Geshan Zhang

High performance sulfur, nitrogen and carbon doped mesoporous anatase–brookite TiO2 photocatalyst for the removal of microcystin-LR under visible light irradiation

Carbon, nitrogen and sulfur (C, N and S) doped mesoporous anatase-brookite nano-heterojunction titania photocatalysts have been synthesized through a simple sol-gel method in the presence of triblock copolymer Pluronic P123. XRD and Raman spectra revealed the formation of anatase and brookite mixed phases. XPS spectra indicated the presence of C, N and S dopants. The TEM images demonstrated the formation of almost monodisperse titania nanoparticles with particle sizes of approximately 10nm. N2 isotherm measurements confirmed that both doped and undoped titania anatase-brookite materials have mesoporous structure. The photocatalytic degradation of the cyanotoxin microcystin-LR (MC-LR) has been investigated using these novel nanomaterials under visible light illumination. The photocatalytic efficiency of the mesoporous titania anatase-brookite photocatalyst dramatically increased with the addition of the C, N and S non-metal, achieving complete degradation (∼ 100 %) of MC-LR. The results demonstrate the advantages of the synthetic approach and the great potential of the visible light activated C, N, and S doped titania photocatalysts for the treatment of organic micropollutants in contaminated waters under visible light.

Degradation Mechanism of Cyanobacterial Toxin Cylindrospermopsin by Hydroxyl Radicals in Homogeneous UV/H2O2 Process

The degradation of cylindrospermopsin (CYN), a widely distributed and highly toxic cyanobacterial toxin (cyanotoxin), remains poorly elucidated. In this study, the mechanism of CYN destruction by UV-254 nm/H2O2 advanced oxidation process (AOP) was investigated by mass spectrometry. Various byproducts identified indicated three common reaction pathways: hydroxyl addition (+16 Da), alcoholic oxidation or dehydrogenation (-2 Da), and elimination of sulfate (-80 Da). The initiation of the degradation was observed at the hydroxymethyl uracil and tricyclic guanidine groups; uracil moiety cleavage/fragmentation and further ring-opening of the alkaloid were also noted at an extended reaction time or higher UV fluence. The degradation rates of CYN decreased and less byproducts (species) were detected using natural water matrices; however, CYN was effectively eliminated under extended UV irradiation. This study demonstrates the efficiency of CYN degradation and provides a better understanding of the mechanism of CYN degradation by hydroxyl radical, a reactive oxygen species that can be generated by most AOPs and is present in natural water environment.

The effect of basic pH and carbonate ion on the mechanism of photocatalytic destruction of cylindrospermopsin

This study investigated the mechanistic effects of basic pH and the presence of high carbonate concentration on the TiO2 photocatalytic degradation of the cyanobacterial toxin cylindrospermopsin (CYN). High-performance liquid chromatography combined with quadrupole time-of-flight electrospray ionization tandem mass spectrometry (LC/Q-TOF-ESI-MS) was employed for the identification of reaction byproducts. The reaction pathways were proposed based on the identified degradation byproducts and radical chemistry. In high pH system (pH = 10.5) similar reaction byproducts as those in neutral pH system were identified. However, high pH appeared to inhibit sulfate elimination with less sulfate elimination byproducts detected. In the presence of carbonate in the photocatalytic process, hydroxyl radical reaction would be largely inhibited since carbonate ion would react with hydroxyl radical to form carbonate radical. The second order rate constant of carbonate radical with CYN was estimated to be 1.4 × 10(8) M(-1)s(-1), which is much smaller than that of hydroxyl radical. However, the more significant abundance of carbonate radical in the reaction solution strongly contributed to the transformation of CYN. Carbonate radical has higher reaction selectivity than hydroxyl radical and hence, played a different role in the photocatalytic reaction. It would promote the formation of byproduct m/z 420.12 which has not been identified in the other two studied photocatalytic systems. Besides, the presence of carbonate ion may hinder the removal of toxicity originated from uracil moiety due to the low reaction activity of carbonate radical with uracil moiety in CYN molecule. This work would further support the application of photocatalytic technologies for CYN treatment and provide fundamental information for the complete assessment of CYN removal by using TiO2 photocatalysis process.

CHAPTER 1:Photocatalytic Degradation of Organic Contaminants in Water: Process Optimization and Degradation Pathways

This chapter discusses thoroughly the outcomes of the TiO2 photocatalytic degradation of organic contaminants of emerging concern, including manmade (insecticides, organochlorinated compounds, and antibiotics) and naturally occurring compounds (cyanotoxins and taste and odor compounds). Specifically, information is provided on the degradation of various organic contaminants in actual water samples, their corresponding reaction kinetics, the individual effects of water quality parameters (including pH, natural organic matter, and alkalinity) and nano-interfacial adsorption phenomena. Emphasis is given to the mechanisms of photocatalytic degradation of organic contaminants based on their structural differences and the corresponding transformation products formed.