Oxygen-promoted low-temperature hydrothermal conversion of Chinese medicinal herbal residues into humic acids

Abstract The synthesis of value-added humic acids from biomass and industrial wastes, such as traditional Chinese medicinal herbal residues (CMHR), represents a sustainable approach for mitigating the adverse environmental impacts associated with these materials. Among the various methods for converting CMHR, hydrothermal technology is gaining increased attention due to its high conversion efficiency; however, this approach is constrained by relatively high temperature requirements. Here, we propose a low-temperature method for the hydrothermal conversion of CMHR into mineral-like artificial humic acids (HA a ). Utilizing a temperature of 150 °C in the presence of added oxygen, we achieved a yield of humic acids of 31.25%. Time-resolved Fourier-transform infrared spectroscopy showed the HA a was comprised of carbonyl functionalities and aromatic structural characteristics that closely resembled those of natural humic acids. Investigations by 13 C NMR and XPS revealed that oxygen injection facilitated the oxidation of carbon species and formation of carboxylic acid groups to promote the stabilization of nitrogen functionalities through the Maillard reaction. These results show that CMHR can be utilized as a feedstock for humic acid synthesis reactions that leverage oxygen to enhance hydrothermal humification. The hydrothermal performance of this approach is comparable to the highest yields of HA a reported using high temperatures (> 180 °C) and long reaction times (> 7 h), demonstrating that this method can be used for the sustainable production of high-quality artificial humic analogs.