Salicylaldehyde-grafted magnetic chitosan/nanoclay composites for efficient paraquat adsorption

Pesticide contamination in water poses serious environmental and health risks, highlighting the need for efficient adsorbents. Chitosan-based composites are promising due to their biodegradability, functionality and recoverability. In the present investigation, magnetic chitosan grafted with salicylaldehyde (MG-CS-SAL) and magnetic chitosan nanoclay grafted with salicylaldehyde composites (MG-CS-SAL/NCLA) were hydrothermally synthesized and their adsorption efficiencies were evaluated for pesticides (carbofuran, 2,4-dichlorophenyl acetic acid, nitenpyram, cypermethrin, chlorpyrifos, paraquat, glyphosate and atrazine) adsorption. MG-CS-SAL/NCLA showed the highest paraquat removal efficiency, which was and further studied for the optimization of adsorption conditions. The synthesized composites were analyzed using FTIR, SEM-EDX and XRD techniques. The adsorption conditions were optimized using the Taguchi design (OA25) and the point of zero charge (pH pzc ) was determined to be 6.75. Under adsorption circumstances of pH 7, an adsorbent dosage of 0.01 g/50 mL, a contact period of 40 min and an initial concentration of 100 mg/L at 60 °C, a paraquat removal efficiency of 95% was attained (483 mg/g). The paraquat adsorption data aligned to a pseudo-second-order kinetic model (R² = 0.99) and a Langmuir isotherm model (R² = 0.97). Thermodynamic study indicated that the adsorption process was spontaneous (ΔG° < 0), endothermic (ΔH° = 0.17 kJ/mol) and associated with an increase in entropy (ΔS° = 0.04 J/mol·K) at the solid-liquid interface. The synthesized composite showed 95.24% adsorption and 93.33% desorption in the initial cycle of NaOH, which validates the successful recovery of paraquat. Nevertheless, the reusability was limited, as the adsorption decreased to 41.86% in the second run because of loss of active sites, structural degradation and low dosage of the adsorbent (0.01 g/50 mL). The optimum conditions were employed to treat paraquat-contaminated wastewater from rice fields and 94.97% removal was observed, which revealed a promising application of the composite for paraquat removal from wastewater. These findings indicate that MG-CS-SAL/NCLA is a highly efficient and promising adsorbent for paraquat removal from contaminated water, with potential applicability for the remediation of other structurally similar pesticides.