Experimental Study On Degradation Of Organic Pollutants In Water By Activated Persulfate With Modified Iron-based Materials

With the progress of society and the great development of industry and agriculture,more and more wastewater from various industries with high resistance to degradation and toxicity are produced during the production process,including printing and dyeing wastewater,pesticide and papermaking wastewater.There are many organic materials which are difficult to biodegrade,and have stable structure,large molecular weight and strong toxicity.It is seriously harmful for groundwater,surface water body,flora and fauna,and even endanger human social activities with discharging into the environment water body and no effective treatment.Iron-based materials,not only as a catalyst to activate H₂O₂ and persulfate for advanced oxidation,but also as an adsorbent to remove heavy metal ions.Besides,Iron-based materials which include magnetite,hematite and pyrite exist widely in the world.They also have excellent physical and chemical properties such as stable structure and low cost.Especially,in the recent years with the rise of advanced oxidation processes,AOPs based on iron-based catalysts have been favored and studied by many scholars due to the various advantages of the above iron-based catalysts.In this paper,three types of iron-based oxides are modified by hydrothermal co-precipitation method,which are three catalysts:sulfur-modified iron oxide,porous sulfur-modified iron oxide and metal-cerium-doped iron oxide.They are used to catalyze persulfate to produce high redox potential sulfate radicals?SO₄^-·?to degrade simulated dye wastewater and highly toxic phenol wastewater,respectively.At last explore there catalytic degradation mechanism.The main conclusions are follows:?1?Preparation of sulfur-modified iron oxide and degradation of orange AO7 wastewater by S-Fe/Na₂S₂O₈ system.At the mole ratio 1:2 of M?S?:M?Fe?,the sulfur-modified iron oxide catalyst S-Fe was synthesized via co-precipitation and high temperature calcination method.The structure and performance of S-Fe had been characterized by the methods of Scanning electron microscope?SEM?,X-ray diffraction?XRD?,X-Ray Photoelectron Spectroscopy?XPS?.The results showed that the S-Fe was successfully prepared by the co-precipitation and high temperature calcination method,which was aggregated in a columnar shape and mainly composed of Fe₁₂S₁₁O₅₁,FeO?OH?,FeS,Fe₃O₄ and FeS.The S-Fe catalyst was used in the S-Fe/Na₂S₂O₈system to degrade acid orange AO7 wastewater.The reaction effect was affected by the reaction conditions such as the amount of oxidant added,the amount of catalyst added,pH value,and initial substrate concentration.During the experiment,it was found that with the increase of the amount of catalyst and oxidant the degradation efficiency of acid orange was achieved more perfect.Another,when the initial concentration of AO7 was 50mg/L,the dosage of Na₂S₂O₈ was 2.0g/L,the dosage of S-Fe was 1.0g/L,and the pH was 3,the removal rate of acid orange AO7 reached 82.9%and 0.0342 min^-1 reaction rate.It was found that?SO₄·^-?was the main free radical,according to the magnitude difference of the reaction rates of free radicals with tert-butanol?TBA?and ethanol?EtOH?respectively,through the quenching experiment of free radicals,tert-butanol?TBA?and ethanol?EtOH?were added during the degradation process.Besides,after repeated experiments,S-Fe still showed significantly great catalyst activity.?2?Preparation of porous sulfur-modified iron oxide and degradation of phenol wastewater by porous S-Fe/Na₂S₂O₈ systemOn the basis of sulfur modified iron-based catalyst,oxalic acid as a pore expander was added to expand structure of S-Fe.Porous,as catalyst,to activate persulfate for the degradation of phenol wastewater was studied.The effects of oxidant dosage,catalyst dosage,and pH value on the degradation efficiency of phenol wastewater were investigated.It showed that under the condition that the oxidant dosage was 2.0g/L,the catalyst dosage was2.0g/L,and the pH value was 5,the removal efficiency of phenol reached 60%after 1 hour.At the same time,Scanning electron microscope?SEM?,X-ray diffraction?XRD?,X-Ray Photoelectron Spectroscopy?XPS?and other characterization methods were used for characterization of porous S-Fe,which showed that the synthesized catalyst was full of developed voids.According to the magnitude difference of the reaction rate of free radicals with tert-butanol?TBA?and ethanol?EtOH?respectively,it was confirmed that the main free radical was?SO₄·^-?through quenching experiments of free radicals in the porous S-Fe/Na₂S₂O₈ system.Anymore,compared with S-Fe catalysts,the stability of the synthesized porous S-Fe catalyst was worse after three or four runs experiments.?3?Preparation of cerium-doped iron oxide and degradation of rhodamine RHB wastewater by Ce-Fe/Na₂S₂O₈ systemAt a certain mole ratio of M(Ce⁴⁺):M(Fe²⁺+Fe³⁺),the cerium-doped iron oxide catalyst Ce-Fe was synthesized via co-precipitation and high temperature calcination method.Characterization was performed by means of scanning electron microscope?SEM?,X-ray diffraction?XRD?,and X-ray photoelectron spectroscopy?XPS?,which indicated that Ce-Fe composites were successfully synthesized.The composite was used in the Ce-Fe/Na₂S₂O₈system to degrade rhodamine RHB wastewater.The effects of oxidant dosage,catalyst dosage,cerium doping amount and pH value on the degradation efficiency and the removal rate of rhodamine RHB were investigated.When the oxidant dosage was 1.0 g/L,the catalyst dosage was 1.0 g/L,the cerium doping ratio was 1/25,and the pH value was 7 for 50 minutes,the degradation rate of rhodamine RHB reached 89%.The first-order reaction kinetic rate constant was 0.0442 min^-1.According to the magnitude difference of the reaction rate of free radicals with tert-butanol?TBA?and ethanol?EtOH?respectively,it was confirmed that the main free radical was?SO₄·^-?through quenching experiments of free radicals in the porous Ce-Fe/Na₂S₂O₈ system.Comparison experiments were carried out in each chapter,which showed that iron-based catalysis was difficult to be used for activating persulfate to generate free radicals with high redox potentials lonely.After non-metal S element was added to become modification,the catalytic activity of the obtained S-Fe catalyst would be significantly improved;meanwhile,using oxalic acid an s hole expander made the voids of synthesized iron-based catalyst developed significantly,thereby the catalyst would exposed at persulfate solution more easily.Finally,the rare earth metal doped Ce-Fe catalyst was used.Because the rare earth cerium itself has catalytic effect,after be synthesized with another metal Fe,the conversion between electrons was accelerated,so that the active site of cerium-doped iron oxide Ce-Fe.At last,it can be seen from the comparison experiments that the rare earth metal cerium-doped Ce-Fe catalyst has the best reactivity.