Correlating the chemical properties and bioavailability of dissolved organic matter released from hydrochar of walnut shells

Large amounts of lignocellulosic biomass are discarded, whereas the carbon source of sewage is deficient. This situation greatly impairs the efficiency of wastewater treatment. To address this concern, we evaluate the feasibility of using hydrochar as a potential carbon source by systematically investigating the effects of hydrothermal carbonization (HTC) conditions on the composition, content, and chemical structure of dissolved organic matter (DOM) released from hydrochar. Results show that the most important factor that affects the properties of hydrochar and DOM is temperature, followed by heating rate. Under optimal HTC conditions, the growth of Bacillus subtilis increased by 18.32% in hydrochar aqueous solution in comparison with the 6.64% growth of the untreated biomass group. Excitation emission matrix-parallel factor analysis and UV-vis analyses confirm that the DOM released by hydrochar produced at a low temperature mainly contains protein substances, which promote the growth of microorganisms. The DOM released by hydrochar at a high temperature mainly contains humic substances with an aromatic structure; such substances are toxic to microorganisms. This study demonstrates that hydrochar obtained under optimized conditions can be a potential carbon source of wastewater treatment plants.