Experimental and quantum chemical investigation for the single and competitive adsorption of cationic dyes onto activated carbon

Abstract

Single and competitive adsorption studies were performed to scrutinize the removal of two cationic dyes, namely, crystal violet (CV) and malachite green (MG) by adsorption onto activated carbon (BAC) derived from Balanites aegyptiaca seed shell. The BAC was characterized via scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and pH of point of zero charge (pHpzc) analysis. The physicochemical parameters influencing the adsorption process, namely, pH, contact time, adsorbent weight, initial concentration, temperature and ionic strength were examined. Moreover, Density Functional Theory (DFT) studies were performed to investigate the chemical reactivity of the dye molecules. Experimental results indicated that maximum adsorption of both dyes was achieved at pH 8.0 and equilibrium was attained after contact time of 50 min for MG and 60 min for CV. The competitive adsorption results showed lower adsorption capacities as compared to the single adsorption results indicating antagonistic interaction. Isotherm and kinetic models were employed for fitting the experimental data. The sorption kinetics was found to obey the pseudo second order model. The equilibrium data suggests that Freundlich model could represent the dyes uptake onto the adsorbent. Thermodynamic analysis revealed that the adsorption is a spontaneous and endothermic process. The quantum chemical investigation performed on the tested dyes using DFT method have affirmed that the MG molecules are more reactive (ΔE = 1.236 eV), electrophilic and have the capacity to adsorb strongly on the BAC surface compared to the CV (ΔE = 1.476 eV). The results attested that BAC has great potential for cationic dyes adsorption from aqueous environment