Catalytic Degradation of Organic Contaminants by Microwave-Assisted Persulfate Activation System: Performance and Mechanism

Abstract

In this study, a nickel ferrite (NiFe₂O₄) system was constructed to purify a phenol solution in water. During the process, the influences of several critical operating parameters including the NiFe₂O₄ amount, PS dosage, MW power, initial pH value, and different natural water anions were systematically studied. The results indicated that the constructed system performed excellently regarding the removal efficiency (97.74%) of phenol within 30 min. Meanwhile, the influence of co-existing anions such as Cl⁻, NO₃⁻, H₂PO₄⁻, and HCO₃⁻ was also studied, which displayed an inhibiting action on phenol degradation, while HA facilitated it. To explore the reaction mechanism of this system, major free radical quenching experiments were conducted, and it was confirmed that both SO₄•⁻ and HO• were primary radicals. Moreover, stability experiments confirmed the apt stability of the NiFe₂O₄ system. Besides, the mineralization and toxicity analysis performed during phenol degradation also confirmed the superiority of the as-constructed system. Furthermore, the possible degradation mechanism of phenol was proposed. Hence, this system could be applied in advanced wastewater treatment.

1. Introduction

People are increasingly concerned about environmental pollution caused by recalcitrant organic contaminants such as antibiotics, pesticides, surfactants, dyes, and hydrocarbons, which may include phenols, ammonia, cyanides, sulfides, and aromatic compounds [1,2,3]. Most phenols and their derivatives are persistent organic pollutants and may harm organisms even in low doses [3,4]. Therefore, attention has been increasingly paid to phenolic wastewater treatment [5]. PS can be seen as the product of the substitution of two hydrogen atoms in H₂O₂ by sulfonyl-SO₃H. It is a white crystal and is very soluble in water. Figure 1 shows the PS structure.

Microwaves (MWs) activate PS to degrade organics by different mechanisms. In this process, in addition to the widely accepted thermal effect, the specific effect also plays an important role [3,6,7]. This feature could reduce the number of intermediate products from organic matter degradation and is conducive to the removal of refractory organic matter [8,9]. Compared with traditional heating methods, the MW thermal effect has the superiority of no heat transfer process demands, simultaneous heating of the whole system, outstanding heating uniformity, and no secondary pollution [10,11]. Moreover, MW could create more micropores on the surface of the material, thus improving the adsorption and PS activation properties of the material [12,13]. For instance, a significant study on ethyl-parathion removal from soil was performed employing PS activation using MW and proved the activation ability of the PS/MW system [14]. Besides, it was proven to be an effective method by using magnetic Fe₃O₄ as a PS activator under microwave irradiation to remove p-nitrophenol (PNP) in solution and also presented a potential application prospect in wastewater treatment [15]. In addition, compared to the MW-H₂O₂ system and ozonation process, the MW-PS system exhibited an excellent degradation result. For example, utilizing the MW/PS system to treat refractory organics in strongly alkaline dinitrodiazophenol wastewater, the removal rates of COD and chromaticity in 16 min were 74.07% and 99.40%, respectively [16]. Conclusively, the exceedingly good catalytic performance of MW is receiving increasing attention.

Recently, advanced oxidation processes (AOPs) have displayed the advantages of lower energy consumption, lower secondary pollution, and higher oxidation ability than traditional wastewater treatment technology. AOPs have attracted increasing attention due to their ability to generate reactive oxygen species (ROS, such as SO₄•⁻ and HO•) for oxidating or degrading refractory organic pollutants [17]. The method of generating HO• (E₀ = 2.8 V) and SO₄•⁻ (E₀ = 2.5–3.1 V) is activating persulfate (PS) using a series of methods, including heating, ultrasound, ultraviolet, and a transition metal oxide [18,19,20,21,22]. Transition metals, such as iron and cobalt, etc., have been utilized as chemical activation methods [20,21,22]. For instance, the Co₂FeAl-LDO/PMS system was used to degrade carbamazepine (CBZ). The results indicated that the system showed excellent degradation efficiency of CBZ (>99%) in 30 min, and the reaction rate constant was 0.2103 min⁻¹ [23]. Besides, it is meaningful to study recalcitrant organic contaminants (ROCs) removal using cobalt ferrite CoFe₂O₄. This process achieved better destruction of four ROCs and offered a latent alternative method in water processing [24]. Therefore, a material with excellent PS activation and magnetic properties was developed and is presented in this study [17].

In this study, the application potential of NiFe₂O₄ was explored as a PS activator with the assistance of microwaves. The objective was to determine the function of ROSs during phenol degradation, explore the possible mechanism, and evaluate the safety of intermediate products.

2. Results and Discussion

2.1. Physicochemical Properties of NiFe₂O₄ Sample

The SEM technique was utilized to investigate the morphology of NiFe₂O₄, which is exhibited in Figure 2a. The SEM images showed that the catalyst NiFe₂O₄ had a uniform nanospherical structure, which coincided with previous studies.

The structure of the NiFe₂O₄ was determined by X-ray diffraction (XRD). As shown in Figure 2b, the diffraction peaks of NiFe₂O₄ appeared at 2θ = 18.4°, 30.3°, 35.7°, 43.4°, 53.8° 57.4°, and 62.9°, which corresponded to the Bragg planes of (220), (311), (222), (400), (422), (511), and (440), respectively. This is consistent with the standard based on their JCPDS cards, which proved that NiFe₂O₄ had been successfully fabricated [17,25,26].

Besides, the magnetic property of the NiFe₂O₄ sample was measured by a Vibrating Sample Magnetometer (VSM) at room temperature, and the results are exhibited in Figure 2c. The magnetization curve revealed a typical S-shape with the external magnetic field, indicating that NiFe₂O₄ is a ferromagnetic material [27]. It is of great significance for the recovery and reuse of the catalyst.

2.2. Phenol Degradation in the NiFe₂O₄ System

Batch control experiments were carried out to compare the phenol degradation ability of different reaction systems. As shown in Figure 3a, phenol using NiFe₂O₄, MW, and PS systems reached degradation rates of 6.84%, 35.78%, and 16.05% within 30 min, respectively. That is because NiFe₂O₄ possessed poor adsorption capacity for phenol, and it is difficult to exert the catalytic ability of pure PS effectively. For MW, based on previous studies, it has both thermal and non-thermal effects to remove phenol, so it shows a slightly stronger removal effect [10]. The treatment of phenol via using NiFe₂O₄/PS, systems reached degradation rates of, because PS only activated SO₄•⁻ and HO• on the surface of the catalyst [28]. However, in some combinatorial systems, such as MW+PS and MW+ NiFe₂O₄+PS, the removal effects of phenol were quite obvious, reaching 76.66% and 97.74% after a 30min reaction. In conclusion, relative to a single system (NiFe₂O₄, MW, and PS), the NiFe₂O₄ hybrid system had a more powerful ability to remove phenol. This could be explicated by the fact that NiFe₂O₄ and MW possess a synergy effect when they come together so they can display an excellent ability to activate PS for the degradation of phenol.

2.3. Influences of Several Parameters on Phenol Degradation

2.3.1. Influence of PS Dosage

Figure 3b shows the relationship between the decomposition of phenol and PS dosages from 0.2 to 1 g/L. In the PS-catalytic process, the reactive radicals are related to the PS dosage [16]. As shown, the degradation rate of phenol was directly proportional to the dosage of PS. Specifically, when the PS dosage increased from 0.2 g/L to 0.8 g/L, the degradation growth rate of phenol increased from 76.5% to 97.7% within 30 min. The removal of phenol was obviously accelerated with the increasing PS concentration owing to the generation of more radicals [16]. Unfortunately, when the PS concentration continued to rise from 0.8 g/L to 1.0 g/L, the phenol degradation rate remained essentially constant (increased by 2%). The reaction rate increases with the increase in PS dosage. This phenomenon might be attributable to the fact that the high PS concentration could have a scavenger effect on SO4•⁻ as expressed in Equations (1) and (2) [6].

SO₄•⁻+ SO₄•⁻ → S₂O₈²⁻

(1)

SO₄•⁻+ S₂O₈²⁻→S₂O₈•⁻+ SO₄²⁻

(2)

Thus, considering the economy and degradation efficiency, 0.4 g/L was considered to be the optimum PS concentration for later experiments.

2.3.2. Influence of NiFe₂O₄ Dosage

It is widely known that the dosage of the catalyst is vital for the removal experiment of pollutants in the PS system. Consequently, a series of tests were carried out to examine the effect of the NiFe₂O₄ dosage with no variation of other factors. As Figure 3c illustrated, the removal efficiency of phenol increased with the growth of the NiFe₂O₄ dosage in the experiments. The reaction rate increases with the increase in NiFe₂O₄ dosage. That may be due to the fact that when the catalyst concentration increased, the active sites, which were used to activate PS in the reaction system, were correspondingly added [29,30]. So, taking into account the cost of the NiFe₂O₄/PS system, 0.6 g/L seemed to be the optimum catalyst concentration.

2.3.3. Influence of the MW Power

In the NiFe₂O₄ system, based on previous research, the power of MW also plays an important role in phenol degradation [15]. So, the influence of microwaves was also discussed in this study, and the results are exhibited in Figure 3d. As we can see, when the power of MW increased (10 W, 30 W, 50 W, and 90 W), the phenol degradation rate also increased (94.3%, 95.4%, 97.7%, and 99.4% within 30 min) and the reaction rate increased with the increase in MW power. This could be explained by the fact that (i) the reaction system could be heated by the MW and led to the system temperature increasing, which could accelerate the phenol degradation reaction. (ii) Previous research also proved that MW could accelerate the dispersion and mass transfer of catalysts [15]. (iii) In the system, the synergetic effect between the MW and PS occurred for phenol degradation, which was strengthened as the MW power increased [12,15].

2.3.4. Roles of the Initial pH in Phenol Degradation

In fact, the initial pH of practical water is diverse in different environments and plays a crucial role in the production and behavior of free radicals [31]. So, it has an important sense to explore the impact of the initial system pH in degradation experiments. As observed from Figure 4a, after a 30 min reaction, more than 95% phenol could be removed in all the initial pH situations (pH = 2, 4, 6, 8,m and 10). Noticeably, when the system’s initial pH rose to 6, the degradation rate speeded up markedly. The rate constant reached the maximum of 0.06844 s⁻¹. That indicated the NiFe₂O₄ system had a wide range of pH adaptability. The majority of natural water has a pH range of 5-9 [32], so the NiFe₂O₄ system would be of great significance in practical applications.

2.3.5. Influence of the Co-Existing Anions

The ion condition in natural water is very complex. So, for the sake of the practicability of the system, some common and abundant anions in natural water were appended to the simulated sewage (such as Cl⁻, H₂PO₄⁻, HCO₃⁻, NO₃⁻, and HA). As shown in Figure 4b, Cl⁻, H₂PO₄⁻, HCO₃⁻, and NO₃⁻ played a negative role in the reaction, and the inhibitory effects were H₂PO₄⁻ (5.99%) > HCO₃⁻ (19.95%) > NO₃⁻ (66.02%) > Cl⁻ (91.29%) after the 30 minute reaction. On the contrary, the degradation of phenol was enhanced, and the removal efficiency achieved 99.94% with the presence of HA. This phenomenon could be explained by the fact that anions could react with SO₄•⁻ in the system and this would cause a negative effect on phenol decomposition. Moreover, as a kind of natural organic matter, HA is rich in water and its active sites could promote the generation of SO₄•⁻ from PS. The corresponding reactions are listed below (Equations (3)–(11)) [17,23,33,34,35,36,37]:

SO₄•⁻ + Cl⁻ → SO₄²⁻ + Cl•

(3)

Cl• + Cl⁻ →Cl₂•⁻

(4)

•OH + Cl⁻→ClOH•⁻

(5)

SO₄•⁻ + HCO₃⁻ → SO₄²⁻ + HCO₃•

(6)

NO₃⁻ + SO₄•⁻ →NO₃• + SO₄²⁻

(7)

NO₃⁻ + •OH →OH⁻ + •NO₂

(8)

H₂PO₄⁻ + SO₄•⁻ →H₂PO₄• + SO₄²⁻

(9)

HCO₃⁻+ SO₄•⁻→SO₄²⁻+ CO₃•⁻+H⁺

(10)

HCO₃⁻+ •OH →CO₃•⁻+H₂O

(11)

2.4. Mineralization Ability and Toxicity Analysis

TOC is often utilized to detect the degree of contaminant mineralization in the catalytic reaction [2]. In this study, a change in phenol TOC was also observed, and the results are exhibited in Figure 4c. It was clear that the TOC degradation efficiency increased slowly over the period of 30 min, and a 93.04% mineralization efficiency was reached for phenol after the reaction in the NiFe₂O₄ system. In addition, in the whole reaction process, the toxicity of the system was reduced, which indicated that the toxicity of the decomposition intermediates had decreased [12,13]. All the analyses and results proved that the NiFe₂O₄ system could effectively reduce the toxic intermediates during the degradation of the LVF solution.

2.5. Mechanism Analysis

In order to explore the PS activation mechanism in the NiFe₂O₄ system, radical inhibition tests were carried out to detect the function of HO• and SO₄•⁻ in the degradation of phenol. As is well known, ethanol (EtOH) and tertiary butanol (TBA) are considered common inhibitors in experiments [13,38,39,40,41]. The reaction constant between TBA and HO• is (4–9.1) ×10⁵ M⁻¹•s⁻¹ and that between TBA and SO₄•⁻ is (3.8–7.6) × 10⁸ M⁻¹•s⁻¹. There is a big difference between the two reaction constants, so TBA is utilized as a scavenger for HO•. EtOH can quickly react with both SO₄•⁻ ((1.6–7.7) ×10⁷ M⁻¹•s⁻¹) and HO• ((1.2–2.8) ×10⁹ M⁻¹•s⁻¹). As seen in Figure 4d, the phenol degradation efficiency was 97.74% in the non-inhibitor system within 30 min; however, it dropped to 20.8% and 7.86%, respectively, after the participation of TBA and EtOH. The results meant that both the decomposition rate of phenol was reduced after the addition of EtOH and TBA and the inhibition influence was higher in the EtOH-PMS system. To summarize, both HO• and SO₄•⁻ participated in the reaction, and SO₄•⁻ played a larger role in the decomposition reaction system.

Based on the above discussion and results, the mechanism of enhanced PS activation with the NiFe₂O₄/MW was proposed and exhibited in Figure 5. The addition of NiFe₂O₄, Fe³⁺, and Ni²⁺, which acted as activation centers on the catalyst surface, activated PS to generate SO₄•⁻. SO₄•⁻mainly performed electronic transfer and HO• played the role of addition and hydrogen absorption [16]. Furthermore, Fe³⁺ was able to become Fe²⁺ when the system was in the presence of Ni²⁺. In addition, the existence of MW could boost the above reaction [37,42,43]. The thorough oxidation reactions are displayed below:

Ni²⁺ + S₂O₈²⁻ → Ni³⁺ + SO₄•⁻ + SO₄²⁻

(12)

Ni²⁺ + Fe³⁺ → Ni³⁺ + Fe²⁺

(13)

Fe²⁺ + S₂O₈²⁻ →Fe³⁺ + SO₄•⁻ + SO₄²⁻

(14)

2H₂O+S₂O₈²⁻ → HSO₅⁻ + HSO₄⁻

(15)

Ni³⁺ + HSO₅⁻ → Ni²⁺ + SO₅•⁻ + H⁺

(16)

SO₄•⁻ + H₂O → SO₄²⁻ + 3H⁺ + HO•

(17)

SO₄•⁻ + OH⁻ → SO₄²⁻ + HO•

(18)

Phenol + SO₄•⁻ + HO• → products

(19)

2.6. Stability of NiFe₂O₄

In addition, as an important concern for potential applications, the catalytic stability of the catalyst was estimated through consecutive repeated degradation reactions in identical experimental conditions utilizing recycled material, and the results are displayed in Figure 6. Compared with the fresh catalyst, the removal of phenol in 30 min was able to reach 96% after the eighth run. Compared with the first time, there was almost no change in the amount of phenol removal the second time. In the third to eighth trials, a slight decrease in the removal of phenol was observed. Obviously, the NiFe₂O₄ system exhibited excellent degradation performance and stability on the whole. Therefore, according to these outcomes and the XRD atlases, we could come to the conclusion that the prepared NiFe₂O₄ catalytic material possessed outstanding structure and catalytic stability, so it is a promising catalyst. The thermal analysis (Figure 7) shows that there is no weight change in the temperature range of 0–1000 °C, which proves the stability of the material.

3. Experiments

3.1. Chemicals

Iron chloride (FeCl₃·6H₂O), nickel chloride (NiCl₂·6H₂O), ferrous chloride (FeCl₂·4H₂O), ethylene glycol (C₂H₆O₂), sodium chloride (NaCl), sodium bicarbonate (NaHCO₃), sodium nitrate (NaNO₃), sodium dihydrogen phosphate (NaH₂PO₄), phenol (C₆H₅OH), potassium persulfate (PS), and absolute ethanol (C₂H₅OH) were purchased from Kermel Chemical Reagent Co., Ltd., (Tianjin, China). Tert-Butanol (C₄H₁₀O (TBA)) was obtained from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). In addition, phenol was provided by Aladdin Chemistry Co., Ltd., (Shanghai, China). All the chemicals were of analytical grade and used without further purification. Deionized water (DI) was used throughout this experiment.

3.2. Synthesis of NiFe₂O₄ Sample

In this study, Nickel ferrite (NiFe₂O₄) was synthesized according to a previous study. In detail, 1.63 g FeCl₃·6H₂O and 0.72 g NiCl₂·6H₂O were dissolved in a 60 mL ethylene solution and sonicated for half an hour. Afterwards, the mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave, sealed, and heated at 200 °C for 12 h. At given time intervals, the autoclave was cooled to room temperature and then filtered. Subsequently, the obtained samples were washed by DI and EtOH repeatedly (four times in this case). Eventually, NiFe₂O₄ was dried at 60 °C for 3 h.

3.3. Characterizations

The morphology and elemental analysis of NiFe₂O₄ were detected by SEM (Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan). The XRD patterns of the catalyst were obtained on a D/Max-2004 X-ray powder diffractometer (Bruker, Germany) with Cu Kα radiation (λ = 0.15426 nm). Besides, the magnetic properties of the NiFe₂O₄ samples were measured by VSM (Lake Shore 7303) at 26 °C.

3.4. Degradation of Phenol Solution

In this study, phenol degradation experiments were conducted in 150 mL conical flasks, in which a certain amount of the sample and PMS were placed into the flasks, which had a 50 mL 10 mg/L phenol solution to start the degradation reaction. Then, they were placed in a microwave instrument with a certain power. After a certain intermission, 5 mL of the sample was collected from the reaction solution and filtered through a 0.22 μm filter. Finally, the concentration of phenol was measured using the colorimetric method of 4-aminoantipyrine at the characteristic wavelength of 510 nm [3]. It should be noted that the mineralization ability and toxicity analysis were evaluated by a TOC analyzer and activated sludge process, respectively. Finally, the major reaction factors of phenol degradation were also discussed. In particular, the complete degradation was conducted in duplicate to reduce errors.

4. Conclusions

To summarize, the NiFe₂O₄ system was excellent in the degradation of phenol. In this process, several methods (XRD, SEM, and VSM) were carried out to characterize NiFe₂O₄. Then, phenol removal tests were conducted in different situations with the NiFe₂O₄ system to evaluate the activation properties of this system. The results were as follows: (i) A series of characterization detections proved that NiFe₂O₄ was successfully synthesized. (ii) Multiple sets of experiments proved that the NiFe₂O₄ system possessed excellent removal effectiveness for phenol, and achieved a removal efficiency of 97.74% within 30 min. Besides, the ideal degradation conditions were also found, regarding the PS dosage (0.4 g/L), NiFe₂O₄ concentration (0.6 g/L), and MW power (50 W). Moreover, the NiFe₂O₄ system had good adaptability across a wide range of pH values. The quenching experiment confirmed both the HO• and SO₄•⁻ played a vital role in the experiments, and H₂PO₄⁻, HCO₃⁻, NO₃⁻, and Cl⁻ could hinder the reaction process but NOM could promote it. (iii) Through the mineralization ability and toxicity analysis, phenol was proven to be almost completely mineralized, and the toxicity was greatly reduced by the NiFe₂O₄ system. Reusability and stability experiments confirmed the excellent stability and cyclicity of the NiFe₂O₄ catalyst. (iiii) In particular, based on the results of the experiments, the possible mechanism of phenol degradation was proposed.