Enhanced Performance Of CNTs Loaded With Co₃O₄ To Activate PMS And The Degradation Mechanism Of SMX

The emergence of emerging pharmaceuticals and personal care products（PPCPs）in water bodies has attracted much global attention in recent decades.Conventional wastewater treatment technologies do not completely eliminate PPCPs from wastewater.While advanced oxidation techniques such as electrochemical oxidation,ultrasonic treatment and ionising radiation can effectively eliminate PPCPs from wastewater.The advanced oxidation technology based on persulfate which has a high oxidation capacity and a wide pH range has received increasing attention in recent years.The development of efficient catalysts in the activation process of persulfate and the degradation mechanism of persulfate on pollutants are currently the hot and difficult research areas.Therefore,in this paper N-CNT-Co₃O₄,CNT-Co₃O₄-in and CNT-Co₃O₄-out were prepared by equal volume impregnation method.They were added to the PMS system to activate the PMS for the degradation of the typical PPCPs pollutant-PMS,to optimize their activation conditions for PMS and investigate the degradation mechanism of SMX.The main research work of this paper is as follows.（1）N-CNT-Co₃O₄ was prepared by isovolumetric impregnation method in which nitrogen-doped carbon nanotubes（N-CNT）was the carrier and cobalt particles were loaded on the N-CNT.A series of characterizations of the catalysts were carried out by XRD,XPS,Raman,TG,TEM and BET.The results showed that the cobalt particles loaded on N-CNT was Co₃O₄,and the content of Co on the catalyst was about 9%,which was mainly distributed on the surface of N-CNT.The degree of graphitization,the number of micropores and the specific surface area of the catalyst increased after the loading of Co₃O₄.The above prepared catalyst was used to activate PMS to degrade SMX.When pH=5.3,catalyst=0.4 g/L,[PMS]₀=0.16 m M,[SMX]₀=20 mg/L,the N-CNT-Co₃O₄/PMS system could completely degrade SMX within 30 min.the N-CNT/PMS system could degrade about 78%of SMX,and the only PMS system could only degrade 32%of SMX.It indicated that the introduction of Co₃O₄ as a carrier of N-CNT significantly improved the catalytic activity of the catalyst.In this paper,the effects of N-CNT-Co₃O₄ dosing,the concentration of PMS,the concentration of SMX,the initial pH of the solution and humic acid on the degradation of SMX by the N-CNT-Co₃O₄/PMS system were investigated.The effect of inorganic anions(HPO₄^2-,CO₃^2-,Cl^-,SO₄^2-,NO₃^-)on the degradation of SMX by the N-CNT-Co₃O₄/PMS system was also investigated in this paper.The results showed that:Cl^-and HPO₄^2-had a facilitative effect on the degradation of SMX by the system;SO₄^2-has an inhibitory effect on the degradation of SMX by the system;CO₃^2-and NO₃^-had almost no effect on the degradation of SMX.（2）The CNT-Co₃O₄-in and CNT-Co₃O₄-out were prepared by isovolumetric impregnation method using CNTs as the carrier with restricted domain loading of cobalt particles,with in and out indicating the location of loading.A series of characterizations of the catalysts were carried out by XRD,XPS and so on.The results showed that the cobalt particles loaded on CNTs was Co₃O₄,and the Co₃O₄ in CNT-Co₃O₄-in was mainly distributed inside the tube of CNTs,and the loading of Co was about 8.5%;the Co₃O₄particles in CNT-Co₃O₄-out was mainly distributed outside the tube of CNTs,and the loading of Co was about 9.5%.The graphitization degree and specific surface area of CNT-Co₃O₄-in were larger than those of CNT-Co₃O₄-out.The CNT-Co₃O₄-in and CNT-Co₃O₄-out prepared above were used to activate PMS to degrade SMX.The experimental results demonstrated that the loading position of Co₃O₄ had an effect on the catalytic activity of the catalysts,and the catalytic activity of CNT-Co₃O₄-in was better than that of CNT-Co₃O₄-out.Under the same experimental conditions:pH=5.3,catalyst=0.4 g/L,[PMS]₀=0.12 m M,[SMX]₀=20 mg/L,CNT-Co₃O₄-in was able to degrade SMX completely in 16 min,while CNT-Co₃O₄-out required 30 min to degrade SMX completely.The results of single-factor effect experiments for both CNT-Co₃O₄-in/PMS and CNT-Co₃O₄-out/PMS systems showed that the catalyst dosage,the concentration of PMS,the concentration of SMX and humic acid all had effects on the degradation of SMX in the systems.The initial pH did not have a significant effect on the degradation of SMX by the systems,and the systems were highly adaptable to pH.The effect of inorganic anions(HPO₄^2-,CO₃^2-,Cl^-,SO₄^2-,NO₃^-)on the degradation of SMX by the two systems were investigated.The results showed that:Cl^-and HPO₄^2-had a facilitative effect on the degradation of SMX by the system;CO₃^2-and SO₄^2-had an inhibitory effect on the degradation of SMX by the system;NO₃^-had almost no effect on the degradation of SMX.This paper also investigated the mechanism of SMX degradation by the N-CNT-Co₃O₄,CNT-Co₃O₄-out and CNT-Co₃O₄-in activated PMS.The radical burst experiments combined with ESR results revealed that the main active substances in the three systems of N-CNT-Co₃O₄/PMS,CNT-Co₃O₄-out/PMS and CNT-Co₃O₄-in/PMS were ¹O₂,SO₄^●-and·OH.In addition,the electrochemical experimental analysis revealed that there was also an electron transfer phenomenon in the system,which can also promote the degradation of SMX.Thus there were both free radical and non-free radical pathways for the degradation of SMX by N-CNT-Co₃O₄,CNT-Co₃O₄-out and CNT-Co₃O₄-in activated PMS.The stability of the catalyst and the mineralization effect on SMX were also investigated in this paper.The results showed that all three catalysts were relatively stable and the degradation of SMX was around 80%after four cycles;the catalysts had a good effect on the mineralization of SMX,and the mineralization rates were all greater than 70%.