Role of plants in treating metal-polluted waters in wetlands

Abstract: The overall aim of this thesis was to evaluate the role of wetland plants in wetland treatment of metal-polluted waters. The specific aims were to investigate metal uptake in various wetland plant species; whether and how wetland plants tolerate high metal concentrations in their immediate surroundings, and how the pH and metal concentrations of polluted waters influence metal uptake in wetland plants were also investigated. Acid mine drainage with low pH (3.5) and high metal concentrations, and stormwater with a near neutral pH and lower metal concentrations were the two waters treated. The submerged plant species Elodea canadensis and Sparganium hyperboreum, the floating-leaved plant species Potamogeton natans, and the emergent plant species Carex rostrata, Eriophorum angustifolium, and Phragmites australis were exposed to concentrations of cadmium (Cd), copper (Cu), iron (Fe), lead (Pb), and zinc (Zn) naturally found in these waters. Low pH and high metal concentrations in the polluted waters were demonstrated to have a decreased effect on the treatment efficiency. The presence of submerged and floating-leaved plant species increased the treatment efficiency of a wetland by taking up dissolved metals, while metal uptake by emergent plant species was minor. The wetland plant species displayed only few signs of stress when exposed to Zn, Cu, and Cd. Thiol-rich peptides, increased cell wall binding, and rising pH in the medium are possible mechanisms whereby Cd is detoxified in the plant tissue.The overall conclusion was that using wetlands to treat polluted waters is possible, although treatment efficiency is influenced by the pH and metal concentrations in the polluted water. The role of plants was dependent on the character of the waste water and on the specific plant species used. Wetland plants tolerate metal concentrations in their tissues by activating internal mechanisms that detoxify the metals.