Degradation of oxytetracycline by the in situ hydrogen peroxide-producing photo-Fenton system g-C3N4/CQDs/Fe2+: Mechanism, degradation pathway and toxicity change analysis
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摘要: 芬顿反应是一种通过Fe2+催化H2O2分解生成羟基自由基(•OH)的氧化工艺，因其•OH生成速率快和操作简单等优点被广泛应用于废水中有机污染物的降解，然而存在需要外部加入H2O2和pH适用范围窄的缺陷限制其进一步应用。为解决以上问题，通过外加Fe2+活化复合光催化剂石墨相氮化碳/碳量子点(g-C3N4/CQDs)原位生成的H2O2，构建了g-C3N4/CQDs/Fe2+光芬顿体系，避免了H2O2在储存和运输过程中的潜在风险，并扩宽了反应的pH值范围。该体系对土霉素(OTC)的降解表现出优异的活性，在g-C3N4/CQDs投加量为120 mg，OTC初始浓度为20 mg·L−1，Fe2+投加量为0.36 mmol·L−1，OTC的初始pH为7时，其降解效率高达97%。基于自由基捕获、•OH的生成变化测定及与H2O2生成过程的对比等实验，证明g-C3N4/CQDs/Fe2+光芬顿降解OTC优异的活性，主要来源于Fe2+活化原位H2O2产生的•OH活性物种。进一步地，OTC降解的中间产物也通过质谱联用仪检测分析，并推断出OTC被降解的可能路径。此外，通过测量光密度(OD 600)值获得细菌的生长曲线，结果表明随着g-C3N4/CQDs/Fe2+降解OTC的进行，其反应溶液的毒性是逐渐减小的。最后，与外加H2O2传统芬顿体系降解OTC进行了对比分析，结果表明该光芬顿体系基本能够达到传统芬顿降解OTC的效果，且不受pH值范围的限制，为改善芬顿反应在实际废水处理中的应用提供了新的思路。Abstract: Fenton reaction is an oxidation process that generates hydroxyl radicals (•OH) through the decomposition of H2O2 catalyzed by Fe2+, which is widely used for the degradation of organic pollutants in wastewater because of its fast •OH generation rate and simple operation. In this study, the g-C3N4/CQDs/Fe2+ photo-Fenton system was constructed by adding Fe2+ activated composite photocatalyst graphite phase carbon nitride/carbon quantum dots (g-C3N4/CQDs) to generate H2O2 in situ, which avoids the potential risk of H2O2 during storage and transportation, and broadens the pH range of the reaction. The system showed excellent activity for the degradation of oxytetracycline (OTC) with a high degradation efficiency of 97% at a g-C3N4/CQDs dosing of 120 mg, an initial OTC concentration of 20 mg·L−1, an Fe2+ dosing of 0.36 mmol·L−1, and an initial pH of 7 for OTC solution. The excellent activity of g-C3N4/CQDs/Fe2+ photo-Fenton degradation of OTC based on experiments such as radical capture, determination of changes in the generation of •OH and comparison with the H2O2 generation process was demonstrated to be mainly derived from the •OH active species generated by Fe2+ activation of in situ H2O2 production. Further, the intermediate products of OTC degradation were detected and analyzed by mass spectrometry, and the possible degradation paths of OTC were inferred. In addition, by measuring the optical density (OD600) value to obtain the growth curve of bacteria, the results showed that the toxicity of the reaction solution gradually decreased with the degradation of OTC by g-C3N4/CQDs/Fe2+. Finally, compared with the conventional Fenton system with the addition of H2O2 to degrade OTC, the results showed that the photo-Fenton system can basically achieve the effect of conventional Fenton, and it is not limited by the pH range which provides a new idea for improving the application of Fenton reaction in actual wastewater treatment.
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Key words:
- photo-Fenton /
- photocatalysis /
- in-situ H2O2 /
- oxytetracycline /
- degradation path /
- toxicity analysis
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图 1 实验装置
1. Light source switch; 2. Magnetic stirrer; 3. Reaction solution (Graphite phase carbon nitride/carbon quantum dots (g-C3N4/CQDs)-oxytetracycline (OTC)-Fe2+); 4. Photocatalytic reaction chamber; 5. Xenon lamp steady current power supply
Figure 1. Experimental apparatus
图 2 g-C3N4和g-C3N4/CQDs的XRD图谱(a)和FTIR图谱(b)；g-C3N4 (c)和g-C3N4/CQDs (d)的TEM图像
Figure 2. XRD patterns (a) and FTIR spectra (b) of g-C3N4 and g-C3N4/CQDs; TEM images of g-C3N4 (c) and g-C3N4/CQDs (d)
图 3 (a) g-C3N4/CQDs光催化生产H2O2的能力；g-C3N4/CQDs/Fe2+体系对OTC的降解曲线(b)和降解效率(c)
The reaction conditions is g-C3N4/CQDs 120 mg, initial pH=7; C0—Initial concentration of OTC; C—Concentration of OTC after time t
Figure 3. (a) H2O2 photocatalytic productivity of g-C3N4/CQDs; Degradation curves (b) and degradation rate (c) of OTC by g-C3N4/CQDs/Fe2+ system
图 4 不同g-C3N4/CQDs投加量对g-C3N4/CQDs/Fe2+体系降解OTC的降解曲线(a)、二级动力学方程(b)、降解效率(c)和不同投加量产H2O2的能力(d)
k—Second order rate constant
Figure 4. Degradation curves (a), second-order kinetic equation (b), degradation rate (c) of OTC with different dosage of g-C3N4/CQDs for g-C3N4/CQDs/Fe2+ system and ability to produce H2O2 in different dosage (d)
图 5 不同OTC的初始浓度对 g-C3N4/CQDs/Fe2+体系降解OTC的降解曲线(a)、二级动力学方程(b)和降解效率(c)
Figure 5. Degradation curves (a), second-order kinetic equation (b) and degradation rate (c) of OTC with different initial concentration ofOTC for g-C3N4/CQDs/Fe2+ system
图 6 不同Fe2+浓度对g-C3N4/CQDs/Fe2+体系降解OTC的降解曲线(a)、二级动力学方程(b)和降解效率(c)
Figure 6. Degradation curves (a), second-order kinetic equation (b) and degradation rate (c) of OTC with different concentration of Fe2+ for g-C3N4/CQDs/Fe2+ system
图 7 不同OTC的初始pH对g-C3N4/CQDs/Fe2+体系降解OTC的降解曲线(a)、二级动力学方程(b)和降解效率(c)
Figure 7. Degradation curves (a), second-order kinetic equation (b) and degradation rate (c) of OTC with different initial pH for g-C3N4/CQDs/Fe2+ system
图 8 g-C3N4/CQDs/Fe2+体系对比传统外加H2O2芬顿体系降解OTC的降解曲线(a)、二级动力学方程(b)和降解效率(pH=7，0.36 mmol·L−1 Fe2+，0.17 mmol·L−1 H2O2) (c) ；(d) g-C3N4/CQDs/Fe2+体系降解OTC的稳定性测试结果
Figure 8. Degradation curves (a), second-order kinetic equation (b) and degradation rate of OTC with g-C3N4/CQDs/Fe2+ system compared with the traditional plus H2O2 Fenton system (pH=7, 0.36 mmol·L−1 Fe2+, 0.17 mmol·L−1 H2O2 ) (c) ; (d) Stability test results of g-C3N4/CQDs/Fe2+ system for degradation of OTC
图 9 不同捕获剂对g-C3N4/CQDs/Fe2+降解OTC的影响(a)和g-C3N4/CQDs产 H2O2的影响(b)；(c) •OH的生成变化测定实验；(d) g-C3N4/CQDs产H2O2、g-C3N4/CQDs/Fe2+体系•OH生成与OTC降解各时间段效果比例图
AO—Ammonium oxalate; p-BQ—p-benzoquinone; IPA—Isopropyl alcohol
Figure 9. Effects of different trapping agents on OTC degradation in g-C3N4/CQDs/Fe2+ (a) and production of H2O2 in g-C3N4/CQDs (b); (c) Experiment for the determination of changes in the generation of •OH; (d) Proportion of the results of the H2O2 production capacity of g-C3N4/CQDs, the •OH generation capacity and the effect on degrading OTC of g-C3N4/CQDs/Fe2+ in each time period
图 10 g-C3N4/CQDs/Fe2+体系对OTC的降解机制
Figure 10. Mechanism of degradation of OTC by g-C3N4/CQDs/Fe2+ system
图 11 g-C3N4/CQDs/Fe2+体系对OTC的HPLC-MS和质谱分析
Figure 11. HPLC–MS and mass spectra of OTC by g-C3N4/CQDs/Fe2+ system
图 12 g-C3N4/CQDs/Fe2+体系对OTC的降解途径
Figure 12. Proposed degradation pathways of OTC by g-C3N4/CQDs/Fe2+ system
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