Iron-based Photocatalytic System Enhanced By Graphite-like Carbon Nitride For Removal Of Emerging Contaminants

Water pollution poses a serious threat to the survival of aquatic organisms and human health.Among them,emerging contaminants are difficult to completely remove by traditional water treatment technology due to their complex structure and low concentration.Advanced oxidation pocesses（AOPs）can generate active free radicals with strong oxidizing ability,these free radicals has a good effect on the removal of difficult-to-degrade emerging contaminants.In recent years,due to fewer restrictions on the use of peroxymonosulfate（PMS）,Fenton-like systems based on sulfate radical（SO₄·^-）instead of Fenton systems have been studied extensively.There are also several shortcomings of Fe³⁺/PMS system,such as the use of large amount of reagents,the proportion of reagents needs to be strictly controlled,and the production of a large amount of iron mud after the reaction.The use of iron ions to activate PMS requires optimization of the catalytic system to achieve efficient degradation of organic pollutants in wastewater.As an environmentally friendly purification technology,photocatalytic technology has a good application prospect.Graphite-like carbon nitride（g-C₃N₄）is widely used as a photocatalyst due to its moderate band gap width（approximately 2.7 e V）and physical and chemical stability.Combining the advantages of photocatalysis and advanced oxidation technology,using the mutual influence of metallic iron and g-C₃N₄ to better activate PMS.From the perspective of Fe³⁺being in a homogeneous or heterogeneous state,this paper designs photocatalytic systems that efficiently activate PMS to degrade organic pollutants,and use carbamazepine（CBZ）as a substrate to explore the photocatalytic activity of these systems.The main contents include:（1）Designed Fe³⁺/g-C₃N₄/PMS photocatalytic system that can efficiently activate PMS to degrade organic pollutants with trace Fe³⁺.The Fe³⁺concentration used in the experiment was6.17×10^-6 M and the PMS concentration was 0.2 m M.The amount of reaction reagents is extremely low,which can avoid the production of a large amount of"iron mud".When changing the[Fe³⁺]:[PMS]ratio,it was found that when the ratio was 1:32 or 20:1,CBZ was completely degraded within 30 minutes,breaking the restriction of the strict ratio of metal ions to oxidant.The radical trapping experiments,DMSO oxidation experiment,EPR detection confirmed that¹O₂,O₂·^-,·OH,SO₄·^-and a small amount of Fe^V=O active species were involved in the system.Among them,O₂·^-and ¹O₂ played a major role,SO₄·^-and·OH jointly promoted the degradation of CBZ.Finally,the intermediate products and final products of CBZ degradation were analyzed by ultra-performance liquid chromatography and mass spectrometry（UPLC-MS）,and the possible degradation pass of CBZ was proposed.（2）To improve the existence state of Fe³⁺in the reaction system,the composite photocatalystβ-FeOOH@g-C₃N₄ was prepared by microwave method.According to the characterization results of XRD,XPS and TEM,the chemical structure of β-FeOOH@g-C₃N₄was obtained:In the initial stage of the microwave process,the free Fe³⁺was first coordinated with the nitrogen atom on g-C₃N₄,and then the O atom was coordinated with Fe³⁺combined with nitrogen atom,resulting in in situ growth of nano-β-FeOOH on g-C₃N₄.Compared with the system that usingβ-FeOOH or g-C₃N₄ as the catalyst alone,the efficiency of CBZ degradation of β-FeOOH@g-C₃N₄ was increased by 5-10 times under solar irradiation,and the mineralization rate of CBZ was as high as 92%.Secondly,β-FeOOH@g-C₃N₄ still maintained high catalytic activity after acid treatment or repeated degradation of CBZ and almost no Fe³⁺was detected in the reaction solution,indicating thatβ-FeOOH@g-C₃N₄ had excellent stability and repeated usability.Through free radical detection,theβ-FeOOH@g-C₃N₄/PMS system under solar irradiation produced ¹O₂,O₂·^-,·OH and SO₄·^-four free radical active species.The DMSO oxidation experiment also proved that Fe^V=O was produced.Based on these results,we proposed a possible synergistic mechanism for theβ-FeOOH@g-C₃N₄/PMS photocatalytic system:In the initial stage of the reaction,a large amount of ¹O₂,O₂·^-and Fe V=O contributes to the oxidative degradation of CBZ,while deep oxidation and the increased TOC removal rate was mainly attributed to·OH and SO₄·^-.