Adsorptive removal of Congo red dye from industrial effluent using cotton calyx iron oxide (CC-Fe3O4) composite

Environmental pollution is an emerging issue in the areas of South Asia and the burning of crop residues is one of the major contributors to smog/pollutants production. In recent work, the residues of cotton crop, i.e., cotton calyx (CC), have been converted into a valuable and eco-friendly adsorbent at zero cost for the refining of polluted waters. Furthermore, cotton calyx composite was synthesized with iron oxide (CC-Fe₃O₄) to improve its sorption potential for the mitigation of selected pollutants, i.e., Congo red (CR) dye. By using FTIR, SEM, TGA, and XRD, the newly synthesized biosorbents were characterized. SEM–EDX and FTIR analyses revealed that both biosorbents (CC and CC-Fe₃O₄) have a porous surface along with various functional groups, which is an indication of an ideal adsorbent for the sorptive removal of pollutants like dyes. The effect of the operating parameters (dye concentration, adsorbent dosage, temperature, contact time, and pH of the dye solution) on the sorption efficacy was studied to identify optimal conditions. The highest percentage of CR removal (99%) was achieved in an acidic medium using 0.6 g CC-Fe₃O₄ composite in 60 min at 20 °C. Isothermal modeling of the parameter’s optimization data proved that the Langmuir model (R = 0.97–0.99) is more able to explain the sorption process than Freundlich indicates the monolayer sorption process. Adsorption kinetics professed that pseudo-second-order rate law effectively represented the ongoing adsorption system. The thermodynamic studies revealed that the sorption process was spontaneous at room temperature as ∆H° was negative. The maximal sorption capacity among the sorbents is 20.66 mg g⁻¹ for CC-Fe₃O₄ composite, which is higher than CC. In conclusion, CC-Fe₃O₄ composite proved an efficient biosorbent for the mitigation of CR dye from wastewater.

Graphical Abstract