Fang Yanfen

Unique Ability of BiOBr To Decarboxylate D-Glu and D-MeAsp in the Photocatalytic Degradation of Microcystin-LR in Water

Bismuth oxide bromide, BiOBr, was used to catalyze the degradation of microcystin-LR (MC-LR) in water at neutral pH under visible light. During the investigation, twelve intermediates from MC-LR decomposition were identified by LC-MS. In addition to attacking MC-LR at the typically susceptible sites (i.e., the conjugated double bond of the Adda chain and terminal unsaturated bond of the Mdha chain), the BiOBr photocatalyst has the remarkable ability to decarboxylate the free acid groups on d-glutamic acid (Glu) and methyl-d-aspartic acid (MeAsp). This reactivity has not been previously observed with TiO2 photocatalysis or with other MC-LR treatments in which decarboxylation does not occur until the MC-LR ring has been cleaved or mineralized to CO2. Some expected intermediate products were detected with oxygen-18 labeling by using H2(18)O as the solvent to confirm that the decarboxylation process is mediated by BiOBr. Results from characterizing the intermediates as well as oxygen-18 labeling studies indicates that oxidative decarboxylation of MC-LR by BiOBr photocatalysis is not always initiated by hydroxyl radical attack (and/or interaction with a hole followed by hydrolysis) proposed mechanism in TiO2 photocatalysis, whereas likely caused by a direct interaction between photoinduced hole of BiOBr and free carboxyl groups of MC-LR. This unusual decarboxylation behavior seems to be associated with the particular valence band and conduction band state of BiOBr photocatalyst. Also under BiOBr catalysis, a very stable guanidine group of l-arginine (l-Arg) that is nonreactive with TiO2 photocatalysis is converted to an amino group and subsequently oxidized to a nitro group during the decomposition of MC-LR. This reaction sequence is also related to decarboxylation because the guanidine conversion requires a completely or partially decarboxylated precursor. Our results indicate that BiOBr, a photocatalyst that selectively destroys sites crucial to MC-LR toxicity, shows great promise as a means of effectively treating drinking water.

Heterogeneous degradation of toxic organic pollutants by hydrophobic copper Schiff-base complex under visible irradiation

A hydrophobic complex of Cu2+[bis-salicylic aldehyde-o-phenylenediamine], Cu-SPA, was prepared and used as a heterogeneous photocatalyst to degrade organic pollutants in water under visible irradiation (λ ⩾ 420 nm) at neutral pH. The structure of complex was characterized by using nuclear magnetic resonance (NMR), elemental analysis, IR and UV-vis spectrometries. Degradation of Rhodamine B (RhB), Sulforhodamine B (SRB) and Benzoic acid (BA) in water were used as model reactions to evaluate the photocatalytic activities of Cu-SPA. The results indicated that RhB and SRB were easily adsorbed on the hydrophobic surface of Cu-SPA from aqueous solution (the maximum adsorption amount: Qmax = 11.09 and 8.05 μmol/g, respectively). Under visible irradiation, RhB and SRB were decolorized completely after 210 and 240 min, respectively, and BA was removed completely after 5 h. The efficiency of H2O2 was > 95%, in contrast to that of the reaction without catalyst or light (< 20%). In water soluble medium, the hydrophobic Cu-SPA can be used more than 6 cycles. ESR results and the behavior of cyclic voltammetry showed that, in the reaction process, Cu2+-SPA was reduced to intermediate state Cu+-SPA firstly, which was extremely unstable and reacted rapidly with H2O2, leading to high reactive oxygen species (·OH radical ) to degrade the substrate.