Absence Of Gradients And Nernstian Equilibrium Stripping Research Articles

Release of indium from In2O3 nanoparticles in model solutions and synthetic seawater

Indium oxide (In2O3) nanoparticles (NPs) are used in electronic devices, from which indium (as its nanoparticulate form or as other generated chemical species) can be released to natural waters. To assess for the impacts of such releases (e.g. toxic effects), information on the kinetics and thermodynamics of the In2O3 dissolution processes is key. In this work, the evolution with time of the dissolution process was followed with the technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) by measuring the free indium concentration ([In3+]). AGNES can determine the free ion concentration in the presence of nanoparticles without a prior separation step, as shown in the case of ZnO nanoparticles, a procedure that is more accurate than the typical sequence of centrifugation+filtration+elemental analysis. Excess of indium oxide NPs were dispersed in 0.1 mol L−1 KNO3 at various pH values ranging from 2 to 8. Additional dispersions with bulk In2O3 at pH 3 or NPs in synthetic seawater at pH 8 were also prepared. The temperature was carefully fixed at 25 °C. The dispersions were continuously stirred and samples were taken from time to time to measure free indium concentration with AGNES. 180-day contact of In2O3 to solutions at pH 2 and 3 was not enough to reach equilibrium. The dissolution of the NPs at pH 3 was faster than that of the bulk (i.e. non nanoparticulate) material. Equilibrium of the NPs with the solution was reached at pH 4 and 5 in KNO3 and at pH 8 in seawater, in shorter times for higher pH values, with free indium concentrations decreasing by a factor of 1000 for each increase in one pH unit. The solubility products of In(OH)3 and In2O3 were compared. Equilibration of NPs with synthetic seawater took <18 days, with an average free [In3+] (up to 196 days) of 1.03 amol L−1.

Absence of Gradients and Nernstian Equilibrium Stripping (AGNES): An Electroanalytical Technique for Chemical Speciation: A Tutorial Review

Free metal ion concentrations of amalgamating elements such as Zn, Cd, In, or Pb can be determined with absence of gradients and Nernstian equilibrium stripping(AGNES) in a variety of matrices, ranging from seawater to wine or dissolving nanoparticles. In this hands-on paper, we review the fundamental concepts and provide the practical steps to implement AGNES, including ready-to-run files for the software controlling the potentiostat, computation spreadsheets, step-by-step laboratory protocols, etc. Two case studies with a free Zn concentration determination are discussed: (i) a synthetic solution with the ligand oxalate and (ii) a natural sample of the Segre river (Catalonia, Spain). Suggestions for the extension of AGNES to other systems are indicated.

Direct determination of free Zn concentration in samples of biological interest

The speciation of essential metal ions in biological fluids, such as blood plasma and serum, is of fundamental importance to understand the homeostasis of these elements. The activity of metal ions such as Zn2+ in extracellular media is thought to affect their interaction with membrane-bound transporters, and thus is critical for their cellular uptake. Previous approaches to determine “free” Zn2+ (i.e. the hexa-aquo ion) are based on separation by either chromatography or ultrafiltration, or on metallochromic dyes. However, both types of approach are prone to affect the relevant equilibria. These drawbacks can be circumvented with the electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping), since it can measure free zinc concentration without perturbing the sample speciation. Here, a Bovine Serum Albumin (BSA) + Zn synthetic mixture and Fetal Bovine Serum (FBS) are analyzed as proof of concept. Adsorption of BSA on the surface of the Hanging Mercury Drop Electrode (HMDE), despite the advantage of its renewal, is so intense that it blocks appropriate attainment of the required equilibrium, and only estimations of [Zn2+] can be derived. In contrast, a rotating disc electrode with a thin mercury film deposited on it (TMF-RDE) is advantageous because of its small volume and enhanced mass transfer. Protein adsorption can be prevented by covering the TMF-RDE with Nafion. A free Zn concentration [Zn2+] = 2.7 nmol L−1 was found at pH 7.0, total Zn 20 μμmol L−1 and BSA 600 μμmol L−1. A sample of FBS with fixed pH 7.2 (MOPS 0.08 mol L−1) yielded [Zn2+] = 0.25 nmol L−1. This methodology opens the way to free metal concentration determinations in biological fluids.

AGNES in irreversible systems: The indium case

• In 3+ /In 0 irreversibility is by-passed due to the lability of In hydroxy species. • AGNES conditions are met in the Nernstian region of the foot of an SSCP wave. • They can be recognized as a linear segment when plotting ln (τ/τ*) vs E d. Free indium concentrations can be determined using AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) with no relevant hindrance from the irreversibility of the In 3+ /In° redox couple at a mercury electrode. The electroactivity, high lability and mobility of the In hydroxy species help in reaching AGNES conditions for a relatively moderate ratio (deposition time):(gain), which decreases with increasing pH due to the growing contribution of the hydroxy species. In the related technique SSCP (Scanned Stripping ChronoPotentiometry), points corresponding to more positive deposition potentials are said to be “at the foot of the wave”. Some of these points can reach equilibrium along the fixed SSCP deposition time, so that these points are effectively AGNES experiments, and free metal ion concentrations (such as those of free indium) can also be computed from them provided sufficiently accurate data are available. A new representation of the SSCP wave allows for the diagnosis of the potential region where AGNES conditions are fulfilled.

Effective concentration signature of Zn in a natural water derived from various speciation techniques

The uptake of nutrients or toxicants by different organisms in aquatic systems is known to correlate with different fractions of the nutrient's or toxicant's total concentration. These fractions can be provided by different analytical techniques, from which the better correlation is expected to be found for those with a characteristic length comparable to that in the considered organism uptake. An effective concentration signature can be built up with the concentration values associated to the availability (i.e. fluxes in dynamic techniques) of the nutrient or toxicant measured by various analytical techniques with different characteristic lengths. Here, this new representation was obtained for the pool of Zn complexes in the Mediterranean stream Riera d'Osor (Girona, Catalonia, Spain) with a suite of four analytical techniques. Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) and Polymer Inclusion Membrane (PIM) devices provided the free Zn concentration. Linear Anodic Stripping Voltammetry provided a labile fraction (defined here as cLASV, higher than the free concentration), related to the diffusion layer scale. Diffusion Gradients in Thin-films provided higher labile fractions (known as DGT concentrations, cDGT) connected to the different characteristic lengths of different configurations (e.g. one or two resin discs) longer, in any case, than that corresponding to LASV. The combination of the information retrieved by the techniques allowed to quantify lability degrees of the pool of Zn complexes and to build up the effective concentration signature for this water.

Full wave analysis of stripping chronopotentiometry at scanned deposition potential (SSCP): Obtaining binding curves in labile heterogeneous macromolecular systems for any metal-to-ligand ratio

Abstract The different deposition potentials applied in Scanned Stripping ChronoPotentiometry (SSCP) probe different values of the free metal and complex concentrations at the electrode surface. The knowledge of these concentrations gives access to a relevant window of the binding curve of a metal to a homogeneous or heterogeneous ligand. Here, a suitable mathematical treatment for the determination of these surface concentrations, when using a mercury thin film rotating disk electrode, is reported. The proposed procedure does not require ligand excess conditions and takes advantage of the knowledge of the free metal ion concentration in the bulk solution provided by the technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). It is experimentally shown that the information derived from the SSCP points (i.e. at different deposition potentials in a unique solution) is consistent with the speciation results yielded by the technique AGNES in pertinent solutions prepared within a range of metal-to-ligand ratios. Successful examples with polystyrene sulfonate, Laurentian Fulvic acid and a peat humic acid are reported. It is concluded that, by running consecutively SSCP and AGNES in a single solution, speciation information corresponding to different compositions (i.e. those locally generated at the electrode surface by the end of the deposition step) can be retrieved, this enabling an easy determination of the binding curve.

Antimony speciation in aqueous solution followed with AGNES

Determining antimony speciation is essential to understand its environmental behaviour and potential (eco)toxicological effects. Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) is an electroanalytical technique that has been applied up to date to the determination of the free metal ion concentrations of Zn(II), Cd(II), Pb(II) and In(III). In this work, the first application of AGNES to the measurement of the concentration of one particular species of Sb(III) in aqueous solution is developed. The extensive hydrolysis of this element, even at low pH, suggests the convenience of performing calibrations and AGNES measurements in terms of the Sb(OH)3(aq) concentration, rather than the “free” ion. A new preconcentration factor, Y′, is also introduced as the ratio between [Sb0] in the amalgam and [Sb(OH)3(aq)] in the solution. Y′ dependence on pH is evaluated with calibrations at different pH values. The experimental values of Sb(OH)3(aq) obtained with AGNES in the titration of antimony with the organic ligand oxalate agree very well with the values predicted with the complexation constants recently reported. This promising methodology opens the way to tackle antimony speciation with AGNES in other systems.

New methodology to measure low free indium (III) concentrations based on the determination of the lability degree of indium complexes. Assessment of In(OH)3 solubility product

The vast industrial and consumer application of indium (solar cells, displays, etc.) risks its eventual leakage to the environment. Given the relevant role of speciation on the ecotoxicological effects of a certain total amount of indium, it is crucial to develop proper techniques to determine free concentrations of indium. The electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) has already proved useful in such a goal. However, the optimization of suitable deposition times in some conditions might be laborious. This work presents a new strategy, based on the technique ADLC (Accumulation under Diffusion Limited Conditions), to determine lability degrees, which apart from their intrinsic physicochemical interest — by comparing reaction kinetics with diffusion rates—, provide useful guidelines for AGNES deposition times. The suggested novel methodology is illustrated with: i) the computation of lability degrees of indium-oxalate complexes (unreported up to date) and ii) measuring free concentration of indium (for the first time as low as pmolL−1) in precipitated solutions (i.e. containing precipitated indium hydroxide in the electrochemical cell). The method takes advantage of the fact that the addition of a suitable concentration of oxalate does not change the free indium concentration (which is buffered due to the existing precipitate at a fix pH), but creates a large amount of labile and mobile indium complexes that contribute to the desired accumulation at very large gains (preconcentration factors). Results confirm the validity of the solubility product of indium hydroxide reported in the database NIST 46.7.

Dissolution and Phosphate-Induced Transformation of ZnO Nanoparticles in Synthetic Saliva Probed by AGNES without Previous Solid-Liquid Separation. Comparison with UF-ICP-MS.

The variation over time of free Zn2+ ion concentration in stirred dispersions of ZnO nanoparticles (ZnO NPs) prepared in synthetic saliva at pH 6.80 and 37 °C was followed in situ (without solid-liquid separation step) with the electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). Under these conditions, ZnO NPs are chemically unstable due to their reaction with phosphates. The initial stage of transformation (around 5-10 h) involves the formation of a metastable solid (presumably ZnHPO4), which later evolves into the more stable hopeite phase. The overall decay rate of ZnO NPs is significantly reduced in comparison with phosphate-free background solutions of the same ionic strength and pH. The effective equilibrium solubilities of ZnO (0.29-0.47 mg·L-1), as well as conditional excess-ligand stability constants and fractional distributions of soluble Zn species, were determined in the absence and presence of organic components. The results were compared with the conventional ultrafiltration and inductively coupled plasma-mass spectrometry (UF-ICP-MS) methodology. AGNES proves to be advantageous in terms of speed, reproducibility, and access to speciation information.

Towards improving the electroanalytical speciation analysis of indium

The geochemical fate of indium in natural waters is still poorly understood, while recent studies have pointed out a growing input of this trivalent element in the environment as a result of its utilisation in the manufacturing of high-technology products. Reliable and easy-handling analytical tools for indium speciation analysis are, then, required. In this work, we report the possibility of measuring the total and free indium concentrations in solution using two complementary electroanalytical techniques, SCP (Stripping chronopotentiometry) and AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) implemented with the TMF/RDE (Thin Mercury Film/Rotating Disk Electrode). Nanomolar limits of detection, i.e. 0.5 nM for SCP and 0.1 nM for AGNES, were obtained for both techniques in the experimental conditions used in this work and can be further improved enduring longer experiment times. We also verified that AGNES was able (i) to provide robust speciation data with the known In-oxalate systems and (ii) to elaborate indium binding isotherms in presence of humic acids extending over 4 decades of free indium concentrations.The development of electroanalytical techniques for indium speciation opens up new routes for using indium as a potential tracer for biogeochemical processes of trivalent elements in aquifers, e.g. metal binding to colloidal phases, adsorption onto (bio)surfaces, etc.

Comparison of different speciation techniques to measure Zn availability in hydroponic media

Four analytical techniques are compared: AGNES (Absence of Gradients and Nernstian Equilibrium Stripping), LASV (Anodic Stripping Voltammetry with Linear stripping), DGT (Diffusive Gradients in Thin films) and PIM (Polymer Inclusion Membranes). These techniques have been designed to provide the free ion concentration or a labile fraction, complementarily contributing to an integrated description of speciation and availability. Their simultaneous application to the determination of free Zn concentrations or labile fluxes in seven solutions of a hydroponic medium reveals characteristics of each technique and correlations between their results. All dynamic results can be interpreted in terms of a general theoretical framework on fluxes. Indeed, in techniques under diffusion-limited conditions in the sample, the flux can be split into the free contribution (linearly proportional to the free fraction), plus the contribution of the complexes (where mobility, lability and abundance of complexation are intertwined). A methodology to compute lability degrees is developed. Measurements with PIM devices confirm that diffusion in the sample solution is not rate limiting, so its flux is proportional to the free metal in the donor solution. A proportionality between the responses of any given two techniques is observed, which suggests that, for the low ligand-to-metal concentration ratios used in the present work, any of these techniques would correlate similarly with uptake, toxic or nutritional measurements.

Metal (Pb, Cd, and Zn) Binding to Diverse Organic Matter Samples and Implications for Speciation Modeling.

This study evaluated the influence of dissolved organic matter (DOM) properties on the speciation of Pb, Zn, and Cd. A total of six DOM samples were categorized into autochthonous and allochthonous sources based on their absorbance and fluorescence properties. The concentration of free metal ions ( CM2+) measured by titration using the absence of gradients and Nernstian equilibrium stripping (AGNES) method was compared with that predicted by the Windermere humic aqueous model (WHAM). At the same binding condition (pH, dissolved organic carbon, ionic strength, and total metal concentration) the allochthonous DOM showed a higher level of Pb binding than the autochthonous DOM (84- to 504-fold CPb2+ variation). This dependency, however, was less pronounced for Zn (12- to 74-fold CZn2+ variation) and least for Cd (2- to 14-fold CCd2+ variation). The WHAM performance was affected by source variation through the active DOM fraction ( F). The commonly used F = 1.3 provided reliable CPb2+ for allochthonous DOMs and acceptable CCd2+ for all DOM, but it significantly under-predicted CPb2+ and CZn2+ for autochthonous DOM. Adjusting F improved CM2+ predictions, but the optimum F values were metal-specific (e.g., 0.03-1.9 for Pb), as shown by linear correlations with specific optical indexes. The results indicate a potential to improve WHAM by incorporating rapid measurement of DOM optical properties for site-specific F.

Free indium concentration determined with AGNES

Indium is increasingly used in electronic devices, from which it can be mobilized towards environmental compartments. Speciation of In in waters is important for its direct ecotoxicological effects, as well as for the fate of this element in the environment (e.g. fluxes from or towards sediments). Free indium concentrations in the environment can be extremely low due to hydrolysis, especially important in trivalent cations, to precipitation and to complexation with different ligands. In this work, the free indium concentration (which is a toxicologically and geochemically relevant fraction) in aqueous solutions at pH3 has been measured with an adapted version of the electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). Speciation measurements in mixtures of indium with the ligands NTA (nitrilotriacetic acid) and oxalate indicate that the values of their stability constants in the NIST46.6 database are less adequate than those published in some more recent literature. The extraordinary lability and mobility of In-oxalate complexes allow the measuring of free indium concentrations below nmol/L in just 25s of deposition time.

Speciation of Zn, Fe, Ca and Mg in wine with the Donnan Membrane Technique

Free concentrations of Zn2+, Fe3+, Ca2+ and Mg2+ in a red wine (Raimat, Catalonia, Spain) have been determined, with the Donnan Membrane Technique (DMT) for the first time. The required equilibration time benefits from the acceptor solution including major cations. K+ and Na+, mainly unbound to any ligand in the sample, have been identified as suitable reference ions. A free Zn concentration of 1.76μmolL−1 determined with DMT was in excellent agreement with the free Zn concentration independently provided by the electroanalytical technique Absence of Gradients and Nernstian Equilibrium Stripping (AGNES), 1.7μmolL−1, amounting to 14.4% of the total Zn. The free concentrations found in this wine were 1.79μmolL−1 Fe3+, 1.11mmolL–1 Ca2+ and 3.4mmolL−1Mg2+ (8.82%, 40% and 57% of their total concentrations). Prior to the application of the techniques to the red wine, they had been cross-validated in Zn-tartrate solutions.

A review on electrochemical methods for trace metal speciation in environmental media

Summary Trace metal speciation is key to understand/predict bioavailability and potential toxicity of metals to biota and will, undoubtedly, be incorporated in future regulations. Electroanalytical methods have a role to play in such development: they offer a wide range of advantages such as speed, portability, economy, solid interpretation backgrounds and low limits of quantification. This review focusses on three selected stripping techniques: Competitive-Ligand Exchange-Cathodic Stripping Voltammetry (CLE-CSV), Scanned Stripping Chrono-Potentiometry (SSCP) and AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) reporting their working principles, characteristics (strong and weak points) and recent applications to systems of environmental relevance (such as seawaters, freshwaters or soil extracts).

AGNES at vibrated gold microwire electrode for the direct quantification of free copper concentrations

The free metal ion concentration and the dynamic features of the metal species are recognized as key to predict metal bioavailability and toxicity to aquatic organisms. Quantification of the former is, however, still challenging. In this paper, it is shown for the first time that the concentration of free copper (Cu2+) can be quantified by applying AGNES (Absence of Gradients and Nernstian equilibrium stripping) at a solid gold electrode. It was found that: i) the amount of deposited Cu follows a Nernstian relationship with the applied deposition potential, and ii) the stripping signal is linearly related with the free metal ion concentration. The performance of AGNES at the vibrating gold microwire electrode (VGME) was assessed for two labile systems: Cu-malonic acid and Cu-iminodiacetic acid at ionic strength 0.01 M and a range of pH values from 4.0 to 6.0. The free Cu concentrations and conditional stability constants obtained by AGNES were in good agreement with stripping scanned voltammetry and thermodynamic theoretical predictions obtained by Visual MinteQ. This work highlights the suitability of gold electrodes for the quantification of free metal ion concentrations by AGNES. It also strongly suggests that other solid electrodes may be well appropriate for such task. This new application of AGNES is a first step towards a range of applications for a number of metals in speciation, toxicological and environmental studies for the direct determination of the key parameter that is the free metal ion concentration.

Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) for the determination of [Zn2+] in estuarine waters

Zinc (Zn) has been classified as a “Specific Pollutant” under Annex VIII of the EU Water Framework Directive by two thirds of the EU member states. As a result, the UK Environmental Quality Standard (EQS) for Transitional and Coastal (TrAC) Waters has been reduced from 612 nM to 121 nM total dissolved Zn. It is widely accepted that the free metal ion ([Zn2+]) is the most bioavailable fraction, but there are few techniques available to determine its concentration in these waters. In this work, Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) has been applied, for the first time, to determine [Zn2+] in estuarine waters. The AGNES method had a mean RSD of ±18%, a (deposition time dependent) limit of detection of 0.73 nM and a [Zn2+] recovery of 112 ± 19% from a certified reference material (BCR-505; Estuarine Water). AGNES results for 13 estuarine samples (salinity 0.1–31.9) compared well (P = 0.02) with Competitive Ligand Exchange Cathodic Stripping Voltammetry (CLE-AdCSV) except for one sample. AGNES requires minimal sample manipulation, is unaffected by adsorption of interfering species at the electrode surface and allows direct determination of free zinc ion concentrations. Therefore AGNES results can be used in conjunction with ecotoxicological studies and speciation modelling to set and test compliance with water quality standards.

Towards field trace metal speciation using electroanalytical techniques and tangential ultrafiltration

In this work we propose a trace metal speciation methodology to determine the total, free and ultrafiltered (<1KDa) metal fractions using electrochemical methods (SCP and AGNES) and tangential ultrafiltration (UF) experiments that can easily be carried out on-site. We tested our methodology spiking Cadmium ions into two natural waters samples from Itapanhau and Sorocabinha rivers in Sao Paulo State, Brazil. The limits of detection (LOD) was 1.6×10−9M for the total Cd2+ determination performed by Stripping Chronopotentiometry (SCP) in the source and acidified ultrafiltered solution and 1.9×10−9M for the free Cd2+ determination using Absence of gradients and Nernstian equilibrium stripping (AGNES), using a thin mercury film electrode. The total metal determination was performed by SCP in acidified samples and the results compared with graphite furnace atomic absorption spectroscopy (GF-AAS). The SCP results were adequate with a 96% of recovery from the known metal spike for the 12 samples tested. For the Itapanhau sample the free metal determined by AGNES and the ultrafiltered fraction are identical, while for the Sorocabinha the free metal in the source is significantly smaller than the ultrafiltered fraction, indicating that this sample must be rich in metal complexes with small inorganic ligands that are able to permeate the 1kDa membrane. The proposed metal speciation methodology validated in the laboratory combining UF and SCP/AGNES is able to be used in on-site experiments providing valid information regarding the total and free metal concentrations and additionally some insight on the role of small inorganic ligands to the metal complexation.

Free Zn2+ determination in systems with Zn-Glutathione

Abstract Zn-Glutathione speciation was studied applying the electrochemical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) to determine the free zinc concentration. In titrations varying either pH, total concentration of glutathione ( c T,GSH ) or total concentration of Zn ( c T,Zn ), free Zn concentrations determined with AGNES were compared with the values predicted from previously reported complexation constants. The speciation of Zn was studied in a real sample of root extracts of Hordeum vulgare where the c T,Zn had been determined by ICP-MS and c T,GSH by HPLC. The free [Zn 2 + ] was measured with AGNES using a special device where a mixture of N 2 /CO 2 saturated in milliQ water controls the pH and avoids the evaporation of the sample. The lower free zinc concentration determined with AGNES, in comparison with the predicted one assuming the literature complexation constants and taking into account only the presence of Zn and GSH, indicates that it is necessary to include more ligands apart from GSH (as other phytochelatins) in the speciation model.

Adsorption of Cadmium on Titanium Dioxide Nanoparticles in Freshwater Conditions – A Chemodynamic Study

AbstractThe adsorption of Cd onto two titanium dioxide nanoparticles (nTiO2) with distinct crystalline phases and sizes was investigated in conditions that can be found in freshwater systems (I=0.01 M, pH 6.0–8.5). Cd dynamic speciation was assessed by two distinct electroanalytical techniques: absence of gradients and nernstian equilibrium stripping (AGNES) and scanned stripping chronopotentiometry (SSCP). With both NPs, 1 : 1 CdTiO2 homogeneous and labile surface complexes are formed. To our knowledge this is the first study about the dynamic nature of the nTiO2 surface complexes formed in the presence of Cd.