Although the number of vessels with exhaust gas cleaning systems (EGCSs or scrubbers) has sharply increased to comply with strengthened regulations for marine environment, secondary pollutions are caused by discharged polycyclic aromatic hydrocarbons (PAHs).
While conventional processes, such as physical processes, chemical processes, and biological processes, have been applied to remediate PAH-contaminated environments, drawbacks are still existing, such as the need of additional treatment for degradation, the use of chemical reagents, neutralization, and long treatment time.
Here, two approaches are investigated to remediate water contaminated with PAHs using liquid phase plasma (LPP); one of them is direct decomposition of PAHs by LPP under diverse electrical discharge conditions, and the other one is a degradation of PAHs by photocatalysts synthesized using LPP.
For the direct decomposition of PAHs, the performance of LPP is evaluated with 10-30 kHz of frequency and 1-3 μs of pulse width. The increased frequency and pulse width enhanced the degradation efficiency, and 93.3, 90.7, 86.0, and 85.4% for Nap, Ace, Flu, and Phe, respectively, are degraded at a frequency of 30 kHz and pulse width of 3 μs. Considering physical condition of the plasma, long pulse width accelerated electrons, leading to increased generation of active species from intensified collision between electrons and surrounding molecules. Conversely, high frequency decelerated electrons due to the excessive changes in the polarity. However, the increased number of plasma discharges results in the generation of numerous active species. Generations of •OH and O radicals are confirmed by optical emission spectrometer and electron paramagnetic resonance. In addition, changes in functional groups which are corresponding to hydroxyl and oxygen groups are identified by Fourier transform infrared spectroscopy.
For the secondary degradation of PAHs by LPP, BL-ZnO and Ag/BL-ZnO photocatalysts are synthesized using Zn and Ag electrodes at 30 kHz of frequency and 3 μs of pulse width with 10 min of treatment time. The chemical structure and states of synthesized photocatalysts are analysed by X-ray diffraction and X-ray photoelectron spectroscopy. The morphology and chemical composition are examined by scanning electrode microscope and energy dispersive spectrometer. The optical properties of photocatalysts are studied using UV-Vis diffuse reflectance spectrometer and photoluminescence spectrometer. In addition, synthetic mechanism is suggested based on the erosion of electrodes, generated active species, and formation of functional groups on synthesized ZnO. BL-ZnO shows enhanced performance due to the oxygen vacancy which prohibits the recombination of electrons and holes. Ag/BL-ZnO achieves the highest performance and it is assumed that the Ag plays a role of electron acceptor and forbids recombination of electron and hole.