Electrochemical sensors for iron speciation in artificial water systems
Résumé
Iron is a very abundant chemical element naturally present in the Earth’s crust [1]. It plays an important role and has major effects in environment (like in fresh- and seawater), in industry (for instance in corrosion problems) or in humans (as it is implied in many biological or metabolic systems). Iron is mainly present as ferrous Fe(II) and ferric Fe(III) in aqueous media. However, Fe(II) is spontaneously oxidized in the presence of oxygen whereas Fe(III) is weakly soluble and forms stable complexes and precipitates at neutral pH [2]. Furthermore, the permissible values of iron concentration in drinking water is between 0.3 – 3 mg L-1 (i.e. 5.3 10-6 – 5.3 10-5 mol L-1). Consequently, the assay of total iron at low concentration as well as the determination of iron speciation is of key interest and still represent an analytical challenge. Many analytical methods have been developed in routine to detect and quantify iron in natural water involving chromatography, spectrophotometry, fluorescence, chemiluminescence and so on. However, all these techniques are expensive, bulky and time-consuming. Alternatively, electrochemical methods have been advantageously considered since they are low-cost, user-friendly, produce results in real time and can be used in situ [3-4]. If polarographic methods are currently not used anymore for evident toxicity reasons, mercury-free electrodes techniques based on adsorptive stripping voltammetry have been extensively developed these last ten years [5-6]. Nevertheless, these imply chelating agents including catechol or cupferron, thus making the procedure complicated and chemical-consuming. In this work reagentless electrochemical sensors have been considered for the detection of Fe2+ and Fe3+. Two amperometric sensors have been developed based on glassy carbon electrodes modified by deposition of a Nafion® cation exchange resin film containing sulfonic groups. Fe2+ and/or Fe3+ were pre-concentrated at open circuit conditions during 5-10 minutes, allowing ion exchange with protons. Electrochemical detection was then performed by cyclic or square wave voltammetry. In both techniques signals related to iron were recorded, the peak currents of which being proportional to Fe2+ and Fe3+ concentrations, respectively. Satisfactory calibration curves were achieved in the micromolar concentration range when analyzes were made in synthetic solutions. Limits of detection down to 0.1 and 0.01 µmol L-1 were reached for Fe2+ and Fe3+ respectively and the sensors exhibited a reproducible response for at least 15 measurements. Furthermore, accuracy was verified by comparing electrochemical results with ICP-OES analyzes. Finally, promising results obtained with real water samples from artificial circuits will be presented. [1] S.R. Taylor and al., The Continental Crust. Its composition and Evolution, Blackwell Scientific, Oxford 1985. [2] X.W. Liu and al., Mar. Chem., 2002, 77, 43-54 [3] M. Lu and al., Electroanal., 2012, 24, 1693–1702 [4] L.M. Laglera and D. Monticelli, Curr. Op. Electrochem., 2017, 3, 123-129. [5] M.M. Abualhaija and al., Mar. Chem., 2014, 164, 60-74. [6] F. Sanvito and al., Electroanalysis, 2019, 31, 212-216.