid phase microextraction (SPME) is applied in the dynamic speciation analysis of the pesticide atrazine in an aqueous medium containing sorbing latex nanoparticles. It is found that the overall rate of extraction of the analyte is faster than in the absence of nanoparticles and governed by the coupled diffusion of free and particle-bound atrazine toward the solid/sample solution interface. In the eventual equilibrium the total atrazine concentration in the solid phase is dictated by the solid phase/water partition coefficient (K(sw)) and the concentration of the free atrazine in the sample solution. These observations demonstrate that the nanoparticles do not enter the solid phase. The experimental data show that the rate of release of sorbed atrazine from the latex particles is fast on the effective time scale of the microextraction process. A lability criterion is derived to quantitatively describe the relative rates of these two processes. All together, the results indicate that SPME has a strong potential for dynamic speciation analysis of organic compounds in media containing sorbing nanoparticles.

Metal binding by heterogeneous ligands: kinetic master curves from SSCP waves

Stripping chronopotentiometry at scanned deposition potential, SSCP, is shown to be a powerful tool for determination of the distribution of metal dissociation rate constants, kd, for heterogeneous ligands. SSCP effectively scans a range of metal-to-ligand ratios from the foot to the plateau of the wave, thus allowing a portion of the distribution of k(d) values to be determined for a given bulk solution composition. In particular, the nature of the measurement renders accessible very low metal-to-ligand ratios which are otherwise difficult to attain by a bulk solution methodology, such as a potentiometric titration. In the presence of kinetic currents, the shape of the SSCP wave is modified as compared to the labile case. A data interpretation framework is developed which well-describes the SSCP wave for heterogeneous complexes in the kinetic current regime. The analysis utilizes the Freundlich binding isotherm together with the Koutecký-Koryta approximation, i.e., assuming a spatially discontinuous transition from labile to nonlabile behavior. For the case of Pb(II) and Cu(II) complexation by a peat fulvic acid, the existing body of data is drastically expanded to the low to very low metal-to-ligand ratio domain, thereby greatly improving the quality of the derived distribution parameters.

Study of interactions of concentrated marine dissolved organic matter with copper and zinc by pseudopolarography

The interaction of dissolved organic matter (DOM) with copper and zinc in a concentrated seawater sample was characterised by pseudopolarography. Measurements performed at increased concentrations of copper(II) ions showed successive saturation of active DOM sites which indicate possible partition of copper between (i) free or labile complexes, (ii) reduced and released within the potential window of the method, and (iii) electroinactive copper complexes. Pseudopolarograms measured at pH 4 indicate a release of copper which was bound to the active sites of DOM that formed non-labile complexes. Variation of the peak position and half-peak width along the scanned deposition potentials and with the increasing concentration of copper bear the information about the complex electrochemical processes at the electrode surface and in the bulk of the solution. Pseudopolarograms of zinc showed a strong dependence of the peak current and the peak position along the scanned deposition potentials on pH values, indicating preferentially complexation of zinc with carboxylic-like active sites of DOM in the measured sample. Pseudopolarography is a valuable method in the trace metal complexation and speciation studies, serving as a fingerprint of the analysed sample.

Stability of lead(II) complexes of alginate oligomers

The current work reports on the Pb(ll) complexes formed with oligomeric uronic acids (carboxylated saccharide residues) found polymerized in the cell walls and envelopes of algae and bacteria alike. The application of partial acid hydrolysis, size-exclusion chromatography (SEC), 1H NMR, and scanned deposition stripping chronopotentiometry (SSCP) has permitted the determination of stability constants for Pb(II) with both mannuronic (M) and guluronic (G) acid oligomers ranging from the dimer to the pentamer. The determined logarithm of the stability constants range between 4.11 +/- 0.05 and 5.00 +/- 0.04 mol(-1) x dm3 for the eight oligomers studied (pH 6; I = 0.1 mol x dm(-3)). Additional experiments under the same experimental conditions employing galacturonic and glucuronic acid oligomers yielded slightly lower values (2.19 +/- 0.10 to 4.02 +/- 0.07 mol(-1) x dm3) that were expected based on their structure, whereby the monomers which were not included in the alginate oligomer series (unavailable by SEC), yielded the lowest stability constants. This work demonstrates the applicability of the SSCP technique for the determination of stability constants for metal-ligand complexes in which the ligands display relatively low molecular mass. Previous studies on heavy metal interaction with the matrix polysaccharide alginate have largely been restricted to the whole polymer that forms a gel upon binding to network bridging ions such as calcium. The results will be discussed in this context with the emphasis being placed on the relevance of these findings to processes occurring at the biointerface and results from the relevant literature.

Dynamic Speciation Analysis of Heterogeneous Metal Complexes with Natural Ligands by Stripping Chronopotentiometry at Scanned Deposition Potential (SSCP)

Stripping chronopotentiometry at scanned deposition potential (SSCP) allows chemical heterogeneity in metal speciation to be unambiguously identified. In the labile regime, use of the Freundlich binding isotherm allows straightforward determination of parameters to describe the apparent stability and heterogeneity of metal complexes with humic substances. The extent of heterogeneity of metal binding by several humic substances follows the order Cu(ii) >> Pb(ii) > Cd(ii). The lability of metal complexes decreases from the foot to the top of the wave, and the greater the degree of heterogeneity, the more readily lability is lost. In the kinetic current regime, the Koutecký–Koryta approximation allows an expression to be obtained for the SSCP wave that provides a good estimate of the experimental data for metal complexes with moderate degrees of heterogeneity.

Electrochemical metal speciation analysis of chemically heterogeneous samples: the outstanding features of stripping chronopotentiometry at scanned deposition potential

The application of depletive stripping chronopotentiometry at scanned deposition potential (SSCP) to metal ion speciation analysis of chemically heterogeneous complex systems is described. In this electroanalytical stripping technique, metal which is accumulated in the electrode during the deposition step is quantitatively reoxidized during the detection step. SSCP is able to provide an unambiguous measure of heterogeneity that is not attainable by other nondepletive electroanalytical stripping techniques that measure only a proportion of the accumulated metal. SSCP data are not affected by the secondary interferences that plague conventional electrochemical techniques, that is, induced metal adsorption and insufficient ligand excess during stripping. Furthermore, the effect of heterogeneity, as manifested in flattening of the SSCP wave, can be distinguished from that of electrochemical irreversibility. Data are presented for complexation of Cu(II), Pb(II), and Cd(II) by several humic fractions: distinct heterogeneity differences are observed.