Sulfate radical-based advanced oxidation processes have had considerable attention due to the highly oxidizing function of sulfate radicals (SO4·) resulting in acceleration of organic pollutants degradation in aqueous environments. A Co-Ni mixed oxide nanocatalyst, which was prepared by the sol-gel method, was employed to activate peroxymonosulfate (PMS, HSO5) to produce SO4· with Acid Orange 7 (AO7) selected as a radical probe. The catalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM). The characterization results indicated that the ingredient of the catalyst had been changed and the amount of surface hydroxyl increased significantly with the addition of Ni. Therefore, it proved that Co-NiOx catalyst was more effective than CoOx to activate PMS. Moreover, ultrasound (US) can increase the degradation rate of AO7 and US/Co-NiOx/PMS system. This study also focused on some synthesis parameters and the system reached the maximum efficiency under the condition when [PMS] = 0.4 mM, [catalyst] = 0.28 g/L, Pus = 200 W. The AO7 removal in these systems follows first order kinetics. Last but not least, quenching studies was conducted which indicated that the amount of hydroxyl radicals (·OH) increases with the increase of initial pH and SO4· was the primary reactive oxidant for AO7 degradation.

INTRODUCTION

Organic pollutants from industrial and agricultural production are widespread in natural water, and it is even serious that some organic pollutants are toxic to humans and difficult to be degraded by conventional water treatment processes (Yamada 2007; Schipper et al. 2008). Thus, the degradation of organic pollutants in water treatment has been a promising and eye-catching research topic, and several of oxidation systems have been introduced to degrade organic pollutants in recent years. Among these, advanced oxidation processes (AOPs) were largely investigated resulting from its high oxidation ability (Kishimoto et al. 2015).

AOPs could produce reactive radicals, such as hydroxyl radicals (·OH) and superoxide radical (O2·), to degrade organic contaminants. In recent years, sulfate radicals (SO4·), which can be generated through breaking the two oxygen bond of peroxymonosulfate (PMS), had attracted great interest in the environmental applications due to its high activity (E0 = 2.5–3.1 V) and the wide pH applicability (Cui et al. 2012). PMS ions can be seen as an H-atom in H2O2 that was replaced by SO3 group, the distance of the two oxygen bond in PMS is 1.460 Å (Ghanbari & Moradi 2017). Besides, ·OH is another product of PMS activation, which was also an advantage of PMS (Ghanbari & Moradi 2017). In general, PMS could be activated by heat, UV light, ultrasound (US), and some transition ions (e.g. Fe, Ru, Ce, Co) to induce the generation of sulfate radicals (SO4·) (Anipsitakis & Dionysiou 2004).

Previous literature proved PMS coupled with Co2+ to be an excellent combination of the generation of SO4· (Guo et al. 2015), and Co catalyst also successfully catalyzed PMS to generate SO4· (Shukla et al. 2010a, 2011a, 2011b). Moreover, several supports such as Al2O3, TiO2 had been investigated for Co to carry out the reaction in the presence of PMS (Yang et al. 2007; Liang et al. 2012b). Co catalyst had proved to be an effective activator of PMS to generate SO4·, which could finally degrade organic pollutants. Meanwhile, the catalytic activity of Co catalyst can also be improved with bimetal composite or by using mesoporous materials to support the catalyst (Shukla et al. 2011a). Besides, it has been concluded that microwave (Pang & Lei 2016) and UV irradiation (Xue et al. 2016) were effective methods to enhance the generation of radicals.

In general, the bimetal composite was an excellent method to enhance the activity of the catalyst. Recently, some literature figured out that Fe/Co complex was used for organic pollutants treatment (e.g. phenol, 2,4-DCP) and concluded that bimetal composite more effectively activates PMS than Co alone (Stoyanova et al. 2014; Wang et al. 2014b; Xu et al. 2015). Apart from improving efficiency, the bimetal composite was even more stable than single metal in aqueous solutions (Yang et al. 2009). Many transition metals can combine with Co, such as Mn (Liang et al. 2012a; Yao et al. 2015), and Mg (Hu et al. 2013). Ni can also combine with Co, however there were few studies on Ni complex with Co. Actually, Ni complex can improve catalyst performance effectively. In the H2O2 system, Ni-Fe is even more active than Co-Fe (Sharma et al. 2015).

US was also one of the enhancement methods which can improve the catalytic efficiency. Whether in homogeneous (Su et al. 2012) or heterogeneous systems (Zou et al. 2014), US can accelerate degradation of pollutants. In some experiments, US had significantly improved the heterogeneous processes (Hou et al. 2012; Zou et al. 2014). Moreover, US can also promote degradation of pollutants in a bimetallic catalyst system. Cai found that US can significantly enhance Fe-Co/PMS system to degrade Acid Orange 7 (AO7) (Cai et al. 2015). Thus, the advantages of US include pollution free, non-residue and high efficiency, and it can also combine with other methods easily.

This study aims to investigate AO7 degradation during activation of PMS by Co-NiOx and enhanced effect with US, and especially focuses on effects of PMS dosage, catalyst dosage, and US power, and identify of primary reactive oxidants at pH range from 4.86 to 9.48. Moreover, the change of morphology between CoOx and Co-NiOx catalyst was studied by characterization.

MATERIALS AND METHODS

Materials

Cobalt nitrate (Co2(NO3)3·6H2O), nickel nitrate (Ni(NO3)2·6H2O), sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium bicarbonate (NaHCO3), tert-Butanol (t-BuOH) and ethanol (EtOH) were purchased from Kelong Chemical Co., Ltd (Chengdu, China). AO7 (C16H11N2NaO4S) and potassium iodide (KI) were purchased from Sinopharm Chemical Reagent Co., Ltd. PMS (2KHSO5·KHSO4·K2SO4) was obtained from Sigma-Aldrich Co., Ltd (Shanghai, China). All chemicals were analytical grade reagents or senior and the experimental solutions were prepared by pure water. Egg-white was bought in the market.

Experimental procedure

Catalysts were prepared by the sol-gel process (Ahmadov et al. 2011). Total ion consistence is 1 M, and the consistence ratio of Co/Ni is 1:1, and the proportion of ion solution and egg-white is 1:2.5. All of the catalysts were dried at 80 °C and calcined at 400 °C in the air atmosphere, then reserved at room temperature without water.

All experiments were performed in a 250 mL global glass reactor under constant stirring with a PTFE-coated magnetic stirrer at 20 ± 2 °C. Each run was switched on by simultaneously adding the desired concentration of AO7, PMS, and catalyst. US was furnished by a US generator (SB25-12DTD Infinity, Ningbo Xinzhi Biotechnology Co., Ltd). At different time intervals, 3 mL aliquot was sampled filtered through 0.45 micron glass microfiber filters (Whatman Co.).

Analytical methods

The concentration of AO7 in aqueous solution was analyzed by an UV-Vis spectrophotometer (UV-1800 Infinity, Shanghai MAPADA Instrument Co., Ltd) at 484 nm, and full spectrum scan was made from 200 to 600 nm.

For the whole study, two duplicate experiments were carried out in order to obtain the accuracy value. The data presenting in figures were the average calculated using the three initial values of the experiments, and the experimental errors are less than 5%.

RESULTS AND DISCUSSION

Characterization of CoOx and Co-NiOx

In Figure 1, the X-ray diffraction (XRD) spectra of CoOx and Co-NiOx show some sharp diffraction peaks of metallic, confirming that crystalline metal catalyst was embedded (Qiao et al. 2007). The spectra of CoOx showed distinct peaks of Co3O4 (JCPDS 74-2120) at 19.0, 31.3, 36.8, 44.8, 55.7, 59.4, 65.2 degree, which corresponds to (1 1 1), (2 2 0), (3 1 1), (4 4 4), (4 2 2), (5 1 1), (4 4 0) reflection of Co3O4, respectively. In addition, the characteristic peaks of Co3O4 were also observed in Co-NiOx. Some peaks representing NiO (JCPDS 71-1179) and NiCo2O4 (JCPDS 73-1702) were observed in Co-NiOx. For NiCo2O4, it was considered that Ni replaces Co atom in Co3O4. Therefore, the peak of NiCo2O4 was almost completely overlapping with Co3O4 (Yang et al. 2009).
Figure 1

XRD patterns of CoOx catalyst and Co-NiOx catalyst.

Figure 1

XRD patterns of CoOx catalyst and Co-NiOx catalyst.

X-ray photoelectron spectroscopy (XPS) measurement was applied to investigate the oxidation states and surface chemical compositions of catalysts. As shown in Figure 2, the spectrum of CoOx showed two major peaks of binding energy at 780.1 and 795.5 eV, respectively, which may be characteristic of Co3O4 (Shi et al. 2012; Chen et al. 2014; Xu et al. 2015). For Co-NiOx catalyst, two sub-peaks were resolved with binding energy at 780.1 and 795.6 eV. Combining with XRD analysis, the Co species in Co-NiOx catalyst were effectively coupled with Ni to form NiCo2O4 with intimate Ni-Co interactions resulting in the shift of electron binding energy (Yang et al. 2008). Thus, the peak at 795.6 eV may also belong to Co3O4.
Figure 2

XPS spectra of (a) Co, (b) O.

Figure 2

XPS spectra of (a) Co, (b) O.

The surface hydroxyl of the catalyst can further be verified by inspecting the O spectrum. Both for the CoOx and for Co-NiOx catalyst, a peak at 529.8 eV was found, which was related to lattice oxygen (Yang et al. 2007). A shoulder at a higher binding energy of 531.5 eV was identified to surface hydroxyl groups (i.e. Co–OH or Ni–OH) (Yang et al. 2007). Meanwhile, the subdued XPS peaks at 532.7 and 533.1 eV indicated adsorbed water in the catalyst (Shi et al. 2012).

The result of percentage change of catalyst is shown in Table 1. As can be seen, since the addition of Ni, the content of Co reduced from 100% to 80.7%, meanwhile, the content of Ni increased from 0 to 19.3%. Moreover, Co-NiOx also had more surface hydroxyl groups than CoOx.

Table 1

The percentage change in the Co and O

Catalyst Percentage of surface metal
 
Percentage of total oxygen
 
Cobalt Nickel Lattice oxygen Hydroxyl groups Adsorbed water 
CoOx 100 60.7 26.2 13.1 
Co-NiOx 80.7 19.3 53.9 30.7 15.4 
Catalyst Percentage of surface metal
 
Percentage of total oxygen
 
Cobalt Nickel Lattice oxygen Hydroxyl groups Adsorbed water 
CoOx 100 60.7 26.2 13.1 
Co-NiOx 80.7 19.3 53.9 30.7 15.4 

Figure 3 shows Fourier transform infrared spectroscopy (FT-IR) spectra of catalysts. The absorptions around 3,420 cm−1 imply the existence of surface hydroxyl groups (Co-OH or Ni-OH) (Xu et al. 2015). Apparently, the absorption of hydroxyl groups improved significantly in Co-NiOx resulting in that Co-NiOx showed a more obvious absorption peak than CoOx of surface hydroxyl groups. The peaks at 566 and 663 cm−1 were observed due to vibration of Co-O (Shukla et al. 2011a; Yao et al. 2015), and these results of O and Co were consistent with XPS and XRD analysis.
Figure 3

FT-IR spectra of catalysts.

Figure 3

FT-IR spectra of catalysts.

The transmission electron microscopy (TEM) images of the Co-NiOx are presented in Figure 4. Generally, the dark dot was ascribed to the supported oxide species, and the gray area was considered as support (Stoyanova et al. 2014). As shown in Figure 4, dark and gray areas were evenly distributed, which indicated that the metal oxides were well embedded in the support; this result was also consistent with the XRD. Moreover, the dark dot showed aggregation which could be assigned to the self-interaction of the hydrophilic polar groups on the support (Xu et al. 2015). The sizes of majority of metal oxides were in the range from 10 nm to 30 nm, and this catalyst belongs to the nanocatalyst.
Figure 4

TEM images of catalyst at different scales.

Figure 4

TEM images of catalyst at different scales.

Enhancement of AO7 degradation in the Co-NiOx/PMS/US system

In order to examine the enhancement of oxidation ability during the PMS activation process by Co-NiOx and US, the degradations of AO7 in PMS alone, PMS/US, CoOx/PMS, NiOx/PMS, Co-NiOx/PMS and Co-NiOx/PMS/US system were investigated. As shown in Figure 5(a), for the reaction carried out in the PMS alone and PMS/US, negligible change in AO7 concentration was observed, and only ~2% of AO7 was degraded in 3 min at these systems. However, in the CoOx/PMS, Co-NiOx/PMS, and Co-NiOx/PMS/US systems, the AO7 degradation was significant and 70%, 94%, and even nearly 100% AO7 was degraded in 3 min, respectively. In addition, AO7 was degraded about 11% in 3 min in the NiOx/PMS system. With the extension of reaction time, Co-NiOx/PMS produced ~100% AO7 removal in 5 min but CoOx/PMS in 10 min. It should be noted that AO7 was difficult to be degraded in the PMS alone, PMS/US and NiOx/PMS systems, as the degradation rates were only 2%, 2% and 33% in 15 min, respectively. Therefore, it suggested that the PMS in homogeneous solution could hardly induce significant oxidation of AO7 without catalyst and NiOx was not powerfully catalytically active to activate PMS.
Figure 5

(a) Degradation of AO7 in different systems, (b) concentration of PMS in different systems, (c) UV–vis spectrum of CoOx/PMS system, (d) UV–vis spectrum of Co-NiOx/PMS system, and (e) UV–vis spectrum of Co-NiOx/PMS/US system. [AO7] = 0.032 mM, [PMS] = 0.8 mM, [catalyst] = 0.20 g/L, PUS = 200 W, Initial pH = 6.60.

Figure 5

(a) Degradation of AO7 in different systems, (b) concentration of PMS in different systems, (c) UV–vis spectrum of CoOx/PMS system, (d) UV–vis spectrum of Co-NiOx/PMS system, and (e) UV–vis spectrum of Co-NiOx/PMS/US system. [AO7] = 0.032 mM, [PMS] = 0.8 mM, [catalyst] = 0.20 g/L, PUS = 200 W, Initial pH = 6.60.

In order to further examine the different catalytic performance between CoOx and Co-NiOx and the role of US, the concentration of PMS is monitored and the results are shown in Figure 5(b). It indicated that Co-NiOx was more effective than CoOx, and the US could enhance the process of PMS activation. Moreover, UV-vis spectral scanning of AO7 at CoOx/PMS, Co-NiOx/PMS, Co-NiOx/PMS/US system was carried out. As shown in Figure 5(c)5(e), there was one main band located at 484 nm, which was originated from the azo bond. The absorptions at 310 nm were associated with naphthalene ring structures (Wang et al. 2014a). The absorption peaks at 484 and 310 nm decreased with the reaction time, which proved that the azo structure and naphthalene ring were destroyed. It could be observed that the peaks at 484 nm and 310 nm decreased quickly in Co-NiOx/PMS/US system with a new absorption band around 250 nm appearing. This may be attributed to the formation of some intermediates such as 1-amino-2-naphthol, sulfanilamide and some small molecule carboxylic acid due to the further generation of oxidation intermediates (Shi et al. 2015; Xu et al. 2015). Furthermore, as compared to Figure 5(c), the intermediate peak decreased faster in Figure 5(d). In addition, Figure 5(e) shows the fastest rate of the peak decrease, which represented more pollutants removal.

The reason for the different degradation rate in these systems could be explained by the characterization. On the one hand, the XRD spectra of the Co-NiOx sample showed the existence of NiCo2O4. Ni replaces Co in Co3O4 and may cause lattice defects which may lead to the difference of arrangement of the atoms; these differences may make electron transfer easily, thence more radicals were generated (Yu et al. 2010; Yan et al. 2013). On the other hand, surface hydroxyl groups were considered to be an important factor in affecting the catalyst activity (Shukla et al. 2011b). Surface hydroxyl groups could enhance the ability of oxidizing agent to capture electrons in order to generate more hydroxyl radicals. According to the results of XPS and FT-IR, Co-NiOx also provides more surface hydroxyl groups than CoOx. So the catalytic activity of Co-NiOx was better than CoOx. Moreover, the role of US in this experiment was to promote mixing and cavitation which will be discussed in detail in the subsequent chapter.

Effect of PMS and catalyst dosages and US power

To further investigate the degradation of AO7 in the Co-NiOx/PMS system, the effect of PMS dosage, catalyst dosage, and US power (Pus) were explored as shown in Figure 6. Figure 6(a) shows the effect of PMS dosage on AO7 degradation, the degradation rate was improved by increasing PMS dosage and a plateau was reached after the addition of 0.4 mM PMS in solution. Further increase in PMS dosage resulted in the decrease of the initial degradation rate constant (k) of AO7. In Supplemental Material (SM) Figure 1(a), the initial k in different PMS dosages were 0.342, 0.637, 0.753, 0.992, 1.000, 0.974, 0.895 min−1, respectively (SM Figure 1 is available with the online version of this paper). Since PMS was the root source of SO4· (Shukla et al. 2011a), the increase of PMS dosage could obtain more SO4·. Moreover, the loss in AO7 degradation rate may be caused by the self-quenching of sulfate and hydroxyl radicals (·OH) (Shukla et al. 2010b). As in Equations (1) and (2), higher PMS dosage will generate more HSO5, which could consume the active SO4· and result in a lower degradation rate. 
formula
1
 
formula
2
Figure 6

The impact on various factors, (a) PMS dosage, (b) catalyst dosage, (c) Pus ([AO7] = 0.032 mM, Initial pH = 6.60, T = 20 ± 2 °C).

Figure 6

The impact on various factors, (a) PMS dosage, (b) catalyst dosage, (c) Pus ([AO7] = 0.032 mM, Initial pH = 6.60, T = 20 ± 2 °C).

As shown in Figure 6(b), higher Co-NiOx dosage in solution resulted in higher AO7 degradation rate. When the catalyst dosage varied from 0.28 g/L to 0.32 g/L, the degradation rate was not accelerated obviously. As shown in SM Figure 1(b), the initial k in different catalyst dosages were 0.228, 0.400, 0.704, 0.947, 1.023, 1.309, 1.323, respectively. Generally, the increase of degradation rate was clearly attributed to the increased availability of active sites in the solution for the reaction with PMS which generates more SO4· (Shukla et al. 2011a). Nevertheless, when catalyst dosage increased to a certain amount, the number of active sites did not impact the degradation rate as much as the other factors such as PMS dosage.

Figure 6(c) displays the degradation of AO7 with different Pus. It was concluded that the US accelerated the rate of heterogeneous interfacial reactions. However, excess amount of Pus led to a slight inhibition of AO7 degradation. The initial k was shown in SM Figure 1(c). The role of US was generally explained as follows. First, in US conditions, micro-stream was formed which contributed to the full mixture of the heterogeneous catalysts and liquid. Additionally, the micro-stream continuously cleaned the surface of catalysts for further reactions. The instantaneous high temperature and pressure which were produced by cavitation bubble would also accelerate the rate of degradation (Cai et al. 2015), but for continuously increased Pus, the benefit was not observed. It was because that further increase in Pus led to the collapse of the cavitation bubbles and the production of an acoustic screen that would terminate the transmission of US in the water (Hou et al. 2012; Wang et al. 2014a; Zou et al. 2014).

Identification of reactive species at different initial pH

According to the previous literatures, SO4· and ·OH were likely to be produced in the PMS activation processes (Yang et al. 2007; Xu et al. 2015; Yao et al. 2015). In order to confirm the contribution of the oxidizing radical species, t-BuOH and EtOH were employed. According to the literature report, the reaction constant of t-BuOH with ·OH was 3.8–7.6 × 10−8 M−1s−1, EtOH contains α-hydrogen, the reaction constant of EtOH with SO4-· and ·OH were 1.6–7.7 × 10−7 and 1.2–2.8 × 10−9 M−1s−1, respectively, which acted as a quencher of both the SO4· and ·OH while t-BuOH was generally used as ·OH quencher (Hou et al. 2012).

Figure 7(a)7(d) illustrate that AO7 removal rate did not change significantly with the increase of initial pH in the non-scavenger system and ~100% AO7 removal could be reached in less than 3 min. However, in past studies, some researchers found that the pH of the system exerts a significant influence on the reaction (Hazime et al. 2013; Ahmadi et al. 2015). This difference could be explained in the following respects.
Figure 7

Inhibition of AO7 degradation by quenchers at different initial pH. [AO7] = 0.032 mM, [PMS] = 0.4 mM, [quencher]: [PMS] = 1000:1, [catalyst] = 0.28 g/L, T = 20 ± 2 °C. Initial pH (a) 4.86, (b) 6.60, (c) 8.16, (d) 9.48.

Figure 7

Inhibition of AO7 degradation by quenchers at different initial pH. [AO7] = 0.032 mM, [PMS] = 0.4 mM, [quencher]: [PMS] = 1000:1, [catalyst] = 0.28 g/L, T = 20 ± 2 °C. Initial pH (a) 4.86, (b) 6.60, (c) 8.16, (d) 9.48.

SO4· could be converted into ·OH in the present of OH and H2O (Liang et al. 2007). The ·OH could also degrade AO7 effectively. Thus, the degradation rate did not change significantly with initial pH raise. 
formula
3
 
formula
4
Furthermore, no buffer solution was added which avoided the effects of the buffer solution. Thus, some of the reactions (Equations (5) and (6)) (Kusic et al. 2011) and by-products such as small molecule carboxylic acids could reduce the pH of the system (Shi et al. 2015; Xu et al. 2015). 
formula
5
 
formula
6
The final pH did not show an obvious difference with different initial pH value (about 4.82–5.08) which caused that the effect of initial pH was continuously weakened during the reaction.

Compared with t-BuOH, EtOH had played a more important role in suppressing the process. The result showed merely 10% AO7 removal in 15 min in all systems after added EtOH. However, addition of t-BuOH would slow down the rate of degradation and ~100% AO7 removal could be reached in 7–10 min.

Furthermore, from Figure 7(a)7(d), it also found that with the initial pH increase, the more obvious suppression was observed within t-BuOH. According to Equations (3) and (4), it demonstrated that SO4· was considered as the dominant species in the system, but did not exclude the existence and action of ·OH, especially in high initial pH.

Stability of the catalyst

In order to explore the stability of catalyst, the catalyst was multiple test and the Co and Ni ions concentration were detected at the end of the reaction. Figure 8(a) shows the reusability of Co-NiOx catalyst. During multiple tests, Co-NiOx showed deactivation but not obvious, nearly 100% AO7 was removed in 3 min, but the initial k in multiple tests were 1.374, 1.171, 1.001, respectively. Furthermore, the dissolution of Co and Ni ions is shown in Figure 8(b). The dissolution of Ni ions is 5.89, 4.20, 2.94 mg/L, respectively and the dissolution of Co ions is 3.14, 2.36, 1.46 mg/L, respectively. During the reaction, the ions dissolution of the catalyst was very low which indicated that the stability of Co-NiOx was excellent.
Figure 8

Multiple tests of Co-NiOx catalysts (a) degradation of AO7, (b) ions dissolution.

Figure 8

Multiple tests of Co-NiOx catalysts (a) degradation of AO7, (b) ions dissolution.

CONCLUSIONS

In this study, efficient Co-NiOx nanocatalysts had been applied in heterogeneous activation of PMS to generate SO4· and thus to accelerate the decomposition of AO7. The results of XRD showed that NiCo2O4 and lattice defects were formed with the addition of Ni. Moreover, XPS and FT-IR results demonstrated that Co-NiOx could provide more surface hydroxyl groups. Furthermore, the TEM results illustrated that the size of the catalyst is between 10 and 30 nm. Thus, Co-NiOx was an effective catalyst for heterogeneous catalysis of PMS for oxidation of AO7.

Moreover, this study clearly revealed that PMS dosage, catalyst dosage, and US power were of crucial importance to the decomposition of AO7. The optimum reaction took place under the following conditions: [PMS] = 0.4 mM, catalyst dosage = 0.28 g/L, Pus = 200 W. The studies on radical quenching proved that the co-existence of SO4· and ·OH in Co-NiOx/PMS/US system. SO4· was the main substance formed by the Co-NiOx/PMS interaction, though the number of ·OH was increased with the addition of pH. The multiplex test proved that the stability of catalyst was excellent and the metal dissolution was very low.

As a catalyst, Co-NiOx had a bright prospect because it could effectively produce radicals to degrade organic matter in water and its catalytic effects could be enhanced with US.

ACKNOWLEDGEMENTS

Appreciation and acknowledgment are given to the National Natural Science Foundation of China (No. 51508353).

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Supplementary data