Valorisation of rice straw for lipase immobilisation: toward sustainable treatment of lipid-rich wastewater

This study explores the valorisation of alkali-treated RS as support for lipase immobilisation through physical adsorption, aiming to develop an efficient biocatalyst for treating lipid-rich wastewater. The immobilised lipase was characterised using SEM, TGA, BET, XRD, and FTIR techniques, revealing changes after immobilisation. BET results show an increase in surface area from 1.19 m²/g to 10.95 m²/g following alkali treatment, with a subsequent decrease to 0.35 m²/g after immobilisation. XRD analysis revealed an increase in crystallinity from 69.40% to 83.12% after NaOH treatment, followed by a decrease to 64.83% after lipase immobilisation, indicating effective support modification and enzyme attachment. FTIR analysis revealed a distinct N-H bending vibration corresponding to the amide II band in the immobilised matrix, confirming successful immobilisation of lipase. Kinetic studies revealed an improved performance of the immobilised lipase, with a reduction in Km from 0.438 to 0.330 mM, indicating enhanced substrate affinity, and a 2.18-fold increase in Vmax compared to the free enzyme, reflecting higher catalytic efficiency. The immobilised lipase exhibited superior thermal and pH stability, with storage stability tests showing 59% activity retention after 30 days at 4°C, compared to 19% for the free enzyme. Lipid-rich wastewater treatment using PPL immobilised NaOH-treated RS (lipase loading: 15 mg. g⁻¹ support) achieved 75.60 ± 2.43% lipid degradation, after 48 h at 37°C and 150 rpm, while the free enzyme achieved 16.64 ± 3.33%. These results demonstrate that RS provides a sustainable support for lipase immobilisation, offering an effective strategy for treating lipid-rich wastewater.