Kinetics and application of photo-based advanced oxidation processes for treating organic compounds

Abstract

Advanced oxidation processes (AOPs) are known to be effective treatment options for the removal of organic pollutants from aquatic systems. The hydroxyl radical (·OH) is a key oxidant generated in AOPs that highly and non-selectively reacts with a wide range of organic and inorganic compounds. However, the non-selective character of ·OH could result in a significant radical water matrix demand, which negatively impacts the efficiency of AOPs. In addition, the high reactivity of ·OH causes its extremely low steady-state concentration (≤ 10-12 M), as well as causing difficulty in terms of its direct measurement. Thus, the prediction of the ·OH exposure is important in efficiently controlling AOPs. In this paper, the oxidation mechanisms and kinetics of ·OH and other oxidants in three varieties of AOP, namely photolysis, photocatalytic oxidation with hydrogen peroxide, photocatalytic oxidation with ozone, were investigated. In chapter 2, this study examined the comparative degradation of benzene and phenanthrene using a pulsed ultraviolet light (PUV) reactor. The concentration of free chlorine was determined for various NaCl concentrations (0-10 mM) and electrode distances (10, 20, and 40 cm) in order to investigate the effect of oxidation by free chlorine on degradation. It was observed that the presence of NaCl had a dual effect on benzene removal, while phenanthrene removal increased with decreasing NaCl concentration. Both benzene (0.065 min−1) and phenanthrene (0.24 min−1) pseudo first-order rate constants were highest with a NaCl concentration of 0.25 mM and an electrode distance of 10 cm. The degradation of phenanthrene was much higher than that of benzene due mainly to smaller Dewar’s reactivity values ranging from 1.80 to 2.18 at five different positions for phenanthrene compared to the one position of benzene (2.31), which suggests that phenanthrene is more easily attacked than benzene. In chapter 3, SSNT coupling with GQDs (SSNT@GQD) was successfully synthesized by two-step electrochemical anodization process. SSNT@GQD exhibits great activity to catalytic photodegradation of phenanthrene, indicating that the photochemistry of phenanthrene is of great importance. PHE degradation was investigated in UV/persulfate process under various conditions by experiments and a steady-state kinetic model. The presence of NOM significantly decreased degradation efficiency due to the effects of competitive UV absorption and radical scavenging with the latter one being dominant. The degradation pathway of PHE was proposed based on the identified degradation products. Reaction pathways mainly involving the initial electron oxidation of PHE by SO4·- and further oxidation reactions of the cation intermediate PHE·+ such as ipso-hydroxylation and aromatic ring-cleavage were proposed. In chapter 4, Titanium-doped stainless steel nanotubes (SSNT@Ti) were synthesized by electrochemical anodization and chemical reduction. Benzoic acid was subjected to degradation by photolysis, photocatalysis, photolytic oxidation, and photocatalytic oxidation using SSNT@Ti under simulated solar irradiation in a laboratory scale system. Photocatalytic oxidation employing SSNT@Ti along with hydrogen peroxide (H2O2) as an oxidant exhibited the highest degradation rate (8.08 × 10-3 s-1), while without SSNT@Ti was 5.06 × 10-3 s-1. Pseudo-first-order rate constants were expressed as functions of the initial benzoic acid concentration, irradiation intensity, and initial H2O2 concentration. An empirical kinetic model was developed based on the kinetic equations in terms of each factor, which resulted in good prediction of the photocatalytic oxidative degradation rates (R2 = 0.98). Compared to other photocatalysts reported in the literature, SSNT@Ti showed competitive photocatalytic ability. ·OH played dominant roles for the degradation of benzoic acid, while degradation efficiency was improved by improved decomposition of hydrogen peroxide at N2 gas ambient (absence condition of dissolved oxygen). SSNT@Ti showed the 2 times higher TOC removal efficiency than without SSNT@Ti, indicating that SSNT@Ti had a good mineralization ability. The response surface methodology (RSM) was performed to find a suitable approximating function, and least squares regression was obtained as best-fit model. The predicted versus observed results plot for BA degradation rate showed in the linear regression fit (R2=0.99), and the model is statistically significant with the model F-value of 170.84 and a low p-value (p < 0.05). Overall, the important degree of three variables on BA degradation is: light intensity > H2O2 dosage ≈ initial BA concentration. The optimum conditions were initial BA concentration 0.05 mM, light intensity 330 W·m-2 and H2O2 dosage 3 mM within the range of experiment. In chapter 5, the purpose of this research was to investigate of new material to apply in photocatalytic ozonation processes. Firstly, lab-sacle test was carried out using real plant wastewater (Initial COD = 350-450 mg·L-1). In the dark condition with SSNT, there is no degradation of COD and TOC, indicating that adsorption of the organic compound onto SSNT surface was negligible. In the UV light irradiation condition with SSNT and ozone showed the improved degradation efficiency of COD and TOC. Secondly, pilot-sacle batch and continues test were carried out using same plant wastewater to achieve the goal (final COD = 100 mg·L-1). In batch test, reaction volume acts important parameter during operating condition. Degradation efficiency was increased with decreasing the reaction volume. In continuous system test, the experiment was conducted depending on the UV configuration and circulation stage. As a results, UV configuration affect to organic compound degradation and four UV lamps should be installed at least per each stage in SSNT reactor. O3/CODcr ratio was 1.6 ∼ 1.7 in the photocatalytic ozonation with SSNT, while O3/CODcr ratio is 3 ∼ 5 in general. That is, photocatalytic ozonation system with SSNT is cost-effective and applicable process for degrading the organic compounds.