Ultrasound enhanced heterogeneous activation of peroxymonosulfate by a Co-NiOx catalyst

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.

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.

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.).

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%.

The surface hydroxyl of the catalyst can further be verified by inspecting the O spectrum. Both for the CoO_x and for Co-NiO_x 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.

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.

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).

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).

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.

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.

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