A voltammetric procedure for the determination of total, dissolved, solid phase and nanoparticulate Fe in polluted mine-waters has been developed. Mine-water samples were collected from abandoned mine sites in the UK, which have been designated by the UK Coal Authority for remediation research and for routine monitoring of mine-water quality. Monthly samples were taken over the period March, 2006 to April, 2007 from six different sites in North East England. The samples were analysed directly using differential pulse voltammetry at gold electrodes: separate peaks due to unhydrolysed and hydrolysed Fe aquo ions could be assigned and quantified. The results show that our analysis provides data for total dissolved iron of comparable analytical quality to the established mine-water analysis techniques based on inductively-coupled plasma-optical emission spectroscopy. Further, the difference between differential pulse voltammetry data in acidified and unacidified samples allows quantitation of the Fe-containing solid phase and nanoparticulate Fe oxyhydroxide phases. If samples were prefiltered, then these measurements also allowed us to determine the fraction of Fe present in nanoparticles <450 nm diameter. Finally, steady-state voltammetry at Pt microelectrodes was used to obtain the ratio of Fe(II) and Fe(III) redox states in the soluble Fe fraction.

Cyclic-, Differential Pulse- and Steady-state Microdisc Voltammetry (CV, DPV, SMV) techniques have been used to quantify the occurrence and fate of dissolved Fe(ii)/Fe(iii), nano-particulate and micro-particulate iron over a 12 month period in a series of net-acidic and net-alkaline coal mine drainages and passive treatment systems. Total iron in the mine waters is typically 10-100 mg L(-1), with values up to 2100 mg L(-1). Between 30 and 80% of the total iron occurs as solid phase, of which 20 to 80% is nano-particulate. Nano-particulate iron comprises 20 to 70% of the nominally "dissolved" (i.e. <0.45 μm) iron. Since coagulation and sedimentation are the only processes required to remove solid phase iron, these data have important implications for the generation or consumption of acidity during water treatment. In most waters, the majority of truly dissolved iron occurs as Fe(ii) (average 64 ± 22%). Activities of Fe(ii) do not correlate with pH and geochemical modelling shows that no Fe(ii) mineral is supersaturated. Removal of Fe(ii) must proceed via oxidation and hydrolysis. Except in waters with pH < 4.4, activities of Fe(iii) are strongly and negatively correlated with pH. Geochemical modelling suggests that the activity of Fe(iii) is controlled by the solubility of hydrous ferric oxides and oxyhydroxysulfates, supported by scanning and transmission electron microscopic analysis of solids. Nevertheless, the waters are generally supersaturated with respect to ferrihydrite and schwertmannite, and are not at redox equilibrium, indicating the key role of oxidation and hydrolysis kinetics on water treatment. Typically 70-100% of iron is retained in the treatment systems. Oxidation, hydrolysis, precipitation, coagulation and sedimentation occur in all treatment systems and - independent of water chemistry and the type of treatment system - hydroxides and oxyhydroxysulfates are the main iron sinks. The electrochemical data thus reveal the rationale for incomplete iron retention in individual systems and can thus inform future design criteria. The successful application of this low cost and rapid electrochemical method demonstrates its significant potential for real-time, on-site monitoring of iron-enriched waters and may in future substitute traditional analytical methods.

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