Hydrothermal Co-Liquefaction of Drift Macroalgal Biomass and Single Use Plastic Wastes: Optimizing Aqueous Phase Valorization for Enhanced Energy Recovery

Hydrothermal co-liquefaction of drift macroalgal biomass and single use plastics presents a technically viable route for biofuel production, simultaneously addressing marine biomass overgrowth and plastic waste accumulation. Optimized process parameters (340 °C, 75 min, 1:1 feedstock ratio) yielded 41.2% bio crude. Catalytic enhancement of HTL with diatomaceous earth (DE) increased bio crude yield to 47.83%, while nanoporous zinc oxide (ZnO) produced a comparable yield of 48.1%. Furthermore, various aqueous phase valorization (APV) strategies, such as hydrothermal gasification (HTG) and photocatalytic reforming (PCR), were evaluated, with HTG yielding the highest hydrogen production (62.5%) and PCR producing 42.2% hydrogen. Additionally, aqueous phase (AP) recirculation significantly improved the bio crude yield, reaching 51.6% in ZnO-assisted HTL (6 mL/g) and 51.3% in DE-assisted HTL (10 mL/g), while DE HTL+AP achieved the highest carbon (64.37%) and energy recovery (77.00%), demonstrating the effectiveness of aqueous phase (AP) valorization in improving overall energy recovery and resource utilization efficiency. Each aqueous phase valorization (APV) process was assessed individually based on its quantifiable energy output. The findings identified ZnO HTL+HTG as the most efficient strategy, achieving an NER of 1.83, marking an 87.6% improvement over the base HTL process and demonstrating its superior potential for maximizing energy efficiency and bio crude production.