Synergetic Activation Of Sulfite By Photocatalysis And Transition Metal For PPCPs Removal

In the last couple of decades,organic contaminate originated from persistent use of pharmaceutical and personal care products（PPCPs）have received extensive attention.PPCPs may cause potential negative effects on human health and ecosystems.These adverse effects include acute and chronic toxicity,release of resistance genes and endocrine disruption.PPCPs are inefficiently degraded in traditional wastewater treatment plants.Therefore,PPCPs and their by-products can enter the receiving water bodies and eventually affect water environment.Recently,sulfate radical-based(SO₄^·-)advanced oxidation processes（SR-AOPs）has been widely used in the removal of PPCPs.The main ways to generate SO₄^·-contributing predominantly in SR-AOPs processes are the activation of peroxymonosulfate（PMS）,persulfate（PS）or sulfite[S（IV）].Among them,S（IV）is a by-product of wet desulfurization,which has the characteristics of low cost,low toxicity and has good application prospects.An ultra-small Fe₂O₃ clusters deposited graphitic carbon nitride（g-C₃N₄）is synthesized（catalysts were named CEF-X and X was related to the Fe concentration in synthesis process）in this article and used for S（IV）activation for carbamazepine（CBZ）and diclofenac（DCF）degradation.Moreover,the synergetic effect in contamination degradation has also been studied.For CBZ degradation in S（IV）system,photogenerated holes（h⁺）can transform S（IV）into sulfate radical and photogenerated electrons（e^-）can accelerate Fe（II）/Fe（III）cycle,so that photocatalysis and Fe（II）have a synergistic effect in S（IV）activation.In contrast,the degradation of DCF in S（IV）system depends on the excitation of DCF rather than photocatalyst.Instead of SO₄^·-precursor,S（IV）acts as the electron transfer bridge between excited DCF and photocatalyst.Thus,the deposition of Fe₂O₃ could promote CBZ removal while negatively affects DCF degradation.Density Functional Theory calculation shows that the first excited state rather than the ground state of diclofenac is more suitable for reactive site prediction,due to the unique distribution of HOMO and LUMO,DCF is degraded via a photosensitization-like mechanism.The initial p H mainly affects the degradation of pollutants by changing the surface charge of the catalyst and the existence form of oxidant.Common inorganic anions in wastewater mainly affected the degradation of pollutants through radical scavenging and changing the surface charge of the catalyst.The decreased efficiency of AOPs in real water is generally attributed to two factors:（I）particles or dissolved organic matters（DOMs）which screen the reactive sites of catalysts;（II）substances competing reactive species.Meanwhile,both low concentration and excessive S（IV）can inhibit the degradation of CBZ and DCF.The photocatalyst recycling experiment indicating its excellent reusability.Based on the above results,this study compares the selective mechanism of the catalyst for S（IV）and PS activation.Since the photocatalytic degradation of CBZ is a thermodynamically controlled process,the enhancement of CBZ degradation by different oxidants depends on the utilization of h⁺.S（IV）can form SO₄^·-with higher oxidation potential through h⁺.This reaction removes the thermodynamic limitation and promotes the CBZ degradation.Moreover,e^-can accelerate Fe（II）/Fe（III）cycle.Therefore,photocatalysis and Fe show synergetic effect in S（IV）activation.However,PS cannot be utilized by h⁺and decreases e^-production.Since PS activation,cycle reaction of Fe（III）/Fe（II）and·O₂^-generation all rely on e^-.Therefore,the lack of e^-and the existence of competition leads poor synergetic effect between photocatalysis and Fe（II）in PS activation.In addition,the effect of initial p H has been investigated in different oxidant systems.g-C₃N₄ and CEF-0.05 have different p H dependence in S（IV）system,while they have the same p H dependence in PS system.This result further proves that the deposition of Fe₂O₃ cannot promote CBZ degradation in PS system.