Critical role of macrophytes in achieving low iron concentrations in mine water treatment wetlands.
Aerobic wetlands are increasingly being included in mine water treatment systems which need to achieve low residual iron concentrations (<0.5 mg L(-1)) in final discharges. Traditionally the macrophyte components of such systems have been thought to be insignificant sinks for major contaminants such as iron. However, we report high rates of plant uptake of iron where the latter is present at relatively low concentrations, suggesting that macrophytes may well be critical to achieving low residual iron concentrations in final effluents from such systems. The wetland macrophyte Phragmites australis was grown in waters with a range of iron concentrations (0-50 mg L(-1)). At an Fe supply of 1 mg L(-1) almost 100% of the Fe was taken up into plant tissues. The majority of iron was stored in and around the roots of the plants, which helps allay fears of possible release of contaminants during seasonal die-back of emergent shoots and leaves. The 1 mg L(-1) threshold also proved to be important in terms of plant growth, with significant inhibition (evident in root length and in dry weights of shoots and roots) in plants grown in waters with Fe above this concentration. No direct causal relationship between iron content in aerial tissues and growth inhibition was found, which strongly suggests that iron toxicity cannot explain these results. These results have implications for the design of mine water treatment wetlands, particularly with regard to initial establishment of vegetation and achieving sufficient Fe removal in "polishing" applications (i.e. where it is intended to remove the last few mg L(-1) of Fe).