Remediation of circum-neutral metal mine drainage using laboratory-scale permeable reactive barriers

Abstract

Many former metal mining areas of the UK are characterised by circum-neutral pH, low-Fe drainage with concentrations of Zn, Pb and Cd that exceed EU water quality guidelines. Drainage waters from these former mining areas present a challenging target for remediation. This thesis explores the potential of using a range of permeable reactive materials: Zero valent iron (ZVI), activated carbon, limestone, compost and fly ash to remove metal contaminants from such waters in simulated laboratory-scale permeable reactive barriers. Mine drainage waters were collected from a number of former mining areas identified as having circum-neutral, low-Fe drainage; Nenthead Pb/Zn mine (North Pennines), Silvermines Pb/Zn/BaSO4 mine (southern Ireland) (both carbonate-hosted deposits) and Bwlch Pb/Zn mine (mid Wales) (a siliciclastic-hosted deposit). These waters were used in a series of batch and long-term laboratory columns investigations to assess rates and patterns of Zn, Pb and Cd removal. For comparative purposes, metal removal rates from an Acid Mine Drainage (AMD) water (Parys Mountain Cu mine, Anglesey) were also assessed. Experimental results show that ZVI, activated carbon and fly ash removed up to 100% Zn, Pb and Cd from circum-neutral pH mine discharges over extended periods of up to 4048 pore volumes. Furthermore, systems receiving circum-neutral drainage waters lasted up to 400 times longer and removed up to 170 times more Zn, Pb and Cd than those receiving AMD water. This is primarily a function of the higher influent pH and low-Fe concentrations of the circum-neutral mine waters, which prevent the passivation of reactive materials by Fe (oxy)hydroxide precipitates, therefore increasing the system lifespan. The general order of material performance in terms of metal removal and system longevity was ZVI>peat fly ash>activated carbon>limestone. ZVI demonstrated an extraordinary long-term ability to remove Zn, Pb and Cd from the circum-neutral mine waters, with a 270g column of ZVI being capable of reducing Zn concentrations to below the EU Water Quality Standard of 0.05 mg L-1 for more than 4000 pore volumes (run over a five-and-a-half month period). Peat fly ash also proved an effective material for the rapid removal of contaminant metals from the circum-neutral mine waters, with a 2-minute reaction period being sufficient to remove up to 100% of Pb, Zn and Cd. Activated carbon and limestone were found to be less effective materials for metal removal. Geochemical modelling using Geochemist’s Workbench® was found to be a useful tool for assisting in the interpretation of column experimental results, with speciation calculations allowing identification of oversaturated phases. This research demonstrates the potential of traditional reactive materials to treat a range of circum-neutral mine discharges from areas with differing hydrochemical and geological origins. Furthermore, estimates show that passive treatment plants in areas with circum-neutral, low-Fe drainage could be on average 35 times smaller than those required in areas discharging AMD water.