Remediation of soil contaminated by fluorene using needle-plate pulsed corona discharge plasma

In this study, a pulsed corona discharge plasma system was employed to remediate fluorene-contaminated soil. The effect of positive and negative power, discharge voltage, discharge frequency, electrode gap, soil moisture, soil pH, and initial fluorene concentration on the degradation efficiency of fluorene were studied. Lower moisture levels were more beneficial to fluorene degradation because the soil interstices were occupied by water molecules when soil moisture increased. Neutral or alkaline soil was more beneficial to fluorene degradation. At optimal conditions (discharge voltage 30 kV, discharge frequency 50 Hz and electrode spacing 20 mm), the degradation efficiency of fluorene reached 78.7% after 60 min of treatment. Furthermore, it was proved that in situ discharge is better than ex situ discharge for fluorene degradation. Periodic discharge and washing could be adopted because the degradation efficiency of fluorene dramatically improved after the removal of oxidation by-products; the degradation efficiency reached 99% within 45 min of discharge time after two "washing" processes. The degradation mechanism of fluorene was also studied. The discharge happened directly on the soil surface, and produced active species including electrons, hydroxyl radicals, ozone, ultraviolet light, ionized oxygen, and atomic oxygen, which contributed to the decomposition of pollutants. The possible degradation pathway of fluorene was also studied. Some degradation by-products, such as 1, 2-benzenedicarboxylic acid, 2-propenoic acid, 1-ethyl-4-hydroxybenzene, butyrolactone, and ethylene alcohol were detected. These active species could break the pollutants into smaller molecular by-products and then partially mineralize them.