Biochar for Organic Pollutant Removal from Water and Wastewater: From Preparation to Removal Mechanisms—A Review

Organic contaminants such as dyes, phenols, pesticides, antibiotics, pharmaceuticals and personal care products (PPCPs), and endocrine-disrupting chemicals (EDCs) are widespread in water and wastewater and are highly harmful to human health and the environment. Biochar, a solid carbon material derived from waste biomass under limited-oxygen conditions, is a promising low-cost sustainable adsorbent for their removal. This review critically integrates recent advances in biochar preparation—feedstock selection, thermochemical conversion routes, activation, and modification—with the physicochemical properties that govern performance. Removal mechanisms are systematically analyzed, spanning adsorption (pore filling, hydrophobic partitioning, π–π electron donor–acceptor interactions, hydrogen bonding, electrostatic attraction, cation bridging) and catalytic degradation mediated by persistent free radicals, persulfate/peroxymonosulfate activation, and photocatalysis. Key influencing factors (solution pH, temperature, coexisting ions, natural organic matter, dosage, and particle size), regeneration, and engineering applications are evaluated. Current challenges—heterogeneity and standardization, safety of biochar-borne hazardous constituents, performance in real matrices, and spent material management—and future directions, including machine-learning-assisted design and integrated treatment trains, are identified. This review aims to guide the rational design of biochar-based materials toward practical water and wastewater treatment.