In Situ Enhancement of Alcohol Production by Fe ₃ O ₄ Nanoparticle-Immobilized Yeast to Promote Methanogenesis during Anaerobic Digestion

Pretreating organic wastes with yeast to produce alcohol has been widely recognized as an effective method to stimulate digester communities to continuously perform direct interspecies electron transfer (DIET) during anaerobic digestion. However, the increase in the costs of additional facilities (e.g., pretreatment tank) and the continuous addition of yeast limit its practical applications. In this study, a strategy of in situ enhancement of alcohol production by Candida krusei (CK) immobilized with Fe3O4 nanoparticles (nFe3O4) during anaerobic digestion was proposed. The results showed that CK cells were immobilized with nFe3O4 via physisorption and chemical bonding. Investigations on extracellular polymers via the three-dimensional excitation–emission matrix showed that nFe3O4 promoted the secretion of tryptophan proteins, fulvic acids, and humic acids by CK, indicating an improvement in the cell stability and metabolic activity. With CK-nFe3O4, the efficiencies of alcohol production reached 0.92 mol/molglucose, which was 46.0 ± 1.5% higher than that with only CK (0.63 mol/molglucose). Meanwhile, with CK-nFe3O4, the catalytic periods of cells reached four cycles, which was two-fold that of with only CK (two cycles). Further analysis of diffusion showed that nFe3O4 improved mass transfer, alleviating substrate/product inhibition during alcohol-producing fermentation. Based on these advantages, the cumulative methane production with CK-nFe3O4 was increased by 56.3 and 34.6%, respectively, compared to that without CK-nFe3O4 and with only CK during anaerobic digestion.