Catalytic degradation of methylene blue through activation of bisulfite with CoO nanoparticles

Journal Pre-proofs Catalytic degradation of methylene blue through activation of bisulfite with CoO nanoparticles Ruyan Dou, Hao Cheng, Jianfeng Ma, Yong Qin, Yong Kong, Sridhar Komarneni PII: DOI: Reference:

S1383-5866(19)35477-2 https://doi.org/10.1016/j.seppur.2020.116561 SEPPUR 116561

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

28 November 2019 3 January 2020 12 January 2020

Please cite this article as: R. Dou, H. Cheng, J. Ma, Y. Qin, Y. Kong, S. Komarneni, Catalytic degradation of methylene blue through activation of bisulfite with CoO nanoparticles, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116561

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Catalytic degradation of methylene blue through activation of bisulfite with CoO nanoparticles Ruyan Dou1,2, Hao Cheng2,3, Jianfeng Ma1,2*, Yong Qin1, Yong Kong1, and Sridhar Komarneni4 1 School

2

of Environmental and Safety Engineering, Changzhou University, Jiangsu, 213164, China

Guangxi Key Laboratory of Green Processing of Sugar Resources, College of Biological and

Chemical Engineering, Guangxi University of Science and Technology, Guangxi, 545006, China 3

Province and Ministry Co-sponsored Collaborative Innovation Center of Sugarcane and Sugar

Industry，Nanning 530004，Guangxi, P.R. China 4 Department

of Ecosystem Science and Management and Materials Research Institute, 204

Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA

Abstract A novel catalytic system was developed by activating NaHSO3 with CoO nanoparticles for methylene blue (MB) degradation. The CoO nanoparticles were synthesized by a facile one-pot hydrothermal method followed by calcination. The



Corresponding author

Tel.: 814-865-1542; Fax: 814-865-2326

E-mails: [email protected] and [email protected] 1

crystallinity, morphology and elemental valence of the CoO were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. MB degradation rates of 91.5% in one minute and 99.4% in six minutes were obtained using the CoO/NaHSO3 system, which may provide a more cost-effective and efficient way for decomposition of dye pollutants. The well-known classical quenching tests were used to investigate the free radicals involved in MB degradation along with electron paramagnetic resonance (EPR) spectra, the latter further confirmed the types of free radicals. Sulfate and hydroxyl radicals were proposed to be responsible for the excellent dye degradation achieved with the new CoO/NaHSO3 system. Key words: CoO nanoparticles; NaHSO3 activation; Chemical catalysis; Dye degradation

1 Introduction In recent years, large amounts of organic dye wastes generated in different industries are being discharged into the environmental waters at random, causing increased environmental contamination[1][2], especially in China. Most traditional methods of treatment can achieve a slight decrease in chroma, i.e., slight discoloration because the organic substances are only decomposed into smaller substances[3][4].

2

However, the properties of these decomposition products are very difficult to control and master[5], which may still cause harm to the environment. Therefore, the development of highly effective degradation processes for dyes in wastewater has generated enormous interest recently[6][7]. Advanced oxidation technologies (AOPs) have been one of the most popular and promising methods in wastewater treatment, receiving considerably more attention for the destruction of recalcitrant organic contaminants[8][9]. By generating hydroxyl radicals (•OH) (oxidation potential is 1.8-2.7 V vs. normal hydrogen electrode (NHE) from

hydrogen

reaction[11][12],

peroxide[10], such

as

traditional

Fenton

photo-Fenton[13][14],

reaction

and

Fenton-like

electron-Fenton[15][16],

sono-Fenton[17] and sono-photo-Fenton[18][19], have been widely investigated and applied for the removal of organic pollutants from wastewater. Nevertheless, they still keep facing the limitations, including low optimal reaction pH (2-4), the difficulties about the oxidant in cost-intensive storage and transportation, and production of large amounts of sludge[20]. In recent years, advanced oxidation processes based on sulfate radicals (SO•- 4) (SR-AOPs) have been widely investigated and become a promising alternative method with several advantages for the destruction of recalcitrant organic contaminants. Since sulfate radicals (SO•- 4) have a higher oxidation-reduction potential (E0=2.5-3.1V vs.NHE)[7], which is comparable or even higher than that of the hydroxyl radicals (•OH) 3

(E0=2.7V)[21][22], they were suggested to be more powerful for organic contaminant degradation[23]. Some studies have demonstrated that sulfate radicals (SO•- 4) could have longer lifetime than hydroxyl radicals (•OH), which is helpful to have favorable contact time with organic target pollutants and prolong the oxidation time to achieve excellent electron transfer process[24]. According to the above study, the degradation capacity of sulfate radicals (SO•- 4) is superior to hydroxyl radicals (•OH) in theory under the same conditions. Furthermore, sulfate radicals (SO•- 4) have high oxidation efficiency, leading to slow rate of consumption of stable precursor oxidants and thus, make sulfate radical-based processes very effective to decompose some recalcitrant organic compounds that could not be oxidized by hydroxyl radicals (•OH) in order to achieve higher removal capacity[25]. Generally, sulfate radicals (SO•- 4) could be generated by employing pyrolysis, photolysis, radiolysis, or chemical reagents to activate peroxymonosulfate (PMS, HSO5) or persulfate (PS, S2O2− 8)[26][27][28][29][30]. Great majority of advanced oxidation technologies (AOPs) adopt a combination of catalysts, such as transition metal ions[31] (eg. Co2+[32], Mn2+, Fe2+, Ce3+[21], Cu2+[33]), with strong oxidizing agents, such as O3, H2O2, PMS or PS, in the dark or irradiation by visible light, ultraviolet or ultrasound to achieve higher removal efficiency in wastewater treatment and remediation. Among activation catalysts of the transition metal ions, Co2+-based catalyst is thought of as the optimum reactive catalyst for the activation of strong 4

oxidants. However, Co2+-based homogeneous catalysis is a potential threat to human beings due to the discharge of cobalt ions containing water, limiting the practical value of its application[34][35]. Thus, heterogeneous Co2+-based catalysis seems to be a promising strategy and aroused much attention because of the favorable activity and stability[34]. So far, a variety of heterogeneous cobalt-based catalysts like cobalt oxide, spinel-type ferrite, and immobilized cobalt catalysts[34], have been studied to activate peroxymonosulfate (PMS, HSO- 5) or persulfate (PS, S2O2− 8) for the generation of sulfate radicals (SO•- 4) and hydroxyl radicals (•OH) to effectively oxidize persistent organic contaminants[34]. Among them, the use of CoO nanoparticles appears to be a promising way and therefore, these nanoparticles have enjoyed pervasive attention because of their important role on catalytic performance[36]. Nguyen et al.[34] have synthesized CoO@meso-CN nanocomposite with various CoO loadings by a simple and facile synthesis method, which is useful to the dispersion of CoO and achieve a high surface area for the enhancement of catalytic performance. Oladipo et al.[37] have demonstrated a simple co-precipitation technique for the fabrication of CoO–NiFe2O4 mixed metal oxide catalyst exhibiting a large specific surface area and ferromagnetic behavior. Superior catalytic activity in Fenton-like decolorization of azo dye has been achieved because Co and Fe metal ions possess high surface potential needed for the generation of reactive species. In addition, a favorable recycling capacity is valuable in practical application. Banerjee et al.[38] prepared nickel doped graphitic carbon nitride

5

nanosheets by a simple thermal treatment procrdure to chemical catalytic degrade MO dye. The sample possessed the best catalytic activity and excellent recyclability as it could be reused up to three times with little decrease in the reaction rate. Zhu et al.[39] studied the chemical catalytic system where different crystalline forms of manganese oxides combined with organic acids were used to decolorize the dye. Dai et al.[40] investigated a synergistic catalytic system where Co3O4/GO (GO=graphene oxide) with different Co3O4 loadings was used to activate PMS to produce sulfate radicals for the generation of Orange Ⅱ. However, the study of chemical catalysis using CoO nanoparticles as a catalyst is worthy of further study to determine its performance. Herein, we report the synthesis of CoO nanoparticles via the hydrothermal method followed by calcination in tube furnace. Then, a combination of CoO catalyst and NaHSO3 as the co-catalyst was used in a system where degradation of MB organic contaminant was carried out. The bisulfite (BS) was applied as an additive to make the chemical catalysis become a superior process. A series of experiments have been conducted to examine the chemical catalytic performance of the catalyst in the activation of bisulfite (BS) for the decomposition of MB dye wastewater. At the same time, cycling experiments were also conducted to demonstrate the recyclability and stability. To the best of the authors’ knowledge, there have been no investigations about the use of cobalt oxide/bisulfite (BS) for chemocatalysis of organic contaminants.

6

2 Experimental 2.1 Materials and reagents Cobalt chloride [CoCl2•6H2O], urea [CO(NH2)2], and ascorbic acid [C6H8O6] reagents of analytical grade (AR) were obtained from Shanghai Titan Scientific Co., Ltd. (Shanghai, China) and used for the synthesis of the CoO. These as-supplied chemicals were used in the CoO synthesis. Sodium bisulfite (BS), NaHSO3 of AR grade was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China) and was used as a co-catalyst without any other purification. Deionized water was used throughout all the experiments. 2.2 Synthesis of CoO Although there are various kinds of preparation methods for the synthesis of CoO, nanoparticles of CoO were prepared in this study by a hydrothermal process. The detailed procedure is as follows: 2 g of the CoCl2•6H2O, 2.4 g of the CO(NH2)2, and 2.8 g of the C6H8O6 were dissolved in 60 mL of deionized water by stirring the mixed solution to homogenize. Then, the above solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and was kept at a temperature of 160℃ for 4 hours in an oven. After cooling to room temperature, the obtained solid products were washed with deionized water first and ethanol subsequently. The solid products were first dried at 70℃ in an oven. Finally, these obtained powders were calcined in a tubular furnace

7

for 4 hours at 500℃ without the protection of nitrogen. 2.3 Characterization of samples Powder XRD patterns of the as-prepared sample were acquired with an X-ray diffractometer (Max-2500PC, Rigaku D) equipped with a Cu Kα radiation (40 kV, 100 mA), using a wavelength of 0.154 Å to determine the crystalline phases in the samples. The total surface area of the as-prepared sample was determined with the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption-desorption isotherms on a Micromeritics ASAP 2010C analyzer. The surface morphology of the as-synthesized material was characterized by field emission scanning electron microscope (SEM, HitachiSU8010) with an acceleration electron voltage of 15 kV. The elemental valence state of samples was analyzed by X-ray photoelectron spectrometer (Thermo K-Alpha+). Electron paramagnetic resonance (EPR) spectra were recorded on EPR spectrometer (Bruker, EMX-10) to identify the reactive radical species. 2.4 Catalytic oxidation of MB The catalytic oxidation process of MB measured the catalytic activity of the as-prepared sample through chemical catalysis (only with added NaHSO3). All experiments were carried out in beakers containing 100 mL MB dye solution at 25±2℃ under magnetic stirring (500 rpm). Typically, 60 mg of the as-obtained CoO samples was put into the methylene blue solution (with the concentration of 30 mg/L) under

8

constant stirring. The chemo-catalytic oxidation process of MB was also conducted by adding 200 mg NaHSO3 into the MB solution. The reactions were initiated by simultaneously adding sodium bisulfite and the sample powders. During the reaction, 2 mL of mixture solution was withdrawn every 1 min using a syringe and filtered through a Millipore filter (pore size 0.22 μm) to remove the catalyst powder. Then, the resulting clear solution was analyzed by a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan) to record the concentration changes of MB at a certain wavelength, λ= 664 nm. For the stability and recyclability tests of the materials, experiments were carried out using the same methods as mentioned above. After each catalytic oxidation reaction, the sample was collected by centrifugation and washed by deionized water and ethanol and dried in an oven at 70℃. After that, the dried sample was used for next reuse test under the same experimental conditions.

3 Results and discussion 3.1 XRD analysis The crystalline phases and crystallinity of the nanostructures were determined from XRD patterns. Fig.1 displays the XRD pattern of the synthetic sample. As we can see from the XRD patterns, the diffraction peaks of CoO matched with the standard JCPDS card of CoO (No. 43-1004). It can be seen that only three diffraction peaks at 36.5°, 9

42.4° and 61.5° could be detected in the 2θ range of 5º to 80º and these three peaks could be indexed to CoO. The above peaks correspond to (111), (200) and (220) planes of CoO, respectively. The peaks are very broad and weak, suggesting the formation of nanocrystalline CoO particles[34]. In addition, no other diffraction peaks related to any impurities could be detected.

(200)

Intensity(a.u.)

(111)

(220)

20

30

40

50

60

2

Fig. 1. X-ray diffraction pattern of CoO sample.

3.2 SEM analysis The SEM images of the pristine CoO are presented in Fig.2, displaying the nanoparticulate morphology of the as-prepared CoO catalyst. Fig.2a shows that the 10

catalyst possesses somewhat regular sphere-like morphology and Fig.2b suggests that no aggregation occurred in the catalyst and the sphere-like particles were dispersed relatively evenly and appear generally uniform in size. Such dispersion of particles is expected to increase the specific surface area for greater reactivity. The specific surface area of the as-prepared catalyst was measured by the N2 adsorption-desorption method. The result suggested that the CoO sample has a high specific surface area with 88.7 m2g−1 for favorable activity.

(a)

(b)

100nm

10μm

Fig. 2.

SEM images of pure CoO sample.

3.3 XPS analysis In addition to the structural and morphological features, surface composition and atomic valence state of the as-prepared samples are also important due to their close association with the catalytic performance. The full XPS spectrum as survey scan (Fig.3a) is presented to analyze the coexistence of Co and O elements and verify their

11

presence. There are a set of peaks related to Co 2p and O 1s positioned at 780.08ev and 531.08ev in the scan (Fig. 3a). However, no other peaks related to other species were detected and thus, confirming the absence of impurities. The XPS curves and corresponding fitted data of CoO in the Co and O region are shown in Fig. 3b and c. The Co 2p spectra of the as-prepared sample are shown in Fig.3b, where two peaks centered at 780.5ev and 795.7ev belong to Co2p 3/2 region and Co2p 1/2 region along with two shake-up satellite shoulders at 786.5ev and 803.1ev respectively[41]. With regard to Co2p 3/2 region, the peak separation of Co2p 3/2 indicates that it consists of two peaks of 780.23ev and 780.88ev, which are assigned to Co(Ⅱ) octahedral and tetrahedral positions, respectively[42]. Fig.3c displays the high-resolution spectrum of O1s, which could be deconvoluted into three distinct characteristic peaks located at 529.93ev, 531.33ev, and 532.98ev, all assigned to cobalt (Ⅱ) oxides. The three dominant peaks correspond to the surface lattice oxygen of the metal-oxygen bonds (Olatt), hydroxyl groups (Oads) and surface adsorbed water molecules (OH2O), respectively[43][44]. Moreover, the XPS spectra revealed the presence of cobalt reactive sites that may be relevant to the chemical catalytic oxidation reactions.

12

(a)

Co2p

Co2p

(b)

Co2p3/2

Co(Ⅱ)

Intensity(a.u.)

Co2p1/2

1200

1000

800

Binding Energy (ev)

Intensity(a.u.)

O1s

400 770

600

Co(Ⅱ)

775

780

785

790

795

Binding Energy (ev)

800

805

810

(c)

O 1s

cobalt(‖)oxides

Olatt

cobalt(‖)oxides

Intensity(a.u.)

Oads

OH2O

cobalt(‖)oxides

525

Fig. 3.

530

535

Binding Energy (ev)

540

545

XPS analysis of CoO sample: (a) XPS survey spectrum, (b) Co spectra, and (c) O spectra.

3.4 Catalytic activity 3.4.1 Chemo-catalytic performance testing The chemo-catalytic activity of CoO was tested by determining the degradation efficiency of MB with the assistance of NaHSO3. Fig.4. shows the effect of CoO on the catalytic activation of NaHSO3 towards methylene blue oxidation. As can be seen from Fig.4 MB removal rate was 40% in one minute for the control experiment conducted only in the presence of CoO, probably owing to the favorable adsorption capacity of the 13

catalyst. At the same time, the concentration of MB dye solution slightly and gradually decreased but remained stable in the other control experiment where only NaHSO3 was present. This indicates that bisulfite could be oxidized by dissolved oxygen to form some sulfate radicals with the ability to slightly degrade organic contaminants[45]. However, the removal efficiency of MB was dramatically increased up to 91.5% in one minute when CoO and NaHSO3 were present simultaneously. When the reaction time was increased to six minutes, MB was almost completely degraded to the extent of about 99.4% MB by CoO and NaHSO3. These results imply clearly that CoO material could react with NaHSO3 to generate some highly reactive and strongly oxidizing species such as •OH and SO•4 ― leading to such a very high MB degradation. The MB degradation process was simulated by one-dimensional kinetic equation of 𝐶𝑡

the formula as follows: ― ln 𝐶o=kt, where k is kinetic constant (min−1). Fig.5 shows that the degradation of MB approximately followed pseudo-first-order kinetics model. It is notable that the apparent rate constant presented by CoO/NaHSO3 system is much higher than the sum of the apparent rate constants presented by other processes. This implies that the synergistic effect of CoO and NaHSO3 in the chemical catalytic reaction could contribute to the cooperative degradation of MB.

14

1.0

0.8

C/C0

0.6

0.4

CoO+NaHSO3

0.2

CoO NaHSO3

0.0 0

Fig. 4.

1

2

3

Time(min)

4

5

6

The degradation curves of MB under different conditions: CoO alone, NaHSO3 alone, and

the combination of CoO and NaHSO3. Experimental conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L.

6 CoO+NaHSO3

5

CoO NaHSO3

-ln(C/C0)

4 3 2 1 0 0

1

2

Time(min)

3

4

Fig. 5 Pseudo-first-order kinetic fit for the degradation of MB by CoO/NaHSO3 system.

The influence of initial pH on decomposing MB pollutants by the CoO/BS system 15

was also studied by conducting some experiments under the same conditions as those mentioned above but at different pH of solutions. The experimental results are given in Fig.6, which shows that the initial pH affects the degradation rate of the chemical catalytic system to some extent and that good degradation was achieved both in strongly acidic and strongly alkaline conditions. This good degradation efficiency could probably be ascribed to the CoO metal oxide, which has stronger oxidation performance under strongly acidic medium and forming Co(Ⅱ)/Co(Ⅲ) species by electron transfer to activate sulfites for the generation of active species such as hydroxyl radicals (•OH) and sulfate radicals (SO•- 4). Methylene blue is a cationic dye and therefore, its adsorption is favorable under strongly alkaline conditions because of negative charge development on the CoO surface, which may make it easier for the decomposition of MB dye readily by this catalysis system.

(a)

1.0

pH=3 pH=6 pH=8

0.8

0.8

0.6

0.6

pH=8 pH=10 pH=12

C/C0

C/C0

(b)

1.0

0.4

0.4

0.2

0.2

0.0

0.0 0

1

2

3

Time(min)

4

5

6

0

1

2

3

Time(min)

4

5

6

Fig. 6. Influence of initial pH on MB degradation under the CoO/BS system. Experimental 16

conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L.

3.4.2 Proposed chemical catalysis mechanism In order to identify the main oxidative active species in the CoO/BS system, some free radical quenching experiments were carried out to check the mechanism of the superior catalytic activity in the system. As depicted in Fig.7, classical quenching tests were carried out to significantly inhibit the degradation of MB pollutants via applying ethanol (EtOH) and tert-butyl alcohol (TBA). It is well-known that both ethanol (EtOH) and tert-butyl alcohol (TBA) are favorable quenching agents for the inhibition of hydroxyl (•OH) and sulfate radicals (SO•- 4). The degradation of MB was slightly more inhibited with the presence of 3 mL TBA when compared with the addition of 3mL EtOH under the same conditions mentioned above without light irradiation. The results show that the degradation rate of MB chemical pollutants decreased compared with no scavengers, which illustrated the existence of •OH and SO•- 4. To further investigate the dominant radicals responsible for MB degradation, EPR tests using DMPO as spin trapping agent was also conducted to detect the radical species. As can be seen from Fig.8, when CoO was added together with DMPO, BS and MB solution, both •OH and SO•- 4 appeared as determined by the characteristic peaks of DMPO-SO•- 4 and DMPO-•OH adducts within the first 45 seconds of the chemical catalytic reaction. Overall, hydroxyl and sulfate radicals work together on this reaction. Sulfate radicals apparently played the major role in promoting the degradation of MB due to their better 17

reactivity.

1.0

(C0-C)/C0 (%)

0.8

0.6

0.4

0.2

0.0

Fig. 7.

no scavenger

tBuOH

EtOH

Degradation of MB solution under the CoO/BS system. Experimental conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L.

18

Intensity(a.u.)

45 s

3460

3480

3500

3520

3540

3560

Magnetic field (G)

Fig. 8. EPR spectra of DMPO adducts ( : DMPO- SO•- 4;

: DMPO-•OH).

A possible mechanism for MB degradation in the CoO/BS system along with the change of initial pH is proposed here. As shown in Fig.6a, when the initial pH of the solution was 3.0, removal rate of 90% could be achieved in six minutes. This level of degradation could be attributed to the appearance of Co(Ⅱ)/Co(Ⅲ) species owing to their higher oxidation potential in acidic media. The participation of these species could result in the production of a majority of sulfate radicals (SO•- 4) according to Eq. 1-5. However, a slight decrease in degradation rate occurred when the initial pH was 6.0, which could be attributed to the decline of oxidation of metal ions and thus, resulting in the slower degradation reaction rate at pH 6. Co2 + + HSO3― →Co3 + + •SO3― + H +

(1)

19

•SO3― + O2→•SO5―

(2)

•SO5― + HSO3― →•SO4― + SO24 ― + H + (3) Co3 + + e ― →Co2 +

E0 = 1.81v

Co2 + + •SO5― + H2O→Co3 + + •SO4― + 2OH ―

(4) (5)

When the initial pH was 8.0, a removal rate of 80% was achieved after six minutes. This increase in degradation from pH 6 to pH 8 could be due to a portion of sulfate radicals (SO•- 4) reacting with water to form hydroxyl radicals (•OH) in alkaline media according to Eq. 6-9[46], which could remove contaminants more thoroughly by electron transfer and proton capture due to the non-selective oxidation capacity. The co-existence of sulfate radicals (SO•- 4) and hydroxyl radicals (•OH) improved the removal compared with the initial pH of 6.0. Olmez-Hanci and Arslan-Alaton[24] have shown that sulfate radicals (SO•- 4) possess a longer lifetime than that of hydroxyl radicals (•OH) after they were produced, which suggests that sulfate radicals (SO•- 4) have more time to contact with much more organic substance and decompose it faster than hydroxyl radicals (•OH) under the same conditions. With the increase of the solution pH gradually (Fig.6b), a majority of sulfate radicals (SO•- 4) were apparently converted into hydroxyl radicals (•OH), resulting in a slower degradation rate at this pH. •SO4― + H2O→SO24 ― + •OH + H +

(6)

•SO4― + OH ― →•OH + SO24 ― (7) •SO5― + O2― →•SO4― + O2

(8)

2HSO3― + O2→SO•4 ― + SO24 ― +2H + 20

(9) However, the degradation rate became faster when the initial pH was 12.0. This phenomenon could be attributed to metal ions and their oxides reacting with redundant hydroxyl ions to produce electrons, which reacted with dissolved oxygen and hydrogen ions in solution to generate hydrogen peroxide, which led to the faster reaction rate (Eq. 10-14)[21][47]. Co2 + + OH ― →Co(OH)2

(10) (11)

Co(OH)2→CoO + H2O Co(OH)2 + OH ― →CoO(OH) + H2O + e ―

(12)

CoO + OH ― →CoOOH + e ― (13) O2 + 2H + + 2e ― →H2O2

(14)

3.5 Evaluation of the stability and recyclability of CoO To determine the stability for practical application of CoO/BS system, the performance of the CoO sample in multiple cycles was determined (Fig.9). Cycling experiments for MB degradation were conducted under CoO/NaHSO3 system for three cycles (Fig.9). As summarized in Fig.9, little or no change occurred in the chemo-catalytic performance between fresh and recycled CoO catalyst. The removal rate of MB still remained at 96.9% after three cycles, suggesting that the CoO possesses excellent catalytic activity and high stability. Also, because of the weak magnetism of the material, it can be easily recovered. Fig.10 shows the XRD patterns of the fresh CoO sample and the used CoO sample. It can be observed that there is no significant 21

difference between the fresh and used sample, implying that the crystalline phase was unchanged due to the excellent stability of the catalyst. The present results indicate that the CoO sample could be an ideal chemo-catalyst for the practical applications in wastewater treatment based on its superior recyclability and catalytic activity.

RUN2

RUN1

1.0

RUN3

0.8

C/C0

0.6

0.4

0.2

0.0 0

Fig. 9.

3

6

9

Time(min)

12

15

18

Repeated catalytic degradation of MB solution under the combination of CoO and NaHSO3. Experimental conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L.

22

Intensity(a.u.)

Fresh catalyst

Recycled catalyst

20

30

40 2

50

60

Fig. 10. X-ray diffraction patterns of the fresh CoO sample and used CoO sample.

4 Conclusions A novel heterogeneous chemical catalytic system was developed by the activation of bisulfite (NaHSO3, BS) with CoO nanoparticles. The catalyst system of bisulfite and CoO nanoparticles exhibited superior decomposition capacity of MB pollutants and led to a degradation efficiency of up to 91.5% in one minute while the highest degradation rate of up to 99.4% was achieved when the reaction time was for six minutes without any other energy input. The results of the recycling experiments demonstrated a very high and satisfactory MB removal rate of 96.9% even after three cycles. Thus, the novel CoO/BS catalyst system not only showed excellent chemical catalytic activity but also showed an optimum stability and considerable recyclability. Hence, the CoO/BS system 23

has potential applications for catalytic decomposition of organic chemical contaminants. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 20968005), “Qing Lan Project” of Jiangsu Province, “333 Project" of Jiangsu Province, the High Levels of Innovation Team and Excellence Scholars Program in Colleges of Guangxi and the Opening Project of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (KF201812-4). One of us (SK) was supported by the College of Agricultural Sciences under Station Research Project No. PEN04705.

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Figure captions Fig. 1.

X-ray diffraction pattern of CoO sample.

Fig. 2.

SEM images of pure CoO sample.

Fig. 3.

XPS analysis of CoO sample: (a) XPS survey spectrum, (b) Co spectra, and (c) O spectra.

Fig. 4.

The degradation curves of MB under different conditions: CoO alone, NaHSO3 alone, and the combination of CoO and NaHSO3. Experimental conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L.

Fig. 5

Fig. 6.

Pseudo-first-order kinetic fit for the degradation of MB by CoO/NaHSO3 system.

Influence of initial pH on MB degradation under the CoO/BS system. Experimental conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L. 29

Fig. 7.

Degradation of MB solution under the CoO/BS system. Experimental conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L.

Fig. 8.

EPR spectra of DMPO adducts ( : DMPO- SO•- 4;

: DMPO-•OH).

Fig. 9.

Repeated catalytic degradation of MB solution under the combination of CoO and NaHSO3. Experimental conditions: MB 30 mg/L, CoO catalyst 0.6 g/L, NaHSO3 2 g/L.

Fig. 10. X-ray diffraction patterns of the fresh CoO sample and used CoO sample.

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Highlights:     

Removal rate of 99.4% in six minutes was obtained by novel CoO/NaHSO3 system An excellent removal rate of 99% was still achieved after the third circle The catalyst could be easily collected and reused with magnets for next recycle The CoO/NaHSO3 system could show high efficiency across a wide range of pH A possible mechanism is proposed based on EPR spectra and trapping experiments

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Authors’ contributions: The conceptual idea is from Jianfeng Ma. Ruyan Dou and Hao Chengperformed the research. All authors, Ruyan Dou, Hao Cheng, Jianfeng Ma, Yong Qin, Yong Kong, and Sridhar Komarneni contributed in writing the manuscript.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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