Degradation of antibiotic norfloxacin by solar light/visible light-assisted oxidation processes in aqueous phase

Abstract

In recent years, the contamination of water sources by antibiotics is one of the most concerned environmental issues worldwide due to the widespread application of antibiotics in both human and animals health care. Among the existing antibiotics, norfloxacin contributes to a significant portion of the water environment and thus has to be removed prior to the discharge of the treated water. The presence of norfloxacin in aquatic environment has been reported to interfere with bacterial DNA replication, be toxic to aquatic plants and organisms, and contribute to bacterial geno-toxicity. Therefore, the development of treatment technologies operating in low cost and with high efficiency becomes an important and attractive issue for the removal of antibiotics from aqueous medium. Till now, little is known about the degradability and reaction products of norfloxacin by solar light or visible light excited photocatalysis. The knowledge regarding the norfloxacin decomposition by UV, ozonation, and ClO₂ oxidation processes is limited although these processes have shown good performance in terms of norfloxacin decay in some studies. The objectives of this dissertation are to explore and evaluate three processes including simulated solar light mediated bismuth tungstate process (SSL/Bi₂WO₆), SSL/Bi₂WO₆ with the assistance of hydrogen peroxide (SSL/Bi₂WO₆/H₂O₂), and visible light mediated carbon doped titanium dioxide process (Vis/C-TiO₂) for the elimination of norfloxacin in water and waste waters. For the degradation of norfloxacin via SSL/Bi₂WO6 process, the effects of environmental parameters including catalyst dosage, probe concentration, and solution pH on the process performance were investigated systematically. At weak alkaline, an optimal decay performance was obtained which can be attributed to the beneficial effect of hydroxyl ions. The reaction rationale and related constants were determined by Langmuir-Hinshelwood (LH) model combined with the experimental results. Our results showed that the adsorption of norfloxacin onto Bi₂WO6 benefited the SSL/Bi₂WO6 process, and the Bi₂WO6 maximum adsorption capacity was determined to be 3.20× 10-3 mmol/g. In the SSL/Bi₂WO6 process, the degradation of norfloxacin was accompanied by the decomposition of germicidal group and benzene ring, as well as gradually generation of inorganic ions of NH4+ and F-. Furthermore, the toxicity of initial norfloxacin solution was efficiently eliminated by the SSL/Bi₂WO6 process. The practical oxidative species in the SSL/Bi2WO6 process for the removal of antibiotic norfloxacin was investigated in details. The norfloxacin decay performance under the influence of different radicals scavengers were carried out to determine the effective oxidative species and the hydroxyl radical was confirmed to be the key oxidative species. The effect of inorganic salts on the decay performance was also investigated. The overall norfloxacin decay can be characterized into photolysis, photocatalysis-via hydroxyl radical, and photocatalysis-via direct hole oxidation; it was calculated that their contribution will be 29.7, 65.5, and 4.8% respectively for norfloxacin removal. The degradation mechanism of norfloxacin through the SSL/Bi₂WO₆ process was further proposed based on the LC/MS analysis. The SSL/Bi₂WO₆ process was effective at a pH range from 5.0 to 9.0 for norfloxacin decay. To further broaden the sensitive/workable pH range of SSL/Bi₂WO₆ process, a buffer system and Fe³⁺ salt was introduced at extreme basic and acidic pH, respectively. The buffer system continuously supplies higher dosage of hydroxyl ion for active radical generation and prevents the acidification of the solution, resulting in a better norfloxacin and TOC removal at alkaline condition. The Fe³⁺ salt on the other hand, offered an additional homogeneous photo-sensitization pathway and altered the surface property of heterogeneous Bi₂WO₆. The former will assist the norfloxacin decay and the latter can increase the collision between the photo-generated hole and hydroxyl ions. The combination of both will significantly improve the decay of norfloxacin at acidic pH range. To improve the decay of norfloxacin, the SSL/Bi₂WO₆/H₂O₂ process was carried out for the removal of antibiotic norfloxacin in aqueous solution. The degradation kinetics can be characterized by a two-stage pseudo first-order reaction kinetics under various reaction conditions. In general, a fast first-stage was observed and followed by a slower second-stage. The effects of the norfloxacin initial concentration (0.05 - 0.30 mM), catalyst dosage (0.5 - 3.0 g/L), and hydrogen peroxide dosage (0.2 - 20.0 mM) were investigated, and the process was optimized at 2g/L Bi₂WO₆ and 10 mM hydrogen peroxide. The kinetic model derived for the SSL/Bi₂WO₆/H₂O₂ process can predict the removal of norfloxacin successfully. In addition, the degradation mechanism of norfloxacinin during the process was also proposed based on the identified six aromatic intermediates, in which the reaction is initiated by the attack of ·OH on either the piperazine or quinolone moieties as discussed in detail in this study. Finally, a Vis/C-TiO₂ process was used for the degradation of norfloxacin. An original model for deciphering the surface property of the catalyst was proposed and confirmed by various tests. The decay rates fit a pseudo first-order kinetics and can be described by the LH model. The effects of C-TiO₂ dosage, norfloxacin initial concentration, and pH levels were investigated and optimized. The optimal pH level was found to be located at weak basic condition; out of this range, the decay rate will be reduced. This is due to the change of charges on C-TiO₂/norfloxacin interface, effective particle size and surface property of C-TiO₂. To get a deep insight of the process, the norfloxacin decay performance was investigated with the addition of hydroxyl radical scavenger and electron acceptor. Hydroxyl radicals were verified to play a major role in the decomposition of norfloxacin by applying methanol as scavenger. In the presence of electron acceptor (IO₃⁻), the norfloxacin decay was improved at low [IO₃⁻], but the effect was attenuated at higher concentrations. Moreover, the effect of various inorganic ions was also studied: the hole scavenger of ammonium did not show negative influence, whereas the fluoride presented a unique restriction in the norfloxacin decay. The reuse and sedimentation properties of C-TiO₂ was evaluated for practically application as well.