Photocatalytic oxidation of ammonia in wastewater has been abundantly investigated in lab − scale, but there are still many issues to be solved towards practical application. Herein, we have immobilized Cu/TiO2 photocatalyst on different solid substrates in order to practically utilize and recycle the photocatalyst during wastewater treatment, on the basis of exploring the effects of different influencing factors such as pH, temperature and salinity on the photocatalytic oxidation of ammonia in this work. The performance of Cu/TiO2 films was evaluated by circulated treatment of different types of wastewater including high salinity ammonia wastewater, copper − ammonia wastewater and liquid − ammonia mercerization wastewater. The characters of wastewater matrices significantly influence the performance for ammonia oxidation. Different from the slurry test of photocatalyst power that operated in a closed reactor, it is importantly found that oxygen in air plays significant role in photocatalytic oxidation of ammonia into dinitrogen in the aerobic oxidation process, when the Cu/TiO2 films were employed. The possible oxidation mechanism has been proposed to elucidate the ammonia oxidation process.
Research Article
Photocatalytic Treatment of Different Types of High−Concentration Ammonia Wastewater by Cu/TiO2 Film and its Mechanism Study
https://doi.org/10.21203/rs.3.rs-688668/v1
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Cu/TiO2 films
photocatalytic oxidation
high concentration ammonia removal
N2 formation
wastewater treatment
There are a variety of methods to remove ammonia from contaminated wastewater, including ion exchange, breakpoint chlorination, adsorption, biological treatment, chemical oxidation, membrane separation, and advanced oxidation processes (Charrois and Hrudey 2007; Song et al. 2019; Wang et al. 2014; Zeng et al. 2019). Generally, sewage plants use cost − effective biological methods to remove ammonia nitrogen in sewage through nitrification and denitrification. In this microbial − based process, NH4+/NH3 is converted to NO2− and (or) NO3− under aerobic condition, and then reduced to N2 under anoxic condition (Fei et al. 2020). However, some wastewater with high − concentration ammonia is not suitable for biochemical treatment due to the limited microbial activities under specific condition. For example, high salinity ammonia nitrogen wastewater (NaCl and Na2SO4) will inhibit the microbial activity of the biochemical system and reduce the abundance and diversity of microbial communities, especially high concentrations of Cl− will reduce the biodegradation efficiency of ammonia nitrogen (Chen et al. 2018; Feng et al. 2020; Hong et al. 2013; Li et al. 2020; She et al. 2016). Copper − ammonia complex polluted wastewater not only has a high − concentration ammonia and high pH, but also contains copper ammonia complex (Cu(NH3)42+), which are very stable and can poison microorganisms, so the copper ions in water need to be removed before subsequent biochemical treatment (Chai et al. 2017; Dai et al. 2013; Peng et al. 2017; Yang and Kocherginsky 2007). The high pH, high − concentration ammonia and low C/N ratio in liquid − ammonia mercerization wastewater produced in the textile industry strongly inhibit biological activity (RigoniStern et al. 1996), which results in high cost of biological treatment through complete nitrification and denitrification (Zheng et al. 2020).
Among the different physicochemical treatment approaches, heterogeneous photocatalytic oxidation (PCO) could be considered for removing high − concentration ammonia in wastewater and there are many literature research about this. The final products of photocatalytic conversion of ammonia by TiO2 under the UV irradiation are mainly N2 and NO3−, depending on the cocatalyst on TiO2. Some studies have shown that loading precious metals on TiO2 can significantly improve the photocatalytic activity (Takata et al. 2019). For example, Pt/TiO2 photocatalyst shows significantly improved activity for ammonia oxidation and N2 selectivity (Takata et al. 2019). We also found that 0.3 wt% Cu loaded TiO2 photocatalyst shows comparable photocatalytic activity with Pt/TiO2 for oxidizing low concentration of ammonia, but exhibits much higher photocatalytic activity for oxidizing high concentration of ammonia, primarily ascribed to the Cu metal mediated complexation effect (Feng et al. 2021). In order to practically employ the photocatalyst powder, immobilization of powder onto solid substrate or deposition onto a film is necessary to improve its stability and recyclability (Zhou et al. 2019). In this work, in order to test the Cu/TiO2 photocatalyst for the treatment of wastewater with high − concentration ammonia, the experimental influencing factors, oxidation efficiency and products, photocatalyst immobilization methods and treatment of different types of wastewater were explored.
Preparation of Cu/TiO2 photocatalyst film
According to our previous work, the copper (0.325 wt%) − loaded TiO2 (P25) was obtained by photoreduction deposition. The Cu/TiO2 photocatalyst film was prepared by the drip coating method. Titanium sheets, copper sheets, glass, and ITO conductive glass were selected as the supports of the photocatalyst film. The supports were immersed in deionized water for ultrasonic cleaning for 20 minutes and dried. An appropriate amount of catalyst powder was mixed with ethanol to form a drop coating slurry (100 mg/mL), and ultrasonically dispersed for 60 minutes. The slurry was slowly dropped on the horizontally placed support until the slurry evenly covers its surface. Finally, it was dried at 60 oC for 24 h to form the homogeneously dispersed photocatalyst film.
Photocatalytic oxidation of ammonia nitrogen
Influencing factors of slurry experiments
The slurry experiments of photocatalytic degradation of ammonia nitrogen under UV light irradiation (Xe lamp (PLS − SEX300)) in the presence of photocatalyst powder was carried out in a quartz reactor coated with a quartz glass from top to avoid ammonia valorization. The pH of the solution was adjusted by using 1.0 M NaOH and 1.0 M HCl. The reaction temperature was maintained at about 20 oC by a cryostat. The sample was taken out from the sampling port and centrifuged at 14000 rpm for 15 minutes so that the supernatant can be obtained to test.
Activity test of photocatalyst film
As shown in Fig. 1, the photocatalytic activity of film was tested in a water − circulated condition with UV light irradiation from the top of the photocatalyst film. The prepared photocatalyst film was placed on the bottom of an open reactor, and the water circulated between the wastewater tank and the open reactor to allow the water flow on the surface of the photocatalyst film. Three typical high − concentration ammonia nitrogen wastewaters are selected to investigate the performance of the photocatalyst film. The simulated wastewater characters are shown as follows. High salinity ammonia nitrogen wastewater: [NH3 − N] = 500 mg/L, [SO42−] = 10000 mg/L and [Cl−] = 10000 mg/L. Copper − ammonia complex wastewater: [NH3 − N] = 500 mg/L, and [Cu2+] = 200 mg/L. Liquid − ammonia mercerization wastewater: [NH3 − N] = 500 mg/L, and [CODCr] = 200 mg/L (sodium acetate as the donor).
Detection and analysis methods
Surface chemical composition of the photocatalyst and valence state of the copper were analyzed by the X − ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe II). The reaction liquid sample was taken out from the sampling port at regular intervals to detect the concentration of ammonia nitrogen (NH4+/NH3 − N), nitrite nitrogen (NO2−−N), nitrate nitrogen (NO3−−N) and copper ions. The concentration of ammonia nitrogen, nitrite nitrogen and nitrate nitrogen in the solution is detected by spectrophotometry with an ultraviolet − visible spectrophotometer (Agilent, Cary 5000). The copper ion concentration was quantitatively analyzed by inductively coupled plasma emission spectrometer (ICP − OES, Optima 8000).
Effect of the initial pH on the removal of ammonia nitrogen
The photocatalytic conversion of ammonia nitrogen (500 mg/L) in aqueous solution with different initial pH is shown in Fig. 2. The initial pH greatly affected the photocatalytic oxidation of ammonia nitrogen. As shown in Fig. 2a, the ammonia removal rate was improved with the increase of the initial pH, which reaches an optimum pH value of 10.0 − 11.0. However, when the pH is 7.0, the ammonia nitrogen concentration is basically unchanged. It is known that when the solution is acidic and neutral, the ammonia nitrogen exists in the form of NH4+ which hydroxyl radicals generated by photocatalysis cannot easily attack (Lee et al. 2002; Yao et al. 2020). Fig. S1 shows that under different pH conditions, the process of photocatalytic ammonia nitrogen is fitted with the first − order kinetic reaction process. The pH not only influences on the conversion of ammonia nitrogen, but also the oxidation products. It can be seen from Fig. 2b that when the pH increases from 7.0 to 10.0, the amount of gaseous − N species generated increases. However, the conversion rate of ammonia nitrogen into N2 and other gases decreases with the increase of pH. Table S1 shows that the gas conversion rate is 94.0% under the pH = 8.0 and the gas conversion rate drops to 61.2% under the pH = 11.0. The reason is probably that the increase in pH facilitates generation of hydroxyl radicals ●OH, which mainly produces nitrite and nitrate nitrogen. According to the previous work (Feng et al. 2021), the key to generation of gaseous − N species is superoxide radical ●O2−. As the pH increases, superoxide radical ●O2− was gradually depleted due to the limited dissolved oxygen in the closed reaction system. The generated ionic species were also monitored. As shown in Fig. 2c and Fig. 2d, nitrite ions were found as the major products and nitrate ions are negligible. With the increase of pH, more nitrite ions were produced in 6 h reaction.
Effect of temperature on the removal of ammonia
The photocatalytic conversion of ammonia nitrogen in aqueous solution with different temperatures is shown in Fig. 3. As shown in Fig. 3a, the removal efficiency of ammonia nitrogen does not change significantly when the temperature rises from 20 oC to 30 oC. However, when the temperature is increased to 40 oC and 50 oC, the removal efficiency of ammonia nitrogen is improved. Fig. S2 shows that under different temperature conditions, the process of photocatalytic ammonia nitrogen is consistent with the first − order kinetic reaction process. When the temperature is greater than 30 oC, the reaction rate increases with the increase of the reaction temperature. Therefore, it can be considered that the increase in temperature promotes the reaction rate mainly because of the increase in the number of molecular collisions, rather than the increase in the number of thermally activated molecules. As the reaction temperature increases, the amount of ammonia nitrogen converted into N2 and other gases gradually increases, probably due to the reduced gas solubility in water accelerates the release of gaseous products into gas phase.
According to the data in Table S1, the Arrhenius Eq. (1) can be used to calculate the activation energy of photocatalytic ammonia nitrogen (Jung and Kruse 2017).
In the formula, k is the reaction rate constant at temperature T (h− 1), Ea is the activation energy of the reaction (J/mol), R is the molar gas constant (J/mol·K), and T is the absolute temperature (K). According to the Arrhenius Eq. (1), with ln(k2/k1) as the ordinate and (1/T2 − 1/T1) as the abscissa, using the least squares method to fit, the slope − Ea/R can be obtained, and the activation energy of the reaction is finally calculated to be 8.421 kJ/mol.
Effect of the high salinity on the removal of ammonia nitrogen
Generally, industrial wastewater with high concentration of ammonia nitrogen has a high salinity. Therefore, the effect of excessive Cl− and SO42− on the photocatalytic oxidation of ammonia nitrogen was investigated. It can be seen from Fig. 4a that the removal of ammonia nitrogen is basically not affected by the high concentration of SO42−. Under alkaline conditions, part of SO42− could be converted into sulfate radicals (SO4●−) by photogenerated ●OH (Wang et al. 2021). Although high concentration of SO42− consumes part of ●OH, SO4●− has a higher oxidation − reduction potential (E0 = 2.5 − 3.1 eV), and its oxidation capacity is equivalent to ●OH (E0 = 2.8 eV) (Song et al. 2019; Wang et al. 2019; Wang et al. 2019). Therefore, the high concentration of SO42− basically does not affect the removal rate of ammonia nitrogen. As shown from Fig. 4b, under the condition of high concentration of Cl−, the removal effect of ammonia nitrogen is obviously suppressed and as the concentration of Cl− increases, the removal rate of ammonia nitrogen gradually decreases. The reason may be that Cl− has the ability to scavenge ●OH, which convert part of Cl− into chlorine radical Cl● (E0 = 2.4 eV) that can not efficiently degrade ammonia nitrogen directly, thereby inhibiting the removal rate of ammonia nitrogen (Han et al. 2021; Liou and Dodd 2021).
Treatment of wastewater by photocatalyst film
The Cu/TiO2 photocatalyst powder was immobilized onto some solid substrates including titanium sheet, copper sheet, glass and ITO conductive glass. Figure 5a shows that the removal rate of ammonia nitrogen on glass substrate is the highest. In contrast, the efficiencies of photocatalytic oxidation of ammonia using titanium sheet, copper sheet and ITO supported photocatalytic films are relatively lower. Unlike the slurry tests in the closed reactor, the circulated film test system is open to the air, which is benefit for O2 supply and improved mass transfer between aqueous and gaseous phases. It can be seen from Fig. 5b and Fig. S3 that only a small part of NH3 is converted into NO2− and NO3−, and most of NH3 is removed in the form of gaseous species. This is because superoxide radicals ●O2− are proposed as the key oxidant to gas production (Feng et al. 2021), and the increased supply of oxygen in the open reactor system will increase the superoxide radicals ●O2−, and thus N2 production will be increased.
The flow rate of circulated water on the surface of catalyst film has strong influence on its removal efficiency of ammonia nitrogen and catalyst stability. It can be seen from Fig. 6a and Fig. S4 − S6 that when the flow rate is 40 mL/min, the morphology of Cu/TiO2@Glass before and after the photocatalytic reaction is basically unchanged, and the removal efficiency of ammonia nitrogen is the highest. When the flow rate increases, the catalyst film on the glass surface is damaged due to the excessive surface velocity, which leads to the weakening of the photocatalytic efficiency. However, when the flow rate decreases, the catalyst film on the glass surface is also damaged. The reason may be that the residence time of the solution on the surface of the catalyst film increases, and anions tend to accumulate on the surface of TiO2 and destroy the bonding structure of TiO2/SiO2, thereby reducing the removal rate of ammonia (Levchuk et al. 2019). Therefore, the flow rate 40 mL/min was selected for subsequent experiments.
The circulated treatment of different types of high − concentration ammonia wastewater by Cu/TiO2@Glass was investigated. In the control experiment (Fig. 7a), the ammonia was rapidly removed in the first hour, and then the oxidation rate gets slower. Because solution pH gradually decreases along with the ammonia removal, the photocatalytic oxidation of NH4+ in acidic or neutral condition is inhibited. Compared with Fig. 7a, the removal of ammonia in high salinity wastewater decreased significantly (Fig. 7b), the ammonia no longer drops after 4 h and about 165 mg/L of NH3 − N can be removed finally. The reason is that excessive Cl− consumes the h+ and ●OH on the surface of the photocatalyst film, resulting in a decrease of photocatalytic oxidation efficiency (Wang et al. 2021). Figure 7c shows that the NH3 − N in the copper − ammonia complex wastewater no longer drops after 2 h and about 160 mg/L of NH3 − N can be removed. The reason is that copper ions play a role in capturing e− in the photocatalytic process, inhibiting the generation of ●O2−, resulting in a decrease of photocatalytic oxidation efficiency. However, Cu2+ ions in water can be reduced to Cu+ or Cu depositing on photocatalyst film by the photogenerated electrons, which can be considered as simultaneous removal of NH3 − N and Cu2+ according to the Fig. S7. Figure 7d shows that the NH3 − N in the liquid − ammonia mercerization wastewater was slowly removed during 6 h reaction and about 135 mg/L of /NH3 − N can be removed finally, since the organic matters in liquid − ammonia mercerization wastewater consume the h+ and ●OH produced on the surface of the photocatalyst film.
To investigate the stability of Cu/TiO2@Glass during treatment of different wastewater, the experiments were repeated five times and the removal rates were shown in Fig. 8a. Cu/TiO2@Glass film before and after the reaction was weighed, and the mass change was calculated as shown in Fig. 8b. For high salinity ammonia wastewater, as the number of reuses of photocatalyst film increases, the removal rate of NH4+/NH3 − N gradually decreases and it is dropped by about 6% after five repeated experiments. In addition, the mass of photocatalyst film is continuously decreased because the excessive anions damages the surface structure of the film. Regarding the liquid − ammonia mercerization wastewater, although the organic matter has a greater inhibitory effect on the removal efficiency of NH3 − N, the removal rate was only dropped by about 1.5% after five repeated experiments, indicating that organic matter has a small impact on the reusability of Cu/TiO2@Glass. For copper − ammonia wastewater, as the number of reuses of Cu/TiO2@Glass increases, the removal rate of NH3 − N increases unpredictably. The removal of NH3 − N increased from 56.6–67.7% during the first three cycles and then slightly decreased, but the removal rate is still higher than that of the first photocatalytic process. Besides, as the number of reuses increases, the mass of Cu/TiO2@Glass continues to increase, because copper ions in the solution are continuously reduced and deposited on the surface of photocatalyst film. The deposited Cu species could be work as cocatalyst to enhance the photocatalytic performance (Mingmongkol et al. 2021).
Mechanism of photocatalytic conversion of ammonia
There are many pathways involved for photocatalytic oxidation of ammonia on Cu/TiO2 as illustrated in Fig. 9. In the closed reactor condition, it has been proposed that ●OH, h+ and ●O2− are main oxidants for ammonia conversion, in which ●O2− radical strongly influences the gaseous products (pathway I). However, due to the depletion of dissolved O2 content, the reactions prefer to further proceed via pathways II and III, thus the conversion rate and selectivity to N2 gradually decreases. In contrast, in the open system, the NH3 abatement resulting from both the large portion of volatilization and photocatalytic oxidation would decrease the pH of the solution gradually, so that the reaction driven by ●OH (pathway II and III) are limited. Moreover, due to the continuous supply of O2 within 6 hours, the ammonia conversion driven by ●O2− (pathway I) would be proceeded continuously, and the selective oxidation to gaseous products was enhanced as shown in Fig. 9. It can be found that no matter what kind of Cu/TiO2 films used in the open system, the removal of ammonia and the selectivity of gaseous products are higher than those of Cu/TiO2 powder in the closed reactor, which further demonstrates the role of dissolved O2 in the ammonia conversion.
The influencing factors such as pH, temperature and salinity strongly influence the photocatalytic oxidation of ammonia by Cu/TiO2 photocatalyst. The results showed that the optimal initial pH is 10.0 under the high − concentration ammonia condition. After 6 hours, about 140 mg/L NH4+/NH3 − N can be removed, of which 65.6% of ammonia was converted into N2 and other gases. Increasing reaction temperature over 40 oC can markedly improve the photocatalytic oxidation efficiency. Excessive SO42− (50000 mg/L) did not affect the removal effect of ammonia, but excessive Cl− (50000 mg/L) inhibited the removal effect of ammonia. Furthermore, it was found that Cu/TiO2 particles have strong affinity with glass substrate as the Cu/TiO2@Glass film provides good stability compared to other solid supports during multiple repeated tests. In addition, the photocatalytic performance of Cu/TiO2@Glass film, was excellent for treating different types of wastewater including high salinity ammonia wastewater, copper − ammonia wastewater and liquid − ammonia mercerization wastewater due to the continuous oxygen input from air, the improved photons absorption efficiency and strong stability. More importantly, the photocatalytic oxidation performance of Cu/TiO2 film was superior to that of the Cu/TiO2 powder, highlighting the advantages of Cu/TiO2 film for practical application.
Supplementary Information
The online version contains supplementary material available at http://doi.org/xx/
CRediT author statement
Jianpei Feng: Methodology, Investigation, Data analysis. Guan Zhang, Supervision, Writing, Funding acquisition. Xiaolei Zhang, Supervision, Funding acquisition. Ji Li, Supervision, Resources.
Data availability
All data generated or analyzed during this study are included in this published article and supplementary information files.
Acknowledgments
The authors thank the financial supports from NSFC (no. 51978197), the Shenzhen Science and Technology Innovation Commission (Grant No. JCYJ20180306171722756, JCYJ20180306172051662) and Pearl River talent plan (Young Scholar) of Guangdong Province.
Ethics approval and consent to participate
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Declaration of competing interest
The authors declare that there is no other interests or relationships that could have appeared to influence the work reported in this paper.
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