Artificial humification of lignin architecture: Top-down and bottom-up approaches

• Published in: Biotechnology advances Vol. 37; no. 8; p. 107416
• Main Authors: Lee, Jeong Gu, Yoon, Ho Young, Cha, Joon-Yung, Kim, Woe-Yeon, Kim, Pil Joo, Jeon, Jong-Rok
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 01.12.2019; Elsevier Science Ltd
• Subjects: Addition polymerization; Animals; Antioxidants; Architecture; carbon; Cellulose; fish feeding; humans; Humic Substances; Humification; Lignin; Lignin - chemistry; Lignin valorization; Lignite; Lignocellulose; markets; Microorganisms; Nutrition; organic fertilizers; Oxidizing agents; Phenols; physicochemical properties; plant growth; plant nutrition; plant residues; Polymer engineering; R&D; Refineries; remediation; Research & development; research and development; Residues; Soil; Soil fertility; Stimulants; Structure-function relationship; Transformations; xenobiotics; Humification; Lignin valorization; Humic substances; Structure-function relationship; Polymer engineering
• ISSN: 0734-9750; 1873-1899; 1873-1899
• DOI: 10.1016/j.biotechadv.2019.107416

Humic substances readily identifiable in the environment are involved in several biotic and abiotic reactions affecting carbon turnover, soil fertility, plant nutrition and stimulation, xenobiotic transformation and microbial respiration. Inspired by natural roles of humic substances, several applications of these substances, including crop stimulants, redox mediators, anti-oxidants, human medicines, environmental remediation and fish feeding, have been developed. The annual market for humic substances has grown rapidly for these reasons and due to eco-conscious features, but there is a limited supply of natural coal-related resources such as lignite and leonardite from which humic substances are extracted in bulk. The structural similarity between humic substances and lignin suggests that lignocellulosic refinery resulting in lignin residues as a by-product could be a potential candidate for a bulk source of humic-like substances, but structural differences between the two polymeric materials indicate that additional transformation procedures allowing lignin architecture to fully mimic commercial humic substances are required. In this review, we introduce the emerging concept of artificial humification of lignin-related materials as a promising strategy for lignin valorization. First, the core structural features of humic substances and the relationship between these features and the physicochemical properties, natural functions and versatile applications of the substances are described. In particular, the mechanism by which humic substances stimulate the growth of plants and hence can improve crop productivity is highlighted. Second, top-down and bottom-up transformation pathways for scalable humification of small lignin-derived phenols, technical lignins and lignin-containing plant residues are described in detail. Finally, future directions are suggested for research and development of artificial lignin humification to achieve alternative ways of producing customized analogues of humic substances. •Artificial humifications of lignin for versatile applications are discussed.•Top-down ways: alkaline oxidative hydrolysis, composting, vermicomposting, Fenton reaction and Mn-based oxidation.•Bottom-up ways: oxidative polymerization and sodium borohydride reduction.•Potential humification methods to be considered: ozonation, thermal degradation and fermentation with specific microbes.