Combustion of Salix used for phytoextraction : The fate of metals and viability of the processes

Abstract : Phytoextraction may reduce the environmental risk posed by contaminated sediments while simultaneously providing an economic return via bioenergy production. There is a lack of information on the combustion of metal enriched willows used for phytoextraction. This work aimed to determine the Cd phytoextraction efficiency of Salix on a case study in France and elucidate the distribution of metals in the end products of the combustion process. Three willow clones were cultivated in short rotation coppice (SRC) on a metal contaminated dredged sediment landfill site. Combustion assays were performed in a biomass boiler of 40 kW with a Zn and Cd enriched Salix wood 'Tora' harvested from a part of the SRC and a commercial 'Tora', for comparison. In a best-case scenario, phytoextraction could reduce total Cd burden of the sediment from 2.39 mg kg-1 DW to 2 mg kg-1 DW in 19 years. Combustion experiments showed that Cd and Zn occurred at the highest concentrations in the particulate fraction of the flue gas (flyash), rather than in the bottom ash. Combustion of 'Tora' from phytoextraction resulted in Cd and Zn concentrations in flue gas emissions that exceeded French regulation. This was also observed for Cd in the flue gas of the commercial 'Tora'. Irrespective of the wood provenance, the use of industrial or collective boilers, equipped with efficient filters, is required to minimize air pollution. Given this constraint, wood produced during phytoextraction should be usable for bioenergy production. The possible uses of bottom ash are discussed.