Kinetics of alizarin dye hydrolysis in alkaline medium for wastewater treatment

Abstract

The kinetics of the hydrolysis of Alizarin dye (ALZ) in the basic medium is investigated for the purpose of wastewater treatment. ALZ represents a group of aromatic dyes, that are heavy, toxic, and non-biodegradable. The kinetics of the reaction was followed by UV-Vis spectrophotometry and ab-initio computational methods. The effects of initial concentration, ionic strength, and temperature were studied. The kinetic salt effect (ionic strength) demonstrated that OH– is part of the rate-determining step of the reaction. Unlike common reactions, anti-Arrhenius behavior was observed within the temperature range of 25-50°C. Therefore, the apparent activation energy was determined to be-23.91 kcal/mol. Using theoretical quantum calculations, the reaction under study was investigated using the density functionals B3LYP and B97D3, and final energies were obtained using the 2nd order Moller-Plesset (MP2) theory. A complex reaction mechanism is suggested that involves the formation of an intermediate that combines the ALZ anion and water molecule attached by H-bonding. The mechanism accounted for the anti-Arrhenius behavior and the negative Ea. The standard reaction enthalpy (ΔH298) obtained using the B97D3 Grimme’s functional was within the range of the experimental Ea value.