Phenol removal from wastewater by CWPO process over the Cu-MOF nanocatalyst: process modeling by response surface methodology (RSM) and kinetic and isothermal studies

Abstract

Water-stable metal–organic frameworks (MOFs), which possess unique porous structures, have attracted attention from scientists exploring novel and efficient methods for the elimination of phenol compounds from aqueous media. The numerous properties of MOFs such as tunable porosity, hierarchical structure, immense pore volume, and specific surface area, together with their excellent adsorption and recyclability performances offer new insight compared to traditional catalysts. Herein, Cu-MOF was synthesized and characterized via FTIR, BET, XRD, TEM, and SEM. Results indicated the formation of nanostructured Cu-MOF with mesoporous and macroporous characteristics. Cu-MOF was employed as a new catalyst in the catalytic wet peroxide oxidation (CWPO) of phenol. The central composite design of the RSM (response surface methodology) approach was used for the design of the CWPO process in the statistical study of the removal of phenol from wastewater. The RSM methodology predicted that the optimal conditions for the phenol degradation occured at phenol concentration 400 ppm, Cu-MOF amount (1.5 g L⁻¹) at 50 °C for 30 min. The phenol removal percentage under optimal conditions was predicted by RSM to be 91.4% where experimental test resulted 91.87% removal of phenol. The order of the relative significance of variables predicted by the Pareto analysis was as follows: temperature (X₃) > concentration (X₁) > adsorbent dosage (X₂) > contact time (X₄). Furthermore, the isotherms (Langmuir and Freundlich) and kinetics of phenol oxidation in the CWPO process were investigated for the adsorption of phenol on Cu-MOF. The average values of the empirical constant, adsorption constant (saturation coefficient) and R² for the Langmuir equation were q_m =  500 mg g⁻¹, K_L  =  0.19 L mg and 0.88, respectively. The average values of the Freundlich adsorption constant, empirical coefficient and R² were K_f  =  1.44 mg g⁻¹, n  =  0.66 L mg⁻¹ and 0.94, respectively. The results indicated that the data was better fitted with the Freundlich model. Finally, the kinetics of the process was confirmed to correspond to the pseudo-second-order equation.