Laccase modified fulvic acid-like substance from paper spent cooking liquor: Chemical structure optimization, functional group reconstruction and bioactivity improvement

Abstract

Identifying a recovery technology for paper spent cooking liquor and converting it into useful material is important for the sustainable development of paper industry. Previous research reported that paper spent cooking liquor-derived fulvic acid-like substances (molecular weight < 5 kDa, PFA) promotes rice growth under salt stress but not as well as natural fulvic acid, which restricts its application as a fertilizer synergist. The ability of PFA to promote rice growth was positively associated with functional groups content. Laccase was used to modify the chemical structure of PFA to further improve its bioactivity. Results showed that laccase modification increased carboxyl and phenolic group contents through oxidation and demethylation with 60 min as optimal reaction time. Under this condition, carboxyl and phenolic group content were increased by 33 %-68.9 % and 19.1 %-208.4 %, respectively. In addition, laccase lowered the molecular weight of PFA (the fraction of molecular weight < 1 kDa increased by 21 %-35.76 %), promoted the degradation of lignin, and increased the relative abundance of tannins (containing phenolic compounds) by 42.89 %-89.41 %. Moreover, laccase-modified PFA (PFA-Ms) promoted rice growth under salt stress than that of PFA. Correlation analysis verified that the promoting effect of PFA-Ms on the elongation of rice roots was associated with carboxylic-C and tannin contents. The antioxidant and root growth-promoting functions of the phenolic and carboxyl groups were confirmed by experiments using phenol and acetic acid, respectively. This study provides an attractive perspective for the sustainable utilization of paper spent cooking liquor as a fertilizer synergist in agriculture.