Performance Of Nanothermally Treated Pyrite Catalyzed Electro-Fenton Treatment Of Simulated Wastewater And Landfill Leachate

In recent years,with the rapid development of industrialization and social economy,the increasing demand and consumption of living materials by residents has led to the rapid growth of the output of urban solid waste.The harmless treatment methods of former household refuse mainly included incineration,sanitary landfill,and composting,among which sanitary landfill was a more common treatment method,and the resulting landfill leachate had become an urgent problem to be solved.Landfill leachate usually contained high concentration of COD,nitrogen,phosphorus,heavy metals,and other pollutants.Direct discharge will cause pollution to the water environment and soil,and seriously affect the ecological environment and human health.The main treatment methods of landfill leachate included microbial degradation,physical adsorption and membrane technology,coagulation and flocculation,chemical precipitation,and advanced oxidation technology,etc.,but there were generally large reagent consumption and sludge production,serious membrane pollution and other problems.The use of nano-heat treatment of natural pyrite catalyzed electrochemical advanced oxidation technology to treat landfill leachate can improve the degradation efficiency and reduce the use of catalysts and oxidants.In this thesis,the treatment of simulated Rhodamine B（Rhb）wastewater,humic acid（HA）wastewater and landfill leachate by heat treatment modified pyrite catalyzed electro Fenton（EF）process was researched.The heat treatment modified pyrite was characterized,and the effects of pyrite heat treatment temperature,anode type,current density,catalyst concentration and p H on the degradation of Rhb and humic acid were investigated.The current mineralization efficiency（MCE）of the degradation of Rhb and energy consumption ratio（EC）of humic acid wastewater by anode type and current density were analyzed.Finally,the feasibility of the process for landfill leachate treatment was analyzed.The main research results were as follows:（1）The pyrite ball after heat treatment was ground to nano particles and a series of characterizations were carried out.The results show that the heat treatment increases the specific surface area of pyrite,and the maximum value was 21.69 m²/g at 700℃.The sample（Py6）obtained by heat treatment of pyrite（Py）at 600℃did not undergo phase transformation,and both exhibited a granular structure.The sample（MPy）began to undergo phase transition at 700℃,showing a complex of pyrite and pyrrhotite,which was lumpy and covered with powder particles.The sample（Pyr）completely changed into pyrrhotite with massive structure at 800℃.Fe-S bond,Fe-SO₄ bond and Fe-O-OH bond existed in Py,Py6,MPy and Pyr,and the bond force constants of Fe-S and Fe-SO₄changed obviously during heat treatment.Ferrous iron in Py and Py6 were mainly distributed in the binding energy of 710e V,and the sulfur element mainly existed in the form of sulfate.Ferrous ions in MPy and Pyr were mainly distributed in the binding energy of 710e V and 707e V,and the sulfur element was mainly in the form of polysulfide.The surface morphology of the catalyst was observed by SEM,and it was found that Py and Py6 were granular,MPy was massive and covered with powdery particles,and Pyr was mostly massive.The characterization results indicated that the heat treatment successfully made the transformation of pyrite,resulting in the change of the pyrite appearance morphology and the increase of the relative specific surface area.The change of the force constant of the chemical bond increased the saturation susceptibility and the relative mass fraction of ferrous ions,resulting in the transformation of sulfate to polysulfide.（2）In the experiment of Rhb wastewater degradation by EF process catalyzed by pyrite with nano-heat treatment.The degradation performance of pyrite catalyzed EF system was higher than that of electrocatalysis and traditional EF.The order of degradation performance of heat-treated pyrite catalyst was MPy>Py6>Py>Pyr.Under optimized operating conditions（BDD anode,1g/L MPy,30m A/cm²,p H 5）,97.85%of Rhb was degraded.It was proved that heat treatment improved the chemical activity of pyrite,more ferrous and polysulfide content can improve the Fenton reaction rate,and nanoparticles overcame the problem of insufficient dispersion of catalyst.Increasing the MPy concentration provided more ferrous ions to improve the Fenton oxidation rate and accelerate the degradation of Rhb,and the catalyst was fully cycled under acidic conditions.Because BDD anode generated and physically adsorbed more heterogeneous hydroxyl radicals（HO^·）and indirectly promoted the generation of sulfate radicals(SO₄^·-),both reactive oxygen species can oxidize Rhb.It made BDD show higher MCE and lower decay than DSA and Graphite.The degradation rate of Rhb increased with the increase of current density,but high current density will lead to self-depletion of reactive oxygen species.The MCE was higher at low current density,but the removal efficiency of Rhb was lower.At a current density of 30m A/cm²,a relatively low attenuation of current mineralization efficiency was obtained,and 77.06% of TOC is mineralized.After five cycles,the degradation ability of magnetic pyrite was not significantly lost.The magnetic pyrite after the reaction was transformed back to pyrite,and the relative content of iron did not decrease obviously,but the loss of sulfur was great.Since HO^·was the main reactive oxygen species in the system,followed by SO₄^·-,sulfur loss has little effect on the performance of the catalyst.Finally,the possible catalytic mechanism in MPy-EF system was proposed.（3）In the experiment of HA wastewater degradation by EF process catalyzed by pyrite with nano-heat treatment,the order of catalytic performance was MPy>Pyr>Py6>Py.Under the optimal conditions（BDD anode,30m A/cm²,1 g/L MPy,p H 2.5）,the TOC in HA was removed by 90.63%,and a lower energy consumption ratio was obtained.More ferrous ions and polysulfide content in heat treated pyrite can significantly improve the catalytic activity of pyrite.Higher specific surface area and magnetic susceptibility can also enhance the adsorption and chemical precipitation of the catalyst,further promoting the removal of HA.MPy concentration and p H had significant effects on the degradation of HA.Increasing catalyst concentration can significantly accelerate the decomposition and chemical precipitation of HA,and low p H can effectively reduce the sedimentation of iron ions and promote the formation of H₂O₂ at the cathode.Because BDD anode can produce more reactive oxygen species,BDD showed better anodic oxidation capacity than DSA and Graphite.The degradation rate of TOC in HA wastewater increased with the increase of current density,but too high current density will lead to a higher EC.A faster degradation rate and a relatively lower EC were obtained at a current density of 30 m A/cm².Finally,the optimized EF-MPy process was used to treat the landfill leachate.TOC and COD were removed 70%and 66%respectively within 6h,which improved the biodegradability of the landfill leachate.