Hydrothermal synthesis of similar mineral-sourced humic acid from food waste and the role of protein

Food waste is a challenging biomass resource due to its high moisture content, low calorific value, and complex composition. Natural humification of animal and plant residues is highly related to microorganism activity, but natural hydrothermal conditions are also speculated to play a significant role. In this work, a novel method for the conversion of food waste into artificial humic acid (HAa) under hydrothermal conditions is proposed. The results revealed that an optimum HAa yield of 43.5% from food waste was successfully obtained at 215 °C for only 1 h. Detailed analyses, including elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), and Fourier transform infrared (FT-IR) spectroscopy, showed that the produced HAa had similar structures and compositions with natural HA extracted from minerals. Moreover, the proteins contained in the food waste significantly promoted HA formation through the reaction of saccharides with amino acids, in which Maillard-like reactions were the key steps. These results not only provide experimental evidence for verifying the role of hydrothermal reactions in transforming food waste into humic acid but also provide insight into effective resource utilization of food waste.