**4. Methods for heavy metal recovery**

So far there have been numerous studies on the phytoremediation process, having examined the species of plants that have a greater ability to accumulate heavy metals, factors affecting the results of phytoremediation and areas that should be remediated with plants. In terms of treatment, storage or heavy metal recovery from the biomass resulted from the phytoreme‐ diation process the reference studies are scant.

Delplanque et al. [17] conducted a study on the behavior of metals during the combustion of leaves and shoots of *Salix* (grown for phytoremediation), describing the type of ash obtained from the use of biomass as an energy source and the level of heavy metal concentration. At the end of their study, the authors concluded that the combustion of biomass obtained from phytoremediation reduces the waste volume, but ash cannot be used as fertilizer in agriculture due to high levels of heavy metals.

The international researches in the last decade deal with developing several techniques and methods of incineration and recovery of heavy metals from industrial waste [18,19], garbage [20] and vegetable waste resulting from agriculture [21,22,23]. Unlike industrial waste and garbage, the biomass obtained from the phytoremediation process has a higher content of organic matter and low ash content, only 5% of the dry matter [24]. Compared with agricultural waste, collected from the fields containing heavy metals within normal levels, biomass resulting from phytoremediation process contains much higher concentrations of heavy metals. Considering these aspects and morphological differences between the species used in phytoremediation, heavy metal recovery methods must be adapted for each type of biomass used (grass species, tree species), according to the concentration and metal mixture.
