Catalytic Mechanism Of Iron-based Heterogeneous Catalysts In Fenton-like And Electro-Fenton Oxidation Processes For Dyeing Wastewater

Reactive dyes are usually used for the dyeing of cotton cellulose fiber,protein fiber,nylon and viscose fiber.Because of the low dying rate,there are usually residual dyes in the dyeing wastewater.The dyeing wastewater must be properly treated before being discharged,otherwise it will pollute the water body.Fenton oxidation of dyeing wastewater has disadvantages such as large consumption of chemicals,narrow p H range,and producing large amount of sludge.Therefore,it is of great significance and scientific value to explore more efficient,convenient and low-cost advanced oxidation technology.In this paper,heterogeneous Fenton and electro-Fenton advanced oxidation system is constructed through replacing traditional soluble iron(II)salts by iron-based catalysts,chalcopyrite,pyrite,iron-carbon composite and sponge iron,to treat simulated wastewater using Reactive Red X-3B as model dye,and to conduct industrial experiments on real dyeing wastewater treatment.The main results of the research are as follows:(1)The Fenton system catalyzed by iron-based catalyst,i.e.iron-carbon composites,sponge iron,chalcopyrite,and pyrite,can effectively treat Reactive Red X-3B simulated wastewater.For the 1.0 g/L Reactive Red X-3B simulated wastewater,the removal rate of Reactive Red X-3B can reach 100%,and the removal rate of COD can reach 90%.Comparing the four catalysts,the use of iron-carbon composite has the best COD removal efficacy,and the use of chalcopyrite and pyrite is less affected by initial p H.The Fenton-like oxidation efficacy is weakened under strong acid(p H=2)and weakly alkaline(p H=8)conditions,and the optimal initial p H is 3?4.The larger the catalyst dosage,the better the COD removal efficacy is.The Fe²⁺dissolved from iron-based catalyst homogeneously catalyze H₂O₂ to generate·OH,thereby oxidizing the dye.The traditional homogeneous Fenton has a lower COD removal rate than the iron-based heterogeneous Fenton,so the surface iron of the solid catalyst has contribution to degradation efficacy.Experiments show that the heterogeneous catalysis of iron on the surface of chalcopyrite and pyrite is more important than the homogeneous catalysis of dissolved Fe²⁺.The Behnajady-Modirshahla-Ghanbery(BMG)mathematical model t/(COD₀-COD)=m+nt can describe and predict the COD removal kinetics of degradation of Reactive Red X-3B by Fenton-like oxidation.(2)For the 1.0 g/L Reactive Red X-3B simulated wastewater,electrochemical methods can effectively oxidize and degrade dyes.Adding Fe²⁺to the electrolyte can improve dye degradation through electro-Fenton oxidation.In the electro-Fenton system,the use of graphite anodes can achieve a better removal efficacy than the use of DSA anodes.Using iron-based heterogeneous catalyst instead of soluble ferrous salt,the efficacy is slightly better than homogeneous electro-Fenton,and its outstanding advantages are wider working p H range and less sludge.The iron-carbon composite has the best removal effect,which can achieve 100%Reactive Red X-3B and 80%COD removal rate,with a unit energy consumption of 0.31 k Wh/g COD.In the cyclic test,although the reaction rate decreased slightly,the removal of Reactive Red X-3B was basically maintained for all four iron-based catalysts in the 5 cycles of the electro-Fenton process.The COD removal using iron-carbon composite and sponge iron decreased by 20?24%,the COD removal using chalcopyrite and pyrite did not significantly change.XRD analysis showed that the crystal structures of chalcopyrite and pyrite did not significantly change after Fenton oxidation.The XPS test results show that the chalcopyrite after use is oxidized to a certain extent,especially the valence of S element changes greatly;the chemical form of pyrite has almost no change after use.Chalcopyrite and pyrite have better stability and industrial application value.(3)In the advanced oxidation based on iron-based catalyst,catalysts slowly release metal ions into water.In this process,pyrite and chalcopyrite cause a p H drop,and iron-carbon composites and sponge iron cause a p H rise.The dissolved Fe²⁺participates in the homogeneous Fenton reaction.Pyrite and chalcopyrite have a low concentration of dissolved metal ions,and the contribution rate of homogeneous catalysis is small.The heterogeneous catalysis at the solid-phase interface is dominant.The surface iron and copper catalyze the decomposition of H₂O₂ to generate·OH through the heterogeneous Fenton reaction.The electron spin resonance test confirmed the existence of·OH and ~1O₂.Adding free radical scavengers to evaluate the relative importance of reactive oxygen species(ROS)in the removal of pollutants,experiments have found that ROS such as·OH,~1O₂ and·O₂^-play a key role in the oxidative degradation of dye.(4)The presence of Cl^-in the electrolyte accelerates the removal of Reactive Red X-3B and COD,owing to free chlorine generated in situ at anode.The chlorine evolution capacity of the DSA anode is much higher than that of the graphite anode.The total active chlorine concentration in the solution using DSA is more than 5 times when using graphite anode.In the heterogeneous electro-Fenton system with iron-based catalysts,the removal rate of Reactive Red is improved under the conditions of using chalcopyrite and pyrite,but is reduced under the conditions of using sponge iron and iron-carbon composite;the initial COD removal rate is improved under the conditions of using chalcopyrite,pyrite and iron-carbon composite catalysts,but almost unchanged using sponge iron.In the presence of iron-based catalysts,the final COD removal rate is higher,and the unit energy consumption for pollutant removal is significantly reduced.Active chlorine has an oxidizing effect on the catalyst.When the concentration of chloride ions in the wastewater is high,a large amount of active chlorine can be generated in a short time,and react with catalyst(ferrous ion/iron-based catalyst)not conducive to the degradation and mineralization of the dye in the wastewater.(5)Pyrite catalyzed Fenton oxidation is applied to the treatment of real wastewater from dyeing factory.Industrial experiments show that the combination of coagulation and pyrite catalyzed Fenton oxidation process has a good treatment efficacy on wastewater,and the average COD and color removals are 89%and 94%,respectively.The operating cost of the treatment and reuse process is about CNY 1.5/t.Pyrite catalysed Fenton reaction has the advantages of wide p H range,reusable catalyst,and simple operation.In summary,the Fenton-like and electro-Fenton method based on iron-carbon composites,sponge iron,chalcopyrite and pyrite,are effective ways to remove reactive dyes in dyeing wastewater,which can improve efficiency and reduce costs.This study reveals the mechanism of iron-based catalysts and clarifies the oxidation degradation pathway of Reactive Red X-3B dye,which has reference value for the practical application of heterogeneous Fenton and electro-Fenton oxidation technologies.Pyrite catalyzed Fenton oxidation has good prospects for popularization and application in treating real dyeing wastewater.