The ozonation efficiency for removal of recalcitrant organic pollutants in alkaline wastewater is always low because of the presence of some hydroxyl radical scavengers. To solve this problem, the O3/Ca(OH)2 system was put forward, and p-nitrophenol (PNP) was chosen to explore the mechanism of this system. The effects of key operational parameters were studied respectively; the Ca(OH)2 dosage 3 g/L, ozone inlet flow rate 3.5 L/min, ozone concentration 65 mg/L, reactor pressure 0.25 MPa, and temperature 25 °C were obtained as the optimal operating conditions. After 60 min treatment, the organic matter mineralized completely, which was higher than the sum of the ozonation-alone process (55.63%) and the Ca(OH)2 process (3.53%). It suggests that the calcium hydroxide in the O3/Ca(OH)2 process possessed a paramount role in the removal of PNP. The liquid samples and the precipitated substances were analyzed by gas chromatography mass spectrometry, X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy; it was demonstrated that Ca(OH)2 could accelerate the generation of hydroxyl radical and simultaneously in situ separate partial intermediate products and CO32− ions through some precipitation reactions.
As a green and efficient advanced oxidation process (AOP), ozonation has been considered a promising method for wastewater treatment as it is flexible in operation (Shahidi et al. 2015); meanwhile, it is unlimited by the colloid substance and has easy operation (Bauman et al. 2011; Kumar et al. 2017). Also, due to its strong oxidation-reduction potential, ozonation can degrade macromolecular organic matter into smaller molecules and achieve the high oxidation and decolorization efficiency (Zeng et al. 2012). However, several disadvantages limit its application, such as: (i) short half-life and easy decomposition, leading to the high cost (Cheng et al. 2016); (ii) the low reaction rate and limited mass transfer resulting from the relatively low solubility and stability in water (Izadifard et al. 2018); (iii) selectivity between ozone and organic molecules, hindering the ozonation process (Xiong et al. 2016). Therefore, it is essential to accelerate the mass transfer rate of ozone and generate the hydroxyl radical; catalytic ozonation has been investigated as an efficacious way for oxidation (Cheng et al. 2016).
On the one hand, the pH of the liquid media has a significant impact on the process of catalytic ozonation, which can be divided into two common aspects. Firstly, catalysts could be utilized to facilitate ozone decomposition and strengthen the formation of active radicals (Wang & Bai 2016), but it is important to keep the catalytic ozonation process in the acidic media, and that carbon dioxide as mineralized product can overflow to the reaction system, without consuming hydroxyl radicals (Nawrocki & Kasprzyk-Hordern 2010). However, the stability of catalysts should be further considered. The leaching of catalytic active components and the easy contamination of catalysts are problems that must be investigated in actual wastewater treatment (Hu et al. 2016). Secondly, ozone decomposition in water is greatly decided by pH in the solution and occurs faster when the pH value increases to 10. Basic pH is favorable for ozone decomposition; in general, the decomposition of O3 in water is initiated by OH− (e.g. conventional O3/H2O2 process) (Yu et al. 2011; Asgari et al. 2013). Nevertheless, there are HCO3− and CO32− ions that will generate in alkaline condition due to the pollutant mineralization, resulting in the consumption of hydroxyl radicals by some free radical scavengers and decline of the oxidation efficiency (Kuosa et al. 2015).
Calcium hydroxide is a kind of cheap and environmental-friendly chemical with a moderate solubility in water. Calcium hydroxide in aqueous solution first ionizes into calcium ions and hydroxyl ion; the pH value in solution could be maintained by Ca(OH)2 to promote ozone decomposition. And then the Ca2+ would react with HCO3− and CO32− ions; therefore, the CO32− ions could be simultaneously precipitated and separated due to the presence of Ca2+ ions, resulting in high ozonation efficiency.
On the other hand, in order to improve the ozone–water mass transfer efficiency in engineering applications, many strengthening measures were explored by numerous scholars, such as an oscillatory baffled reactor (Al-Abduly et al. 2014), planar falling film reactor (Mahyar et al. 2017), and rotating packed bed reactor (Zeng et al. 2012). It has been reported that microbubbles can improve the gas–liquid mass transfer rate, and the ozone-microbubble in the liquid phase will generate hydroxyl radical during shrinks and burst instantaneously (Takahashi et al. 2012). Compared with a conventional bubble reactor, the gas–liquid mixing pump was utilized to disperse the mixture gas of oxygen and ozone into numerous micro gas bubbles with microscale in the microbubble reactor, realizing the compulsory mixing of ozone and wastewaters, therefore enhancing utilization of ozone and improving the mass transfer rate.
Phenols are generally considered to be one of the most important organic pollutants discharged into the environment, causing unpleasant taste and odor of drinking water (Alshehri et al. 2014; Al-Kahtani et al. 2018). p-Nitrophenol (PNP) is an important chemical raw material used as a typical phenolic compound to produce explosives, preservatives, pharmaceuticals, pesticides and dyes (Wang et al. 2015). In a typical chemical industrial organic wastewater, it is difficult to degrade. Thus, this work mainly aimed to evaluate the ozonation efficiency using PNP as a model pollutant and explore the mechanism of the O3/Ca(OH)2 process in a microbubble reactor. Effects of Ca(OH)2 dosage, pressure, temperature, initial PNP concentration, ozone concentration and inlet flow rate on the degradation and mineralization of PNP are discussed respectively. Furthermore, the mechanism of PNP mineralization is discussed thoroughly through the in situ separation effects of Ca(OH)2 towards hydroxyl radical scavengers and some ozonation intermediate products.
MATERIALS AND METHODS
Details of reagents used in this study are provided in the Supporting information (available with the online version of this paper).
The experimental setup for the ozonation experiments is shown in Figure 1. More details about this setup are given elsewhere (Quan et al. 2017). The samples were collected at regular intervals during the ozonation process, and the solid–liquid mixture was separated. The liquid sample was used to determine the PNP concentration and total organic carbon (TOC) and the intermediate products in the id sample were analyzed by gas chromatography-mass spectrometry (GC-MS); the solid phase sample was dried at 105 °C and used for analysis by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy.
Details of analytical methods used in this study are provided in the Supporting information.
RESULTS AND DISCUSSION
Superiority and synergetic effect of the O3/Ca(OH)2 system and microbubbles
In order to evaluate the synergetic effect of the O3/Ca(OH)2 process and microbubbles, comparative experiments and analyses using (a) bubble–O3, (b) microbubble–O3, (c) Ca(OH)2, (d) O3/Ca(OH)2 and (e) O3/NaOH were set up to treat PNP solution, and the results are shown in Figure 2. A lower degradation efficiency of PNP was obtained by ozone alone in the traditional bubble rector compared to the microbubble reactor. The enhancement of the mass transfer rate and utilization of ozone in the liquid led to the high ozonation rate in the microbubble reactor.
Otherwise, for the sake of highlighting the superiority of the O3/Ca(OH)2 process, compared to the process of (b), (c), (d), a high PNP removal was obtained by O3/Ca(OH)2 process, while a low mineralization rate was realized by ozonation alone. It has been reported that the O3/OH− process could generate hydroxyl radicals by ozone decomposition; therefore, Ca2+ ions and Na+ ion were compared to verify the effect of Ca2+ ions in the solution. The PNP degradation rate was fast in the two O3/OH− processes because of the efficient decolorization rate of ozone; however, the mineralization rate in the O3/NaOH process was far lower than that in the O3/Ca(OH)2 process, and even a little lower than that in the alone O3 process after 40 min treatment, indicating that the free radical scavengers such as CO32− ions or HCO3− ions were produced.
Even though the CO3−. could react with the organic molecule as an oxidant, its oxidation potential (E0 = 1.78 V, pH 7.0) is less than that of the hydroxyl radicals (E0 = 2.3 V, pH 7.0) (Khodami & Nezamzadeh-Ejhieh 2015; Senobari & Nezamzadeh-Ejhieh 2018). Thus, there is a competitive reaction in the degradation of organic matter between the carbonate radical anion and hydroxyl radical, resulting in the low removal efficiency.
In addition, research on variation of CO32− ions in the reaction were studied by Quan and colleagues in prior work (Quan et al. 2017); the results showed that the CO32− ions concentration in the O3/Ca(OH)2 process was 13.58 mg/L after 35 min treatment, which decreased as the reaction proceeded. However, the CO32− ions concentration in O3/NaOH process was 229 mg/L, and there was an increasing trend as the reaction proceeded; the hydroxyl radicals were consumed greatly, resulting in the ozonation efficiency being reduced.
As results show, the maximum degradation and mineralization, with the maximum reaction rate constant of 0.34795 min−1 and 0.12535 min−1 respectively, were observed in the presence of Ca(OH)2. These results confirm that O3/Ca(OH)2 process enhances the ozonation efficiency and exhausts hydroxyl radical scavengers.
Effect of operating parameters
Effect of Ca(OH)2 dosage
Figure 3 shows the effects of different Ca(OH)2 dosage on PNP degradation and mineralization, and the pH of the solution was monitored in ozonation process. It is noted that the Ca(OH)2 has promotional effects on PNP degradation; nevertheless, varying the Ca(OH)2 dosage has little influence on the PNP degradation. It may be attributed to the effect of the microbubble gas–liquid reactor, which improves the mass transfer rate between the gas phase and liquid phase, leading to the enhancement of ozonation efficiency: hence, PNP could be eliminated completely with 30 min ozonation treatment alone. For the ozone-alone process, increasing pH in the solution could enhance the degradation rate as reported by previous researches (Li et al. 2017). Higher initial pH is beneficial to the production of hydroxyl radicals. When Ca(OH)2 dosage was 0.5 g/L, the pH in the phase decreased gradually due to the generation of a small amount organic acid, the pH value remained below 10 after 10 minutes of reaction, and the free radicals produced in the solution were sufficient for the complete degradation of PNP. Therefore, variation of the Ca(OH)2 dosage has little influence on removal of PNP (Wang et al. 2016), so PNP could be degraded rapidly.
However, Figure 3(b) indicates that Ca(OH)2 dosage had a promoting influence on mineralization: the TOC removal rate increased from 62.33% to 99.46% after 50 min treatment when Ca(OH)2 dosage was increased to 3 g/L. At the same time, the reaction rate constants of the degradation and mineralization processes were calculated by the pseudo-first order reaction kinetics equation and the results are listed in Table S1 (available with the online version of this paper). According to the results, the reaction rate constants increased with the increasing Ca(OH)2 dosage. Meanwhile, Figure 3(c) shows that the pH in solution increased with increasing Ca(OH)2 dosage during the process, finally, the pH value remained constant. However, the high concentration of OH− may cause the deactivation of .OH (Nezamzadeh-Ejhieh & Khorsandi 2010). The carbonate will generate in the high alkaline environment due to the mineralization of organic matter; this process will produce some radicals which have lower oxidation potential than .OH, which will hinder the reaction process. Thus, the addition of Ca(OH)2 has a dual function on the intensification of the ozonation process. For one thing, enough alkalinity is needed to promote the ozone decomposition and maintain the concentration of radical in solution. For another, enough Ca2+ ions are needed to eliminate some intermediate product and radical scavengers, and maintain the concentration of .OH in the liquid phase. Hence, the Ca(OH)2 dosage chosen was 3 g/L.
Effect of pressure
Effects of reactor pressure on the degradation and mineralization of PNP were researched as shown in Figure S1 and the reaction rate constants can be seen in Table S1 (both available online). There are some differences in the Table S1; however, it can be seen that the reactor pressure has a barely noticeable difference on degradation and mineralization of PNP from Figure S1. Generally speaking, a vigorous agitation will be generated between the gas phase and liquid phase due to the enhancement of reactor external pressure. Meanwhile, the gas–liquid mass transfer resistance decreases, resulting in an increase in ozone mass transfer. However, the gas–liquid mixer in the microbubble reactor has great superiority for dispersion of the micro gas bubbles, and it could greatly intensify the ozone mass transfer efficiency because of the effect of the internal pressure caused by micro gas bubbles. Therefore, the external pressure imposed on the reactor has little influence on the degradation of organic pollutants (Zhang et al. 2016).
Effect of temperature
However, in the O3/Ca(OH)2 system, the ozone could decompose rapidly and generate the free radicals due to the high pH value; the enhancement of ozonation efficiency in this process is far higher than the effect of temperature. Therefore, from the macroscopic aspect, increasing the temperature has negligible influence on the organic matter removal as shown in Figure S2. As reported by Lu & Pei (2016), 25 °C is the optimal reaction temperature.
Effect of initial PNP concentration
Effects of initial concentration of PNP on the degradation and mineralization of PNP were investigated. Figure S3 (available online) shows that the degradation and mineralization decrease with increasing initial PNP concentration in solution. The degradation efficiency for initial concentration values in the first 15 min were 98.57%, 89.71%, 71.92% and 61.11% respectively, moreover, the TOC removal efficiency in the first 25 min were 96.88%, 84.31%, 62.69% and 51.63% respectively. The process of removal rate decline with increase in initial PNP concentration is due to constant ozone dosage (Wang & Bai 2016).
Effect of ozone concentration
Effects of ozone concentration on the degradation and mineralization of PNP were investigated as shown in Figure S4 (available online) and the reaction rate constants can be seen in Table S1. It can be seen from Figure S4(a) that PNP was basically degraded after 20 minutes of reaction, but it can be seen from Table S1 that the reaction rate constant k1 increases rapidly from 0.163 min−1 to 0.369 min−1 when the concentration of ozone increases from 30 mg/L to 65 mg/L, and decreases to 0.314 min−1 when the concentration of ozone increases from 65 mg/L to 75 mg/L. Table S1 shows that the variation tendency of the reaction rate constant k2 is the same as that for k1. Figure S4(b) shows that the TOC removal rate increased from 61.98% to 96.67% after 30 minutes of reaction when the concentration of ozone increased from 30 mg/L to 65 mg/L, while the removal rate of TOC increased slowly to 97.85% when the concentration of ozone was further increased to 75 mg/L. A potential explanation is that the ozone concentration in liquid achieves a saturation, and the organic matter was degraded by enough ozone. Therefore, it was not necessary to increase the ozone concentration to 75 mg/L.
Effect of inlet ozone flow rate
In addition, the effects of ozone flow rate on the degradation and mineralization of PNP were investigated as shown in Figure S5 (available online) and the reaction rate constants can be seen in Table S1. The variation tendency was consistent with that of the inlet ozone concentration. The increase of ozone dosage accelerates the ozonation process. However, at the same inlet ozone concentration and the same ozonation time period, with the increase of the inlet gas flow rate, the flow rate dissolved in the liquid phase increases, and the number of bubbles in the liquid phase increases. Therefore, the mass transfer efficiency increases. However, with the increase of the flow rate, the size of bubbles increases, which reduces the specific surface area. As a result, the ozone utilization rate is reduced; so increasing the inlet ozone flow rate has no obvious promotion effect.
in situ separation of hydroxy radical scavengers
Liquid phase analysis
The ultraviolet (UV) adsorption spectra of the wastewater before and after the O3/Ca(OH)2 process with different treatment time (0–60 min) is shown in Figure 4. The absorption peak at 400 nm mainly contributed to the formation of p-nitrophenolate ions in alkaline condition (Bae et al. 2015). The figure shows that the peaks of PNP at 400 nm decreased quickly with the ozonation reaction proceeding, and disappeared at treatment time of 20 min and longer, which indicated that the -NO2 on the molecular structure of PNP was degraded effectively by O3/Ca(OH)2 system (Wan et al. 2017); at the same time, some new small peaks appeared at 200–300 nm with increasing reaction time. However, the absorbance intensity of the peaks at the range of 200–300 nm almost disappeared after 25–30 min treatment, which indicated that PNP in solution was almost mineralized completely, resulting in its high TOC removal efficiency. The results of the UV-vis analysis were consistent with the variation of degradation and mineralization.
In order to further identify degradation intermediates of PNP and their variation by O3/Ca(OH)2 process, the intermediates in the three experiments contrasting the bubble system, microbubble system and O3/Ca(OH)2 system were analyzed by GC-MS analysis as shown in Figure S6. To confirm the structure of intermediate products, Figure S7 shows the chromatogram of intermediates of PNP degradation after reaction time of 20 min and 40 min in the bubble reactor (Figures S6 and S7 are available online).
As seen in Figure 5, there are some of the same intermediate products obtained by the three different processes. Several organic compounds, including 1,4-cyclohexanedione, 2-propanone,2-(1-methylethylidene) hydrazone, pentane, 2,2,3-trimethyl and 1-hexene-2,4-dimethyl-, were removed from the microbubble reactor. It is attributed to the effect of the microbubble rector. Otherwise, it is observed that the organic compounds in the O3/Ca(OH)2 process were less than half of those in microbubble process: only isopropyl cyclopropane carboxylate, butane, 2,2′-oxybis-, Ethyl butyrate and ethyl butyrate remained in the solution and mineralized completely after 60 min treatment. However, hydroquinone, detected in the bubble process and microbubble process, was not found in the O3/Ca(OH)2 process, which suggests that the ozonation process was accelerated by the addition of Ca(OH)2; in other words, PNP could be oxidized into small molecules in a short period of time by the O3/Ca(OH)2 process. In addition, for example, isopropyl cyclopropane carboxylate was detected in 40–60 min ozonation treatment by microbubble process, while it was detected in 0–20 min ozonation treatment in the presence of Ca(OH)2. The chromatogram of intermediates of PNP degradation of reaction time 20 min and 40 min in the bubble reactor are shown in Figure S7. These results demonstrate that Ca(OH)2 could facilitate ozone decomposition effectively and generate hydroxyl radical, thus accelerating the reaction rate and increasing the TOC removal efficiency.
Solid products analysis
The structures and crystallinity of solid products in the ozonation process were studied by XRD and the results are depicted in Figure 6. It can be seen that the crystalline product mainly including calcium hydroxide (Ca(OH)2), calcium carbonate (CaCO3), and calcium oxalate (CaC2O4). The new solid phase products were produced in the ozonation treatment from 0 min to 60 min, and it can be explained from the following two aspects. (i) The precipitation was formed between the direct oxidation by-products and calcium, such as calcium oxalate. In ozonation, ozone molecules attack PNP molecules, causing the partial degradation of PNP and forming some intermediates, such as fumaric acid, maleic acid, acrylic acid, and oxalate (Bakheet et al. 2013); this result is in accordance with the decrease of the pH in liquid phase in Figure 3. (ii) CO32− ions were generated by the production of CO2 in alkaline condition because of the mineralization of organic pollutants, forming the precipitation of CaCO3.
As seen in Figure 6(a), the diffraction of Ca(OH)2 disappeared gradually with prolonging of the ozonation time, illustrating that the Ca2+ ions and OH− ion were consumed. Diffraction peaks assigned to calcium oxalate at 2θ = 14.3°, 20.1° and 32.24° appear at 20 min ozonation treatment. When PNP degraded into small molecular organics completely, it is indicated that oxalic acid in solution combined with calcium ions, forming precipitation of calcium oxalate that was detected in the XRD pattern. Also, diffraction peaks of calcium carbonate at 2θ = 29.44°, 35.94°, 39.4° and 47.68° exist in the precipitation and strengthen gradually. And Figure 6(b) displays the diffraction peak of the precipitated substances including CaC2O4 and CaCO3.
To further research the surface properties, the surface characteristics of solid products after 60 min treatment were observed by SEM and FTIR as shown in Figure 7. Figure 7(a) shows the surface morphology of solid substances, some of which have a cube structure, which is a common calcite, and one of the most stable crystalline structures of calcium carbonate. Some have an irregular-shaped polygonal structure with rough surfaces and particles attached to crystals. It agrees with the structure of calcium oxalate.
Figure 7(b) shows the FTIR absorption spectrum of solid substance after PNP mineralization. The band at 3,447 cm−1 is attributed to the O-H stretching vibration; the absorption peak of OH is likely to come from the COOH, alcohol or phenol (Naushad et al. 2017). The band at 2,964 cm−1 is attributed to the -CH3 antisymmetric stretching vibration. The band at 1,639 cm−1 is attributed to the double band stretching vibration. The bands at 1,024, 1,094, 1,261, 1,325, and 1,458 cm−1 are attributed to the C-O stretching vibration, including carbohydrates, esters, ethers, which illustrates that COO- is generated by organic matter degradation (Albadarin et al. 2016). And the band at 873 cm−1 is attributed to the CO32− out-of-plane deformation vibration; the band at 712 cm−1 is attributed to O-C-O in-plane deformation vibration. The characteristic absorption peaks of CO32− and COO− in IR spectrums verified the production of calcium oxalate and calcium carbonate.
In the O3/Ca(OH)2 process, the pH value of solution kept to 12.47 and reached saturation, which facilitates the effect of OH− for the ozone decomposition and generation of hydroxyl radical. This verified that the oxalic acid and CO32− ions were precipitated by the Ca2+ ions in the O3/Ca(OH)2 system, and this should be the mechanism of Ca(OH)2-intensified mineralization of PNP solution. The whole mechanism could be summarized as Figure 8.
The ozonation of PNP by O3/Ca(OH)2 process in a microbubble reactor was discussed. PNP could be removed and mineralized efficiently in the presence of 3 g/L Ca(OH)2. After 60 min treatment, the organic matter was almost mineralized completely, and the mineralization rate also was much higher than those of the O3 process of 55.63% and Ca(OH)2 process of 3.53% under certain conditions, which confirmed the superiority of the O3/Ca(OH)2 process. The removal rate increased with increasing ozone concentration up to 75 mg/L, ozone flow rate and 4 L/min, and decreasing initial PNP concentration; in addition, it was insignificantly affected by the external pressure and temperature.
Ca(OH)2 combined with ozone could intensify the ozonation process, and the mechanism was explored through GC-MS analysis of the liquid samples and characterization analysis of the precipitated substances. The results show that the microbubble process improves the utilization of ozone, reducing the types of organic matter of PNP degradation compared to the bubble process, and the O3/Ca(OH)2 process could maintain the pH value in solution, accelerating the generation of hydroxyl radical, and thus leading to PNP being completely mineralized. Furthermore, the ozonation efficiency can be intensified by in situ precipitation and separation of hydroxyl radical scavengers and the intermediate product oxalic acid.
This work was supported by the National Science Foundation of China (grant number 21176273); the personnel project of Chongqing Science and Technology Committee (grant number 2014); and Chong Qing Municipal Solid Waste Resource Utilization & Treatment Collaborative Innovation Center.