Water pollution and storage is becoming an important problem to the people worldwide. Conventional water treatment processes, such as chlorination and ozone oxidation suffer a lot of limitations. The application of pulsed electrical discharges in water solutions for degradation of organic pollutants offers the advantage of simultaneous using of different physical effects (UV light) and chemical effects by the active species like OH, H, O, HO₂, H₂O₂, O₃ able to decompose the harmful organic compounds. In this study we analyzed the degradation of the benzoquinone molecule by using in-situ Coherent anti-Stokes Raman Spectroscopy (CARS) in order to monitor the active species during the decomposition process.

In the present experiment a pulsed plasma in an aqueous benzoquinone solution was used. The plasma was generated in a reaction glass cell, between two rods electrodes. The plasma electrical characteristics were: the peak to peak voltage of 2 – 4 kV, the pulse frequency of 10 – 15 kHz and the pulse width of 2 μs. The reaction cell was set on the optical microscope stage of the CARS system. Benzoquinone was dissolved in a potassium sulfate solution (4 g/L) and the pH was adjusted with dilute sodium hydroxylate or dilute sulfuric acid.

The CARS benzoquinone signal was observed at 1230 cm^-1, 1050 cm^-1 and 1667 cm^-1 corresponding to COH bending CH, bending and CC stretching vibrations, respectively. Degradation of the benzoquinone molecule was monitored by the peak at 1667 cm^-1, characteristics to benzene ring vibration. The shift and the amplitude of the absorption peak at 427 nm were observed by ex-situ measurements by the UV-visible spectroscopy.

The degradation of benzoquinone molecule depends on solution pH and plasma electrical characteristics.