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Photocatalytic Degradation Kinetics and Mechanism of Model Organic Pollutants in Water Environment

Author: AnJiBin
Tutor: FengHuiXia；AnTaiCheng
School: Lanzhou University of Technology
Course: Applied Chemistry
Keywords: Photocatalytic Kinetics Mechanism Central Composite Design Dimethyl Phthalate Lamivudine Acyclovir
CLC: X13
Type: Master's thesis
Year: 2011
Downloads: 229
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Abstract

Organic pollutants are ubiquitous in the global water environment. These pollutants, which can be bio-accumulated through food chain, may pose hazardous threats to eco-environment and human health, due to their long residual life, high bio-accumulative ability and potential toxicity. Thus, it is very necessary to conduct researches to eliminate these pollutants from water environment.In present, the emergent environment pollutants such as pharmaceuticals and endocrine disrupters have attracted a broad interests. However, most of work about photocatalytic degradation of these pollutants mainly focus on the degradation kinetics. However, the single-variable-at-a-time （SVAT） method has lots of drawbacks in relation to not only time-consuming, but also the absence of interactions between different variables as well as the inefficiency to predict the true optimal photocatalytic process. An alternative method, central composite design （CCD） based on response surface methodology （RSM） can overcome all these above mentioned shortcomings.In this thesis, one endocrine disrupters （dimethyl phthalate） and two antiviral pharmaceuticals （lamivudine and acyclovir） were choosing as model organic pollutants. Both SVAT and CCD experiment approaches were respectively employed to assess the photocatalytic degradation and mineralization process. The findings are summarized as following:（1）TiO₂ photocatalysis is an effective advanced oxidation technology for the decontamination of dimethyl phthalate, and the degradation half-life was 35.9min.The distribution experiments of five reactions specious (such as e_cb^-,h_vb⁺ ,·OH ,O₂^·- and H₂O₂) during the photo-catalytic process were investigated systemically, and the results indicated that the ?OH is the main reaction species, and the other reaction species play minor role in the photocatalytic process.（2）Compared SVAT with CCD experiment approach, the CCD based on response surface methodology is a powerful tool to optimize and assess the individual and interaction effects of variables on the photo-catalytic process. The optimal conditions were: TiO₂ at 1.00 g L^-1, pH value at 6.7, and initial lamivudine concentration of 60μM L^-1. A tentative mechanism of photocatalytic degradation of lamivudine was proposed from HPLC/MS-MS results coupled with the calculated data of the FEDs and point charges. The hydroxylation addition reaction （major） and the direct photo-hole oxidation （minor） are considered as two initial predominant attack pathways for photocatalytic degradation of lamivudine. Further oxidation of early cleavage intermediates and the hydroxylation products would be decontaminated into CO₂, H₂O and other inorganic ions (NH₃/NH₄⁺, NO₃^-, SO₄^2-)（3）Two semi-empirical expressions were obtained and successfully applied to optimize the photocatalytic degradation and mineralization process. And the optimal experiment condition for acyclovir degradation and mineralization were acyclovir concentration at 50μmol L^-1, TiO₂ concentration at 0.5g L^-1, initial pH value at 6.37 and light intensity at 2.0 mW cm^-2. In addition, the seven intermediate compounds of acyclovir degradation were identified by HPLC/MS/MS, and the tentative pathway was also proposed support by FEDs results.

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