Activation of peroxymonosulfate by metal (Fe, Mn, Cu and Ni) doping ordered mesoporous Co₃O₄ for the degradation of enrofloxacin

Abstract

Various transition metals (Fe, Mn, Cu and Ni) were doped into ordered mesoporous Co₃O₄ to synthesize Co₃O₄-composite spinels. Their formation was evidenced by transmission electronic microscopy (TEM), X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) analysis. It was found that Co₃O₄-composite spinels could efficiently activate peroxymonosulfate (PMS) to remove enrofloxacin (ENR) and the catalytic activity followed the order Co₃O₄–CuCo₂O₄ > Co₃O₄–CoMn₂O₄ > Co₃O₄–CoFe₂O₄ > Co₃O₄–NiCo₂O₄. Moreover, through the calculation of the specific apparent rate constant (k_sapp), it can be proved that the Co and Cu ions had the best synergistic effect for PMS activation. The Co₃O₄-composite spinels presented a wide pH range for the activation of PMS, but strong acidic and alkaline conditions were detrimental to ENR removal. Higher reaction temperature could promote the PMS activation process. Sulfate radical was identified as the dominating reactive species in Co₃O₄-composite spinel/PMS systems through radical quenching experiments. Meanwhile, the probable mechanisms concerning Co₃O₄-composite spinel activated PMS were proposed.

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1. Introduction

During the past years, sulfate radical (SO₄⁻˙) based advanced oxidation processes (SR-AOPs) have attracted an increasing interest among researchers owing to their great potential in degradation or even mineralization of recalcitrant organic pollutants.¹ Compared with the hydroxyl radical (˙OH), SO₄⁻˙ possesses a longer lifespan, higher independence of pH and higher selectivity of oxidation.^2,3 Peroxymonosulfate (PMS), a precursor of SO₄⁻˙, is deemed as a cost-effective and environmental-friendly oxidant.⁴ PMS remains stable in aqueous solution and barely decomposes into SO₄⁻˙ by itself, but it can be activated to produce SO₄⁻˙ by the use of UV, transition metals, and some nonmetal catalysts.^5–7 Among different activation technologies, transition metals have attracted much attention due to their lower energy consumption and higher activation efficiency. Actually, many transition metal ions such as Co²⁺, Mn²⁺, Ni²⁺, Fe²⁺, Ru³⁺, Ce³⁺ and so forth, have been proved as qualified catalysts for PMS activation.² Of note, Co²⁺ has been found to possess the highest reactivity.⁸ Unfortunately, the Co²⁺/PMS process is unfavorable in practical application because of the toxicity of Co²⁺.

In order to relieve the secondary pollution, heterogeneous cobalt-based catalysts have become a research hotspot. Anipsitakis et al. firstly employed Co₃O₄ to activate PMS and found Co₃O₄ presented an excellent catalytic behavior in the activation of PMS.⁹ Chen et al. successfully prepared nanoscale Co₃O₄ and tested its catalytic performance in PMS solution, results showed that 0.2 mM acid orange 7 (AO7) can be completely degraded within 30 min by 2 mM PMS in the presence of 0.5 g L⁻¹ Co₃O₄.¹⁰ Pu et al. fabricated three types of Co₃O₄ using different metal organic frameworks, and found that all the Co₃O₄ exhibited outstanding catalytic activity and the difference in catalytic ability can be attributed to the difference in specific surface area.¹¹ Consequently, Co₃O₄/PMS system is quite acceptable from the view of application due to the high activation efficiency and limitation of cobalt leaching. However, on the basis of the underlying threat of cobalt ions, it is essential to take measures to further limit the cobalt leakage during PMS activation.

It is reported that bimetallic oxides may be desirable catalysts to ease the conflict between catalytic performance and metal ions leaching, because intimate interactions between two metals can effectively suppress the leakage of metal ions, such as Fe–Co interactions in CoFe₂O₄.² Moreover, bimetallic oxides are also prominent PMS activators. Su et al. synthesized a series of Co_xFe_3−xO₄ nanoparticles and found that the higher cobalt content in Co_xFe_3−xO₄ showed the higher catalytic activity towards PMS.¹² The high catalytic behavior of CoFe₂O₄ was also illustrated in our previous study.¹³ Yao et al. reported that CoMn₂O₄ showed stronger catalytic activity than Co₃O₄, Mn₂O₃ and their physical mixture due to the synergistic effects of Co and Mn species.¹⁴ Similarly, CuCo₂O₄ also exhibited high catalytic performance and low metal leachability in PMS solution.¹⁵ In our previous research, order mesoporous Co₃O₄ (OM-Co₃O₄) was fabricated and showed superior catalytic ability toward PMS than its spinel counterpart, but the leakage of cobalt was up to 77.74 μg L⁻¹ which was higher than conventional Co₃O₄ nanoparticles.¹⁶ Therefore, it can be reasonably speculated that cobalt leaching will reduce if some transition metals are doped to OM-Co₃O₄ to form mixed spinels with cobalt.

Herein, diverse transition metals (i.e., Fe, Mn, Cu and Ni) was introduced into OM-Co₃O₄ to synthesized a series of Co₃O₄-composite spinels which were characterized by transmission electronic microscope (TEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and Zeta potential analysis. Due to the ubiquitous detection in aquatic environment,¹⁷ enrofloxacin (ENR) was selected as target pollutant in this study. The catalytic activities of as-prepared Co₃O₄-composite spinels were systematically compared through apparent rate constant, PMS consumption, intensity of electron paramagnetic resonance (EPR) signal and specific apparent rate constant. Moreover, the effects of initial pH and reaction temperature during PMS activation were also investigated. Finally, a possible mechanism of PMS activation was proposed through quenching tests. To be best of our knowledge, it is the first time to apply order mesoporous Co₃O₄-composite spinels as effective PMS activators for the control of organic pollutants.

2. Materials and methods

2.1 Chemicals

ENR, acetonitrile (HPLC grade), Oxone (KHSO5·0.5KHSO4·0.5K2SO4, PMS, KHSO5 ≥ 47%) and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The silica templates KIT-6 were purchased from Nanjing XFNANO Materials Tech Co., Ltd. (Jiangsu, China). Other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water (18 MΩ cm) was produced from an Ulupure water purification system (Shanghai, China).

2.2 Catalysts preparation and characterization

OM-Co₃O₄ was synthesized using nanocasting route with KIT-6 as hard template, and the procedure was conducted as described before.¹⁶ For the preparation of Co₃O₄-composite spinels, 1.0 g OM-Co₃O₄ was dispersed in 2.5 mL ethanol of Fe(NO₃)₃·9H₂O, Mn(NO₃)₂·4H₂O, Cu(NO₃)₂·3H₂O and Ni(NO₃)₂·6H₂O, respectively, and Co/M molar ratio was controlled at 3. After magnetic stirring for 1 h, the mixture was dried overnight at 60 °C and then calcined at 450 °C for 5 h (the heating rate was set at 2 °C min⁻¹). Finally, the obtained composite spinels were referenced as Co₃O₄–CoM₂O₄ (M = Fe, Mn) and Co₃O₄–MCo₂O₄ (M = Cu, Ni) which depended on the oxidation state of dopant.

The crystal structures of catalysts were characterized by X'Pert PRO diffractometer (PANalytical, Holland) with Cu Kα radiation. The morphologies and structures of Co₃O₄-composite spinels were observed using transmission electron microscopy (TEM) (Philips, Holland). N₂ adsorption and desorption isotherms were measured using ASAP 2010 analyzer (Micromeritics, USA) at liquid nitrogen temperature (−196 °C). The pH at point of zero charge (pH_pzc) was determined by Zetasizer Nano analyzer (Malvern, UK).

2.3 Catalytic experimental procedure

The catalytic degradation experiments were performed with a 100 mL ENR solution at 10 mg L⁻¹ in 250 mL brown glass bottles, which were installed in a controlled temperature water bath stirring apparatus. In a typical run, specific amount of catalysts was added to ENR solution to receive adsorption–desorption equilibrium, followed by pH adjustment with H₂SO₄ and NaOH solution (100 mM) to ensure a desirable pH value after PMS addition. Subsequently, an appropriate amount of PMS was charged into the reaction solution to initiate experiment. At defined time intervals, 1 mL samples were collected and quenched by 0.1 mL Na₂SO₃ (100 mM). The resulting mixtures were immediately filtered by a 0.22 μm syringe filter for further analysis. All experiments were carried out in duplicates and the mean values were reported (with error bar).

2.4 Analytical methods

ENR concentrations were measured through a high-performance liquid chromatograph (HPLC, Agilent 1200, USA) with an Eclipse XDB-C18 column (5 μm particle, 150 × 4.5 mm), the concentrations were measured at λ = 278 nm using a mobile phase consisting of a mixture of acetonitrile and phosphoric acid (pH = 2.5) (v/v = 20 : 80) at a flow rate of 1.0 mL min⁻¹. The PMS concentrations were measured by the method of Waclawek et al.¹⁸ EPR analysis were performed on a Bruker A300 spectrometer (Germany) with DMPO as a spin-trapping agent. The parameters of EPR spectrometer were center field was 3360.67 G, sweep width was 100 G, static field was 3310.66 G, microwave frequency was 9.42 GHz, microwave power was 2.03 mW, modulation amplitude was 1.0 G and sweep time was 30.72 s.

3. Results and discussion

3.1 Characterization

The crystalline phases of OM-Co₃O₄ and Co₃O₄-composite spinels were displayed in Fig. 1. It was worth noting that the precursors and OM-Co₃O₄ would result in the formation of composite spinels at high temperature. The lattice of Co₃O₄ would host other cations through the replacement of cobalt cations.¹⁹ However, no significant difference can be observed between OM-Co₃O₄ and Co₃O₄-composite spinels in Fig. 1. The well-defined diffraction peaks of 2θ = 19.00°, 31.27°, 36.85°, 38.54°, 44.81°, 55.66°, 59.36° and 65.24° were corresponded to (111), (220), (311), (222), (400), (422), (511) and (440), respectively. This might be ascribed to that the unit cell parameters of CoFe₂O₄, CoMn₂O₄, CuCo₂O₄ and NiCo₂O₄ were very close to that of Co₃O₄,¹⁹ thus these phases cannot be distinguished through XRD analysis. But according to Debye–Scherrer equation, the calculated mean crystallite sizes of OM-Co₃O₄, Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ were 17.43, 25.32, 22.87, 27.35 and 24.86 nm, respectively. Compared with OM-Co₃O₄, the increase of mean crystallite sizes in Co₃O₄-composite spinels indicated that the introduction of metal dopants destroyed original structure of OM-Co₃O₄.

Fig. 1 XRD patterns of OM-Co₃O₄ and Co₃O₄-composite spinels.

The TEM and HR-TEM images of OM-Co₃O₄ and Co₃O₄-composite spinels were showed in Fig. 2. It can be clearly seen that OM-Co₃O₄ showed a highly ordered mesoporous structure, and the spacing distances between two fringes are 0.285 and 0.467 nm, which were in conformity with (220) and (111) planes, respectively. Obviously, after the introduction of metal dopants, ordered mesoporous structure was partly or completely destroyed, which may be attributed to the formation of Co₃O₄-composite spinels. The lattice fringes can be clearly observed in HR-TEM images, indicating that highly crystalline nature of Co₃O₄-composite spinels, which was corresponded to the strong and sharp diffraction peaks in XRD analysis. Similarly with OM-Co₃O₄, the spacing distances between two fringes in Co₃O₄–CoFe₂O₄ were 0.281 and 0.471 nm, corresponding to (220) and (111) planes, respectively. And that in Co₃O₄–CoMn₂O₄ were 0.279 and 0.469 nm, which were also assigned to (220) and (111) planes, respectively. However, as for Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄, the (111) plane was not observed, and the lattice spacing of 0.275 and 0.286 nm was corresponded to (220) plane.

Fig. 2 TEM and HR-TEM images of OM-Co₃O₄ and Co₃O₄-composite spinels. (a)–(c) OM-Co₃O₄; (d)–(f) Co₃O₄–CoFe₂O₄; (g)–(i) Co₃O₄–CoMn₂O₄; (j)–(l) Co₃O₄–CuCo₂O₄; (m)–(o) Co₃O₄–NiCo₂O₄.

The surface areas and pore size distributions of OM-Co₃O₄ and composite materials were investigated by N₂ adsorption–desorption isotherms. As shown in Fig. 3(a), all materials showed type IV isotherms, and the specific surface areas of OM-Co₃O₄, Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ were 66.91, 52.34, 50.92, 30.39 and 29.53 m² g⁻¹, respectively. The reduction of specific surface areas in Co₃O₄-composite spinels was ascribed to the deterioration of ordered mesoporous structure after the impregnation of metal cations, which was in accordance with the observation of TEM images. Moreover, compared with Co₃O₄–CoFe₂O₄ and Co₃O₄–CoMn₂O₄, the more significant decrease of specific surface areas in Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ may be related to the oxidation state of the doped metal ions. The formation of Co₃O₄–MCo₂O₄ consumed more Co₃O₄ than Co₃O₄–CoM₂O₄,¹⁹ thus the deterioration of ordered mesoporous structure in Co₃O₄–MCo₂O₄ was more significant than that of Co₃O₄–CoM₂O₄, which can also be observed from TEM images. The changes of pore volume and pore diameter also demonstrated the conclusion, as seen in the Fig. 3(b), the pore volume and pore diameter all followed the order of OM-Co₃O₄ > Co₃O₄–CoM₂O₄ > Co₃O₄–MCo₂O₄. The textural parameters of OM-Co₃O₄ and Co₃O₄-composite spinels were summarized in Table 1.

Fig. 3 (a) Nitrogen adsorption–desorption isotherms of OM-Co₃O₄ and Co₃O₄-composite spinels and (b) their pore size distributions.

Table 1 Physicochemical properties of OM-Co₃O₄ and Co₃O₄-composite spinels

Samples	XRD	N2 adsorption–desorption			pHpzc
	Crystallite size (nm)	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
OM-Co3O4	17.43	66.91	0.135	8.08	5.85
Co3O4–CoFe2O4	25.32	52.34	0.105	7.19	4.21
Co3O4–CoMn2O4	22.87	50.92	0.094	6.32	3.93
Co3O4–CuCo2O4	27.35	30.39	0.083	5.91	4.76
Co3O4–NiCo2O4	24.86	29.53	0.086	6.21	5.37

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XPS analysis can be used to determine the surface composition and chemical oxidation states of OM-Co₃O₄ and Co₃O₄-composite spinels. In XPS spectra of OM-Co₃O₄ (Fig. 4(a)), the sharp peak emerged at 779.6 eV was assignable to Co 2p_3/2, which could be deconvoluted into octahedral Co³⁺ at 779.4 eV and tetrahedral Co²⁺ at 780.7 eV.²⁰ The proportions of Co²⁺ and Co³⁺ were determined to be 63.01% and 36.99%, respectively. The O 1s envelope (Fig. 4(b)) could be deconvoluted into two parts, namely the lattice oxygen (O_latt) at 529.2 eV and surface adsorbed oxygen (O_ads) at 530.8 eV.¹⁶ Based on this deconvolution, the proportions of O_latt and O_ads were found to be 50.86% and 49.14%, respectively. After the doping of iron into OM-Co₃O₄, the content of Co²⁺ increased from 63.01% to 65.34%, which could be ascribed to the substitution of Co³⁺ with Fe³⁺ in OM-Co₃O₄. It was worth noting that the content of O_ads increased from 50.86% to 51.74%, which was conductive to the PMS activation.⁴ As seen in Fig. 4(c), the doped iron existed in the form of positive trivalent.

Fig. 4 XPS survey spectrum of Co 2p_3/2 (a), O 1s (b) and Fe 2p_3/2 (c) for OM-Co₃O₄ and Co₃O₄–CoFe₂O₄.

3.2 Catalytic activity of Co₃O₄-composite spinels

ENR removal in different systems was presented in Fig. 5(a). Adsorption tests showed that all the catalysts exerted a low efficiency in ENR adsorption, and the highest efficiency was received by Co₃O₄–CoFe₂O₄ with 3.22% of ENR adsorption within 30 min, which may be attributed to the largest specific surface area. Although PMS is a strong oxidizing agent with oxidation potential of 1.82 V,²¹ only 16.36% ENR could be removed by PMS in the absence of activator. However, the ENR degradation was greatly enhanced in the presence of both Co₃O₄-composite spinels and PMS. ENR can be completely removed in Co₃O₄–CuCo₂O₄/PMS and Co₃O₄–CoMn₂O₄/PMS systems, and the removal efficiencies were 96.37% and 94.56%, respectively. Furthermore, the ENR degradation well followed a pseudo-first-order kinetics pattern:where [ENR]₀ is the initial ENR concentration, [ENR] is the concentration of ENR at time t, and k_app is the apparent rate constant. As seen in Fig. 4(b), the fitting k_app for Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ are 0.122, 0.255, 0.273 and 0.097 min⁻¹, respectively, which illustrated that the catalytic activity abided by the order of Co₃O₄–CuCo₂O₄ > Co₃O₄–CoMn₂O₄ > Co₃O₄–CoFe₂O₄ > Co₃O₄–NiCo₂O₄. In addition, Fig. 4(b) provided the PMS consumption during ENR oxidation. It can be seen that Co₃O₄–CuCo₂O₄ consumed the maximum PMS concentration (0.4 mM), which was 1.05, 1.74 and 2.00 times higher than Co₃O₄–CoMn₂O₄, Co₃O₄–CoFe₂O₄ and Co₃O₄–NiCo₂O₄, respectively. This result was also confirmed the sequence of catalytic activity of Co₃O₄-composite spinels.

Fig. 5 (a) ENR degradation in different systems; (b) values involved in different Co₃O₄-composite spinels/PMS systems; (c) EPR spectra in different composite spinels/PMS systems (DMPO = 25 mM). Experimental condition: [ENR] = 10 mg L⁻¹, [catalyst] = 0.1 g L⁻¹, [PMS] = 1 mM, pH₀ = 6, T = 25 °C.

EPR experiments were also performed for the comparison of the catalytic activity of Co₃O₄-composite spinels. As presented in EPR spectra (Fig. 5(c)), there was no distinctive EPR signal obtained by PMS alone. Nevertheless, simultaneous use of Co₃O₄-composite spinels and PMS could lead to obvious EPR signals. These EPR signals indicated the formation of 5,5-dimethyl-2-oxo-pyrroline-1-oxyl (DMPOX),^22,23 which was ascribed to the fast activation of PMS and efficient oxidation of DMPO,²⁴ also proofing the high catalytic performance of Co₃O₄-composite spinels. In addition, the signals of DMPOX caused by Co₃O₄–CuCo₂O₄/PMS and Co₃O₄–CoMn₂O₄/PMS systems were stronger than that caused by Co₃O₄–CoFe₂O₄ or Co₃O₄–NiCo₂O₄ activated PMS system (Fig. 5(b)), further authenticating the order of catalytic activity of Co₃O₄-composite spinels.

It was suggested that the catalytic performance of Co₃O₄-composite spinels not only depended on the specific surface area, but relied on dopant itself. In order to eliminate the difference in the specific surface area, the specific apparent rate constant k_sapp which defined as the ratio of k_app to the BET surface area was introduced:

where k_sapp is the specific apparent rate constant, and S_BET is the specific surface area of composite spinels. As shown in Fig. 5(b), the k_sapp of Co₃O₄–CuCo₂O₄ (8.983 × 10⁻³ g (m² min⁻¹)) was still the highest one which was 3.85, 1.79, 2.73 times higher than that of Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄ and Co₃O₄–NiCo₂O₄, respectively, indicating that Co and Cu ions possessed the best synergistic effect for PMS activation.

3.3 Effect of initial pH

The influence of initial pH ranging from 3 to 11 on ENR degradation was investigated in the PMS activation process, and the results were displayed in Fig. 6. From Fig. 6(a)–(e), it can be seen that four Co₃O₄-composite spinels all showed a wide pH range for PMS oxidation and higher ENR removals were obtained in pH range of 5 to 9 while lower removals occurred at strong acidic and alkaline conditions. Similar results were also reported by the previous investigations, such as degradation of orange II in MnFe₂O₄/PMS process and removal of acetaminophen in Fe₃O₄/PMS system.^25,26 The ENR degradation was significantly inhibited at strong acidic condition might be originated from the attachment of H⁺ to the peroxide bond (O–O) of PMS (eqn (3)) and the change of catalyst surface charge (eqn (4)), so that the interfacial repulsion would result in a weaker catalytic performance.²⁷ The retardation of ENR removal at strong alkaline condition can be ascribed to the following reasons: (1) the increase of catalyst surface negative charges. The pH_pzc of Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CoCo₂O₄ and Co₃O₄–NiCo₂O₄ was 4.21, 3.93, 4.76 and 5.37, respectively (Table 1). The surface charges of catalysts were negative when solution pH was higher than pH_pzc. Higher solution pH would cause higher amount of negative charges on catalyst surface, which could enhance the electrostatic repulsion between catalyst surface and PMS anions. Consequently, the catalytic performance decreased at strong alkaline condition. (2) The transform of dominant PMS species. Given that pK_a1 of H₂SO₅ was less than 0 and pK_a2 was 9.4, SO₅²⁻ would replace HSO₅⁻ and become dominant PMS species when solution pH was higher than 9.4. Compared with HSO₅⁻ (E⁰(HSO₅⁻/SO₄²⁻) = 1.75 V), SO₅²⁻ (E⁰(SO₅²⁻/SO₄²⁻) = 1.22 V) was less oxidative and more difficult to react.²⁸ Additionally, SO₅²⁻ could also lead to a stronger electrostatic repulsion between catalyst surface and PMS anions.

SO₂ − O − O − H + H⁺ → SO₂ − O − O − H₂⁺ (3)

[Cat − OH] + H⁺ ↔ [Cat − OH₂⁺] (4)

Fig. 6 Effect of initial pH on ENR degradation in different Co₃O₄-composite spinels/PMS systems: (a) Co₃O₄–CoFe₂O₄; (b) Co₃O₄–CoMn₂O₄; (c) Co₃O₄–CuCo₂O₄; (d) Co₃O₄–NiCo₂O₄. Values involved in different pH and Co₃O₄-composite spinels/PMS systems: (e) k_app; (f) k_sapp. Experimental condition: [ENR] = 10 mg L⁻¹, [catalyst] = 0.1 g L⁻¹, [PMS] = 1 mM, T = 25 °C.

The values of k_sapp were also calculated and the results were presented in Fig. 6(f). It could be more intuitionistic to compare the catalytic performances of four Co₃O₄-composite spinels in different conditions due to the elimination of difference in specific surface area. As shown in Fig. 5(f), Co₃O₄–CuCo₂O₄ maintained the highest k_sapp values with pH varied from 3 to 11, suggesting that Co and Cu species were the best combination for PMS activation among these four spinels. However, it should be noted that the lowest k_sapp value was showed by Co₃O₄–CoMn₂O₄ at pH 11 other than Co₃O₄–CoFe₂O₄ or Co₃O₄–NiCo₂O₄. This might be closely related to pH_pzc of catalysts, the pH_pzc of Co₃O₄–CoMn₂O₄ was 3.93 which was much lower than that of other three Co₃O₄-composite spinels. It manifested that Co₃O₄–CoMn₂O₄ could present lower performance at strong alkaline condition than others, thus Co₃O₄–CoMn₂O₄ possessed the highest k_sapp value.

3.4 Effect of temperature

The effect of reaction temperature (25, 35, 45 and 55 °C) on ENR removal in the process of PMS activation was studied. As displayed in Fig. 7(a–d), the ENR degradation in four Co₃O₄-composite spinels/PMS systems presented the similar trend, catalytic performances of Co₃O₄-composite spinels significantly increased with the increase of reaction temperature. As reaction temperature increased from 25 to 55 °C, k_app values of Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ increased from 0.122, 0.255, 0.273, 0.097 min⁻¹ to 0.496, 0.872, 0.898, 0.498 min⁻¹, respectively. This result may be due to the fact that higher reaction temperature simplified the rupture of O–O bond and generation of SO₄⁻˙.^29,30 In addition, higher reaction temperature was beneficial for reactant molecules to overcome activation energy barrier.¹⁷ The activation energy (E_a) could be determined by plotting ln k_app against 1/T based on Arrhenius equation (Fig. 7(e)). The obtained E_a values were 40.73, 33.85, 33.07 and 45.51 kJ mol⁻¹ in Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ activated PMS systems, respectively. The lower E_a value signified the higher catalytic reactivity, and the order of E_a was well corresponded to the sequence of catalytic activity. Moreover, all the E_a values were much higher than that of the diffusion-controlled reactions, which usually ranged from 10 to 13 kJ mol⁻¹.³¹ This implied that the apparent reaction rate for ENR removal during Co₃O₄-composite spinels activated PMS processes was dominated by the rate of intrinsic chemical reactions on the catalyst surface. It was reported that out-sphere interactions were usually diffusion-controlled reactions, thus PMS activation by Co₃O₄-composite spinels was most likely an inner-sphere electron-transfer process.³² The consumption of PMS during ENR oxidation processes was monitored (Fig. 7(f)). Similar with the trend of k_app, PMS consumption also increased as the reaction temperature increased. The PMS consumption caused by Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ increased from 0.230, 0.372, 0.400 and 0.200 mM to 0.796, 0.954, 0.968 and 0.696 mM with the reaction temperature increased from 25 to 55 °C, suggesting that higher reaction temperature was conducive to PMS activation and ENR degradation, which was well correspond to the conclusions by the observations of k_app values. In addition, the higher PMS consumption also reflected higher catalytic reactivity. From Fig. 7(f), the PMS consumption caused by Co₃O₄-composite spinels always showed the sequence of Co₃O₄–CuCo₂O₄ > Co₃O₄–CoMn₂O₄ > Co₃O₄–CoFe₂O₄ > Co₃O₄–NiCuFe₂O₄ even in different reaction temperatures, indicating that Co₃O₄–CuCo₂O₄ possessed the highest catalytic activity among the four Co₃O₄-composite spinels.

Fig. 7 Effect of reaction temperature on ENR degradation in different Co₃O₄-composite spinels/PMS systems: (a) Co₃O₄–CoFe₂O₄; (b) Co₃O₄–CoMn₂O₄; (c) Co₃O₄–CuCo₂O₄; (d) Co₃O₄–NiCo₂O₄. Parameters involved in different reaction temperatures and Co₃O₄-composite spinels/PMS systems: (e) Arrhenius curves; (f) ΔPMS. Experimental condition: [ENR] = 10 mg L⁻¹, [catalyst] = 0.1 g L⁻¹, [PMS] = 1 mM, pH₀ = 6.

3.5 Radical identification and catalytic mechanism

Three different scavengers, tert-butyl alcohol (TBA), ethanol (EtOH) and phenol were employed to identify the dominant radical species in Co₃O₄-composite spinels/PMS systems. TBA can rapidly react with ˙OH (k_˙OH = 3.8–7.6 × 10⁸ M⁻¹ s⁻¹) but has a much lower reactivity with SO₄⁻˙ (k_SO4⁻˙ = 4–9.1 × 10⁵ M⁻¹ s⁻¹),³³ and EtOH is a well scavenger for ˙OH and SO₄⁻˙ (k_˙OH = 1.2–2.8 × 10⁹ M⁻¹ s⁻¹, k_SO4⁻˙ = 1.6–7.7 × 10⁷ M⁻¹ s⁻¹).³⁴ Phenol can also react with ˙OH and SO₄⁻˙ at a high rate (k_˙OH = 6.6 × 10⁹ M⁻¹ s⁻¹, k_SO4⁻˙ = 8.8 × 10⁹ M⁻¹ s⁻¹).³⁵ In view of the difference in reaction rates, using TBA, EtOH and phenol as scavengers was a feasible program for the identification of primary active species.

As presented in Fig. 8(a–d), only a slight reduction of ENR removal could be obtained in the presence of 10 or 100 mM TBA, implying that ˙OH was involved in Co₃O₄-composite spinels activated PMS processes. With the addition of 10 mM EtOH, the ENR degradation was significantly inhibited and the removal efficiencies in Co₃O₄–CoFe₂O₄, Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ activated PMS processes were decreased from 97.37%, 100%, 100%, 94.56% to 66.84%, 69.92%, 82.42%, 57.81%, respectively. More addition of EtOH (100 mM) would cause further reduction of ENR removal efficiency with 47.90%, 47.70% 53.96% and 40.43%, respectively. In order to further confirm the contribution of SO₄⁻˙, 10 mM phenol was introduced into solutions which resulted in a more significant inhibition to ENR removal, less than 18% of ENR could be decomposed in the four Co₃O₄-composite spinels/PMS processes. The quenching tests clearly suggested that SO₄⁻˙ was the primary reactive species during PMS activation by Co₃O₄ composite spinels and ˙OH was also involved in these processes.

Fig. 8 Effect of quenchers on ENR degradation in different Co₃O₄-composite spinels/PMS systems: (a) Co₃O₄–CoFe₂O₄; (b) Co₃O₄–CoMn₂O₄; (c) Co₃O₄–CuCo₂O₄; (d) Co₃O₄–NiCo₂O₄. Experimental condition: [ENR] = 10 mg L⁻¹, [catalyst] = 0.1 g L⁻¹, [PMS] = 1 mM, pH₀ = 6, T = 25 °C.

XPS analysis of Co₃O₄–CoFe₂O₄ before and after catalytic oxidation was also performed to illustrate the heterogeneous catalytic mechanism (Fig. 4). As shown in Fig. 4(a), before catalytic oxidation, the contents of Co²⁺ and Co³⁺ was determined to be 65.34% and 34.66%. After catalytic oxidation, the proportions of Co²⁺ and Co³⁺ were changed to 61.93% and 38.07%, respectively. The partial increase of Co³⁺ was ascribed to the electrons donating of Co²⁺ during the oxidation process. In the case of O 1s spectra (Fig. 4(b)), the content of O_latt decreased from 48.26% to 46.35%, and the proportion of O_ads increased from 51.74% to 53.65%. The increment of O_ads can be attributed to the generation of Co–OH or O₂ adsorbed on the surface of Co₃O₄–CoFe₂O₄. It has been reported that ≡Co²⁺ − ⁻OH was the critical species for the generation of radicals during the process of PMS activation.³⁶ Of note, the Fe 2p_3/2 envelope could be deconvoluted into Fe²⁺ at 709.5 eV, which indicated that the redox reactions between Co and Fe were involved in the PMS activation.

Based on the results of quenching experiments and XPS analysis, the plausible mechanisms of Co₃O₄-composite spinels activated PMS were put forward. Taking Co₃O₄–CoFe₂O₄/PMS system as example, H₂O molecules were firstly physically absorbed on the part of ≡Co²⁺ sites to generate ≡Co²⁺–⁻OH. Then, ≡Co²⁺ − ⁻OH would react with HSO₅⁻ to form SO₄⁻˙ after introduction of PMS (eqn (5)), and could regenerate through the reaction between formed ≡Co³⁺ − ⁻OH species and HSO₅⁻ (eqn (6)). Similarly, ≡Fe³⁺ could also combine with dissociative adsorption of H₂O molecules to form ≡Fe³⁺–⁻OH, which would transform to ≡Fe²⁺ − ⁻OH (eqn (7)) and generate SO₄⁻˙ by reacting with HSO₅⁻ (eqn (8)). In addition, due to the standard redox potential of ≡Co³⁺/≡Co²⁺ was 1.92 V,⁸ while the standard redox potential of ≡Fe³⁺/≡Fe²⁺ was 0.77 V,³⁷ the reduction of ≡Co³⁺ by ≡Fe²⁺ was thermodynamically feasible (eqn (9)). The efficient regeneration of surface ≡Co²⁺ by this process may be able to remain the stability and high efficiency of Co₃O₄–CoFe₂O₄. Besides, ≡Co²⁺ or ≡Fe²⁺ on catalyst surface could also react with PMS to produce ˙OH (eqn (10) and (11)), which could also be generate by the transformation of SO₄⁻˙ (eqn (12) and (13)). Co₃O₄–CoMn₂O₄, Co₃O₄–CuCo₂O₄ and Co₃O₄–NiCo₂O₄ activated PMS processes all presented the similar mechanism with Co₃O₄–CoFe₂O₄ activated PMS process, and the reactions which were involved in Co₃O₄-composite spinels/PMS systems were listed in Table 2.

Table 2 Reactions involved in composite spinels/PMS systems

Systems	Reactions
Co3O4–CoFe2O4/PMS	≡Co^2+ − ^−OH + HSO5^− → ≡Co^3+ − ^−OH + SO4^−˙ + OH^−	(5)
	≡Co^3+ − ^−OH + HSO5^− → ≡Co^2+ − ^−OH + SO5^−˙ + H^+	(6)
	≡Fe^3+ − ^−OH + HSO5^− → ≡Fe^2+ − ^−OH + SO5^−˙ + H^+	(7)
	≡Fe^2+ − ^−OH + HSO5^− → ≡Fe^3+ − ^−OH + SO4^−˙ + OH^−	(8)
	≡Fe^2+ + ≡Co^3+ → ≡Fe^3+ + ≡Co^2+	(9)
	≡Co^2+ + HSO5^− → ≡Co^3+ + SO4^2− + ˙OH	(10)
	≡Fe^2+ + HSO5^− → ≡Fe^3+ + SO4^2− + ˙OH	(11)
	SO4^−˙ + H2O → HSO4^− + ˙OH	(12)
	SO4^−˙ + OH^− → SO4^2− + ˙OH	(13)
Co3O4–CoMn2O4/PMS	≡Co^2+ − ^−OH + HSO5^− → ≡Co^3+ − ^−OH + SO4^−˙ + OH^−
	≡Co^3+ − ^−OH + HSO5^− → ≡Co^2+ − ^−OH + SO5^−˙ + H^+
	≡Mn^3+ − ^−OH + HSO5^− → ≡Mn^2+ − ^−OH + SO5^−˙ + H^+
	≡Mn^3+ − ^−OH + HSO5^− → ≡Mn^4+ − ^−OH + SO4^−˙
	≡Mn^2+ − ^−OH + HSO5^− → ≡Mn^3+ − ^−OH + SO4^−˙
	≡Mn^4+ − ^−OH + HSO5^− → ≡Mn^3+ − ^−OH + SO5^−˙ + H^+
	≡Mn^2+ + ≡Co^3+ → ≡Mn^3+ + ≡Co^2+
	≡Mn^3+ + ≡Co^3+ → ≡Mn^4+ + ≡Co^2+
	≡Co^2+ + HSO5^− → ≡Co^3+ + SO4^2− + ˙OH
	≡Mn^2+ + HSO5^− → ≡Mn^3+ + SO4^2− + ˙OH
	SO4^−˙ + H2O → HSO4^− + ˙OH
	SO4^−˙ + OH^− → SO4^2− + ˙OH
Co3O4–CuCo2O4/PMS	≡Co^2+ − ^−OH + HSO5^− → ≡Co^3+ − ^−OH + SO4^−˙ + OH^−
	≡Co^3+ − ^−OH + HSO5^− → ≡Co^2+ − ^−OH + SO5^−˙ + H^+
	≡Cu^2+ − ^−OH + HSO5^− → ≡Cu^+ − ^−OH + SO5^−˙ + H^+
	≡Cu^2+ − ^−OH + HSO5^− → ≡Cu^3+ − ^−OH + SO4^−˙ + OH^−
	≡Cu^+ − ^−OH + HSO5^− → ≡Cu^2+ − ^−OH + SO4^−˙ + OH^−
	≡Cu^3+ − ^−OH + HSO5^− → ≡Cu^2+ − ^−OH + SO5^−˙ + H^+
	≡Cu^3+ + ≡Co^2+ → ≡Cu^2+ + ≡Co^3+
	≡Cu^+ + ≡Co^3+ → ≡Cu^2+ + ≡Co^2+
	≡Co^2+ + HSO5^− → ≡Co^3+ + SO4^2− + ˙OH
	≡Cu^2+ + HSO5^− → ≡ ^3+ + SO4^2− + ˙OH
	SO4^−˙ + H2O → HSO4^− + ˙OH
	SO4^−˙ + OH^− → SO4^2− + ˙OH
Co3O4–NiCo2O4/PMS	≡Co^2+ − ^−OH + HSO5^− → ≡Co^3+ − ^−OH + SO4^−˙ + OH^−
	≡Co^3+ − ^−OH + HSO5^− → ≡Co^2+ − ^−OH + SO5^−˙ + H^+
	≡Ni^2+ − ^−OH + HSO5^− → ≡Ni^3+ − ^−OH + SO4^−˙ + OH^−
	≡Ni^3+ − ^−OH + HSO5^− → ≡Ni^2+ − ^−OH + SO5^−˙ + H^+
	≡Co^2+ + HSO5^− → ≡Co^3+ + SO4^2− + ˙OH
	≡Ni^2+ + HSO5^− → ≡Ni^3+ + SO4^2− + ˙OH
	SO4^−˙ + H2O → HSO4^− + ˙OH
	SO4^−˙ + OH^− → SO4^2− + ˙OH

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3.6 Reusability and stability of different Co₃O₄-composite spinels

The reusability of heterogeneous catalysts is an important indicator to assess the industrial application potential of heterogeneous catalysts. In this study, the reusability of different Co₃O₄-composite spinels was evaluated and the results were presented in Fig. 9(a). As shown, after being reused for three times, all the Co₃O₄-composite spinels still remained high catalytic activity toward PMS. Taking Co₃O₄–CoFe₂O₄ as example, after three consecutive runs, just a slight reduction in ENR removal was observed and their values were 94.89%, 91.32% and 89.27%, respectively. The slight loss of catalytic activity was mainly ascribed to the leaching of metal ions during the consecutive runs. Therefore, the stability of different Co₃O₄-composite spinels was further investigated and the results were reported in Fig. 9(b). As exhibited, the leaching of cobalt was clearly observed after each run. With the increase of cycle times, the leaching concentration decreased. Of note, after doped different transition metals into OM-Co₃O₄, the cobalt leakage can be effectively controlled, which was attributed to the intimate interactions between two metals.² Therefore, Co₃O₄-composite spinels are ideal PMS activator for environmental remediation.

Fig. 9 (a) Reusability of different Co₃O₄-composite spinels as catalyst for the degradation of ENR; (b) cobalt leaching concentrations in different Co₃O₄-composite spinels/PMS systems. Experimental condition: [ENR] = 10 mg L⁻¹, [catalyst] = 0.1 g L⁻¹, [PMS] = 1 mM, pH₀ = 6, T = 25 °C, reaction time = 25 min.

4. Conclusion

Co₃O₄-composite spinels were successfully synthesized through doping transition metals (Fe, Mn, Cu and Ni) to ordered mesoporous Co₃O₄. The obtained Co₃O₄-composite spinels all showed outstanding catalytic activity toward PMS. Co₃O₄–CuCo₂O₄ exhibited the highest catalytic performance in PMS solution, followed by Co₃O₄–CoMn₂O₄, Co₃O₄–CoFe₂O₄ and Co₃O₄–NiCo₂O₄. ENR degradation would be retarded in strong acidic and alkaline conditions, the improvement of reaction temperature could significantly accelerate ENR decomposition. Sulfate radical was confirmed to be the primary reactive species in Co₃O₄-composite spinels activated PMS processes and hydroxyl radical was also involved in these processes. The synergistic effect between two metals in Co₃O₄-composite spinels was the vital reason for the high catalytic reactivity. Co₃O₄-composite spinels displayed satisfactory reusability and doping different transition metals into OM-Co₃O₄ can effectively control the cobalt leaching. In consideration of cost and toxicity, Co₃O₄-composite spinels might have great potential in pollution control than OM-Co₃O₄.

Conflicts of interest

No conflict of interest exists in the submission of this manuscript and manuscript is approved by all authors for publication.

Acknowledgements

The authors are grateful for the financial support of this study by the National Natural Science Foundation of China (No. 51508509), China Postdoctoral Science Foundation (No. 2015M581936), Natural Science Foundation of Zhejiang Province (No. LY18E080036) and State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF16017).

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