Efficient adsorption of anionic azo dyes on porous heterostructured MXene/biomass activated carbon composites: Experiments, characterization, and theoretical analysis via advanced statistical physics models

In this paper, a porous adsorbent synthesized from biomass activated carbon and MXene, named as CMAC composite, was utilized for the removal of three anionic azo dyes, allure red (AR), congo red (CR) and sunset yellow (SY). The formation of this heterostructure adsorbent was achieved by electrostatic self–assembly of negatively charged 2D MXene nanosheets and activated carbon with the assistance of a cationic surfactant (CTAB) solution. This method impeded the re–stacking of MXene nanosheets, effectively reduced the multilayer plate structure of MXene and enlarged the layer spacing, thus promoting the exposure of available active sites to further enhance the adsorption performance. The CMAC was physicochemically characterized via different analytical techniques and the dye adsorption isotherms at three temperatures were quantified. The experimental results showed that CMAC displayed excellent adsorption efficiency for CR with adsorption capacities above 1400 mg/g. The adsorption of the dyes coincided with the Langmuir model, pseudo–second order kinetic model and intraparticle diffusion model. A multilayer statistical physical model was employed to explain the adsorption mechanism between the tested dyes and CMAC. The simulation results provided the possible adsorption directions of the dye molecules on the adsorbent surface under different operating conditions, and the decrease of the active sites density DM indicated that the aggregation of dye molecules existed only when CMAC adsorbed SY. The adsorption energy calculations showed that the adsorption of AR and CR by CMAC was heat–absorbing and the adsorption of SY was exothermic. The adsorption mechanism can be attributed to the synergistic effect of physical adsorption, hydrogen bonding and electrostatic interactions.