連續流光催化系統降解油品污染地下水之研究

高雄市地下水污染以油品污染為最大宗，此計畫研製可見光光觸媒材料應用於油品污染地下水之整治技術。本研究以官能基化奈米碳管與二氧化鈦複合成固定化可見光光觸媒(CNT/TiO_2)，利用LED燈源模擬太陽光源，建立批次及連續流系統進行油品污染地下水之降解實驗。批次系統研究結果顯示，無陰離子環境本光催化系統其苯、甲苯、乙苯之降解效能為42.8～57.4%，然當環境中存在Cl^-與SO_4^(2-)約100 mg/L以上，則對光催化效果產生抑制現象，降解效能降至30.1～38.1%，因水中陰離子為氫氧自由基之抑制劑，於陰離子環境中添加低濃度(10^(-3)～10^(-4) M)之過氧化氫，可減少陰離子抑制之影響並且提升降解效率至79.9～83.5%。連續流系統結果顯示，處理流速為0.282 cm/s(相當於30 min停留時間)時，苯、甲苯及乙苯可由3倍法規標準值之起始濃度分別降解至1.68倍、0.741倍及0.585倍、處理流速降低至0.071 cm/s (相當於120 min停留時間)時，甲苯及乙苯已降解至符合法規標準值。而於系統中添加10^(-3) M之過氧化氫，處理流速為0.282 m/s環境下即可將苯、甲苯、乙苯分別降解至法規標準值之0.9、0.24及0.21倍。因此本研究研發之CNT/TiO_2固定化複合光觸媒光催化技術可有效於太陽光環境中降解油品污染地下水，並解決受地下水陰離子抑制之問題，後續可有效將此技術應用於實場整治場址中。

關鍵字

連續流反應系統；功能性奈米碳管；複合光觸媒；油品地下水；二氧化鈦

並列摘要

1. Introduction/Background: The major contaminants in the groundwater in Kaohsiung, Taiwan, are petroleum products. Photocatalysis with TiO_2 as the photocatalystis known to be an attractive and efficient process to remove contaminants from aquifers. In spite of the advantages of this process, three mainproblems still need to be overcome: fast recombination between electron-hole pairs, the mismatch between the band gap of TiO_2 and the solar spectrum, and the recycling of TiO_2 nanoparticles. Recently, carbon nanotubes (CNTs) have been used as a support for the dispersion of functional materials in order to give these additional functionalities, such as a large surface area, large electron-storage capacity, superior metallic conductivity, and wide visible light absorption. The possible mechanisms are that CNT can act as both a stimulator enhancingtheinjection of photo-excited electrons from TiO_2 andanelectron sink effectively separating electron-hole pairs, as well as serve as a dispersing template to reduce the agglomeration of TiO_2 nanoparticles. It was reported that introducing carboxyl acid (-COOH) and acyl chloride (-COCl) on the CNTs would increase the roughness of the surfaces andbe beneficial for binding chemicals on CNTs. Moreover, for economic reasons, an immobilized TiO_^(2-)based photocatalyst is needed. This study aimed to investigate the photodegradation performance of petroleum-contaminated groundwater when using functionalized carbon nanotube/TiO_2 hybrid photocatalysts in a continuous photocatalytic system with LED light. The effects of chloride and sulfate anions on the photodegradationwere also investigated. 2. Materials and Methods: The CNT/TiO_2 photocatalysts were prepared with titanium-isopropoxide (TTIP), functionalized CNTs, followed by a sol-gel hydrolysis method. Two functionalized CNTs, CNT-COOH and CNT-COCl, were used in this study, which will have more homogeneous adherence with base materials and higher reactivity. Specific amounts of TTIP were mixed with CNT-COOH and CNT-COCl in an isopropanol solution. 0.5 mL of this sol-gel solution (functionalized CNT: TiO_2 = 1:99 by weight) was immobilized on a Raschig ring (5 mm in diameter, 10 mm in length) using a dipping method, which resulted in a concentration of 0.25 g/g. The immobilized glass Raschig ring was moved into an 80°C oven for 1 hr and cooled to room temperature. The immobilized Raschig rings were ready for use after being calcined in an oven at 550°C for 1 hr. The photo reactor irradiated with 24 LED chips of 410 nm, with a total light intensity of 40 mW/cm^2, was established for batch photocatalytic experiments. The batch experiments were conducted for 4 hr. A quartz tube of 45 mm in diameter 60 mm in length 2 mm in thickness was established for conducting continuous flow photocatalytic experiments under a simulated sunlight source. The sunlight source was simulated with 21 LED chips consisting of 310 nm × 3,410 nm × 9,455 nm × 3,510 nm × 3, and 590 nm × 3, whichsurrounded the outside of the quartz tube. The power of each LED chip was 10W, and this resulted in a light intensity of 40 mW/cm^2. The continuous flow experiments were conducted at 0.071 ~ 0.564 cm/s for 4 hr, corresponding to a retention time of 12 ~ 15 min. 3. Results and Discussion: In batch systems, 42.8 ~ 57.4% of degradation for benzene, toluene, and ethylbenzene were found after 4 hrs irradiation. The photodegradation efficiency decreased as the anion concentration increased. With addition of 1,000 mg/L Cl^- and SO_4^(2-), the degradation efficiency decreased to 30.1 ~ 38.1%, and the inhibition effect was significant. This worse photodegradation performance might be due to theconsumption of hydroxyl radical by anions. Further, with addition of 10^(-3) ~ 10^(-4) M H_2O_2, the degradation efficiency increased up to 79.9 ~ 83.5%. For a continuous flow system, 21% ~ 64% of benzene, 62% ~ 85.4% of toluene, and 70% ~ 87.6% of ethylbenzene was degraded at a flow rate of 0.071 ~ 0.564 cm/s. Better degradation performances of toluene and ethylbenzene were found in the continuous flow system, although this was not the case for benzene. The methyl group and ethyl group in toluene and ethylbenzene, respectively, might be easily attacked by hydroxyl radicals, thus resulting in better degradation performance. With addition of 1,000 mg/L of Cl^- and SO_4^(2-), the degradation efficiency was 20%, 83% and 86% for benzene, toluene, and ethylbenzene at a flowrate of 0.282 cm/s, respectively. The inhibition effect of anions in a continuous system was not significant, and this might be because more fresh hydroxyl radicals were supplied in this system. With the addition of 10^(-3) M H_2O_2, the degradation of benzene, toluene, and ethylbenzene was 21%, 90% and 92.5%, respectively, better than the figures seen in a batch system, except for benzene. The lower degradation efficiency of benzene in a continuous flow system might result from the higher mass effect of the other two compounds. 4. Conclusion: 1. The results showed that the chloride would inhibit the photodegradation, but the sulfate would enhance it in the case of a low concentration. With a higher concentration of 100 mg/L, both chloride and sulfate would inhibit photodegradation performance. This might result from coverage of the photocatalyst surface by anions. With addition of H_2O_2, the photodegradation efficiency of the contaminants would be enhanced, and thus the inhibition effect of anions would be overcome. 2. The degradation efficiency of benzene, toluene, and ethylbenzene was 43%, 77% and 80% at a flowrate of 0.282 cm/s in continuous flow system, respectively. With the addition of 10^(-3) M H_2O_2, the degradation efficiency would enhance and the concentrations of three compounds would less than the regulation limits. 3. It was concluded that the prepared functionalized CNT/TiO_2 photocatalysts could effectively degrade benzene group compounds in a continuous flow photocatalytic system.

並列關鍵字

continuous flow reactor ； functional carbon nanotube ； hybrid photocatalyst ； petroleum groundwater ； titanium dioxide

國際替代計量

連續流光催化系統降解油品污染地下水之研究

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