Recent progress in bio-based SiO₂–metal oxide composites for water treatment applications: A review

• Bio-based SiO₂ extracted from agricultural waste enables sustainable water treatment materials. • Bio-based SiO₂-metal oxide composite system enhances adsorption, photocatalytic and antibacterial performance. • Composite structure governs efficiency and stability in pollutant removal. • Green synthesis routes enhance scalability and environmental compatibility. • Future challenges and prospects for real-world water treatment are discussed. Developing green, efficient, low-cost, and sustainable multifunctional water treatment materials has become an important research direction. Bio-based SiO₂ has attracted extensive attention due to its abundant sources, low cost, and high specific surface area. However, its limited functionality makes it difficult to achieve efficient removal, degradation, and antibacterial performance in water treatment. By incorporating metal oxides, the composites exhibit synergistic adsorption, photocatalytic, and antibacterial performance, with removal performance 1.5 to 5 times higher than that of pure SiO₂ and maintain good removal performance across various pollutant systems compared with traditional adsorbents, while enhancing their reusability. The review explores the research and application of bio-based SiO₂-metal oxide composites in water treatment. To this end, the influence of different methods for extracting bio-based SiO₂ from agricultural waste on the structure and properties of the materials is systematically surveyed. The preparation and characterization of bio-based SiO₂-metal oxide composites for structural optimization direction and performance regulation mechanisms are elaborated on heavy metal removal, organic pollutant degradation, and antibacterial purification. Emphasis is placed on structure–performance relationships, synergistic mechanisms, and engineering feasibility. Overall, this review presents a systematic research approach for the combination of metal oxides and bio-based SiO₂ to guide the direction toward sustainable water purification technologies.