Combustion outperforms anaerobic digestion for bioenergy recovery from Mediterranean plants used in phytostabilization of metal-contaminated soil

Phytoremediation of soils contaminated with metals and metalloids can be considered a sustainable way to provide these soils a fruitful use, such as the production of energy crops. This is particularly relevant for phytostabilisation, where the plants normally show low metal accumulation. This study assessed the bioenergy potential of the biomass generated in an off-site open air phytostabilisation experiment that used Mediterranean plant species ( Brassica juncea , Cynara cardunculus , Atriplex halimus , Dittrichia viscosa , Retama sphaerocarpa and Salvia rosmarinus ) grown in two metal-contaminated soils (agricultural and mine affected). Plant growth, biomass characteristics, and bioenergy production potential through thermochemical (combustion) and biochemical (anaerobic digestion) conversion of the plant biomass were evaluated. Cynara cardunculus had the greatest concentration of soluble carbohydrates, which resulted in the highest biogas production potential (biomethane potential 357 mL CH 4 g -1 volatile solids) despite its low bioenergy (biogas) yield (41 GJ ha -1 y -1 ). Brassica juncea showed also high biomethane potential (239-250 mL CH 4 g -1 VS) due to its low lignin content, with high biochemical energy yield when grown in the agricultural soil (135 GJ ha -1 y -1 ), which was lower in the highly polluted mine soil due to the limited biomass produced. The higher heating values (HHV) for combustion were very similar for all plant biomasses (15-20 MJ kg -1 ). However, the elevated biomass in mine soil of D. viscosa showed this species as the most efficient for bioenergy yield through thermochemical conversion (310 GJ ha -1 y -1 ). Brassica juncea in the agricultural soil and D. viscosa in the mine soil showed low plant metal concentrations and appeared as the most suitable options for bioenergy production and fossil fuels partial substitution in a phytostabilisation programme. This reinforces the well-documented phytoremediation potential of both species. • Plant biomass from phytostabilisation was found suitable for bioenergy production • Combustion energy was higher than that coming from biogas (anaerobic digestion) • Phytostabilisation-plants derived bioenergy could partially substitute fossil fuels • Best suited plant for phytoremediation-bioenergy varied with soil contamination level • Overall, Brassica juncea and Dittrichia viscosa showed as the most suitable species