Degradation Of Polutants And The Production Of Chlorine By Fluidized Bed Photocatalytic System

Recently,the discharge of wastewater has been increasing year by year,the pollutants in which have been more and more complicated.The existing wastewater treatment process meets difficulty to handle with the severe water pollution.Therefore,there is an urgent need for the development of a high-efficiency and energy-saving wastewater treatment technology to alleviate the pressure caused by the shortage of water resources.Photocatalytic technology is a stable,efficient and green technology with a bright prospect in the field of energy development and environmental protection.In particular,its ability to oxidize and degrade pollutants in water has been widely recognized.However,the application of the photocatalyst in the wastewater treatment has been limited due to the performance defects of the photocatalyst,the difficulty of expanding the scale and the low reaction efficiency in high-salinity water.In this study,an internal circulating fluidized bed photocatalytic reaction system was developed,and the removal of refractory pollutants in water with different salinity was achieved through the preparation of different photocatalysts and the adjustment of reactor operating parameters.For water with low salinity,bismuth vanadate（Bi VO₄）was modified with graphene as the carrier to improve the photogenerated electron separation efficiency of Bi VO₄ using the excellent conductivity of graphene.In addition,magnetic particles of Fe₃O₄were used to improve the separation ability of the photocatalyst.The Bi VO₄/Fe₃O₄/r GO ternary photocatalytic magnetic material was prepared by a one-step hydrothermal method.The photocatalytic activity was judged by the degradation efficiency of the common pollutants（rhodamine B and sulfapyridine）.The experimental results showed that the ternary materials were successfully prepared with good photoresponse and electrochemical properties,which could be quickly separated from the solution under an external magnetic field.The photocatalytic degradation rates of rhodamine B（Rh B）and sulfapyridine（SPY）with the ternary materials reached 0.0584 min^-1 and 0.0197 min^-1,respectively.The degradation efficiencies of them exceeded 95%within 120 min and 180 min,respectively.The photocatalytic activity of the ternary materials could be maintained after 5 cycles of experiments,which proved that Bi VO₄/Fe₃O₄/r GO material were highly stable.The mechanism of photocatalytic degradation of pollutants by ternary materials was analyzed using the electronic self-selected resonance instrument（ESR）and the capture agent of oxidized active substances.The results showed that the photocatalytic degradation of Rh B and SPY was dominated by light to holes and superoxide radicals.In addition,the results of the above static experiments served as the basic parameters for the design and operation of the reactor.With the prepared ternary materials as the catalyst,the other operating parameters of the reactor were optimized to achieve the high degradation efficiency of SPY in water.Moreover,the ternary materials had higher p H fitness for the solution with higher photocatalytic efficiency between 4.26 and 9.76.However,the photocatalytic degradation process of pollutants was greatly affected by the salinity in the solution,and the operating efficiency of the reactor decreased significantly with the increase of salinity.In order to develop a suitable treatment technology for high-salinity water,which contain the high content of chloride ions,the reaction system of converting chloride ions into Reactive Chlorine Species（RCS）using photocatalyst under visible light conditions and degrading pollutants was constructed.Here,the binary and ternary visible light photosensitizers anthraquinone-2-sulfonic acid/graphene（AQ2S/GR）and anthraquinone-2-sulfonic acid/reduced graphene oxide/ferric oxide（AQ2S/r GO）/Fe₃O₄)were prepared byπ-πmethod.The two photosensitizers had high efficiency of photosensitive chlorination degradation of sulfapyridine under visible light conditions,and the reaction kinetic constants reached 0.353 h^-1 for binary materials and 0.1694 h^-1 for ternary materials,respectively.The p H of the reaction system,the dosage of photosensitizer and the concentration of chloride ion all had significant influence on the degradation of sulfapyridine.In addition,the capture experiment of oxidative active substances and the theoretical calculation revealed that the degradation of sulfapyridine was dominated by dichloro radicals and superoxide radicals.Based on the above results and the characteristics of the fluidized bed reactor,an internal circulating fluidized bed reactor for the photocatalytic chlorination degradation of SPY was constructed.Through optimizing the dosage of the catalyst,the incident light intensity,the aeration flow rate,and the chloride ion concentration,the efficiently removal of SPY in high-salinity water was achieved in this reactor.At the end of the reactor operation,Free Available Chlorine（FAC）was detected in the solution,indicating that when the concentration of pollutants in the solution dropped to a certain level,the dichloro radicals generated in the reactor self-quenched to produce chlorine.Based on the above results,a photosensitive chlorination reaction under visible light irradiation was developed.By optimizing the reaction conditions of chlorine production,the continuous chlorine production process for 120 h was realized in the photosensitive reaction solution composed of photosensitizer of 1 mmol/L and hydrochloric acid solution of 1 mol/L in the split photosensitive chlorine production reactor.When AQ2S was replaced by the efficient photosensitizer anthraquinone-2-methyl sulfonic acid（AQ2MS）as the photosensitizer,the chlorine production reached114.59 mg in the reaction system after 120 h.In order to achieve the portability of the reaction system,a tube and a disc type of one-piece photosensitive chloride reactor were developed with relatively stable capacity of chlorine production.However,due to the lack of aeration module,a large amount of chlorine was decomposed by hydrogen peroxide in the solution,reducing the reaction efficiency of the one-piece reactor.