Activated Carbon Supported Mn₃O₄ Ultrasonic Catalytic Degradation Of Organic Pollutants In Water

Water is the basis of all life forms on our planet.With the rapid growth of population,economic development and the increasing industrialization of human society,water quality began to decline.The increase of water consumption leads to water pollution,in which industrial wastewater plays a leading role.Most of the water is highly toxic and mineralized,so the treatment of wastewater is urgent.Aldehydes and phenols are widely used in pesticide,petrochemical and other fields,which are highly toxic and difficult to degrade.Therefore,ultrasonic degradation of organic pollutants,an advanced oxidation process,has gradually attracted widespread attention and began to be used in the degradation of pollutants in water.In this paper,the supported catalyst?Mn₃O₄/AC?was prepared by chemical impregnation method and ultrasonic technology was used to degrade aldehydes?p-hydroxybenzaldehyde,salicylaldehyde?and phenols?2,6-dimethylphenol,catechol and methylhydroquinone?.When the sound pressure generated by ultrasonic wave propagates through the liquid medium,cavitation bubbles are generated rapidly due to the oscillation.The rupture of cavitation bubbles and the subsequent high temperature and pressure environment accelerate the oxidation reaction between catalyst and organic matter.?1?Preparation and characterization of Mn₃O₄/AC catalystThe supported catalyst?Mn₃O₄/AC?was prepared by chemical impregnation method.According to the factors affecting the catalyst activity?including the concentration of the impregnation solution,the calcination temperature and the amount of activated carbon?,the orthogonal experiment with 3 factors and 3 levels was designed.The best degradation catalysts were characterized by X-ray diffraction?XRD?,cold field emission scanning electron microscope?SEM?and specific surface area analyzer?BET?.At the same time,the loading capacity of catalyst no.1-9 was measured.The results showed that the diffraction peak position of Mn₃O₄/AC was the same as that of the standard card,and the part of Mn₃O₄particles was tetragonal crystal structure,and most of them were uniformly loaded on the surface of activated carbon Under the same preparation conditions,there are different loads.?2?Degradation of pollutants by ultrasonic catalysisThe degradation of aldehydes and phenols in water by Mn₃O₄/AC combined with ultrasound was studied.The degradation conditions were optimized?in terms of catalyst type,catalyst dosage,ultrasound power,pH value and initial concentration?.Under the optimum conditions,the degradation rates of p-hydroxybenzaldehyde and salicylaldehyde were 99.53%,95.23%,and the degradation rates of 2,6-dimethylphenol,catechol and methylhydroquinone were 99.49%,97.62%,98.15%.?3?Mechanism of degradation of aldehydes and phenols in wastewaterUnder the optimal experimental degradation conditions,different types and concentrations of free radical scavengers were added to each organic pollutant.According to the degree to which the free radical scavengers inhibited the oxidation reaction in the ultrasonic catalysis process,the reaction types during the ultrasonic degradation were speculated,and it was concluded that most of the reactions of the studied aldehydes and phenols during the degradation process were pyrolysis reactions at high temperature,and a very small part of them were the oxidation reactions of hydroxyl radicals.High performance liquid chromatography?HPLC?was used to detect the intermediate products produced during the molecular degradation of each organic substance,and the reaction mechanism of the five organic pollutants in the catalytic degradation process was also speculated.In aldehydes,the reaction pathway of p-hydroxybenzaldehyde may be that the aldehyde group on the benzene ring is oxidized to carboxyl group,hydroquinone is generated after CO removal,then p-benzoquinone is further oxidized to generate enoic acid,then oxalic acid,and finally small molecules CO₂ and H₂O are formed by mineralization;The reaction pathway of salicylaldehyde can be divided into two types.One is the unstable aldehyde group and hydroxyl group on the benzene ring,which can be easily removed to form benzoquinone at high temperature.The benzoquinone is unstable to remove co to form cyclopentane-2,4-dienone.The oxidation ring opens to form cis-butyric acid,and finally mineralization to form small molecular substances such as H₂O.In the other,the unstable aldehyde carbon in the benzene ring is first removed to form the carbonyl group,and then the carbonyl carbon is fractured at high temperature and oxidized to form the small molecules acrylone,ethylene and CO.In phenolic contaminants,the reaction pathway of 2,6-dimethylphenol may be the dehydrogenation of phenol hydroxyl on benzene ring to carbonyl,the attack of hydroxyl radical on p-carbon to 2,6-dimethylbenzoquinone,then the oxidation of ring opening to maleic acid,lactic acid,and finally to CO₂ and H₂O;the reaction process of catechol is the high temperature removal of aldehyde and hydroxyl on benzene ring to o-carbonyl,and the removal of CO to cyclopent-2,4-dienone.At last,1,3-butadiene is oxidized to form small molecules such as ethylene;the reaction process of methylhydroquinone is hydroxyl radical replacing the methyl on the benzene ring to produce 1,2,4-phenylenol,which is decomposed at high temperature and oxidized to ethylene glycol and 4-hydroxy-2-butanone,and finally oxidized to propylene ketone,ethylene and CO and other small molecules.