Electrochemical studies of the kinetics and mechanism of aqueous iron reactions in mine-water

Abstract

This work has been conducted to investigate the chemical processes involved in the remediation of acidic mine drainages (AMD) using electroanalytical methods to study the chemistry of Fe(III) in pure laboratory solutions and samples obtained from Shilbottle Colliery (Northumberland, UK), which are heavily contaminated with soluble Fe leached from the spoil heap. One method for remediation is to raise the pH of the waters and precipitate Fe(III) in an artificial wetland; however microelectrode voltammetry shows significant soluble Fe(III) remains. The characterization of hydrolysed species of Fe(III) in pure laboratory solutions and Shilbottle samples has been studied using voltammetry, Raman spectroscopy, electrospray ionization - mass spectrometry (ESI-MS) and X-ray diffraction (XRD). The voltammetric and Raman studies show that, there is some similarity in square wave voltammograms and Raman spectra between Shilbottle solutions and aqueous laboratory Fe(III)-sulfate solutions. ESI-MS of laboratory Fe(III) solutions with non-coordinating, or weak coordinating anions such as perchlorate shows the presence of monomers, dimers, trimers and tetramers of soluble Fe(III) complexes. However no Fe-related peaks were observed in sulfate-containing media and the Shilbottle samples due to the presence of neutral species such as FeSO4OH. XRD data shows formation of basic sulfate complexes in the case of Fe(III)-sulfate and no XRD peaks can be assigned to Fe(III) in perchlorate media. Determination of soluble fractions of [Fe(III)/Fe(II)] and [SO42-] in Shilbottle samples have been performed using developed voltammetric methods. The total soluble [Fe] varied between 281 and 446 ppm and soluble [SO42-] between 3460 and 7400 ppm. A standard geochemical model (PHREEQC) has been used to study chemical speciation in AMD; a comparison of the model and our measurements indicates that the Shilbottle samples are clearly out-of-equilibrium. Kinetic studies of hydrolysis of Fe(III) in pure laboratory solutions and Shilbottle samples have been performed using potentiometric, voltammetric and Raman spectroscopy techniques. The obtained data show that the hydrolysis of Fe(III) upon raising the pH by addition of NaOH is second order in [Fe]; the rate constants were measured and a mechanism which can rationalize the rate constants is proposed. Also, ii effects of sulfate ions on the hydrolysis of Fe(III) in aqueous solutions have been investigated. The study shows that the SO42- ions catalyse formation of soluble dimers, oligomers and basic sulfate complexes. Furthermore, FeSO4+ stays dissolved in the solution longer than simple Fe(III) species such as Fe3+, Fe(OH)2+ and Fe(OH)2+.