Phytoextraction of Cadmium: Feasibility in Field Applications and Potential Use of Harvested Biomass
Résumé
Cadmium is one of the most threatening soil contaminants because of its high toxicity and widespread anthropogenic distribution. Therefore, remediation of Cd-polluted soils is urgently required. Cropping Cd-accumulating plants appears to be the most relevant approach for removing this pollutant from large soil surfaces. Several field studies have shown the potential of phytoextraction to clean-up moderately Cd-contaminated sites, but this process still has important limitations. Hyperaccumulating plants such as Noccaea caerulescens and Sedum plumbizincicola show considerable Cd removal rates due to their extraordinary accumulation capacities, but commonly suffer from strongly limited biomass production, especially N. caerulescens. Interesting results were also found for other plants, e.g. some cultivars of 'indica' rice Oryza sativa or Solanum nigrum, but in these cases, further studies are required for confirmation. Some fast-growing willow clones, cultivated in short rotation coppice, offer a possibility to produce energy biomass on contaminated soils more than by decontaminating them, because of their low Cd extraction rate. Relatively little is known about the use of harvested biomass produced through phytoextraction. A few studies showed that combustion is a feasible option, because Cd is retained mainly in the fly ash, whereas the bottom ash contains relatively low amounts of Cd and could even be used as a fertilizer. Further investment is required to evaluate the possibility of producing high-performance cultivars of the best Cd hyperaccumulators. If this approach yields positive results, the complete process chain will need to be tested on a large scale, including the valorisation of biomass.