| dc.description.abstract | Due to the efficiency of conventional drinking and wastewater treatment processes to remove endocrine disruptors is frequently unsatisfactory, other technology units such as advanced oxidation processes oftentimes are needed in order to achieve ideal discharge quality. One of the appealing technologies is the UV/TiO2 photocatalytic decomposition process, as it has the capacity to nonspecifically oxidize organic compounds ultimately to CO2 and H2O, hence being able to successfully eliminate recalcitrant organic pollutants from the solution.
In this study, photo-degradation experiments of nonylphenol (NP) were conducted in batch reactors using P-25 Degussa particles and UV-A lamps with an emitting wavelength of 352 nm as the TiO2 and UV sources, respectively. In addition to investigating the effects of TiO2 dosage, solution pH and the ionic strength on NP removal during the photocatalytic decomposition process, the potential of applying immobilization techniques in this process was also assessed.
Experimental data showed that photo-degradation efficiency of NP increased with increasing amounts of TiO2, resulting in 100 mg/L as the optimal TiO2 dosage that caused 100% of NP removal in 2 hr. Further, the pseudo first order model was successfully used to explain the kinetic behavior of NP destruction in the solution as the model fitted the data well. However, results of TOC analysis on samples taken after 2-hr reactions showed that 23% of organic carbons remained in the solution, suggesting that some intermediates were produced and retained over the course of the experiment. When the pH of the system varied, higher NP removal was observed at pH 6-7 than at pH 3 and pH 11. This can be explained by the nature of the heterogeneous photocatalytic degradation process, which involves an initial contact between pollutants and the surface of TiO2 before the degradation kicks in. Given that the point of zero charge (pHpzc) of TiO2 is approximately 6.5 and the pKa of NP is 10.28 or so, under alkaline conditions both TiO2 and NP would have negatively-charged surfaces and may experience electrostatic repulsion when they proceed to each other; therefore, ineffective NP photo-degradation resulted from a decrease in the sorption of NP to TiO2 is anticipated. On the other hand, under acidic and neutral conditions, TiO2 particles and NP may have a better contact chance through the electrostatic pairing force and the van der Waals attractive force, therefore resulting in higher photo-decomposition rates; nonetheless, compared to acidic conditions, neutral conditions still have relatively high reduction rates because of more hydroxyl free radicals generated under such circumstances. Addition of ionic compounds such as NaCl and Na2SO4 did not lead to significant aggregation of of TiO2, nor was the competition between the ions and the pollutant (i.e., NP) for the active sites of TiO2 surface observed. In other words, the degradation efficiency was not affected to a great deal after the ionic compounds were supplemented into the system. Lastly, NP removal through immobilization of TiO2 particles using alginate or chitosan was worse than the use of the powder-formed TiO2 in the photo-degradation system. Scanning electron microscopic analysis of these immobilized particles indicated that the majority of TiO2 powders were entrapped inside the structure of the immobilized particles, thus preventing TiO2 from the contact of pollutants and further resulting in decreases in the degradation efficiency. | en_US |