Degradation Of Dyes And Sludge Dewatering By Fenton-like Systems Based On Iron-based Metal-organic Frameworks

Organic textile effluents and excess sludge originated from printing and dyeing industry bring huge environmental pollution and health risks to people.Fenton oxidation technology is a highly efficient effluent treatment technology.However,Fenton oxidation also suffers from intrinsic shortcomings such as rigorous operating pH,non-reusability,the secondary pollution,i.e.,which restricts its practical application.With the development of Fenton oxidation,heterogeneous Fenton-like oxidation has attracted considerable attention in the water treatment fields.Meanwhile,the relatively low catalytic efficiency has been choke point for most heterogeneous Fenton-like catalysts.Therefore,it is necessary to search for an efficient,stable,and environmentally friendly Fenton-like system to water treatment.Metal-organic frameworks（MOFs）are an emerging category of organic-inorganic hybrid crystalline materials.MOFs have potential application value due to the unique properties,including desirable porosity and high surface area,easy functionalization,the tunability of their pore size.Recently,research effort clearly indicated that the addition of proper regulating agent could enhance the oxidation efficiency of heterogeneous catalytic process.Therefore,we have constructed the Fenton-like oxidation systems for the degradation of textile effluents and the dewatering performance of excess sludge using MOFs and their derived as heterogeneous Fenton-like catalysts,hydroxylamine hydrochloride as regulating agent.The main research contents are as follows:Firstly,MOF-235（Fe）was prepared by a hydrothermal method and evaluated as a potential Fenton-like catalyst for degradation of methyl orange（MO）.The prepared MOF-235（Fe）was characterized by using scanning electron microscope（SEM）,X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,and N₂ adsorption-desorption isotherms.Together with the characterization results,it can be inferred that the MOF-235（Fe）has been synthesized successfully.The introduction of hydroxylamine hydrochloride can strongly promote the catalytic oxidation of the Fenton-like system,more than 53.4%of MO was degraded within 20 min.The effects of different parameters such as hydroxylamine hydrochloride concentration,initial pH,H₂O₂ concentration,catalyst dosage and reaction temperature on the degradation efficiency of MO were assessed.The results showed that under optimal conditions,about 100%MO and 81.3%TOC of a MO solution could be removed after 30 min.Moreover,catalyst retained the catalytic performance in a wide pH range of 3¹1 with a removal efficiency of over 96%.Furthermore,we studied the adsorption kinetics and isotherm of the reaction system and found that the MO adsorption process followed the pseudo-second-order kinetics model and Langmuir isotherm model.Moreover,the mechanism in MOF-235（Fe）/H₂O₂/NH₂OH·HCl system was identified.The results showed that the main active species was hydroxyl radical（·OH）.Our results show the application potential of MOFs on heterogeneous Fenton-like catalysts.Seconedly,a novel magnetic carbon was prepared by one-step pyrolysis of MIL-101（Fe）.The obtained magnetic carbon was characterized by using scanning electron microscope（SEM）,vibration sample magnetometer（VSM）,X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,and N₂ adsorption-desorption isotherms.Fenton-like reaction system was constructed using magnetic carbon as Fenton-like catalyst,hydroxylamine hydrochloride as regulating agent,acid orange 7（AO7）as target pollutant.The effects of common influencing factors such as initial pH,H₂O₂ concentration,hydroxylamine hydrochloride concentration,and reaction temperature on the degradation of AO7 were investigated.The results showed that under optimal conditions,complete degradation of AO7 was achieved in 10 min,and the TOC removal can reach about 67.0%after 40 min.Moreover,the magnetic carbon retained good stability.The leaching of iron in the reaction solution was 1.55 mg/L.After five successive catalytic cycles,the magnetic carbon exhibited no significant loss of activity.Furthermore,the free radical capture experiment verified that the main free radical in the reaction system is·OH.The above results indicate that an efficient and stable Fenton oxidation system has been successfully constructed for the removal of organic pollutants from water.Lastly,the MIL-101（Fe）/H₂O₂/NH₂OH·HCl Fenton-like system was combined with the skeleton builder CaO to condition the excess sludge generated during the post-treatment of the sewage.The addition of CaO can adjust the pH,and form a rigid porous structure,reduce the compressibility of the sludge,and facilitate the separation of sludge and water.The effects of initial pH and the dosage of conditioner on sludge dewatering performance were studied in single-factor experiments by using specific resistance to filtration（SRF）and capillary suction time（CST）as evaluation indexes.SRF reduction rate achieved 85.2%,CST reduction rate achieved 57.9%when process parameters were as follows:Fenton-like’s reaction time of 30 min,initial pH of 3,and dosages（calculated by sludge dry solids）of H₂O₂,NH₂OH·HCl,CaO,and MIL-101（Fe）dosage of 5,20,400,and 60 mg/g,respectively.After the composite conditioning,sludge particles turned smaller,the pore size became smaller,the morphology became irregular,loose pores and the TOC content of the supernatant increased significantly.The results indicate that the composite conditioner can effectively crack the extracellular polymeric substances（EPS）,and then enhance the dewatering performance of sludge.