Modification Of The Graphitic Carbon Nitride Photocatalysts For The Removal Of Aqueous Organic Pollutants And Mechanism Study

In recent decades,researchers have been looking for green and clean technologies to solve the environmental crisis.Among many strategies,graphitic carbon nitride（g-C₃N₄）based semiconductor photocatalysis is of great potential in practical application for the wide raw material source,good ecosystem compatibility,and stable physical and chemical properties of g-C₃N₄.To overcome the intrinsic disadvantage of g-C₃N₄,narrow light absorption range,and easy recombination of photocarriers,this study has made many efforts including biochar modification,step-scheme heterojunction construction,bimolecular modification,and synergistic piezo-photocatalysis to optimize the light response,reduce the recombination of photocarriers,and boost the high efficient directional movement of photoelectrons,improving the photocatalytic contaminant degradation efficiency of the g-C₃N₄-based photocatalysts.The composition and the properties of the photocatalysts have been investigated via systematic characterizations and theoretical calculation.The promoted photocatalytic performance was evaluated via photodegradation of tetracycline hydrochloride or atrazine.The reactive species,photocatalytic mechanism,and the degradation pathway of contaminants were analyzed.The detailed experiment results are summarized below.（1）To widen the light absorption range of g-C₃N₄,the Enteromorpha biochar has been fabricated and combined with g-C₃N₄ to construct the biochar/g-C₃N₄ composite.The photoresponse of the biochar/g-C₃N₄ was promoted.Besides,a partial graphitic structure was synthesized in biochar during the calcination to construct a conjugation system with the g-C₃N₄.The conjugated structure in biochar/g-C₃N₄ enhanced the migration of photoelectrons and boost its photocatalytic efficiency.Under visible light,the biochar/g-C₃N₄ composite can effectively activate PMS（0.2 g/L）to realize nearly 90%tetracycline hydrochloride（10 mg/L）removal efficiency.Given the quenching experiments,ESR spectra,and LC-MS/MS results,the photocatalytic mechanism and degradation pathway of tetracycline hydrochloride have been proposed.While it was found that,even though this system can effectively remove antibiotics,its efficiency towards refractory herbicide atrazine was limited.（2）To realize the effective separation of photoelectrons and holes and further enhance the redox ability of the catalyst,g-C₃N₄ has been hybridized with pyromellitic diimide（PDI）and Ti₃C₂ to construct the step-scheme PDI-g-C₃N₄/Ti O₂@Ti₃C₂ heterojunction.The staggered band configuration and intimate contact between PDI-g-C₃N₄ and Ti O₂@Ti₃C₂ can induce the formation of step-scheme heterojunction with good redox ability,where the photogenerated electrons and holes can be effectively separated for they would respectively migrate to the PDI-g-C₃N₄ and Ti O₂@Ti₃C₂.The step-scheme PDI-g-C₃N₄/Ti O₂@Ti₃C₂ can activate PMS（0.2 g/L）under visible light to remove 75%atrazine（10 mg/L）within 1 h,with a 46.08%total organic carbon removal rate.The PDI-g-C₃N₄/Ti O₂@Ti₃C₂ showed stable photocatalytic performance under various simulated environmental conditions,and its structure shows no obvious change after the reaction.Besides,it was found that the PDI-g-C₃N₄/Ti O₂@Ti₃C₂ can generate H₂O₂ during the photocatalyst to boost atrazine degradation.（3）The directional movement of photoelectrons and holes can further improve the photocatalytic performance of the g-C₃N₄.Therefore,the g-C₃N₄ has been modified with triazole（TA）and PDI to construct a donor-π-acceptor electron structured TA-CN-PDI.The as-prepared TA-CN-PDI photocatalyst can realize the establishment of the electron enrichment region and the electron depletion region and induces the directional movement of photocarriers.Without the assistance of PMS,the TA-CN-PDI can achieve 95%atrazine（5 mg/L）removal within 1 h under visible light.The Density Functional Theory（DFT）calculation verified that the electrons are rearranged in HOMO and LUMO,where they can be effectively separated to boost their directional movement.In the TA-CN-PDI,the electron enrichment region and the electron depletion region are located around PDI and TA-CN,respectively.（4）The ultrasonic-assisted piezocatalysis can induce the formation of a polarization field in the g-C₃N₄,which can offer the external force and boost the separation of photoelectrons and holes.Hence,the multi-scale modification strategy was applied on g-C₃N₄to construct the g-C₃N₄/PDI-g-C₃N₄ homojunction with piezoresponse.Under simultaneous ultrasonic and light irradiation,the synergistic effect of piezocatalysis and photocatalysis can be realized to boost the degradation of contaminants.On the morphological scale,the multi-step thermal condensation can induce porous structure to promote mass transfer;on the molecular scale,the modification of PDI and enhance the polarity of the catalyst to boost its piezoresponse;on the electronic scale,the construction of homojunction can reduce the energy loss during the electron transfers in the different interfaces.The g-C₃N₄/PDI-g-C₃N₄ showed excellent piezo-photocatalytic efficiency under p H=2.97,achieving 94%atrazine removal（10mg/L）in 1 h.In ultrapure water,the system can effectively produce H₂O₂ with 625.54μM·h^-1.The Finite Element Method（FEM）simulation has confirmed the strengthened piezocatalytic activity in g-C₃N₄/PDI-g-C₃N₄.The piezo-photocatalytic mechanism and atrazine degradation pathway were investigated.Moreover,the TEST assessment of the intermediates has further confirmed the effective toxicity reduction of atrazine.