Biowaste-based biochar: A new strategy for fermentative bioethanol overproduction via whole-cell immobilization

This work explores the potential use of biochar as a microbial cell carrier enhancing the efficiency of alcoholic fermentations. Olive kernels, vineyard prunings, sewage sludge and seagrass residues were applied as biowaste for biochar production through pyrolysis at two different temperatures (250 °C and 500 °C), while a commercial type of non-biomass char was also employed for benchmarking purposes. Apart from vineyard prunings pyrolyzed at 250 °C, all other carbonaceous materials presented crystalline phases including halite, calcite, sylvite and/or silicon. Moreover, increase in pyrolysis temperature enhanced biochar’s porosity and BET-specific surface area, which reached 41.7 m2 g−1 for VP-based biochar remaining at lower levels (0.15–5.3 m2 g−1) in other specimens tested. Elemental analysis demonstrated reduction in oxygen and increase in the carbon content of biochars produced at elevated temperatures, while biochar from seagrass included residues of chloride (0.3–5.14%). Three major yeasts were immobilized on materials exhibiting the highest surface areas and applied in repeated batch fermentations using Valencia orange peel hydrolyzates as feedstock. The biocatalysts developed using S. cerevisiae and K. marxianus immobilized on vineyard prunings-based biochar exhibited exceptional ethanol productivities as compared to the relevant literature, which reached 7.2 g L−1 h−1 and 7.3 g L−1 h−1 respectively. Although the aforementioned strains improved biofuel production by 36–52% compared to the conventional process, P. kudriavzevii KVMP10 was not efficient following immobilization on biochar. The approach constitutes an innovative method for bioenergy production, demonstrating a novel application of biochar in industrial biotechnology which incorporates important technological advances such as enhanced biofuel production and biomass recycling.