KOH catalyzed oxidation of kraft lignin to produce green fertilizer

Abstract

Due to the population growth, there is an urgent need for sustainable technologies and products to address the imbalance between the availability of arable land and growing food needs. Increasing crop productivity in a cost-efficient and environmentally-friendly pathway is one approach to address this issue. In this study, lignin, a low-cost and underutilized by-product of the pulping industry, was converted to a fertilizer via oxidation in the presence of KOH. The oxidation of lignin in water (15 wt%) under the conditions of 195 °C temperature, 300 psi pressure, and KOH dosage of 30 wt% (based on dried lignin) generated water-soluble lignin enriched with carboxylate groups. The X-ray photoelectron spectroscopy (XPS) and proton nuclear magnetic resonance (1H NMR) analyses confirmed the introduction of carboxylate groups to lignin, while 31P NMR and heteronuclear single quantum coherence (HSQC) NMR studies confirmed the alterations in the aliphatic and aromatic structures of lignin. The fertilizing effects of oxidized lignin were investigated on Zea mays (maize) plants. The results confirmed that the average length and dry weight of the plants grown in the presence of oxidized kraft lignin were 27% and 92% greater than those produced without lignin, and they were 12% and 81% higher than those grown in the presence of humic acid, respectively. After 30 days, the plants grown in the presence of oxidized kraft lignin contained 14% and 32% more chlorophyll than those generated in the presence of humic acid and control samples, respectively. Finally, the ash content analysis of the plants shows that applying oxidized kraft lignin as a fertilizer can reduce the ash content and increase the organic content of plants. These results confirmed that the oxidation of kraft lignin in the presence of KOH could be a strategy to induce a green fertilizer for crop cultivation.