Insight into the Mechanism of Humic Acid’s Dissolution Capacity for Lignin in the Biomass Substrates

The poor solubility of lignin in most solvents causes the biggest challenges in lignin valorization to improve overall biorefinery economics. In this work, humic acid (HA), a natural surfactant, was investigated with respect to its ability to assist in biomass delignification. After obtaining a baseline level of delignification using standard HA, a mechanism of action was proposed in which the lignin removal was driven by hydrophobic interactions. To explore this hypothesis, HA was modified in two ways. One batch was sulfonated to increase HA hydrophilicity, while the other was esterified to increase HA hydrophobicity. Treatment of the same substrates with the modified HA materials revealed that elevated hydrophobicity provided a further increase in the level of delignification. It was found that the solubility of lignin in the HA solution was increased with the enhancement of HA hydrophobicity from 12.2% (S-HA) to 16.8% (E-HA). Therefore, the initial hypothesis was confirmed that HA-derived delignification improvements appear to be a function of hydrophobic interactions. These results should aid further investigation into choosing surfactants (ideally those that are biobased) for utility in biorefinery processes.