Engineering artificial humic substances derived from organic wastes: A comprehensive review of pathways, technologies and sustainability challenges

The increasing accumulation of organic wastes, together with the progressive degradation of soils and depletion of humic substances (HS), represents one of the major environmental challenges facing modern agriculture. Natural humification processes, which require hundreds to thousands of years, are insufficient to meet the growing demand for soil restoration and sustainable production. Therefore, the development of efficient and sustainable technologies for producing artificial humic substances (AHS) has become an important research priority. This review summarizes current knowledge and engineering strategies for artificial humification (AH), emphasizing the mechanistic roles of lignin, cellulose, and hemicellulose as key precursors in HS formation. Bio-assisted, chemical, and thermochemical pathways for AHS generation are discussed, including hydrothermal treatments, wet and dry torrefaction, pyrolysis, oxidation, hydrolysis, Maillard and Fenton reactions, modified composting, anaerobic fermentation, and enzymatic processes. Special attention is focused on emerging approaches involving low-cost and “green” catalysts and hybrid methods that integrate multiple technologies. Comparative and critical analysis reveals substantial differences among reported methods in terms of efficiency, substrate compatibility, and product characteristics. Current studies indicate significant economic, technical, and environmental limitations. These challenges primarily include low process yields, insufficient product stability and functionality, high energy requirements, the use of aggressive chemical reagents, the potential formation of toxic by-products, the lack of standardized criteria for product quality assessment, and limited scalability of certain approaches. Despite these challenges, AH presents significant future potential. Strategies such as biomass pretreatment, the use of green additives, and the integration of catalytic and biotechnological processes may represent next-generation strategies for the high-efficiency, economically viable, and environmentally friendly conversion of organic wastes into valuable AHS.