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Title: Photocatalytic degradation of pesticides using N-doped TiO₂ nanoparticles activated by visible LED irradiation
Authors: Abdelhaleem, Amal Sayed Moustafa
Degree: Ph.D.
Issue Date: 2019
Abstract: The escalating contamination of aquatic environment by pesticides has received considerable attention because of their persistence, toxicity, and negligible biodegradability. Hence, the accumulation of pesticides in surface and/or groundwater may cause drastic effects on the human health and aquatic lives. Therefore, the development of effective advanced oxidation processes (AOPs) for degradation and mineralization of pesticides is a crucial issue. Since visible light represents about 45% of the solar energy, visible light mediated photocatalysts could be an ideal option. However, the popular TiO₂ photocatalyst has a limited absorption under visible light due to its large band gap. Accordingly, nitrogen doping into TiO₂ lattice (N-doped TiO₂) was proposed in this study to enhance the photocatalytic degradation of pesticides under visible light. In addition, commercial visible light emitting diode lamps (Vis LED) were utilized for the first time as an alternative light source to conventional visible light sources due to its availability, energy efficiency, low cost, and eco-friendliness. The main objective of this study is to degrade various pesticides, including 4-chlrophenoxyacetic acid, monuron, diphenamid, and carbofuran using sole N-doped TiO₂ or after its combination with other oxidants and/or FeIII to enhance its performance under Vis LED. N-doped TiO₂ (N-TiO₂) and FeIII impregnated N-doped TiO₂ (FeN-TiO₂) were synthesized and characterized using XRD, XPS, UV-vis spectroscopy techniques, BET SSA, TEM, EDX, density of hydroxyl groups, and point of zero charge. Firstly, 4-chlorophenoxyacetic acid (4-CPA) was degraded using N-TiO₂/Vis LED process. The synergetic effect of N-TiO₂/Vis LED process was studied in detail by varying reaction conditions including the initial concentration of 4-CPA, catalyst dosage, light intensity, and initial pH. Additionally, the influence of inorganic anions and radical scavengers on the performance of the N-TiO2/Vis LED process was also evaluated. The N-TiO₂/Vis LED process was found to be a promising approach in terms of mineralization of 4-CPA. It is interesting to note that the performance of this process was not reduced after successive usage of the recycled catalyst; instead, the reaction rate of 4-CPA decay actually increased by using the spent catalyst. Secondly, monuron decay was achieved via peroxymonosulfate (PMS) activation by N-TiO₂ under Vis LED. The N-TiO₂/PMS/Vis LED hybrid process was found to be an effective approach under a wide solution pH range of 2.5-9.2 (> 80% decay). Interestingly, the highest efficiency was observed at pH 9.2 due to the contribution of PMS/Vis LED process by generating both ·OH and SO₄-· at alkaline pH. However, the decay rate of monuron was inhibited at pH 11.6 due to the dissociation of ·OH into O-· and the electrostatic repulsion among reagents. The process was also suitable for ion-rich wastewater since no significant reduction in the performance was induced in the presence of inorganic anions. Furthermore, the process was proven to be a promising approach for mineralization of monuron and its intermediates. Twenty reaction intermediates were detected and five of them are newly reported. A novel mathematical model was established based on reaction intermediates using a parallel-serial-irreversible reaction approach, which is helpful in predicting the detoxification extent of hazardous intermediates.
Thirdly, a combined process was proposed for carbofuran (CBF) degradation through PMS activation under Vis LED by FeN-TiO₂. An in-depth investigation was conducted to examine the synergistic effect of FeN-TiO₂/PMS/Vis LED process under various reaction conditions. An increase in the rate constant was observed with the increment of pH from 2.4 to 7.4, implying the feasibility of the process at neutral pH. A further increase in pH from 8.9 to 11 showed a sudden drop in the rate constant (if the role of base activation is ignored). The efficiency of CBF degradation is still more than 70% after adding NO₃⁻, SO₄²⁻, HCO₃⁻, and Cl-, suggesting the applicability of the process for anion-rich wastewater. Interestingly, the efficiency was not influenced even after the further increment of nitrate concentration. Furthermore, the high chloride concentrations caused an enhanced efficiency due to the generation of reactive halogens through two-electron transfer. Sixteen major intermediates were recognized and eight of them were never reported in the previous studies. Surprisingly, a new degradation pathway was noted in this study via H-abstractions and cyclization mechanisms. The FeN-TiO₂/PMS/Vis LED process exhibited a dual functionality in terms of mineralization efficiency and about 90% TOC reduction can be achieved. Therefore, these findings provide new insights into the mechanism of PMS activation under Vis LED after coupling non-metal doped TiO₂ photocatalyst with a metal component, and its implications for degradation of refractory and non-biodegradable pollutants in wastewater. Furthermore, a photo-Fenton like process (FeN-TiO₂/H₂O₂/Vis LED) was employed for CBF degradation. Accordingly, a novel kinetic model was proposed to predict CBF degradation in the FeN-TiO₂/H₂O₂/Vis LED process under high Vis LED light intensities based on intrinsic reaction parameters, including the Vis LED light intensity, FeN-TiO₂ dosage, initial H₂O₂ concentration, and ·OH generation. The developed model was verified and validated under various reaction conditions. The involvement of ·OH generation in the kinetic model could be beneficial in comparing the obtained results in different experimental setup designs, thereby providing new insights for understanding the photocatalytic mechanism. Finally, degradation of diphenamid (DPA) was examined by a novel process through sulfite activation by FeN-TiO₂ under visible LED (Vis LED). Thirty intermediates were identified and twenty-four of them are newly reported. A new pathway was reported for the first time in the DPA studies through the rupture of benzene ring linkage. A higher mineralization degree was achieved using the FeN-TiO₂/sulfite/Vis LED process, which is not in accordance with previous reports on sulfite-based processes. The absence of sulfate adducts could provide a rational explanation of the higher mineralization degree during DPA degradation. The mechanism of sulfite activation in the FeN-TiO₂/sulfite/Vis LED process was proposed. In addition, the effect of anions was studied in detail. Based on reusability test, the DPA degradation efficiency increased after successive usage of the FeN-TiO₂. Furthermore, a novel two-phase mathematical model was derived to predict the DPA and decay of its intermediates. After the complete degradation of DPA, the leached Fe-ions were found to be negligible and sulfite was completely depleted. Considering several factors such as the cheap source of sulfite (an air pollutant waste from flue-gas desulfurization process), low cost of Fe, negligible leaching of Fe-ions, and high energy efficiency of Vis LED light, the FeN-TiO₂/sulfite/Vis LED process could be a practical and green technology for the removal of wastewater contaminants.
Subjects: Hong Kong Polytechnic University -- Dissertations
Water -- Purification -- Photocatalysis
Pesticides -- Environmental aspects
Pages: xxxi, 276 pages : color illustrations
Appears in Collections:Thesis

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