Rapid humification of biomass via hydrothermal conversion: a comprehensive review

Abstract

Humic acid (HA), a crucial substance for maintaining soil fertility and health, plays a vital role in sustainable agricultural development and environmental remediation. Hydrothermal humification (HTH) offers the advantage of producing HA analogues from biomass in a significantly shorter timeframe compared with natural processes, thereby enhancing carbon efficiency. This approach aligns with green chemistry principles by promoting the sustainable utilization of resources while minimizing environmental impacts. However, research on the hydrothermal production of HA is still in its early stages, with the underlying conditions, influencing factors, and conversion mechanisms remaining unclear. Furthermore, the potential applications of hydrothermal HA are not yet fully understood. Drawing from nearly a decade of research, this article addresses the mechanism of hydrothermal conversion of biomass into HA and discusses the impacts of diverse HTH operating parameters such as reaction time, biomass composition, reaction solvent, and reaction temperature on the humification process. Given the current lack of research on the applications of hydrothermal HA, we demonstrated the potential applications and challenges of hydrothermal HA by exploring the use of HA from various other sources in diverse scenarios, including agriculture, environmental protection, functional material preparation and animal husbandry. Furthermore, the challenges and research directions for the commercial application of hydrothermal HA are discussed, aiming to provide a reference for studies on HA derived from biomass via hydrothermal conversion.