Iron detection and speciation in natural waters by electrochemical techniques: A critical review

Abstract

Summary Developments in iron detection and speciation in natural waters by electrochemical methods are reviewed considering latest, post 2010, evolutions. Voltammetric and potentiometric methods have been employed to detect iron and iron species in natural waters, potentiometric methods being devoted to redox speciation only. Regarding iron detection by voltammetry on the mercury electrode, several improvements have been introduced since 2014, leading to a very low detection limit (LOD, 5 pM), reduced analysis time, elimination of the UV pretreatment and a 20-fold reduction in sample size. Chemically modified, mercury free, electrodes have started to be introduced. Among bulk electrodes, bismuth and bismuth alloy offer the best performances in term of detection limit (0.2 nM). Gold or gold nanoparticle electrodes modified with self-assembled monolayers of different thiols are the most promising among chemically modified electrodes: thiols bear an iron complexing moiety that efficiently preconcentrate iron onto the electrode surface with a claimed LOD of 30 pM. Regarding iron speciation analysis, no substantial advances have been recently achieved. Several ionophores were synthesised and tested in potentiometric electrodes for iron redox speciation (for the detection of both Fe(III) and Fe(II) species), but their use is limited to acidic solutions due to sensitivity limitations. Finally, the voltammetric detection of iron complexation by competitive ligand—cathodic stripping voltammetry has made limited methodological progresses since its introduction in 1994, although improved data treatment procedures have been conveniently implemented.