Degradation Characteristics And Mechanisms Of Microcystin-LR By Ag₃PO₄ Photocatalyst

The contamination of algae caused by eutrophication is a big challenge in environmental treatment of China. Aquatic ecological safety and human health have been greatly endangered by microcystin due to the malignant proliferation of algae. Therefore, seeking effective methods to control the eutrophication and harmful algae blooms (HABs) have always been of great interest. Ag3PO4 is an environment-friendly photocatalyst with relatively good degradation capacity for hazardous organic pollutants, which can generate highly reactive oxygen species (e.g., HO·) to mineralize organic substances. In this paper, a new attempt was made to investigate the capacity of the Ag3PO4 photocatalyst in the degradation of microcystin-LR (MC-LR) under visible light. The influenced parameters, including initial pH value, initial Ag3PO4 concentration, initial MC-LR concentration and Ag3PO4 catalyst recycling, were fitted with the pseudo-first-order kinetic model. Meanwhile, the degradation intermediates of MC-LR were examined by liquid chromatography/mass spectrometry (LC/MS) and the degradation mechanism was analyzed.The main contents and achievements of this study are as follows.1. Characteristics of the Ag3PO4 photocatalystAg3PO4 photocatalyst was synthesized by the ion-exchange method and characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (SEM), and UV-Vis spectrophotometer (UV-Vis). All diffraction peaks of the as-prepared Ag3PO4 samples were in line with the standard spectrum (JCPDS card No.06-0505); Ag3PO4 consisted of agglomerated smooth spherical particles with an average particle size of approximately 200 nm to 500 nm. The band gap of Ag3PO4 was approximated as 2.35 eV.2. Drawing of the MC-LR standard curveThe intracellular MC-LR was extracted from Microcystis aeruginosa by freeze thawing-solid phase extraction method. The MC-LR standard curve was established by HPLC and the regression equation was y=32.25x-0.074, R2=0.9998.3. Research of degradation kinetics of MC-LR using Ag3PO4 photocatalystThe degradation parameters including initial pH, initial Ag3PO4 concentration, initial MC-LR concentration, and Ag3PO4 catalyst recycling times were in line with the pseudo-first-order kinetic model. The maximum MC-LR degradation rate of 99.98% can be obtained within 5 h under the following optimum conditions:pH of 5.01, Ag3PO4 concentration of 26.67 g/L, MC-LR concentration of 9.06 mg/L, and cycling run of one time,k=1.52 h-1.The photocatalytic degradation of MCs was pH dependent. The hydrophobicity of MC-LR and solubility and catalytic activity of Ag3PO4 catalysts were factors which led to the low absorption of MC-LR under strong acidic or basic conditions. The data suggested that Ag3PO4 catalysts exhibited the best photocatalytic performance for MC-LR degradation under pH 5.01. The degradation efficiency was decreasing with the MC-LR concentration. A maximum value of the effective area of receiving light was detected in the photocatalytic system. The photocatalytic system reached a saturation condition as the initial concentration of Ag3PO4 was further increased from 26.67 g/Lto 33.47 g/L. Excessive dosage of catalyst led to the scattering effect, which affected the light absorption of the photocatalyst and then decreased the pseudo-first-order kinetic constant. The increased number of cycling runs promoted the deposition of the Ag nanoparticles on the surfaces of the Ag3PO4 particles during the photocatalytic process. The deposited nanoparticles covered the active sites which led the decrease of the pseudo-first-order kinetic constant. Hence, Ag3PO4 exhibited the best photocatalytic performance for MC-LR degradation in the first run.4. Research of intermediates and pathways of MC-LR degradation by Ag3PO4 photocatalystThe degradation products of MC-LR were studied in detail by employing liquid chromatography-mass spectrometry (LC/MS) analysis and 9 intermediates products were found. They are as follows:(M+H)+ 1011.5, (M+H)+1027.5, (M+H)+1029.5, (M+H)+781.3, (M+H)+835.4, (M+H)+1009.5, (M+H)+877.4, (M+H)+764.3, and (M+H)+681.4 respectively. The ketone-derivative (M+H)+781.3 was first found in this research.The main process of MC-LR degradation by Ag3PO4 treatment involved HO attack. The main sites of MC-LR molecule attacked by HO· were the cyclic structure of MC-LR and the conjugated diene bond, benzene ring and methoxy group of the Adda side chain. The following three main degradation pathways were proposed according to the molecular weight of the products and the reaction mechanism between HO· and peptides or proteins.1) Hydroxylation on the aromatic ring of Adda. HO· attacked on the benzene ring and formed benzene hydroxylation and/or benzene dihydroxylation through electrophilic substitution reaction, followed by further oxidation to form aldehyde or ketone peptide residues.2) Hydroxylation on the diene bonds of Adda. HO· attacked on the conjugated diene bond of Adda side chain through electrophilic addition reaction and produced dihydroxylated-MC-LR, and then the hydroxylated C4-C5 or C6-C7 bond of Adda was cleaved through further oxidation to form ketone peptide residues; In the second oxidation route, the Adda chain was completely removed and formed the ketone-derivative (M+H)+781.3. One probable pathway:The hydroxylated C4-C5 or C6-C7 bond of Adda was cleaved through further oxidation to form aldehyde, and then the aldehyde was subjected to a series of oxidation processes, forming the m/z 781.3 ketone derivative with an intact cyclic structure.Another possible route was through the substitution of an HO group to the hydrogen of C7 of the Adda chain. This mechanism formed an enol-MC-LR ((M+H)+ 1011.5) that was isomerized to the ketone-MC-LR. After a series of oxidative-induced bond cleavage steps, the ketone derivative (M+H)+ 781.3 was formed.3) Internal interactions on the cyclic structure of MC-LR. The removal of ammonia from the Arg moiety with MeAsp led to the lost of Leu and Mdha, while, the double hydroxylation on the C6-C7 of the Adda chain led the cleaving of aromatic ring, which ultimate formed the linearization structure of MC-LR.