(Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering North University of China, Taiyuan 030051)

Effects of Coexisting Substances on Nitrobenzene Degradation with O3/H2O2Process in High-Gravity Fields

Zhang Shiguang; Qin Yuejiao; Zhang Dongming; Jiao Weizhou; Guo Liang; Liu Youzhi

(Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering North University of China, Taiyuan 030051)

This study used nitrobenzene as the simulated pollutant to study the effects of common inorganic sodium salts and organics on nitrobenzene degradation by O3/H2O2in high-gravity felds. The experiment results showed that the highgravity technology could increase the nitrobenzene removal rate by improving the ozone transfer effciency and ozone dissolution. Coexisting substances had different effects on the degradation kinetics of nitrobenzene in high-gravity felds. Among such substances, Na2CO3, NaOH, Na3PO4, and NaNO3accelerated the removal of nitrobenzene. The main action principle of nitrobenzene degradation by O3/H2O2is that the additives can increase the pH value of the solution, stimulate ozonolysis, generate hydroxyl radicals (·OH), and improve oxidation efficiency. By contrast, NaCl, NaHCO3, NaHSO4, ethanol (C2H5OH), acetic acid (CH3COOH), formic acid (HCOOH), andtert-butyl alcohol (TBA) inhibited nitrobenzene removal. When NaHCO3, CH3COOH, or HCOOH were added, the pH value of the solution decreased and free radical chain reactions were hindered. However, NaCl, NaHCO3, C2H5OH, and TBA consumed ·OH radicals and inhibited nitrobenzene removal.

wastewater; nitrobenzene; coexisting substance; high gravity; ozone; hydrogen peroxide

1 Introduction

Owing to the advantages of high oxidation efficiency and non-selective oxidation, as well as its ability to treat a great variety of wastewater with different composition and concentration, the advanced ozone oxidation technology has been intensively investigated in environmental protection[1-2]. Given the low solubility of ozone in water, the degradation of most types of pollutants with this technology is controlled by the mass transfer process or is controlled simultaneously by the ozonation reaction rate. Therefore, the mass transfer of ozone from the gaseous phase to the liquid phase becomes the primary control step. The rotating packed bed (RPB) as a novel gas-liquid contactor can significantly intensify the mass transfer[3]and mixing process, and has been used in absorption[4-7], distillation[8-10], green reaction[11], stripping[12], and other reactions. In recent years, scholars have effectively rectifed the low mass transfer rate of ozone from the gaseous phase to the liquid phase by integration with the high-gravity technology[13-15], which is significantly meaningful when ozone is applied to the water treatment process. Ko, et al.[16]has found that the low mass transfer rate of ozone is the major controlling factor in fast chemical reactions for conventional bubble column reactors. By adopting the high-gravity technology, the mass transfer rate of ozone has been effectively improved and the removal rate of guaiacol is greatly increased. Chiu, et al.[17]adopted the RPB-O3process to treat naphthalene-containing solutions and found that the naphthalene removal rate increased when RPB was used as the ozone reactor. Zeng, et al.[18]proved that an O3/H2O2system with high gravity can be used to treat phenolic wastewater and found that the phenol removal rate increases with an increasing rotation speed. Thus, the high-gravity technology can effectively increase the phenol removal rate. Li, et al.[19]reported that the simulated amoxicillin wastewater was treated by the O3/Fenton process in a RPB and the results showed that the O3/Fenton process was the most effective one thanks to the synergistic effect of O3and the Fenton reagent. The COD removal rate achieved in the O3/Fenton process was by 65% higher than that obtained by the Fentonprocess.

Thus far, studies on organic wastewater treatment using the RPB-O3/H2O2process mainly focus on the optimal treatment condition selection and the oxidative degradation mechanism[20]. However, the strengthening mechanism of high-gravity technology is rarely considered. Moreover, in most experiments, the simulated wastewater is taken as the research object[21-22]. However, actual industrial wastewater contains not only model pollutants for treatment but also many coexisting substances. Studies have found that many substances have signifcant effects on the mass transfer of ozone and ozonolysis reaction[23]. Given the complex composition of actual industrial wastewater, the current studies mainly discuss the effects of a few types of common inorganic ions in wastewater. Therefore, on the basis of previous researches[20], this study analyzes the common inorganic and organic substances existing in a large amount of actual nitrobenzene (NB) wastewater. By adding relevant pure substances into the stimulated NB wastewater, this study investigates the effect of each substance on the O3/H2O2reaction kinetics for degrading NB and reveals their action principles in a high-gravity fields. With a particular emphasis on the mechanism of high-gravity strengthened O3/H2O2oxidation, this study provides some theoretical data supports for the subsequent actual complex wastewater studies on the effect of different coexisting substances in the O3/H2O2process.

2 Experimental

2.1 Chemicals

The experiments used the following reagents covering: nitrobenzene (NB, analytical reagent, provided by the Development Centre of Tianjin Kemiou Chemical Reagents Co., Ltd., China), and H2O230% (analytical reagent, provided by the Tianjin Tianli Chemical Reagents Ltd., China). The water used in the experiments was the deionized water. The NB-containing wastewater used in this experiment was prepared by dissolving a specifed amount of NB in the deionized water. Approximately 10 mmol/L each of Na2CO3, NaOH, Na3PO4, Na2SO4, NaNO3, NaCl, NaHCO3, NaHSO4, C2H5OH, CH3COOH, HCOOH, andtertbutyl alcohol (TBA) are added, respectively, to a pH-7.5 NB solution at an initial concentration of 100 mg/L to investigate the effects of different additives on the O3/ H2O2process.

2.2 Experimental procedures

The experimental device is a cross-fow rotating packed bed (RPB) made by our laboratory. The packing is made of the stainless-steel wire gauze. The inner diameter is 40 mm and the external diameter is 75 mm for the rotator, and the height in the axial direction is 75 mm. Instruments include: a Dionex’s UltiMate 3000 liquid chromatograph (LC), a GT 901 pump suction ozone detector (Shenzhen Kernuo Electronic Technology Co., Ltd.), a LBC-50W(S) ozonator (Shandong NIPPON Photoelectricity Equipment Co., Ltd.), and a PHS-3C precision pH meter (Shanghai Jinpeng Analytical Instruments Co., Ltd.).

The setup of NB wastewater treatment with the RPB-O3/ H2O2process is shown in Figure 1. Oxygen (1) produces ozone gas at a certain concentration from an ozonator (2). The ozone gas enters the bottom of the RPB (4) via a gas flowmeter (3) and then flows upward through the wire gauze packing. The NB wastewater is sent from a liquid storage tank (8) to the RPB centre by a pump. Thereafter, the wastewater passes through the wire gauze packing from the inside to the outside along a radial direction. The NB wastewater travels via the cross contact with the ozone in an axial direction to complete the mass transfer and oxidation reaction. The wastewater fows to the liquid storage tank (8) when circulating from the exit of the liquid phase to the bottle wall. Then, the unreacted ozone gas enters the tail gas treatment tank (9). H2O2is frstly added to the wastewater during the experiment, so that the required concentration can be reached.

2.3 Analytical methods

The NB concentration in the wastewater was detected by a Dionex’s Ultimate 3000 HPLC, equipped with a C18reversed-phase column (250 mm×4.6 mm, 5 μm). The UV detection wavelength was 262 nm, and the mobile phase was composed of methanol-water (70:30). The fow velocity was 0.9 ml/min, and the column temperature was 20 ℃, while the sample volume was 20 μL. The calculation formula for the NB removal rate is expressed as fol-lows:

whereC0andCtare the NB concentration in the wastewater before and after treatment, respectively. The gas phase ozone concentration is measured by iodometry. The measurement of the liquid-phase ozone concentration uses the indigo method for detection[24].

Figure 1 Experimental process fl ow diagram

3 Results and Discussion

3.1 NB removal ef fi ciency of RPB-O3/H2O2oxidation

The high-gravity factorβ[25], the ratio between the average centrifugal acceleration and gravitational acceleration of the RPB, is a dimensionless parameter that measures high gravity. The effect of high-gravity factorβon the NB removal rate in wastewater can determine whether the high-gravity technology can strengthen O3/H2O2oxidation process to remove NB.

Under the optimal operating conditions mentioned above[20], this study investigated the effects of different high-gravity factors on the removal of NB. The results are shown in Figure 2. The NB removal rate increased signifcantly when the high-gravity factor increased from 0 to 83.2. When the high-gravity factor reached more than 83.2, the NB removal rate tended to be stable (Figure 2). The ozonation process of NB was simultaneously affected by the mass transfer process of ozone in water and the ozonization reaction. Given that the mass transfer process of the ozone was subject to liquid flm control, the highgravity factor increased with an increasing rotation speed, thus strengthening the shearing and separating force of packing against the wastewater, and cutting the liquid into tiny liquid drops, wires, and flms to greatly increase the gas–liquid contact area.

Figure 2 Effect of high-gravity factorβon the NB degradation by O3/H2O2

Moreover, the liquid suffered from high-frequency impact in the complex packing, thus causing the quick renewal of the gas-liquid interface and accelerating the mass transfer rate of ozone from the gaseous phase to the liquid phase. Figure 3 shows the effect of the highgravity factor on the liquid-phase ozone concentration in the ozone experiment. Furthermore, the comparison between the liquid-phase ozone concentration in the packed bed (PB, in which the high gravity factor was equal to 0) and the RPB indicated that the concentration of the liquid-phase ozone increased significantly with an increasing high-gravity factor. When the highgravity factor increased from 0 to 108.6, the concentration of the liquid-phase ozone increased from 0.84 to 2.54 mg/L. When the high-gravity factor reached above 83.2, the slow trend of increase in the concen-tration of the liquid-phase ozone became consistent with the results of the correlation experiment on the high-gravity factor and the NB removal rate. Thus, the ozone dissolution rate in the high-gravity fields was increased to improve the ozone dissolution in a unit of time and then was combined with the hydrogen peroxide contained in the wastewater to generate more hydroxyl radicals (·OH). The high ozone dissolution rate enhanced the oxidation and degradation of NB to improve the NB removal rate.

Figure 3 Effect of high-gravity factorβon the ozone concentration

3.2 Effect of coexisting substance on NB degradation in the high-gravity fi eld

The NB removal rate can be effectively improved by increasing the mass transfer rate of ozone from the gaseous phase to liquid phase and the concentration of the liquidphase ozone. However, many other substances exist in actual wastewater besides NB. These substances can usually infuence the mass transfer of ozone and ozonolysis reaction. During the NB degradation reaction with the RPB-O3/H2O2process, some common inorganic sodium salts and organics were added. Thereafter, their effects on O3/H2O2ability to degrade NB in the high-gravity field were investigated. Earlier studies found that the process of degrading NB by O3/H2O2was in accordance with the pseudo-first order reaction kinetics either in the highgravity feld or in the low gravity feld:

In Formulas (2) and (3),kandkblankrepresent the reaction rate constant of NB in the high-gravity feld with or without the addition of coexisting substances (min?1), respectively. Without the addition of coexisting substances, the linear equation of NB degradation obtained from the RPB-O3/ H2O2process isy=0.062 83x-0.022 57, withR2=0.998. Figure 4 shows the effect of the RPB-O3/H2O2process on the degradation kinetics of NB with the addition of coexisting substances. If the enhancement factorAis higher than 1 (A> 1), these substances can promote NB degradation; otherwise, it can inhibit degradation (Figure 5). Figure 4 and Figure 5 show that the addition of Na2CO3, NaOH, Na3PO4, Na2SO4, and NaNO3facilitated the NB removal (Figure 4(a)), whereas the addition of NaCl, NaHCO3, NaHSO4, C2H5OH, CH3COOH, HCOOH, and TBA hindered the NB removal (Figure 4(b)).

Figure 4 Relationship between ln(C0/Ct) andt

Figure 5 Effect of coexisting substances on the enhancement factor (A)

3.2.1 Facilitation of NB removal by coexisting substances

Figure 6 shows that the addition of Na2CO3, NaOH, Na3PO4, Na2SO4, and NaNO3in the RPB-O3/H2O2process facilitated the NB removal. Moreover, NaOH showed a highestfacilitation ability. The effects of different pH conditions on the NB removal rate were discussed. Figure 7 shows that the ability of the RPB-O3/H2O2process to degrade NB improved signifcantly under alkaline conditions. H2O2, as a weak binary acid, could ionize gradually to generate H+and. Increasing the pH value was benefcial to ionization of H2O2and generation of. OH-could react with O3to generate. In the free radical chain reaction of O3/H2O2system,was the primary accelerant of ozonolysis and ·OH generation. Therefore, an increase in the pH value was benefcial to ozonolysis and the generation of strong oxidizing ·OH, thus improving the oxidation effciency. The reaction mechanism[26]is shown in Formulas (4)–(6) .

Figure 6 Effects of coexisting substances on the facilitation of NB removal rate by RPB3/H2O2

Figure 7 Effect of pH value on NB degradation by RPB-O3/H2O2

Carbonate is a common inorganic ion detected in natural water and is generally used as a radical scavenger in the advanced ozone oxidation process in order to directly inspect the oxidation process. Formulas (7) and (8) are the relevant equations of this process relating to its reaction with ·OH and the speed constant[27]. Research results indicate that in the presence of carbonate ions, the pollutant degradation in the advanced ozone oxidation process is inhibited[28]. Formulas (7) and (8) show that the addition of carbonate can result in a competing reaction with NB. The consumption of ·OH decreases the NB removal rate. However, this experiment shows that, in a high-gravity feld, the addition of a small amount of Na2CO3can actually accelerate the NB degradation, which is similar to the result of Lin, et al[13]. A comparison between the effects of different concentrations of Na2CO3on the NB removal rate in high gravity and normal gravity fields (PB) is shown in Figure 8.

Figure 8 Effect of Na2CO3concentration on the NB removal rate

Figure 8 shows that the NB removal rate in the high-gravity field at first increased and then gradually decreased with a rising Na2CO3concentration. However, in a normal gravity feld, the NB removal rate decreased with an increasing Na2CO3concentration. The effect of the highgravity factor on the liquid-phase ozone concentration (Figure 3) shows that, in a high-gravity field, the mass transfer rate of the ozone increased significantly and a large amount of ozone entered the liquid phase within a short time. However, the concentrations of OH-and HO2-(generated from the ionized H2O2) were not high enough to react with all the water-soluble ozone molecules togenerate ·OH. Many ozone molecules in the solution failed to be converted into strong oxidizing ·OH radicals, and the reaction rate constant between the ozone and NB was only 0.09 L/(mol·s)[29]. Moreover, the reaction rate was slow. Na2CO3was a type of basic salt that could increase the pH value even in small amounts (with the pH value of the NB solution increasing from 7.5 to 8.6). The ionization of H2O2was then accelerated and a large number ofradicals were formed. Thus, O3was decomposed to generate ·OH (Formulas (4)—(6)).

Moreover, the state of NB dissociation was improved with an increasing pH value, and the reaction rate among dissociated organics, O3, and ·OH species was higher than undissociated organics[27]. Therefore, the NB removal rate was effectively increased. Although the carbonate ions would compete with NB in consumption of·OH radicals when the concentration of Na2CO3was <15 mmol/L, the facilitating effect was actually greater than the inhibiting effect which could accelerate the removal of NB. When the concentration of Na2CO3was >15 mmol/L, the inhibiting effect was greater than the facilitating effect, resulting in a decreased NB removal rate, and the degradation process was hindered. The experiment on absorption of ozone in water (Figure 3) indicates that the mass transfer rate of ozone was slower in the normal gravity feld than in the high-gravity field. Under this condition, the volume ofgenerated by OH-and H2O2ionization in the solution was large enough to decompose the liquid-phase O3rapidly and maintain the ozone concentration in the solution at a low level. At this stage, the addition of Na2CO3rather than the signifcant facilitation of ozonolysis and the generation of ·OH radicals would consumes a large number of ·OH radicals. Therefore, this process decreased the NB removal rate and its effect was generally inhibited.

The addition of Na3PO4could facilitate the O3/H2O2process and remove NB in the high-gravity feld because Na3PO4increased the pH value of the solution, and accelerated the decomposition of O3to generate ·OH. On the other hand, the phosphate radical, the accelerant of O3decomposition, and the generation of ·OH could improve the performance of RPB for NB removal. The action principle of additive NaNO3for facilitating the O3/H2O2process might be the result of the direct and indirect reactions of the promoted ozone[30]. Thus, the performance of the process for NB removal was improved. However, the additive Na2SO4did not infuence the O3/H2O2process.

The analysis of the effect of adding Na2CO3, NaOH, Na3PO4in the RPB-O3/H2O2process on the NB removal rate revealed that the pH value of the water sample could signifcantly infuence the NB degradation by the O3/H2O2process. These additives (Na2CO3, NaOH) increased the pH value of the water sample, accelerated the ozonolysis, generated the strong oxidizing ·OH radicals, and improved the oxidation effciency.

3.2.2 Inhibiting effect of coexisting substances

The addition of NaCl, NaHCO3, NaHSO4, C2H5OH, CH3COOH, HCOOH, and TBA in the RPB-O3/H2O2process exerted an inhibiting effort on NB removal (Figure 9). When the reaction rate between ·OH and the coexisting substance was greater than the reaction rate of NB, a thorough inhibiting effect appeared. The reaction rate of CH3COOH[31]with NB differed insignificantly from the reaction rate of ·OH with NB (Formulas (7) and (9)). A small volume of added CH3COOH consumed a part of ·OH radicals in the course of competition with NB, thus decreasing the NB removal rate. The NB removal rate would decrease to some extent after adding a small volume of NaCl, because chloridion Cl was a scavenger of ·OH[32]. Furthermore, the ·OH concentration was reduced, which could infuence the NB removal rate.

Figure 9 Effects of coexisting substances on the NB removal rate by RPB-O3/H2O2

Figure 10 shows the effect of adding NaHSO4, CH3COOH, and HCOOH on the solution pH value; these substances reduced the initial pH value of the solution from the initial value of 7.5 to 6.4, 4.3, and 3.1, respectively. The effectof pH value on the experiment showed that the acidic environment was not conducive to either ozonolysis or ·OH generation. Moreover, as the intermediate products of NB degradation, CH3COOH and HCOOH hindered the NB degradation[33].

As the ·OH trapping agents, TBA, and NaHCO3could greatly consume the ·OH radicals generated from ozonolysis, interrupt the free radical chain reaction, and reduce the performance of RPB for nitrobenzene removal[27](Formulas (10) and (11)).

Figure 10 Effects of coexisting substances on the initial pH value

By analyzing the effect of adding NaCl, NaHCO3, NaHSO4, C2H5OH, CH3COOH, HCOOH, and TBA in the RPB-O3/H2O2process on the NB removal, the inhibiting effect can be divided into two cases, namely: (1) the additives such as NaHSO4, CH3COOH, and HCOOH can decrease the solution pH value and infuence H2O2ionization and ozonolysis; and (2) the additives such as NaCl, NaHCO3, C2H5OH, and TBA can compete with NB and consume ·OH radicals .

In summary, the degradation of nitrobenzene with the O3/H2O2oxidation technology in the high-gravity field is a complex process. However, the experiments which were simplifed by adding specifc inorganic sodium salt or low molecular weight organic substance into the stimulated wastewater could not thoroughly reflect the actual processes involving multiple factors. Therefore, further strengthening the study on the mechanism and effciency of coexisting substances in order to accelerate ozonation in the actual wastewater environment is necessary because relevant theoretical support can be provided in the future discussion about the contribution rate of direct and indirect ozone oxidation in high-gravity felds.

4 Conclusions

1）During the RPB-O3/H2O2oxidation process, the highgravity technology intensifed the ozone mass transfer, increased the ozone dissolution within a unit time, strengthened the ozonolysis reaction, and improved the rate of nitrobenzene removal.

2）The addition of Na2CO3, NaOH, Na3PO4, and NaNO3into the RPB-O3/H2O2process could facilitate the NB removal. The major facilitating mechanism was to improve oxidation effciency by increasing the pH value of the solution, promoting ozonolysis, and generating the ·OH chain reaction. The addition of NaCl, NaHCO3, NaHSO4, C2H5OH, CH3COOH, HCOOH, and TBA inhibited the NB removal. Besides, the addition of NaHCO3, CH3COOH, and HCOOH could significantly reduce the solution pH value and hindered the free radical chain reaction. NaCl, NaHCO3, C2H5OH, and TBA were considered as the·OH depleting agents which would compete with the NB removal.

3）In the high gravity and the normal gravity felds, when the Na2CO3concentration was <15 mmol/L, the experiment of the O3/H2O2process in degrading NB showed different results. Thus, as compared with the normal gravity fields, the high-gravity fields could strengthen the mass transfer rate of the ozone and increase the ozone concentration in water. After Na2CO3was added, the increase in the solution pH value accelerated the H2O2ionization, generated, and promoted ozonolysis to generate ·OH radicals. The facilitating effect is stronger than the inhibiting effect, as evidenced by the increase in the NB removal rate. When the Na2CO3concentration was >15 mmol/L, the inhibiting effect was stronger than the facilitating effect.

Acknowledgements: This work was supported by the Natural Science Foundation of China (21206153,U1610106), the Excellent Youth Science and Technology Foundation of Province Shanxi of China (2014021007), and the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (201316).

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Received date: 2016-05-10; Accepted date: 2016-09-12.

Dr. Jiao Weizhou, Telephone: +86-35-13921986; Fax: +86-351-3921497; E-mail:jwz0306@126.com.