Optimization of toxic metal adsorption on DEA-calix[4]arene appended silica resin using a central composite design

Abstract

The present study is devoted to the application of Central Composite Design (CCD) to purify water contaminated with toxic metal ions (Pb²⁺, Cd²⁺ and Hg²⁺) using a p-tert-diethanolaminomethylcalix[4]arene (DEA-C4) based silica resin. The synthesized material was characterized using SEM, FTIR and EDX techniques. Three effective parameters (pH, adsorbent dosage and concentration of metal ion) were optimized by assessment with CCD. The data was analyzed using the quadratic model and the obtained results revealed the highest % adsorptions for Pb²⁺ (92.7), Cd²⁺ (93.7) and Hg²⁺ (88.2) at the optimum points for pH (5.4, 3.18 and 3.89), adsorbent dosage (23.8, 25.5 and 26.12 mg) and metal ion concentration (5.3, 7.62 and 7.18 ppm), respectively. Analysis of variance (ANOVA) results show a good fit for the regression model, showing R² values of 0.99 for Pb²⁺, 0.99 for Cd²⁺ and 0.98 for Hg²⁺. Furthermore, the results from Freundlich adsorption isotherms suggest its favorability, as the values for n are >1 (i.e., 7.06, 8.85 and 13.2) and there are higher values for R² = 0.97, 0.99 and 0.98 for Pb²⁺, Cd²⁺ and Hg²⁺, respectively. However, for the Langmuir isotherm, the value of the separation factor R_L is >1 (1.01, 1.02 and 1.05 for Pb²⁺, Cd²⁺ and Hg²⁺, respectively), revealing its un-favorability. In kinetics, the pseudo-second-order kinetic model (Ho and McKay) shows R² values of 0.998, 0.999 and 0.999 for Pb²⁺, Cd²⁺ and Hg²⁺, suggesting the chemical reaction seems significant in the rate controlling step. The thermodynamic results concluded that the reaction is spontaneous and exothermic in nature.