Biochar-Supported Photocatalysts for the Removal of Emerging Organic Pollutants from Water: Mechanisms, Challenges, and Future Perspectives

Semiconductor-based photocatalysts have gained substantial research interest for water treatment applications owing to their economic feasibility and practical applicability, excellent optical properties, and environmental compatibility. Nevertheless, the practical applicability of semiconductor photocatalysts is often restricted by rapid electron–hole recombination and inefficient solar-energy harvesting, which arise from the predominant UV-light activation required by wide-band-gap semiconductors. To address these limitations, various modification strategies, including elemental doping, heterojunction construction, and the incorporation of carbon-based support materials, have been extensively investigated. Among these materials, biochar has emerged as a low-cost, eco-friendly, and sustainable support owing to its high porosity, large specific surface area, and abundant surface functional groups. The integration of biochar with semiconductor photocatalysts enhances pollutant adsorption, facilitates charge separation and transfer, suppresses electron–hole recombination, and improves visible-light utilization, thereby significantly enhancing photocatalytic performance. This review presents a comprehensive overview of photocatalytic degradation mechanisms and the diverse semiconductor materials employed for environmental remediation, with particular emphasis on biochar-based photocatalytic systems. The fundamental science of biochar is critically discussed, including feedstock selection, production technologies, factors influencing biochar properties, and strategies for synthesizing biochar-supported photocatalysts. Particular attention is devoted to the multifunctional role of biochar in promoting photocatalytic activity through enhanced electron transport, charge separation, electron storage, and increased surface active sites. Furthermore, recent applications of biochar-supported photocatalysts in the degradation of synthetic dyes, antibiotics, and petroleum-derived pollutants are critically evaluated. Finally, the review highlights the key operational parameters governing photocatalytic efficiency, identifies current challenges and knowledge gaps, and outlines future research directions toward the design of efficient, stable, and scalable biochar-based photocatalytic systems for sustainable water purification.