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

Highlights

• Complexation information can be retrieved from each SSCP point.

• Bulk c_M from AGNES is a robust input in the mathematical treatment.

• An SSCP wave generates a binding curve c_ML vs. c_M (at the electrode surface).

• Successful examples of this strategy include humic and fulvic acids.

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.

Introduction

Speciation is key to unravel trace metal ions mobilities in natural media and their bioavailability/toxicity to living organisms. The description of aqueous metal complexation with ligands is complicated by the extraordinary diversity of complexants present in the environment, ranging from small chemical molecules to larger reactive particles such as natural organic matter (NOM), mineral clay systems or polymeric-like materials of biological origin (e.g. exopolysaccharides, peptides, proteins, etc.) [1,2]. Heterogeneity is a defining feature for most of the ligands which arises from (i) the “chemical” properties, including polyfunctionality (i.e. diverse nature of the complexing sites) and polyelectrolytic features of the reactive interfaces (due to the presence of electric charges) and the possible formation of multidentate complexes, (ii) the “physical” characteristics related to the geometry of the particle, such as the anisotropy of the shape, and (iii) the structural organisation of the reactive interface [3,4]. Metal complexation is strongly impacted by the heterogeneity properties of the ligands or suspended matter and this will give rise to a distribution of involved stability constants of the metal complexes as a function of a degree of sites' occupation.

Amongst the current analytical tools to finely address metal speciation in particulate dispersions, the electroanalytical technique Scanned Stripping Chronopotentiometry (SSCP) [5] is very well adapted to provide information on the dynamic behaviour, the thermodynamic nature [[6], [7], [8], [9]] and the heterogeneity degree of the metal ion complexes [10] and exhibits the ability to achieve very low detection limits (nanomol per litre concentrations). SSCP signal is constructed from the individual measurement of the Stripping ChronoPotentiometric (SCP) runs at various deposition potential (E_d). The resulting analytical signal is directly proportional to the reduced metal concentration inside the mercury electrode accumulated during the deposition step. By plotting the SCP measurements upon E_d variation, the SSCP signal takes the form of a wave curve which contains a significant amount of heterogeneity information due to the variation of the metal-to-ligand ratio at the electrode surface for each deposition potential. In cases where the solution contains one type of homogeneous ligand, the slope of the SSCP wave is simply defined by a Nernstian relationship [5], whereas for situations where the solution contains a mixture of metal complexes arising from chemical/physical/structural heterogeneity of the ligands, the description of the slope becomes more involved. In this latter case, the SSCP wave is elongated with respect to the potential axis (i.e. abscissae) [11], as compared to that measured for the chemically homogeneous case.

A few methods have been used to extract the heterogeneity information from SSCP curves, from a simple Freundlich type model [10,12,13] to the detailed analysis of the full SSCP wave proposed by Serrano et al. [14]. Concerning the Freundlich approach, the average heterogeneity parameter, Γ, is obtained from the SSCP curve slope. The second approach consists in a computational method to convert the experimental SSCP data to the corresponding concentrations of free and complexed metal species at the electrode surface. The method was applied to small complexes in excess of ligand conditions [14], when no information about heterogeneity can be derived. Although the second approach produced good results in presence of small complexes probed with the Hanging Mercury Drop Electrode, its application to more complex systems has been hindered by the difficulty of correctly estimating the free and bound metal concentrations in the bulk.

To overcome this difficulty, we propose to directly measure the bulk speciation using Absence of Gradients and Nernstian Equilibrium Stripping, (AGNES), a technique that has been able to robustly determine free concentrations of Cd and Pb in a number of systems, including synthetic solutions with dissolved organic matter [[15], [16], [17], [18], [19]].

The objective of this work is to build up binding isotherms for the trace-metal interaction with heterogeneous macromolecular ligands from the SSCP wave. To achieve this goal, the full SSCP wave analysis model of ref. [15] is extended to tackle heterogeneous macromolecular labile systems for any metal-to-ligand ratio probed with the Rotating Disc Electrode, simplifying the computations by directly measuring the free metal ion concentration in the bulk with the complementary use of AGNES.