Catalytic Ozonation of Succinic Acid in Aqueous Solution by Co/Al 2 O 3 -EPM: Performance,Characteristics,and Reaction Mechanism

Abstract

In this study, the high-reactive Co/Al₂O₃-EPM (prepared by electroless plating method) were used for catalytic ozonation. On account of the powerless degradation of small molecule acid by ozone alone, succinic acid (SA), a kind of small molecule acid, was selected as the simulated pollutant to evaluate the performance of Co/Al₂O₃-EPM in Co/Al₂O₃-EPM/O₃ system. First, several key preparation parameters of Co/Al₂O₃-EPM and operational parameters in Co/Al₂O₃-EPM/O₃ system for SA removal were optimized. The maximum SA removal (100%) and the total organic carbon (TOC) removal (66.9%) were obtained under the optimal conditions. Second, the superior performance of Co/Al₂O₃-EPM/O₃ system for SA removal was confirmed by four control experiments (i.e., Co/Al₂O₃-EPM alone, O₃ alone, Co/Al₂O₃-IM alone, and Co/Al₂O₃-IM/O₃ systems). The conversion and mineralization of SA were investigated by high-performance liquid chromatography and TOC analyzer, respectively. The stability of different systems was evaluated by cobalt leaching in the effluent. Besides, X-ray diffraction, Fourier transform infrared, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller were used to analyze the characteristics of Co/Al₂O₃-EPM and Co/Al₂O₃-IM (prepared via impregnation method). The results show that Co/Al₂O₃-EPM had a more uniform and dense cobalt film than Co/Al₂O₃-IM. In addition, in the Co/Al₂O₃-EPM/O₃ system, quenching experiments were conducted, and the dynamic concentration of HO^• in the bulk solution was detected. Finally, a possible reaction mechanism for SA degradation in Co/Al₂O₃-EPM/O₃ system was proposed. In brief, all results suggest that Co/Al₂O₃-EPM is a promising catalyst with high catalytic activity and strong stability for ozone decomposition.

Introduction

For decades, advanced oxidation processes (AOPs) have been admittedly identified as one of the most efficient methods for mineralization of refractory pollutant in water treatment (Boczkaj and Fernandes, 2017). AOPs include Fenton-based reactions, radiation, sonolysis, photolysis, photocatalysis, ozonation processes, and electrochemical oxidation (Kusic et al., 2006; Wang and Xu, 2012; Cheng et al., 2016; Moreira et al., 2017). Among them, the research of ozonation in wastewater treatment has attracted more and more researchers' attention due to the strong oxidizing property of ozone (Gomes et al., 2017).

Nevertheless, the degradation efficiency of organics by ozone alone is not satisfactory because ozone itself can hardly degrade refractory contaminants completely. If catalysts were added in ozonation processes, ozone would be decomposed to produce free radicals such as HO^•, which has a stronger oxidizing property and can improve the efficiency of ozone oxidation (Kasprzyk-Hordern et al., 2003). Although homogeneous catalysis and heterogeneous catalysis of ozone can both greatly improve ozonation processes, homogeneous catalytic ozonation is limited because of secondary pollution to the environment (Álvarez et al., 2007).

Therefore, heterogeneous catalytic ozonation, on account of the segregative characteristic, has caught much more attention in industrial wastewater treatment. Many studies of heterogeneous catalytic ozonation have been conducted in recent years. So far, there are a lot of effective solid catalysts used in ozonation system. Especially, metal supported catalysts for ozonation become more popular among researchers (Kasprzyk-Hordern et al., 2003).

In previous studies, most researchers used the impregnation method (IM), deposition method, melt method, ion exchange method, and other methods to prepare a variety of loading type catalysts, aiming to achieve high removal efficiency in industrial wastewater (Beltrán et al., 2002; Kowalczyk et al., 2017; Majeed et al., 2017; Sanjana et al., 2017). Among them, IM is one of the most popular methods due to its simple operation. However, in some cases, the catalysts produced by IM show bad repeatability and rapid deactivation due to unstable properties (Trovarelli, 2002). How to improve catalyst catalytic activity and stability are intricate problems.

Electroless plating method (EPM), also known as autocatalytic plating, is a plating technology without impressed current (Ren and Lai, 2016). Namely, several redox reactions carry through in the aqueous solution without the external electrical power (Guo et al., 2017). The process depends on the presence of reducing agents. For example, sodium hypophosphite monohydrate (NaH₂PO₂·H₂O), as a reducing agent, can deposit metal from the bath onto support surface (Aal et al., 2008; Wang et al., 2011; Islam et al., 2012). Although this method has been widely used in aerospace and automotive industry (Tsai et al., 2007) and printed circuit board industry (Wu and Sha, 2009) owing to its strong hardness of surface, strong corrosion resistance, and high bonding force between the plating film and support substrates, few studies evaluate the performance of catalyst prepared by EPM in wastewater treatment.

Cobalt oxides are p-type oxides with the ability to adsorb oxygen and yield electron-rich adsorbed species (i.e., −O⁻ and −O²⁻) (Dhandapani and Oyama, 1997). In some literatures, researchers have confirmed that cobalt-based catalyst is an appropriate material for catalytic ozonation. For example, Álvarez et al. (2007) found Co/Al₂O₃ catalyst prepared by IM can accelerate the decomposition of ozone and largely increase the removal rate of pyruvate. Besides, Xu et al. (2016) studied degradation of bezafibrate by catalytic ozonation system and found cobalt doped red mud catalyst (Co/RM) has an excellent capacity to aid ozone generate HO^•.

In this study, to overcome the defects of catalysts prepared by IM, EPM, a relative new way, was used to synthesize Co/Al₂O₃ (Co/Al₂O₃-EPM) for heterogeneous catalytic ozonation. Further, succinic acid (SA), a raw material being difficult to be degraded by ozone alone, was selected as the model pollutant to evaluate the activity of the Co/Al₂O₃-EPM in Co/Al₂O₃-EPM/O₃ system. The main aims of this work were to (i) optimize several preparation parameters of Co/Al₂O₃-EPM and key operational parameters in Co/Al₂O₃-EPM/O₃ system; (ii) investigate the different properties between Co/Al₂O₃-EPM and Co/Al₂O₃-IM (Co/Al₂O₃ prepared by IM) and prove superior performance of Co/Al₂O₃-EPM for the decomposition of ozone; and (iii) investigate the reaction mechanism for SA removal in Co/Al₂O₃-EPM/O₃ system.

Materials and Methods

Reagents

Details about the main reagents are presented in the Supplementary Data.

Catalyst preparation

Two kinds of Co/Al₂O₃ samples were prepared by different synthetic methods, that is, the new electroless plating method and the conventional IM.

(i) electroless plating method: Al₂O₃ particle and cobalt sulfate heptahydrate (CoSO₄·7H₂O) were the main raw materials for the formation of Co/Al₂O₃. The particle size of Al₂O₃ was about 50 mm. Before plating, chloride sensitization and palladium chloride activation pretreatments must be done for the formation of active sites and ensure that the extra-thin cobalt film successfully loads on the Al₂O₃ (Qi et al., 2017). The sensitization, activation, and plating baths formulas are summed up in Supplementary Table S1. The concrete operating procedures were described in our previous study (Ren and Lai, 2016). The cobalt concentration in plating bath before and after use was detected by inductively coupled plasma-mass spectroscopy (ICP-MS), respectively. Finally, the obtained concentration difference in plating bath before and after use was the amount of cobalt loaded on the Al₂O₃. The samples were separated from plating baths and then were calcined by muffle furnace with desired temperature (300–1,200°C) and time (0.5–4.0 hours).

(ii) IM: Al₂O₃ particle and cobalt nitrate salt (Co (NO₃)₂) were the main raw materials for the formation of Co/Al₂O₃. This method involved the addition of known amounts of Al₂O₃ to Co (NO₃)₂ solution (the same amount of cobalt which loads on Al₂O₃ by EPM) and then stirred by agitation (300 rpm) for 24 h (Cooper and Burch, 1999). Then, the impregnated Al₂O₃ particle was filtered and dried at 105°C ± 1°C before it was calcined by muffle furnace (the calcination temperature and time were same as Co/Al₂O₃-EPM).

To distinguish the two kinds of catalyst, the nomenclature is used as follows: X°C-Co/Al₂O₃-Y, where X represents the calcination temperature and Y stands for the synthetic method. Therefore, Co/Al₂O₃-EPM is the catalyst prepared by electroless plating method, while Co/Al₂O₃-IM is the catalyst prepared by IM.

Experiment procedure

SA powder was simply dissolved in deionized water to prepare SA stock solution (200 mg/L). The whole reaction time is 120 minutes. For each run, 300 mL of SA solution was poured into t500 mL glass beaker. The catalytic ozonation reaction was initiated by the addition of catalysts and ozone. The ozone was produced by the ozone generator, and its flow rate was controlled by the rotor flow meter. During the reaction, the solution was stirred continuously at 300 rpm by a mechanical stirrer, and temperature inside the reactor was kept at 20°C ± 1°C by water batch heating. Then, the samples, taken out and filtered through a PTFE syringe filter disc (0.45 μm) at predetermined intervals, were analyzed by high-performance liquid chromatography (HPLC), total organic carbon (TOC) analyzer and ICP-MS. Necessarily, excess tert-butyl alcohol (TBA) and sodium thiosulfate (NaS₂O₃) were added into samples to scavenge HO^• and residual O₃.

The experiment included three aspects: first, to investigate the performance of Co/Al₂O₃-EPM for ozonolysis, several preparation parameters (i.e., calcination temperature, calcination time, and cobalt loads) of Co/Al₂O₃-EPM were optimized. Then, some significant operational parameters (i.e., initial pH value, ozone flow rate and Co/Al₂O₃ dosage) on SA removal in Co/Al₂O₃-EPM/O₃ system were also optimized. Besides, four control experiments with Co/Al₂O₃-EPM alone, O₃ alone, Co/Al₂O₃-IM alone, and Co/Al₂O₃-IM/O₃ systems were set up. The SA removal, TOC removal and cobalt leaching in different systems were detected respectively to confirm the superiority of Co/Al₂O₃-EPM/O₃ system.

Also, characterizations of Co/Al₂O₃-EPM and Co/Al₂O₃-IM were evaluated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET), respectively. In addition, the quenching tests were conducted, and the dynamic amount of HO^• was quantified in Co/Al₂O₃-EPM/O₃ system.

Analytical method

The detail analytical methods in this study are presented in the Supplementary Data.

Quantification of HO^• in aqueous solution

The dynamic concentration of HO^• in aqueous solution was detected by HPLC, which was explained in previous literatures (Lindsey and Tarr, 2000; Sung et al., 2006; Wang et al., 2014). In this study, excess (1 g/L) benzoic acid (BA) was added into deionized water, and then appropriate amount of catalyst and O₃ was injected to initiate the reaction. At predetermined intervals, 2-mL samples were taken out and filter through a PTFE syringe filter disc (0.45 μm). Meanwhile, TBA and NaS₂O₃ were immediately added into sample to scavenge free radical and O₃. In literature, it was reported that HO^• can rapidly react with BA (K_HO^•_+BA = 4.2 × 10⁹ M⁻¹ s⁻¹) and form p-hydroxybenzoic acid (p-HBA) (Lindsey and Tarr, 2000). The p-HBA can be used as a probe for HO^• because per mole p-HBA is corresponding with 5.8 mole HO^• (Lindsey and Tarr, 2000).

The dynamic changes of p-HBA in the reaction process were analyzed by reversed-phase HPLC chromatography system (Agilent USA) equipped with the Eclipse XDB C-18 (5 μm, 4.6 × 250 mm) column and an absorbance detection system set at 265 nm. The binary phases were (A) water with 0.1% H₃PO₄ and (B) acetonitrile, and the eluent was A and B (60:40, v/v) with a flow rate of 1.0 mL/min. The concentration of p-HBA during the treatment process can be calculated out by standard curve of p-HBA (not shown). Finally, the concentration of HO^• in the reaction process can be obtained according to stoichiometric ratio.

Results and Discussion

Optimization of preparation parameters

To optimize the preparation parameters of Co/Al₂O₃-EPM and investigate its performance in Co/Al₂O₃-EPM/O₃ system, three single-factor experiments about preparation parameters were carried out. The SA removal and cobalt leaching were used to evaluate the performance of Co/Al₂O₃-EPM. Also, XRD and Fourier transform infrared (FTIR) were used to identify the active species on the Co/Al₂O₃-EPM.

Effects of the calcination temperature

Effect of calcination temperature on the performance of Co/Al₂O₃-EPM was evaluated in Co/Al₂O₃-EPM/O₃ system. Figure 1a shows that the SA removal efficiency is reduced (from 100% to 18.0%) with the increase of calcination temperature (from 300°C to 1,200°C). Specifically, SA can be completely removed only when the calcination temperature is comparatively low (i.e., below 800°C). However, the SA removal efficiency is obviously decreased (53.9%, 32.4% and 18.0%, respectively) when the calcination temperature is above 800°C (1,000°C, 1,100°C, and 1,200°C, respectively). In literature, it was reported that different calcination temperature may lead to the formation of different active species on Co/Al₂O₃ (Kasprzyk-Hordern et al., 2003). Therefore, XRD and FTIR were used to identify the active species of Co/Al₂O₃-EPM at different calcination temperature.

FIG. 1.

Effects of (a) calcination temperature, (b) calcination time, (c) cobalt loads on the catalytic activity of Co/Al₂O₃-EPM; (d) Effects of cobalt loads on absorption and catalytic activity of Co/Al₂O₃-EPM. ([SA]₀ = 200 mg/L, initial pH 3.6, ozone flow rate = 300 mL/min and Co/Al₂O₃-EPM dosage = 10 g/L). EPM, electroless plating method.

Supplementary Figure S1a shows that all the Co/Al₂O₃-EPM catalysts have the same compounds composition (including Al₂O₃, Co₃O₄ and CoAl₂O₄) except 1,200°C-Co/Al₂O₃-EPM. No obvious characteristic diffraction peaks of Co₃O₄ and CoAl₂O₄ are detected in 1,200°C-Co/Al₂O₃-EPM. Presumably, the cobalt plated on the surface of Al₂O₃ drops off and the residual constituent is α-Al₂O₃ when the calcination temperature is 1,200°C. The similar result was also obtained by Horváth et al. (2017) who found that the cobalt loading decreases with the increase of Co/Al₂O₃ calcination temperature. Since the standard peaks of Co₃O₄ and CoAl₂O₄ have the almost same diffraction peak positions (Wang and Chen, 1991; Busca et al., 1992; Chokkaram et al., 1997), it is hard to distinguishing them through XRD patterns. Thus, FTIR was used to further discern Co₃O₄ and CoAl₂O₄ (Ji et al., 2000).

Supplementary Figure S1b shows the FTIR spectra of Co/Al₂O₃-EPM prepared at different calcination temperature (i.e., 300°C, 500°C, 800°C, 1,000°C, 1,200°C). All catalysts except the 1,200°C-Co/Al₂O₃-EPM display two strong distinct peaks in the scope of 700–500 cm⁻¹ wavenumber, which are the typical Co–O bond vibrations according to previous study (Ji et al., 2000). The peaks locating at around 665 and 573 cm⁻¹ are attributed to Co₃O₄ (Rygh et al., 2000). The peaks locating at 675 and 561 cm⁻¹ are attributed to CoAl₂O₄ (Busca et al., 1992) as well. The results suggest that Co₃O₄ is the active specie at the lower calcination temperature (300–800°C), while CoAl₂O₄ is the active specie at the higher calcination temperature (1,000°C). In literature, it is reported that the catalytic activity of Co₃O₄ is better than other cobalt-oxides matters in catalytic ozonation system (Álvarez et al., 2007).

Further, Supplementary Fig. S2a shows that cobalt leaching in effluent increases rapidly with the decreased calcination temperature. Specially, the extremely high cobalt leaching (91.62 mg/L) is found in 300°C-Co/Al₂O₃-EPM/O₃ system. Besides, when the calcination temperature is at 400°C, 500°C, and 600°C, the cobalt leaching was 5.75, 3.10, and 2.69 mg/L, respectively. Only calcination temperature ≥800°C, the cobalt leaching in effluent is below 1 mg/L. Besides, wastewater discharge standard of China regulates that the concentration of cobalt in the wastewater must be controlled in 1 mg/L (Wang et al., 2017). Combined the data about the SA removal and cobalt leaching in different Co/Al₂O₃-EPM/O₃ systems, 800°C should be selected as the optimal calcination temperature in the following experiments.

Effects of the calcination time

Calcination time of Co/Al₂O₃-EPM may influence the species of cobalt oxides. Therefore, effect of calcination time on the performance of Co/Al₂O₃-EPM was evaluated in Co/Al₂O₃-EPM/O₃ system. Figure 1b shows that SA removal ascends (from 33.1% to 100%) when the calcination time increases from 0.5 to 2.0 hours and then it descends (from 100% to 41.5%) with the further increased calcination time (from 2.0 to 4.0 hours). The different performance of Co/Al₂O₃-EPM may reveal that the species of cobalt oxides is vital in catalytic ozonation system.

The phenomenon may get a reasonable explanation as follows: (i) The short calcination time (<2.0 hours) leads to inadequate oxidation of cobalt, which may result in the formation of low-active components such as CoO. (ii) Overlong calcination time (>2.0 hours) causes cobalt layer to fall off from Al₂O₃ carrier because of different expansion coefficient between Co and Al₂O₃ (Thieme and Rüssel, 2014; Hirata et al., 2016). Only the appropriate calcination time (i.e., 2.0 hours) is in favor of the formation of Co₃O₄. Therefore, 2.0 hours was chosen as the optimal time of calcination in following experiments.

Effects of the cobalt loads

The content of cobalt oxide may effectively influence the Co/Al₂O₃ catalytic activity for ozonation. Therefore, the effect of cobalt loads on the performance of Co/Al₂O₃-EPM was evaluated in Co/Al₂O₃-EPM/O₃ system. The cobalt loads are controlled by the concentration of CoSO₄·7H₂O in the plating bath, which could be quantified by ICP-MS. When the concentration of CoSO₄·7H₂O changes, the cobalt loads on the Co/Al₂O₃-EPM will change. In this study, the concentration of CoSO₄·7H₂O in the plating bath varied from 2.8 to 25.2 g/L, which caused the cobalt loads on the Co/Al₂O₃-EPM to change from 2% to 18 wt%.

According to Fig. 1c, SA removal increases (from 67.6% to 100%) with the increase of cobalt loads (2% to 18 wt%). To be specific, the SA can be removed absolutely in 120 minutes reaction when the cobalt loads are above 10 wt%. It is worth noting that, regardless of cobalt loading, the removal rate of SA is very high in the early stage of reaction (about the first 15 minutes). In literature, it was reported that if catalysts had high adsorption capacity, the pollutants would be degraded rapidly in the early stage of the reaction (Munoz et al., 2016). Thus, the adsorption capacity of 800°C-Co/Al₂O₃-EPM cannot be neglected. The data of adsorption experiment are displayed in Fig. 1d, which suggest that the adsorption capacity of 800°C-Co/Al₂O₃-EPM drops (approximately attained 51.5%, 37.5%, 25.1%, 15.1%, 7.1%, and 3.1%, respectively) with increased content of cobalt (2, 6, 10, 12, 16, and 18 wt%). The results illustrate that cobalt layer wrapping on the Al₂O₃ may result in the decrease of pore sizes, which inhibits the adsorption capacity of 800°C-Co/Al₂O₃-EPM.

In addition, Supplementary Fig. S2b shows that cobalt leaching in the effluent is over 1 mg/L (2.73, 10.16, and 13.21 mg/L, respectively) when the cobalt load is above 10 wt% (12, 16, and 18 wt%, respectively). Although increase of cobalt loads would produce more catalytic sites for ozonolysis (Gruttadauria et al., 2007), the excessive cobalt loads might cause excessive dissolution of cobalt ions in aqueous solution. Thus, the homogeneous catalysis would be as a dominant way to degrade organic pollutant (Beltrán et al., 2003a). Further, high cobalt leaching in the effluent causes serious secondary pollution of environment. From the three aspects that have been discussed above, the 10 wt% cobalt loads of 800°C-Co/Al₂O₃-EPM was selected as the optimal metal loads in the following experiments.

Optimization of operational parameters

The crucial operating parameters in Co/Al₂O₃-EPM/O₃ system are also very vital for the SA degradation. Thus, effects of initial pH value, ozone flow rate, and 800°C-Co/Al₂O₃-EPM dosage on SA removal in 800°C-Co/Al₂O₃-EPM/O₃ system were investigated, respectively.

Effects of the initial pH value

Figure 2a shows the effect of initial pH (from 3.6 to 11.0) on SA removal in 800°C-Co/Al₂O₃-EPM/O₃ system. It is noted that the SA removal efficiency varies with the changes of initial pH values. SA removal efficiency attains 100% at the initial pH of 3.6 or 11.0. However, the SA removal efficiency only reaches 70.6%, 56.5%, and 80.1% at the initial pH of 5.0, 7.0, and 9.0, respectively. It is well known that ozonation reaction follows different mechanism at different pH value. In literature, it was reported that decomposition rate of ozone in water is accelerated with the increase of pH value (Staehelin and Hoigne, 1985; von Gunten, 2003; Jung et al., 2017). As shown in Equations (1 )–(3), HO^• can be produced largely under alkaline conditions to react with pollutants nonselectively (Westerhoff et al., 1997).

FIG. 2.

Effects of (a) initial pH value, (b) ozone flow rate (c) 800°C-Co/Al₂O₃-EPM dosage on SA removal in 800°C-Co/Al₂O₃-EPM/O₃ system ([SA]₀ = 200 mg/L, calcination temperature = 800°C, calcination time = 2.0 hours, and cobalt loads = 10 wt%). SA, succinic acid.

Further, pH value determines the surface properties of catalysts. As seen in Supplementary Fig. S3, the pH_zpc of 800°C-Co/Al₂O₃-EPM is about 7.2. The surface of catalyst is in neutral state at the pH ≈ pH_zpc. Besides, when the pH is not around the pH_zpc, the surface of 800°C-Co/Al₂O₃-EPM is protonated (pH < pH_zpc) or deprotonated (pH > pH_zpc) (Ikhlaq et al., 2012). Specifically, at alkali pH, the 800°C-Co/Al₂O₃-EPM is deprotonated, which could enhance the electrostatic interaction between 800°C-Co/Al₂O₃-EPM and O₃. Therefore, 800°C-Co/Al₂O₃-EPM/O₃ system shows the good efficiency for SA degradation at the initial pH of 11.0. Besides, at the initial pH of 3.6, the 800°C-Co/Al₂O₃-EPM is protonated and the carboxyl group in SA is negatively charged. Thus, SA can be largely adsorbed on the surface of 800°C-Co/Al₂O₃-EPM and then further to be degraded.

Further, Supplementary Fig. S4a shows cobalt leaching in effluent at different initial pH value. The cobalt leaching is all below 1 mg/L (0.91, 0.66, 0.55, 0.39, and 0.21 mg/L, respectively) with the variation of initial pH value. Although the 800°C-Co/Al₂O₃-EPM/O₃ system shows the best performance on SA removal at the initial pH of 11.0, the reagent cost is high because of pH adjustment. In addition, the pH value in effluent is close to neutral condition (about 6.2) when initial pH is not adjusted (i.e., initial pH 3.6). Therefore, the initial pH at 3.6 was selected as the optimal initial pH in the following experiments. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} {{ \rm{O}}_3} + {{ \rm{H}}_2}{ \rm{O}} \to 2{ \rm{H}}{{ \rm{O}} ^\cdot } + {{ \rm{O}}_2} , k = 1.1 \times {10^{ - 4}} \;{{ \rm{M}}^{ - 1}} {{ \rm{s}}^{ - 1}} \tag{1} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} {{ \rm{O}}_3} + { \rm{O}}{{ \rm{H}}^ - } \to {{ \rm{O}}_2}{ ^\cdot - } + { \rm{H}}{{ \rm{O}}_2} ^\cdot , k = 70 \;{{ \rm{M}}^{ - 1}} {{ \rm{s}}^{ - 1}} \tag{2} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { { \rm { O } } _3 } \; + { \rm { H } } { { \rm { O } } _2 } ^\cdot \mathbin \leftrightarrows 2 { { \rm { O } } _2 } + { \rm { H } } { { \rm { O } } ^\cdot } , k = 1.6 \times { 10^9 } \; { { \rm { M } } ^ { - 1 } } { { \rm { s } } ^ { - 1 } } \tag { 3 } \end{align*} \end{document}

Effects of ozone flow rate

The effect of ozone flow rate on SA removal is depicted in Fig. 2b. The removal of SA increases from 75.8% to 100.0% by increasing ozone flow rate from 100 to 300 mL/min. However, SA degradation efficiency almost stays unchanged when ozone flow rate further increases (>300 mL/min). The phenomenon suggests that mass transfer rate of ozone in aqueous solution rises with the increase of ozone concentration when the ozone flow rate is below 300 mL/min, which ultimately facilitates catalytic ozonation process.

Similar results were also reported by Beltran et al. (2002) who found that oxalic acid degradation increases with the ozone concentration increases from 7 to 55 mg/L in TiO₂ catalytic ozonation system. Whereas, Pan et al. (2012) deem that ozone concentration in the liquid phase will approach its maximum value, if ozone flow rate reaches a certain point. Thus, in this study, ozone concentration in the water may reach the saturation point at the 300 mL/min ozone flow rate. Therefore, 300 mL/min was selected as the optimal ozone flow rate in the following experiments.

Effects of the 800°C-Co/Al₂O₃-EPM dosage

The effect of 800°C-Co/Al₂O₃-EPM dosage on SA removal is depicted in Fig. 2c. It can be seen that SA removal rises from 41.0% to 100.0% with the increased 800°C-Co/Al₂O₃-EPM dosage (from 2.5 to 5.0 g/L). The result indicates that active sites increase with the increased catalyst dosage. However, SA removal decreases rapidly when 800°C-Co/Al₂O₃-EPM further increases from 5.0 to 20 g/L. In literatures, it was found that volumetric liquid side mass transfer coefficient (k_La) will decline with the increased catalyst dose (Ferreira et al., 2010). Therefore, gas–liquid mass transfer may be affected by the amount of 800°C-Co/Al₂O₃-EPM, thereby inhibiting the reaction between HO^• and SA. Also, the increased 800°C-Co/Al₂O₃-EPM may generate excessive HO^•, which will either recombine to create peroxide [Eq. (4)] or consume ozone to produce HO₂^• [Eq. (5)] (Ahmadi et al., 2017).

Moreover, Supplementary Fig. S4b shows that the cobalt leaching increases (from 0.51 to 3.56 mg/L) with the increased 800°C-Co/Al₂O₃-EPM dosage (from 2.5 to 20.0 mg/L). It can be explained that sufficient contact between 800°C-Co/Al₂O₃-EPM and SA make the corrosion of 800°C-Co/Al₂O₃-EPM more serious. Beltrán et al. (2003b) found the similar result in their previous study. In summary, considering the maximum removal efficiency of SA and the cobalt leaching in the solution, 5.0 g/L of catalyst was selected as the optimum dosage and used in all the following experiments. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{H}}{{ \rm{O}} ^\cdot } + { \rm{H}}{{ \rm{O}} ^\cdot } \to {{ \rm{H}}_2}{{ \rm{O}}_2} \tag{4} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{H}}{{ \rm{O}} ^\cdot } + {{ \rm{O}}_3} \to {{ \rm{O}}_2} + { \rm{H}}{{ \rm{O}}_2} ^\cdot \tag{5} \end{align*} \end{document}

Control experiment

To validate the better performance of 800°C-Co/Al₂O₃-EPM/O₃ system, four control experiments (i.e., 800°C-Co/Al₂O₃-EPM alone, O₃ alone, 800°C-Co/Al₂O₃-IM alone, and 800°C-Co/Al₂O₃-IM/O₃ systems) under optimal conditions were set up.

Figure 3a shows that SA removal obtained in 800°C-Co/Al₂O₃-EPM/O₃ system (100% after 120 minutes treatment) is much higher than other systems. However, at the early stage of the reaction, higher SA removal rate is obtained by 800°C-Co/Al₂O₃-IM/O₃ system. This result can be illustrated as that the adsorption capacity of 800°C-Co/Al₂O₃-IM is better than 800°C-Co/Al₂O₃-EPM. The data of BET surface area, pore volume, and average pore of 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM were depicted in Supplementary Table S2, respectively. It is clear that the pore size of 800°C-Co/Al₂O₃-IM (8.43 nm) is bigger than the 800°C-Co/Al₂O₃-EPM (8.18 nm). Similar result was found by Xue et al. (2017) who found that catalysts with larger pore size could promote the adsorption of thiophene compounds. Further, it is clear that only 5.5% SA was removed in O₃ alone system.

FIG. 3.

Effect of different systems on (a) SA removal and (b) TOC removal ([SA]₀ = 200 mg/L, calcination temperature = 800°C, calcination time = 2.0 hours, cobalt loads = 10 wt%, ozone flow rate = 300 mL/min, initial pH 3.6 and catalyst dosage = 5 g/L). TOC, total organic carbon.

All the phenomena suggest that small molecule acid can hardly be removed by ozone alone. Besides, the catalytic capacity of 800°C-Co/Al₂O₃-EPM for catalytic ozonation is much better than that of 800°C-Co/Al₂O₃-IM.

Figure 3b reveals TOC removal in 800°C-Co/Al₂O₃-EPM/O₃ and 800°C-Co/Al₂O₃-IM/O₃ system after 60 and 120 minutes reaction, respectively. The mineralization efficiency of SA in different system follows the same trend as SA removal. In the 800°C-Co/Al₂O₃-EPM/O₃ system, TOC removal (53.5% and 66.9%) is higher than that of the 800°C-Co/Al₂O₃-IM/O₃ system (35.5% and 41.8%). However, the mineralization degree is lower than SA removal, indicating that there may be the formation of some intermediates. However, in this study, we failed to identify the SA degradation intermediates by HPLC-MS. It could be investigated carefully in the further study. In addition, cobalt leaching (5.67 mg/L) in the effluent of 800°C-Co/Al₂O₃-IM/O₃ system is seven times as much as that (0.80 mg/L) of 800°C-Co/Al₂O₃-EPM/O₃ system.

Above all, all the results suggest that stability and catalytic activity of 800°C-Co/Al₂O₃-EPM is much better than 800°C-Co/Al₂O₃-IM.

Characterization of two kinds of Co/Al₂O₃ catalysts

To further investigate the active species, surface morphology, and elementary composition of 800°C-Co/Al2O3-EPM and 800°C-Co/Al2O3-IM, the XRD, XPS, FTIR, SEM and EDS of two kinds of catalysts were detected, comparatively.

XRD, FTIR, and XPS analysis

The above experimental results show that different cobalt-based compound causes different catalytic activity and stability of Co/Al₂O₃-EPM. For the further contrastive study on the performance of 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM, cobalt forms on the catalyst carrier were detected by XRD, FTIR, XPS.

Figure 4a shows the XRD patterns of the 800°C-Co/Al₂O₃-EPM and the 800°C-Co/Al₂O₃-IM. The results indicate the two catalysts have a similar diffraction peak, which corresponds to CoAl₂O₄ or Co₃O₄ (Wang and Chen, 1991; Busca et al., 1992; Chokkaram et al., 1997). The crystallite size of Co₃O₄ or CoAl₂O₄ can be calculated via the Scherrer equation [Eq. (6)], where D represents the crystallite size, β is the full width at half maximum of the diffraction peak, K is a constant (0.89), θ is the Bragg angle, and λ is the X-ray wavelength (0.15418 nm).

FIG. 4.

(a) XRD patterns of 800°C-Co/Al₂O₃-IM and 800°C-Co/Al₂O₃-EPM; (b) FTIR of 800°C-Co/Al₂O₃-IM and 800°C-Co/Al₂O₃-EPM (I) represents 800°C-Co/Al₂O₃-IM and (II) 800°C-Co/Al₂O₃-EPM; calcination time = 2 hours and cobalt loads = 10 wt%). FTIR, Fourier transform infrared; XRD, X-ray diffraction.

After calculation, the crystallite size of active species on the 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM is 18.1 and 20.2 nm, respectively. In literature, it was reported that smaller crystallite size of active species may have better catalytic activity compared to larger crystallite size of catalyst (Manjunathan et al., 2015). Therefore, 800°C-Co/Al₂O₃-EPM may have better performance than 800°C-Co/Al₂O₃-IM. Because it is hard to distinguishing CoAl₂O₄ and Co₃O₄ through XRD patterns, 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM were further detected by FTIR. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} D = { \frac { K \lambda } { \beta \cos \theta } } \tag { 6 } \end{align*} \end{document}

FTIR spectra of 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM are shown in Fig. 4b. In the scope of 700–500 cm⁻¹ wavenumber, 800°C-Co/Al₂O₃-IM catalyst has two strong peaks at 662 and 571 cm⁻¹ wavenumbers, while 800°C-Co/Al₂O₃-EPM catalyst has two strong peaks at 671 and 561 cm⁻¹ wavenumbers. According to the analysis in Effects of the Calcination Temperature section, it can be concluded that cobalt-based compound on 800°C-Co/Al₂O₃-EPM is Co₃O₄ and cobalt-based compound on 800°C-Co/Al₂O₃-IM is CoAl₂O₄. Therefore, the 800°C-Co/Al₂O₃-EPM has better catalytic activity than the 800°C-Co/Al₂O₃-IM.

Figure 5 displays the photoelectron spectra of Al 2p, Co 2p, and O 1s of two kinds of catalysts. The number of (i) and (ii) represents the 800°C-Co/Al₂O₃-EPM and the 800°C-Co/Al₂O₃-IM, respectively. The corresponding binding energies data of some Co-containing compounds found in literature are also presented in Supplementary Table S3 (Wang and Chen, 1991).

FIG. 5.

(a) Full-range scan of the samples, (b) Co 2p core level, (c) Al 2p core level and (d) O 1s core level. XPS spectra of (i) 800°C-Co/Al₂O₃-EPM and (ii) 800°C-Co/Al₂O₃-IM (cobalt loads = 10 wt% and calcination time = 2.0 hours; calcination time = 2 hours, and cobalt loads = 10 wt%). XPS, X-ray photoelectron spectroscopy.

As shown in Fig. 5, XPS analytical results of two kinds of catalysts on Al 2p, and O 1s binding energies are the same. Al 2p binding energy of 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM is 74.6 and 74.7 eV, respectively, indicating that Al is Al (III) species (Kasprzyk-Hordern et al., 2003). The peak at 529.7–530.1 eV corresponds to lattice oxygen indicating O 1s component at 530.1 eV is O²⁻ (Zsoldos and Guczi, 1993; Piumetti et al., 2015).

In addition, for 800°C-Co/Al₂O₃-EPM, the Co 2p_3/2 spectrum is at 780.6 eV, which proves that the surface cobalt oxide corresponds to Co₃O₄ or CoO, whereas, for 800°C-Co/Al₂O₃-IM, Co 2p_3/2 component is at 781.2 eV manifesting that the cobalt is largely presented as CoAl₂O₄ (Wang and Chen, 1991). Further, according to the Supplementary Table S3, the shake-up satellite peak of (i) is weak, indicating that the species on 800°C-Co/Al₂O₃-EPM is Co₃O₄ rather than CoO. The result is highly consistent with the previous studies, which reported that Co₃O₄ has a weak shake-up satellite peak, while CoO has a strong shake-up satellite peak (Kasprzyk-Hordern et al., 2003; Lv et al., 2012; Horváth et al., 2017). Therefore, under the same calcination time and cobalt loads, the catalytic activity of 800°C-Co/Al₂O₃-EPM is much better than that of 800°C-Co/Al₂O₃-EPM.

SEM analysis

Figure 6 shows the SEM images of Co/Al₂O₃-IM and Co/Al₂O₃-EPM. The different prepared methods have significant differences on Co/Al₂O₃ catalysts morphology. Figures 6a–d, respectively, present SEM images of Co/Al₂O₃-IM and Co/Al₂O₃-EPM before calcination. Cobalt film deposited on the surface of Co/Al₂O₃-EPM is dense, shipshape, and ball-like. Besides, particles of cobalt are nanoscale, which can increase the active sites to a great extent. Nonetheless, cobalt layer on Co/Al₂O₃-IM cannot be seen before calcination because the precursor of cobalt oxides is cobaltiferous salt.

FIG. 6.

SEM images of (a, b) Co/Al₂O₃-EPM before calcination, (c, d) Co/Al₂O₃-IM before calcination, (e, f) 800°C-Co/Al₂O₃-EPM after calcination, and (g, h) 800°C-Co/Al₂O₃-IM after calcination (calcination time = 2 hours and cobalt loads = 10 wt%). SEM, scanning electron microscopy.

In addition, SEM images of 800°C-Co/Al₂O₃-IM and 800°C-Co/Al₂O₃-EPM (i.e., after calcination) are also revealed in Fig. 6e–h. It is clear that cobalt oxide film is still shipshape deposited on the 800°C-Co/Al₂O₃-EPM, while lumpy and bulky cobalt oxide particles is nonuniformly deposited on the 800°C-Co/Al₂O₃-IM.

The results suggest that the cobalt layer on the 800°C-Co/Al₂O₃-EPM is well dispersive, which will increase the active sites for ozone decomposition. In addition, for 800°C-Co/Al₂O₃-IM, cracks and holes are both found in cobalt oxide particles and Al₂O₃ substrate. But for 800°C-Co/Al₂O₃-EPM, cracks and holes are only found in cobalt oxide film. In literature, it was reported that transverse drawing stress of crack tip is not enough to the expansion at high temperature, which makes material have cracks and holes after calcination (Krautgasser et al., 2016).

SEM-EDS analysis

Supplementary Figure S5 shows elemental composition of 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM through SEM-EDS. Supplementary Figures S5a–a′ and b–b′ show that raw 800°C-Co/Al₂O₃-EPM catalyst has higher content of cobalt (10.11 wt%) than 800°C-Co/Al₂O₃-IM (7.46 wt%). Supplementary Figures S5c–c′ and d–d′ show that there is a slight loss of cobalt in both kinds of catalysts (800°C-Co/Al₂O₃-EPM with 9.28 wt% Co and 800°C-Co/Al₂O₃-IM with 4.78 wt% Co) after 120 minutes reaction. But the amount of cobalt loss in 800°C-Co/Al₂O₃-EPM is obviously lower than that in 800°C-Co/Al₂O₃-IM.

The results manifest that 800°C-Co/Al₂O₃-EPM has the firmly stuck interaction between cobalt layer and the support, which hardly makes the cobalt film fall off from the Al₂O₃ in both calcining and reaction process. However, due to the weak adhesion between cobalt-based matter and Al₂O₃, 800°C-Co/Al₂O₃-IM has serious abscission in not only the calcined process, but also the reaction process. As a result, the concentration of cobalt ions in the solution of 800°C-Co/Al₂O₃-IM system is larger than 800°C-Co/Al₂O₃-EPM system.

EDS-mapping analysis

Supplementary Figure S6 shows the EDS-mapping images of 800°C-Co/Al₂O₃-EPM and 800°C-Co/Al₂O₃-IM. Supplementary Figures S6a–d show that almost no Al element is detected while Co element distributes equably on the surface of 800°C-Co/Al₂O₃-EPM. This is because the uniform cobalt layer closely wraps around Al₂O₃. On the contrary, Al element is easy to be observed on the catalyst, but only a small quantity of Co element is observed on 800°C-Co/Al₂O₃-IM catalyst. The result suggests that 800°C-Co/Al₂O₃-EPM catalyst has more dense and uniform cobalt particles on the carrier than 800°C-Co/Al₂O₃-EPM. Therefore, the catalytic capacity of 800°C-Co/Al₂O₃-EPM is much stronger than that of 800°C-Co/Al₂O₃-IM.

Possible reaction mechanisms

Radical scavenging experiment

In literature, inorganic ions (HCO₃⁻, CO₃²⁻, H₂PO₄⁻, and HPO₄²⁻) and alcohols (tertiary butanol, methyl alcohol, and isopropanol) can act as HO^• scavengers to hinder the catalytic activity of catalyst in heterogeneous ozonation systems (Sui et al., 2010; Fischbacher et al., 2013). Generally, alcohols are usually used to quench HO^• in the aqueous solution. Meanwhile, inorganic ions were easily adsorbed on the catalyst surface to substitute for surface hydroxyl groups. Thus, inorganic ions were usually used to identify the existence of surface hydroxyl groups. To study the contribution of HO^• on SA removal in the Co/Al₂O₃-EPM/O₃ system, excess (50 mM) TBA and phosphate (KH₂PO₄) were used as quenchers in Co/Al₂O₃-EPM/O₃ system.

Figure 7a shows that SA removal efficiency decreases from 100% to 67.3% in the presence of TBA. Meanwhile, SA removal efficiency decreases from 100% to 26.5% with the addition of KH₂PO₄. In brief, SA removal efficiency hindered by KH₂PO₄ is more obvious than by TBA. The results suggest that surface hydroxyl groups on the surface of 800°C-Co/Al₂O₃-EPM play an important role for reactive radical generation and SA removal.

FIG. 7.

(a) Effect of scavengers (TBA and KH₂PO₄) on SA removal in 800°C-Co/Al₂O₃-EPM/O₃ system; (b) Quantification of HO^• during reaction process in 800°C-Co/Al₂O₃-EPM/O₃ and O₃ systems. TBA, tert-butyl alcohol.

Specifically, a large proportion of HO^• may be formed on the surface of 800°C-Co/Al₂O₃-EPM due to the interaction between the surface hydroxyl groups and ozone [Eqs. (7 )–(9)]. However, after addition of KH₂PO₄, H₂PO₄⁻ occupies large surface-active site of catalyst leading to the decrease of surface hydroxyl groups, finally, resulting in poisoning of the surface sites. The similar finding was also confirmed by Pi et al. (2003) that phosphate has a strong affinity on the surface of CuO/Al₂O₃ to substitute for surface hydroxyl groups, and then significantly reduces the catalytic activity of CuO/Al₂O₃ on oxalic acid degradation in ozonation system.

In addition, a part of HO^• in liquid phase may come from solid–liquid mass transfer or homogeneous catalysis. Therefore, the addition of TBA also has some inhibiting effect on SA removal. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{M}} \hbox{-} { \rm{OH}} + 2{{ \rm{O}}_3} \to { \rm{M}} \hbox{-} {{ \rm{O}}_2} ^\cdot + { \rm{H}}{{ \rm{O}}_3} ^\cdot + {{ \rm{O}}_2} \tag{7} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{H}}{{ \rm{O}}_3} ^\cdot \to { \rm{H}}{{ \rm{O}}^\cdot } + {{ \rm{O}}_2} \tag{8} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{M}} \hbox{-} {{ \rm{O}}_2} ^\cdot + {{ \rm{O}}_3} + {{ \rm{H}}_2}{ \rm{O}} \to { \rm{M}} \hbox{-} { \rm{OH}} + { \rm{H}}{{ \rm{O}}_3} ^\cdot + {{ \rm{O}}_2} \tag{9} \end{align*} \end{document}

Quantification of dynamic HO^• concentration

To further confirm the existence and contribution of HO^• in 800°C-Co/Al₂O₃-EPM/O₃ system, the HO^• generated during the treatment process in 800°C-Co/Al₂O₃-EPM/O₃ and O₃ alone systems was quantified by HPLC. The concentration of HO^• based on this method is obtained via stoichiometric ratio between HO^• and p-HBA. Besides, this method is particularly regarded as p-HBA cannot adsorb strongly on the surface of 800°C-Co/Al₂O₃-EPM (Sung et al., 2006).

Seen from Fig. 7b, in O₃ alone system, the concentration of HO^• during the reaction process is very low and almost unchanged in the whole reaction process. However, the amount of HO^• raises during the whole treatment process in 800°C-Co/Al₂O₃-EPM/O₃ system demonstrating the generation of HO^• might be significantly influenced by the reaction time. Above all, the result manifests that there is indeed a true catalysis for ozone decomposition and HO^• radical is an important factor for the SA removal in 800°C-Co/Al₂O₃-EPM/O₃ system.

Reaction mechanism

According to the above mentioned analysis, there would be four pathways for SA removal in 800°C-Co/Al₂O₃-EPM/O₃ system, and the reaction mechanisms were illustrated in detail as follows (Fig. 8):

FIG. 8.

Reaction mechanism for succinic acid removal in 800°C-Co/Al₂O₃-EPM/O₃ system.

Direct oxidation

From the control experiments, the SA removal efficiency is 5.5% after 120 minutes treatment process in O₃ alone system. Therefore, it could be concluded that a small part of SA could be directly removed by ozone molecules.

Heterogeneous oxidation

The experimental results show that there is a catalytic reaction in 800°C-Co/Al₂O₃-EPM/O₃ system to promote the degradation of SA. Specifically, when ozone molecules are adsorbed on the surface of 800°C-Co/Al₂O₃-EPM, it can interact with surface hydroxyl groups, causing HO^• generated on the surface of catalyst. In other words, SA could be removed in situ. In addition, heterogenous oxidation is an intricate process. In this system, the heterogenous oxidation process also contains following mechanisms: (i) SA is adsorbed on the surface of 800°C-Co/Al₂O₃-EPM and then further be removed directly by gaseous or aqueous ozone; (ii) SA and O₃ are both adsorbed on the surface of 800°C-Co/Al₂O₃-EPM before reacting with each other, and then they interact on the surface of 800°C-Co/Al₂O₃-EPM.

Homogeneous oxidation

In 800°C-Co/Al₂O₃-EPM/O₃ system, the amount of cobalt leaching and HO^• are detected, which illustrated that a part of HO^• could be formed by the reaction between dissolved cobalt and O₃. In other words, there is a homogeneous oxidation process for SA degradation.

Adsorption

In the 800°C-Co/Al₂O₃-EPM alone system, the SA could be removed by 14.5%, suggesting that the absorption effect cannot be ignored.

Conclusions

This work mainly focuses on preparing a new type of catalyst, that is, Co/Al₂O₃-EPM, to study its catalytic activity for ozone decomposition. First, several single-factor experiments were conducted to optimize the preparation parameters of Co/Al₂O₃-EPM. Also, some key operational parameters in 800°C-Co/Al₂O₃-EPM/O₃ system for SA removal were also optimized. Under the optimal conditions (i.e., plating time = 10 minutes, calcination temperature = 800°C, calcination time = 2.0 hours, cobalt loads = 10 wt%, initial pH 3.6, ozone flow rate = 300 mL/min, and catalyst dosage = 5.0 g/L), the Co/Al₂O₃-EPM shows the optimal catalytic activity and stability for SA removal.

Then, through control experiment, 800°C-Co/Al₂O₃-EPM catalyst showed better performance than 800°C-Co/Al₂O₃-IM according to the SA and TOC removal. Further, according to the analysis of BET, SEM, EDS, EDS-mapping, XRD, FTIR, and XPS, the 800°C-Co/Al₂O₃-EPM catalyst not only shows more uniform scatter degree of cobalt but also generates higher catalytic activity matter (i.e., Co₃O₄) than the 800°C-Co/Al₂O₃-IM catalyst. Besides, the data of cobalt leaching in two systems confirms the fact that the stability of 800°C-Co/Al₂O₃-EPM is better than 800°C-Co/Al₂O₃-IM.

In addition, quenching tests and the measurement of dynamic HO^• concentration in the 800°C-Co/Al₂O₃-EPM/O₃ system suggest that the surface hydroxyl groups play an important role for SA removal and there is indeed a true catalysis for ozone decomposition. In brief, all the results indicate the new type of catalyst, Co/Al₂O₃-EPM, has a better catalytic activity for catalytic ozonation than Co/Al₂O₃-IM. Therefore, new Co/Al₂O₃-EPM should be used as a promising catalyst for the catalytic ozonation in the field of wastewater treatment.

Footnotes

Acknowledgments

The authors acknowledge the financial support from Fundamental Research Funds for the Central Universities (No. 2015SCU04A09) and Science and Technology Project of Sichuan Province (2016JY0154).

Author Disclosure Statement

No competing financial interests exist.

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Supplementary Material

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Catalytic Ozonation of Succinic Acid in Aqueous Solution by Co/Al 2 O 3 -EPM: Performance,Characteristics,and Reaction Mechanism

Abstract

Abstract

Introduction

Materials and Methods

Reagents

Catalyst preparation

Experiment procedure

Analytical method

Quantification of HO• in aqueous solution

Results and Discussion

Optimization of preparation parameters

Effects of the calcination temperature

Effects of the calcination time

Effects of the cobalt loads

Optimization of operational parameters

Effects of the initial pH value

Effects of ozone flow rate

Effects of the 800°C-Co/Al2O3-EPM dosage

Control experiment

Characterization of two kinds of Co/Al2O3 catalysts

XRD, FTIR, and XPS analysis

SEM analysis

SEM-EDS analysis

EDS-mapping analysis

Possible reaction mechanisms

Radical scavenging experiment

Quantification of dynamic HO• concentration

Reaction mechanism

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Supplementary Material

Quantification of HO^• in aqueous solution

Effects of the 800°C-Co/Al₂O₃-EPM dosage

Characterization of two kinds of Co/Al₂O₃ catalysts

Quantification of dynamic HO^• concentration