Degradation of organic pollutants combining plasma discharges generated within soil with TiO2 and ZnO catalysts: Comparative analysis, optimization and mechanisms

An advantageous plasma-catalytic setup was developed and applied towards the degradation of trifluralin in soil. For the first time, TiO2 and ZnO catalysts were compared when employed in conjunction with micro-discharges generated directly into the interconnected soil channels. In the presence of catalysts, a significant increase in degradation efficiency was observed; after 5 min, trifluralin degradation increased from 66.5% (plasma alone) to 94.2% and 93% with the addition of TiO2 and ZnO, respectively. In terms of degradation kinetics, TiO2 was a slightly superior catalyst compared to ZnO whereas both catalysts performed better under oxygen than in air atmosphere. Compared to plasma alone, the plasma-catalytic treatment considerably increased (∼3fold) the process energy efficiency. Moreover, the inhibitory effect of soil moisture was less pronounced during TiO2 plasma-catalysis where a reduction of ∼19% in pollutant degradation was observed at 5 wt% soil moisture compared to a ∼54% reduction during plasma alone. The addition of TiO2 and ZnO resulted in a significant increase in NO2 concentration and a noticeable reduction in O3 generation associated with an increase of certain plasma species concentration and the generation of additional and more active ROS, respectively. Liquid chromatography (UPLC/MS) data at the early stages of the trifluralin degradation revealed similar intermediates and degradation processes between plasma-alone and plasma-catalysis. The present effort supports the potential of future implementation of a plasma-catalytic soil remediation method being a rapid, highly efficient, low energy demanding and green method.