In this study, bio-nanocomposite hydrogel was synthesized based on starch grafted poly (Acrylic Acid) reinforced by cellulose nanofiber to improve swelling and mechanical properties of hydrogel. This hydrogel was used as an adsorbent, which has the high ability to hold a large amount of water inside its three dimensional network, for removing chromium and cadmium ions from aqueous solutions. The morphology of this bio-nanocomposite hydrogel was assessed before and after adsorption of chromium and cadmium ions by using scanning electron microscopy. The response procedure based on Box- Behnken design was used to evaluate the effect of independent variables on response performance (Cr(VI) and Cd(II) ions removal efficiency) as well as predicting the best removal rate. In this study, the effect of different parameters such as pH, initial concentration of chromium and cadmium ions, and flow rate through the adsorption column on process efficiency were investigated. The concentration of Cr(VI) ion was measured by UV-VIS spectrophotometer at 540 nm and the concentration of Cd(II) ion was determined by atomic absorption. The results showed that the removal efficiency is highly dependent on the pH of the solution. The highest percentages removal was obtained to be 50.26% for chromium ion at pH=6, initial concentration of 20 (mg/L), and flowrate of 10 (mL/min), and 82.50% for cadmium ion at pH=5, initial concentration of 10 (mg/L), and flow rate of 5 (mL/min). Langmuir and Freundlich adsorption isotherm models were employed. Correlation coefficients comparison for both chromium and cadmium ions showed that Langmuir isotherm model has good agreement with experimental data. In order to study the kinetics of the process, Thomas, Yoon- Nelson, and Clark models were employed. For both ions, experimental data was in good agreement with all three models. One of the most important features of the adsorption process is the possibility of desorption. In order to recover synthesized bio-nanocomposite hydrogel, hydrochloric acid solution was used. First, in the batch system, the desorption potential and the appropriate concentration of acid as the reducing agent were determined. Finally, the saturated bed of cadmium ion was restored by the reducing agent and the adsorption-desorption process was performed on three successive stage on adsorbent bed.
