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

Abstract

11Trace metal speciation is key to understand/predict bioavailability and potential toxicity 12 of metals to biota and will, undoubtedly, be incorporated in future regulations. 13Electroanalytical methods have a role to play in such development: they offer a wide 14 range of advantages such as speed, portability, economy, solid interpretation 15 backgrounds and low limits of quantification. This review focusses on three selected 16 stripping techniques: Competitive-Ligand Exchange -Cathodic Stripping Voltammetry 17 (CLE-CSV), Scanned Stripping Chrono-Potentiometry (SSCP) and AGNES (Absence 18 of Gradients and Nernstian Equilibrium Stripping) reporting their working principles, 19 characteristics (strong and weak points) and recent applications to systems of 20 environmental relevance (such as seawaters, freshwaters or soil extracts). 21 22 23 24 Keywords: speciation, availability, electroanalysis, electrochemistry, heavy metal, 25 CLE-CSV, SSCP, AGNES 26 27 published in Current Opinion 3 (2017) 144-162 33 Speciation (i.e. the distribution of an element among its different chemical forms[1]) 34 has been recognised as a key factor for trace metal (bio)availability to biota, as 35 postulated by hegemonic ecotoxicological paradigms such as the Free Ion Activity 36 Model (FIAM) [2] or the Biotic Ligand Model (BLM)[3], both assuming that the free 37 ion is the only bioavailable species. The limits of these models have recently been 38 reviewed [4], highlighting numerous cases where specific fractions of metal complexes 39 can be internalized, either directly or through the dissociation of labile complexes. Thus, 40there is a need to design, develop, apply and interpret analytical techniques able to 41 provide reliable speciation information [5][6][7] that can be used by regulatory bodies. 42Although there are numerous non-electrochemical techniques [5][6][7][8][9][10][11][12][13], those that are 43 provide attractive possibilities such as speed of analysis, low-cost and miniaturization in 44 view to on-site (and in-situ) measurements [14], and can also provide thermodynamic 45 (i.e. equilibrium) information (such as the amount of the free metal, the amount of 46 complexing sites, the stoichiometry of complexes and their stability constants [15]) as 47 [16;17]. Stripping techniques, due to their low detection limits, are well suited for 50 environmental analysis. They are two-stage techniques, where the first stage is a 51 deposition step that accumulates the analyte (at the electrode surface or in its volume), 52 followed by a second stage that quantifies this analyte. Anodic Stripping Voltammetry 53 (ASV), in its multiple variants, is the most popular and has the advantage to possibly be 54 measuring a potentially bioavailable fraction of the metal, as shown again recently 55 [18;19]. 56 57 published in Current Opinion 3 (2017) 144-162 This critical evaluation focusses on three techniques that are known powerful tools for 58 metal speciation: AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) 59 and SSCP...

Towards improving the electroanalytical speciation analysis of indium

“…SCP is a stripping technique, thus presenting a very low detection limit, typically on the nanomolar level, and furthermore is not affected by the adsorption of organic ligands at the surface of the working electrode [28]. The second technique exploited in this work, AGNES [29] is employed for the direct determination of the free metal concentration, thus providing an estimation of the stability constants of the complexed metal species in solution.…”

Especiação Dinâmica De Metais Traço Em Ambiente Aquático Usando Cronopotenciometria De Redissolução Anódica

“…Algumas revisões críticas foram publicadas sobre SCP, 88 comparando-a com outras técnicas não-eletroquímicas 89,90 ou com outras técnicas eletroanalíticas. 86,91 Todavia, o número de estudos em que o foco é a especiação dinâmica é menor, devido à maior dificuldade da interpretação desses resultados e ao menor número de modelos teóricos adequados a descrevê-los. Uma boa parte destes estudos tem sido realizada para comprovar as teorias que abordamos no tópico anterior, não sendo necessariamente repetida aqui.…”

Assessment of labilities of metal complexes with the dynamic ion exchange technique

“…Metal speciation provides crucial information for current paradigms of ecotoxicity (such as the Free Ion Activity (Anderson et al 1978) and Biotic Ligand (Paquin et al 2002) Models), and thus several techniques have been developed along the years for this purpose. Among the most recent ones, we can quote voltammetric techniques such as Absence of Gradients and Nernstian Equilibrium Stripping (Companys et al 2017), membrane-based techniques such those using Polymer Inclusion Membranes (Companys et al 2018;Vera et al 2018), Donnan Membranes (Temminghoff et al 2000) and dialysis membranes (Berggren 1989;Franklin et al 2007), as well as techniques based on functionalised resins (Alberti et al 2007;Pesavento et al 2009;Sunda 1984;Tantra et al 2016). Among the latter, special mention must be made of the Ion Exchange Technique (IET), developed by Cantwell (Cantwell et al 1982) and later applied by many others, including Campbell and Fortin (Crémazy et al 2015;Fortin and Campbell 1998).…”