Photocatalytic degradation of nonylphenol ethoxylate and its degradation mechanism
Introduction
Alkylphenol ethoxylates (APEOs) are the second largest commercial nonionic surfactants in the world, among which octylphenol ethoxylates (OPEO) and nonylphenol ethoxylates (NPE10O) are the most widely used [1,2]. NPE10O is the main component of textile auxiliaries such as emulsifiers, penetrants and detergents. It is widely used in leather and textile printing and dyeing [3]. After entering the water environment, NPE10O wastewater can be converted into short chain nonylphenol ethoxylates and more toxic nonylphenol (NP) major metabolites. Studies have shown that compared with the parent compound, these refractory, intermediate biometabolism with strong bioaccumulation and toxicity can interfere with human endocrine secretion, which is carcinogenic and teratogenic to humans [[3], [4], [5], [6]]. At present, researchers are actively looking for alternatives to NPE10O, and more promising are fatty alcohol ether and isomeric alcohol ether, but from a completely alternative point of view, fatty alcohol ether and isomeric alcohol ether still have some difficulties. By 2017, NPE10O still accounts for a large proportion of the nonionic surfactant market. Therefore, it is extremely important to study the deep degradation and degradation mechanism of NPE10O. In recent years, processes for the advanced treatment of NPE10O wastewater, such as activated carbon [7,8], resin adsorption [9,10], photocatalytic degradation [1,[11], [12], [13]] and biodegradation [[14], [15], [16]], have attracted much attention. The relative cost of activated carbon and resin is relatively high, and biodegradation takes a long time, while photocatalytic degradation with high selectivity, wide application range, has become a research hotspot in sewage treatment in recent years. Semiconductor materials that have been studied as photocatalytic materials include TiO2 [17,18], WO3 [19], ZnO [20], Nb2O5, α-Fe2O3 [21], g-C3N4 [22,23], BiOX [24], BiVO4 [25], Ag3PO4 [26], Ag2CO3 [27], AgBr, AgI, etc. Among them, TiO2 is the earliest photocatalyst used to degrade organic pollutants, and has been enduring even after years of research based on low price, good selectivity, and high photocatalytic activity. However, the large band gap and the high recombination rate of photogenerated electron-hole pairs of pure TiO2 will severely restrict its use, therefore it needs to be modified to improving photocatalytic activity. To date, it can effectively control the electron transport of TiO2 by means of metal/non-metal doping [28,29], noble metal surface deposition [30], semiconductor recombination, and organic dye [31]/quantum dot sensitization [32], thereby reducing the recombination rate of charge carriers. Herein, the emphasis is on the sensitization of carbon quantum dots, which is called the most promising semiconductor photosensitizer.
Carbon quantum dots (CQDs), as an excellent photosensitive material, can prevent the recombination of energy in addition to the sensitive visible light response. In addition, there is an electronic coupling between the π orbit of CQDs and the conduction band of TiO2, and electrons will be transferred at the interface between CQDs and TiO2, which demonstrates unique superiority in inhibiting the recombination of electrons and holes. CQDs can be more detailedly divided into graphene quantum dots (GQDs), carbon nanodots (CNDs) and polymer dots (PDs) [33,34]. Rajender et al. [35] reported that the graphene quantum dots (GQD) were hybridized with TiO2 nanoparticles to form heterojunctions and used for photocatalytic degradation of MB. The results show that the TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation. Al-Kandari et al. reported the preparation of nanocomposites (rGOTi) by loading 0.33 wt% of reduced graphene oxide (rGO) on commercial TiO2 nanoparticles using a hydrothermal method, and used to study photocatalytic degradation of phenol, p-chlorophenol and p-nitrophenol. The result shows that rGOTi with much higher photocatalytic activity degrade efficiently a mixture of three phenolic compounds [36]. Therefore, it is more wise to choose CQDs as photosensitizers for TiO2 [[37], [38], [39], [40], [41]]. In the work, TiO2@g-CQDs photocatalyst was synthesized by one-step sol-gel method. The sample was characterized by HRTEM, XRD, FT-IR, UV-DRS and XPS and applied to degrade NPE10O simulated wastewater. The effects of photocatalyst concentration, g-CQDs doping amount, NPE10O initial concentration and source power factor on the degradation rate of NPE10O were discussed. At the same time, the photocatalytic degradation path and mechanism of NPE10O was speculated.
Section snippets
Chemicals and materials
Graphite (99.95%) and tetrabutyl titanate (99.8%) were respectively purchased from Aladdin and Tianjin Kermel Chemical Reagent Co., Ltd. Nonylphenol ethoxylates was purchased from Beijing Sage Chemical Co., Ltd. Hydrochloric acid and absolute ethanol were attained from Shanghai Sinopharm Chemical Reagent Co., Ltd. Isopropanol (IPA), vitamin C (VC) and ethylenediaminetetraacetic acid (EDTA) were bought from Sinopharm Chemical Reagent Co. Ltd. The above reagents were of analytical grades without
HRTEM studies
The morphology and particle size of the prepared samples were studied by HRTEM. Fig. 1(a–b) showed HRTEM images and particle size distribution of g-CQDs prepared by mixing acid reflux after optimization. It can be seen that the g-CQDs have good dispersibility, the particle size ranges from 1 to 3 nm, and the average diameter is 1.83 ± 0.36 nm. Fig. 1(c–d) represented the HRTEM image of TiO2, which exhibited morphology of irregular block-like with size of 30–40 nm. And it can be seen from Fig. 1
Conclusion
In this study, successful synthesis of photocatalyst TiO2@g-CQDs was confirmed by HRTEM, XRD, FT-IR and XPS, UV-DRS characterization. In the process of photocatalytic degradation of NPE10O, the results show that TiO2@g-CQDs has excellent photocatalytic performance, and it can completely degrade NPE10O within 60 min. Compared with pure TiO2, 20-TiO2@g-CQDs improves the degradation efficiency of NPE10O by 55.6%. The photocatalyst concentration, g-CQDs doping amount, NPE10O initial concentration
CRediT authorship contribution statement
Huiqin Liang: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xiumei Tai: Resources, Writing - review & editing, Supervision, Data curation. Zhiping Du: Resources, Supervision, Data curation.
Acknowledgements
The work has been supported by the National Natural Science Foundation of China (No. U1610222) and the National Key Research & Development Plan (2017YFB0308704). We would like to thank the project of JALA Research Funds (JALA2017).
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