Highly efficient adsorption of direct Scarlet dye using guanidinium-based covalent organic polymer

Ionic covalent organic polymers are promising water pollutant adsorbents with enhanced adsorption potential compared to their neutral counterparts benefiting from both electrostatic attractions and the ion-exchange process. This work deals with the construction of a cationic COP through a facile direct approach and assessment of its performance for the scavenging of Direct Scarlet 4BS (DS-4BS) anionic dye. Many analytical techniques including FT-IR, TGA, BET, XRD, zeta potential, and FE-SEM/EDS were conducted to validate this cationic polymer formation. As results revealed the maximum adsorption capacity (q_max) was obtained 236.4 mg/g under pH = 2, adsorbent quantity = 0.005 g, dye concentration = 250 ppm, and time = 3.5 h. Based on the regression coefficient (R²) values, experimental data were suitably matched with the Langmuir model, indicating monolayer adsorption and the best-fitted kinetics model was pseudo-second-order. Also, according to the calculated adsorption energy (Ea = 4.5 kJ/mol), the dye adsorption mechanism was mainly governed by the physisorption process. Additionally, thermodynamic investigations revealed that according to the negative values of the standard free Gibb's energy (∆G⁰), the adsorption process is spontaneous. Also, the positive value of the standard enthalpy (∆H⁰ = 38.5 kJ/mol) indicated the endothermic nature of this adsorption, which means adsorption capacity increases with the increase in temperature.