A circular approach for the phytostabilization of soils containing potentially toxic elements at different levels of contamination

The management of the plant biomass generated during the phytostabilization of soils contaminated with potentially toxic elements (PTEs) has shown to be crucial for the implementation of these technologies. Its use as a source for bioenergy production is one of the most appealing options for valorization. In this work, 7 different plant species, including perennial bushes, halophytic and herbaceous species and crops, were cultivated combined in pairs of similar characteristics, in two soils (mine affected and agricultural) differently contaminated with PTEs (mainly Pb, > 2000 kg^-1 in both soils, and Zn, ca. 14,000 mg kg^-1 in the mine soil). A novel circular approach for phytostabilization processes was evaluated through the characterization of the ashes obtained in the combustion of the plant biomass, and the sequential extractability of PTEs and nutrients in these ashes, to determine their possible use as amendments or fertilizing materials back into contaminated soil remediation processes. The approach studied improved soil quality (increased microbial biomass related parameters) and reduced toxicity (as for plant germination and Crustacea mortality tests results). Plant ashes showed high PTEs content, indicative of low loss of contaminants during combustion. Water soluble PTEs and P concentrations were found to be very low in all the ash samples, whereas that of K was very high; Cd and Zn were mostly extracted with ammonium citrate (39-94 % of their total concentration), and Pb (and also P) was majorly extracted (43-85 %) with Na-EDTA. This differential extractability may allow the use of these ashes for nutrient or PTE extraction. The ashes could be also used as soil amendment or inorganic fertilizer substitute when coming from plants with low to moderate PTE content, this opening their possibilities of reintroduction in the soil, which would result in a novel and potentially circular soil remediation process.