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Preliminary evaluation of a constructed wetland for treating extremely alkaline (pH 12) steel slag drainage

Lookup NU author(s): Dr Will Mayes, Jon Aumonier, Dr Adam Jarvis

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Abstract

High pH (>12) leachates are an environmental problem associated with drainage from lime (CaO)-rich industrial residues such as steel slags, lime spoil and coal combustion residues. Recent research has highlighted the potential for natural (‘volunteer’) wetlands to buffer extremely alkaline influent waters. This appears ascribable to high CO2 partial pressures in the wetland waters from microbial respiration, which accelerates precipitation of calcium carbonate (CaCO3), and the high specific surface area for mineral precipitation offered by macrophytes. The research presented here builds on this and provides preliminary evaluation of a constructed wetland built in March 2008 to buffer drainage from steel slag heaps in north-east England. The drainage water from the slag mounds is characterised by a mean pH of 11.9, high concentrations of Ca (up to 700 mg/L), total alkalinity (up to 800 mg/L as CaCO3) and are slightly brackish (Na = 300 mg/L; Cl = 400mg/L) reflecting native groundwaters at this coastal setting. Documented calcite precipitation rates (mean of 5g CaCO3/m2 /day) from nearby volunteer sites receiving steel slag drainage were used to scale the constructed wetland planted with Phragmites australis; a species found to spontaneously grow in the vicinity of the discharge. Improved performance of the wetland during summer months may at least in part be due to biological activity which enhances rates of calcite precipitation and thus lowering of pH. Secondary Ca-rich precipitates also serve as a sink for some trace elements present at low concentrations in the slag leachate such as Ni and V. The implications for scaling and applying constructed wetlands for highly alkaline drainage are discussed.


Publication metadata

Author(s): Mayes WM, Aumonier J, Jarvis AP

Publication type: Article

Journal: Water Science and Technology

Year: 2009

Volume: 59

Issue: 11

Pages: 2253

Print publication date: 01/01/2009

ISSN (print): 0273-1223

Publisher: IWA Publishing

URL: http://dx.doi.org/10.2166/wst.2009.261

DOI: 10.2166/wst.2009.261


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