Study On Degradation Of 2.4-D And Reduction Of Hexavalent Chromium By Liquid-phase RF Discharge

As a new type of highly efficient advanced oxidation and reduction process,liquid-phase RF discharge has attracted people's attention due to its stable discharge,easy excitation under normal temperature and pressure,and being able to operate in a wide range of solution conductivity.A coaxial electrode system was designed based on its self-shielding effect.Basic physical and chemical characteristics of the discharge of the electrode system were studied in a self-made liquid phase RF discharge reactor.In addition,the characteristics of 2,4-dichlorophenoxyacetic acid(2.4-D)oxidation and Hexavalent Chromium(Cr(VI))reduction in the reactor under different experimental conditions were investigated,respectively.The main conclusions were as follows:(1)The liquid-phase RF discharge plasma showed continuous and periodic patterns in the macroscopic morphology;the discharge produced free radicals such as oxidizing hydroxyl radicals(·OH),reducing hydrogen atoms(·H),and molecular hydrogen peroxide(H2O2).The amount of H2O2 increased gradually with increasing discharge time.At pH 2.00,the amount of H2O2 formation reached 76.36 mg·L-1 at 100 min.(2)Acidic conditions favor the degradation of 2.4-D;Due to the production of H2O2 and H during the discharge,addition of iron salts(Fe2+/Fe3+)had significant catalytic effect on the 2.4-D degradation.The optimal dosage was 0.3 mM and 0.5 mM for Fe2+and Fe3+,respectively.The catalytic efficiency of the iron salts on 2.4-D degradation was the highest at pH 2.00.The degradation of 2.4-D increased with increasing temperature;Addition of free radical scavengers(Isopropanol)decreased 2.4-D degradation,indicating that OH was the main active substance responsible for the 2.4-D degradation.The main aromatic intermediate product of 2.4-D degradation was 2.4-dichlorophenol.With the elapse of discharge time,its concentration increased initially and then decreased.Chloride and some small organic acids such as formic acid,glycolic acid and oxalic acid were generated.By analyzing the distribution of the 2.4-D degradation products,it was speculated that the OH preferentially attacks the C1-O and the unsubstituted position of the benzene ring.The dechlorination reaction mainly occurred in the chlorine-containing intermediate product.(3)According to the calculation of the energy efficiency of the discharge system,it was found that 175 W is the best incident power for reducing Cr(?);Decreasing solution pH and conductivity enhanced the removal of Cr(?).When the initial pH was 4.00 and the conductivity was 200 ?S·cm-1,the removal of Cr(?)can reach 99.66%.The analysis of Cr(?)reduction products by infrared spectroscopy revealed the stretching vibration peaks of-OH,Cr3+and hydroxyl groups,Cr and oxygen atoms,respectively,indicating that Cr(?)was reduced to trivalent chromium.The addition of OH scavenger enhanced the removal of Cr(?),where the enhancing efficiency of formic acid is higher than that of methanol.