Novel approach to zinc removal from circum-neutral mine waters using pelletised recovered hydrous ferric oxide
Introduction
In river basins affected by historic metal mining, long-standing Zn pollution of surface waters can have significant impacts on ecology and pose a threat to compliance with surface water quality standards, such as those set out in Europe by the EU Water Framework Directive (2000/60/EC). In mining settings, Zn pollution arises principally due to the oxidative dissolution of sphalerite (ZnS) in both subterranean (e.g. mine shafts) and surface (e.g. waste rock heaps) settings and can be discharged to surface waters via a range of point and diffuse pathways [1]. Although zinc is an essential trace element for plants and mammals [2], it can be toxic to sensitive aquatic life (e.g. salmonid fish) at low concentrations reflected in the maximum acceptable Zn concentration of 8–125 μg L−1 in the UK (hardness-dependent (as mg L−1 CaCO3) national environmental quality standard (EQS)).
Treatment options for Zn-rich mine waters are established for acidic mine waters where Zn is prevalent in the form Zn2+. Here, active dosing with lime or caustic magnesia removes Zn as a hydroxide solid [3], while passive systems such as using a Dispersed Alkaline Substrate (DAS) of fine-grained alkaline material (e.g. calcite or caustic magnesia) on a coarse woodchip matrix have shown promise in laboratory and preliminary field trials [4]. Alternatively, Zn can be immobilised as a sulphide in bioreactors where sulphate reducing bacteria are present in substrates usually comprising a mix of organic and calcareous alkalinity-generating media (e.g. Reducing and Alkalinity Producing Systems (RAPS); [5]). At circum-neutral mine water discharges, which occur in many of the metal mining areas in the UK where mineral veins are hosted in Carboniferous strata [6], Zn is present predominantly as the complex . will not readily react to form non-carbonate solids. As such, attempts to employ aerobic passive treatment systems such as wetlands have not been very successful [3]. Similarly, efforts to remove Zn as smithsonite (ZnCO3) using anoxic limestone drains to elevate to pH sufficient for ZnCO3 precipitation yielded only a 22% mean reduction in Zn during 3-month pilot-scale trials at Nenthead, Northumberland [7]. While alkali dosing could be used in net alkaline waters, the comparatively high operating costs of this form of treatment preclude its deployment at most long-abandoned mine sites. Additionally, the nature of many metal mine waters, which discharge directly into rivers in steep-sided valleys where low gradient terrain is scarce, prompts the need for treatment technologies that have a small land ‘footprint’. This demand limits the potential for wetland or RAPS-based treatment systems in many situations.
The use of low-cost, high surface area adsorbents such as zeolites [8], red mud [9], algae [10] and moss [11] for removing metals from wastewater streams has been widely investigated in recent years. The effectiveness of these materials can vary greatly with factors such as pH, influent metal concentration, and local availability of these materials which affect their potential for widespread usage. The research presented here provides a pilot-scale evaluation of HFO pellets (locally called ‘ochre’) as a high surface area sorbent for removing Zn from metal mine discharges. The HFO pellets comprise poorly crystalline ferric oxyhydroxides (e.g. Fe(OH)3 and FeO·OH – [12]) which have been recovered from coal mine water treatment systems. HFO is a major waste stream from coal mine water treatment facilities in the UK and Europe, with over 1200 tonnes of Fe recovered annually in the UK alone [13]. Although several end uses for recovered HFO have been investigated and promising rates of removal of phosphorus from agricultural and sewage wastewaters have been documented [14], no single end use has so far been found to consume the current and projected supply of HFO in Europe. As such, large quantities are currently stockpiled pending disposal via landfill.
This current research builds on these developments to assess whether HFO is similarly effective as a sorbent for problematic metals (Zn in this case) from metal-rich discharges as it is for P. Widely cited literature [15] provides theoretical and laboratory-based indications of the effectiveness of hydrous ferric oxides as a sorbent for metals, particularly at the circum-neutral pH values encountered here. This is due in a large part to their high specific surface area and strong sorptive interactions with metal ions which adsorb through the formation of surface complexes. While these indications relate in large part to pure synthetic ferric oxides, laboratory studies that inform this current work have highlighted the potential for pelletised recovered HFO to effectively immobilize Zn and Pb from solution [16]. Treatment efficiency in excess of 99% was found in continuous flow column experiments with influent concentrations of Zn and Pb at 3.0 and 2.5 mg L−1, respectively. Zn and Pb removal was rapid, with >99% removed within a 2 h contact time with surface sorption the dominant mode of removal from solution. The maximum Zn adsorption capacity of the pellets has also been estimated at 36.0 mg Zn g−1 through batch experiments and observing breakthrough in continuous laboratory flow trials ([16] and unpublished data of the authors). This compares favourably with other potentially low cost sorbents such as blastfurnace slag (17.7 mg Zn g−1; [17]), red mud (12.6 mg Zn g−1; [17]) and activated carbon (13.8 mg Zn g−1; [18]). This current study builds on the theoretical and laboratory background to assess whether the promising performance of pelletised recovered HFO as a Zn sorbent in laboratory studies is translated to field conditions in a pilot-scale field trial at a polluting metal mine discharge. This will also assist in resolving the scale-dependence of geochemical processes occurring in treatment units between laboratory and field studies.
Section snippets
Study site and pilot system set-up
The North Pennines Orefield was mined intensively for lead (Pb), Zn and fluorspar between the 17th century and the 1970s. Despite the long timescales since abandonment of the majority of the mine facilities by the 1920s, several catchments draining the mineralised orefield continue to be afflicted by high dissolved Zn concentrations in breach of EQS [1], [19]. The Scraithole mine water discharge emerges from the Scraithole Low Level (54°49′01″N; 002°18′21″W) on the steep western bank of the
Water chemistry
Summary physico-chemical data describing the composition of the Scraithole mine water and effluent water from the treatment tank during the course of the trial are presented in Table 1. The mine water displays hydrochemical facies typical of mine waters draining the North Pennine Orefield, with the dominant ions Ca2+, SO42− and HCO3− [18], [24]. Zn is present in concentrations between 0.4 and 2.2 mg/L and the mine water is generally seen to be of consistent quality (hence relatively low standard
Conclusions
- 1.
This study has highlighted the potential for using a waste stream generated from coal mine water treatment to be developed as a low-cost, small footprint treatment option for circum-neutral metal mine waters. The pilot-scale field trials showed the HFO pellet drain to have a mean treatment efficiency of 32%, at an average residence time of 49 min.
- 2.
However, more instructive insight as to the performance of the system can be gained from the area- and volume-adjusted removal rate. With a mean
Acknowledgements
The authors gratefully acknowledge the funding provided by the Environment Agency under project SC030136/2. The views expressed in this article are not necessarily those of the Environment Agency. We are indebted to Jon Aumônier (Mineral Industry Research Organisation, UK) for his underpinning work on the development and manufacture of HFO pellets, offering technical advice and assisting with the construction of the pilot system. The authors also wish to thank Emma Gozzard (Newcastle
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