High salinity tolerance of Zn-rich CNx in the photocatalytic treatment of wastewater

1. Introduction

With the gradually expandssion of the paper, printing and dyeing, pharmaceutical and pickled food industries, a large and complex amount of high salt organic wastewater was produced [1,2]. As reported, the wastewater produced during the production of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) contained 2,4-dichlorophenol (2,4-DCP) up to 800-1200 mg/L, while the NaCl concentration reached 55.0-65.0 g/L [3]. In view of the highly toxicity, ecologically persistence, accumulation and mobilit, it was harmful to the ecosystem and public health, which needed to be strictly controlled and properly handled in China, the United States and other European countries [3]. However, its high salt content characteristics had a significant negative impact on aerobic and anaerobic bacteria to limi the application of biological treatment processes [4].

At present, a number of physicochemical methods were considered to have an advantage for the treatment of this type of wastewater, such as fenton, membrane, evaporative crystallization, electrocatalysis and photocatalysis, etc. It has been demonstrated that the decomposition of H₂O₂ catalyzed by Fe²⁺ produced ·OH (E⁰=2.70 V_NHE) with strong oxidizing ability to indiscriminately attack organic matter, and the dissolved oxygen gained electrons under catalytic conditions to produce ·O₂^- (E⁰=1.06 V_NHE) to selectively attack electron-rich groups [5,6]. Physical processing enabled efficient phase transfer of organic matter. Efficient removal of organic matter without being affected by salinity was the goal pursued. However, the material had a high surface energy, which tended to induce salt precipitation and salt crystallization in the indication. This phenomenon was observed in applications such as membrane filtration and electrocatalysis [7,8]. The Fenton method and evaporation crystallization required a large amount of reagents and additional energy. Compared with the above, photocatalytic technology could continuously and efficiently decompose organic matter in water under the condition of lower energy consumption, which demonstrates certain advantages in salt-containing wastewater treatment.

However, there were some disadvantages of photocatalytic process for treating high salt wastewater. Specifically, the active sites on the catalyst surface was used to enhance the capture and oxidative degradation of organic matter in water on the one hand, and the capture of active species in water to enhance the generation of free radicals on the other hand [9]. Under high salt stress, salt occupied the surface active sites and prevented subsequent adsorption-catalyzed degradation of organic matter. This was a typical catalyst deactivation phenomenon [10]. Reducing the adsorptive active sites while enhancing the generation of free radicals was the focus of current catalyst research and development.

As previously reported, we loaded metals onto carbon nitride by thermal condensation method to prepare stable composite catalysts with high catalytic activity for 2,4-DCP removal from highly chlorine environment. In this study, effects of other parameters such as catalyst, salinity, pH value, 2,4-DCP concentration on the degradation process were evaluated and the degradation pathway of 2,4-DCP was deduced. Further, the reactive oxygen species evolution and the 2,4-DCP degradation pathways were proposed based on quenching experiment and product analysis. In conclusion, this study revealed the possible mechanism of high-salt wastewater treatment using photocatalysis.

2. Materials and Method

2.1. CNs Synthesis

The CNs were synthesized as follows. First, mixed 6.82 g ZnCl₂ and 25.20 g melamine at the mol ratio of 4:1 in 300 mL 0.35 wt.% HCl solution at 90-95 ^oC and then agitated for 30 min. Second, freeze-dried the mixture for 2 days, and then calcinated at 400-500 ^oC for 4h, to produce powdered material. Third, washed the materials by water and ethanol several times, and then dried overnight. A pink powder was generated and named as Zn-CN_x. In the control experiment, the added Zn salt was replaced by Co, Ag, Mo, and Bi salts, respectively, according to the above steps. The corresponding products were named as Co-CN_x, Mo-CN_x, Ag-CN_x and Bi-CN_x.

Figure 1. Flow chart for preparing metallic CNs particles.

Figure 1. Flow chart for preparing metallic CNs particles.

2.2. Photocatalytic Experiment

The prepared CN_x was used for removing 2,4-DCP from salt wastewater. First, mixed 20 mg CNx into 200 mL wastewater containing 100 mg/L 2,4-DCP. Second, agitated the wastewater at 500 rpm for 30 min. Third, opened the mercury lamp to start the photocatalytic experiment, and then took 1 mL wastewater samples at a given interval, following by quenching with methanol. The rest 2,4-DCP concentration was determined by HPLC. Control experiments were performed by varying the salt dosage, the initial pH and the 2,4-DCP concentrations in the range of 0.58‰-11.70‰, pH 3.0-11.0 and 20-200 mg/L, respectively. The experiment was repeated three times, and the average data was reported.

2.3. Characterization

The prepared CN_x were characterized by SEM, XRD, FTIR, UV-vis, PL and XPS, separately. The generated intermediates in the degradation experiment were analyzed by using UPLC.

3. Results and Discussion

3.1. Metallic CNs Characterization

Six CN_x were prepared in the experiment and showed typical peaks of CN_x at 2θ= 10^o and 80^o (Figure 2a). However, sharp peaks of CoO, elemental Ag, MoO₃, elemental Bi and ZnO were found on the corresponding CN_x. This revealed the formation of metallic CN_x. After the treatment for 120 min, nearly 75.60% 2,4-DCP were removed in the presence of Zn-rich CN_x, higher than these with the addition of pure CN_x, and Co, Ag, Mo, and Bi doped CN_x (Figure 2b). Such degradation date fitted good with the pseudo-first kinetic model with the regression coefficients of > 0.99. However, the corresponding degradation rates by adding Co, Ag, Mo, and Bi-doped CN_x were in the range of 0.0044-0.0051 min^-1, nearly half of that with the addition of Zn-rich CN_x. This demonstrated that the doping of Zn apparently enhanced the photocatalytic performance of CN_x.

3.2. Photocatalytic Conditions Optimization

The Zn-rich CN_x showed superior performance in the degradation of 2,4-DCP in salt wastewater. Figure 3a showed that the degradation efficiency of 2,4-DCP slightly varied in the salt dosage of 0.58‰-11.7‰, demonstrating that Zn-rich CN_x showed good stability in the high-salt wastewater. However, with the decrease of pH from 9 to 3, the degradation efficiency also dropped from 84.66 to 70.08%. But when the initial pH was increased from 9 to 11, the degradation efficiency was considerably increased to 99.41% (Figure 3b). After reaction, the final pH was also equilibrized in the range of 6.01-6.59 (Figure 3c). Without the addition of Zn-rich CN_x, the equilibrium pH was steadily dropped. The related mechanism will be discussed in Section 3.4.

As the increase of the initial 2,4-DCP concentration in the range of 20.00-200.00 mg/L, the corresponding degradation efficiency was also dropped from 100.00 to 51.90% (Figure 3d). The high removal of 103.81 mg/L 2,4-DCP occurred with the addition of 200 mg/L 2,4-DCP (Figure 3e). Without the photocatalyst, the direct adsorption of 2,4-DCP onto the Zn-rich CN_x were 2.09, 9.00, 12.19 and 13.00 with the addition of 20.00, 50.00, 100.00 and 200.00 mg/L 2,4-DCP (Figure 3f), in accordance with the removal of 2.09%, 1.80%, 1.19% and 0.68% 2,4-DCP from the wastewater. This demonstrated that the photocatalytic degradation played a key role in the removal of 2,4-DCP from salt wastewater.

3.3. Zn-Rich CNs Materials Characterization

The Zn-rich CN_x was aggregated particles (Figure 4a), and kept sharp peaks of CN_x at the wavenumbers of 1451, 1400 and 1316 cm^-1 (Figure 4b). However, the peaks at 1100, and 733 cm^-1 shifted to low-value and few peaks at 3080, 1240 and 805 cm^-1 disappeared, belong to the conjunction of ZnO onto CN_x via −NH and −OH bonds. The XPS spectra showed two peaks of Zn 2p at 1022.0 and 1045.1 eV (Figure 3c). For C 1s, the peak at 288.4 and 284.8 eV became intensified (Figure 3d), demonstrating that more −COOH and C−C sites were exposed with the addition of Zn. According, the N1s peaks of C−N=C bond at 398.7 eV also raised (Figure 3e), and the peak of N−(C)₃ bond at 400.4 eV disappeared. However, for O1s, a new peak at 533.4 eV belonged to adsorbed O₂ and/or water. Such results demonstrated the doping of Zn into CN_x and the corresponding variation of functional sites.

3.4. Photocatalytic Mechanism Analysis

In the photocatalysis system, the photon was adsorbed by CN_x, and then converted into a pair of electron-hole. The generated hole only oxidized the adsorbed organics on CN_x surface. But the electron was transited to CN_x surface and then captured by OH^- and oxygen groups, with the formation/releasing of free radicals into the wastewater. This started the oxidation of organics in wastewater. Given that the direct adsorption of 2,4-DCP on CN_x was low (Figure 3f), the generated free radicals played positive roles in the degradation of 2,4-DCP in the salt wastewater.

The doping of Zn into CN_x showed two advantages. First, the photo absorbance of CN_x increased and strengthened. The doped ZnO restructured the surface sites of CN_x, and then more ZnO and oxygen vacancy sites were exposed to surface. Accordingly, the band-gap energy raised from 2.74 eV to 3.35 eV (Figure 5a), and the UV-vis absorbance in the range of 300-360nm also became intensified (Figure 5b), demonstrating that more photons were adsorbed by Zn-rich CN_x. Second, a heterojunction structure was formed in Zn-rich CN_x and steadily increased the production of free radicals. For example, an electron was generated on the conduction band of ZnO, but moved to the valence band of CN_x in the heterojunction structure, not to that of ZnO (Figure 5c). This extended the transportation distance of photo-induced electron/hole pairs. Thereby, more photo-induced electron transported to the surface and then involved in the production of free radicals. No heterojunction structure was formed in the doping of Co, Ag, Mo and Bi into CN_x, and thereby the corresponding removal efficiencies of 2,4-DCP were low.

2,4-DCP was targeted to determine the photocatalytic performance of CN_x. It has a benzene ring with two chlorine and one hydrogen group, and was also adsorbed onto the Zn-rich CN_x. Such adsorption performance was mainly affected by the experimental conditions, e.g., the salt condition, the initial pH, and the initial concentration. For instance, at high salt condition, the free anions and cations were also adsorbed onto the CN_x surface, in comparison with 2,4-DCP. This led to the decrease of the adsorption capacity of 2,4-DCP. Similarly, the initial pH also varied the structure and the pH_zpc of 2,4-DCP. However, the removal efficiency of 2,4-DCP onto the Zn-rich CN_x via adsorption, was relatively low, comparing with that of photocatalytic degradation. This demonstrated that the oxidation of free radicals predominated the removal of 2,4-DCP.

The quench experiment showed that only two free radicals of ·O₂^- and ·OH were generated (Figure 5d). Especially, the radical ·O₂^- occupied by 88% of 2,4-DCP removal, whilst the radical ·OH only contributed the removal of 7% 2,4-DCP (Figure 5e). The radical ·OH can be converted to the radical Clž, to continue the oxidation of 2,4-DCP. Thereby, the photocatalytic degradation of 2,4-DCP by the Zn-rich CN_x was stable in high salt wastewater. With the raise of wastewater pH, more OH^- were involved in the production of free radicals, and then steadily decreased the equilibrium pH.

The degradation of 2,4-DCP mainly included two ways. One of which is the direct decomposition by UVA, which did second role in the 2,4-DCP removal. The adjacent position of hydroxyl group on 2,4-DCP, was firstly activated by UVA, to form two intermediates, 3,5-Dichloro-2-(2,4-dichlorophenoxy)phenol and 2,4-Dichlorophenoxycarboxylic acid. The intermediates were unstable and then converted to P-chlorophenol and o-chlorophenol , and p-phenol and o-phenol, respectively. Under such condition, the benzene ring did not open, instead, was combined in the form of 3,5-Dichloro-2-(2,4-dichlorophenoxy)phenol. The following decomposition commonly experienced several hours. The other is the photocatalytic degradation by UVA in the presence of Zn-rich CN_x, predominating the 2,4-DCP removal. With the treatment, the adequate of free radicals, e.g., ·O₂^- and ·OH, were generated and attacked randomly the chlorine and hydroxyl groups. This simultaneously started the dechloridation and the ring opening of benzene ring, with the generation of abundant byproducts, e.g., maleic/fumaric acids, trihydroxyethylene, propanoic acid, acetic acid and oxalic acid. As the generation of free radicals continued, the rapid decomposition of 2,4-DCP and its byproducts also proceeded, to release free chloridion and carbonate into wastewater.

Figure 6. Derivation of 2,4-DCP degradation pathways in photolysis (UVA) and photocatalysis (Zn-CN/UVA) systems.

Figure 6. Derivation of 2,4-DCP degradation pathways in photolysis (UVA) and photocatalysis (Zn-CN/UVA) systems.

3.5. Prospectives

The Zn-rich CN_x showed desirable photocatalytic performance in the degradation of organics from high-salt wastewater. In high-salt wastewater, the salt often occupied the surface sites of catalyst, to stop the adsorption-oxidization of organics, resulting in the catalyst deactivation. To reactivate catalyst, it is washed several times with deionized water. In this study, the Zn-rich CN_x showed a low adsorption capacity of 2,4-DCP, revealing that it had a few surface adsorption sites. Thereby, it also kept good photocatalytic performance in high-salt wastewater.

The Zn-rich CN_x had a broad absorbance spectrum, and exhibited ability to adsorb more photon with different energy in such range. In addition, it had a ZnO-CN_x heterojunction, to extend the migration distance of the light-induced electron-hole pair. This promoted the separation of light-induced electron-hole pair, resulting in the generation of adequate free radicals for oxidation/decomposition of organics. Thereby, the Zn-rich CNx showed high photocatalytic degradation rate of 2,4-DCP, in comparison with the other synthesized catalysts, e.g., Pb@BC-20%TiO₂, Cu/ZnS, ZnO, CuPc/g-C₃N₄ and Au/TiO₂/PO₄ (Table 1). In the photocatalytic degradation, the free radicals unselectively attacked 2,4-DCP and other organics, including rhodamine B, atrazine and aniline pollutants . Thus, the Zn-rich CN_x posed bright application prospect in the wastewater treatment in paper, printing and dyeing, pharmaceutical and pickled food industries [11,12].

Even though the Zn-rich CN_x had many advantages in high-salt wastewater treatment, some investigation also should be taken in future. First, the stability of Zn-rich CN_x should be optimized, including the leaching of Zn and the reusability in wastewater treatment. Second, the synthesis of Zn-rich CNx was tedious and energy-consuming. New synthesis method should be developed at mild condition to simplify the synthesis process and save energy. Third, costly reagents were used in the synthesis. The liquid reagents should be recycled, whilst the metallic chemical can be extracted from spent waste.

4. Conclusions

In summation, the process of high salinity tolerance of Zn-rich CNx in the photocatalytic treatment of wastewater had been studied and further corroborated. The results of the study showed that the degradation and Kobs of 2,4-DCP removal increased slightly with increasing salt content. Quenching experiment shown tha ·O₂^- and ·OH played a important role. Combined with product analysis, the 2,4-DCP dehalogenation and ring-opening pathway was proposed. Moreover, the Zn@CN photocatalysis system also exhibited stability under initial pH interferences, 2,4-DCP concentration change. Overall, this study introduced a promising avenue for high salt organic wastewater treament in photocatalysis systems.