Assessment of the potential of sunflower grown in metal-contaminated soils for production of biofuels

Environmental biotechnology needs solutions that are associated with a low budget and cleaner remediation, and which are connected to resources and energetic valorization, to be able to encourage a circular bioeconomy. A prospective resolution for heavy-metal-contaminated soils is the application of phytoremediation approaches merged with bioenergy generation using the resulting biomass. Sunflower (Helianthus annuus) has been studied as a feedstock for biodiesel generation, and appears to be very attractive for biogas and bioethanol production. The current study reports an innovative energetic valorization approach of H. annuus biomass derived from the application of a phytoremediation strategy devised to remove Zn and Cd from an industrially contaminated soil (599 mg Zn kg−1 and 1.2 mg Cd kg−1)—and its comparison to the analysis of the same energetic valorization pathway for sunflower plants growing in an agricultural non-contaminated soil. After plant harvesting, bioethanol was produced from the aboveground tissues, and applied in the transesterification of the oil obtained through seed extraction for the generation of biodiesel. Also, biogas production was assessed through the root’s biomass anaerobic digestion. Similar yields of oil extraction—0.32 and 0.28 mL g−1 DW—were obtained when using seeds from H. annuus cultured in contaminated and non-contaminated soils, respectively. The production yield of bioethanol was superior using biomass from the agricultural non-contaminated soil (0.29 mL g−1 DW) when compared to the industrial metal-contaminated soil (0.20 mL g−1 DW). Zinc was measured in minor levels in bioethanol and oil (ca. 1.1 and 1.8 mg mL−1, correspondingly) resulting from the biomass cultivated in the industrialized soil, whereas Cd was not detected. The production yield of biogas was superior when using root biomass from H. annuus cultivated in agricultural non-contaminated soil (VS max. ca. 104 mL g−1) when compared to the one deriving from the industrial contaminated soil (VS max ca. 85 mL g−1). Generally, results demonstrate that substantial production yields of the tested biofuels were attained from biomass resulting from phytoremediation, corroborating this integrated original approach as a valuable alternative for the phytoremediation of HM-polluted soils and as an important strategy for plant biomass valorization.