Accelerated photocatalytic degradation of diclofenac by a novel CQDs/BiOCOOH hybrid material under visible-light irradiation: Dechloridation, detoxicity, and a new superoxide radical model study
Graphical abstract
Introduction
Semiconductor photocatalysis have garnered intensive interest as a green technology that utilizes sustainable solar energy for the degradation of organic pollutants, toward the mitigation of environmental degradation and non-renewable energy issues. In the last few years, bismuth-based semiconductors (Bi2Ti2O7, BiVO4, BiOCl, BiFeO3, Bi2WO6, (BiO)2CO3, and BiOCOOH etc.) have been synthesized for photocatalytic applications [1], [2], [3], [4], [5], [6], [7], [8]. Among them, BiOCOOH is comprised of a layered structure that is similar to BiOX (X = Cl, Br, I), which can be applied as a photocatalyst [9]. With additional “green” elements (e.g., C, H, and O) rather than toxic halogens, BiOCOOH will not generate secondary pollution in ecosystems, and thus has excellent potential for environment remediation applications. However, BiOCOOH is only responsive under UV light, which limits its use in the conversion of solar energy [10]. In order to exploit this photocatalyst system working under visible light, non-metal (i.e. reduced graphene) and narrow band gap semiconductors (i.e. BiOI, Ag3PO4, and Br−) doping were employed by researchers to modify these wide band gap materials [2], [11], [12].
A novel class of nanocarbons, referred to as carbon quantum dots (CQDs), have recently attracted much attention. They possess rich photophysical properties, such as being non-metallic and non-toxic, and exhibit upconversion photoluminescence behavior and broadband optical absorption [13], [14]. Thus, CQDs have been employed to modify semiconductors to enhance their photocatalytic activity through possessing excellent upconversion, photoluminescence, and electron transfer. On this point, we anticipated that CQDs modified BiOCOOH composites (CQDs/BiOCOOH) would exhibit good performance in the elimination of trace pollutants. As far as we know, no previous studies regarding CQDs embedded within BiOCOOH materials, or their applications for environmental remediation have been reported.
In order to clearly elucidate and detail the photocatalytic performance of CQDs/BiOCOOH composites, diclofenac (DCF) was employed as a target contaminant. DCF has emerged as a frequently detected pharmaceutical micropollutants in secondary effluent, surface water, and drinking water at concentrations range of from 1.2 to 4.7 μg·l−1, respectively [15], [16]. Due to its low biodegradability and sorption character onto the activated sludge, only thirty percent of DCF is typically eliminated via conventional sewage treatment plants [17]. It is recognized that DCF and its transformation byproducts pose a serious threat to public health and ecosystems according to the persistence and ecotoxicity in the water [18]. Thus, it is necessary to increase the removal efficiency (detoxicity, dechloridation and mineralization) of DCF through water treatment techniques such as photocatalysis.
Previous researches involving the elimination of organic pollutants with CQDs-based semiconductor materials has been conducted over that last few years [19], [20]. However, the detailed upconversion effect of CQDs on these materials has not been systematically reported, particularly when employing different excitation light sources, for example blue, green or near-infrared light. During the photocatalytic process, reactive oxygen species (ROS, e.g. OH and O2−) are generated [21], [22]. A model study might be more explicit toward elucidating the significant role of ROS. Further, an exploration of the interrelation between the ROS and various byproducts would be more useful. In addition to the correct characterization of byproducts, quantum chemical calculations were employed to verify ROS-induced degradation mechanisms in detail [23], [24].
In the present study, a visible light-driven composite catalyst CQDs/BiOCOOH was prepared via a facile hydrothermal-sintering process. The structure, morphology, and spectral characteristics of CQDs/BiOCOOH composites were investigated and the photocatalytic activity for the degradation of DCF was studied under visible light irradiation. The effects of CQDs and ROS induced degradation pathways and O2− model were evaluated. The relationships between byproduct toxicity, total organic carbon (TOC), and chloridion (Cl−) were investigated during the photocatalytic degradation of DCF. Finally, the stability and potential applications (for the degradation of other emerging pollutants) of CQDs/BiOCOOH composites were elucidated as a sustainable visible light-driven photocatalyst for water purification.
Section snippets
Chemical reagents
Sodium formate dihydrate (HCOONa·2H2O), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), and ethylene glycol were obtained from TCI Reagent Co. Ltd. (Shanghai, China). DCF and sulfadimidine (SM) (98% purity), were purchased from J&K Chemical Co. Ltd. (Beijing, China). Rhodamine B (RhB) was obtained from Aladdin Co. Ltd. Other reagents were of analytical grade and required no further purification.
Preparation of materials
The preparation method for BiOCOOH was referenced from Xiong et al. [10], where 1 mM Bi(NO3)3·5H2O was
Characterization of the CQDs/BiOCOOH samples
In the present study, CQDs were successfully prepared. As shown in Fig. 2a, the CQDs was excited by the wavelength light range of from 550 to 900 nm, and the upconverted emission spectral were located in the visible light range (350–570 nm), suggesting that the lower energy long-wavelength could be converted to higher energy short-wavelength via the CQDs. The results were similar to the previous reports [30], [31]. Thus, the CQDs were used to modify BiOCOOH. The phase structures of the
Conclusion
Based on a structural understanding, a new CQDs/BiOCOOH photocatalyst was successfully synthesized via a facile hydrothermal route. Under visible light irradiation, the CQDs/BiOCOOH composites showed superior catalytic activity and stability in the degradation of DCF. The electron transfer ability and upconversion of the CQDs were beneficial toward enhancing this photocatalytic activity. Typically, the rate constants of DCF degradation could be fitted as a first-order function of the initial
Acknowledgments
The authors wish to thank the National Natural Science Foundation of China (21677040 and 21377031), the China Postdoctoral Science Foundation (No. 2015M582188), and the Innovative Team Program of High Education of Guangdong Province (2015KCXTD007).
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