A comprehensive review on hydrothermal humic acids: synthesis, characterization, applications, and perspectives

Abstract

Humic acids (HAs) are important components of soil organic matter, playing a critical role in maintaining soil moisture and fertility. Due to their unique chemical properties, including redox activity, complexation ability, biological activity, and buffering capacity, HAs have been widely applied in various fields such as soil improvement, environmental remediation, and medicine in recent years. The artificial synthesis of HAs is the most promising strategy to satisfy the substantial market demand. Compared with composting technology, hydrothermal humification can rapidly convert biomass waste (BW) into hydrothermal humic acids (HHAs) without relying on microbial processes, thus avoiding associated microbial risks. This paper first reviews the various hydrothermal synthesis methods and mechanisms of HHAs from BW reported in recent years. Secondly, a comprehensive analysis of the characterization techniques for revealing the structural characteristics and synthesis mechanisms of HHAs is summarized. Finally, the applications of HHAs in soil improvement and carbon sequestration, crop growth, and environmental management are reviewed. Based on the above discussion, this paper analyzes the current technical challenges in BW hydrothermal technology and proposes future development prospects. The review aims to provide valuable insights into the current research on the synthesis and application of HHAs.