Preparation Of Porous Carbon Coating And Its Performance In Activated Persulfate Degradation Of Organic Pollutants

In recent years,using transition metal activated persulfate to produce sulfate radical(SO₄^·-)to degrade organic matter has been a research hotspot in the field of organic wastewater treatment.Among them,cobalt(Co²⁺)and its compounds have the highest catalytic efficiency for potassium bisulfate（PMS）,but the precipitation of Co²⁺in the catalyst will cause secondary pollution to the water environment,which limits its application.ZIF-67 is a metal-organic framework material containing Co²⁺,which has a large specific surface area.It is often used to adsorb pollutants in water and activate PMS.However,ZIF-67 has poor water stability,and its structure will gradually disintegrate during the reaction process,resulting in the release of Co²⁺.At the same time,ZIF-67 is viscous after being put into water and cannot be recovered.Therefore,how to strengthen the water stability of ZIF-67 and solve its recovery problem is the key to improve its practicability.In this paper,ZIF-67 was prepared into porous carbon ZIF-67（C）by high temperature pyrolysis,and then further etched and mixed with epoxy resin（EP）to form a coating loaded on glass,which solved the problems of powder recovery and Co²⁺leakage.At the same time,a series of characterization methods were used to analyze the materials,and the catalytic effects of several materials on PMS were compared through the degradation of methyl orange（MO）in water,and their catalytic mechanisms were discussed.Finally,the catalytic coating was applied to the degradation of tetracycline and ciprofloxacin in water,and good results were obtained.Specific research contents are as follows:（1）In order to solve the problem that ZIF-67 can not be recovered,it was carbonized to form porous carbon-ZIF-67（C）.The effects of molar ratio of metal to organic ligands(Co²⁺:2-HMIM),solvent type and carbonization temperature（T）on the synthesis of ZIF-67（C）were discussed.ZIF-67（C）exhibited the best PMS activation performance when the preparation conditions were Co²⁺:2-HMIM=1:8,methanol as solvent and T=500℃;When the initial concentration of MO is 10 mg/L,the dosage of ZIF-67（C）is 0.2 g/L,and the dosage of PMS is 0.1 g/L,the removal rate of MO can reach 84.4%within one hour.Through free radical quenching experiment,it can be seen that the active species（SO4）plays a leading role in the whole degradation system,and there is also a small amount of hydroxyl radical（OH）.Compared with ZIF-67,the Zeta potential of ZIF-67（C）increased from+0.443 m V to+14.9 m V,indicating that ZIF-67（C）had better dispersion in water.XPS results show that there is highly crystalline Co₃O₄ in ZIF-67（C）,which is the active site of PMS catalyzed by ZIF-67（C）and the reason of magnetic properties produced by ZIF-67（C）.After reusing ZIF-67（C）for 4 times,the removal rate of MO is still 78%,and the mass loss is almost no,which shows that ZIF-67（C）has strong stability,but the leakage of Co²⁺in aqueous solution after reaction is as high as 63.3 mg/L..（2）In order to reduce the amount of Co²⁺in the aqueous solution after ZIF-67（C）reaction,"etched ZIF-67（C）porous carbon"was formed by etching ZIF-67（C）with tannic acid and then carbonizing ZIF-67（C）.The effects of the amount of tannic acid,etching time and T on the etching of ZIF-67（C）porous carbon were investigated.When the amount of tannic acid is 0.17 g,the etching time is controlled at 10 min and T=500℃,"etched ZIF-67（C）"has the strongest activation effect on PMS.TEM photos show that"etched ZIF-67（C）"has a cavity inside,but still retains the outer frame of ZIF-67.Under the same reaction conditions as（1）,the degradation rate of MO was68.5%when"etched ZIF-67（C）"was used as catalyst.The activation effect of"etched ZIF-67（C）"on PMS was decreased due to tannic acid etching off part of Co in ZIF-67,and the magnetic property became weaker and the recoverability became worse.However,when the dosage of"etched ZIF-67（C）"was increased to 0.4 g/L,the degradation rate of MO increased to 93%,and the leakage of Co²⁺in the solution decreased to 0.19 mg/L.（3）In order to further strengthen the recoverability of"etched ZIF-67（C）",the coating made by mixing EP with"etched ZIF-67（C）"powder was cured on glass.The effects of EP dosage and coating mode on coating synthesis were investigated.Under the same reaction conditions as（1）,the degradation rate of MO was 74.3%when"etched ZIF-67（C）"porous carbon coated glass was used as catalyst,which was due to the increase of adsorption rate of MO by the coating and the fact that EP did not cover the active site of"etched ZIF-67（C）"after forming the coating.When the loading of"etched ZIF-67（C）"increased to 0.3 g/L,no Co²⁺was detected in the solution after the reaction,which proved that the coating had good stability.Finally,the porous carbon coating of"etched ZIF-67（C）"was applied to activate PMS to remove antibiotics tetracycline and ciprofloxacin in water.It was found that under neutral conditions,when the loading of"etched ZIF-67（C）"was 0.4 g/L and the dosage of PMS was 10 mg/L,the degradation rates of tetracycline and ciprofloxacin were 72%and 58.5%,respectively.It has a certain application prospect in antibiotic wastewater treatment.