Surface characterization and adsorption mechanism of carboxymethyl cellulose for cadmium(II) removal from aqueous solution

A comprehensive analytical investigation was undertaken into the mechanistic process behind the adsorption of cadmium (II) onto the carboxymethyl cellulose (CMC) using scanning electron microscopy (SEM), Fouriertransform infrared spectroscopy (FTIR), X-ray diffraction (XRD), equilibrium isotherm modeling, and adsorption kinetic analysis.The batch experiments were done under the previously optimized conditions (contact time = 50 min, pH = 6, T = 25 °C, C ∘ = 20 mg/L, adsorbent dose = 1 g/L), at which the reference removal efficiency was 88%, and the equilibrium adsorption capacity was 17.6 mg/g.FTIR studies showed that the adsorption of Cd(II) led to a characteristic shift of the asymmetric (-COO⁻) stretching band from 1622.13 to 1608.63 cm -1 , which is consistent with surface complexation between Cd(II) and the carboxymethyl functional groups of CMC.The presence of new sharp diffraction peaks in the XRD spectra of the Cd(II) loaded CMC was indicative of deposition of crystalline cadmium-containing phases over the adsorbent surface, and the SEM images showed a significant morphological change from an open fibrous network to a close-packed lamellar structure with near-cubic crystalline surface deposits.The Freundlich isotherm (R 2 = 0.997, and RMSE = 0.612 mg/g) was the best model to define the equilibrium data, while the Langmuir model yielding a theoretical maximum monolayer adsorption capacity of q max = 35.0mg/g and separation factor R L = 0.091, indicating favorable adsorption within the Langmuir model framework.The pseudo-second-order model best fit the data gotten from kinetic analysis, with R 2 = 0.998 and (qe) cal = 18.87 mg/g, which indicates that is consistent with a chemisorption-dominated mechanism and is consistent with the FTIR mechanistic evidence.A non-zero intraparticle diffusion intercept confirmed the multi-step rate control that included film and pore diffusion.Convergent evidence of all the characterization and modeling methods confirms that CMC is a well-defined, mechanistically effective, and biodegradable adsorbent, as reported in the literature, for the remediation of Cd(II) in contaminated water systems.