Engineered Biochar for Dye Wastewater Treatment: Adsorption Evidence, Mechanistic Validation, and Scale‐Up Gaps

ABSTRACT Dye‐bearing industrial effluents contain chemically diverse colorants whose treatment is complicated by salinity, surfactants, dissolved organic matter, auxiliaries, and fluctuating operating conditions. Engineered biochar is widely investigated as an adsorbent and as a support for catalytic or hybrid treatment; however, reported capacities are often compared across incompatible pH, dose, concentration, temperature, and matrix conditions. This systematic review evaluates evidence of adsorption, mechanism validation, and scale‐up gaps for biochar‐based dye wastewater treatment. Descriptive synthesis shows that most primary treatment evidence still relies on synthetic or simulated aqueous matrices: Real textile or dyeing wastewater occurred in 1/50 (2.0%) treatment records, regeneration or reusability in 6/50 (12.0%), and toxicity‐related testing and leaching/release assessment in 2/50 (4.0%) each; continuous‐flow evidence was not sufficiently reported for quantitative synthesis. Chemical activation, heteroatom doping, metal/oxide incorporation, and biopolymer compositing can improve uptake or selectivity. But the benefits are conditional and may be offset by reagent demand, pore blockage, dopant release, secondary waste, and regeneration losses. Mechanistic claims require convergent evidence from pH‐edge behavior, surface charge, spectroscopy, competitive sorption, mass balance, and transformation‐product analysis; isotherm fit or pseudo‐second‐order kinetics alone cannot prove monolayer adsorption or chemisorption. Machine learning and sustainability claims are promising but require transparent datasets, study‐wise validation, uncertainty analysis, functional units, and benchmarked life‐cycle or techno‐economic boundaries. The review therefore frames engineered biochar not as a universally superior material, but as a condition‐dependent treatment platform requiring reproducible reporting and stage‐gated engineering validation.