以紫外光/臭氧程序增進光觸媒對室內揮發性有機物三氯乙烯去除效率之研究

Abstract

室內空氣污染之一的揮發性有機物(Volatile organic compounds, VOCs)，與室內空氣品質不良有關，如對人體造成不良之健康影響與病態大樓症候群。本研究所選用之目標污染物為三氯乙烯，為微量存在於潤滑劑或修正液等常用物質中。使用光觸媒(Photocatalytic Oxidation, PCO)程序對於處理多數室內揮發性有機物相當有效，同時具室溫操作、完全礦化成水及二氧化碳最終產物等優點，而成為近年來廣泛使用之室內空氣清淨技術。過去研究指出添加臭氧至光觸媒反應，可避免中間產物造成光觸媒失活之可能性且具增進效應；然而，臭氧於室內環境有其建議值，其尾氣不得有影響人體健康之虞。因此，本研究之目的亦探討臭氧對於光觸媒於室內揮發性有機物轉化之中間產物控制之影響與臭氧之去除效率。 本實驗乃於一光反應器進行，內部為石英管組成(長45 cm、寬2.5 cm)，並紫外光光源為15 W、波長254 nm之長型燈管，並選用Degussa P25 TiO2商用化光觸媒為研究對象。而本實驗所選擇之影響因子包含TCE進流濃度、濕度、氣體流率與臭氧濃度等，各實驗進行二重覆試驗，以確保實驗之再現性。 實驗結果顯示，氣體流率大於1,600 L/min 時，其氣相質傳效應可忽略，而此時光觸媒反應之動力模式符合雙分子之Langmuir-Hinshelwood (L-H) 模式。光觸媒反應之TCE轉化率與二氧化碳產出率隨TCE濃度增加而減少，但其氧化速率之趨勢則為相反。光觸媒反應之中間產物的殘餘量隨著進流之TCE濃度增加而增加。TCE之光觸媒反應常數介於10.7至11.3 μ-mole m-2s-1間，且與氫氧自由基反應常數(kOH•)呈線性關係。VOCs與水分子之 Langmuir 吸附常數分別介於 0.61至0.70 ppm-1 及8.33×10-3至1.67×10-2 ppm-1 之間。VOCs之Langmuir 吸附常數的倒數與亨利定律常數(Henry’s Law constant)相關密切；相反地，水分子之Langmuir吸附常數的倒數與亨利定律常數線性關係較低，乃因TCE轉化後之中間產物與水反應，使水分子消耗。 濕度對於光觸媒具有增進氫氧自由基生成之正效應及競爭吸附之負效應。實驗結果指出，相對濕度之增加，中間產物之控制能由約60%減少至40%。而值滯留時間愈長，中間產物能與水分子反應，獲得較充分之分解，因此，可微幅提升二氧化碳之產率。添加臭氧至UV/TiO2系統對去除TCE並無顯著提升，當添加濃度由100 ppb提升至500 ppb(亦即O3/TCE為0.1)，可微幅提升中間產物之控制(約5%)，但當添加濃度高於此一比值，即出現抑制效應。最終添加至1,000 ppb時，仍殘存約35%(UV/TiO2/O3)與40%(UV/O3)之中間產物殘餘率。 臭氧濃度對UV/O3與UV/TiO2/O3兩程序具抑制效應，添加量愈高則氧化速率隨之降低。此乃釋放之氯自由基(Cl•)，與OH•發生一系列競爭反應，如TCE或臭氧分子，Cl•之反應速率則較臭氧與OH•高兩個數量級(128倍，8.7×10-12 vs. 6.8×10-14)，顯示臭氧遭受破壞，此結果亦與臭氧增進效應指標為負值相呼應。而O3/TCE為一重要比值，當大於0.1即增進效應不再顯著。 總結來說，不論於何種程序下進行，皆會導致CO與CO2產生率提升(惟UV/O3不產生CO)，顯示TCE分解為中間產物後，發生水解效應產生CO與CO2。然而，UV/O3程序則發生無CO產生，推論因CO產率過低，造成Q-trak無法偵測出其讀值。

Volatile organic compounds (VOCs), one of indoor air pollutants, are related to aggravation of indoor air quality (IAQ), such as health effect to human and Sick Building Syndrome (SBS). This study selected trichloroethylene (TCE) as model pollutant with respect to the common substance in indoor environment, such as lubricants or correction fluids. The advantages of Photocatalytic Oxidation (PCO) listed as followed: available to several compounds, operation in room temperature, and to be mineralized to H2O or CO2. It has becoming tendency for this process applied to indoor air clean technology. The intermediates, which generated from the PCO process, may raise the concerns of photocatalyst deactivation. Previous studies indicated that the ozone addition has the enhancement effect on this process, however, the ozone concentration has a recommended value of indoor environment because of the negative effect to human health. Therefore, one of the objectives of this study is to investigate of ozone addition to PCO process, the intermediates control ability and ozone removal efficiency. The experiments were conducted in a photoreactor, with a rectangular quartz plate (45 cm in legth and 2.5 cm in width). The UV light source was controlled at 254 nm and 15 W. Also, Degussa P25 TiO2 was chosen as the catalyst in this study. The influential factors including TCE concentration, relative humidity, gas flow rate and ozone concentration were evaluated and performed twice for reproductive assurances. The effect of gas-phase mass transfer was negligible when gas flow rate exceeded 1,600 ml/min. And the PCO kinetics fitted Langmuir-Hinshelwood model well for bimolecular competitive adsorption type. The TCE conversion and CO2 evolution decreased with increasing TCE concentrations. However, the TCE oxidation rate showed the opposite effect. The residual intermediates increased with increasing TCE concentrations. The PCO rate constants ranged from 10.7 to 11.3 μ-mole m-2s-1, and linear-correlated to the VOCs-hydroxyl radical rate constants (kOH•). The Langmuir adsorption constants of VOCs and water ranged from 0.61 to 0.70 ppm-1 and 8.33×10-3 to 1.67×10-2 ppm-1, respectively. A linear positive relationship was found between the reciprocal of Langmuir adsorption constants and Henry’s Law constants of TCE. However, the reciprocal of Langmuir adsorption constants of water showed a negative relationship with Henry’s Law constants of TCE. This might be due to the intermediates, converted form TCE, reacted with water, and caused the dissipation of water. Relative humidity has a enhancement effect to hydroxyl radicals and retardation effect because of competitive adsorption. Results indicated that the increase of water vapor, the intermediates could decrease from 60% to around 40%. Also, the longer the retention time, the higher the CO2 yield rate. The reason was the contact ability between intermediates and water. The ozone addition to the process of UV/TiO2 exhibited slightly enhancement. When the addition from 100 to 500 ppb of ozone, namely the ratio of O3/TCE is 0.1, the intermediates could slightly controlled around 5%. However, the ratio higher than 0.1, it started to retard. When the addition was 1000 ppb, it still remained about 35% (UV/TiO2/O3) and 40% (UV/O3) of residual intermediates, respectively. The ozone concentration showed retardation effects to UV/O3 and UV/TiO2/O3 processes. The oxidation rate decreased when the addition concentration increased. That was due to the released of Cl•and competed with OH•. These two radicals competed for TCE and water molecules. Owing to the higher reaction rate of Cl•with ozone than that of OH•with ozone by two magnitudes (128 times), ozone has devastated. This results also corresponded to the negative value of enhancement indices of ozone. Therefore, it seems that the enhancement effect was not found at the ratio of O3/TCE (0.1). In summary, under whether processes (UV/TiO2, UV/TiO2/O3 or UV/O3), the CO and CO2 yield rate increased (only under UV/O3 process gained no CO). It showed that TCE converted to residual intermediates and accompanied the reaction of hydrolysis. Therefore, the end products would be CO and CO2. However, the CO yield of UV/O3 was too low to be detected by the Q-trak.