Design and analysis of microwave-assisted systems for aqueous phase carbofuran removal

• Main Authors: Remya, Neelancherry, 瑞米亞
• Other Authors: Lin, Jih-Gaw
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/07494123019266066667

博士 === 國立交通大學 === 環境工程系所 === 99 === Carbofuran is a broad spectrum insecticide, which is widely used in agricultural cultivations for preventing the loss of plants cultivated for economic purpose. Carbofuran usage has received intensive concern not only due to its heavy dosage but also due to its oral toxicity. In the recent years, contamination of surface water and groundwater systems with different pesticide formulations including carbofuran has increasingly been reported. Although several physico-chemical methods have been applied for the degradation/removal of carbofuran from aqueous and soil systems, their applicability is restricted sometimes by the economic considerations or by the stringent operating conditions. On the other hand, carbofuran exhibits a special refractory character to biodegradation methods. Thus, developing a highly effective and speedy treatment technique for aqueous carbofuran removal is one of the most important research issues. Therefore, this study focused on the design and analysis of a microwave (MW)-assisted systems for effectively removing carbofuran from aqueous phase. The MW-assisted experiments (batch flow) were conducted at 100 mg/L carbofuran concentration using a modified-MW reactor (750 W power and 2450 MHz frequency) under pulsed and continuous-modes with different reaction conditions. As a first step, the pulsed-mode experiments were carried out in the presence of granular activated carbon (GAC)/zero valent iron (ZVI)/hydrogen peroxide (H2O2) under different reaction temperatures, i.e. 30?aC, 50?aC and 80?aC, and at varying pHs, i.e. 2, 4, 6, 8 and 10. The lower MW-reaction temperatures, i.e. 30?aC and 50?aC, have produced poor carbofuran degradation efficiencies whereas complete carbofuran degradation (100%) was observed at 80?aC and pH 6 under all the MW-assisted systems. Carbofuran degradation rate accelerated under the alkaline pHs, i.e. pH 8 and 10 (at 80?aC); however, complete and very rapid carbofuran degradation (within 5 min) was accomplished in the MW-assisted GAC and ZVI systems under pH 10 and 80?aC. On the other hand, insignificant carbofuran removal/degradation (2-24%) was observed in the presence of GAC/ZVI/H2O2 without MW. Carbofuran removals in the MW-assisted systems were modeled using the first-order reaction kinetics and a maximum removal rate constant of 4.17/min was obtained in the MW-assisted ZVI system. In the pulsed-mode experiments, a maximum of 86% carbofuran mineralization was achieved in the MW-assisted GAC system. Carbofuran degradation products in the MW-assisted systems were identified through the mass spectrometric analyses. The results indicated the formation of 2,3-dihydro-2,2-dimethylbenzofuran-7-ol, 3-Hydroxy-2,2-dimethyl-2,3-dihydro-1-benzofuran-7-yl-methylcarbamate, 2,2-Dimethyl-3-oxo-2,3-dihydro-1-benzofuran-7-yl methylcarbamate, cyclohexene carboxylic acid and cyclohexylacetate. As a second step, the reaction process underlying in the MW-assisted carbofuran degradation were investigated through scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDAX) and X-ray diffraction (XRD) analyses. These analyses along with other basic MW-assisted system investigations revealed the following observations: (1) exposure to MW increased more micro-pores on GAC surface, (2) the metal ion reduced quickly to the corresponding zero oxidation state of the metal at the end of MW experiments, (3) H2O2 rapidly dissociated faster to hydroxyl radical (•OH) in the presence of MW and (4) the GAC and ZVI could be recycled for MW-assisted experiments without much decrease in carbofuran degradation efficiency. Consequently, continuous-mode MW-H2O2 experiments were conducted under different MW-power levels (300-900 W) at 100 mg/L carbofuran concentration. The MW-H2O2 system operated at 750 W has showed rapid carbofuran degradation, i.e. 30 sec, with the highest first-order removal rate constant of 25.82/min. However, 97% COD removal was observed in the same system after 30 min. In comparison, 100% carbofuran removal and 49% COD removal were observed in the pulsed-mode MW-H2O2 system after 10 and 30 min, respectively. From the experimental outcomes, energy consumption in the MW-assisted systems were worked out and compared with some of the previously reported carbofuran removal techniques, which demonstrated that MW-assisted systems are highly efficient and cost-effective for carbofuran degradation and mineralization. Finally, a series of statistically designed carbofuran degradation experiments based on the central composite design (CCD) with five levels with three factors, i.e. carbofuran concentration ~ 50-250 mg/L; MW power ~ 300-900 W; MW-reaction time ~ 0.5-5 min, were performed under the MW-GAC system. The experimental results indicated that the MW power was the most significant factor, followed by carbofuran concentration. Overall, the MW-assisted system is a powerful tool for carbofuran bearing water/wastewater treatment.