|Author:||Lin, Koon Yee|
|Title:||Degradation of triclosan in water by heterogeneous advanced oxidation process activated by magnetic nanoparticles|
|Subject:||Water -- Purification -- Materials.|
Organic water pollutants -- Biodegradation.
Nanostructured materials -- Industrial applications.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Civil and Environmental Engineering|
|Pages:||vii, 110 pages : color illustrations|
|Abstract:||The major aim of this dissertation is to investigate the degradation of a pharmaceutical and personal care product (PPCP) and endocrine disrupting chemical (EDC), triclosan (TCS), by an advanced oxidation process (AOP) using magnetic nanoparticles, Fe₃O₄ and MnFe₂O₄, especially their performance in activation of oxidants, hydrogen peroxide, peroxymonosulfate (PMS) and persulfate (PS). Effect of different experimental parameters including pH and dosage on the TCS adsorption and degradation by Fe₃O₄ was studied. The adsorption of TCS onto Fe₃O₄ was affected by the pH of the system and the Fe₃O₄ dosage. Maximum TCS adsorption capability was observed at pH 2-6. Increase in Fe₃O₄ dosage led to increase in TCS adsorption. Fe₃O₄/PMS was found to be superior in terms of TCS degradation efficiency than Fe₃O₄/PS or Fe₃O₄/H₂O₂ especially in neutral pH range and thus become the focus of the investigation. At pH 2, both Fe(II) and Fe(III) were detected in the solution throughout the reaction, suggesting the reaction mainly proceeds through homogenous mechanism. However, no iron species were detected in the solution for the other cases with pH level above 2, and optimum treatment performances were found to be in the neutral pH range (6-9). Under these circumstances, the reactions mainly occur on or nearby the surface of Fe₃O₄ through the heterogeneous mechanism. Optimal TCS to PMS dosage was 1:25.|
MnFe₂O₄ system was more complex than Fe₃O₄ system as MnFe₂O₄ alone can oxidize TCS at pH 3.3 or below likely through non-radical pathway involving the complexation of TCS with Mn(III) on the surface of MnFe₂O₄ and subsequent electron transfer. The addition of PMS or PS further enhances the removal efficiency of TCS at all pH levels tested but H₂O₂ inhibits it. PMS produced the most significant improvement with the reaction time cut from 4 hours to less than 20 minutes. Effect of pH and dosages and their kinetics were studied in detail for the degradation of TCS by MnFe₂O₄/PS and MnFe₂O₄/PMS. Optimal reaction pH for MnFe₂O₄/PS was 3.3 while optimum PMS and MnFe₂O₄ dosages were at 0.5 mM and 0.5 g/L respectively. Reaction rate decreased as pH increased. Autocatalysis was observed in MnFe₂O₄/PS system at pH 4.0 and above and a mathematical model was developed for pH 4.0. For MnFe₂O₄ /PMS system, optimal performance were at pH 5-9, and 0.5 mM PMS and 0.75 g/L MnFe₂O₄. As the most efficient system in this study, MnFe₂O₄/PMS was also evaluated for practical application. MnFe₂O₄ exhibited good recyclability with improvement in degradation efficiency in the second and third cycle before stabilizing for the fourth and fifth. Besides, MnFe₂O₄ retained TCS degradation capability even without addition of PMS in the second cycle. Degradation of TCS was slower in secondary effluent than in distilled and deionized water (DDW). Toxicity assessment was performed on TCS degradation by MnFe₂O₄ at pH 3.3 and MnFe₂O₄/PMS at pH 7.0. The treatment with MnFe₂O₄ alone at pH 3.3 exhibited lower toxicity for brine shrimp than MnFe₂O₄/PMS at pH 7.0. A new experimental approach was explored to eliminate the use of in situ oxidants to lower toxicity and take advantage of PMS superior power in TCS degradation at the same time. MnFe₂O₄ pre-activated by acid, PMS and PS can remove TCS at neutral pH without any in situ oxidants. The major degradation pathway is believed to be non-radical reactions while oxidation by sulfate radicals is considered to be the minor pathway.
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