Activation Efficiency And Mechanism Of Nonmetallic Doped Iron-based Carbon Nanotubes On Sodium Persulfate

Among the water treatment technologies,the advanced persulfate oxidation technology,which possessed strong oxidation,outstanding mineralization ability and strong environmental adaptability,has a wide application prospect in improving the biodegradability of wastewater and the advanced treatment of wastewater.Ascribed to high catalytic activity and fast reaction rate,metal-based catalysts are commonly used in advanced oxidation technology.However,the use of metal-based catalysts in the application always causes leaching of metal ions.In order to obtain catalysts with high catalytic activity and low metal dissolution,it is necessary to combine the characteristics of carbon-based materials and metals by loading or embedding the metals into carbon materials.In this paper,we studied the effect of non-metallic element S doping on the catalytic ability of sodium persulfate（PDS）by self-made Fe-based carbon nanotubes,the effect on the activation mechanism of PDS,and the main active sites of the catalyst were explored.The main results are as follows:（1）The doping of sulfur（S）atoms into Fe-based carbon nanotubes was successfully done via calcination and nitrogen and sulfur co-doped Fe-based carbon nanotubes（Fe₃C@NSCNTs）were prepared.By electron microscopy,we found that Fe₃C@NSCNTs was curved nanotubes.The nanotubes were made of nonmetallic elements,mostly carbon,and soybean-like iron carbide is wrapped by the nanotubes.（2）After doping S,Fe₃C@NSCNTs showed a better catalytic ability of sodium persulfate than nitrogen-doped Fe-based carbon nanotubes（Fe₃C@NCNTs）,commercial carbon nanotubes and iron carbide.Under the conditions of 1 m M PDS and0.2 g L^-1 Fe₃C@NSCNTs,20 mg L^-1 bisphenol A was almost completely degraded in30 min,and the mineralization rate was high.（3）Through quenching experiment and electron paramagnetic resonance spectroscopy,it was found that Fe₃C@NCNTs could activate PDS to produce free radicals and nonradicals（singlet oxygen）,while after the doping of S,the Fe₃C@NSCNTs activated PDS to mainly form electron transfer complexes,without free radicals and singlet oxygen in the system.The doping of S changed the activation was of catalysts for PDS.（4）Fe₃C@NSCNTs/PDS system performed a high BPA degradation rate under the interference of a variety of anions,natural organic matter and real water,and it was less disturbed by p H with less metal leaching.The carbon layer wrapping round Fe₃C greatly reduced the dissolution of iron ions.（5）The main catalytic activity sites of Fe₃C@NSCNTs were in the outer carbon layer,and the internal Fe₃C is the key to the catalytic activity.The outer carbon layer acted as an electron shuttle,and improving the electron transfer capacity of the carbon layer was beneficial to improving the catalytic capacity of the catalyst.In addition,the outer carbon material isolates the contact between Fe₃C and PDS,which might be the reason for the less free radicals.The Fe₃C@NSCNTs-PDS electron transfer complexes had a longer life than the free radicals and could continue to degrade BPA for a period without additional PDS supply.