Mechanism Research For The Photochemical Oxidation Of Microcystins By Natural Minerals

The eutrophication caused algal blooms, and the frequent eruption of the algal bloomscaused more serious environmental problems. Microcystis aeruginosa a class ofcyanobacteria, is the most common, frequent and harmful in water. Algal toxins producedby many cyanobacteria are the algae secondary metabolites, and they are the hepatotoxicitythreat to human health. And it （Microcystins MCs） has been become a common concernedresearch object. So removing the MCs with green and effective methods is a hot research.Mineral catalyzed is a heterogeneous catalytic reaction occurred on the mineral surface,and it takes a full advantage of the natural self-purification. Natural iron-bearing mineralsas heterogeneous catalysts used in Fenton reaction system have been studied widely,referred as a mineral Fenton-like system. Indeed, these materials provide an attractivealternative for the decontamination of soils, underground waters, sediments, and industrialeffluents for the natural, abundant, inexpensive, and environmentally friendly characteristic.Minerals used as photochemical catalyst to degrade the toxic organic pollutants can beused for contaminant in situ chemical oxidation technology a more and more populartechnology. It has practical application value that natural minerals catalytic degradation ofmicrocystins under the light irradiation. Natural mineral used to degrade microcystins wasstill rare. Discussing the intermediates products and degradation pathways in degradationprocess of algal toxins was another hot spot, which has an important significance toanalyse toxicity of degradation product and selectivity of the catalyst.In this paper, standard natural minerals, provided by Cental-South Research Instituteof Metallurgical Geology, Institute of Geology and Mineral Resources （502）, were used ascatalyst to degrade the Rhodamine B,2,4-Dichlorophenol and Microcystins under thevisible light irradiation （λ>420nm）, and then the photochemical properties during thephotochemical oxidation of organic pollutants process were studied. The catalyticmechanism was discussed and splitting of the MCs mechanism was proposed by trackingintermediates products and oxide species in the different catalytic systems process.1. Standard natural minerals were used as environmental treatment reagent, to degradeorganic dye RhB。The adsorption and degradation capability of the minerals under visiblelight irradiation （λ>420nm） were studied. Correlation between the photocatalyticproperties and composition and structure of natural minerals were investigated. And thedegradation mechanism of MCs by high activity catalysts was also studied. 2. Standard natural minerals were selected to degrade the organic dye Rhodamine B（RhB） by activating H₂O₂under the visible light irradiation （λ>420nm）. Natural ore（maghemite） with higher activity to degrade RhB and small molecular compound2,4-dichiorophenol （DCP） was filtrated, and the influencing factors and stability werestudied. The results showed that the RhB and DCP were degraded effectively by themaghemite, activating H₂O₂at the conditions of pH6.8under the visible light irradiation.Maghemite has been used to degrade MC-LR. The concentration of MC-LR was detectedby High Performance Liquid Chromatography, the structure elucidation of reactionintermediates was accomplished by LC-MS/MS. At last, the mineralization effect wasdetected by Total Organic Carbon （TOC） tester. Simultaneously, photocatalytic oxidationspecies was tracked by Electron Spin Resonance （ESR）. Eight hours later, the degradationrate reached to100%, and the mineralized rate of MC-LR was58%after25h. And thehydroxyl radical （·OH） oxidation species was detected in the system. The fragments m/z982.5，m/z825.5of m/z1029were detected, which were significantly different with thedegradation of·OH leading. Since MLCT of≡Fe（III）（OH） on the sueface of maghemitewas well known that it has no obvious excitation under visible light irradiation, even in thepresence of H₂O₂. The results indicated that it was a possible ways that visible lightinduced≡Fe （III）（OOH） to generate Fe （IV）=O, then leading the degradation of MC-LR..3. Pyrite was selected to degrade MCs, at the conditions of pH6.8under the visiblelight irradiation. The concentration of MC-LR was detected by High Performance LiquidChromatography, and the structure elucidation of reaction intermediates was accomplishedby LC-MS/MS. Simultaneously, photocatalytic oxidation species was tracked by ElectronSpin Resonance （ESR）. At last the ion states of pyrite before and after reaction werecarried out using X-Ray Photoelectron Spectroscopy （XPS）. The MC-LR and MC-RRwere degraded effectively. After fourteen hours and ten hours, the degradation rate ofMC-LR and MC-RR reached to100%, respectively. The investigation present that thisgreen environmental photochemical degradation was the oxidation process involvedwith·OH. Double bonds between C4and C5（C6and C7） of Adda and the ethylenic bondof Mdha were easy to be attacked and oxidized. Some peptide bonds were broken downtoo with longer reaction time.4. Hematite was taken as the catalyst to degrade the Rhodamine B and2,4-dichlorophenol under the visible irradiation. The influence factors pH, catalyst dosageand dissolved iron were studied, also the reaction mechanism were discussed though the determination of degradation process by UV-vis spectrophotometer, infrared spectrometer,fluorescence spectrophotometry and COD. The result indicated that RhB could bedegraded effectively in the Cata/RhB/H₂O₂/vis system. The optimum condition werecatalyst dosage0.6g/L; pH3.0; concentration of H₂O₂1.5×10^-3mol/L, under that RhBcould be discolored after180min, and the degradation rate of2,4-DCP was up to56%after24h. The result also suggested that the content of Fe ion leaching in the solution play asmaller part in the process, and it has a good stability. The·OH was determined whichdemonstrated the process of deep oxidation and the oxidative process was predominatedmainly by the hydroxyl radical （OH）.5. Kaoline was modified with mixture of chitosan and hydroxyl aluminum chloride（Al₂ClH₅O₅H₂O） modifier after the simple treatment. Then flocculation to remove themicrocystis aeruginosa was studied. The results showed that the remove rate was up to100%after30min, with20mg/L modified clay under the natural conditions （pH nature）.Compared with other modified clay, it has higher removal efficiency, and the requireddosage was also less than others. The results also indicated that this modified clay cancatch the algae cells to cause the flocculation through the bond bridging function ofchitosan and hydroxyl aluminum chloride. On the other hand, the surface of chitosan andhydroxyl aluminum chloride carried the positive charge, while the surface of algal cellscarried the negative charge, so the algae were flocculated.