In-situ fabrication of g-C3N4/MIL-68(In)-NH2 heterojunction composites with enhanced visible-light photocatalytic activity for degradation of ibuprofen
Introduction
In recent years, pharmaceutical drugs such as anti-inflammatories, antibiotics, and antidepressants, have been frequently detected in surface/underground water and wastewater with concentrations in the range of µg–ng/L [1], [2], [3]. These compounds have been categorized as emerging pollutants due to their high consumption, continuous release, biorefractory nature, potential hazard, and widespread distribution in environment [4], [5], [6], [7]. Ibuprofen (IBP), a widely used nonsteroidal anti-inflammatory drug with considerable production of a few kilotons per year in the world, has been recognized as a typical emerging pollutant [8], [9]. Although its toxicity and concentration are relatively low, IBP exhibits excellent stability, non-photolysis, and non-biodegradation, causing accumulation in aquatic environment and organism and thus poses severe threat to human health [10], [11], [12]. In addition, previous studies showed that conventional processes for wastewater plants were not efficient enough to remove IBP completely [2], [13], [14], [15]. Therefore, it is essential to develop new alternative technologies to solve the anti-inflammatory drug contamination problems in water.
Visible-light-driven photocatalysis, an environmentally friendly and highly effective method for the degradation of persistent organic pollutants, has been evaluated to treat IBP contaminated wastewater. It can convert IBP to biodegradable compounds, CO2, H2O, and inorganic ions by utilizing abundant solar energy [16], [17], [18], [19]. Extensive studies have been conducted to synthesize and optimize photocatalysts for the complete degradation of IBP [20], [21], [22], [23], [24]. However, the applications of these photocatalysts are rather limited by low solar energy conversion efficiency, easy photocorrosion, easy agglomeration, and unstable state in water. Consequently, it is necessary to explore novel photocatalysts without these shortcomings for IBP degradation.
Metal–organic frameworks (MOFs), constructed with organic ligands and metal-oxo clusters, have gained much attention owing to their widespread application prospect. It has been proved that MOFs can serve as light-sensitive semiconductors under solar illumination since the pioneering study on the photoactivity of MOF-5 [25]. Moreover, as a porous inorganic–organic hybrid material with infinite three-dimensional networks, MOFs possess stable crystalline structures, high pore volume, and large specific surface area, beneficial for overcoming the disadvantages of conventional photocatalysts, such as poor adsorption performance, low stability, and easy agglomeration [26], [27], [28]. Among all the MOFs synthesized in previous studies, MIL-68(In)-NH2, first reported by Farrusseng’s group, is considered to be a novel photocatalyst with excellent visible-light response [29]. According to Liang et al., MIL-68(In)-NH2 exhibited high photocatalytic activity for the reduction of Cr(VI) [30]. However, its application in the photocatalytic degradation of persistent organic pollutants is still limited by two key factors: (1) visible-light absorption edges and (2) separation efficiency of photogenerated electrons and holes. One of the most effective methods to overcome these limitations is to construct a heterojunction system by incorporating MOFs and semiconductors with well-matched band structures [31], [32], [33].
Thanks to appropriate energy levels and band structures, g-C3N4 and MOFs have been used to construct highly efficient heterojunction photocatalysts for wastewater treatment. Wang et al. [34] fabricated a novel g-C3N4/MIL-125(Ti) mesoporous photocatalyst using solvothermal method. Owing to the introduction of g-C3N4, the visible-light absorption range of g-C3N4/MIL-125(Ti) composites was significantly extended, leading to improved performance for Rhodamine B degradation. Lei et al. [35] successfully synthesized CNNSs/MIL-88B(Fe) hybrid nanocomposites by doping g-C3N4 nanosheets on the surface of MIL-88B(Fe). A high photocatalytic activity was achieved for the degradation of MB and reduction of Cr (VI). This can be attributed to the formed heterojunction. Yuan et al. [36] also reported an attractive heterojunction photocatalyst composed of g-C3N4 nanosheets and ZIF-8, which both enhanced the transfer rate of photogenerated carriers and the stability of materials. It can be concluded that doping g-C3N4 was a great way to improve the photocatalytic activity of parent materials, which successfully solved the problem that current photocatalyst materials were not effective enough to utilize solar energy for the degradation of contaminants. Meanwhile, high stability and efficient removal of pollutants with few catalysts, short operation time and low energy consumption were achieved, beneficial for practical application. Moreover, the complementarity and advantages of g-C3N4 and MOF were fully utilized. Combining highly photocatalytic activity with excellent adsorption capacity can greatly optimize the properties of catalysts and broaden their application fields [37]. Motivated by above studies, in this work, the novel visible-light-driven g-C3N4/MIL-68(In)-NH2 heterostructure composites with highly photocatalytic activity and adsorption capacity were developed. To the best of our knowledge, there have been no reports focus on the construction of type II heterojunction between g-C3N4 and MIL-68(In)-NH2, and this is the first study to report g-C3N4/MOF materials applied for the degradation of pharmaceutical and personal care products (PPCPs).
Herein, several g-C3N4/MIL-68(In)-NH2 composites were fabricated using a typical in-situ solvothermal method assisted with ultrasonication. Various characterization methods have been used to study the surface morphology, crystalline structure, chemical components, and electronic and optical properties of g-C3N4/MIL-68(In)-NH2. Furthermore, the g-C3N4/MIL-68(In)-NH2 was applied for the degradation of IBP under visible-light irradiation, and 93% removal rate was achieved in 120 min with few catalysts, low energy consumption, and no additional oxidants, which showed significant advantages comparable to previous reports [17], [38], [39]. Moreover, the effects of pH, IBP concentration and catalyst dosages on the photodegradation performance of composites were also discussed in detail. Finally, the stability and reusability of photocatalysts, possible photocatalytic mechanism, and degradation pathways of IBP were also explored to better understand the photocatalytic system. It can provide some references for practical application of materials and complete degradation of IBP.
Section snippets
Materials
Indium nitrate hydrate (In(NO3)3·xH2O, 99.99%, AR), 2-aminoterephthalic acid (H2BDC-NH2, 98.0%, AR), melamine (C3H6N6, 99.0%, AR), N′N-dimethylformamide (DMF, 99.5%, AR), ibuprofen (C13H18O2, 98.0%, AR, CAS: 15687-27-1), and sodium sulfate (Na2SO4, 99%, AR) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH, 96.0%, AR), hydrochloric acid (HCl, 36%, AR), and ethylenediamine tetra acetic acid disodium (C10H14N2Na2O8·2H2O, 98%, AR) were supplied
X-ray diffraction (XRD) patterns
Fig. 1 shows the XRD patterns of as-synthesized samples. The diffraction peaks of MIL-68(In)-NH2 are consistent with those reported previously, indicating high crystallinity of materials [40]. The XRD pattern of g-C3N4 shows two typical diffraction peaks at 13.1° and 27.5°, corresponding to the interlayer stacking of g-C3N4 and stacking of conjugated aromatic system, respectively [41]. More importantly, the main characteristic peaks of parental MIL-68(In)-NH2 appeared in all the XRD patterns of
Conclusions
In summary, g-C3N4/MIL-68(In)-NH2 heterostructure composites were synthesized for the first time and applied to the photodegradation of IBP under visible-light irradiation. With the construction of the heterojunction at the interface between g-C3N4 and MIL-68(In)-NH2, significant increase in the photocatalytic activity of composites was achieved. Under the optimal experimental conditions, g-C3N4/MIL-68(In)-NH2-3 showed the highest degradation ratio (93%) and TOC removal efficiency (70%) towards
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was financed by the Social Science and Technology Development Project of Dongguan (No. 20185071631595), the NSF of China (No. 21976060), Fundamental Research Funds for the Central Universities (D2192900), and Applied Science and Development Project of Guangdong Province (No. 2016B020240005).
References (75)
- et al.
The fate of pharmaceuticals and personal care products (PPCPs), endocrine disrupting contaminants (EDCs), metabolites and illicit drugs in a WWTW and environmental waters
Chemosphere
(2017) - et al.
Occurrences and removal of pharmaceuticals and personal care products (PPCPs) in drinking water and water/sewage treatment plants: a review
Sci. Total Environ.
(2017) - et al.
Pharmaceuticals and personal care products (PPCPs): a review on environmental contamination in China
Environ. Int.
(2013) - et al.
A review on emerging contaminants in wastewaters and the environment: current knowledge, understudied areas and recommendations for future monitoring
Water Res.
(2015) - et al.
Pharmaceuticals as emerging contaminants and their removal from water. A review
Chemosphere
(2013) - et al.
Occurrence, distribution, and seasonality of emerging contaminants in urban watersheds
Chemosphere
(2018) - et al.
Occurrence and fate of emerging contaminants in water environment: a review, Groundwater for
Sustainable Dev.
(2018) - et al.
Toxicological interactions of ibuprofen and triclosan on biological activity of activated sludge
J. Hazard. Mater.
(2017) - et al.
Eco-pharmacovigilance of non-steroidal anti-inflammatory drugs: necessity and opportunities
Chemosphere
(2017) - et al.
Toxicity, degradation and metabolic fate of ibuprofen on freshwater diatom Navicula sp
J. Hazard. Mater.
(2017)