Valorization of mixed shellfish waste into a ZnO-CaO hybrid catalyst for sustainable biodiesel production

This work presents a green bioprocessing route for transforming shellfish waste into viable catalysts for sustainable biodiesel production. It focuses on the valorization of marine bioresources by employing mixed shells from the local clam Anadara granosa (known as ' kerang' ) and the shrimp Penaeus merguiensis —abundant seafood industry wastes—as sustainable and low-cost sources of calcium carbonate for the synthesis of calcium oxide (CaO). This was used to create a ZnO-CaO heterogeneous catalyst for biodiesel production from waste cooking oil (WCO) via transesterification. The catalyst, synthesized via calcination at 1000°C, was characterized by SEM and FTIR, confirming a porous surface with active chemical groups for catalysis. The process was optimized by varying parameters including shell ratio, ZnO-to-CaO molar ratio, methanol-to-oil ratio, temperature, and time. A maximum biodiesel yield of 95.34% ± 1.15% was achieved under optimal conditions: a 75:25 shrimp: kerang shell blend, ZnO:CaO = 4:1, methanol:oil = 20:1, at 65°C for 2 h. GC-MS confirmed the fatty acid methyl ester (FAME) profile, and the fuel met ASTM D6751 standards, with a density of 0.7932 g/cm³ ± 0.0013 g/cm³ and viscosity of 2.37 mm²/s ± 0.35 mm²/s. The catalyst also demonstrated reusability, maintaining over 75% yield for three cycles. This work demonstrates the viability of waste-derived ZnO-CaO catalysts as cost-effective, environmentally friendly alternatives for sustainable biodiesel production.