Mo-promoted Degradation Of Dyes By Fenton And UV-Fenton Systems

Rhodamine B(Rh B),as one of the common organic dyes,has attracted extensive attention in the field of water environment pollution control because of its"carcinogenicity,mutagenicity and deformity".However,the conventional Fenton water treatment process is limited by treatment efficiency and cost,so it is difficult to achieve a good removal effect on Rh B.Fenton's degradation principle was to attack some chromogenic groups by producing a large number of active free radicals,but Fenton itself has some constraints.In recent years,in order to break the limiting conditions of Fenton reaction and achieve better and faster reaction,some new treatment technologies derived from Fenton technology have been widely used in the degradation of printing and dyeing wastewater.Molybdenum(Mo)assisted catalytic Fenton Method and Mo assisted catalytic ultraviolet(UV)-fenton method are new water pollution degradation technologies.Different from the traditional Fenton Method,they play a major role in the whole reaction system in the form of singlet oxygen(~1O₂).~1O₂is an unstable excited oxygen molecule,which is similar to superoxide radical(·O_2-),hydroxyl radical(·OH),hydrogen peroxide(H₂O₂)and other reactive oxygen species(ROS),and plays an important role in sewage treatment.In this thesis,Mo Fenton system and Mo UV Fenton system were tried to be constructed and used to degrade organic dye RHB.In this paper,through the comparison of Rh B removal efficiency among the single Fenton system,Mo Fenton system and Mo UV Fenton system,the effects of different influencing factors,water background components and actual water bodies in the two systems on Rh B removal efficiency are studied.Through the capture test of different free radicals in the two systems,the differences of Rh B removal efficiency by different active components are further explored,Finally,the degradation pathway and oxidation products of Rh B in the system were investigated to analyze the degradation pathway of Rh B in Mo Fenton system and Mo UV Fenton system.The results are as follows:(1)In pure water system,compared with Fenton degradation alone,Mo-Fenton system and Mo-UV-Fenton system can significantly improve the removal efficiency of Rhodamine B.Under the same conditions,the degradation efficiency of Rhodamine B was 55.17%,71.26%and97.77%respectively.In the discussion of free radicals,by adding·OH scavenger(tert butyl alcohol(TBA)),~1O₂capture agent(tryptophan(TRP))and·O₂^-capture agent(p-benzoquinone(BQ))respectively,it wass concluded that ~1O₂(mainly),·OH played a role in the degradation of Rhodamine B.The existence of ultraviolet(UV)accelerated the decomposition rate of H₂O₂and increased the concentration of·OH in the system,which was more conducive to the action of Mo on·OH and then converted to ~1O₂,so as to promoted the rapid degradation of Rhodamine B.Due to the different amount of Mo cocatalyst,the removal efficiency of Rhodamine B was different.When the amount of Mo increased,on the one hand,it would promote the conversion of Fe²⁺/Fe³⁺and produce a large amount of·OH.On the other hand,under the action of Mo,·OH would produce ~1O₂through disproportionation reaction,so as to accelerate the degradation of Rhodamine B;By changing the p H of the reaction system,acid or alkali would inhibit the formation of reactive free radicals in the reaction,so as to inhibit the degradation of dyes;In addition,the change of H₂O₂dosage directly affects the overall content of oxidant in the whole system.Too little content would lead to low free radical concentration in the reaction system.When the content was too high,it would consume·OH and affect the dye degradation effect;Fe²⁺ion provided by ammonium ferrous sulfate((NH₄)₂Fe(SO₄)₂·6H₂O)was an important catalyst.With the increase of dosage,the dye degradation effect was better,but too much dosage would lead to too fast production rate of·OH,consumption of·OH and capture of·OH by too much Fe²⁺.In addition,chloride ion(Cl^-)(0-10 mm)has a slight inhibitory effect on Rhodamine B through the capture of·OH;On the one hand,humic acid(NOM)would complex with Fe³⁺in the system and affect the conversion between Fe²⁺/Fe³⁺,on the other hand,NOM would have a certain shielding effect on UV,affect the photolysis of H₂O₂and the concentration of·OH in the system;In the application of practical water body,Rhodamine B also has obvious degradation,but it would be affected by other components in the actual water body.The cycle efficiency of Mo as cocatalyst in the system was also studied.After five cycle experiments,the effect of Mo on the system was not attenuated and the stability was good.(2)In this study,UV-Vis full scan,EPR characterization and TOC determination were carried out for the reaction system.It is considered that the decolorization principle of Rhodamine B is mainly due to the N-de ethylation and the fracture of chromogenic group under the attack of free radical,as well as the ring opening,mineralization and carboxylation of aromatic ring.In addition,in order to further verify the oxidative degradation pathway of Rhodamine B,the products of the system were analyzed by high performance night spectrum LC-MS/MS.There may be 7 intermediates for Rhodamine B in Mo Fenton system and 15intermediates for Rhodamine B in Mo UV Fenton system.The first step in the degradation process often involves N-deethylation or attacking the N-ethyl at the N position.The decolorization of Rhodamine B is mainly the result of the structural fracture of chromogenic conjugated groups.At the same time,the conjugated system of dye molecules is destroyed,and free radicals can directly attack the central carbon of Rhodamine B and decolorize it quickly.