A novel environmental risk-free microwave discharge electrodeless lamp (MDEL) in advanced oxidation processes: Degradation of the 2,4-D herbicide

A novel microwave discharge electrodeless lamp (MDEL) has been developed for wastewater treatment with Advanced Oxidation Processes (AOPs) using environment risk-free gases (e.g., xenon, nitrogen, helium, oxygen, hydrogen and argon alone or a mixture thereof) that provide the needed light plasma source when microwave irradiated. The MDEL was optimized through an examination of the light intensity of the emitted radiation in the UV–visible spectral region at controlled pressures and gas-mixture ratios and to test whether the gases self-ignite on irradiation with microwaves. The usefulness of the MDEL was assessed by examining the degradation of aqueous 2,4-dichlorophenoxyacetic acid (2,4-D) in the absence and presence of TiO2 particles irradiated simultaneously by both microwave (MW) and by UV radiation emitted from the microwave-triggered (2.45 GHz) electrodeless lamp. The decomposition efficiencies for the disposal of the 2,4-D herbicide are compared with the MDEL in close contact (inside) with the 2,4-D solution or the 2,4-D/TiO2 dispersion and with the MDEL located outside the reactor. Degradation of 2,4-D with the MDEL was monitored spectroscopically and by the loss of total organic carbon (TOC) using no less than seven different protocols, namely (i) MW irradiation alone, (ii) MDEL (outside), (iii) Hg lamp/TiO2, (iv) Hg lamp/MW/TiO2, (v) MDEL (outside)/TiO2, (vi) MDEL (inside), and (vii) MDEL (inside)/TiO2. Most efficient in the degradation of 2,4-D were the MDEL/TiO2 systems with the MDEL lamp inside the reactor in contact with the 2,4-D/TiO2 aqueous dispersion followed closely by the MDEL alone (no TiO2) also in contact with the 2,4-D solution.