Biosilica from Agricultural Residues for Hazardous Contaminant Removal and Soil Remediation: Synthesis Routes, Structure–Property Relationships, and Environmental Relevance

The conversion of agricultural residues into functional silica materials offers opportunities to combine waste valorization with environmental remediation. This review critically examines biosilica production from rice husk and straw, corn, sugarcane, wheat, and selected fruit and potato wastes, focusing on sol–gel processing, hydrothermal synthesis, and acid leaching. To address feedstock heterogeneity, reported silicon and silica contents are distinguished according to their analytical basis, including biomass and ash fractions. Synthesis routes are compared using available quantitative indicators, including silica yield, purity, specific surface area, and processing conditions, while accounting for differences in reporting practices and incomplete datasets. Particular attention is given to the relationships between precursor composition, synthesis parameters, surface chemistry, and contaminant-removal performance. The analysis highlights the importance of accessible surface sites, functional groups, and contaminant speciation, demonstrating that specific surface area alone cannot predict adsorption efficiency. Although biosilica shows potential for water treatment and soil remediation, practical implementation remains constrained by feedstock variability, limited testing under realistic conditions, regeneration challenges, and uncertainties regarding long-term safety. The review identifies key research priorities for developing reproducible, economically viable, and environmentally sustainable biosilica technologies.