Enhancement of the hydrothermal carbonization of pig manure using bio-based surfactants: Product distribution and characteristics, pollutant control, and preliminary mechanisms

Surfactants have shown broad application prospects in enhancing the hydrothermal carbonization (HTC) of biomass. Current research in this field mainly focuses on the screening and optimization of petroleum-based surfactants, and the potential application of sustainable bio-based surfactants in this field remains unexplored. Hence, this research explored the application of common bio-based surfactants in the HTC of pig manure. Two bio-based surfactants, that is, phosphatidylethanolamine (a biosurfactant, PE) and sodium cholate (a green surfactant, SC), showed superior performance in promoting the HTC of pig manure, which raised the yield of hydrochar from 54.48 wt% (no surfactant) to 66.86 wt% (PE) and 66.27 wt% (SC), respectively. The introduction of PE and SC surfactants promoted the carbonization degree and organic matter content of hydrochar, resulting in a higher calorific value (20.20–20.25 MJ/kg versus 16.85 MJ/kg). While SC improved the pyrolysis performance of hydrochar, PE enhanced its combustion performance. The application of PE enhanced the formation of acids and the migration of esters to hydrochar. In the case of SC, enhancing the formation of esters and ketones was the main pathway for promoting the formation of hydrochar. The introduction of PE/SC surfactants also mitigated the enrichment of heavy metals in hydrochar. Furthermore, the adsorption/fixation of PAHs in hydrochar was enhanced. The application of advantageous bio-based surfactants can effectively enhance the HTC of pig manure, and it is worth further optimizing the corresponding processes.