Development And Application Of Visible Light-based Peroxide Oxidation Processes For Degradation Of Dyes In Aqueous Solution

Fenton technology is one of advanced oxidation processes (AOPs), which finds wide application in the treatment of organic wastewater. This process involves the hydroxyl radical (·OH) formation in Fenton reaction (Fe2+ and H2O2) at acidic condition. The ·OH with high redox potential can oxidize a broad range of organic pollutants effectively. Similar to Fenton process, transition metal ions can also activate persulfate (PS) to generate sulfate radical (SO4·-). The SO4·- is another powerful oxidant and can oxidize most of organic contaminants. This SO4·--generating process is called as Fenton-like process. In a conventional Fenton reaction, the Fe2+/Fe3+ cycle is a key step. The ultraviolet (UV) light irradation can significantly boost the Fe2+/Fe3+ cycle and thus accelerate the Fenton reaction, which is known as UV-Fenton. However, it is well-known that only 4% of solar energy belongs to UV light, whereas visible light covers about 45% of the energy. Therefore, the development of visible light-based peroxide oxidation processes is of great significance. In this dissertation, the visible-light-irradiated peroxide oxidation processes using light-emitting diode (LED) as irradiation source were developed by means of photoexcitation of dyes and visible-light-harvesting of photocatalysts for the degradation of dyes in aqueous solution. The removal efficiency of dyes and reaction mechanism as well as the reusability and stability of heterogeneous catalysts were investigated.(1) Fe-supported bentonite (Fe-B) was successfully fabricated as a low-cost heterogeneous catalyst for adsorption and visible light photo-Fenton degradation of Rhodamine B (RhB) from aqueous solution. The support of iron rendered a significant, increase in specific surface area and a slight increase in interlayer spacing of bentonite, thus improving the adsorption performance of Fe-B. The maximum adsorption capacity of RhB onto Fe-B was calculated to be 227.27 mg g-1 by the fitting of Langmuir adsorption isotherm. The catalytic activity of Fe-B was evaluated by the degradation of RhB in the presence of hydrogen peroxide (H2O2) under visible light irradiation. The estimated quantum yield of photocatalytic degradation of RhB was calculated as (1.66 ±0.21)×10-2. The results showed that the Fe-B catalyst demonstrated good performance in the removal of RhB by combination of adsorption and degradation with optimum operating conditions of Fe-B 0.25 g L-1, H2O212 mM and natural pH 4.2. The Fe-B/H2O2/Vis process was effective toward RhB removal in a wide pH range from 3.0 to 9.0 and the Fe-B catalyst exhibited good chemical stability after five consecutive adsorption-degradation cycles.(2) Fe-B was used as an inexpensive adsorbent to rapidly remove RhB from aqueous solution and the spent Fe-B was regenerated in the visible light photo-Fenton process via the photoexcitation of RhB. The heterogeneous visible light photo-Fenton process was found to be effective for the regeneration of the spent FeMB in the pH range from 3.0 to 9.0. Furthermore, the regeneration efficiency of as high as 79% was still achieved after 5 consecutive adsorption-regeneration cycles when operated at circumneutral pH (pH= 6.0). Considering that, the visible light photo-Fenton approach could be applied as an excellent alternative for regenerating clay-based adsorbents by avoiding the use of dissolved iron salts.(3) A novel kaolinite-supported iron oxide (Fe-K)/PDS/Vis process for the degradation of RhB from aqueous solution is reported. It was found that although peroxydisulfate (PDS) can degrade RhB via a non-radical reaction, the excited RhB molecule (RhB*) and the Fe(?) species formed on the catalyst surface can effectively activate PDS to generate radicals which degrade RhB under visible light irradiation. On the basis of quenching experiments and electron paramagnetic resonance (EPR) studies, it is suggested that the free radicals produced from PDS coupled with the surface-adsorbed radicals formed on the catalyst were responsible for the degradation of the dye via RhB*. Moreover, the Fe-K catalyst showed excellent reusability and stability with a low level of iron leaching.(4) A new source of ·OH and/or SO4·- generated from visible-light-induced activation of peroxides via photoexcited electron transfer from organic dyes is reported, where RhB and Eosin Y (EY) were selected as model dyes. The formation of ·OH and/or SO4·- in the reactions and the electron transfer from the excited dyes to peroxides were validated by means of EPR, photoluminescence (PL) spectra and cyclic voltammetry (CV). The performance of the peroxide/dye/Vis process was demonstrated to vary depending on the types of peroxide and target substrate, for instance, the excited state of RhB (RhB*) was active for PDS and peroxymonosulfate (PMS) activation but inactive for H2O2 activation, whereas the excited state of EY (EY*) was capable of activating H2O2, PDS and PMS. Meanwhile, the peroxide/dye/Vis process was effective for simultaneous decolorization of dyes and production of active radicals under neutral even or basic conditions, without the need of any catalysts.(5) An earth-abundant Fe-containing MOF material, namely MIL-53(Fe), was synthesized via hydrothermal method. MIL-53(Fe) shows photocatalytic activity for the degradation of Acid Orange7 (AO7) from aqueous solution under visible light irradiation, yet the photocatalytic performance of bare MIL-53(Fe) was not satisfactory due to the fast recombination of photoinduced electron-hole pairs. This can be effectively overcome by adding PDS to the catalytic process. The accelerated photocatalytic degradation of AO7 is demonstrated by the result that the degradation efficiency of AO7 in the MIL-53(Fe)/PDS/Vis process reached almost 100% within 90 min as compared to only 24%under the identical experimental conditions for the MIL-53(Fe)/Vis process. To investigate the mechanism of the MIL-53(Fe)/PDS/Vis process, PL spectra, electrochemical measurements and EPR analysis were performed. It was deduced that the efficient separation of photogenerated electrons and holes by the introduced PDS and the subsequent formation of reactive radicals resulting from the activation of PDS by photogenerated electrons accounted for the accelerated photocatalytic degradation of AO7 in the MIL-53(Fe)/PDS/Vis process. Furthermore, the applicability of MIL-53(Fe) used in the peroxydisulfate-mediated photocatalytic process was systematically investigated in terms of the identification of reactive radicals, the reusability and stability of the photocatalyst, as well as the effect of operating parameters.