Efficient removal of organic pollutant by activation of persulfate with magnetic Co3O4/CoFe2O4 composite

Huixuan Zhang; Li-chao Nengzi; Yu Liu; Yingjie Gao; Xiuwen Cheng

doi:10.1016/j.arabjc.2020.03.012

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Original article

13 (

); 5332-5344

doi:

10.1016/j.arabjc.2020.03.012

Efficient removal of organic pollutant by activation of persulfate with magnetic Co₃O₄/CoFe₂O₄ composite

Huixuan Zhang^{^a}, Li-chao Nengzi^{^b}, Yu Liu^{^a}, Yingjie Gao^{^a}, Xiuwen Cheng^{^a,^b,⁎}

a Key Laboratory of Western China's Environmental Systems (Ministry of Education) and Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China

b Academy of Economics and Environmental Sciences, Xichang University, Xichang 615000, China

⁎Corresponding author at: College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China. chengxw@lzu.edu.cn (Xiuwen Cheng)

Received: 2019-8-13, Accepted: 2020-3-15, Published: 2020-05-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

In this study, magnetic spinel Co₃O₄/CoFe₂O₄ composite were synthesized by the mechanical mixing of both powdered pristine samples. Then the catalyst was characterized by TEM, SEM, XRD, BET, XPS and VSM measurement. Next, Co₃O₄/CoFe₂O₄ composite was applied to degrade rhodamine B (RhB) in water by activating persulfate. Results showed that Co₃O₄/CoFe₂O₄ composite exhibited high efficiency for removal of RhB, and 95.59% of it could be degraded in 45 min. Besides, the effects of parameters, such as initial pH, PS dosage, Co₃O₄/CoFe₂O₄ composite dosage, initial concentration of RhB and temperture were studied. Also, the effects of coexisting anions on RhB degradation were observed and explained. Furthermore, we conducted the quenching experiment and found that sulfate radical and hydroxyl radicals were the main active radicals in the degradation process. Finally, recycle experiments proved that Co₃O₄/CoFe₂O₄ had a good stability for RhB degradation. In short, Co₃O₄/CoFe₂O₄ composite is a promising catalyst for wastewater treatment.

Keywords

Co3O4/CoFe2O4 composite

Persulfate

Rhodamine B

Sulfate radicals

Hydroxyl radicals

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1 Introduction

Today, water environment pollution is one of the major environmental problems in the world. Among various types of polluted water, printing and dyeing wastewater, as one of the hard-degradable wastewater, is gradually receiving more and more attention (Hou et al., 2016; He et al., 2018). Due to the processing of chemical fiber and other spinning products, the printing and dyeing wastewater contains a large amount of dyes, oils, starch alkalis, resins and inorganic salts, etc. It is very difficult to deal with. Most of the dyes are easily dissolved in water in form of cationic (rhodamine B, malachite green, methylene blue, and methyl violet) and anionic (Congo red, methyl orange, and rose Bengal) (Nafiey et al., 2017).

Advanced oxidation technology is a new technology which can efficiently remove difficult-to-degrade pollutants from water. In this method, highly active radicals (HO^•, SO₄^•−, etc.) can be generated through various routes to degradate pollutants, including Fenton method (Wang et al., 2009), photocatalytic oxidation method (Cheng et al., 2015), electrochemical oxidation method (Li et al., 2013), etc. Among these approaches, SO₄^•−-based technology has received increasing attention due to its higher redox potential (2.5–3.1 V) based on the activation methods compared with HO^• (Lutze et al., 2015; Ghauch and Tuqan, 2012). Apart from these, SO₄^•− has higher selectivity and longer half-life over HO^• in some cases (Ahmed et al., 2012). It can be generated by persulfate (PS) and peroxymonosulfate (PMS) activation through heat (Antoniou et al., 2010; Huang et al., 2002), ultraviolet (Chu et al., 2016; Sharma et al., 2016; Sharma et al., 2015 ), ultrasound (Gayathri et al., 2010; Chakma et al. 2017; Babu et al. 2017; Neppolian et al., 2010 ), electron beam (Criquet and Leitner, 2011; Furman et al., 2010); electrochemical (Ahmadi and Ghanbari, 2016; Ghanbari and Martinez-Huitle, 2019; Popat et al., 2019 ); zero valent metal (Khatri et al., 2018; Devi et al., 2016) and transition metal ions (Xie et al., 2016; Rastogi et al., 2009; Ayoub and Ghauch, 2014; Senthilnathan and Nachiappana, 2015; Sarath et al., 2014; Nidheesh and Rajan, 2016; Asha et al., 2017 ). Particularly, potassium persulfate (K₂S₂O₈), as one of the common source of SO₄^•−, attached much attention due to its moderate cost, high stability, high solubility in aqueous solution, and it exists in the form of solid at room temperature (Zacharias et al., 2016; Lin et al., 2011). In view of economy and operation, choosing transition metal to activate PS is a good choice because it doesn't require energy. Furthermore, the Co²⁺ was proven to be most effective transition metal ions (Anipsitakis and Dionysiou, 2003). However, Co²⁺ is highly soluble in water environments, which could induce health problems (Huang and Huang, 2009; Yang et al., 2009; Ji et al., 2016 ). In order to overcome this shortcoming, heterogeneous Co catalysts such as CoO, CoO₂, Co₂O₃, and Co₃O₄, have been used to activate PS. Among them, Co₃O₄, as one of the P-type metal oxide semiconductors, attracted wide attention due to its magnetic (Wang et al., 2015). Besides, owing to its low valence band maximum (VBM), Co₃O₄ has a superior oxidation capacity (Bautista et al., 2008). What's more, spinel ferrites (MFe₂O₄ (M = Fe, Cu, Co, Ni, etc.)) have high catalytic activity, stable crystalline structure, extremely low solubility and good magnetic in terms of activating PS in water environment. So they have been taken as very promising candidates (Yang et al., 2009; Zhang et al., 2013; Deng et al., 2013 ). In particular, spinel cobalt ferrite (CoFe₂O₄) has been confirmed to be one of the efficient catalyst which can activate PMS to degrade organic compounds due to its excellent magnetic property and surface active Co2+ (Ren et al., 2015; Liu et al., 2015). Sun et al. (2016) fabricated an octahedral CoFe/CoFe₂O₄ submicron composite by a flexible one-pot hydrothermal, which can efficient activate PMS to decompose methyl orange. Li et al. (2017) successfully prepared polypyrrole/CNTs-CoFe₂O₄ magnetic nanohybrid (CNTs-CoFe₂O₄@PPy), which was used as a superior adsorbent and catalyst to remove anionic and cationic dyes according activating PMS. However, at present, there is less research on activating PS with CoFe₂O₄ to produce sulfate radicals for degradation of pollutants. If combine Co₃O₄ with CoFe₂O₄, the cobalt leaching will reduce and magnetic will strengthen, thus catalysts can be better recycled.

In this study, we first prepared Co₃O₄ by calcination, then syntheized CoFe₂O₄ by a simple one-step hydrothermal method. Subsequently, we combined the two of catalysts by mechanical stirring. Next, the physicochemical properties of Co₃O₄/CoFe₂O₄ composite were analyzed. After this, we evaluated the catalytic activity of Co₃O₄/CoFe₂O₄ by degradation of RhB via the formation of sulfate radicals and hydroxyl radicals by activating PS. Finally, we proposed the possible reaction mechanism towards the degradation of RhB.

2 Materials and methods

2.1

2.1 Materials

Ferric nitrate (Fe(NO₃)₃·6H₂O), cobalt nitrate (Co(NO₃)₂·6H₂O), and potassium persulfate(K₂S₂O₈) were purchased from Tianjin Kemiou Chemical Reagent Co. LTD. Cobalt chloride (CoCl₂·6H₂O), urea (CO(NH₂)₂) and ascorbic acid (C₆H₈O₆) were provided by Sinopharm Chemical Reagent Co. Ltd. Ultra-pure water was used throughout the whole experiment.

2.2

2.2 Synthesis of Co₃O₄ powder

In a typical experiment, 5 g of Co(NO₃)₂·6H₂O was placed in a closed ceramic crucible and heated in a muffle furnace at around 400 °C for 4 h. After the furnace cooling to room temperature, Co₃O₄ powder was obtained. All chemicals were of analytical reagent grade and used without any further purification. Ultra-pure water was used throughout this experiment.

2.3

2.3 Synthesis of Co₂Fe₂O₄ powder

CoFe₂O₄ were synthesized by a simple one-step hydrothermal method. 3.9975 mmol Fe(NO₃)₃·9H₂O, 1.995 mmol CoCl₂·6H₂O, 4.26 mmol C₆H₈O₆ and 15 mmol CO(NH₂)₂ were slowly added to the 100 mL breaker which contained 60 mL distilled water. Afterwards, it was stirred under magnetic for 30 min until dissolved. Subsequently, the mixture was transferred into an autoclave and treated at 160 °C for 6 h. After centrifuging and washing with water and ethanol for several times, the black precipitate was dried at 80 °C for 3 h. Finally, it was calcined in static air at 500 °C for 4 h.

2.4

2.4 Synthesis of Co₃O₄/Co₂Fe₂O₄ composite

The Co₃O₄/CoFe₂O₄ composite was fabricated by the mechanical mixing method. In detail, a certain amount of Co₃O₄ and CoFe₂O₄ powders were dispersed in absolute ethanol and stirred for 2 h under magnetic. After, the powders were dried at 70 °C for 4 h.

2.5

2.5 Characterization

In this paper, the surface morphology of Co₃O₄/CoFe₂O₄ composite catalyst was observed by scanning electron microscopy (SEM) at 20 kV. Transmission electron microscopy (TEM) was conducted on a FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. The crystal structure, composition and lattice constant of catalysts were measured on a Rigaku D/MAX III-3B X-ray diffractometer (XRD) with Cu kα (λ = 0.15418 nm) radiation. And the accelerating voltage and applied current were held at 40 kV and 200 mA, respectively. The specific surface area and pore size distribution of samples were measured on an AUTOSORB-1 (Quantachrome Instruments) at 77 K. X-ray photoelectron spectroscopy (XPS) analysis of the catalysts obtained were conducted on PHI-5700 ESCA apparatus with an Al Kα X-ray source to explore the elements of the surface, and the binding energies were referenced to adventitious carbon for 284.62 eV. Lakershore Model 7304 instrument was used to measure the magnetic properties of catalysts. Magnetic measurement was performed at room temperature by using a physical property measurement system (PPMS, 730 T, LAKESHORE, USA)

2.6

2.6 Pollutants degradation experiment

As for the organic pollutant degradation tests, RhB was selected to be a typical model contaminant. In each run, a certain amount of catalysts above was added into 50 mL of RhB solution (10 mg·L⁻¹) under stirring. The initial pH value of RhB solution solution was about 5.49 without adjustment. Before to reaction, the suspension was kept in dark for 20 min to ensure the establishment of adsorption/desorption equilibrium between catalysts and rhodamine B. Next, a fixed amount of PS was added to initiate the catalytic reaction. At given time intervals, the collected samples after centrifugation and filtration (through a 0.22 μm membrane) were measured at the characteristic peak of 550 nm by using a T6 ultraviolet-visible spectrophotometer.

3 Results and discussion

3.1

3.1 SEM and TEM analysis

The morphological observations clearly indicated the successful synthesis of Co₃O₄/CoFe₂O₄ composite. As shown in Fig. 1a, a large amount of nanoparticles gathered together. However, it was difficult to distinguish Co₃O₄ and CoFe₂O₄ since both of them were spherical particles. From the TEM image (Fig. 1b), a very serious aggregation phenomenon was also exhibited. Besides, EDXA elemental mapping and the selected energy dispersive spectrum (EDS) were also analyzed in order to see the element dispersion of the Co₃O₄/CoFe₂O₄ composite. As displayed in Fig. 1c–f, the composite was composed of Co, Fe and O elements.

TEM image (a), SEM image (b), EDXA elemental mapping (Co, Fe and O elements (c–e)) and EDS spectroscopy (f) of Co3O4/CoFe2O4 composite.

3.2

3.2 XRD analysis

XRD patterns further indicated the crystal structure of the catalysts. Seen from Fig. 2, The diffraction peaks of pure Co₃O₄ appearing at 18.9°, 31.2°, 36.8°, 45.2°, 59.6° and 65.2° could be perfectly assigned to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) crystal planes of Co₃O₄ respectively. For single CoFe₂O₄, the peaks at 30.2°, 35.5°, 42.3°, 57.2°, and 62.9° were detected, which could be indexed to the cubic spinel structure of cobalt ferrite (Li et al., 2011). As for pure Co₃O₄/CoFe₂O₄ catalyst, all the characteristic peaks belonging to Co₃O₄ didn’t shift. However, due to the lower doping content of CoFe₂O₄ nanoparticles, only the peak at 35.5° could be observed, which could be attributed to the (3 1 1) crystal plane of cobalt ferrite, the other diffraction peaks of CoFe₂O₄ were not be directly detected.

XRD patterns of Co3O4, CoFe2O4 and Co3O4/CoFe2O4 samples.

3.3

3.3 Nitrogen adsorption-desorption

It is well known that the catalytic properties of materials are also closely related to their specific surface area and porosity. We studied the structural properties of pure Co₃O₄ and Co₃O₄/CoFe₂O₄ composites by nitrogen adsorption analysis. As shown in Fig. 3, according to the IUPAC classification, the isotherms of pure Co₃O₄ and Co₃O₄/CoFe₂O₄ were type IV, characteristic for mesoporous structures, with relatively similar H₂ hysteresis loops. The specific surface area of Co₃O₄/CoFe₂O₄ was calculated to be 13.894 m²·g⁻¹, which was larger than that of pure Co₃O₄ (12.143 m²·g⁻¹). For pure Co₃O₄, the BJH pore size analysis on the desorption branch exhibited only a peak centered at 35.04 nm. The pore size distribution of Co₃O₄/CoFe₂O₄ composite also with one peak at 36.09 nm. Both of the samples had the good homogeneity of the pores. Similarly, the Co₃O₄/CoFe₂O₄ composite had a bigger pore volume (0.146 m³·g⁻¹) compared with pure Co3O4 (0.132 m³·g⁻¹). Large accessible surface area and numerous pores made the catalyst to provide more reactive sites, thus enhanced the catalytic performance.

N2 adsorption and desorption isotherm and BJH pore-size distribution of Co3O4 (a) and Co3O4/CoFe2O4 (b).

3.4

3.4 XPS measurement

The XPS spectra of the Co₃O₄/CoFe₂O₄ composite were recorded and given in Fig. 4. As shown in Fig. 4a, the de-convolution of the Co 2p peak in CoFe₂O₄ exhibited four peaks. From CoFe₂O₄, the peak at 780.7 eV was attributed to Co 2p_3/2, while the peak at 795.65 eV was caused by Co 2p_1/2. What's more, the peaks of the satellites at about 785.75 eV and 802.67 eV were two shake-up type peaks of Co at the high binding energy side of the Co 2p_3/2 and Co 2p_1/2 edges. However, when Co₃O₄ was combined with CoFe₂O₄, the de-convolution of the Co 2p peak in Co₃O₄/CoFe₂O₄ composite exhibited six peaks. The peaks at 779.59 eV and 794.36 eV were assigned to Co³⁺, whereas the peaks at 781.07 eV and 795.97 eV were attributed to Co2+ (Li et al., 2011). Besides, the peaks at 788.97 eV and 803.99 eV belonged to the shake-up satellite peaks (Deng et al., 2016). This suggested that the Co species existed in the form of Co²⁺ and Co³⁺ in the Co₃O₄/CoFe₂O₄ composite. In addition, from the peak area, it could be determined that the content of Co³⁺ was much more than that of Co²⁺, which also indicated that the content of Co₃O₄ was much higher than that of CoFe₂O₄, which was consistent with the results of SEM and TEM. The Fe 2p XPS spectra of the CoFe₂O₄ and the Co₃O₄/CoFe₂O₄ samples were presented in Fig. 4b. Two peaks at a binding energy of 710.75 eV and 724 eV, corresponding to Fe 2p_3/2 and Fe 2p_1/2, respectively, which suggested the presence of Fe³⁺. Furthermore, the peak at 717.35 eV and 732.66 were assigned to shake-up satellite peaks (Salunkhe et al., 2015). However, it could be observed that the binding energy of Fe 2p_1/2 shifted from 724 eV of the CoFe₂O₄ to 722.35 eV of the Co₃O₄/CoFe₂O₄ composite. Seen from Fig. 4c, the high resolution XPS spectrum of O1s (CoFe₂O₄) included two peaks with binding energy at 529.59 and 531.31 eV, corresponding to the surface lattice oxygen species (O_latt) and the surface adsorbed oxygen species (O_ads), respectively. But it had a slightly decrease in Co₃O₄/CoFe₂O₄. Compared with pure CoFe₂O₄, the varied binding energy values of Co 2p, Fe 2p and O 1s for the Co₃O₄/CoFe₂O₄ composite meant the rapid transfer of electrons.

High-resolution XPS spectrum of Co 2p (a), Fe 2p (b) and O 1 s (c) for CoFe2O4 and Co3O4/CoFe2O4 samples.

3.5

3.5 VSM analysis

As is known to all, pulverous catalysts were difficult to separate and recycle from the slurry system, which caused the practical bottleneck of applications. In this paper the magnetic property of the Co₃O₄/CoFe₂O₄ composite was confirmed by the M−H analysis. As given in Fig. 5, the saturation magnetization (MS) value of Co₃O₄/CoFe₂O₄ composite was 6.13 emu·g⁻¹, which was lower than that of pure Co₃O₄/CoFe₂O₄ sample (59.62 emu·g⁻¹). This decrease could be explained that the introduced Co₃O₄ sample was non-magnetic. Despite this, Co₃O₄/CoFe₂O₄ composite still performed good magnetic property.

Hysteresis loop diagram of CoFe2O4 (a) and Co3O4/CoFe2O4 (b) sample.

3.6

3.6 Degradation performance

3.6.1

3.6.1 Activation of PS by Co₃O₄/CoFe₂O₄ composite for RhB degradation

Fig. 6 exhibited the degradation of RhB under different conditions. As seen from Fig. 6, the catalyst alone did not substantially remove RhB, only 6.61%, 2.09% and 8.08% of RhB was removed by CoFe₂O₄, Co₃O₄ and Co₃O₄/CoFe₂O₄ composite, respectively. The low removal efficiencies might be due to the weak adsorption capacity of these materials. Besides, 32.15% RhB could be removed by PS in the absence of activator owing to the high oxidation potential of PS (2.01 V). Also, 36.56% of RhB was removed in 45 min in CoFe₂O₄/PS system, suggesting that CoFe₂O₄ alone could hardly efficiently activate PS. However, the degradation efficiency of RhB was greatly enhanced in the presence of both Co₃O₄ spinel and PS. The Co₃O₄/CoFe₂O₄ composite performed best degradation of RhB, which could almost completely remove the RhB (95.59%), which was attributed to the coupling effect of Co₃O₄ and CoFe₂O₄.

Removal curves of RhB under different reaction conditions. Experimental conditions: catalyst = 0.5 g·L−1, PS = 0.5 g·L−1, pH = 5.49 (natural) and RhB = 10 mg·L−1.

3.6.2

3.6.2 Effects of persulfate concentration, catalyst dosage, solution pH and reaction temperature

The degradation of RhB in Co₃O₄/CoFe₂O₄/PS system with different PS doses was investigated and exhibitied in Fig. 7a. The removal efficiency of RhB increased from 8.08% to 96.64% with the PS dosage increasing from 0 to 0.9 g·L⁻¹, respectively. This showed that PS provided a source of SO₄^•− and RhB removal was indeed caused by oxidative degradation, rather than a simple adsorption by the catalyst. However, excessive persulfate would cause self-quenching of SO₄^•− (Liang et al., 2004) (reaction (1) and (2)), the degradation efficiencies of RhB were similar when the amount of PS was 0.5 g·L⁻¹ and 0.9 g·L⁻¹. In addition, the reaction stoichiometric efficiency (RSE) was used to evaluate the utilization efficiency of PS according previous studies (Jaafarzadeh et al., 2017; Amasha et al., 2018). The calculated RSE% was the largest when the PS concentration was 0.5 g·L⁻¹. Thus 0.5 g·L⁻¹ was selected as the optimal concentration of PS and used in later experiments.

(1)

{S O}_{4}^{∙ -} + {S O}_{4}^{∙ -} \to S_{2} O_{8}^{2 -}

(2)

{S O}_{4}^{∙ -} + S_{2} O_{8}^{2 -} \to S_{2} O_{8}^{∙ -} + {S O}_{4}^{2 -}

Influence of initial pH (a) amount of persulfate (b) amount of catalyst (c) initial temperature (d) on RhB degradation. Experimental conditions: catalyst = 0.5 g·L−1, PS = 0.5 g·L−1, pH = 5.49 (natural) and RhB = 10 mg·L−1.

Fig. 7b showed the degradation of RhB under different Co₃O₄/CoFe₂O₄ dosages (0–0.9 g·L⁻¹). Generally speaking, higher the Co₃O₄/CoFe₂O₄ dosage resulted in higher RhB degradation efficiency, which could be attributed to the increased number of active sites. As exhibited in Fig. 6b, the degradation efficiency of RhB increased as the amount of catalyst increased to 0.5 g·L⁻¹, and then hardly increased. Although a large number of catalysts could provide more active sites to activate PS, it would also capture the active radicals (Wang et al., 2018). In addition, high concentrations of catalysts would gather together, thus reduced the available amount of active sites.

Due to the wide range of pH in natural water bodies, it was very important to study the optimal reaction conditions for the removal of refractory organic pollutants using Co₃O₄/CoFe₂O₄/PS system. In this study, the effects of initial solution pH on RhB removal were tested in a broad pH ranging from 3.0 to 11.0 at PS dosage of 0.5 g·L⁻¹ and the Co₃O₄/CoFe₂O₄ dosage of 0.5 g·L⁻¹(Fig. 7c). Among them, 5.49 was the original pH of the RhB solution formulated. It was noted that both extremely acidic (pH = 3) and alkaline (pH = 11) conditions were unfavorable for the degradation process. Co₃O₄/CoFe₂O₄ composite was easily dissolved to release metal ions under acidic solution, as a result, the activity of the catalyst was greatly reduced (Wang et al., 2011), which only degraded 64.28% of RhB (pH = 3). In addition, hydrogen ion might scavenge SO₄^•− and •OH (reaction (3) and (4)) (Ahmadi et al., 2017; Ghanbari et al., 2019). The best degradation of RhB was achieved at pH of 5 in a Co₃O₄/CoFe₂O₄/PS system, then started to fall. The k_app values of RhB degradation with initial pH of 3, 5, 5.49, 7, 9 and 11 were 2.07 × 10⁻², 6.89 × 10⁻², 6.65 × 10⁻², 5.80 × 10⁻², 4.94 × 10⁻² and 3.82 × 10⁻³ min⁻¹, respectively. According to the literature, the pH_pzc of CoFe₂O₄ and Co3O4 was 5.96 and 8.5, respectively (Du et al., 2016; Zhang et al., 2016). When the Co₃O₄/CoFe₂O₄ composite was dispersed in water (pH < 5.96), the surface of the catalyst was positively charged, which would increase the coverage of the hydroxyl group (–OH) from H₂O, which was beneficial to the formation of active radicals. At higher pH values, the electrostatic repulsion between the S₂O₈²⁻ and the Co₃O₄/CoFe₂O₄ surface might limit the contact of the PS onto the catalyst surface. Besides, SO₄^•− could be converted into •OH by hydrolysis. For aromatic compounds, SO₄^•− reacted with them by means of electron transfer, and SO₄^•− was more prone to electron transfer reactions than •OH (Neta et al., 1977; Buxton et al., 1988). So only 49.95% of RhB was removed when pH was 11.

(3)

{S O}_{4}^{∙ -} + H^{+} + e^{-} \to H {S O}_{4}^{∙ -}

(4)

∙ O H + H^{+} + e^{-} \to H_{2} O

What's more, the effect of temperature on RhB degradation was also evaluated by varying the reaction temperatures, which was exhibited in Fig. 7d. When the temperature was increased from 25 °C to 55 °C, it could be observed that the degradation efficiency of RhB was greatly improved. When the temperature was 55 °C, the degradation efficiency of RhB could reach nearly 100% within 30 min. These results indicated that the degradation of RhB depended on the temperature. A higher temperature could accelerate the decomposition of PS and the movement of oxidant, intermediates, RhB molecules (Chen et al., 2014). According to the Arrhenius equation (5):

(5)

\ln (k) = l n A - \frac{E_{a}}{RT}

Among them, A is the Arrhenius constant, E_a is the apparent activation energy (kJ·mol⁻¹), R is the universal gas constant (8.314 × 10⁻³ kJ·mol⁻¹·K⁻¹), and T is the absolute temperature (K). In this experiment, the activation energy of the reaction was calculated as 25.32 kJ·mol⁻¹, which implied the intrinsic chemical reaction of the Co₃O₄/CoFe₂O₄/PS system. Furthermore, compared with other similar materials (Deng et al., 2018; Rida et al., 2018), the Co₃O₄/CoFe₂O₄ in this article exhibited the lower activation energy, indicating it could more easily activate PS.

3.6.3

3.6.3 Effect of co-existing ions

In practice, a large amount of salts were usually added in various dyeing processes and might affect the active radical chain reaction in SO₄^•− based AOP (Li et al., 2015). In this paper, the effects of Cl⁻, NO₃^– and HCO₃^– that commonly occurred in real dye wastewater on RhB degradation were evaluated. As shown in Fig. 8, different inorganic anions exhibited different inhibiting effects on the RhB degradation. As seen from Fig. 8a, chloride ion inhibited the degradation of RhB, and the degradation efficiency of RhB decreased with increasing chloride ion concentration. Compared to the case where Cl⁻ was not added, only 88.18% of RhB was degraded in the existence of 40 mM Cl⁻. It had been reported that chloride ions could act as sinks for HO^• and SO₄•⁻ to produce less reactive chlorine species Cl₂, HOCl^•−, Cl^• and Cl₂^•− (reaction (6), 7 and 8), thereby severely inhibiting the degradation process of RhB (Du et al., 2016; Sharma et al., 2015; Feng et al., 2017 ). What’s more, NO₃^– also had an apparently inhibition effect towards the removal of RhB. This was mainly because NO₃^– can react with the catalyst, resulting in a decrease in catalytic efficiency (Li et al., 2013; Gordon et al., 2013). It was worth noting that the most significant decrease was observed after adding HCO₃^– to the reaction system. In fact, HCO₃^– was an inorganic radical scavenger, which could react with HO^• (k = 8.5 × 10⁶ M⁻¹·s⁻¹) and SO₄^•− (k = 1.6 × 10⁶ M⁻¹·s⁻¹) quickly, thus producing less reactive carbonate radicals (reaction (9), 10, 11 and 12) (Lin and Wu, 2014; Zuo et al., 1999).

(6)

{S O}_{4}^{∙ -} + {C l}^{-} \to {S O}_{4}^{2 -} + C l ∙

(7)

∙ O H + {C l}^{-} \to {C l O H}^{∙ -}

(8)

C l ∙ + {C l O H}^{∙ -} \to {C l}_{2} ∙ + {O H}^{-}

(9)

{S O}_{4}^{∙ -} + {H C O}_{3}^{-} \to H^{+} + {S O}_{4}^{2 -} + {C O}_{3}^{∙ -}

(10)

{S O}_{4}^{∙ -} + {C O}_{3}^{2 -} \to + {S O}_{4}^{2 -} + {C O}_{3}^{∙ -}

(11)

∙ O H + {H C O}_{3}^{-} \to H_{2} O + {C O}_{3}^{∙ -}

(12)

∙ O H + {C O}_{3}^{2 -} \to {O H}^{-} + {C O}_{3}^{∙ -}

Effect of (a) Cl−, (b) NO3– and (c) HCO3– on the degradation of RhB. Experimental conditions: catalyst = 0.5 g·L−1, PS = 0.5 g·L−1, pH = 5.49 (natural) and RhB = 10 mg·L−1.

3.6.4

3.6.4 Free radical quenching studies

In order to reveal the activation mechanism, we need to identify the main radical species in the Co₃O₄/CoFe₂O₄/PS system for the degradation of RhB. It had been reported that activation of PS mediated by transition metals or metal oxides produced two main types of active radicals, namely SO₄•⁻ and •OH. As we all know, Methanol (MeOH) is an effective scavenger both for OH (4.6–9.7 × 10⁸ M⁻¹·s⁻¹) and SO₄•⁻ (1.1–2.5 × 10⁷ M⁻¹·s⁻¹). Tert-butanol (t-BuOH) could quickly capture •OH (k_•OH = (3.8–7.6) × 10⁸ M⁻¹·s⁻¹) (Chen et al., 2005). However, it is reported that isopropanol (IPA) could react with both free and adsorbed hydroxyl radicals (Rivas et al., 2015), which had a more significant inhibiting effect for the degradation of pollutant. In this study, we choose isopropanol and methanol as scavengers to conduct quenching experiments. As seen in Fig. 9, when IPA and MeOH were added, the degradation efficiency of RhB were only 82.46% and 57.23%, respectively. Clearly, the inhibition of MeOH was significantly higher than that of IPA, which indicated that SO₄•⁻ and •OH were the main radical in this system, and the contribution of SO₄•⁻ was larger than •OH.

Effects of different scavengers on PS oxidation for RhB degradation. Experimental conditions: catalyst = 0.5 g·L−1, PS = 0.5 g·L−1, pH = 5.49 (natural) and RhB = 10 mg·L−1 (The inset shows pseudo-first-order rate constants).

3.7

3.7 Mechanism analysis

Fig. 10a and b presented the XPS of Co 2p and O 1s of the catalysts before and after oxidation process. After PS reacted with Co₃O₄/CoFe₂O₄ composite, the position of the Co 2p peak had a little shift and the area of peaks decreased, indicating that the fraction of Co²⁺/Co³⁺ changed after used. From the deconvolution of Co 2p envelop (Fig. 10a), the proportion of Co²⁺ dropped from 47.96% to 45.27% of Co specie, which implied that the valence state of cobalt increased from Co²⁺ to Co³⁺ due to transferring electrons to PS to generate SO₄•⁻. Fortunately, the binding energy of the two main peaks was basically unchanged, which indicated that the catalyst was well regenerated. In the case of O 1s spectra (Fig. 10b), the O_ads/O_latt ratio had changed and the proportion of O_ads had increased from 31.17% to 35.09% and the proportion of O_latt reduced from 68.83% to 64.91%, indicating that both of them are involved during the degradation process. The increase of O_ads concentration might be ascribed to the formation of Co-OH and Fe-OH groups adsorbed on the Co₃O₄/CoFe₂O₄ surface, which might contribute to the enhancement of the PS activation process.

(a) Co 2p and (b) O 1s XPS envelop of the fresh and used Co3O4/CoFe2O4 catalyst; (c) Mechanism for RhB degradation by Co3O4/CoFe2O4/PS system.

Based on XPS analysis and other findings, the catalytic mechanism of PS activation by Co₃O₄/CoFe₂O₄ composite was proposed. As shown from Fig. 10c, Co²⁺ and Fe³⁺ were main active centers in Co₃O₄/CoFe₂O₄/PS system. Generally speaking, PS could react with Co²⁺ and Fe²⁺ to produce SO₄•⁻, and simultaneously generate Co³⁺ and Fe³⁺ (reaction (13) and (14)). In addition, the conduction band of CoFe₂O₄ was more negative than the Co₃O₄. When Co₃O₄ combined with CoFe₂O₄, the heterojunction was rapidly formed and electrons were more easily transferred from CoFe₂O₄ to Co₃O₄, which promoted the regeneration of Co²⁺ (reaction (15)), which was consistent with the XPS analysis. However, due to the transfer of electron from CoFe₂O₄ to Co₃O₄, only a few portion of Fe³⁺ could be converted to Fe²⁺ to activate PS (reaction (16)). Fortunately, S₂O₈²⁻ could produce HSO₅⁻ by hydrolysis under acidic condition, therefore Co³⁺ and Fe³⁺ could react with HSO₅⁻ to achieve the regeneration of Co²⁺ and Fe³⁺ (reactions (17), (18) and (19)). In addition, SO₄•⁻ could also react with H₂O to produce HO^• (reaction (20)). Ultimately, all these radicals generated further degraded RhB.

(13)

{C o}^{2 +} + S_{2} O_{8}^{2 -} \to {S O}_{4}^{∙ -} + {S O}_{4}^{2 -} + {C o}^{3 +}

(14)

{F e}^{2 +} + S_{2} O_{8}^{2 -} \to {S O}_{4}^{∙ -} + {S O}_{4}^{2 -} + {F e}^{3 +}

(15)

{C o}^{3 +} + e^{-} \to {C o}^{2 +}

(16)

{F e}^{3 +} + e^{-} \to {F e}^{2 +}

(17)

H_{2} O + S_{2} O_{8}^{2 -} \to {H S O}_{5}^{-} \to {H S O}_{4}^{-}

(18)

{C o}^{3 +} + {H S O}_{5}^{-} \to {S O}_{5}^{∙ -} + H^{+} + {C o}^{2 +}

(19)

{F e}^{3 +} + {H S O}_{5}^{-} \to {S O}_{5}^{∙ -} + H^{+} + {F e}^{2 +}

(20)

{S O}_{4}^{∙ -} + H_{2} O \to ∙ O H + {S O}_{4}^{2 -} + H^{+}

3.8

3.8 Stability of the catalyst

The repeatability and stability of catalysts are key factors in actual application of persulfate activation. In this study, the recycling experiment was conducted by the sample recovery method of centrifugation and drying. Fig. 11a exhibited the RhB removal in successive four cycles with recycled Co₃O₄/CoFe₂O₄ composite. It was clearly that the Co₃O₄/CoFe₂O₄ composite maintained its performance among the four cycles. Co₃O₄/CoFe₂O₄ composite catalyst could still degrade 85.49% RhB after 4 cycles. In addition, we tested the XRD pattern of the fresh and used Co₃O₄/CoFe₂O₄ composite, which were exhibited in Fig. 11b. Obviously, the used Co₃O₄/CoFe₂O₄ composite maintained the stable crystal structure without phase transition. These indicated that the Co₃O₄/CoFe₂O₄ composite catalyst as-prepared had a good practical potential, which could provide the possibility of practical application in future.

(a)Reusability of Co3O4/CoFe2O4 composite for RhB degradation. Experimental conditions: catalyst = 0.5 g·L−1, PS = 0.5 g·L−1, pH = 5.49 (natural) and RhB = 10 mg·L−1; (b) XRD patterns of fresh and used Co3O4/CoFe2O4.

4 Conclusion

In the present study, a novel magnetic catalyst was synthesized, which was applied for PMS activation to achieve effective RhB degradation. It was found that the degradation of RhB was highly pH-dependent with a higher degradation efficiency near the weakly acidic pH. The maximum degradation efficiency was obtained at pH of 5, where 96.05% of RhB was removed (the persulfate concentration of 0.5 g∙L⁻¹, catalyst dosage of 0.5 g∙L⁻¹ and reaction temperature of 25 °C). What’s more, naturally occurring species (Cl⁻, NO₃⁻ and HCO₃⁻) exhibited inhibitory effects on RhB degradation, especially at higher concentrations. Quenching experiment showed that sulfate radicals were the main active species. Furthermore, Co₃O₄/CoFe₂O₄ composite performed good magnetic property, which could be easily separated through external magnetic and recycled from the aqueous solution. In addition, recycle experiments was conducted, in which the degradation efficiency of RhB could still reach 85.49% after 4 cycles. Finally, the enhanced mechanism was proposed. Overall, this paper provided a viable strategy for the fabrication of high-efficient recyclable catalysts which could be used in the treatment of printing and dyeing wastewater.

Acknowledgment

This work was kindly funded by National Natural Science Foundation of China (51978319), Fundamental Research Funds for the Central Universities (lzujbky-2017-it98), National College student innovation and Entrepreneurship training program of Lanzhou University and Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lake, Chinese Academy of Sciences.

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Show Sections

[1] Ahmadi M., Ghanbari F., . Optimizing COD removal from greywater by photoelectro-persulfate process using Box-Behnken design: assessment of effluent quality and electrical energy consumption. Environ. Sci. Pollut. Res.. 2016;23:19350-19361.
[Google Scholar]

[2] Ahmadi M., Ghanbari F., Alvarez A., Martinez S.S., . UV-LEDs assisted peroxymonosulfate/Fe²⁺ for oxidative removal of carmoisine: the effect of chloride ion. Korean J. Chem. Eng.. 2017;34:2154-2161.
[Google Scholar]

[3] Ahmed M.M., Barbati S., Doumenq P., Chiron S., . Sulfate radical anion oxidation of diclofenac and sulfamethoxazole for water decontamination. Chem. Eng. J.. 2012;197:440-447.
[Google Scholar]

[4] Amasha M., Baalbaki A., Ghauch A., . A comparative study of the common persulfate activation techniques for the complete degradation of an NSAID: the case of ketoprofen. Chem. Eng. J.. 2018;350:395-410.
[Google Scholar]

[5] Anipsitakis G.P., Dionysiou D.D., . Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci. Technol.. 2003;37:4790-4797.
[Google Scholar]

[6] Antoniou M.G., de la Cruz A.A., Dionysiou D.D., . Degradation of microcystin-LR using sulfate radicals generated through photolysis, thermolysis and e(-) transfer mechanisms. Appl. Catal. B. 2010;96:290-298.
[Google Scholar]

[7] Asha T.T., Gandhimathi R., Ramaesh S.T., Nidheesh P.V., . Treatment of stabilized leachate by ferrous-activated persulfate oxidative system. J. Hazard. Toxic Radioact. Waste. 2017;21:04016012.
[Google Scholar]

[8] Ayoub G., Ghauch A., . Assessment of bimetallic and trimetallic iron-based systems for persulfate activation: application to sulfamethoxazole degradation. Chem. Eng. J.. 2014;256:280-292.
[Google Scholar]

[9] Babu S.G., Aparna P., Satishkumar G., Ashokkumar M., Neppolian B., . Ultrasound-assisted mineralization of organic contaminants using a recyclable LaFeO3 and Fe3+/persulfate Fenton-like system. Ultrason. Sonochem.. 2017;34:924-930.
[Google Scholar]

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