Batch adsorption of safranin dye from aqueous solution using activated groundnut pods (AGP)

Abstract

This work aimed on the adsorption of safranin dye from wastewater using activated groundnut pods (AGP). The raw pods were collected, crushed into particle size of about 400 - 550 μm and modified in-situ with ZnCl2 to prepare AGP which was characterized using analytical technique; Fourier Transform Infra-Red (FTIR), Energy Dispersive X-ray (EDX) spectroscopy, Thermo gravimetric analyser (TGA) and Scanning Electron Microscopy (SEM). The absorbance of the dye solution was monitored at 530 nm with UV-Visible spectrophotometer. FTIR analysis showed the vibration frequency for C‒H, O‒H, C=O and C‒O stretches at 2900, 3420, 1730, and 1100 cm-1 respectively. SEM results revealed the AGP has a porous surface with heterogeneous pores which became compact after dye adsorption. EDX confirmed the presence of C, O, Zn and Cl in the adsorbent. The suitability of the pseudo-first, pseudo second and Elovich kinetic models for the sorption of safranin onto AGP was examined. The equilibrium data were subjected to Langmuir, Freundlich, Tempkin and Dubinin-Radushkevich isotherm models. Adsorption factors were optimized by Box-Behnken design using response surface methodology (RSM). The pseudo-second order kinetic model provided the best correlation and was found to be more statistically significant. Langmuir model was found to fit well based on the high values of the coefficient of regression R2 and low % standard error values. The monolayer adsorption capacity Qmax was found to be 45.45 mgg-1. Thermodynamic adsorption processes showed the spontaneous, endothermic and randomness of the systems with free energy change less than zero, enthalpy change (∆H) of 66.43 kJmol-1and entropy change (∆S) of 145.37 Jmol-1K-1. Response Surface Methodology (RSM) showed that 0.55 g was the optimum dosage with 92.50 % removal of safranin dye concentration of 200 mgL-1 at pH 10.