Unusual Catalytic Effect of Fe 3+ on 2,4-dichlorophenoxyacetic Acid Degradation by Radio Frequency Discharge in Aqueous Solution.

2,4-dichlorophenoxyacetic acid (2,4-D) is a widely used herbicide for controlling broad-leaved weeds. The development of an efficient process for treating the refractory 2,4-D wastewater is necessary. In this study, liquid-phase degradation of 2,4-D induced by radio frequency discharge (RFD) was studied. Experimental results showed that the degradation was more effective in acidic than in neutral or alkaline solutions. During the degradation, a large amount of hydrogen peroxide (H₂O_2, 1.2 mM/min, almost equal to that without 2,4-D) was simultaneously produced, and catalytic effects of both ferric (Fe³⁺) and ferrous (Fe²⁺) ions on the degradation were examined and compared. It was found that 2,4-D degraded more rapidly in the case of Fe³⁺ than the that of Fe²⁺. Such a scenario is explained that Fe³⁺ was successively reduced to Fe²⁺ by the atomic hydrogen (•H) and •OH-adducts of 2,4-D resulting from RFD, which in turn catalyzed the H₂O₂ to form more •OH radicals through Fenton's reaction, indicating that Fe³⁺ not only accelerates the degradation rate but also increases the amount of •OH available for 2,4-D degradation by suppressing the back reaction between the •H and •OH. 2,4-dichlorophenol, 4,6-dichlororesorcinol, 2-hydroxy-4-chloro- and 2-chloro-4-hydroxy- phenoxyacetic acids, hydroxylated 2,4-Ds, and carboxylic acids (glycolic, formic and oxalic) were identified as the byproducts. Energy yields of RFD have been compared with those of other nonthermal plasma processes. [ABSTRACT FROM AUTHOR]