Interpretation of the adsorption mechanism of Reactive Black 5 and Ponceau 4R dyes on chitosan/polyamide nanofibers via advanced statistical physics model

Herein, a chitosan/polyamide nanofibers (CPN)-based adsorbent is employed to interpret the adsorption mechanism of two dyes, namely Reactive Black 5 (RB5) and Ponceau 4R (P4R), at different temperatures. Based on experimental results, the adsorption isotherms of RB5 and P4R dyes indicate that the separation process tends to a saturation plateau at high concentration, also suggesting that the dyes adsorption process occurs with the formation of a finite number of layers. To provide a reasonable explanation of the dyes adsorption mechanisms, three statistical physics models are used and discussed. The advanced modelling analysis reveals that the adsorption of both dyes occurs by the formation of two different adsorbed layers on CPN surface. Based on the application of the best statistical physics adsorption model, it was deduced that both dyes were anchored with horizontal and non-horizontal positions on CPN surface depending on temperature, which was concluded by investigation the captured number of dye molecules per CPN adsorbent site during the adsorption process. Based on these theoretical results, it was noted that the adsorption capacities at saturation followed this sequence: Q (P4R) > Q (RB5) for all investigated temperatures (i.e., 222.4, 261.3, 352.8, 482.2 mg/g for P4R and 198.6, 267.7, 288.2, 352.5 mg/g for RB5), indicating that CPN adsorbent was more effective for the P4R dye removal. The adsorption energies are studied to explain the difference in the interactions between dyes and adsorbent surface (P4R-CPN, RB5-CPN), and between these dyes (P4R-P4R, RB5-RB5), indicating that the removal of these adsorbates was based on a physisorption mechanism where a desorption process is possible. Comparing the estimated parameters of the best statistical physics model, it was concluded that both steric and energetic factors are mainly responsible for the adsorption of P4R and RB5 dyes on CPN.