Construction Of CaO₂-based Advanced Oxidation Process Systems For Organic Pollutant Degradation

In multitudinous advanced oxidation process（AOP）systems,liquid H₂O₂ is one of the most commonly used oxidant,due to high oxidation efficiency and producing non-toxic by-products.However,the inherent drawbacks of H₂O₂,such as instability,short lifespan,and narrow operating p H range,greatly limit the application of H₂O₂-based AOPs in environmental remediation.Stable,cheap and environmentally friendly CaO₂ is considered as an ideal substitute for H₂O₂,the development of CaO₂-based AOPs and their degradation performance of organic pollutants have received considerable attention.In the reported CaO₂-based AOPs,iron ions(Fe²⁺and Fe³⁺)are most commonly used to activate CaO₂ for organics degradation.Unfortunately,the alkalinity of CaO₂ inevitably leads to the precipitation of iron ions,which not only affects the activation performance,but also produces massive environment-harmful ferric sludge.Hence,in this work,three different CaO₂-based AOPs systems were constructed to systematically explore the degradation performances and degradation mechanisms towards organic pollutants,expecting to provide new strategies and methods for the application of CaO₂ in the environmental remediation.The main contents and results are listed as follows:（1）A CaO₂-based AOP system was constructed for the degradation of organics,using CaO₂ as oxidant and Cu/g-C₃N₄ composites（Cu-CN）as heterogeneous catalyst.CaO₂ and Cu-CN composites were prepared by one-step precipitation method and two-step calcination method,respectively.Among a series of Cu-CN composites prepared with different raw materials,precursor ratio and calcination temperature,the Cu-CN（U550-1）composites had the best performance for activating CaO₂.The CaO₂+Cu-CN（U550-1）system could effectively degrade methylene blue（MB）in a wide p H range（4～12）and temperature range（5～35°C）,and the degradation efficiency was hardly affected by common matrix species(Cl^-,NO₃^-,SO₄^2-,HCO₃^-and HA)and water sources.In addition,Cu-CN（U550-1）composites possessed excellent catalytic cycling stability,and the degradation effiency of MB could still reach more than 80%after 5 cycles.Mechanistic studies showed that·OH and ¹O₂ were generated in the system,and Cu⁺and the Cu-N bond in the Cu-CN（U550-1）composites was the key factor in the radical and non-radical activation of CaO₂ nanoparticles,respectively.（2）An environmentally friendly CaO₂-based AOP system was constructed by using the Mg NCN/Mg O composite prepared by one-step calcination as the activator.The CaO₂+Mg NCN/Mg O system exhibited excellent degradation performance for for tetracycline（TC）and MB over a wide p H range（3～12）,accompanied with the weak influences of coexisting anions and natural water sources.The slow-release characteristic of CaO₂endowed the AOP system with a long-term oxidation ability for continuous degradation of organic pollutants.Mechanistic studies confirmed that ¹O₂ was the main reactive oxygen species（ROS）responsible for the organics degradation,and the coordination between Mg NCN and Mg O in Mg NCN/Mg O composites played a crucial role in the non-radical activation of CaO₂.In addition,the CaO₂+Mg NCN/Mg O system also possessed the ability to simultaneously remove organic dye pollutants and heavy metal ions.100 mg/L CaO₂ and 100 mg/L Mg NCN/Mg O could successfully remediate the complex pollution consisting of 10 mg/L MB and 100 mg/L Cd²⁺.（3）The dual-oxidant AOP system containing CaO₂ and peroxymonosulfate（PMS）was constructed for rapid and efficient degradation of antibiotic TC.The CaO₂+PMS system possessed excellent degradation performance,and could rapidly degrade more than93%of TC（100 m L,50 mg/L）within 8 min.Kinetic studies showed that the second-order kinetic model was more suitable for the degradation of TC in the system,and the kinetic rate constant was 2.522 L·mg^-1·min^-1 under the optimal conditions.More importantly,the system exhibited strong anti-interference ability to common coexisting species(Cl^-,NO₃^-,SO₄^2-,HCO₃^-and HPO₄^2-).Even in a high concentrations（500 mmol/L）,the system could also effectively and quickly degrade TC.In addition,the system was highly adaptable to initial solution p H（3～12）,degradation temperature（15～35°C）,various water sources and organics.Mechanistic investigation showed that that the synergy between CaO₂ and PMS was responsible for the generation of ROS,and ¹O₂ was the main ROS,accompanied by a small amount of O₂^·-.