Title

Simulated solar light assisted photocatalytic degradation of organic matters

Date of Award

2015

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

Faculty

Faculty of Health, Engineering and Science

First Advisor

Associate Professor Lai-Chang Zhang

Second Advisor

Dr Mehdi Khiadani

Abstract

The increasing human population combined with overexploitation of water resources for domestic purposes, industry, and irrigation, has resulted in a shortage of freshwater supply in many parts of the world. Advanced oxidation processes (AOP) have been applied widely to water and wastewater treatment, especially heterogeneous and homogeneous photocatalysis. The photocatalytic reactions take place under ambient operating conditions, but the photoactivity is usually constrained by the narrow wavelength spectrum for photonic activation of catalysts. The higher-end of ultraviolet (UV) spectrum required for catalyst activation is usually accompanied by high operation costs. From the viewpoint of solar energy utilization, the development of photocatalysis under ultraviolet-visible (UV-Vis) light is emerging as an important research direction in this field, and would be perceived as an advantage offering an energy-cost reduction for the AOPs.

The main goal of this study was to enlarge the existing knowledge in heterogeneous and homogeneous photocatalysis under UV-Vis light application for water decontamination. Commercial dyes, especially azo dyes, were chosen as target compounds, to investigate the degradation and mineralization by nanosized semiconductors or sodium hyperchlorite (NaClO) under the irradiation of UV-Vis from the solar simulators.

Acid red 14, a type of azo dye containing sulfonic groups and being commonly used as a textile dye, was chosen as a target compound, to investigate the degradation by a nanosized commercial TiO2 under simulated solar light. The characteristics such as phase features, specific surface area, bandgap energy, etc., of the nanosized TiO2 were investigated. The color removal rate, decrease rate of total organic carbon (TOC), and production rate of sulfate ions in aqueous solution were determined. Acid red 14 could be decolorized and mineralized efficiently with this nanosized TiO2 under simulated solar light. Acidic or neutral conditions were beneficial to the decolorization of acid red 14 aqueous solutions. Acid red 14 was nearly 100% mineralized after 60-min irradiation by simulated solar light under certain experimental conditions. The photocatalytic degradation efficiency increased with increasing irradiation intensity of solar simulated light. TiO2 dosage, pH value of aqueous solution, and initial concentration of acid red 14, had significant effects on the decolorization efficiency. It is feasible to use photocatalysis with this nanosized TiO2 under simulated solar light to efficiently degrade and mineralize acid red 14. The high photocatalytic efficiency of this TiO2 under simulated solar light might be related to its lower bandgap energy (Eg) and the relatively higher fraction of anatase phase.

The photocatalytic degradation of mordant black 11 under UV–Vis light, was investigated using a type of commercial nanosized zinc oxide (ZnO) as photocatalyst in aqueous solutions. Nanosized ZnO was characterized by different methods, including UV–Vis diffuse reflectance spectrum (UV–Vis DRS), X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) method, etc. The effects of influential experimental parameters on decolorization rate, including catalyst dosage, initial dye concentration, initial pH value, irradiation intensity, etc., were studied in details. It was found that after 60-min irradiation, the decolorization rate was only 58.4% at pH 2, but above 98% at natural (pH ~ 4.27) and alkaline conditions. Mordant black 11 could be nearly 100% decolorized after 60 min irradiation by UV–Vis light at natural/alkaline pH and 0.3 g/L dosage. It was found that alkaline pH increased the decolorization rate; an increase in irradiation intensity of UV–Vis light enhanced the degradation rate of the dye significantly at low dosage, while little influence as 0.3 g/L dosage.

The homogeneous photocatalytic degradation of Cibacron Brilliant Red 3B-A (BR 3B-A) in aqueous solution was investigated using sodium hypochlorite (NaClO) as the photocatalyst under the irradiation by UV-Vis light, including simulated solar light and that stronger in UV region. Compared with hydrogen peroxide (H2O2) and Fenton’s reagent at pH 3, NaClO showed much better photocatalytic performances: the decolorization rate reached 97% as dye concentration 100 ppm at the 1.0 g/L NaClO dosage and natural pH. Acidic conditions were more beneficial to the decolorization of BR 3B-A aqueous solution.

Ball milling (BM) was performed by changing the milling time from 0 to 12 h, and revolution speed fixed at 200 rpm. Ball milled TiO2 (A) was used to photocatalytically degrade three commercial azo dyes with different chemical structures. Firstly, BR 3B-A was photocatalytically degraded with the TiO2 (A) ball milled for different time periods. It suggested that among the samples, TiO2 (A) ball milled for 2 h showed the best photocatalytic degradation of BR 3B-A: decolorization rate reached 94.4% at 30 min, compared with 30.0% of the original TiO2 (A). Then TiO2 (A) ball milled for 2 h (BM2 TiO2 (A) )were chosen to photcatalytically degrade and mineralize BR 3B-A, Cibacron Brilliant Yellow 3G-P (BY 3G-P), and Astron Pink FG, with Chloride (Cl) linking to different kinds of Carbon atoms. As known from the ion chromatography (IC) results, the Chlorides broke apart from the aliphitic carbon much more easily and quickly than those connecting with Benzene and Thiazine. Cl- production rates were 99.7%, 78.1%, and 73.2%, respectively, for Astron Pink FG, BY 3G-P, and BR 3B-A at the same interval, after 60-min irradiation. An increase in the amorphous phase in a certain degree was presumably responsible for the increase in photocatalytic performance of TiO2 (A) ball milled for 2 h.

As clearly shown from the results, it is flexible to employ heterogeneous/homogeneous photocatalysis under simulated solar light and UV-Vis light stronger in UV region to degrade and mineralize the organic matters. It will be beneficial for decreasing the energy requirements, the decontamination cost, and the secondary pollution due to the relatively higher mineralization degree. Future study should be emphasized on the photocatalytic degradation of emerging organic pollutants, such as natural organic matters (NOM) like humic acid, pharmaceuticals, pesticides, etc., and prospect the possible mechanisms

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