There is a current debate on whether the toxicity of engineered ZnO nanoparticles (NPs) can be traced back to their nanoscale properties or rather to the simple fact of their relatively high solubility and consequent release of Zn2+ ions. In this work, the emerging electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping), which is specially designed to determine free metal ion concentration, is shown to be able to measure the Zn2+ concentration resulting from dissolution of ZnO nanoparticles dispersed in aqueous salt solutions. Three NP samples from different sources (having average primary particle diameters of 6, 20, and 71 nm) were tested and compared with bulk ZnO material. The enhanced solubility of the nanoparticles with decreasing primary radius allows for an estimation of the surface energy of 0.32 J/m2. AGNES also allows the study of the kinetics of Zn2+ release as a response to a change in the solution parameters (e.g., pH, ZnO concentration). A physicochemical model has been developed to account for the observed kinetic behavior. With this model, only one kinetic parameter is required to describe the time dependence of the free Zn2+ concentration in solution. Good agreement with this prediction is obtained when, starting from an equilibrated NP dispersion, the pH of the medium is lowered. Also, the independence of this parameter from pH, as expected from the model, is obtained at least in the pH range 7–9. When dissolution is studied by dispersing ZnO nanoparticles in the medium, the kinetic parameter initially decreases with time. This decrease can be interpreted as resulting from the increase of the radius of the clusters due to the agglomeration/aggregation phenomena (independently confirmed). For the larger assayed NPs (i.e., 20 and 71 nm), a sufficiently large pH increase leads to a metastable solubility state, suggesting formation of a hydroxide interfacial layer.

Measurements of trace metal species in situ in a softwater river, a hardwater lake, and a hardwater stream were compared to the equilibrium distribution of species calculated using two models, WHAM 6, incorporating humic ion binding model VI and visual MINTEQ incorporating NICA−Donnan. Diffusive gradients in thin films (DGT) and voltammetry at a gel integrated microelectrode (GIME) were used to estimate dynamic species that are both labile and mobile. The Donnan membrane technique (DMT) and hollow fiber permeation liquid membrane (HFPLM) were used to measure free ion activities. Predictions of dominant metal species using the two models agreed reasonably well, even when colloidal oxide components were considered. Concentrations derived using GIME were generally lower than those from DGT, consistent with calculations of the lability criteria that take into account the smaller time window available for the flux to GIME. Model predictions of free ion activities generally did not agree with measurements, highlighting the need for further work and difficulties in obtaining appropriate input data.

AGNES (absence of gradients and Nernstian equilibrium stripping) is a stripping technique consisting of two conceptual steps: (i) application of a potential program (e.g. a step at a fixed potential) generating a known concentration gain between the outer and inner concentrations of the metal at the mercury electrode surface together with null gradients of the concentration profiles (inside and outside the mercury electrode) and (ii) determination of the concentration of reduced metal inside the amalgam in a stripping step. In the present implementation, the stripping step under diffusion limited conditions leads to a measured current just proportional to the free metal ion concentration. In this paper we present the basic principles of the technique, analytical expressions for a simplified model of its voltammetric implementation and a numerical study for a more refined model together with preliminary experimental results in the Cd(II)/nitrilotriacetic acid system showing how this technique can be used as an alternative to other techniques (such as ion selective electrodes) in order to determine free metal activities or concentrations in the presence of complex mixtures avoiding complications such as electrodic adsorption or complexation kinetics.

Trace metals were measured in situ in a freshwater river draining a peat catchment (DOC = 15 mg L(-1)) using diffusive gradients in thin-films (DGT) devices with a range of gel layer thicknesses (0.16-2.0 mm). The reciprocal of the accumulated mass of each metal varied linearly with the thickness of the diffusive layer. These plots allowed calculation of the thickness of an apparent diffusive boundary layer (ADBL). A constant value was obtained from the plots of Cd, Pb, and Zn. The observed increase in the ADBL for the other metals (Mn

Several techniques for speciation analysis of Cu, Zn, Cd, Pb, and Ni are used in freshwater systems and compared with respect to their performance and to the metal species detected. The analytical techniques comprise the following:  (i) diffusion gradients in thin-film gels (DGT); (ii) gel integrated microelectrodes combined to voltammetric in situ profiling system (GIME−VIP); (iii) stripping chronopotentiometry (SCP); (iv) flow-through and hollow fiber permeation liquid membranes (FTPLM and HFPLM); (v) Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE−SV). All methods could be used both under hardwater and under softwater conditions, although in some cases problems with detection limits were encountered at the low total concentrations. The detected Cu, Cd, and Pb concentrations decreased in the order DGT ≥ GIME−VIP ≥ FTPLM ≥ HFPLM ≈ DMT (>CLE−SV for Cd), detected Zn decreased as DGT ≥ GIME−VIP and Ni as DGT > DMT, in agreement with the known dynamic features of these techniques. Techniques involving in situ measurements (GIME−VIP) or in situ exposure (DGT, DMT, and HFPLM) appear to be appropriate in avoiding artifacts which may occur during sampling and sample handling.

Under conditions of bulk depletion, the speciation and bioavailability of trace metals must be considered at two different time scales: (i) the time scale of the biouptake flux, as determined by diffusion of the bioactive free metal, dissociation of the bioinactive complex species, and the internalization rate; and (ii) the time scale of depletion of the bulk medium. The implications of these two time scales for the speciation dynamics are discussed in terms of experimental conditions. The geometry of the system is taken into accountvia a spherical cellular model. It considers a spherical organism depleting a spherical volume in a nonstirred medium and assumes linear adsorption of the metal atthe biointerface and first-order internalization kinetics. In cases where the rate of biouptake is fully controlled by the internalization step, concentration gradients in the medium are insignificant. Then the biouptake becomes independent of the geometry of the system, and the model has a much simpler solution. Examples of trace metal uptake by microorganisms are analyzed: (i) cobalt uptake by Prochlorococcus in the presence of NTA, under conditions where bulk depletion is the controlling process due to the large number of organisms and high internalization rates, (ii) silver uptake by Chlamydomonas reinhardtii with significant effects of bulk depletion, due to the high internalization rate; (iii) lead uptake by Chlorella vulgaris with pratically negligible bulk depletion due to the low internalization rate of the metal; and (iv) lead uptake by intestinal Caco-2 cells, illustrating the simplification of the bulk depletion model for a system with different geometry where internalization is the rate-controlling step.

The size of a microelectrode can have a dramatic impact on the relative importance of the diffusional and kinetic contributions to the voltammetric current of an electroactive metal ion in a complexing medium. Decreasing the radius enhances the diffusional contribution and, as a consequence, the complex system tends to move away from labile behaviour (where an equilibrium relationship holds). Therefore, sufficiently small microelectrodes (either or not combined with short measuring times) should be able to directly sense free metal concentration for not too fast association/dissociation kinetics.The particular case of steady state spherical (or hemispherical) diffusion under ligand excess (pseudo-first order kinetics) is solved analytically. The ensuing lability criterion is shown to be in accordance with a geometrical derivation based on an analysis of the Published in Journal of Electroanalytical Chemistry 2001, vol 505, p 85-94 Voltammetric lability.. 2/25random walk of the free metal ions produced by dissociation of the complex. It is shown that, for a generated metal ion, the probability of reaching the microelectrode surface can be quite different from the planar case. Alternatively, the classical reaction layer concept can be used in the derivation of the lability criterion for spherical geometry as it is shown in this work. All treatments quantitatively show how the lability of metal complexes is reduced with decreasing the dimension of the microelectrode. [85][86][87][88][89][90][91][92][93][94] Voltammetric lability.. inert total kin J J J + ,The thickness of the reaction layer µ is a kinetic parameter [34][35][36][37][38][39], defined such that:

The bipolar faradaic depolarization of the interface metal/solution is examined for the situation in which the transversal electron transfer is limited by mass transfer of the components of a reversible redox couple. Transversal diffusion of the electroactive species to and from the surface and lateral convective mass transport, resulting from a pressure gradient applied along the surface, are taken into account. The analysis first focuses on the case in which the lateral electric field required for bipolar behavior is externally applied through the solution. Numerical analysis of the intrinsic nonlinear coupling between the convective-diffusion equation and the Poisson equation for finite currents allows derivation of the spatial distribution of the potential and the concentration profiles of the electroactive species. The corresponding distribution of the local faradaic current density along the metallic surface and the ensuing overall bipolar current are obtained. Characteristics of the conductivity curves, bipolar current versus applied field, are given for different sets of electric and hydrodynamic parameters. Then, on the basis of these results, the analysis of bipolar faradaic depolarization process is extended to electrokinetic phenomena, in particular streaming potential.