Heterogeneous Activation of Peroxymonosulfate by a Spinel CoAl2O4 Catalyst for the Degradation of Organic Pollutants

Abstract

Bimetallic catalysts have significantly contributed to the chemical community, especially in environmental science. In this work, a CoAl₂O₄ spinel bimetal oxide was synthesized by a facile co-precipitation method and used for the degradation of organic pollutants through peroxymonosulfate (PMS) activation. Compared with Co₃O₄, the as-prepared CoAl₂O₄ possesses a higher specific surface area and a larger pore volume, which contributes to its becoming increasingly conducive to the degradation of organic pollutants. Under optimal conditions (calcination temperature: 500 °C, catalyst: 0.1 g/L, and PMS: 0.1 g/L), the as-prepared CoAl₂O₄ catalyst could degrade over 99% of rhodamine B (RhB) at a degradation rate of 0.048 min⁻¹, which is 2.18 times faster than Co₃O₄ (0.022 min⁻¹). The presence of Cl⁻ could enhance RhB degradation in the CoAl₂O₄/PMS system, while HCO₃⁻ and CO₃²⁻ inhibit RhB degradation. Furthermore, the considerable reusability and universality of CoAl₂O₄ were testified. Through quenching tests, ¹O₂ and SO₄•⁻ were identified as the primary reactive species in RhB degradation. The toxicity evaluation verified that the degraded solution exhibited lower biological toxicity than the initial RhB solution. This study provides new prospects in the design of cost-effective and stable cobalt-based catalysts and promotes the application of PMS-based advanced oxidation processes for refractory wastewater treatment.

1. Introduction

A wide range of organic wastewater has been discharged into the natural water environment with the dramatic development of the printing and dyeing, textile, and medicine industries. These kinds of wastewater are characterized by stable composition, high pollutant concentration, massive variation of water quality and quantity, non-biodegradation [1], as well as high teratogenicity, carcinogenicity, and biotoxicity, seriously threatening human health and the aquatic ecosystem [2,3]. Conventional water treatment technologies, including adsorption [4], photocatalysis [5], and biological process [6], are generally plagued with drawbacks such as high operating cost, low degradation efficiency, and incomplete degradation [7]. Therefore, it is necessary to explore novel, low-cost, and efficient wastewater treatment techniques.

Recently, sulfate radical (SO₄•⁻)-based advanced oxidation processes (SR-AOPs) have gained enormous attention in environmental remediation owing to their low cost and excellent oxidizing capability at a wide pH range [8,9]. SR-AOPs can generate SO₄•⁻ that exhibits a higher redox potential (E⁰ = 2.5–3.1 V) and a longer half-life (30–40 μs) than hydroxyl radical (•OH) and is independent of solution pH [10,11,12]. Among persulfate salts, peroxymonosulfate (PMS) with an asymmetric structure (HO−O−SO₃⁻) and a longer superoxide bond (lo−o = 1.326 Å) can be more easily dissociated to generate more reactive oxygen species than peroxydisulfate (PDS) (lo−o = 1.322 Å) [13,14,15,16]. PMS can be activated in the presence of ultraviolet, heating, bases, ultrasound, electrochemical processes, and transition metal ions [17,18,19,20,21], among which, transition metal ions, especially Co²⁺ and cobalt oxides, demonstrate the best performance in PMS activation [22,23,24]. However, homogeneous PMS activation systems suffer from formidable separation and secondary pollution, while Co single−atom catalysts exhibit shortcomings of poor stability and low degradation efficiency, and these deficiencies remarkably restrict their practical application [25,26]. Thus, further exploration of efficient, stable, and reusable cobalt-based catalysts is still indispensable.

Bimetallic oxides have attracted extensive interest due to their dense active sites, synergistic effects between metal atoms, stable structure, and high catalytic activity [27,28,29]. For instance, Chen et al. [30] demonstrated that CuCo₂O₄ could effectively activate PMS for the degradation of sulfadiazine, which was superior to CuO and Co₃O₄, and the degradation efficiency was 98% within 30 min. Additionally, previous studies have shown that CoFe₂O₄ and NiCo₂O₄ exhibited enhanced mineralization of organic pollutants in the presence of PMS [31,32]. Since cobalt aluminate (CoAl₂O₄) possesses acid and alkali resistance, excellent chemical reactivity, prosperous optical properties, and thermal stability [33,34], it is extensively employed in the fields of high-temperature resistant coatings, plastics, ceramics, enamels, glass and coloring, paint, photocatalyst, and magnetic resonance imaging, etc. [35,36,37]. In our previous study [38], we synthesized CoAl₂O₄ by a combustion method, which exhibited remarkable performance in activating PMS to degrade organic pollutants. However, the synthesis of CoAl₂O₄ suffers from a complex process, high cost, and the degradation mechanism of PMS activation using CoAl₂O₄ needs further exploration.

Herein, CoAl₂O₄ was prepared via a cost-effective co-precipitation procedure and used as an efficient PMS activator to degrade multiple organic pollutants (rhodamine B (RhB), tetracycline hydrochloride (TCH), methylene blue (MB), acid orange (ARG), and acid orange (MO)) in an aqueous solution. The nanostructure, morphology, catalytic performance, university, and stability of CoAl₂O₄ were evaluated. A comprehensive study was conducted to determine the effect of various factors (including calcination temperature, catalyst dosage, PMS concentration, initial pH, and co-existing ions) on the CoAl₂O₄/PMS system against RhB. The degradation mechanism of CoAl₂O₄ by PMS activation was further clarified by the combination of quenching experiments and XPS analysis. Our study details insights into cobalt-based catalysts for efficient and sustainable remediation of organic pollutants.

2. Results and Discussion

2.1. Characterization of Catalysts

The XRD patterns of CoAl₂O₄-500 and Co₃O₄-500 prepared via the co-precipitation method are shown in Figure 1a. The characteristic peaks of CoAl₂O₄-500 appear at 2θ = 19.1°, 31.3°, 36.8°, 44.7°, 59.6°, and 65.3°, which correspond to the (111), (220), (311), (400), (511), and (440) lattice planes of CoAl₂O₄ (JCPDS No. 44-0160) [39]. The crystal size of CoAl₂O₄-500 was calculated to be 16.0 nm according to the Scherrer’s equation. A characteristic peak of Co₃O₄-500 is located at 2θ = 38.5°, which can be attributed to the (222) lattice planes of Co₃O₄ (JCPDS No. 43-1003) [40]. These results indicate that CoAl₂O₄ and Co₃O₄ were successfully synthesized.

As illustrated in Figure 1b, the SEM micrograph shows the morphology of CoAl₂O₄-500. The CoAl₂O₄-500 particles presented irregular blocky morphology with a relatively smooth surface and a non-uniform particle size distribution ranging from 0.1 to 0.4 μm. It was granular with a coarse surface and dispersed distribution.

N₂ adsorption–desorption isotherms were employed to determine the specific surface area and total pore volume of CoAl₂O₄-500 and Co₃O₄-500. As explicated in Figure 1c, the typical type IV profiles with H3 type hysteresis loop for CoAl₂O₄-500, according to the IUPC classification, indicate the presence of well-defined mesoporous structure in CoAl₂O₄-500 [41]. It could be seen that the pore size of Co₃O₄-500 was radically distributed in the range of 40–50 nm (inset in Figure 1c), while the pore structure of CoAl₂O₄-500 was primarily at the mesoporous region in the range of 2–4 nm and 20–25 nm. Compared with Co₃O₄-500, CoAl₂O₄-500 possesses a higher specific surface area (112.90 vs. 16.88 m²/g) and a larger total pore volume (0.2694 vs. 0.0470 cm³/g). Such a high surface area and porous nature of CoAl₂O₄-500 could potentially provide a higher quantity of active sites, lower mass transport resistance, and enhance PMS activation whereas vice in the case of Co₃O₄-500 [42].

The chemical state of Co was investigated via XPS analysis. The Co 2p spectrum possesses two major peaks at around 780.4 and 795.7 eV (Figure 1d), which are assigned to Co 2p_3/2 and Co 2p_1/2 peaks, respectively [43,44]. The peaks at 786.4 and 803.0 eV are indexed to the shake-up satellite of Co 2p_3/2 and Co 2p_1/2 peaks, respectively [45]. The Co 2p_3/2 and Co 2p_1/2 peaks for CoAl₂O₄-500 present a splitting value of 15.3 eV assigned to Co²⁺ [46,47]. There is no peak around 780.0 eV, implying the absence of Co₂O₃ or Co₃O₄ phases [48].

2.2. Catalytic Performance of Catalysts

2.2.1. Effect of Different Reaction Systems

RhB, an artificial synthetic dye and a common component in dye wastewater, was employed as the simulated pollutant to investigate the catalytic performance of CoAl₂O₄ for PMS activation. As illustrated in Figure 2a, RhB can be barely degraded without the presence of PMS and PMS could oxidize only 18.46% of RhB within 120 min without adding a catalyst. In contrast, PMS was effectively activated by CoAl₂O₄-500 with 99% RhB elimination efficiency after 100 min, which was much higher than that of Co₃O₄-500. Meanwhile, the degradation rate constant of the CoAl₂O₄-500/PMS system was about 2.18 times higher than that of the Co₃O₄-500/PMS system (inset in Figure 2a). Moreover, the degradation of RhB by catalytic activation of PMS with bimetallic materials in recent years was summarized. As shown in Table 1, CoAl₂O₄ exhibited better catalytic performance for PMS activation at a wide pH range than the other bimetallic oxides.

2.2.2. Effect of Calcination Temperature

Figure 2b displays the degradation rate of RhB over the CoAl₂O₄ samples calcined at different temperatures. Notably, CoAl₂O₄-300, CoAl₂O₄-500, and CoAl₂O₄-700 attained 84.32%, 94.00%, and 90.82% RhB degradation in 60 min with the rate constants of 0.040, 0.048, and 0.045 min⁻¹ (inset in Figure 2b), respectively. The results indicate that the degradation of RhB was slightly influenced by various calcination temperatures, and CoAl₂O₄-500 exhibited the best catalytic performance in RhB degradation. This might be because, with the increase in calcination temperature, the crystallinity and stability of the catalyst were enhanced [52]. However, when the calcination temperature was further elevated from 500 °C to 700 °C, the active components were over-sintered, and the agglomerated particle diameter size increased, inducing the reduction in active sites for PMS activation to degrade RhB.

2.2.3. Effect of Catalyst Dosage

Reasonable control of catalyst dosage can avoid side reactions and metal dissolution caused by excessive addition, thereby avoiding the degradation efficiency decline and secondary pollution. As observed in Figure 2c, when the dosage of CoAl₂O₄-500 increased from 0.05 to 0.10 g/L, the degradation efficiency of RhB increased from 96.27% after 120 min to 98.51% within 80 min, while the corresponding reaction rate constant of RhB noticeably increased from 0.026 to 0.048 min⁻¹ (inset in Figure 2c). Increasing the catalyst dosage provides a higher total surface area and availability of abundant active sites on the surface of the catalyst for PMS activation to yield more reactive oxygen species [53]. When the catalyst dosage further increased to 0.15 g/L, RhB was almost completely degraded within 80 min, but the degradation rate increased insignificantly. As the fixed PMS dosage, the amount of generated active species is assumed constant. Hence, 0.10 g/L was selected as the optimized amount of CoAl₂O₄-500.

2.2.4. Effect of PMS Dosage

The effect of oxidant amount is also crucial, as this is the producer of active species and directly affects the degradation efficiency. As displayed in Figure 2d, when the PMS concentration increased from 0.05 g/L to 0.10 g/L, the degradation efficiency of RhB enhanced from 91.05% at 120 min to 94.13% within 60 min, and the rate constant gradually increased from 0.020 to 0.048 min⁻¹ (inset in Figure 2d). The positive correlation between PMS dosage and RhB removal could be assigned to the more active species activated from increased PMS that facilitate RhB degradation [54]. However, no remarkable enhancement was observed when PMS concentration was further increased to 0.15 g/L. Although increased PMS could generate more oxidant species, the excess PMS would compete with target compounds for the reactive oxygen species [55]. Hence, the follow-up experiments were conducted with 0.10 g/L PMS as an optimum dosage for achieving the highest RhB degradation rate in the CoAl₂O₄/PMS system.

2.2.5. Effect of Initial pH

The pH in wastewater occasionally varies rapidly, and pH plays a key factor affecting physicochemical properties of reactants, possess of radical generation as well as the speciation of PMS [30]. Therefore, the effect of the solution pH on the CoAl₂O₄-500/PMS system was investigated over a wide range of pH from 3.44 to 10.22. As indicated in Figure 3a, the CoAl₂O₄-500/PMS system presented appreciable dramatical RhB degradation efficiency in a wide pH range, and the optimal pH range of the system was neutral or weak alkaline. Complete RhB degradation was achieved only in 20 min at pH = 9.46, and the reaction rate constants were 3.35, 1.31, 1.12 and 1.16 times of those at pH = 3.44, 4.73, 7.36 and 10.22, respectively (inset in Figure 3a). When the solution pH was 10.22, the degradation efficiency dramatically decreased, which might be because both OH⁻ and H₂O consumed SO₄•⁻ to produce •OH with lower activity and a shorter lifetime [56]. Meanwhile, Co(OH)₂ might also be formed on the catalyst surface under strongly alkaline conditions [57], thus inhibiting the catalytic performance. When the solution pH was 3.44, the degradation efficiency of RhB was 91.87% at 120 min. In acidic pH conditions, strong H−bound could be formed between H⁺ and the O−O group in PMS (stabilization effect) [58,59], thus hindering the interaction between CoAl₂O₄-500 and PMS. Additionally, PMS is mainly present in the form of H₂SO₅ under strongly acidic conditions, rather than HSO₅⁻ [60], thus the activation of PMS could be inhibited and the degradation efficiency under acidic conditions was lower than that under alkaline conditions.

2.2.6. Effect of Different Anions

Inorganic anions are widely presented at various concentrations in actual wastewater, which also react with the reactive oxygen species and affect the catalytic reactions in the PMS oxidant system. Figure 3b–d reveal the effects of Cl⁻, HCO₃⁻, and CO₃²⁻ on the degradation efficiency of RhB in the CoAl₂O₄/PMS system, respectively. Compared with the oxidation system in the absence of inorganic anions, Cl⁻ markedly facilitated the catalytic process, while HCO₃⁻ and CO₃²⁻ inhibited the RhB degradation. As exhibited in Figure 3b, 97.13% of RhB removal occurred in the presence of 0.1 M of Cl⁻ after 10 min, and the reaction rate constant was 2.46 times that without Cl⁻ (inset in Figure 3b). Generally, in the presence of Cl⁻, the sulfate radical-based system is prone to form halogen radicals (e.g., Cl₂•⁻, Cl•) with a higher oxidative potential. Cl₂•⁻and Cl• can react to generate Cl₂, further producing strong oxidizing ClO⁻ [61]. With the addition of 0.1 M of HCO₃⁻ and CO₃²⁻, the RhB degradation declined from 99.51% to 42.13% and 12.15% within 120 min, respectively. The reaction rate constants without anions were 12 and 53 times of those with HCO₃⁻ and CO₃²⁻, respectively (inset in Figure 3c,d). Additionally, this phenomenon can be attributed to the reactive radicals scavenging ability of HCO₃⁻ [62], while CO₃²⁻ can not only directly quench SO₄•⁻ but also quench •OH and SO₄•⁻ through hydrolysis to generate HCO₃⁻ [63]. Therefore, both HCO₃⁻ and CO₃²⁻ can inhibit the degradation of the system, and the inhibitory effect of CO₃²⁻ is more intense than that of HCO₃⁻.

2.3. Versatility and Reusability of CoAl₂O₄

Versatility is one of the most crucial parameters for a catalyst with regard to its practical application. Therefore, to further explore the adaptability of CoAl₂O₄-500, the degradation efficiencies of TCH (an antibiotic), MB (a cationic dye), and ARG (an azo dye) by the CoAl₂O₄-500/PMS system were also evaluated. It can be seen from Figure 4a that the degradation efficiency of TCH and MB achieved 95.0% and 89.9% within 120 min, respectively, while ARG was basically completely degraded after 60 min. These results show that CoAl₂O₄-500 exhibits excellent degradation performance on TCH, MB, and ARG, indicating that the catalyst possesses remarkable universality. The results also reveal that CoAl₂O₄-500 as a heterogeneous catalyst has enormous application potential in the practical treatment of activated PMS degradation of most refractory organic wastewater.

For sustainable application in industrial fields, the stability and reusability of a catalyst are vital indicators. Therefore, the reusability of CoAl₂O₄-500 was evaluated by performing RhB degradation with the recovered catalyst. As demonstrated in Figure 4b, the regenerated CoAl₂O₄-500 provided complete RhB oxidation in eight consecutive cycles without significant loss of catalytic activity. In the CoAl₂O₄-500/PMS system, the concentration of the leached cobalt ions was determined to be 0.29 mg/L, which is lower than that in the Co₃O₄-500/PMS system (0.35 mg/L) and below the permissible limit of cobalt in livestock watering (1 mg/L) [64]. These results confirm that CoAl₂O₄-500 possesses excellent recyclability and practical application potential for wastewater treatment.

2.4. Activation Mechanism

Quenching tests employing anhydrous ethanol (C₂H₅OH, EtOH), tertiary butanol (C₄H₉OH, TBA), trichloromethane (CHCl₃), and furfuryl alcohol (FFA) as scavengers have been conducted to determine the dominated reactive oxygen species in the CoAl₂O₄-500/PMS system. EtOH can be used as a scavenger for both SO₄•⁻ and •OH, while TBA is only efficient for quenching •OH, CHCl₃ is employed to verify the role of O₂•⁻, and FFA is used to verify the contribution of ¹O₂ [65,66]. As depicted in Figure 5a, in the presence of 1 M of EtOH, the removal rate of RhB dropped from 99.76% to 74.92% after 120 min. When further increasing EtOH concentration from 1 to 2 M, the removal efficiency was suppressed dramatically, only 50.96% of RhB was degraded after 120 min. When 1 M of TBA was added, the reaction system degraded 98.49% of RhB after 120 min and the RhB removal efficiency was still higher than that of 1 M of EtOH. Nevertheless, with the concentration of TBA further increased, no enhanced inhibition of TBA toward RhB removal was observed, verifying that •OH exhibited limited suppression. However, stronger suppression to RhB removal was observed for 2 mM FFA compared to the case of 2 M of EtOH or 2 mM of CHCl₃, whose reaction rate constant was the lowest (0.0004 min⁻¹) (Figure 5b). This implies that ¹O₂ played a more significant role in RhB removal than SO₄•⁻ or O₂•⁻. These results implied that O₂•⁻, ¹O₂, SO₄•⁻ and •OH simultaneously contributed to the RhB degradation in the CoAl₂O₄-500/PMS system, while SO₄•⁻ and ¹O₂ were evidenced to be the decisive reactive oxygen species in RhB degradation.

Based on the above analysis and discussion, the electron transfer between Co atoms in CoAl₂O₄-500 was the primary process for PMS activation by CoAl₂O₄-500 in the decomposition of RhB. Specifically, ≡Co(III)/≡Co(II) serve as primary catalytic sites in PMS activation, which are involved in the activation of PMS to generate SO₄•⁻, ¹O₂, and SO₅•⁻ to degrade organic pollutants [67]. The specific reaction mechanism can be summarized as follows: firstly, RhB molecules are adsorbed to the surface by entering the mesopores of CoAl₂O₄-500. Subsequently, ≡Co(II) reacts with PMS on the surface of CoAl₂O₄-500 to generate ¹O₂ and SO₄•⁻ (Equations (1)–(3)) [68]. H₂O₂ and ¹O₂ are formed via the reaction of H₂O and PMS (Equations (4) and (5)). Then, the reactions of H₂O₂ and ≡Co(III) result in the formation of ¹O₂ (Equations (6) and (7)) [69]. Then, SO₄•⁻ can react with H₂O or OH⁻ to form •OH (Equations (8) and (9)). Lastly, RhB molecules adsorbed on the catalyst surface are oxidized to CO₂, H₂O, NO₃⁻, and NH₄⁺ under the combined action of SO₄•⁻ and •OH.

≡Co(II) + HSO₅⁻ → ≡Co(III) + SO₄•⁻ + OH⁻(1)

≡Co(II) + SO₅•⁻ → ≡Co(III) + SO₄²⁻ + 1/2¹O₂(2)

≡Co(III) + HSO₅⁻ → ≡Co(II) + SO₅•⁻ + H⁺ (3)

H₂O + 2SO₅•⁻ → 2HSO₄⁻ + 3/2¹O₂(4)

H₂O + HSO₅⁻ → H₂O₂ + HSO₄⁻ (5)

≡Co(III) + H₂O₂ → ≡Co(II) + O₂•⁻ + H⁺ + H₂O(6)

2O₂•⁻ + 2H⁺ → H₂O₂ + ¹O₂(7)

SO₄•⁻ + H₂O → H⁺ + SO₄²⁻ + •OH(8)

SO₄•⁻ + OH⁻ → SO₄²⁻ + •OH(9)

2.5. Toxicity Evaluation

Mung bean seeds were cultivated for 9 days and their roots, stems, and leaves were measured to explore the ecotoxicity of the RhB degradation intermediates. Their growth states are exhibited in Figure 6a–d, the mung bean seeds cultivated in degraded solution are flourishing more than those grown in the initial solution. Additionally, the average lengths of the roots, stems, and leaves of the mung bean seeds cultivated in degraded solution were computed to be 9.35, 13.36, and 2.50 cm, which are longer than those developed in degraded solution (5.75, 12.53, and 2.30 cm) (Figure 6e). These results verify that the degraded solution exhibits lower biological toxicity than the initial RhB solution [70,71].

3. Materials and Methods

3.1. Materials and Reagents

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), aluminum nitrate hexahydrate (Al(NO₃)₃·9H₂O), RhB, CHCl₃, FFA, and EtOH were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). PMS and TBA were obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). ARG, TCH, and MB were purchased from Shanghai Macklin in Biochemical Reagent Co., Ltd (Shanghai, China). All reagents were analytical grade, and solutions were prepared with deionized water.

3.2. Preparation of CoAl₂O₄

According to the molar ratio of Co²⁺:Al³⁺ = 1:2, 20 mM of Al(NO₃)₃·9H₂O and 10 mM of Co(NO₃)₂·6H₂O were dissolved in deionized water (30 mL), respectively, at room temperature. Under continuously magnetic agitation, the Al(NO₃)₃ solution was added dropwise to the Co(NO₃)₂ solution. Subsequently, NaOH solution was added dropwise to the obtained solution until the pH value reached 12. The mixed solution was stirred for 1 h to generate precipitation and then separated by centrifugation. The resulting precipitate was washed with deionized water and dried at 100 °C. Finally, the sample was further grounded and calcinated in a muffle furnace at a certain temperature for 3 h to obtain the CoAl₂O₄ samples. To evaluate the impact of calcination temperature, the calcination temperatures were set to 300, 500, and 700 °C, respectively, to obtain CoAl₂O₄-X, where X represents the calcination temperature. For comparison, Co₃O₄ was prepared under the same procedure without adding Al(NO₃)₃·9H₂O.

3.3. Characterization

The morphology and structure of the catalysts were examined by X-ray diffraction (XRD, D/MAX-RB, Rigaku, Tokyo, Japan). The software JADE (Shuyun Instruments, Shanghai, China) is employed for X-ray diffraction pattern analysis. The surface morphology of the material was analyzed by Scanning Electron Microscopy (SEM, FEI-Quanta 200, FEI Company, Eindhoven, The Netherlands). The specific surface areas and pore size distribution of the samples were measured by Brunauer–Emmett–Teller (BET, ASAP2020 HD88, Micromeritics, Shanghai, China) produced by the American Mike Instrument Company. Meanwhile, X-ray photoelectron spectroscopy (XPS, ESCALAB Xi, Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA) equipped with Al Kα source was used to determine the surface elemental compositions and valence states of fresh and used CoAl₂O₄-500.

3.4. Experimental Procedure

All the degradation experiments were conducted in a 250 mL glass beaker containing 200 mL of RhB (10 mg/L) solution at room temperature under magnetic agitation. The degradation process was initiated by adding a certain amount of catalyst and PMS. At predetermined time intervals, 3 mL of solution was withdrawn, centrifuged, and analyzed immediately by measuring the absorbance of the supernatant at ${\lambda }_{max}$ = 554 nm using a UV-vis spectrophotometer (UV-1990PC, AoE Instrument Co., Ltd., Shanghai, China). NaOH and HNO₃ solutions (0.01 M) were utilized to adjust the initial pH value of the RhB solution. A certain amount of NaCl, NaHCO₃, and Na₂CO₃ was added into the CoAl₂O₄/PMS system to investigate the effects of different anions. The degradation conditions of TCH, MB, and ARG were similar to those of RhB, and the concentrations of TCH, MB, and ARG were measured by absorbance at 356, 664, and 505 nm, respectively. Unless otherwise specified, the catalyst dosage and PMS concentration were both 0.10 g/L. The concentration of the leached cobalt ions was determined through an atomic absorption spectrometer (SP-3520AA, Spectrum Instruments, Shanghai, China). To investigate the primary active species, different scavengers such as EtOH, TBA, CHCl₃, and FFA were employed before the addition of PMS.

4. Conclusions

In summary, a mesoporous CoAl₂O₄ spinel catalyst was successfully synthesized via a facile co-precipitation approach and applied to activate PMS to degrade RhB, TCH, MB, and ARG. CoAl₂O₄ exhibited much higher catalytic activity and stability than Co₃O₄ in activating PMS to degrade RhB due to its mesoporous and high specific surface area. Under optimal conditions (calcination temperature: 500 °C, catalyst: 0.1 g/L, and PMS: 0.1 g/L), the CoAl₂O₄ catalyst could degrade over 99% of RhB at a degradation rate of 0.048 min⁻¹, which is 2.18 times faster than Co₃O₄ (0.022 min⁻¹). RhB was efficiently degraded by the CoAl₂O₄-500/PMS over a wide pH range (3.44–10.22) and the catalyst possessed high stability for eight consecutive degradation cycles with insignificant cobalt leaching. The degraded solution exhibits lower biological toxicity than the initial RhB solution. Quenching tests confirmed that ¹O₂ and SO₄•^– were the primary reactive oxygen species in the CoAl₂O₄/PMS system. This work provides new insights into expanding the application of cobalt-based catalysts for environmental remediation.

Author Contributions

Conceptualization, S.G.; funding acquisition, S.G. and H.Z.; formal analysis, M.C.; investigation, L.Z.; project administration, S.G.; supervision, S.G., H.F. and H.Z.; software, L.Z. and M.C.; validation, F.A.; visualization, H.F. and F.A.; writing—original draft preparation, L.Z.; writing—review and editing, S.G. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are openly available.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. (a) XRD patterns of CoAl2O4-500 and Co3O4-500, (b) SEM image of CoAl2O4-500, (c) N2 absorption-desorption isotherms and pore size distributions (inset) of CoAl2O4-500 and Co3O4-500, and (d) Co 2p XPS spectrum of CoAl2O4-500.

Figure 2. (a) RhB degradation in various systems; effects of (b) calcination temperature; (c) catalyst dosage, and (d) PMS dosage on the RhB degradation and their corresponding pseudo-first-order model kinetic results (inset) in the CoAl2O4-500/PMS system.

Figure 3. Effects of (a) initial pH and different anions (b) Cl−, (c) HCO3−, and (d) CO32− on RhB degradation and their corresponding pseudo-first-order model kinetic results (inset) in the CoAl2O4-500/PMS system.

Figure 4. (a) Degradation of TCH, MB, and ARG in the CoAl2O4-500/PMS system, (b) cycling experiments of CoAl2O4-500 toward RhB degradation in the presence of PMS.

Figure 5. (a) Effects of different capture agents for RhB degradation in the CoAl2O4-500/PMS system, (b) comparison of the reaction rate constant under different capture agents.

Figure 6. Ecotoxicity tests with 10 mg/L of RhB solution (a,c) and the degraded solution (b,d). (e) Lengths of the roots, stems, and leaves of mature mung beans in the initial and degraded solution after 9 days.

Table 1

Comparison of RhB degradation by bimetallic oxides through PMS activation.

Order	Catalyst	RhB (mg/L)	Catalyst (g/L)	PMS (g/L)	Removal Efficiency	pH Range	Reference
1	MnFe2O4	5	0.2	0.06	90.0% (30 min)	4–10	[49]
2	CoMn2O4	24	0.08	1.8	100% (40 min)	2.7–7.2	[50]
3	CuBi2O4	20	1.0	0.60	96.6% (60 min)	4.9–8.8	[51]
4	NiCo2O4	0.2	0.2	0.30	100% (30 min)	3.28	[31]
5	CoFe2O4	20	0.5	0.50	96.5% (40 min)	3–10	[32]
6	CoAl2O4	10	0.1	0.10	99.0% (120 min)	3.4–10.2	This work

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