Transition Metal-Doped Cobalt Oxyhydroxide Catalysts with Enhanced Peroxymonosulfate Activation for Dye Decolorization ^†

Abstract

Sulfate radical-based advanced oxidation processes (AOPs) are effective for removing organic pollutants from wastewater. In this study, transition metal-doped cobalt oxyhydroxide (CoOOH) was synthesized as an efficient peroxymonosulfate (PMS) activator for the decolorization of rhodamine B. Doping CoOOH with transition metals enhanced its PMS activation performance compared with that of pure CoOOH. Notably, the reaction rate constant in the Ni-CoOOH/PMS system was approximately 4.3 times higher than that in the CoOOH/PMS system. The presence of multiple metals in the catalyst facilitated efficient Co²⁺/Co³⁺ redox cycling, resulting in improved PMS activation and more effective organic pollutant removal. This study highlights the potential of CoOOH-based materials for use in environmental remediation technologies utilizing PMS.

1. Introduction

In recent years, water pollution has become one of the most predominant environmental problems worldwide. Organic refractory pollutants, such as dyes and antibiotics, are difficult to degrade naturally and, therefore, contribute significantly to water pollution. For example, rhodamine B (RhB), a widely used dye, is commonly applied for coloring but is potentially irritating to the respiratory tract and is carcinogenic. To address this issue, the efficient removal of pollutants from wastewater is crucial.

Sulfate radical-based advanced oxidation processes (SR-AOPs) have gained attention as promising technologies for the removal of organic pollutants [1]. SR-AOPs (SO₄^•−, E₀ = 2.5–3.1 V) are preferred over hydroxyl radical-based AOPs (•OH, E₀ = 1.8–2.7 V) because of their higher oxidation potential [2], pH independence, long lifetimes [3], and excellent selectivity [4].

In this study, peroxymonosulfate (PMS) and cobalt oxyhydroxide (CoOOH) were used to achieve efficient pollutant removal. PMS is useful for the degradation of pollutants because it generates reactive species such as singlet oxygen (¹O₂) and sulfate radicals (SO₄^•−). Cobalt is one of the most effective metals for PMS activation. Recent studies have investigated the use of CoOOH as a PMS activator. For example, Zhang et al. reported that the CoOOH/PMS system achieved a high degradation rate for 2,4-dichlorophenol (0.0462 min⁻¹), which was 10 and 4 times higher than those of the Co₃O₄/PMS and CoFe₂O₄/PMS systems, respectively [5]. Zeng et al. prepared a CoOOH microplate (B-CoOOH) from cobalt aluminum layered double hydroxide (CoAl-LDH) and demonstrated that the B-CoOOH/PMS system completely degraded 40 μM sulfamethoxazole within 6 min, with a reaction rate constant of 0.60 min⁻¹, which was 12 times higher than that of the CoOOH/PMS system (0.05 min⁻¹) [6].

To further enhance catalytic activity, several studies have combined CoOOH with other materials. For example, Xi et al. reported that the CoOOH@Bi₂O₃/PMS system exhibited significantly higher rate constants than the CoOOH/PMS system due to the synergistic effect of CoOOH and Bi₂O₃ [7]. Zhang et al. synthesized CoOOH supported on activated carbon (CoOOH@AC), which exposed highly active Co-containing edges in the material. As a result, 96.8% of CIP was degraded within 10 min, which was approximately 11.6 and 9.97 times greater than those of the CoOOH/PMS and AC/PMS systems, respectively [8]. Xing et al. developed a MnOOH/CoOOH composite (B-MCo-0.1), which successfully degraded 50 mL of 25 mg/L phenol solution within 10 min by suppressing catalyst aggregation and increasing the specific surface area [9]. These findings indicate that CoOOH exhibits enhanced activity when combined with other materials. CoOOH has abundant surface hydroxyl groups, active sites, and excellent electron transport properties. However, they are prone to agglomeration and cobalt ion leaching. To overcome these drawbacks and improve PMS activation, CoOOH was doped with transition metals. In this study, a series of catalysts were systematically prepared by doping CoOOH with five different metals (Cu, Ni, Fe, Zn, and Mn), and their performances were evaluated comparatively. Previous studies have mainly focused on single-metal doping or composite materials, and such comprehensive comparisons have rarely been reported. The catalytic activity of the prepared transition metal-doped CoOOH catalysts was evaluated based on the decolorization efficiency of RhB. The results demonstrated that metal doping significantly improved decolorization efficiency compared with that of conventional CoOOH. Furthermore, decolorization experiments were conducted on rhodamine B and other dyes, suggesting the potential for broader applications. These findings provide new insights into enhancing the performance of CoOOH catalysts and their practical applications.

2. Materials and Methods

2.1. Materials

Cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O), nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O), iron(III) nitrate nonahydrate (Fe(NO₃)₂·9H₂O), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), manganese(II) nitrate hexahydrate (Mn(NO₃)₂·6H₂O), sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂), Oxone monopersulfate compound (2KHSO₅·KHSO₄·K₂SO₄), disodium hydrogen phosphate (Na₂HPO₄), and sodium dihydrogen phosphate (NaH₂PO₄) were purchased from Fujifilm Wako Pure Chemicals (Osaka, Japan). Copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O) and RhB (C₂₈H₃₁ClN₂O₃) were purchased from Nacalai Tesque (Kyoto, Japan).

2.2. Methods

2.2.1. Preparation of M-CoOOH

M-CoOOH was prepared using a modified coprecipitation–oxidation method [10] (M = Cu, Ni, Fe, Zn, or Mn). For this, 50 mL of 2.1 M NaOH, 0.45 M Co(NO₃)₂·6H₂O, and 0.05 M of the respective metal nitrates (M) were added in a total volume of 25 mL. The mixture was stirred for 30 min at room temperature. Subsequently, 60 mL of 30% H₂O₂ was added, and the mixture was stirred at 60 °C for 24 h using a hot stirring plate. The resulting product was washed with distilled water and vacuum-dried at 60 °C. A pure CoOOH sample containing only cobalt was synthesized using the same procedure.

2.2.2. Decolorization of Rhodamine B

A 60 mg/L solution of RhB was added to a Pyrex glass reaction vessel for the degradation experiment. The pH was adjusted using a buffer solution. A cobalt oxyhydroxide catalyst was then added, and the mixture was stirred for 30 min at room temperature. Subsequently, a 0.75 mM PMS solution was added to initiate the reaction. At regular intervals, aliquots of the solution were collected, and the reaction was quenched by adding methanol. The catalyst was removed by centrifugation, and the absorbance of the solution was measured using a UV–visible spectrophotometer at 554 nm.

3. Results and Discussion

3.1. Characterization

X-ray diffraction (XRD) analysis was performed on the M-CoOOH samples and the results are shown in Figure 1a. Characteristic peaks corresponding to the (003), (101), (012), (104), (015), (017), (110), and (113) planes were observed [5]. These peaks are indicative of CoOOH, confirming that the synthesized material was CoOOH. Additionally, when Fe, Zn, and Mn were doped into the samples, peaks corresponding to Co₃O₄ were detected [11]. The expanded views of the (003) and (015) plane peaks in Figure 1a are shown in Figure 1b,c, respectively. The peak positions were slightly shifted toward lower angles upon doping with transition metals.

SEM and TEM analyses were conducted to investigate the surface morphologies of the catalysts. The SEM and TEM images of the catalysts are shown in Figure 2 and Figure S1, respectively. The SEM and TEM images confirmed that when CoOOH was doped with the transition metals, the resulting morphology showed that the metals were uniformly attached to the CoOOH surface. The EDS mapping images of the catalysts are presented in Figure S2. Oxygen and cobalt were present and homogeneously distributed in all the catalysts. In addition, the respective metal elements were detected in the M-CoOOH samples.

Figure S3 shows the electrochemical impedance spectroscopy (EIS) results of the CoOOH and Ni-CoOOH catalysts. In the Nyquist plots, a smaller arc radius typically indicates a lower resistance and a higher charge transfer efficiency. As shown in Figure S3, Ni-CoOOH exhibits a smaller arc radius than CoOOH, indicating that Ni doping enhances the charge transfer efficiency. This improved charge transfer efficiency is suggested to contribute to the increased decolorization rate of RhB.

3.2. Performance of Rhodamine B Decolorization

The results of the RhB decolorization experiments using CoOOH doped with different transition metals are shown in Figure 3a,b. RhB was decolorized by 15.2% after 10 min of reaction when the PMS solution was added alone and by 26.3% in the CoOOH/PMS system. The decolorization rates of the M-CoOOH/PMS systems were higher than that of the CoOOH/PMS system. Notably, in the Ni-CoOOH/PMS system, RhB was decolorized by 77.8%. The reaction rate constants were 0.04 min⁻¹ for the CoOOH/PMS system and 0.17 min⁻¹ for the Ni-CoOOH/PMS system, indicating an improvement of approximately 4.3 times.

Figure 4a,b show the results of the decolorization experiments conducted by varying the PMS concentrations. It was confirmed that increasing the PMS concentration led to a higher decolorization efficiency. When the PMS concentration was 1.5 mM, RhB was completely decolorized within 10 min, with a reaction rate constant of 0.47 min⁻¹. Therefore, Ni-CoOOH has a superior performance compared with other studies (Table S1) [12,13,14,15,16,17].

3.3. Reusability

Cyclic experiments were conducted to evaluate the stability of the catalyst. As shown in Figure S4, the RhB decolorization rate after 10 min of reaction remained above 90% in all three cycles. The slight decrease in the decolorization efficiency in the second and third cycles may be attributed to catalyst loss, insufficient washing, or the coverage of active sites due to the adsorption of intermediates onto the catalyst surface. Nevertheless, because the decolorization rate did not significantly decrease after three cycles, the Ni-CoOOH catalyst exhibited good stability.

3.4. Decolorization of Other Dyes

Decolorization experiments were also conducted using Reactive Yellow 86 (RY), Acid Red 88 (AR), and Crystal Violet (CV), in addition to RhB. The results are shown in Figure S5. RY and AR were completely decolorized (100%) within 2 min, whereas CV was decolorized by 94.5% within 10 min. These results indicate that the Ni-CoOOH/PMS system is applicable to dyes other than RhB and demonstrates the promising potential of Ni-CoOOH for wastewater treatment applications.

3.5. Mechanism of M-CoOOH-Activated PMS System

The characteristics of the CoOOH and Ni-CoOOH catalysts before and after the RhB decolorization reaction were analyzed using XPS. The results are shown in Figure 5. Figure 5a shows the survey spectra of CoOOH and Ni-CoOOH before decolorization. A comparison of the Co 2p spectra before and after the reaction revealed a continuous increase in the proportion of Co²⁺ and a decrease in the proportion of Co³⁺ as the reaction progressed (Figure 5b,e). For nickel, the proportion of Ni³⁺ increased, whereas that of Ni²⁺ decreased before and after the reaction. These results indicate that a redox cycle occurs between the catalyst and PMS. Additionally, Co²⁺ is oxidized to Co(IV)=O by PMS, and Co(IV)=O may be reduced back to Co³⁺ after reacting with contaminants [10]. Co(IV)=O is believed to react directly with RhB, leading to the degradation of the ring structure. The percentages of each peak area in Figure 5 are presented in

Table 1. Ratios of chemical species of CoOOH and Ni-CoOOH.

	CoOOH					Ni-CoOOH
	Co3+	Co2+	Oads	M-OH	M-O	Co3+	Co2+	Ni3+	Ni2+	Oads	M-OH	M-O
Before (%)	60.5	39.5	18.1	56.7	25.2	59.3	40.7	23.9	76.1	18.7	52.8	28.5
After (%)	54.6	45.4	11.8	68.9	19.2	47.4	52.6	35.4	64.6	9.1	60.4	30.5

. As shown in Table 1, Ni-CoOOH exhibited a larger percentage change in the metal valence states than CoOOH. This can be attributed to the presence of multiple metals in the catalyst, which enhances the efficiency of the redox reactions.

Figure 6 shows the proposed reaction mechanism for RhB decolorization in this study. As shown in Equations (1)–(10), the reaction of PMS with the catalyst generates singlet oxygen, sulfate radicals, hydroxyl radicals, and high-valence metal oxo species as active decomposition species. We propose that these reactive species attack and break down the chromophore structure of RhB, ultimately leading to its degradation into inorganic substances, such as water and carbon dioxide.

The reaction of CoOOH with PMS is shown in Equations (1) and (2), producing high-valence metal oxo species that directly react with RhB (Equation (3)). The reduction of CoOOH by PMS generates persulfate radicals, as shown in Equation (4). As shown in Equations (5) and (6), the oxidation of CoOOH by PMS produces sulfate and hydroxyl radicals, respectively. Singlet oxygen is formed through the reaction of persulfate radicals with PMS or water (Equations (7) and (8)). Additionally, hydroxyl radicals are generated by the reaction of sulfate radicals with water (Equation (9)). Although cobalt is not easily reduced, the presence of nickel facilitates electron transfer, making reduction more favorable, as shown in Equation (10).

These processes efficiently generate reactive species through repeated redox reactions. Sulfate radicals, hydroxyl radicals, singlet oxygen, and high-valence metal oxo species are believed to contribute significantly to the decolorization of RhB.

≡ M²⁺ − OH + HSO₅⁻ → ≡ M²⁺ − OOSO₃⁻ + H₂O

(1)

≡ M²⁺ − OOSO₃⁻ → ≡ M (IV) = O + SO₄^•−

(2)

≡ M (IV)=O + RhB → ≡ M³⁺ + immediate products + CO₂ + H₂O

(3)

≡ M³⁺ − OH + HSO₅⁻ → ≡ M²⁺ − OH + SO₅^•− + H⁺

(4)

≡ M²⁺ − OH + HSO₅⁻ → ≡ M³⁺ − OH + SO₄^•− + OH⁻

(5)

≡ M²⁺ − OH + HSO₅⁻ → ≡ M³⁺ − OH + SO₄²⁻ + •OH

(6)

SO₅^•− + HSO₅⁻ → ¹O₂ + HSO₄⁻ + SO₄²⁻

(7)

2SO₅^•− + H₂O → 1.5¹O₂ + 2HSO₄⁻

(8)

SO₄^•− + H₂O → •OH + SO₄²⁻ + H⁺

(9)

Ni²⁺ + Co³⁺ → Ni³⁺ + Co²⁺

(10)

4. Conclusions

In this study, we synthesized transition metal-doped CoOOH catalysts that activated PMS. Among them, Ni-CoOOH exhibited the highest decolorization efficiency, completely decolorizing 60 mg/L RhB within 10 min. XPS analysis revealed that the co-existence of multiple metals (Co and Ni) in the catalyst enhanced the redox reaction between PMS and the catalyst, leading to the more efficient generation of reactive decomposition species. In addition, EIS analysis confirmed that Ni doping enhanced the charge transfer efficiency and promoted the reaction between the catalyst and PMS. Therefore, Ni played a crucial role in PMS activation and RhB decolorization. In future work, we aim to expand the application of this catalyst system to the degradation of organic pollutants beyond RhB.