Design of Z-scheme g-C3N4/BC/Bi25FeO40 photocatalyst with unique electron transfer channels for efficient degradation of tetracycline hydrochloride waste
Graphical abstract
Introduction
In recent years, tetracycline antibiotics have been frequently detected in many surface water, groundwater, as well as livestock and poultry aquaculture wastewater around the world, seriously destroying the aquatic ecological environment. Tetracycline hydrochloride (TCH), as a commonly used antibiotic, is widely used in medical, livestock and poultry breeding and aquaculture industries (Garner et al., 2017; Lou et al., 2018). It has good water solubility and is difficult to degrade. Once water containing these antibiotics flow into the aquatic and soil ecosystems they undergo migration and transformation. This will likely cause a negative impact in the diversity of soil microorganisms and insects, as well as other living creature. Additionally, antibiotics may also be transferred to food, fruits, and vegetables, becoming a potential threat to human health (Luo et al., 2011; Yang et al., 2018).
Therefore, there is an urgent need for effective, economical and energy-saving green technologies that can eliminate antibiotics in wastewater to be developed, so that a sustainable use of water resources can be achieved. However, traditional physical, chemical and enhanced advanced oxidation technology used to treat antibiotics in wastewater have several disadvantages, such as high input energy, introduction of various chemical substances, harsh reaction conditions and many by-products, and they are also prone to cause secondary pollution. Therefore, green, energy-saving, high-efficiency, and solar-driven photocatalytic technology are a good option for removing organic pollutants in water (Wilkinson et al., 2017).
The core of photocatalysis technology is to find photocatalysts with excellent performance. Bi25FeO40 belongs to the soft bismuth ore structure semiconductor material. Because of its narrow band gap, non-toxicity and high solar energy utilization, it is considered as a promising candidate material for visible light degradation of organic pollutants (Kalikeri and Kodialbail, 2018). In addition, Bi25FeO40 has superparamagnetic properties (Sun et al., 2013), and as a catalyst, it also has the advantage of easy separation and recovery. However, Bi25FeO40 is prone to photocorrosion (Kolivand and Sharifnia, 2021), so constructing Bi25FeO40 composite semiconductor materialis an effective method to improve its photocatalytic activity and stability. g-C3N4 is considered to be an environmentally friendly material, which will not cause secondary pollution to the aquatic environment during environmental remediation. Additionally, it has high stability at room temperature, a suitable energy band structure and excellent catalytic activity, and is widely used in photocatalytic degradation of water pollutants, organic synthesis reactions and carbon dioxide reduction (Ou et al., 2020). For example, Dong et al. synthesized co-doped carbon nitride with cobalt/vanadium. Based on theoretical and experimental research, it was shown that the co-doping of Co/V elements in g-C3N4 framework can produce a bimetallic synergistic adjustment effect on the electronic structure (Dong et al., 2021). In a published paper from Li et al., the researchers grafted a pyridine ring on the edge of carbon nitride. Experimental studies and DFT calculations showed that the pyridine ring has a dual regulation mechanism. This includes the adjusting of the electronic hybrid structure and the directional transfer of electrons from the center to the edge, which can greatly promote the separation of photogenerated carriers. Furthermore, it can also inhibit the recombination (Li et al., 2021a). Li et al. prepared a NiCoP/g-C3N4 catalyst with high efficiency photocatalytic hydrogen production performance (Li et al., 2021b). Therefore, the new photocatalytic material g-C3N4 and Bi25FeO40 are calcined to prepare a composite g-C3N4 and Bi25FeO40 catalyst, which has a large specific surface area, strong adsorption capacity and a matching band structure (Fu et al., 2019). The band gap can separate photo-generated charge carriers better than a single photocatalyst, thereby producing more active materials (Chen et al., 2020; Wang et al., 2020b). Nevertheless, in practical applications, it is still necessary to further improve the separation and migration rate of the photo-generated charges of the Z-scheme photocatalyst in order to achieve higher photocatalytic activity (Feng et al., 2020; Wang et al., 2020c).
As a common and cheap carbon material, biochar (BC) has the characteristics of containing a carbon matrix, and having high specific surface area, structural defect sites and porous structure. The surface functional groups and condensed aromatic ring structure of BC can accelerate the electron transfer in the photocatalytic process (Mian and Liu, 2018; Xu et al., 2013a). It has attracted widespread attention in its properties for enhancing the activity of photocatalysts (Duan et al., 2017). In addition, black BC has a strong broad-spectrum light absorption capacity (Fu et al., 2016), which is expected to expand the visible light response range of Bi25FeO40 combined with g-C3N4. Previous studies have mostly focused on selecting precious metals, graphene and other materials as conductive channels. In this study, inexpensive and environmentally friendly biomass BC was selected as the conductive channel of the Z-scheme photocatalyst. BC has significant conductivity, which can reduce the electron-hole recombination rate during the photocatalytic process, thereby increasing the oxidation rate of the target compound (O'Connor et al., 2018). According to the research results of Xu et al. and Khanmohammadi et al., the organic carbon content of plant raw material biochar (such as straw) is significantly higher than that of mineral raw material biochar (such as sludge) (Khanmohammadi et al., 2015; Xu et al., 2013b). The higher the carbon content, the better the conductivity (Godlewska et al., 2021). Therefore, in the choice of carbon source, we chose plant-based pomelo peel raw materials to prepare biochar, which is one of the most abundant fruits in the world. The white flocculent layer in pomelo peel contains a lot of cellulose and has a very unique texture and morphological features which is a typical green and renewable material with a wide range of sources (Song et al., 2019).
For single-phase photocatalytic materials, the recombination of photo-generated holes and electrons is still significant, resulting in low photocatalytic degradation efficiency and long-term effects to photocatalyst performance. Therefore, the construction of the Z-scheme structure is of great significance for improving the efficiency of photoelectron-hole separation, thereby promoting e- and h+ to participate in the photocatalytic reaction, and improving the redox capacity and photocatalytic efficiency (Wang et al., 2020a, 2020d). Therefore, in this study, we first load the treated BC onto Bi25FeO40. Then it is covered with flake g-C3N4 on its surface to form a g-C3N4/BC/Bi25FeO40 composite material. BC acts as an intermediate carrier between g-C3N4 and Bi25FeO40 to establish a good transmission channel for light-excited electrons. More importantly, the highly dispersed g-C3N4 and Bi25FeO40 are conducive to exposing more catalytically active sites. The formation of the Z-scheme between g-C3N4 and Bi25FeO40 is promotes the separation of photogenerated electron-hole pairs and possesses strong redox capabilities. Choosing BC as a conductive channel improves the electron transfer and reduces the recombination rate of electrons and holes. This research provides a new strategy for the preparation of Z-scheme photocatalysts and their application in removing organic pollutants.
Section snippets
Materials and reagents
Hydrochloric acid (HCl) and Tetracycline hydrochloride (TCH) were obtained from Aladdin Biochemical Technology (Shanghai, China). Bismuth nitrate (Bi(NO3)3) was obtained from Beilian Fine Chemicals (Tianjin, China). Urea (H2NCONH2),Iron nitrate (Fe(NO3)3), Lithium hydroxide (LiOH), Lithium nitrate (LiNO3),p-Benzoquinone (p-BQ),Ethylenediamine tetraacetic acid (EDTA),Tert-butanol (TBA) and Potassium hydroxide (KOH) were obtained from DaMao chemical reagent (Tianjin, China). All chemicals were of
Characterization of prepared samples
The crystal structure of the prepared material was determined by XRD. As shown in Fig. 1a, the Bragg diffraction peaks of Bi25FeO40 prepared by the microwave method are 24.7°(220), 27.6°(310), 30.3°(222), 32.8°(321), 52.3°(530), 53.9°(600), 55.6°(611) and 61.7°(631), which are consistent with the JCPDS No.46-0416 card (Liu et al., 2018). In Fig. 1b, the main peaks of the g-C3N4sample at the 2θ values of 13.1° and 26.5° correspond to the (001) and (002) crystal planes of g-C3N4, respectively,
Conclusions
We successfully prepared the Z-scheme photocatalyst g-C3N4/BC/Bi25FeO40 by the illumination isoelectric point assisted method, and studied the morphology, optics and photocatalytic mechanism of the catalyst in detail. Within 60 min of light irradiation, the degradation efficiency of the TCH reached 92.2%. The apparent rate constant of degradation of g-C3N4/BC/Bi25FeO40 (0.0341 min−1) is 4.2, 1.4 and 1.27 times the values of TCH degradation for Bi25FeO40, g-C3N4 and g-C3N4/Bi25FeO40. The TOC
Credit author statement
Zhi-Peng Ma: Investigation, Writing- Original draft. Linnan Zhang: Formal analysis, Funding acquisition, Supervision. Xue Ma: Data curation, Formal analysis. Yu-Hang Zhang: Methodology, Writing – review & editing. Fa-Nian Shi; Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no competing interests.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21571132), the young scientific andtechnological talents cultivation project (LQGD2020005) of Departmentof Education of Liaoning Province, China, the key project(ZDZRGD2020033) supported by Shenyang University of Technology and the cultivation project of Shenyang University of Technology, China.
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