Latest development in the fabrication and use of lignin-derived humic acid

Abstract

Humic substances (HS) are originated from naturally decaying biomass. The main products of HS are humic acids, fulvic acids, and humins. HS are extracted from natural origins (e.g., coals, lignite, forest, and river sediments). However, the production of HS from these resources is not environmentally friendly, potentially impacting ecological systems. Earlier theories claimed that the HS might be transformed from lignin by enzymatic or aerobic oxidation. On the other hand, lignin is a by-product of pulp and paper production processes and is available commercially. However, it is still under-utilized. To address the challenges of producing environmentally friendly HS and accommodating lignin in valorized processes, the production of lignin-derived HS has attracted attention. Currently, several chemical modification pathways can be followed to convert lignin into HS-like materials, such as alkaline aerobic oxidation, alkaline oxidative digestion, and oxidative ammonolysis of lignin. This review paper discusses the fundamental aspects of lignin transformation to HS comprehensively. The applications of natural HS and lignin-derived HS in various fields, such as soil enrichment, fertilizers, wastewater treatment, water decontamination, and medicines, were comprehensively discussed. Furthermore, the current challenges associated with the production and use of HS from lignin were described.

Keywords: Fertilizer; Humic acid; Humification; Lignin; Oxidation.

Fig. 1

Polyphenol theory of HS formation from biomass adapted and redrawn from [67]

Fig. 2

Reaction scheme for natural humification adapted and redrawn from [68]

Fig. 3

Chemical structures of Humic acid (HA) and fulvic acid (FA); adapted and redrawn from [67]

Fig. 4

A model structure of lignin and common lignin linkages; adapted and modified from [125]

Fig. 5

A schematic flow diagram of alkaline aerobic oxidation for lignohumate production from lignin

Fig. 6

Reaction pathways for the alkaline aerobic oxidation of lignin adapted from [144, 148, 155, 156]. Route A: degradation (simplification). Route B: undesired coupling)

Fig. 7

A schematic flow diagram of alkaline oxidative digestion for lignohumate production from lignin

Fig. 8

A schematic flow diagram of Fenton reagent-based oxidation for lignohumate production form lignin; RT room temperature

Fig. 9

A schematic flow diagram of oxidative ammonolysis for N-enriched lignohumate production from lignin

Fig. 10

Model reaction scheme for the oxidative ammonolysis of lignin; adapted and redrawn from [139]

Fig. 11

A schematic representation of mineral transportation, soil conditioning, and water retention capabilities of ALH; adapted and modified from [165, 175]