Advances in the Adsorptive Removal of Alizarin Red S from Wastewater: Adsorbents, Mechanisms, Influencing Factors, and Environmental Perspectives

Alizarin Red S (ARS) is a persistent anthraquinone dye whose discharge into aquatic environments represents an important concern due to its chemical stability, intense coloration, and potential ecological impact. Adsorption has emerged as one of the most practical approaches for its removal from aqueous media because of its operational simplicity, adaptability, and compatibility with low-cost and regenerable materials. This review critically examines recent progress in the adsorption of Alizarin Red S using a broad spectrum of adsorbents, including biomass-derived materials, activated carbons, carbon nanotubes, graphene-based systems, polymeric sorbents, metal oxides, metal–organic frameworks, hydrogels, magnetic composites, and hybrid nanostructures. Particular attention is given to the relationship between adsorbent composition, surface chemistry, porosity, functional groups, and adsorption performance. The effects of pH, adsorbent dosage, initial dye concentration, contact time, temperature, and coexisting ions are discussed as key factors controlling dye uptake, equilibrium behavior, and removal efficiency. The reviewed studies indicate that ARS adsorption is commonly governed by electrostatic attraction; hydrogen bonding; π–π interactions; surface complexation; pore diffusion; and, in some systems, dye–metal complex formation. Langmuir and Freundlich isotherms, together with pseudo-second-order kinetics, are the most frequently reported models. Despite significant laboratory-scale advances, challenges remain regarding adsorbent regeneration, selectivity in complex matrices, long-term stability, cost-effective synthesis, and scale-up. Future research should prioritize real wastewater validation, multicomponent systems, circular adsorbent design, computational modeling, machine learning-assisted optimization, and life cycle assessment to support the transition of ARS adsorption technologies toward practical wastewater treatment applications.