Asymmetric post-equilibrium displacement between Congo Red and Alizarin Red S reveals secondary-release risk during competitive adsorption.

Competitive adsorption in multicomponent wastewater can induce post-equilibrium displacement and remobilization of previously adsorbed pollutants. Here, CCS@NH2-MWCNTs was used as a model composite interface to investigate the competitive adsorption and displacement of Congo Red (CR) and Alizarin Red S (ARS). CR exhibited stronger competitive retention than ARS: CR displaced 59.90% and 64.87% of pre-adsorbed ARS at 30 and 40 °C, respectively, whereas reverse displacement remained below 1% at 30 °C and no measurable CR release was detected at 40 °C. Preferential CR retention was maintained over an initial pH range of 4-10. Spectroscopic characterization and DFT/IGMH analyses suggest that charge distribution, extended π-conjugation, multi-site interactions, and interfacial accessibility collectively contribute to the observed retention hierarchy. These findings show that apparent adsorption equilibrium does not necessarily ensure persistent contaminant immobilization and highlight post-equilibrium displacement as a potential secondary-release mechanism relevant to multicomponent wastewater treatment.