Degradation Of Sulfonamides By Mn/Ce Doped Carbon Nanotubes Activated Peroxymonosulfate

In recent years,sulfonamides have been widely used as veterinary drugs in the field of animal husbandry because of their broad antibacterial spectrum and simple methods of use.Their chemical properties are stable and they tend to exist for a long time after entering the water on the earth environment,which introduces much residual of antibiotics to drinking water,groundwater and surface water.The potential risk of water environment increases significantly.Sulfamethoxazole and sulfisoxazole studied in this paper are two common sulfonamides.Therefore,it is urgent to develop efficient technologys for removal of sulfonamides in wastewater.In this study,sulfisoxazole and sulfamethoxazole were selected as target contaminants,cerium doped and manganese/cerium doped carbon nanotubes were fabricated for heterogeneous activation of permonosulfate.The degradation efficiency and influencing factors of sulfisoxazole and sulfamethoxazole by activated PMS with two materials were investigated respectively.The influence factors included dosage of activators,PMS concentration and initial p H,et al.The active species involved in the reaction system were analyzed,and the possible activation mechanism was speculated.The main conclusions are as follows:Cerium-doped carbon nanotubes were prepared by impregnation and calcination method for the degradation of sulfisoxazole.A variety of characterization methods were used,such as TEM,EDS,BET,Raman,XRD and XPS.When the mass ratio of Ce O₂ to CNT was 20%,the Ce-CNT composites got the highest degradation efficiency for sulfisoxazole.When the dosage of activators was 75 mg·L^-1 and the concentration of PMS was 0.3 mmol·L^-1,the degradation rate of sulfisoxazole could reach more than 90%in 30 min and 100%in 50 min,and its degradation rate constant was 3.3 times of that of CNT,which indicated that cerium doping could significantly enhance the performance of carbon nanotube activated PMS.Sulfisoxazole could be removed effectively in the p H range of 3.0-9.0.After 5 cycles of experiments,Ce-CNT-2 still had good activity,stability and recyclability.Electron paramagnetic resonance and radical quenching experiments indicated that ¹O₂,·OH and SO₄·^-were the three working active species in the system.It was postulated that the surface defects of carbon nanotubes participated in the activation of PMS as active centers,which might be related to the formation of ¹O₂,and the direct electron transfer mediated by carbon nanotubes existed in the system.Ce（Ⅲ）/Ce（Ⅳ）performed as active sites and promoted the activation of PMS.The oxygen vacancy also facilitated the electron transfer of the activator.In order to obtain superior activator further,Mn/Ce-doped carbon nanotubes were prepared to degrade sulfamethoxazole on the basis of Ce-CNT-2 by coprecipitation and manganese carbonate pyrolysis.Multiple characterization methods were detected with above test methods.When the molar ratio of Mn/Ce was 7.5:1,the Mn/Ce-CNT composites got the highest degradation efficiency for sulfamethoxazole.When the dosage of Mn/Ce-CNT-7.5and PMS were 100 mg·L^-1 and 3.0 mmol·L^-1respectively,the degradation rate could reach more than 90%in 30 min,and 10 mg·L^-1 sulfamethoxazole could be degraded completely in90 min.The degradation rate was 4.0 and 2.0 times higher than that of CNT and Ce-CNT-2,respectively.The initial p H of the solution had little effect on the degradation efficiency,and sulfamethoxazole could be degraded triumphantly during the p H range of 3.0-11.0.After 5cycles of experiments,the degradation efficiency of Mn/Ce-CNT-7.5 could remain more than98%,demonstrating that it had great stability and recyclability.Electron paramagnetic resonance and radical quenching experiments reflected that the active species in the reaction system were O₂·^-,¹O₂,·OH and SO₄·^-,in which O₂·^-and ¹O₂ played important roles in the degradation of sulfamethoxazole.A large number of oxygen vacancies were formed by manganese doping,which accelerated the charge transfer ability of the material surface.Mn/Ce doping formed effective redox cycles of Ce（Ⅲ）/Ce（Ⅳ）and Mn（Ⅱ）/Mn（Ⅲ）/Mn（Ⅳ）,which reacted with PMS to generate free radicals.The oxygen vacancy not only promoted the interaction between the material and PMS,but also participated in the formation of O₂·^-.