Electrochemical determination of dissolved and particulate iron in mine-waters

Abstract

A voltammetric procedure for the determination of dissolved and colloidal iron in mine-waters has been developed. Whilst mine-waters are of course enriched in iron. we are remarkably ignorant of the physical state and chemical speciation of the iron. This is a problem since the physical and chemical state of iron is central to understanding a range of processes relevant to mine-water geochemistry and remediation. Examples include hydrolysis of dissolved Fe (III) to release protons. the adsorption of trace metals onto iron colloids and the bioavailability of iron within wetlands designed to remediate acidic waters. In this work, we have developed differential pulse voltammetry (DPV) as a rapid and robust method of determining the concentration of truly dissolved and colloidal iron in 0.45 J.lm filtered waters from a series of mine-water discharges and remediation sites in NE, England. Mine-water samples were collected from CoSTaR sites: these are abandoned mine sites in the UK, designated by the UK Coal Authority for remediation research and routine monitoring of water quality. The sites comprise of six full-scale bioreactors receiving a wide range of mine-waters with pH ranging from 3 to 5 and concentrations of < 0.45 Jlm iron between 30 and 800 mg L-1 across the sites. Monthly samples were collected over the period March 2006 to April 2007. The samples were analysed directly using differential pulse voltammetry (DPV) at gold electrode. 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 couple plasma spectroscopy (ICP-OES). The good agreement between the iron concentrations measured in acidified samples electrochemically and by ICPOES validates the accuracy of DPV as an analytical method for iron. Colloidal and particulate iron was also determined since DPV measures only dissolved iron. particulate (>0.45 Jlm) and/or colloidal «0.45 Jlm) iron can then be estimated as the difference between the voltammetric responses of natural samples and samples in which the solid phase iron has been dissolved by the addition of He!. v The percentage dissolved iron ranged from 60-90% (in most cases) in unfiltered samples, while the percentage of colloidal iron varied widely across the sites; from 25-45% in unfiltered samples and 50-75% and 38-85% for dissolved and colloidal iron in the 0.45 Jim filtered samples. The ratio of Fe (II) to Fe (III) in the dissolved fraction was detennined usmg ultramicroelectrodes (UME) method. Iron ratio varied widely for the three sites studied. However, in general, the ratio is 1: 1 for the surface influent waters. 1:3 for the sub-surface waters (underground water-Shilbottle site) and 3: 1 for most of the effluent samples. Results suggest that in general, the influent waters are more oxidised and the effluent more reduced. Finally, characterisation of solid phase iron was done usmg a wide range of spectroscopic techniques. Atomic Force Microscopy (AFM) shows that iron colloids range from nm to Jim for lower pH mine waters; at higher pH, particles mainly aggregates on the Jim to mm scale. FT-IR, XRD, TEM and EDX show that the most common colloidal phase is poorly crystalline Fe oxyhydroxides, however certain unusual crystalline phases, e.g., Schwertmannite were found.