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Photodegradation of emerging contaminants with TiO2 nanoparticles in the presence of electrolytes and quantitative structure-activity relationship
Titanium dioxide (TiO2),emerging contaminants,halide ions,radicals,quantitative structure-activity relationship (QSAR),
|Publication Year :||2019|
|Abstract:||Tris (1,3-dichloropropyl) phosphate (TDCPP)為含磷阻燃劑，其漸漸取代過去的溴化阻燃劑，由於TDCPP的使用量漸增，因此漸漸發現其殘留於環境中被偵測到，且TDCPP對於生物的生殖和神經系統具有負面影響，因此需要將其於環境中移除。光催化反應可以有效地處理水體中汙染物，因為其花費時間短且能將汙染物有效的降解甚至礦化，常見的光催化劑為TiO2，本次研究中所使用的TiO2為Degussa P25。環境中有許多因素可能會影響光催化反應，像是水體的pH、溫度與鹽類，如氯離子與溴離子，其可能來自於海水、垃圾滲出液或含鹽類的沼澤，因此需要分別探討這些環境因子對於光催化反應的影響。之前的研究大多顯示在添加鹽類離子後會抑制光催化劑降解汙染物；但經由我們之前的研究發現添加溴離子反而會促進TiO2光催化降解速率，由於以上研究降解的化合物不同，因此探討化合物的結構是否影響鹽類離子對於P25光催化降解化合物的速率是必要的。P25於酸性環境下其表面電荷帶正電，添加氯離子或溴離子後其表面所帶正電荷減少，表示氯離子或溴離子會中和P25表面的正電荷。另外，由FT-IR測量的結果顯示P25表面的氫氧官能基於添加氯離子或溴離子後減少，進一步地推斷氯離子或溴離子會與P25表面形成鍵結。於紫外光照射下，P25可於60分鐘內降解TDCPP，且在反應過程中TDCPP幾乎礦化並偵測到氯離子的釋出。P25光催化降解TDCPP的效率隨著TDCPP初始濃度增加與P25劑量減少而降低。P25於酸性環境下有較好的降解TDCPP的效率；而溫度對於P25降解TDCPP的速率影響不大；添加10-500 mM 氯化鈉或溴化鈉皆會抑制P25降解TDCPP。為了進一步的探討化合物結構與反應速率之間的關係，以4-chlorophenol (4-CP)、phenol、bisphenol A (BPA)、ethinyl estradiol (EE2)與trimethoprim (TMP)進行試驗。添加 10-500 mM氯化鈉都會 抑制 P25降解 此 5個化合物 的 速 率 ；但添加超過50 mM溴化鈉則會不同程度的促進P25降解此5個化合物的速率。沒有添加溴化鈉時，反應液中有偵測到HO●；但添加溴化鈉後，溶液中則都沒有偵測到HO●的產生，但有偵測到由溴離子與HO●或P25的電洞反應產生之Br●的形成，因此有溴離子存在時，主要以Br●與化合物反應。由定量構效關係分析的結果發現添加溴化鈉時 Hammett σ constant值愈偏負值的化合物其反應速率愈快表示Br●較會與取代基含有推電子基的化合物反應，也證明了Br●會選擇性地攻擊苯環結構上富含電子的化合物。此研究了解了環境因子對於P25光催化降解汙染物的影響、建立偵測Br●的方法、也探討化合物結構與反應速率之間的關係。|
Tris (1,3-dichloropropyl) phosphate (TDCPP), one of phosphorus flame retardants, gradually replaces brominated flame retardants. Due to increasing usage of TDCPP, it is detected in the environment, and TDCPP has adverse effects on animals’ reproductive and neurological system. Therefore, TDCPP has to be removed from the environment. Photocatalytic reactions can efficiently remove contaminants in aquatic environments since it takes short time and contaminants are able to be efficiently degraded or even mineralized by photocatalytic reactions. TiO2 nanoparticles (NPs) are one of common photocatalysts and Degussa P25 TiO2 NP was chosen. Several factors might influence photocatalytic reactions, such as solution pH, temperatures, and halide ions like Cl- and Br-, which are likely from seawater, leachate or salty marshes. Hence, the effects of these factors on the photocatalytic reactions have to be investigated. Most previous studies showed that halide ions would inhibit degradation of pollutants by photocatalysts, while according to our previous study, the presence of halide ions enhanced photodegradation efficiency by TiO2 instead. Since the chemicals utilized in above experiments were different, it is necessary to explore the effects of chemical structures on the photocatalytic degradation rates of chemicals by P25 NPs in the presence of halide ions. The surface of P25 contained positive charges in acid conditions, and the positive charges on P25 surface reduced with Cl- or Br-, indicating that they were neutralized by Cl- or Br-. In addition, the results analyzed by Fourier-transform infrared spectroscopy showed that hydroxyl groups possessed on the surface of P25 decreased after the addition of Cl- or Br-, which further suggested that Cl- or Br- would form bonds with the surface of P25. P25 successfully degraded TDCPP in 60 min irradiated with UV light. Furthermore, TDCPP was almost mineralized during the process of photodegradation by P25, and Cl- released was also detected. The photodegradation rate constants of TDCPP with P25 decreased with increasing the initial TDCPP concentrations, but increased with the increase of P25 dosages. Photodegradation rates of TDCPP decreased when pH increased, and they did not be influenced obviously by different temperatures. The decrease of degradation kinetics of TDCPP in the presence of 10-500 mM NaCl or NaBr was observed. In order to further investigate the relationships between chemical structures and rate constants, rate constants of 4-chlorophenol (4-CP)、phenol、bisphenol A (BPA)、ethinyl estradiol (EE2), and trimethoprim (TMP) by P25 were measured. In the presence of Cl-, the degradation rate constants of 4-CP, phenol, BPA, EE2, and TMP all decelerated; while the photodegradation rates of 4-CP, phenol, BPA, EE2, and TMP increased with over 50 mM NaBr. Without NaBr, HO● formed by P25 NPs, while no HO● detected with NaBr. However, the formation of Br● through the reactions of HO● or hVB+ with Br- from P25 was detected. Thus, the dominant radicals reacting with chemicals were Br● in the presence of Br-. The results from quantitative structure-activity relationship showed that the rate constants of chemicals with negative values of Hammett σ constant was larger, indicating that Br● was more likely to react with chemicals possessing electron-donating substituents. It also confirmed that Br● could attack selectively with electron-rich compounds. This study understood the effects of environmental factors on the photodegradation of contaminants, developed the method of detecting Br●, and investigated the relationships between chemical structures and rate constants.
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