Freeze-drying-induced structural enhancement of graphene quantum dot–modified carboxymethyl cellulose beads for highly efficient cypermethrin removal

The influence of drying procedures on the pore structure and adsorption performance of graphene quantum dot-modified carboxymethyl cellulose (CMC/GQD) beads for cypermethrin removal has not been sufficiently elucidated. Therefore, hydrogel and cryogel forms of CMC/GQD beads were comparatively evaluated to clarify the effect of oven-drying and freeze-drying on cypermethrin adsorption from aqueous media. Freeze-drying preserved the interconnected porous structure and increased the specific surface area from 7.78 m 2 g -1 for the oven-dried hydrogel to 107.86 m 2 g -1 for the cryogel. Taguchi optimization identified an initial cypermethrin concentration of 90 mg L -1 , pH 6, an adsorbent dosage of 1 g L -1 , and a contact time of 120 min as the optimum conditions. Equilibrium analysis showed that the hydrogel and cryogel were better described by the Langmuir (R 2 = 0.9971) and Freundlich (R 2 = 0.9942) models, respectively, indicating predominantly monolayer adsorption on relatively uniform sites for the hydrogel and heterogeneous, multilayer adsorption for the cryogel. The Langmuir model yielded theoretical maximum adsorption capacities of 174.5 and 296.7 mg g -1 for the hydrogel and cryogel, respectively, while the experimentally determined maximum adsorption capacities were 65.4 and 276.12 mg g -1 , respectively. Kinetic analysis indicated that hydrogel adsorption followed the pseudo-second-order model, whereas cryogel adsorption was described by both pseudo-second-order kinetics and liquid-film diffusion. Thermodynamic analysis confirmed spontaneous and exothermic adsorption, with ΔH° = -86.09 kJ mol -1 and ΔS° = -230 J mol -1 K -1 . The cryogel achieved removal efficiencies of 98.7% in deionized water and 98.6% in simulated agricultural runoff. Moreover, using 0.1 M NaOH as the desorption agent, the cryogel maintained a removal efficiency above 75% for six consecutive cycles, although its efficiency decreased from 99.9% in the first cycle to 59.68% after eight cycles. These findings demonstrate that freeze-drying substantially improves the porous structure and adsorption capacity of CMC/GQD beads, providing a reusable adsorbent for cypermethrin-contaminated water.