Peroxymonosulfate activation using CoFe2O4/Fe2O3 nanocomposite for Acid Orange removal

Introduction

Over the decades, the emerging organic contaminants have caused water pollution. The organic pollutants that greatly contribute to water pollution are the discharge of untreated effluent containing organic compounds such as pharmaceuticals, phenols, pesticides and dyes from various industries [1]. Among the pollutants, dyes as the common synthetic organic substances are widely used in the industries. Most of the dyes do not bind to the fabric during the dying process where up to 50% of them are discharged as industrial effluents [2]. Dye persists in wastewater for a long period due to the high photostability property hindering the light penetration and eventually disrupting the photosynthesis of aquatic plants. Acid orange 7 (AO7) is a well–known organic dye that is inexpensive and readily soluble in water. Like most of the azo dyes, AO7 tends to be disposed by the industries and possess threats to human health due to the toxicity, irritation upon exposure and dizziness [3,4].

Various advanced treatment technologies such as adsorption, sewage treatment, and advanced oxidation processes (AOP) have been applied to address the challenges. AOP has been proposed as a possible method to remove the organic pollutants in wastewater without the generation of secondary waste [5,6]. At present, sulfate radical–based AOP (SR–AOP) has drawn greater attention as a more effective solution to the organic pollutant removal in water. By SR–AOPs technology, the persistent organic pollutants are mineralized into smaller molecules and eventually converted into CO₂ and H₂O. Generally, SO₄^•− (+2.6 V_NHE, [7]) is generated via peroxymonosulfate (PMS) activation, and a series of redox reactions is triggered. Several methods can be used for the PMS activation including the use of UV, heat and transition metals [5]. Among the methods, PMS is commonly activated by using transition metals in homogeneous and heterogeneous systems. However, the activation in heterogeneous system is more preferred due to the ease of separation and recovery of the solid catalyst from the treated solution leading to less adverse environmental effects, and minimized operational cost due to nonobligatory need of external energy sources (e.g. heat, irradiation) [8,9].

Cobalt (Co)–based catalysts have been reported as the most efficient catalyst in PMS activation to generate SO₄^•− [10]. To date, a numerous of Co containing materials as heterogeneous catalyst have been studied as PMS activators. CoO, CoO₂, Co₂O₃, CoO(OH) and Co₃O₄ are the five different kinds of cobalt oxide catalysts. Among the catalysts, CoO and Co₃O₄ are more commonly applied in the degradation of organic pollutant via PMS activation [11,12]. However, several studies have observed the leaching of Co during catalytic activation of PMS [13]. To overcome this downside, Co₃O₄ has been deposited on various supports such as metal oxides, industrial wastes, magnetic particles, and adsorbents. Although the supported Co₃O₄ heterogeneous catalysts showed improved catalytic activity, better dispersion and higher recovery from the wastewater, the metal leaching problem was not fully resolved. Among the metal–based catalyst, iron (Fe)–based catalysts are the most popularly studied in the activation of PMS [5,11] . Fe–based nanomaterials such as Fe₂O₃, Fe₃O₄ and zero–valent iron (ZVI) are studied as PMS activators due to the high abundance, low cost, environmental friendliness and performance [14]. Iron oxides are heterogeneous catalysts which are popularly investigated as PMS activators for organic pollutant removal. For Fe₂O₃, the reduction of Fe³⁺ promotes the combination of Fe²⁺ with PMS to produce SO₄^•− [15].

Recently, bimetallic or multimetallic catalysts have become an influencing class of catalysts in the removal of organic contaminants. Binary spinel–type oxides (AB₂O₄) have gained greater attention recently due to the better performance compared to the single–metal oxides mixtures [16]. The spinel ferrites with the advantages of high reusability and easy separation have been considered as effective PMS activators in the persistent organic pollutants removal. In particular, the performances of spinel ferrites can be altered by modifying the redox potential through replacing with other transition metals such as Mn, Co, Cu, Pb [17]. Cobalt ferrite (CoFe₂O₄) is also one of the popular mixed–metal catalyst investigated by the researchers. Notably, the Co leaching could be reduced by the strong Fe–Co interactions in CoFe₂O₄. Although the composition of Fe is higher than Co in the catalyst, Co²⁺ has a major contribution to the PMS activation in CoFe₂O4. Early studies suggested that the phenomenon was due to the higher amount of adsorbed surface –OH groups which was enriched by the presence of Fe. This eventually promoted the formation of Co²⁺–OH complexes, instead of transferring electron from Fe²⁺ to Co³⁺ [18].

In this study, mixed–metal catalysts (CoFe₂O₄/xFe₂O₃; x = 0, 0.25, 0.50, 0.75 and 1) were synthesized via a co–precipitation method and compared for AO7 catalytic removal using PMS activation. The effects of several reaction parameters (i.e. catalyst loading, PMS dosage, initial pH) on the catalytic activity were investigated. Mineralization and reusability studies were conducted to estimate the catalyst practicability. Moreover, the main reactive species generated in the catalytic system were determined by chemical scavenging a PMS activation pathway is proposed.

Materials and Method

Chemicals

The chemicals used in this study are: cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, R&M Chemicals), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, QRëC), sodium hydroxide (NaOH, R&M Chemicals), citric acid monohydrate (C₆H₈O₇·H₂O, R&M Chemicals), Oxone® (PMS, Acros organics), acid orange 7 (AO7, Sigma–Aldrich), ethanol (EtOH, 95%, SYSTERM®), tert–butyl alcohol (TBA, EMSURE® ACS), and deionized (DI) water. All the chemicals were analytical grade and used without further purification.

Nanocomposite Synthesis

The CoFe₂O₄ catalyst was synthesized through a simple co–precipitation method. Firstly, 5 mmol of Co(NO₃)₂·6H₂O and 10 mmol of Fe(NO₃)₃·9H₂O as the metal precursors were added into 50 mL of DI water. Then, 30 mmol of C₆H₈O₇ was added to the mixture as a surfactant to promote the consistency of precipitate formation along with preventing agglomeration. The mixture was heated to 95 °C and 10 M NaOH was added dropwise while stirring vigorously until the pH was adjusted to 10–11. Next, the solution was allowed to stir for 4 h at the same temperature. After cooling the resultant solution, the black precipitate was isolated from the solution by centrifugation and dried in oven at 50 °C for 48 h. Then, the product was grinded into powder using a pestle and mortar until. Thereafter, the resultant product was open–air calcined at 600 °C in a furnace for 4 h to promote crystallinity and to remove the C₆H₈O₇ surfactant. Finally, the catalyst was washed with distilled water until pH 7 followed by drying. For the performance comparison, four other catalysts, namely CoFe₂O₄/xFe₂O₃ (abbreviated by CoFe–x) where x = 0.25, 0.50, 0.75 and 1.00, were prepared similarly. The molar ratio of Co(NO₃)₂·6H₂O:Fe(NO₃)₃·9H₂O:C₆H₈O₇ was increased from 1:2:6 to 1:2.5:7, 1:3:8, 1:3.5:9 and 1:4:10, respectively. The synthesis method is illustrated in Figure 1.

Figure 1.Figure 1. Synthesis of the CoFe catalysts

Characterization Studies

The crystallinity and crystallographic information of the catalyst was studied by obtaining the powder X–ray diffraction (XRD) patterns using an X-ray diffractometer (Bruker D8 Advance), with the scan rate of 0.02° s−1 with Cu-Kα source (λ = 1.5406 Å). The morphology and crystallite size of the catalyst were also observed with a JEOL JSM-6701F field emission scanning electron microscope (SEM) with electron dispersive spectroscopy equipped (Oxford Instruments X-Max 80 mm2 Solid State EDX detector). Moreover, the elemental composition of the sample was also estimated using the EDX. Finally, the Fourier transform infrared (FTIR) spectra of the catalysts were obtained (Perkin Elmer, 2000 FTIR spectrometer) to determine the present functional groups in the catalyst.

Catalytic Performance Evaluation

The performance of all the catalyst was investigated preliminary to determine the catalyst with the highest catalytic activity. The AO7 degradation via PMS activation was conducted by preparing 100 mL of 10 mg L^–1 of AO7 containing 0.2 g L^–1 of PMS. The pH was adjusted immediately to pH 7 after preparing the initial solution. Next, 0.2 g L-1 of the catalyst was added to the solution to initiate the catalytic reaction. At 5 min intervals, 2 mL aliquots were withdrawn and filtered (cellulose acetate filter, 0.45 μm) to obtain the absorbance for the determination of AO7 concentration. The AO7 concentration was determined from the calibration curve of absorbance against the concentration of AO7 by using an UV–Vis spectrophotometer (Hitachi, Model U-2000) at λmax = 485 nm. After determining the optimum catalyst, the effects of catalyst loading (0.05 – 0.40 g L^–1), PMS dosage (0.1 – 0.4 g L^–1) and pH (3–7) on AO7 degradation performance were investigated.

Similarly, the catalyst reusability was investigated by reusing the catalyst over several cycles. After the first cycle, the used catalyst was collected by retaining it via an external magnetic source and decanting of the solution. Then, a freshly prepared AO7/PMS solution was added to the catalyst vessel to commence the reaction.

The dominant radical species generated was identified via the radical scavenger method. EtOH was used as SO₄^•– and ^•OH scavengers whereas TBA was functioned to selectively scavenge ^•OH without affecting the SO₄^•–. EtOH (0.2 M and 2 M) and TBA (1 M) were added to the reaction vessel separately before initiating the catalytic reaction.

Analytical Methods

The total organic carbon (TOC) removal rate was investigated by using a TOC analyzer (Shimadzu TOC-VCPH TOC analyser) to determine the mineralization efficiency of the catalyst. The same condition as the reusability test (0.2 g L^–1 of both catalyst and PMS at pH 7) was employed on the sample preparation for TOC analysis, and the samples were taken at t = 0, 1, 2 and 3 h. The analysis was conducted immediately after the reaction. The catalyst stability was also studied by analysing the metal leaching for Co and Fe metal in the treated solution by ICP–OES (PerkinElmer Optima 800) after 1 cycle of reaction.

Results and Discussion

Characterizations of the Catalysts

The CoFe₂O₄ catalyst with increasing ratio of Fe₂O₃:CoFe₂O₄ ranging from 0:1, 1:4, 1:2, 3:4 and 1:1 were synthesized via co–precipitation method. The FTIR spectra of all the catalysts were presented in Figure 2 indicating that all the FTIR spectra consist of broad band located at about 3400 cm^–1 which was attributed to the O–H stretching. Meanwhile, the peak at about 1600 cm^–1 could be assigned to the O–H bending of the adsorbed H₂O while the two peaks between 400–600 cm^–1 indicated the formation of Fe–O and Co–O bond. The results of the FTIR spectra implied that all the catalyst have the same functional groups, and the formation of Me–O bonds are successful.

Figure 2.Figure 2. FTIR spectra of as-prepared CoFe catalysts

The crystal structure of CoFe₂O₄ was investigated using XRD (Figure 3). Several diffraction peaks were observed at 2θ = 30.2°, 35.7°, 43.4°, 57.0° and 62.8°, which were assigned to (220), (311), (400), (511) and (440) crystal planes of the spinel CoFe₂O₄ respectively (JCPDS No. 22–1086) [19,20]. The results indicated that CoFe₂O₄ nanocomposite has been successfully obtained. The intensity of diffraction peaks were less distinctive compared to the reported XRD patterns by [21,22], suggesting that the formation of CoFe₂O₄ had a low crystallinity. However, the catalysts with more complete crystallinity were claimed to show a weaker catalytic activity [23] . In addition, it has ben reported that the concentration of defect was usually higher in the amorphous phase, resulting in greater conductivity and better catalytic performance [24]. Thus, the mostly amorphous structure of the CoFe₂O₄ could be effective in the pollutant removal.

Figure 3.Figure 3. XRD diffractogram of CoFe₂O₄

The FESEM micrograph revealed the cubic morphology of CoFe₂O₄ catalyst as shown (Figure 4 a ). The EDX elemental mapping (Figure 4 b ) illustrated that both the Co and Fe were homogeneously distributed on the catalyst surface with the ratio of Co:Fe at 1:1.7, which was close to the theoretical molar ratio of 1:2 during the synthesis process. These findings confirmed the formation of CoFe₂O₄ during via the co–precipitation method. In addition, magnetic property could be observed from the CoFe₂O₄ catalyst by using an external magnet, confirming the formation of spinel ferrite.

Figure 4.Figure 4. (a) FESEM micrograph of CoFe₂O₄ and (b) EDX elemental mapping

Catalytic Performance Evaluation

The performance comparison of CoFe₂O₄ and CoFe–x (x = 0.25, 0.50, 0.75 and 1.0) for the AO7 degradation via PMS activation is illustrated in Figure 5 a whereas the removal efficiencies and pseudo–first order rate constant (k) of the mentioned catalysts are presented in Table 1 . The pseudo–first order kinetic is equated as:

\begin{equation} C_{t}=C_{0}\times e^{-kt} \tag{1} \end{equation}

where C_t is the concentration at time t and C₀ is the initial concentration. The pseudo–first order kinetic model that employed to the AO7 degradation illustrated considerably good fittings (R² > 0.96).

Figure 5.Figure 5. (a) AO7 degradation by the as–prepared catalysts. (b) Effect of catalyst loading, (c) effect of PMS dosage, and (d) effect of initial pH (Conditions: pH = 7.0 (for a-c), [PMS] = 0.20 g L⁻¹ (for a, b, d), [catalyst] = 0.20 g L⁻¹ (for a, c, d) , [AO7] = 10 mg L⁻¹).

The transition metal (Meⁿ⁺) present in the metal oxide catalysts can generate SO₄^•− for AO7 degradation via PMS activation by single electron transfer reaction as shown in the equations below:

\begin{equation} Me^{n+}+HSO_{5}^{-}\rightarrow Me^{\left ( n+1 \right )+}+SO_{4}^{\bullet-}+OH^{-} \tag{2} \end{equation}

\begin{equation} Me^{\left ( n+1 \right )+}+HSO_{5}^{-}\rightarrow Me^{+} +SO_{5}^{\bullet-}+H^{+} \tag{3} \end{equation}

\begin{equation} SO_{4}^{\bullet-}+AO7\rightarrow degradation \quad product \tag{4} \end{equation}

From the performance evaluation of various catalysts, the k decreased from 0.1211 min^–1 to 0.0596 min^–1 when the molar ratio of Fe:Co in the catalyst was increased from to 2:1 to 4:1. The increased in Fe resulted a negative effect on the removal efficiency, indicating the superiority of Co over Fe composition in the catalyst during PMS activation. Analogous result of the Co and Fe behaviours was reported by Anipsitakis & Dionysiou [13] in their study on the activation of PMS for the 2,4-dichlorophenol degradation. Moreover, Al–Anazi et al. [25] reported that the removal efficiency decreased from 75% to 24% when Co_1.0Fe_2.0O₄ was replaced with Co_0.1Fe_2.9O₄. The findings further confirmed that the increase in Fe had caused the composition of Co to be lowered and resulted in an adverse effect on the catalytic performance. This can be ascribed to the lower redox potential of Fe (Fe³⁺/Fe²⁺, E₀=+0.77) against Co (Co³⁺/Co²⁺, E₀=+1.82) [26]. Thus, CoFe₂O₄ with the highest removal efficiency in 30 min (97.2%) and the highest k (0.1211 min^–1) was selected to further study the catalytic performance under various conditions due to its outstanding performance compared to other catalysts.

Effect of Reaction Parameters

Figure 5b describes the effect of the catalyst loading on AO7 removal. After 30 min of reaction, the k increased from 0.0337 min^–1 to 0.1362 min^–1 when the CoFe₂O₄ loading was increased from 0.05 to 0.40 g L^–1. The positive correlation between catalyst loading and the catalytic performance is explained by the increase in active sites on the catalyst surface. The density of active sites increases with catalyst loading, resulting in more PMS to be activated and more AO7 to be adsorbed, thus generating more free radicals to degrade AO7 [27]. Nevertheless, the effect of catalyst loading towards the catalytic activity was greater at concentration below 0.20 g L^–1 where the k was doubled with the catalyst loading, but the positive effect was reduced when the catalyst loading was further increased from 0.20 to 0.40 g L^–1, whereby only a slight increment (12%) in the k was observed. This effect might be explained by the magnetic property of the catalyst that causes the particle-particle interactions (aggregation) at higher loadings, which then reduces the specific surface area of the catalyst in the system [28]. In addition, the reduced effect could also be caused by the diffusion limitation in heterogeneous reaction under high concentration of CoFe₂O₄ [22]. These factors would result in the ineffective consumption of PMS on the catalyst surface, reducing the effective collisions between catalyst with PMS and AO7 molecules.

Figure 5 c shows the AO7 removal at different PMS dosages. When the PMS dosage was increased from 0.10 to 0.40 g L^–1, the AO7 removal efficiency increased from 94.5% to 99.4% while the k increased steadily from 0.0948 to 0.1665 min^–1. The rapid increase in the reaction rate is attributed to the higher generation of free radicals when PMS is increased. Previous studies reported that increment in PMS dosage has minor effect in increasing the catalytic removal of organic pollutants which was explained by the excessive PMS serving as a quencher of active radicals, SO₄^•− and formed SO₅^•− that has a weaker oxidizing power. Moreover, SO₄^•− will be self–consumed when this phenomenon occurs, as shown in the equations below [21] :

\begin{equation} 2HSO_{5}^{-} + e^{-}\rightarrow SO_{4}^{\bullet-}+ SO_{4}^{2-}+H^{+} \tag{5} \end{equation}

\begin{equation} SHSO_{5}^{-} + SO_{4}^{\bullet-}\rightarrow HSO_{4}^{-}+ SO_{5}^{\bullet-} \tag{6} \end{equation}

\begin{equation} SO_{4}^{\bullet-} + SO_{4}^{\bullet-} \rightarrow 2SO_{4}^{2-} \tag{7} \end{equation}

However, this effect was not observed in this study implying that the PMS dosage had not reached the saturation level, where the active sites were still available to the PMS and the CoFe₂O₄ is capable of productive utilization of PMS.

The traditional Fenton process requires a strict control of pH (2.5–4.5) while the PMS activation has a wider application due to its functionality under a wider pH condition [29] . Hence, to identify the optimal pH condition for the reaction, the AO7 removal was conducted at the initial pH of 3, 5, 7 and 9. Figure 5 d shows the degradation curve of AO7 at various pH conditions. Initially, the k of AO7 was 0.0423 min^–1 at pH 3. However, when the pH was further increased to 5, 7 and 9, the k was about three times higher (0.1302±0.0091 min^–1). Given that the pKa₂ of PMS is 9.4 and the reactions were conditioned at pH 3–9 (< pKa₂ of PMS), HSO₅^– was the predominant form of PMS species whereas at pH > 9.4, the content of SO₅^2– would sharply increase [30] . Besides, the pH at point of zero charge (pHpzc) of CoFe₂O₄ was reported as 7.7 where the surface charge of catalyst is positive at pH < pHpzc and negative at pH > pHpzc [31]. Hence, the attraction between HSO₅^– with the positively charged CoFe₂O₄ would be greater at pH 3–7, thus promoting the formation of SO₄^•− and ^•OH. However, the removal efficiencies at pH 3 was the lowest despite having the favorable attraction between the negatively charged PMS and the positively charged CoFe₂O₄. This phenomenon was attributed to the stabilization of PMS at acidic condition by the formation of H- bonding between H⁺ from the acidic surrounding and the O–O group of HSO₅⁻, thus resulting in the repulsion between PMS and the positively charged catalyst [30] . The H⁺ in the reaction mixture could also consume and deactivate SO₄^•− and ^•OH especially in low pH condition as shown in the equations below:

\begin{equation} SO_{4}^{\bullet-} + H^{+} + e^{-} \rightarrow HSO_{4}^{-} \tag{8} \end{equation}

\begin{equation} ^{\bullet-}OH + H^{+} + e^{-} \rightarrow H_{2} \tag{9} \end{equation}

The repulsion decreased when pH increased to 5 and 7, where the positively charged catalyst had the favorable attraction towards the negatively charged PMS. At pH 9 condition, the surface of the catalyst became negatively charged and repulsion is expected to occur between the catalyst and PMS. However, the k increased due to the increase in abundance of SO₅^2– that promoted the generation of SO₄^•−. Moreover, the generation of non–selective ^•OH ions (Eq. (10)) at alkaline condition could also promote the AO7 degradation [31] .

\begin{equation} SO_{4}^{\bullet-} + OH^{+} \rightarrow ^{\bullet}OH + SO_{4}^{2-} \tag{10} \end{equation}

Mineralization Studies

To confirm the AO7 degradation was effective instead of only converting it into other secondary organic pollutants, the TOC removal was analyzed. The TOC removal efficiency of the catalyst CoFe₂O₄ was 19.3% at 1 h of reaction time but the efficiency increased to 38% after 3 h of catalytic reaction (Figure 6 ). The low TOC removal observed at the first hour implied that the AO7 molecule was possibly converted into sodium sulfanilamide and 1-amino-2-naphthol resulted from the cleavage of the azo bond (N=N) [32]. The degradation pathway of 1-amino-2-naphthol into CO₂ and H₂O was longer due to the presence of fused benzene rings that were stabilized by the conjugation. However, the increased TOC removal efficiency indicates that the catalyst could mineralize the organic pollutant more effectively when the reaction time was prolonged.

Figure 6.Figure 6. TOC removal by CoFe₂O₄ (Conditions: pH = 7.0, [PMS] = 0.20 g L⁻¹, [CoFe₂O₄] = 0.20 g L⁻¹, and [AO7] = 10 mg L^-1)

Stability and Reusability

The recovery of a catalyst is a critical aspect from the economic and the environmental perspectives. To investigate the reusability, four consecutive cycles of the AO7 removal by PMS activation were carried out using the same catalyst. Figure 7 a illustrates the reusability of CoFe₂O₄ for AO7 removal via PMS activation for 4 cycles. After the first cycle, the removal efficiency of AO7 decreases by about 22%, whereas for the next three consecutive cycles the reduction in rate of AO7 removal decreases (5–10% after each cycle). The reduced performance could be due to the inevitable loss of catalyst during reaction, and the adsorption of AO7 molecules and/or its degradation intermediates on the catalyst active sites which blocking the surface active sites. The adsorption was confirmed by the new absorbance band presents at about 900 cm^–1 in the FTIR spectrum of the catalyst after reaction as shown in Figure 7 b . The new peak could be attributed to the C=C bending which was consistent to the structure of AO7. The catalyst was capable of degrading AO7 continuously for four cycles without washing and drying, implying that the co–precipitation method could synthesize an efficient and reusable catalyst for organic pollutant removals via PMS activation.

Since Co and Fe were the active transition metals participating in the PMS activation, the Co and Fe leaching for the catalyst were analyzed where a trace amount of Co (0.038 mg L^–1) was found in the treated solution after 1 cycle of reaction, whereas the leaching of Fe metal was not detected. The Co leaching was lower than 1 mg L^–1, which falls within the range of the environmental quality standards for surface water (GB 3838–2002). These results indicate that the catalyst is relatively stable after reacting with PMS, despite having a low crystallinity.

Figure 7.Figure 7. (a) Reusability of CoFe₂O₄ catalyst (Conditions: pH = 7, [CoFe₂O₄] = 0.20 g L⁻¹, [PMS] = 0.20 g L⁻¹, [AO7] = 10 mg L⁻¹). (b) FTIR spectra of the CoFe₂O₄ catalyst before and after use.

Identification of Reactive Species

To determine the dominant radical species generated in the PMS/CoFe₂O₄ system, competitive kinetic approach was carried out by conducting radical quenching tests. EtOH was used as the chemical scavenger for both ^•OH and SO₄^•−, whereas TBA was acted as the chemical scavenger for ^•OH. Generally, a chemical scavenger that is selective to a specific radical specie should possess the following properties: a (i) sufficient (≥ 3 order of magnitude) difference in the reactivities between various species present, (ii) inert to other coexisting oxidants such as PMS, and (iii) not interfering the catalytic activity or causing deactivation of catalyst active sites [33] . Given the lower reactivity of TBA towards SO₄^•− (k = 4.0 × 10⁵ M⁻¹ s⁻¹) compared to its reactivity towards ^•OH (k = 6.0 × 10⁸ M⁻¹ s⁻¹) [34,35] which satisfies the properties of a selective chemical scavenger, TBA was used as the •OH in this study. As presented in Figure 8 a, the removal efficiency of AO7 was 97.2% in 30 min without adding any quencher. The addition of 1M TBA has caused a slight decrease (2.2%) in the removal efficiency. However, the removal efficiency reduces significantly to 60.3% when 0.2M EtOH was added. When the concentration of EtOH was further increased excessively to 2M, the catalytic performance was suppressed with the removal efficiency of < 1%. However, the extremely low efficiency can be explained by the increase in viscosity and the decrease in diffusion coefficient due to the excessive EtOH. The factors resulted in the slow diffusion of PMS and catalyst and hence disrupted the system. Furthermore, the rate constants of AO7 with and without ^•OH scavenger were compared to estimate the concentration of radical species as shown in the equation below:

\begin{equation} \%Concentration\ of\ SO_{4}^{\bullet-}=\frac{k\ with\ SO_{4}^{\bullet-}\ scavenger }{k\ without\ scavenger}\times 100 \tag{11} \end{equation}

%concentration of SO₄^•− was calculated as 85% while the %concentration of •OH was calculated as 15%. The results validate that both SO₄^•− and ^•OH were actively involved in degradation of AO7 but SO₄^•− was the dominant active radical generated from the PMS activation.

The possible mechanism for the PMS activated by CoFe₂O₄ is illustrated in Figure 8 b . AO7 molecules initially were adsorbed on the catalyst surface, followed by the redox cycles of Co²⁺/Co³⁺ and Fe²⁺/Fe³⁺. During the redox cycles, PMS could be activated to generate the SO₄^•− and ^•OH for the AO7 degradation. The degradation products were then released into the solution and free active sites of the catalyst were regenerated. The redox process could occur cyclically until the PMS was fully consumed.

Figure 8.Figure 8. (a) Influence of different chemical scavengers on AO7 degradation in PMS/CoFe₂O₄ system (Conditions: pH = 7, [CoFe₂O₄] = 0.20 g L⁻¹, [PMS] = 0.20 g L⁻¹, [AO7] = 10 mg L⁻¹) and (b) proposed mechanism for AO7 degradation in CoFe₂O₄/PMS system.

Conclusion

In this study, a highly stable and magnetic CoFe₂O₄ catalyst was successfully synthesized via co–precipitation method. The identity of CoFe₂O₄ has been confirmed by XRD, FESEM and FTIR spectroscopy. The increase in the ratio of Fe:Co in the catalyst had an adverse effect in the catalytic performance. The CoFe₂O₄ has the highest performance whereas the degradation of AO7 reached 97.2% in 30 min with a k value of 0.1211 min^–1 at neutral pH conditions. The influences of different catalytic reaction parameters (i.e. pH, catalyst loading and PMS dosage) on the rate of removal have also been studied. The catalytic performance increases with the increase in both pH, catalyst loading and PMS dosage. The higher inhibition on AO7 degradation by EtOH than by TBA illustrates that SO₄^•– radical is the dominant species generated from the activation of PMS. Furthermore, the CoFe₂O₄ catalyst is reusable for atleast four consecutive cycles without any treatment on the catalyst, and can continuously mineralize organic pollutants. Trace amount of Co leaching (0.038 mg L^–1) can be detected from the treated solution while Fe leaching was undetected. The CoFe₂O₄ catalyst is practicable due to its magnetic property which provides the advantage of easy separation/recovery after the catalytic treatment of wastewater.

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