Valorization of biomass from plant microbial fuel cells into levulinic acid by using liquid/solid acids and green solvents

Abstract Plant microbial fuel cells are attractive for treating hexavalent chromium contaminated soils while generating renewable electricity. The end-of-service-life plant microbial fuel cells, however, may pose a risk to secondary pollution. In this study, we propose a novel valorization of Chinese pennisetum plant microbial fuel cell waste into levulinic acid using liquid or solid acid catalyst in green solvents, water and/or gamma-valerolactone, under microwave heating. The results showed that the use of 1 M sulfuric acid at 150 °C in 60 min can obtain approximately 15.2 C mol% of levulinic acid. In view of the use of gamma-valerolactone, the addition of H2O showed beneficial effects to improve levulinic acid yield by facilitating cellulose hydration and sugar dehydration-rehydration reactions, for instance, levulinic acid yield increased to 20 C mol% when gamma-valerolactone to water ratio changed from 100/0 v/v to 70/30 v/v. The scanning electron microscope images proved that the cellulose structure was recalcitrant in H2O under the catalysis of Amberlyst 36, while the cellulose structure was altered under gamma-valerolactone to water regardless of the type of catalysts. The X-ray diffraction patterns provided spectroscopic evidence for the enhanced dissolution or hydrolysis of cellulose in a short period of time. The thermal stability of the solid residues after microwave heating at 180 °C was enhanced due to polymerization reaction. The 13C solid-state Nuclear magnetic resonance spectra revealed that remaining gamma-valerolactone and polymerization byproducts were the major residues after the catalytic reaction under pure gamma-valerolactone or at high temperature. The measurement of total chromium and hexavalent chromium distribution confirmed that the metal pre-absorbed in plant biomass all remained in the solid phase, indicating the liquid phase was free of secondary pollution. The proposed valorization plan could innovatively utilize hazardous waste from plant microbial fuel cells to produce value-added chemicals in a sustainable way.