Microwave-assisted remediation of PAHs-contaminated soil using biochar: Process optimization and cost-benefit...

Microwave thermal desorption represents a promising technique for treating polycyclic aromatic hydrocarbons (PAHs)-contaminated soils, but its widespread application is constrained by soil's inherently low microwave absorption. To address this limitation, we propose a biochar-enhanced microwave (BEM) process and systematically evaluate its efficacy. Biochar characterization revealed a well-developed pore structure adsorption capacity for phenanthrene (303.10 mg/g) and pyrene (164.49 mg/g), surpassing that of molecular sieves. Through the multi-factor orthogonal and single-factor experiments, optimal conditions were identified, including 600 W microwave power, 10 min remediation duration, 10% moisture content, and biochar dosage of 5%. Under these conditions, BEM achieved removal efficiencies of 77.33% for phenanthrene and 74.58% for pyrene, with bioavailability removal rates reaching 90.48% and 86.47%, respectively. Mechanistic analysis employing mass balance revealed distinct removal pathways governed by PAH physicochemical properties: phenanthrene removal was dominated by thermal volatilization (86.68%) due to its lower molecular weight and higher volatility, whereas pyrene removal was mainly attributed to degradation (66.73%), favored by its retention in the soil matrix. Vector network analysis demonstrated that biochar improves soil electromagnetic properties, facilitating efficient microwave-to-thermal energy conversion and generating "hotspot" that accelerate PAHs volatilization and degradation. Life cycle assessment and techno-economic analysis confirmed the practical benefits of BEM, revealing substantial cost reduction (10.25-47.66% per ton) and decreased CO₂ emissions (62.78-78.34%) compared to alternative technologies. This study provides a theoretical basis for advancing microwave-based remediation technologies, offering an environmentally sustainable approach for PAH-contaminated soil treatment. This approach aligns with green remediation principles under Dual-Carbon strategy of China.