A model batch and column study for Cd(II) uptake using citric acid cross-linked Salvia spinosa hydrogel: Optimization through Box-Behnken design

Herein, a polysaccharide-based hydrogel from Salvia spinosa (SSH) seeds was isolated and first esterified to citric acid cross-linked Salvia spinosa hydrogel (CL-SSH) using citric acid (CA) as cross-linker and then saponified with aqueous solution of sodium bicarbonate (NaHCO₃) to form adsorbent, i.e., sodium salt of citric acid cross-linked Salvia spinosa hydrogel (Na-CL-SSH). The CL-SSH was characterized using Fourier transform infrared spectroscopy (FTIR), solid-state cross-polarization magic angle spinning nuclear magnetic resonance spectroscopy (CP/MAS ¹³CNMR), and scanning electron microscopy (SEM) analyses. The surface charge on Na-CL-SSH was determined by conductionng point zero-charge pH (pH_ZPC) analysis. The adsorption conditions were pre-optimized by applying response surface methodology Box-Behnken design (RSM-BBD) on the Cd(II) adsorption data. The high value of coefficient of determination (R² = 0.9980), greater F-value (358.61) than p-values (< 0.0001) for ANOVA of the model design, and non-significant lack of fit (LOF) were obtained that indicated the fitness of second-order polynomial equation on the Cd(II) adsorption data. The study of adsorption kinetics, adsorption thermodynamics, and regeneration revealed that the Cd(II) adsorption was a rapid and exothermic process and occurred through ion-exchange mechanism. The column adsorption experiments for Cd(II) adsorption were also conducted and obtained data from the continuous study could be useful for scalability of the Cd(II) adsorption process by Na-CL-SSH to an industrial scale.