Response of Fulvic Acid Linking to Redox Characteristics on Methane and Short-Chain Fatty Acids in Anaerobic Digestion of Chicken Manure

Abstract

Fulvic acid (FA) is formed during the bioconversion of organic matter (OM) to biogas during anaerobic digestion (AD) and has a complex structure and redox function. However, the evolutionary mechanisms of FAs during AD and its interactions with acid and methane production have not been sufficiently investigated, especially at different stages of AD. The results showed that FAs were the main components and their contents decreased gradually in different stages of chicken manure (CM) AD; the results of electrochemical reduction experiments showed that the electron donating capacity (EDC) of FAs fluctuated between 9.76 ~ 45.39 μmole-/(g C), the electron accepting capacity (EAC) fluctuated between 2.55 ~ 5.20 μmole-/(g C), and the electron transfer capacity (ETC) showed an upward trend. The UV-vis and FTIR spectroscopy analysis showed that the structure of FAs changed significantly, and the degree of aromatisation of FAs gradually decreased. In addition, correlation analysis and structural equation modelling showed that EDC was affected by the structures of C-O stretch2, C-H bend1, C-H bend4 and N-H bend during anaerobic digestion of CM, and the molecular weights of the FAs also had a direct effect on the EAC. The EDC of the FAs had an inhibitory effect on the production of propionic acid, whereas the EAC promoted the production of methane. This study provides a theoretical basis for the structural and functional evolution of FAs in CM AD on the mechanism of methane production.

Keywords: anaerobic digestion, Fulvic acids, Chemical structures, Electron transfer capability, Chicken manure