Effect of magnetite particle size on propionate degradation in the propionate-based anaerobic system

The size effect of magnetite (Fe₃O₄) on the degradation of propionate (PA) in the PA-based anaerobic system was investigated. The sequential bench-scale experiments were conducted. Results showed that the effects of different sized magnetite particles on PA degradation varied, and reaction cycles also played a role in substrate removal/degradation. With the increase of reaction cycle, nano-magnetite promoted PA degradation and CH₄ production, which caused faster PA degradation rate (0.997 g/L·d) than the control group (CK) without magnetite (0.834 g/L·d), whereas the groups with micron- and millimeter-sized magnetite had slower PA degradation rates (0.746 and 0.636 g/L·d) than CK group. The particle size or surface characteristics of the magnetite may become the main factor determining the PA degradation rate. Furthermore, the analysis of PA conversion and volatile fatty acids (VFAs) distribution showed the C6-dismutation pathway, which converses PA to butyrate, enhanced by the introduction of magnetite. Microbial community analysis showed that PA was degraded mainly by methyl-malonyl-CoA (MMC) pathway. The relative abundance of Syntrophobacter that catalyze MMC pathway in the group with nano-magnetite were much higher after three reaction cycles at 39 %, as compared to micro-magnetite at 28 %, and millimeter-sized magnetite at 27 %, which contributed to faster degradation of PA. Functional enzyme-encoding genes for the four methanogenesis pathways were identified with reference to KEGG database entries. The methanogenesis pathway using acetate was the most abundant pathway in all groups. The observations have important implications for enhancing the PA removal in PA-inhibited anaerobic digester.