Deciphering the underlying mechanism of MOF-808-based abiotic catalysis enhancing biodegradability of waste activated sludge: Insights from the effects on bioconversion of extracellular organic substances into methane.

Extracellular organic substances (EOSs) usually control sludge biodegradability. Metal-organic framework-808 (MOF-808), a catalyst with a high ability of catalytic hydrolysis and proton transfer, has great potential to enhance biodegradability of EOSs. In this study, the underlying mechanism of MOF-808 enhancing the biodegradability of EOSs via abiotic catalysis was investigated. Experimental results showed that protein-like and humic acid-like substances were the main organic components in EOSs, and the MOF-808 enhanced the disintegration of protein-like substances rather than humic acid-like substances. Analyses of the changes in the functional group, the secondary structure of protein-like substances, and the electron transfer of catalytically degraded EOS samples revealed that the MOF-808 enhanced the hydrolysis of EOSs protein-like substances, loosened their secondary structure, and improved the electron transfer in EOSs. Further analyses of the MOF-808 before and after the catalysis reaction revealed that the coordination of Zr sites with protein-like substance-specific active sites (such as ZrN) in the MOF-808 substantially contributed to the high-efficiency catalysis. Biochemical methane potential assays confirmed that MOF-808-based abiotic catalysis promoted the generation efficiency of methane from the EOSs. These findings can elucidate the abiotic catalytic effect of MOF-808 on sludge biodegradability during the anaerobic digestion process.