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Hollow and oval-configured ultrafine Co3O4 as a highly-efficient activator of monopersulfate for catalytic elimination of Azorubin S

Abstract

Azorubin S (ARS) represents one of the most common and frequently-utilized toxic azo dyes produced from industrial activities. While various conventional treatment techniques could not effectively eliminate ARS from water, heterogeneous metal-based catalyst coupled with monopersulfate (MPS) is a highly-efficient process for eliminating ARS, in which tricobalt tetroxide (Co3O4) has been attracted increasing attention as a preeminent MPS activator due to its outstanding physicochemical properties. However, the nanoscale Co3O4 particles usually pose a limitation of serious agglomeration in the aqueous environment, thus lowering their efficiency. Thus, developing an easy-synthesized and exceptionally efficient Co3O4-based catalyst is crucially paramount. Therefore, in this work, a special hollow-structured oval-like cobalt oxide (abbreviated as HOCO) was successfully constructed using Co-metal organic framework as a precursor, which was then utilized for catalyzing activation of MPS to degrade ARS. This as-obtained HOCO exhibited distinct physicochemical characteristics from commercially-available Co3O4, which subsequently resulted in superior activities for MPS activation in ARS degradation. Specifically, 100% of ARS could be degraded in 30 min with a corresponding reaction kinetic of 0.22 min− 1 by HOCO + MPS system. SO4•– radicals were validated to be primary reactive species for ARS degradation while the degradation pathway of ARS was also elucidated. This study further provides insightful information about the development of novel hollow-structured Co3O4-based catalyst for catalyzing activation of MPS to remove toxic dyes from water.

1 Introduction

Nowadays, dyes are extensively employed as colorants and additives in foods, textiles, cosmetics, and pharmaceuticals [1]. It has been published that more than 10,000 types of dyes are available, in which azo dyes account for over half of the textile dyes employed in the industry [2]. Nevertheless, increasing release of these dye substances from wastewater effluents into water environments may lead to negative effects on ecology in view of their potential toxicities [3]. Among numerous types of dyes, azo dyes represent one of the largest categories [4]; however, most of azo dyes have been validated to exhibit toxicities, even carcinogenicity [5]. Specifically, Azorubin S (ARS), one of the most common red-colored dyes, has been consistently used in many products, but ARS has been considered as a carcinogenic and genotoxic compound [6]. While ARS has been banned in the United States, it is still available and consumed in many countries [7]. Therefore, it is highly crucial to eliminate ARS from contaminated water.

Various techniques are already proposed for removing ARS from contaminated sources, such as adsorption [8], filtration [9], and biological treatments [10]. Unfortunately, these conventional techniques exhibit relatively low removal capabilities especially because of low bio-degradability of ARS [11]. Alternatively, advanced oxidation processes are advantageous techniques for eliminating refractory contaminants, and especially, sulfate-radical (SO4•−)-type techniques are focused recently as sulfate radicals exhibit larger standard redox power, longer half-lives, and higher selectivity [12, 13]. For obtaining sulfate radicals, monopersulfate (MPS) has been increasingly utilized as MPS would be inexpensive and highly-accessible. Nonetheless, the auto-dissociation of MPS is extremely slow; therefore, MPS shall be activated for facilitating generation of sulfate radicals. To date, heterogeneous transition metal-based MPS activation is already validated to be the most practical technique to eliminate refractory organic pollutants [14]. For instance, Wang et al. synthesized a composite of iron, cobalt, and oxygen codoped g-C3N4 (Fe–Co–O–g-C3N4) for activating MPS to degrade sulfamethoxazole (SMX), and 100% of SMX was degraded in 30 min with a reaction rate of 0.085 min− 1 [15]. Moreover, Lim’s group prepared cobalt-doped black TiO2 nanotubes to activate MPS for eliminating 4-chlorophenol (4-CP) in water, in which full 4-CP degradation was afforded within only 20 min with the corresponding degradation kinetic calculated as 0.28 min− 1 [16]. This validates that heterogeneous transition metals would be advantageous for MPS activation. Therefore, the activation of MPS using heterogeneous metal-based catalysts should be also propitious to the elimination of ARS. Amid various transition metals, cobalt (Co)-based catalysts are preeminent MPS activators, in which tricobalt tetroxide (Co3O4) has attracted great attention owing to its outstanding physicochemical properties, redox capability and excellent reactivity [17, 18]. However, the nanoscale of Co3O4 particles usually pose a limitation of serious agglomeration in the aqueous environment [19,20,21], lowering their surface area, then diminishing the catalytic activity. Thus, the development of an exceptionally efficient Co3O4-based catalyst for MPS activation is crucial.

Lately, hollow-nanostructured materials (HNMs) represent a novel class of heterogeneous materials that have been focused and increasingly employed in different catalytic applications [22, 23]. The interesting characteristics of HNMs, such as large contact areas and pore volumes, abundant active sites, high permeability, and productive electron transfer capability make them useful materials for energy storage, catalysis, energy conversion, etc. [24]. More importantly, with the huge internal voids encapsulated by external thin shells, HNMs could offer maximum exposed active surfaces as well as minimize the agglomeration of nanoparticles (NPs) in the aqueous medium [25]. These outstanding features would be afforded by constructing a novel hollow-nanostructured Co3O4-based catalyst for MPS activation in water.

Herein, this work aims to prepare special a hollow-nanostructured Co3O4 with ultrafine Co3O4 NPs confined in an oval-like morphology using a Co-metal organic framework (MOF) as a precursor. Through one-step modification via a chemical-engraving step, then followed by calcination, Co-MOF was completely transformed into a hollow-nanostructured oval-like Co3O4, denoted as HOCO. This as-prepared HOCO was then employed for catalyzing MPS activation to eliminate ARS. Physicochemical properties of HOCO were thoroughly characterized, which were noticeably different with the conventional commercial Co3O4 NPs. Moreover, different influencing parameters, such as MPS dosage, catalyst dosage, temperatures, pH values, co-existing matters on ARS degradation would be also examined. Besides, the reactive oxygen species (ROS) involved in ARS degradation using HOCO-activated MPS were further determined. From the detected intermediate products and the density functional theory (DFT) calculation of ARS, a plausible degradation pathway of ARS was then revealed.

2 Experimental

2.1 Synthesis of HOCO and characterizations

In brief, Co-based MOF with an oval shape was synthesized by coordinating Co with the imidazolate ligand. The resulting Co-MOF was then chemically engraved to transform the oval Co-MOF to a hollow and oval cobaltic precursor, which was thermally oxidized in air to afford the hollow-structured oval Co3O4 (HOCO) as schemed in Fig. 1a. Initially, a Co-MOF with oval-like morphology was firstly prepared, followed by a modification step with tannimum via chemical-etching process before being calcined in air to obtain the final product. In a typical preparation, a solution containing 40 mL of Co (NO3)2 was slowly added into another solution bearing 40 mL of 2-methylimidazole (2-MIM) where the molar ratio of Co precursor to 2-MIM = 1:8. The mixture was then stirred vigorously at ambient temperature for 4 h. Subsequently, the resultant purple-colored precipitate was assembled via centrifugation, rinsed by deionized water, and dried at 65 °C overnight to afford oval-like Co-MOF. Next, a certain amount of the as-prepared oval-like Co-MOF (i.e., 200 mg) was then immersed into a solution containing 300 mL mixture of deionized water and ethyl alcohol (ratio = 1:1 in volume) and tannimum (1 mg mL− 1) for 10 min to complete the chemical-etching process. The resulting product was then collected via centrifugation (8000 rpm), washed thoroughly by ethyl alcohol several times, and dried at 80 °C in a vacuum oven to acquire hollow oval-like Co-MOF. Finally, this as-prepared hollow oval-like Co-MOF was calcined in air at 600 °C for 4 h at a ramping rate of 2 °C min− 1 to afford hollow-structured oval-like HOCO.

Fig. 1
figure 1

(a) Flow-chart of HOCO preparation; (b) and (e) microscopic images of the original oval-like Co-MOF; (c) and (f) microscopic images of hollow oval-like Co-MOF; (d) and (g) SEM and TEM images of HOCO; (h) TEM image of HOCO at a closer magnification (the inset is the particle size distribution); (i) HRTEM image of HOCO; and (j) SAED of HOCO

For material characterizations, the appearance of all materials was determined using Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) (JEOL JSM-7800 F and JEM-1400, Japan). The elemental composition of HOCO was also analyzed using energy-dispersive X-ray spectroscopy (EDS) (Oxford Instruments, UK). Moreover, the crystalline structures of all as-prepared materials were characterized using an X-ray diffractometer, XRD (Bruker, USA). Besides, Raman spectroscopies of HOPO and commercial Co3O4 NP were verified using a Raman spectrometer (TII Nanofinder 30, Japan). The surface chemistry of HOCO was further examined using X-ray photoelectron spectroscopy (XPS) (ULVAC-PHI, PHI 5000, Japan). Additionally, the reducible behaviors of HOCO and commercial Co3O4 NP were further analyzed by H2-temperature-programmed reduction (H2-TPR) profiles using a chemisorption analyzer coupled with a Thermal Conductivity Detector (Anton Paar, ASIQ TPx, Austria). Next, the BET surface area and pore volume of HOCO were determined by N2 sorption isotherms using a volumetric analyzer (Anton Paar Autosorb IQ, Austria). The surface charges of HOCO were also verified using a Zetasizer (Nano-ZS, Malvern Instruments, UK).

2.2 Degradation of ARS

The degradation of ARS using HOCO-activated MPS was conducted using batch-type experiments. Typically, a certain amount of HOCO (i.e., 100 mg L− 1) was firstly added into ARS solution with the initial concentration (C0) of ARS was 50 mg L− 1 for 30 min to verify the adsorption of ARS. Next, 100 mg L− 1 of MPS was instantly introduced to the ARS solution to start the degradation experiment. At a certain reaction time (t), sample aliquots (3 mL) were taken out and filtered to separate HOCO from the ARS solution. The remaining ARS concentration in the filtrate at t min (Ct) was subsequently determined by UV-Vis spectrophotometer with the absorption wavelength of 520 nm. For experimental sections, the effects of different parameters including different HOCO dosages, MPS dosages, various ARS concentrations, initial pH values, co-existing surfactants and salt, and inhibitors were investigated. The recyclability experiments were performed by collecting and re-employing the used HOCO. Moreover, for comparison, commercial Co3O4 NP (size 50 ~ 80 nm) was also used for MPS activation. ROS involving in the degradation of ARS by HOCO + MPS system were revealed by electron paramagnetic resonance (EPR) with a typical radical spin-trapping agent, namely 5,5-Dimethyl-1-pyrroline N-oxide (DMPO). The recyclability of HOCO for multiple ARS degradation cycles was also evaluated by re-using HOCO. Besides, the degradation intermediates produced from ARS degradation were further identified using a mass spectrometer (Thermo Finnigan, LCQ ion-trap mass spectrometer, USA). Total organic carbon (TOC) was analyzed using TOC analyzer (SHIMADZU TOC-5000, Japan).

3 Results and discussion

3.1 Properties of HOCO

First, morphologies of the as-prepared materials were visualized. Figure 1b and e present electronic microscopic figures of the original Co-MOF, indicating successful formation of 2-dimensional oval-like morphology. Its crystalline structure was also determined using XRD analysis as displayed in Fig. 2a, which is consistent with reported literature [26]. After this pristine Co-MOF was modified with tannium via a chemical-engraving process, the resultant product still retained the oval-like appearance of the original Co-MOF (Fig. 1c). However, it can be seen that the interior part of this product was removed, creating a huge hollow-structure inside whereas the thin outer layer (size ca. of 20-25 nm) was remained as shown in Fig. 1f, verifying the successful construction of hollow-structured oval-like Co-MOF. The XRD pattern of this product was also characterized as depicted in Fig. S1 of Supplemental Materials.

Fig. 2
figure 2

(a) XRD pattern of the original oval-like cobaltic MOF; (b) XRD pattern of HOCO; (c) FTIR profiles of the pristine oval-like Co-MOF and HOCO; (d) long-range Raman spectroscopies of Co3O4 NP and HOCO; (e) and (f) Raman spectroscopies at specific regions

When this hollow-structured Co-MOF was thermally-oxidized in air, the resulting substance still showed the oval-like morphology and hollow configuration (Fig. 1c and g). Moreover, densely-packed and ultrafine spherical particles could be observed, which were distributed evenly throughout this calcined product (Fig. 1g); a closer observation in Fig. 1h then shows that the particles would be nano-sized, ranging from 7 to 13 nm (in diameter) with a principal size of 10 nm (Fig. 1h). Besides, its HRTEM (Fig. 1i) unveils two particular d-spacings of 0.29 and 0.24 nm, which corresponded to the plane (2 2 0) as well as (3 1 1) in Co3O4. This result validated that those nano-sized particles or NPs should be Co3O4, demonstrating the successful formation of hollow-structured oval-like cobalt oxide (HOCO).

The crystalline structure of HOCO was also measured (Fig. 2b); and its crystalline patterns exhibited several notable diffraction peaks located at 19, 31.4, 36.8, 38.6, 44.8, 55.5, 59.3, 65.1, 74.1 and 77.3o, which could be well indexed to those signature planes of Co3O4 (JCPDS #42-1467), asserting the full conversion of the oval-like cobaltic MOF to Co3O4. The electron diffraction pattern of HOCO in Fig. 1j reveals the polycrystalline structure of HOCO, which is correlated with its XRD profile. In addition, the elemental composition in HOCO was also identified using EDX analysis in Fig. S2a, where only Co and O could be noted. Besides, the elemental mapping of HOCO was also determined (Fig. S2b-d), showing that oxygen and cobalt were certainly present and uniformly distributed throughout the oval-like outer layer.

Moreover, the functional groups of the pristine oval-like Co-MOF and HOCO were determined through IR profiles (Fig. 2c). For the original Co-MOF, various absorption bands located at 500-1500 cm− 1 shall be attributed to the ligand (i.e., an imidazole ring) [27]. Besides, an absorption band positioned at ~ 1584 cm− 1 could be the C=N bond in 2-MIM, whereas two noticeable peaks located at 2929 and 3138 cm− 1 could be corresponded to the C-H sp3 in the aromatic ring and the C-H sp2 in 2-MIM, respectively, further indicating the formation of Co-MOF [27]. The IR analysis of HOCO showed distinct absorption bands. Particularly, several notable bands detected in 565 and 660 cm− 1 were attributed to Co2+ and Co3+ in cobalt oxide, respectively [28]. Besides, the bands located at 1630/3410 cm− 1 were ascribed to the O-H bond, which was probably due to adsorbed H2O on HOCO [29].

In addition, Raman spectroscopies of Co3O4 NP and HOCO were further characterized as shown in Fig. 2d. It can be noticed that both materials exhibited five noticeable peaks, which could be assigned to the Raman active vibration modes (i.e., A1g + Eg + 3F2g) of Co3O4 [30]. This further validates the formation of Co3O4 in HOCO. However, the peaks at F2g(1) and A1g regions of HOCO (Fig. 2e-f) were shifted to the lower positions than that of commercial Co3O4 NP. Specifically, the band at F2g(1) region of HOCO was switched from 191 to 189 cm− 1, whereas the band at A1g region was moved from 678 to 669 cm− 1. As these two specific regions were assigned to the oxygen mobility of Co3O4 [31], these shifted peaks in HOCO suggested that HOCO might possess more structural defects compared with Co3O4 NP, enabling HOCO to exhibit a more reactive surface [32], thereby enhancing activities of HOCO for MPS activation.

Furthermore, the chemical states of all elements in HOCO were identified by XPS. Figure 3a presents a whole-range scan, showing that only notable Co and O signals were detected, which was in line with EDX/mapping results. Besides, the Co2p spectra of HOCO were then deconvoluted into a number of peaks. The peaks detected in 779.4 and 794.3 eV were assigned to Co3+ species, whereas the peaks located at 781.4 and 796.5 eV were assigned to Co2+ species; and co-existence of these Co species strongly confirm the presence of Co3O4 in HOCO [33]. Moreover, the O1s spectra of HOCO was also examined, and a number of notable peaks were detected at 529.4 and 530.9 eV, derived from O species in metal-O bond (Olatt) and R-O bond (Oads), respectively [34].

Fig. 3
figure 3

XPS analysis of HOCO: (a) whole-range scan, (b) Co2p scan, and (c) O1s scan; (d) TPR analyses of Co3O4 NP and HOCO; (e) physical properties of Co3O4 NP and HOCO; and (f) zeta potential of HOCO at different pH values

Additionally, since both commercial Co3O4 NP and HOCO were comprised Co3O4 and HOCO possessed more reactive surface, it was crucial to probe into the distinct reducibility between these two materials using H2-TPR analysis. As displayed in Fig. 3d, H2-TPR profiles of HOCO and Co3O4 NP are quite different, in which HOCO showed three reduction peaks and Co3O4 NP displayed two reduction bands. Typically, Co3O4 exhibits two reducing steps: the first step is the reducing step from Co3+ to Co2+ and the second step is the reducing step from Co2+ to Co0 [35]. In the case of the commercial Co3O4 NP, these reduction bands were overlapped, in which the band at 465 °C was due to the first step, and the band at 501 °C was due to the second step. On the other hand, TPR analysis of HOCO showed three reductive bands at 296, 334, and 365 °C. Specifically, the peak at 296 °C could be due to the reducing step from Co3+ to Co2+ while these bands at 344 and 365 °C could correspond to the reducing step from Co2+ to Co0 in the cluster form and bulk form, respectively [36]. This result showed that the reduction temperatures of Co species in HOCO were much lower than that of Co3O4 NP, further illustrating the higher degree of active surface in HOCO compared with Co3O4 NP, which subsequently contribute to different activities during oxidative reactions [37].

Next, the physical properties of both HOCO and Co3O4 NP were then characterized in Fig. 3e. HOCO exhibited an isotherm with hysteresis loop, suggesting mesopores contained in HOCO. The pore size analysis can be seen in the inset of Fig. 3e further validated the presence of mesopores and macropores in HOCO. The specific surface area of HOCO was then calculated as 44 m2 g− 1 with a total porous volume = 0.26 cm3 g− 1. On the other hand, the commercial Co3O4 NP showed a relatively low N2 sorption capability; thus, its surface area would be merely 2.5 m2 g− 1 with a certain small total porous volume of 0.001 cm3 g− 1, probably due to the critical aggregation of these Co3O4 NPs as depicted in Fig. S3. This result further unveils the distinct textural properties between HOCO and Co3O4 NP, which would then influence their catalytic activities. These textural properties of HOCO and Co3O4 NP were also summarized in Table S1.

Moreover, since the degradation of ARS would be executed via batch-type degradation in water, zeta potentials of HOCO under numerous pH conditions were necessarily measured. Figure 3f shows the zeta potential profile of HOCO, illustrating that the zeta potentials of HOCO would be positive under acid and neutral conditions; and switched to negative charges at basic conditions. The zeta potential of HOCO would be 23.7 mV at pH = 3, and it would then decrease to 18.3 and 7.4 mV when pH increased to 5 and 7, respectively. When pH = 9, the zeta potential of HOCO would be − 15 mV, and it was further dropped to − 31.6 mV at pH = 11 with its pHzpc = 7.6.

3.2 ARS degradation using HOCO-activated MPS

ARS degradation was investigated by different catalytic systems as shown in Fig. 4a. Initially, it was essential to test if ARS might be eliminated via the adsorption pathway. It can be noticed from Fig. 4a that ARS remained steady within 30 min with HOCO only, demonstrating the negligible adsorptive capability of ARS on HOCO surface. On the other hand, ARS was also hardly degraded in the presence of MPS individually since Ct/C0 just approached 0.9, suggesting that MPS self-activation without catalysts would be ineffective, leading to low ARS degradation efficiency. Nevertheless, when HOCO was employed for MPS activation, ARS would be drastically reduced and wholly removed in 30 min. This result reveals that MPS could be activated by HOCO to result in ROS for degrading ARS. Moreover, as HOCO would be constituted of Co3O4, the commercial Co3O4 was also utilized for comparison with HOCO; and ca. 40% of ARS was eliminated after 30 min, proving that the commercial Co3O4 NP was also able to activate MPS for eliminating ARS. However, the ARS degradation efficiency by the commercial Co3O4 NP + MPS system was relatively low in comparison with that of HOCO + MPS system, indicating the benefit of HOCO for activating MPS to eliminate ARS. Besides, as ARS was efficiently degraded by HOCO coupled with MPS, the total organic carbon (TOC) should be then performed to determine the mineralization degree of ARS. The initial TOC (TOC0) concentration was measured as 19 mg L− 1. As presented in Fig. S4, ca. 60% of TOC could be removed in 30 min, validating the decomposition of ARS using HOCO + MPS system.

Fig. 4
figure 4

(a) ARS elimination by various catalytic systems: MPS only, adsorption, Co3O4 NP + MPS, and HOCO + MPS; (b) the corresponding calculated degradation kinetics (k) (min− 1) and reaction stoichiometric efficiency (RSE) values of different catalytic systems on ARS degradation (Reaction conditions: catalyst = MPS:100 mg L− 1, T = 30 °C)

On the other hand, for further comparing degradation efficiencies of ARS by the above-mentioned catalytic systems, two particular indices (i.e., degradation kinetics (k) (using the pseudo 1st order equation) [38] and reaction stoichiometric efficiency (RSE)) [39] were further calculated as the following equations (Eqs. (1)-(2)):

$${C}_t={C}_0\mathit{\exp}\left(- kt\right)$$
(1)
$$RSE=\frac{ARS\ degraded\ (mol)}{Amount\ of\ MPS\ used\ (mol)}$$
(2)

where k denotes the pseudo 1st-order reaction rate. In Fig. 4b, the corresponding k of HOCO + MPS was calculated as 0.22 min− 1, which would be substantially higher than that of Co3O4 NP + MPS (0.015 min− 1), MPS individually (0.0036 min− 1) and adsorption (0.0017 min− 1). The as-calculated k of HOCO + MPS is much higher than other reported k values for ARS degradation in literature, such as 0.06 min− 1 (FeCF + PC) [40], 0.035 min− 1 (Fc + SPC) [41], and 0.089 min− 1 (CoCNF + Ox) [42]. Besides, HOCO + MPS exhibited the highest RSE value of 0.31, followed by Co3O4 NP + MPS (0.12), MPS individually (0.03) and adsorption (0.001). Moreover, to shed a light on the advantage of this unique characteristic, we also prepared non-hollow structure oval-like Co3O4 NPs (abbreviated as SOCO) followed the same procedure for preparing HOCO, except the chemical-engraving process and employed it for ARS degradation. As depicted in Fig. S5, ca. 80% of ARS was removed when SOCO was coupled with MPS, showing that though SOCO was also capable to activate MPS to degrade ARS, the catalytic activity of SOCO without hollow-structured property was assuredly lower than that of HOCO. This further validated the excellent activity of HOCO to activate MPS, enabling it to be an effective heterogeneous activator for degrading ARS.

3.3 Effects of HOCO and MPS dosages, and different ARS concentrations

Since ARS was effectively degraded using HOCO-activated MPS, the effects of HOCO dosages and MPS dosages should play critical roles in ARS degradation. Thus, different amounts of HOCO and MPS were then investigated to examine their influence for obtaining the optimal condition for further investigations. As shown in Fig. 5a, ARS degradation efficiency would be improved by the increase of HOCO concentrations. Particularly, at 50 mg L− 1 of HOCO, ARS gradually degraded as Ct/C0 approached 0.05 in 30 min, and its corresponding k would be 0.095 min− 1 (the inset of Fig. 5a). As the dosage of HOCO was raised to 100 mg L− 1, ARS would be completely eliminated in 30 min, and it would be reached in an even less reaction time (i.e., 20 min) at 150 mg L− 1 of HOCO. The calculated k would be also measured as 0.22 min− 1 as well as 0.34 min− 1 at the HOCO dosage of 100 and 150 mg L− 1, respectively. This verifies the advantage of higher HOCO concentrations on ARS degradation, owing to the contribution of abundant active sites for accelerating ROS generated from MPS activation.

Fig. 5
figure 5

(a) HOCO, (b) MPS, (c) ARS concentrations, (d) temperatures, (e) pH values, and (f) surfactants and salt. (Reaction conditions: catalyst = MPS: 100 mg L− 1, T = 30 °C)

In addition, the influence of MPS concentrations on ARS degradation was also studied as illustrated in Fig. 5b. As a small dosage of MPS (i.e., 50 mg L− 1) was added, only 60% of ARS was eliminated in 30 min, probably because of insufficient ROS generated. Once MPS concentration was augmented to 100 mg L− 1, complete ARS degradation would be achieved in 30 min. A higher MPS dosage at 150 mg L− 1 slightly enhanced the ARS degradation efficiency (Fig. 5b). This result revealed that the lower dosage of MPS with deficient generated ROS could not fully eliminate ARS while higher MPS dosages could effectively and completely remove ARS owing to sufficient ROS produced. Besides, the corresponding k with various MPS dosage would be also measured inside Fig. 5b. The corresponding k value at 50 mg L− 1 of MPS was computed to be 0.095 min− 1, and it was then substantially raised to 0.22 as well as 0.27 min− 1 at MPS dosage of 100 and 150 mg L− 1, which further validated the enhancing effect of relatively high MPS concentrations on ARS degradation.

Furthermore, it was practical for evaluating the activity of HOCO for catalyzing activation of MPS to eliminate different ARS concentrations. In Fig. 5c, the full degradation of ARS was still afforded in 30 min, even in a high-concentrated ARS solution (200 mg L− 1), confirming a superior performance of HOCO for activating MPS. In addition, the k values (Fig. 5c) were measured to be 0.22, 0.20, and 0.19 min− 1 at the ARS concentration of 50, 100, and 200 mg L− 1, respectively. This further indicates that HOCO could retain its exceptional performance for MPS activation to eliminate ARS at various concentrations.

3.4 Influences of other factors

As ARS was efficiently degraded using HOCO-catalyzed MPS, the effects of other parameters on ARS elimination should be further investigated. First, the effect of temperatures was examined because temperatures are essential in the reaction kinetic (Fig. 5d). Generally, the degradation efficiency of ARS would be significantly boosted when temperature increases. Particularly, at 30 °C (Fig. S6a), the complete degradation of ARS was afforded within 30 min with its corresponding k of 0.22 min− 1. When the temperature raised to 40 and 50 °C, ARS degradation efficiencies were significantly enhanced as Ct/C0 reached zero within shorter reaction times. And the corresponding k values were also drastically increased to 0.31 as well as 0.46 min− 1 at 40 and 50 °C. This result assuredly proves the promoting effect of elevated temperatures on ARS elimination using HOCO-activated MPS.

Moreover, as the corresponding k raised with increasing temperatures, the relationship between k and T would be further correlated via the following equation (Eq. (3)) [43]:

$$\ln k=-\frac{E_a}{RT}+\ln A$$
(3)

where Ea represents the calculated activation energy (kJ mol− 1). The inset of Fig. 5d further shows the relationship between k and temperatures was fitted linearly with R2 = 0.996. Thus, the activation energy (Ea) was then computed as 31 kJ mol− 1. This calculated Ea value from HOCO-activated MPS was comparatively lower than most reported Ea values in literature (Table S2), further validating the exceptional catalytic activity of HOCO, enabling it a highly-competitive catalyst for MPS activation.

The influence of different initial pH values on ARS degradation was then investigated. Figure 5e shows that ARS was completely degraded in 30 min at a neutral condition of pH = 7 with k of 0.22 min− 1 (Fig. S6b). When the pH in ARS solution would be tuned to pH = 5, a weakly acidic environment, ARS degradation was slightly interfered as Ct/C0 approached 0.1 after 30 min. The interference became more pronounced once pH was further tuned to a strongly acidic environment (i.e., pH = 3) since merely 50% of ARS was eliminated, and the corresponding degradation kinetic was significantly reduced to 0.024 min− 1. ARS degradation using HOCO-activated MPS was thus unfavorable in the acidic environment, which possibly due to the fact that MPS was indolent under acidic conditions [44], thereby suppressing MPS activation. Besides, the generated ROS (e.g., SO4•–, OH) might be quickly consumed by free protons as follows (Eqs. (4)-(5)) [45], lowering the degradation of ARS.

$${\textrm{H}}^{+}{+}^{\cdot}\textrm{OH}+{\textrm{e}}^{-}\to {\textrm{H}}_2\textrm{O}$$
(4)
$$\text{H}^++{\text{SO}}_4^{\cdot-}+\text{e}^-\rightarrow{\text{H}\text{SO}}_4$$
(5)

Moreover, as pKa of ARS is 6.5 [46], ARS might be relatively positive due to the protonation. According to the zeta potentials of HOCO, its surface charge seemed to become relatively positive at pH < 6.5. Thus, the repulsion between the protonated ARS, and HOCO grew stronger, constraining their reactions [47]. Furthermore, H+ might react with free radicals at low pH [48], thus suppressing ARS degradation, and hydrogen bonding of O-O and H+ on MPS itself in acidic environments would limit reactions MPS with HOCO [49].

Additionally, the degradation efficiency of ARS would be also affected at higher pH. Particularly, under a weak basic condition of pH = 9, ARS degradation would be minimally influenced as Ct/C0 still reached 0.15 in 30 min (Fig. 5e), and its corresponding k was 0.094 min− 1. Nevertheless, as pH changed to 11, a strongly basic environment, only 30% of ARS was removed whereas the degradation kinetic at pH = 11 was noticeably decreased to 0.011 min− 1. The negative impact of basic environment on ARS degradation, which would be possibly correlated to the much more intensive electrostatic repulsion between the relatively anionic surface of HOCO and SO52− as pH exceeded the pKa of MPS = 9.4, then constricting the production of ROS [20]. Moreover, in the basic conditions, SO4•– radicals would readily react with excessive OH ions to become OH radicals which possess a shorter half-life and lower standard redox potential, leading to fewer SO4•– radicals involved in ARS degradation [20]. Moreover, in the basic conditions, SO4•– radicals would readily react with excessive OH ions (Eq. (6)) to become OH radicals which possess a shorter half-life and lower standard redox potential [50], leading to fewer SO4•– radicals involved in ARS degradation.

$${{\textrm{SO}}_4}^{\cdot -}+{\textrm{OH}}^{-}\to {{\textrm{SO}}_4}^{2\hbox{-} }{+}^{\cdot}\textrm{OH}$$
(6)

On the other hand, in actual water systems, various compounds (including natural and artificial substances) are frequently detected, which potentially interfere ARS elimination using HOCO-activated MPS. Therefore, it was important to scrutinize the effect of these compounds on ARS elimination (Fig. 5f). The effect of inorganic salt was first examined. In this study, sodium chloride (NaCl) at different concentrations was particularly selected as an inorganic salt model. With 500 mg L− 1 of NaCl, ARS was still effectively degraded since almost 100% ARS was achieved in 30 min whereas ARS elimination was negligibly influenced at a higher concentration of NaCl (i.e., 1000 mg L− 1), suggesting the minimal interference of NaCl on ARS degradation using HOCO + MPS. This could be explained that the resultant SO4•– radicals would interact with free Cl ions to produce chloride radicals as following equations (Eqs. (7)-(8)) [51] that possess a relatively low redox power than that of SO4•– radicals, thereby diminishing ARS degradation efficiencies. Nonetheless, HOCO-activated MPS system still efficiently eliminated ARS, even at a higher NaCl concentration, validating that HOCO could retain its preeminent catalytic activity for MPS activation. The corresponding k with NaCl was also calculated as shown in Fig. S6c.

$${{{\textrm{SO}}_4}^{\cdot}}^{-}+{\textrm{Cl}}^{-}\to {{{\textrm{SO}}_4}^2}^{-}+\textrm{Cl}^{\cdot}$$
(7)
$${\textrm{Cl}}^{\cdot }+{\textrm{Cl}}^{-}\to {{{\textrm{CI}}_2}^{\cdot}}^{-}$$
(8)

Moreover, it was also reported that the existence of surfactants may also interfere the activation of MPS, decreasing the degradation effectiveness of the organic pollutants. Thus, it is useful to determine the influence of surfactants in water on ARS elimination using HOCO + MPS. Specific surfactants were chosen, including cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) as shown in Fig. 5f. In the case of CTAB, it is interesting that ARS elimination was improved since Ct/C0 approached zero in a shorter reaction time with k of 0.23 min− 1 (Fig. S6c). This reveals positive influences of CTAB to ARS degradation by HOCO-catalyzed MPS, which was probably because CTAB, a typical cationic surfactant, may occupy HOCO surface, resulting in attraction between positively-charged HOCO surface and ARS molecules where ROS were produced [42], thus noticeably enhancing the degradation of ARS. With SDS (500 mg L− 1), a typical anionic surfactant, ARS degradation efficiency was certainly influenced since solely ca. 50% of ARS was eliminated in 30 min with a much lower k of 0.023 min− 1. The degradation of ARS using HOCO + MPS was much less effective in the co-existence of SDS, which was possibly because the surface of HOCO became negatively after SDS was introduced, creating a stronger electrostatic repulsion between negatively-charged HOCO surface and ARS molecules, then restricting the degradation of ARS [42].

3.5 Recyclability of HOCO

Since HOCO could effectively activate MPS to degrade ARS in water, it would be more intriguing to examine the reusability of HOCO for multiple ARS degradation cycles. It can be noticed from Fig. 6a that almost 100% of ARS was still afforded over 5 degradation cycles, indicating that HOCO could retain its exceptional performance for continuous activation of MPS. Besides, the crystalline pattern of the spent HOCO (Fig. 6b) was consistent with the pristine HOCO, showing that the highly-crystalline structure of HOCO was still retained. This indicated the outstanding durability of HOCO, enabling it to be a useful catalyst for MPS activation to eliminate ARS in water.

Fig. 6
figure 6

(a) recyclability of HOCO over multiple ARS degradation cycles; (b) XRD pattern of the used HOCO; (c) influence of radical scavengers on ARS elimination using HOCO + MPS; (d) EPR analysis. (Reaction conditions: HOCO = MPS: 100 mg L− 1, T = 30 °C)

3.6 Identification of ROS involved in ARS degradation

In order to reveal the generation of potential ROS (e.g., SO4•–, OH) from HOCO-activated MPS as the following equations (Eqs. (9)-(11)) [52, 53], and their contribution to the degradation of ARS, the scavenging test by two specific radical scavengers, such as methyl alcohol (MeOH) and tert-butyl alcohol (TBA), was performed.

$${\textrm{Co}}^{2+}@\textrm{HOCO}+{{\textrm{HSO}}_5}^{-}\to {\textrm{Co}}^{3+}@\textrm{HOCO}+{{{\textrm{SO}}_4}^{\cdot}}^{-}+{\textrm{OH}}^{-}$$
(9)
$${\textrm{Co}}^{3+}@\textrm{HOCO}+{{\textrm{HSO}}_5}^{-}\to {\textrm{Co}}^{2+}@\textrm{HOCO}+{{{\textrm{SO}}_5}^{\cdot}}^{-}{\textrm{OH}}^{-}$$
(10)
$${{\textrm{SO}}_4}^{\cdot -}+{\textrm{H}}_2\textrm{O}\to {{\textrm{SO}}_4}^{2-}{+}^{\cdot}\textrm{OH}+{\textrm{H}}^{+}$$
(11)

Figure 6c shows that ARS elimination was inhibited by either MeOH or TBA. It can be noted that TBA comprises no α-hydrogen in its structure, TBA would promptly react with OH; thus, it was selected to be an OH quencher. Figure 6c reveals that only 65% of ARS was degraded in 30 min by TBA with k decreased to 0.036 min− 1 (Fig. S6d), demonstrating the existence of OH produced during the activation of MPS.

With α-hydrogen in the structure, MeOH can react with both OH and SO4•–. The inhibitory effect became even more remarkable when MeOH was then added as Ct/C0 merely reached 0.98, and k was significantly dropped to 0.0005 min− 1, implying that SO4•– was also generated from HOCO + MPS system. To confirm these ROS, EPR was further adopted using DMPO as a radical trapping probe. From Fig. 6d, it can be observed that there was no obvious pattern when MPS was individually introduced in DMPO solution. Nevertheless, once HOCO and MPS were simultaneously added, notable patterns were particularly detected, which was attributed to DMPO-OH and DMPO-SO4 adducts [54, 55]. This further proves the generation of SO4•– and OH species involved in ARS degradation using HOCO + MPS system (as shown Fig. 6e).

3.7 A potential degradation process for ARS

For elucidating ARS elimination process, computational calculations of ARS molecule according to the DFT were executed. Detailed computational settings were described in SI, and the geometrically-optimized ARS were then revealed in Fig. 7a by atom labels. Moreover, the highest occupied and lowest unoccupied molecular orbitals (MOs) of the geometrically-optimized ARS are also rendered in Fig. S7. Specifically, red and green-colored isosurfaces of these MOs represent electron-abundant and electron-deficient zones, respectively. Moreover, the highest occupied MO located on the naphthalene group bound to the sulfonate group tended to denote electrons. Therefore, ARS would possibly attract electrophilic attacks by the electrophilic ROS.

Fig. 7
figure 7

Mapped isosurface of Fukui function of ARS: (a) labels of atoms, (b) f , (c) f 0, and (d) f+, (e) the front-view, and (f) back-view ESP-mapped surface of ARS

For further examining the degradation pathway by identifying the most-likely reaction spots of ARS, the Fukui function was adopted to provide insights [56]. To this end, the dual descriptor of the Fukui function was calculated, and rendered by Multiwfn to forecast potential reaction spots of ARS [57]. For offereing intuitive visualization of reactive spots of ARS, its corresponding isosurface of electron density derived from the Fukui function is then depicted, and Fig. 7b, c, and d display the f , f 0 and f + isosurfaces of ARS, respectively.

Theoretically, a spot with a huger green-colored isosurface would imply a higher possibility for reactions. Therefore, the electrophilic reaction spots of ARS (e.g., C27, C32, C33, N14, N15, etc.) would withdraw attacks by electrophilic radicals. Besides, Fig. 7c also displays possible spots for non-radical reactions, including C27, N14, and N15. These aforementioned spots might withdraw attacks to initiate the degradation process of ARS. On the other hand, the electrostatic potential (ESP) of ARS (Fig. 7e-f) also indicates that C19, C24, and C27 might exhibit affinities towards the anionic sulfate radical.

To clarifying the degradation process of ARS, degradation derivations from ARS were then determined in Fig. S8 with probable intermediates then outlined (Table S3). According to these intermediates, a probable ARS elimination process using HOCO is illustrated in Fig. 8 [58, 59]. In the beginning, ARS would be transformed to M1 upon dissolving in water, which was then attacked by the electrophilic ROS on those sites adjacent to the sulfonate group to afford an intermediate, M2. Through the cleavage of N=N bond, M2 would be then decomposed to produce M3, and M4. M3 was then further oxidized, and decomposed to generate M5, whereas M4 would be oxidized to result in M6 as well as M7 afterwards. On the other hand, M5 would be continuously oxidized to M6, and then M7, which was eventually decomposed to CO2 and H2O.

Fig. 8
figure 8

A potential degradation pathway of ARS by HOCO-activated MPS

4 Conclusions

In brief, this study has successfully reported a special Co3O4-based catalyst using an oval-like Co-MOF as a precursor. Through a modification step with tannimum via the chemical-engraving process, hollow-structured oval-like Co-MOF was achieved, which was then calcined in air to become hollow oval-like cobalt oxide (HOCO). This HOCO was then adopted for catalyzing MPS activation to eliminate ARS. From material characterization results, HOCO exhibited highly-reactive surfaces and larger structures than commercial Co3O4 NP, contributing to the preeminent catalytic activity of HOCO for MPS activation to eliminate ARS in water. Moreover, HOCO-activated MPS also showed relatively low activation energy (Ea) of 31 kJ mol− 1, which was substantially lower than many reported studies in the literature. Besides, HOCO would also preserve its activities over multiple degradation cycles without any significant loss as well as remaining its highly-crystalline structure. These results certainly confirm that HOCO could be an efficient heterogeneous activator for activating MPS to remove a toxic azo dye, ARS, in water.

Availability of data and materials

All data generated or analyzed during this study are provided.

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Acknowledgements

This work is supported by the Ministry of Science and Technology (MOST)(111-2636-E-005-003-), Taiwan. The authors gratefully acknowledge the use of SQUID000200 of MOST111-2731-M-006-001 belonging to the Core Facility Center of National Cheng Kung University.

Funding

This work is supported by the Ministry of Science and Technology (MOST)(111-2636-E-005-003-), Taiwan.

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Ta Cong Khiem and Duong Dinh Tuan contribute to this study equally. Ta Cong Khiem prepared the draft, Duong Dinh Tuan curated data and prepared the draft, Eilhann Kwon curated data, Bui Xuan Thanh curated data and prepared visualization, Yiu Fai Tsang curated data and investigated, Venkata Subbaiah Munagapati edited the draft, Jet-Chau Wen reviewed the draft, Chechia Hu wrote and edited the draft, Kun-Yi Andrew Lin designed and analyzed experiments. All authors read and approved the final manuscript.

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Correspondence to Chechia Hu or Kun-Yi Andrew Lin.

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Khiem, T.C., Tuan, D.D., Kwon, E. et al. Hollow and oval-configured ultrafine Co3O4 as a highly-efficient activator of monopersulfate for catalytic elimination of Azorubin S. Sustain Environ Res 32, 48 (2022). https://doi.org/10.1186/s42834-022-00158-3

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Keywords

  • Azorubin S
  • Monopersulfate
  • Hollow
  • Cobalt Oxide
  • Activation