Response surface methodology application in optimization of cadmium adsorption by shoe waste: A good option of waste mitigation by waste

Application of shoes waste adsorbents (polyurethane ethylene-shoe material type-I and vinyl acetate-shoe material type-II) were investigated for the removal of cadmium (Cd) from simulated aqueous solution and wastewater. The effect of operating variables such as metal ions concentration (20-820 mg/L), adsorbent dosage (0.1-2.1 g/L) and solution pH (1-9) and contact time (288-1440) were modeled by response surface methodology (RSM). Central composite design (CCD) was used for experimental runs and independent variables i.e., initial metal ion concentration, pH, contact time and adsorbent dose were optimized. The optimum conditions for Cd adsorption were found out to be 305 mg/L Cd(II) ions initial concentration, pH 4.9, contact time 932 min and adsorbent dose 1.3 g for shoe material-I, whereas these values were 402 mg/L Cd(II) ions initial concentration, 5 pH, 881 min contact time and 1.2 g adsorbent dose for shoe material type-II. At conditions, 180.222 mg/g (66.66%) and 396.312 mg/g (94.66%) Cd(II) ions were removed using shoe material-I and II, respectively. The kinetics and isotherms studies showed that pseudo-second-order kinetics model and Freundlich isotherm fitted well to the Cd(II) adsorption data. Besides, at optimized conditions, up to 59.5% and 86.16% Cd(II) ions were removed from industrial wastewater (mixed dyeing, printing and chemical synthesizing industries) onto shoe material type-I and II, respectively. FTIR study revealed the involvement of hydroxyl and carbonyl group in the adsorption of Cd(II) metal ion. The CCD proved to be best statistical model to predict the response with good accuracy and reliability. Results suggest that waste shoe can be used for the Cd(II) adsorption from wastewater and this study is also extendable for the adsorption of other heavy metals since the adsorbent under investigation has not been reported for the adsorption of heavy metals.