Using Myriophyllum aquaticum (Vell.) Verdc. to remove heavy metals from contaminated water: better dead or alive?

This study aimed at investigating the heavy metal removal potential of the invasive macrophyte Myriophyllum aquaticum. The element tested were Cd, Cr, Ni and Zn, in single-metal trials, and experiments were performed with both living and dead biomass of the plant. Regarding metal removal by living plants, the most removed element was Zn, despite Cd showed the highest concentration in plant shoots. The metal negative effect on plant growth was therefore more important than the level of metal concentration in plant tissue in determining the removal percentages. All the metals were mostly accumulated in the roots, where a considerable fraction of the element was simply adsorbed to root cell wall, except in the case of Cr. In shoots, the fraction of the adsorbed metal was extremely lower in respect to roots, thus implying a lower apoplastic binding capacity. Regarding a possible use of the dead biomass for metal removal, we proposed the generation of a hybrid biosorbent enclosing the dried and grounded plant biomass in cotton bags to improve its handling and the adsorption capacity, in the view of a valid alternative to reduce the problems of packed beds. Cadmium and, especially, Zn were the most efficiently removed elements in respect to the other metals. Comparing the removal percentages of the living biomass and the hybrid biosorbent, our data deposed in favor of the use of M. aquaticum as dead biomass for a possible application of this invasive macrophyte in the biological treatment of metal contaminated water. Our findings may be beneficial to metal removal application joint to wetland management, devising a possible use of M. aquaticum waste material after its removal from the invaded habitats.