Enhanced hydrothermal humification of lignocellulosic biomass: Engineering the reaction pathway and economic evaluation

Hydrothermal humification provides a rapid and selective strategy for synthesizing artificial humic acid (HA) from lignocellulosic biomass. However, conventional single-stage processes are often limited by low product yields and incomplete biomass conversion. Here, we engineer the conversion pathway by adopting a sequential acid-alkaline hydrothermal process that promotes staged transformation of biomass components. The method achieved a significantly enhanced HA yield of 34.4 ± 1.7 %, which corresponded to a 2.6–17.4-fold increase over traditional approaches. The resulting HA exhibited higher quality compared with conventional hydrothermal HA and even natural HA, as reflected by its high degree of aromaticity (C-C sp 2 content of 70.2 %) and enhanced redox functionality. Mechanistic analyses (UHPLC-ESI-MS, XPS) showed that the acid stage drives dehydration/condensation of carbohydrate derivatives with lignin fragments to form furan-rich humin-lignin complexes, whereas the alkaline stage induces saponification and nucleophilic substitution, depolymerizing these complexes and releasing phenolic, furanic, and cyclopentanone intermediates. These reactive species polymerize and undergo colloidal self-assembly to reconstruct a highly aromatic HA carbon skeleton. A techno-economic analysis indicated the industrial feasibility of the process, with a projected output of 10.5 tons/day and an estimated daily profit of 118,071 CNY. Overall, this work demonstrated the technical and economic potential of a rationally engineered two-stage hydrothermal pathway for sustainable, large-scale HA production.