Nonmetal-doped Carbon Nitride Activated Peroxymonosulfate And Its Application In Degradation Of Organic Pollutants

Environmental pollution by refractory organics from aqueous medium has become a serious problem facing humanity.The ever-increasing concern over water deterioration has led to enormous efforts in the development of economic and efficient water treatment technologies.In recent years,advanced oxidation processes have held a prominent position in the destruction of refractory organic pollutants not only because they afford the production of reactive oxygen species?ROS?but also because they permit the occurrence of catalytic reaction under reasonably mild conditions.Persulfate such as peroxymonosulfate?PMS,HSO₅^-?is an emerging alternative oxidant that finds pervasive application in advanced oxidation processes for organic contaminant degradation.On peroxy bond activation by various initiators capable of breaking the O-O bond in persulfate,strongly oxidizing radicals involving sulfate radical(SO₄^??)and/or hydroxyl radical?^?OH?can be generated.Among the well-developed persulfate activators,transition-metal-based materials with high redox activity appear to be more efficient heterogeneous catalysts for persulfate activation.Despite the excellent catalytic performance,the deactivation and low stability of transition metal catalysts substantially restrict their implementation for large-scale applications.Therefore,it is imperative to explore active and sustainable catalysts to replace transition-metal-based catalysts for the activation of persulfate.Graphite phase carbon nitride?g-C₃N₄?is composed of carbon and nitrogen elements with abundant reserves of the earth.It is environmentally friendly,has high physical and chemical stability,and is a very promising non-metallic material.However,without light irradiation,the electron transfer rate on the surface of g-C₃N₄ is slow,chemically inert,and its catalytic oxidation efficiency is correspondingly low.In this paper,carbon nitride is doped with nonmetal to prepare an efficient and stable catalyst for activating persulfate to degrade organic pollutants.The main research contents are as follows:?1?Oxygen-doped graphitic carbon nitride?O-CN?was fabricated via a facile thermal polymerization method using urea and oxalic acid dihydrate as the graphitic carbon nitride precursor and oxygen source,respectively.Experimental and theoretical results revealed that oxygen doping preferentially occurred on the two-coordinated nitrogen positions,which create the formation of low and high electron density areas resulting in the electronic structure modulation of O-CN.As a result,the resultant O-CN exhibits enhanced catalytic activity and excellent long-term stability for peroxymonosulfate?PMS?activation toward the degradation of organic pollutants.The O-CN with modulated electronic structure enables PMS oxidation over the electron-deficient C atoms for the generation of singlet oxygen?¹O₂?and PMS reduction around the electron-rich O dopants for the formation of hydroxyl radical?^?OH?and sulfate radical(SO₄^??),in which ¹O₂ is the major reactive oxygen species,contributing to the selective reactivity of the O-CN/PMS system.Our findings not only propose a novel PMS activation mechanism in terms of simultaneous PMS oxidation and reduction for the production of nonradical and radical species but also provide a valuable insight for the development of efficient metal-free catalysts through nonmetal doping toward the persulfate-based environmental cleanup.?2?Herein,we develop a facile copolymerization strategy for the synthesis of carbon and oxygen dual-doped g-C₃N₄ using urea as g-C₃N₄ precursor and ascorbic acid?AA?as carbon and oxygen sources,which induces electronic structure reconfiguration.By replacing AA with other organic precursors,a series of C and O dual-doped g-C₃N₄ are successfully prepared,demonstrating the generality of the developed methodology.As a demonstration,the C and O dual-doped g-C₃N₄ using AA as the organic precursor(CN-AA_0.3)exhibits pronouncedly enhanced catalytic activity in peroxymonosulfate?PMS?activation for organic pollutant degradation without light irradiation compared with pristine g-C₃N₄ and single oxygen-doped g-C₃N₄.Experimental and theoretical results revealed the electron-poor C atoms and electron-rich O atoms as active sites for PMS activation in terms of simultaneous PMS oxidation and reduction.This work offers a universal approach to synthesize nonmetal dual-doped g-C₃N₄ with reconfigured electronic structure,stimulating the development of g-C₃N₄-based materials for diverse environmental applications.