Synthesis, structural characterization, DFT, kinetics and mechanism of oxidation of bromothymol blue: application to textile industrial wastewater treatment

Oxidation of bromothymol blue [BTB] by oxyanion potassium permanganate as a strong oxidizing agent in acid solutions using perchloric acid at a constant ionic strength has been obtained keto bromothymol blue [BTB](Keto) and studied kinetically spectrophotometrically. Different characterization techniques for [BTB] and [BTB](Keto) such as Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and optical properties have been used. The order about the permanganate ion is first, while a fractional first order was released as regards [H+] and [BTB] fractional first order. Dependence on the concentration of hydrogen ions in the concentrations indicates that the oxidation process is acid catalyzed. The absence of either transient Mn-III and/or Mn-IV as involving species in the oxidation process was verified by Mn-II applied to the oxidation reaction. Formation of 1:1 intermediate complex formation kinetically was revealed during the rate-determination step. In the slowest step, two-electron transfer processes of the inner-sphere form have been proposed. A mechanism of the tentative reaction was proposed and explored concerning kinetic parameters. The density functional theory (DFT) by DMol(3) and CASTEP was used for the optimization of dye [BTB] and [BTB](Keto) as an isolated molecule. From the UV-Vis spectrum of [BTB] and [BTB](Keto), the E-Direct(Opt) values are 2.849 eV and 2.35 eV using Tauc's equation, respectively, related to direct transitions of electrons. Using DMol(3) method with DFT simulation, the HOMO and LUMO values for [BTB] and -[BTB](Keto) as isolated molecule are 3.047 eV and 2.869 eV, respectively. The simulated FTIR, molecular electrostatic potential (MEP) and optical properties by Gaussian software and CATSTEP are in great agreement with the experimental study.

Automated summary

The oxidation of bromothymol blue by oxyanion potassium permanganate in acid solution has been studied kinetically, revealing the reaction mechanism and kinetic parameters. Various techniques were used for characterization, showing the formation of keto bromothymol blue and the acid-catalyzed oxidation process. Theoretical simulations with density functional theory and experimental studies showed good agreement regarding the optical properties and molecular structure changes.