Graphitic carbon nitride (CN) was a promising candidate for efficient environmental remediation in the advanced oxidation processes (AOPs). However, whether CN itself had some potential environmental risks, such as affecting the production of disinfection byproducts (DBPs) was still unknown. This study investigated the formation potential of DBPs in the presence of CN. The experimental data revealed that CN had a high potential to form DBPs, and dichloroacetonitrile (DCAN) was the most produced species during the chlorination and chloramination processes. Moreover, the effects of chlorine time, chlorine dosage, pH, and CN dosage during the chlorination process were evaluated to understand the formation pattern of DBPs. The possible mechanism of DBPs formation was deduced by analyzing the results of FTIR, Raman, and XPS before and after chlorination. Finally, the DBPs formation potential and cytotoxicity of the CN leaching solution were investigated, indicating CN could leach the precursors of DBPs and that the potential toxicity of the leaching solution increased with the extension of CN immersion time. In general, this research adds an understanding of the DBP formation of CN in water treatment systems and sheds light on CN's environmental potential risks.
g-C3N4(CN) has a high potential to form DBPs and dichloroacetonitrile (DCAN) was the dominant species.
Chorine dosage, pH, and CN dosage could significantly affect DBPs formation.
Chlorine attacked the N-containing aromatic ring and the surface amino group of CN to produce DBPs.
CN could leach the precursors of DBPs and DCAN was the major contributor to the formed DBPs cytotoxicity.
Controlling the disinfection byproducts (DBPs) formation has always been a difficult task due to their teratogenic, carcinogenic, and mutagenic effects (Wongrueng et al. 2019). Many investigations have demonstrated that the reaction between NOM and chlorine could promote the formation of DBPs (Chen et al. 2017; Li et al. 2021a, 2021b). However, recent research indicated that residual carbon and nitrogen-containing materials were potential precursors of DBPs. Trihalomethanes (THMs) and halo acetic acids (HAAs) could be produced when graphene oxide (GO), activated carbon (AC), biochar, or carbon nanotubes (CNTs) reacted with chlorine or chloramine (Liu et al. 2018a, 2018b; Zhang et al. 2019; Huang et al. 2021). Since g-C3N4 has been unanimously recognized by researchers in water pollution treatment, it is likely to be applied on a large scale in the foreseeable future (Wang et al. 2017; Han et al. 2020). Whether g-C3N4 itself has some byproduct formation potential risks when exposed to chlorine is still unknown. It is imperative to assess the DBPs formation potential of g-C3N4.
Thus, the purpose of this work was to investigate the effects of g-C3N4 on the formation of DBPs. Trichloromethane (TCM), dichloroacetonitrile (DCAN), and dichloroacetic acid (DCAA) were detected to assess the DBPs formation potential of g-C3N4. Cl2 dosage, solution pH, and g-C3N4 dosage were detected to evaluate the factors affecting DBPs formation. The properties of g-C3N4 before and after chlorination were determined to find out the possible mechanism of g-C3N4-formed DBPs. Furthermore, the formation of DBPs and corresponding theoretical cytotoxicity of the g-C3N4 leaching solution were investigated. These researches were significant both theoretically and practically in assessing the possible environmental risks of g-C3N4.
MATERIALS AND METHODS
Reagents and materials
The DBP standards utilized in this study were derived from Anpu Experimental Technology Co. Ltd (Shanghai, China). Anhydrous sodium sulfate (Na2SO4) was obtained from Sinopharm Chemical Reagent Co. Ltd (Tianjin, China). Methyl tert-butyl ether (MTBE) was purchased from Fisher Chemical (China). Sodium hypochlorite stock solution and urea were purchased from Maclin Biochemical Technology Co. Ltd (Shanghai, China). To remove residual chlorine, sodium sulfite (Tianjin Guangfu Fine Chemical Research Institute, China) was utilized.
The g-C3N4 was synthesized from urea. Typically, 15 g of urea was taken in alumina crucibles with a cover and calcined at 550 °C with a heating rate of 5 °C/min for 4 h and allowed to cool naturally to room temperature. The light yellow powders were collected by centrifugation and washed with ultrapure water several times. The sample was named CN.
Firstly, 250 mL ultrapure water mixed with 20 mg/L CN samples was added in a conical flask and the pH was adjusted to 7 with phosphoric acid buffer. The concentration of NaClO (as Cl2) was identical at 1, 4, 10, and 20 mg/L. All samples were performed in the dark for 3 days at room temperature (25 ± 1 °C). After 72 h of chlorination, water samples were filtered by the 0.45 μm membrane. The chlorinated powders were collected and named as CN-Cl. The residual chlorine was quenched by sodium sulfite and the quenched sample was immediately extracted with MTBE. Finally, 1 mL upper layer of MtBE was taken out for gas chromatography (GC) detection. For comparison, the chloramination of CN was investigated. The monochloramine (NH2Cl) solutions were freshly generated by adding NaClO solution gently into a stirred NH4Cl solution with the Cl: N mass ratio of 4:1.
TCM, DCAN, and DCAA were measured according to EPA 551.1 and 552.3 using GC (Agilent 6890N, Santa Clara, CA, USA) equipped with an electron capture detector (Agilent Technologies, Santa Clara, CA, USA). The column used for detection was an HP-5 fused silica capillary column (30 mm × 0.25 mm I.D. with a film thickness of 0.25 mm). The pH of the water samples was measured through a pH meter (PHS-3G, Shanghai Yi Electrical Scientific Instrument Co. Ltd). Residual chlorine was investigated by using the N, N-diethyl-p-phenylenediamine (DPD) method. The dissolved organic carbon (DOC) and the total dissolved nitrogen (TDN) were measured by a TOC/TN analyzer (TOC-5000A; Shimadzu). The concentration of dissolved organic nitrogen (DON) was determined by subtracting inorganic nitrogen (NO3−-N, NO2−-N, NH4+-N) from TDN.
Characterization of CN
The morphologic changes of CN before and after chlorination were seen using a transmission electron microscope (HRTEM, JEM-1200EX). The surface area and pore size of CN were measured using a Brunauer–Emmett–Teller (BET) surface analyser (Quantachrome NOVA1000, USA). The functional groups were determined by using Fourier transform infrared spectroscopy analysis (FTIR, Bruker Tensor 27, Germany) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Fisher Scientific). The variations in thickness were measured by atomic force microscopy (AFM, Bruker Dimension Icon, USA). The Raman spectra of samples were tested with a Raman spectrometer (SR-500I-A, 532 nm).
Analysis of the potential toxic risk of DBPs caused by CN
RESULTS AND DISCUSSION
DBPs formation with CN
Factors affecting the formation of DBPs from CN
Effect of chlorine dosage
Effect of pH
The effect of solution pH on TCM, DCAN, and DCAA formation with CN is depicted in Figure 3(b). The concentration of DCAN and DCAA increased as pH raised from 5 to 7, but decreased obviously when pH reached to 9, the total DBPs followed the same pattern. These results agreed with previous research (Ye et al. 2020), different pHs affected the morphology of HOCl, when pH < 7.5, HOCl was the major species. When pH reached 7.6, OCl− turned into the key species, which had less disinfectant potential, therefore less DCAN and DCAA were formed (Hu et al. 2015).
In addition, the excess hydroxide could assist the hydrolysis reactions of DCAN to produce stable THMs, which would lead to a higher concentration of TCM. Although OCl− had a high proportion under alkaline conditions, HOCl inevitably existed in the solution (Acero et al. 2005). The oxidative decomposition capacity of CN to produce nitrogen organics was greater than the alkaline catalytic hydrolysis ability of DCAN (Liu et al. 2018a, 2018b), thus DCAN was still the dominant species at pH = 9.
Effect of CN dosage
The results described above proved that CN was an important precursor of DBPs. Thus, the dosage of CN was a key factor affecting the formation of DBPs in the chlorination process. As shown in Figure 3(c), the increasing trend of total DBPs slowed down as CN dosage enhanced from 5 to 20 mg/L, which might be ascribed to the depletion of available chlorine. The effect of CN dosage on TCM and DCAA formation was not obvious, the amount of TCM and DCAA was 4.37 and 2.61 μg /L at the lowest CN dosage, respectively. When CN dosage was increased to the maximum dosage, the concentration of TCM and DCAA was 5.05 and 4.25 μg/L. However, the concentration of DCAN enhanced nearly five times when CN dosage changed from 5 to 40 mg/L. The skeleton structure of CN was rich in nitrogen and with the increase of CN dosage, more nitrogen might be introduced to promote the formation of DCAN (Wang & Hu 2018; Cheng et al. 2020; Yan et al. 2022). These results indicated that CN had great potential for forming nitrogenous DBPs and fewer active precursor sites for the formation of carbonaceous DBPs.
Possible mechanism of CN-formed DBPs
XPS was obtained to further investigate the CN surface chemical state variation after the chlorination process. As shown in Table 1, the surface oxygen and carbon properties were increased while the nitrogen content substantially decreased. In addition, similar to previous studies (Zhang et al. 2019; Huang et al. 2021), the surface chlorine content slightly increased after the chlorination process, which further proved the reaction between chlorine and carbon and nitrogen. Figure 5(c) showed the fully scanned spectra of CN and CN-Cl, the samples presented the three main peaks of O, N, and C. Due to the minimal chlorine content, no significant characteristic peaks were observed. Figures 5(d)–(f) showed the high-resolution C1s, N1s, and O1s spectra of CN and CN-Cl samples. The C1s peaks (Figure 5(d)) at 284.8, 285.6, 287.6, and 288.2 eV were attributed to C–C, N–C = N/C–O, C = O, and C–(N)3. The peak corresponding to C–(N)3 reduced, while the peak corresponding to C = O slightly improved in the CN-Cl sample. The N1s XPS spectra in Figure 5(e) were comprised of three peaks at 398.1, 398.8, and 400.4 eV, corresponding to CN = C, N–(C)3 and the amino groups (N–H) at CN terminating edges. The peaks ascribed to N–(C)3 and amino groups were significantly weakened in the CN-Cl sample. The O1s peaks (Figure 5(f)) at 531.6, 532.3, and 533.5 eV ascribed to C = O, surface H2O, and C–O, respectively (Zhou et al. 2023; Xie et al. 2018; Chen et al. 2021). The C = O peak raised observably while the C–O peak decreased after chlorination, indicating oxidation occurred on the surface of CN and formed additional O-containing functional groups, such as ketone, carboxyl, phenolic, alcohol, and ether.
|Sample .||C (%) .||N (%) .||O (%) .||Cl (%) .|
|Sample .||C (%) .||N (%) .||O (%) .||Cl (%) .|
These results suggested that chlorination of CN-produced DBPs might be via two mechanisms: (1) chlorine could destroy the triazine rings of CN by attacking C–(N)3 and N–(C)3 bonds, which promoted the formation of chlorinated organic compounds and (2) chlorine might replace hydrogen of the amino groups on CN terminating edges to form chlorinated amino acids, which resulted in the most DCAN being formed.
DBPs formation and cytotoxicity of CN leaching solution
In this study, the DBPs formation potential of CN was analysed. TCM, DCAN, and DCAN could be produced by chlorination or chloramination of CN while chloramination could considerably lower the amount of DBPs. When chlorination time exceeded 6 h, the concentration of DBPs increased sharply and DCAN was the dominant species. Chlorine dosage, pH, and CN dosage could significantly affect the formation of the total DBPs and the effects on DCAN were the most significant. The destruction of C–(N)3, N–(C)3 bonds, and amino groups of CN by chlorine resulted in the formation of DBPs. Meanwhile, CN was able to leach organics in an aqueous solution, which was an important DBPs precursor. The released organics had a high activity with chlorine to form DBPs, particularly DCAN. The potential toxicity risks caused by leaching solutions were raised due to the increased immersion time of CN. These findings provided a reference for the potential environmental risks of CN; special attention should be paid when CN is exposed to chlorine. Reducing chlorine contacting time, controlling CN dosage, changing pH, reducing immersion time of CN, etc., were considered alternative options to reduce the formation potential of CN-formed DBPs.
This work was financially supported by the National Natural Science Foundation of China (Nos 51878357, 52170002, 51741807).
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.
All authors contributed to the study's conception and design. L.N. conceptualized the system, investigated the study, wrote the original draft, reviewed and edited the file. J.H. developed the methodology, wrote the review, and edited the file. J.M. and H.W. supervised the study, wrote the review, and edited the file. S.L. developed the methodology, characterized the materials, wrote a review, and edited the file. J.L. conceptualized the system, supervised the study, managed project administration, acquired funds, wrote a review, and edited the file.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.