Effect of K2CO3 Activation on Hydrochar Properties and Its Application as Support for Lipase Immobilization

Abstract

Fats, oils, and grease (FOG) pose significant environmental challenges but offer valorization potential through biocatalytic hydrolysis. This study evaluates K₂CO₃-activated canola hydrochar as a suitable support for immobilizing crude fungal lipases from Aspergillus niger and Penicillium chrysogenum. Canola hydrochar from hydrothermal liquefaction (300 °C, 60 min) showed optimal properties (lowest oil sorption: 47.9 ± 4.1 wt %). Subsequent activation with 3.0 M K₂CO₃ increased the BET surface area of canola hydrochar from 19.4 ± 4.0 to 98.5 ± 4.0 m²·g^–1, improving support accessibility for enzyme adsorption. Immobilized lipase achieved a protein loading of 422.5 ± 8.4 mg·g^–1, 67.03 ± 2.4% yield, and 34.79 ± 3.1% expressed activity relative to free enzyme (147 ± 14 μmol·g^–1·min^–1 immobilized vs 195 ± 21 μmol·g^–1·min^–1 free). Kinetic analysis of olive oil hydrolysis yielded a K_m of 12.3 ± 1.2 mg·mL^–1 and a V_max of 245 ± 18 μmol·g^–1·min^–1, indicating effective substrate affinity and catalytic performance after immobilization. The biocatalyst demonstrated reasonable reusability (53% activity retention after 5 cycles) and achieved 92.9% FOG hydrolysis. Under identical assay conditions, the waste-derived hydrochar-supported lipase demonstrated comparable performance to selected commercial Candida rugosa lipases, supporting its potential as a cost-effective and sustainable biocatalyst platform for FOG valorization.