Study on the recycling strategy of wastewater algae by extracting bio-oil from Dunaliella salina and combining it with pyrolysis biochar

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Background The sustainable utilization of Dunaliella salina biomass for biochar and bio-oil production offers a promising approach for pollutant removal and biofuel generation. This study aimed to optimize pyrolysis conditions and evaluate the physicochemical properties of biochar and bio-oil derived from D. salina under real wastewater treatment conditions. Methods D. salina was cultivated in wastewater under controlled aeration (2 L/min) and continuous illumination (100 µmol photons m−2 s−1) for 21 days. Biomass was harvested via centrifugation, dried at 60 °C, and subjected to pyrolysis at temperatures ranging from 400 °C to 900 °C under a nitrogen atmosphere. Biochar yield was determined gravimetrically, and adsorption efficiency was assessed by treating metal-contaminated water with biochar. Bio-oil composition was analysed using GC-MS and NMR spectroscopy, while biochar structural properties were characterized via FE-SEM, XPS, and FTIR. Gas chromatography quantified gas composition, and a life-cycle assessment evaluated environmental impacts. Results Biochar yield decreased from 48.6% at 400 °C to 20.8% at 900 °C, with a corresponding decline in activation energy from 162.3 to 121.9 kJ/mol. DTG analysis showed peak decomposition temperatures shifting from 325 °C to 415 °C, indicating progressive thermal degradation. Bio-oil yield peaked at 45.3% at 550 °C, with dominant compounds including 29.4% aromatics, 22.5% phenols, and 18.3% ketones. Biochar adsorption efficiency reached 95.0% for Pb2+, 96.0% for Cd2+, and 90.0% for As³+, with a maximum surface area of 600 m2/g at 650 °C. Life-cycle analysis indicated a 60.9% reduction in GWP and a 73.3% decrease in fossil resource depletion for biochar compared to bio-oil production. Conclusion Biochar and bio-oil from D. salina demonstrated high adsorption efficiency and fuel potential, supporting their application in environmental remediation and sustainable energy production.