Exothermic Reaction in the Cleaning of Wastewater by a Fe₂O₃/Coconut Shell Activated Carbon/H₂O₂ Heterogeneous Fenton-like System

Abstract

Energy utilization in wastewater degradation has important implications for sustainability; however, efficient multiphase Fenton-like catalysts are still needed. In this study, a heterogeneous Fe₂O₃/coconut shell activated carbon (CSAC) Fenton-like catalyst was prepared and evaluated with respect to degradation performance and exothermic reaction for the treatment of organic wastewater. Fe₂O₃@CSAC retained the porous morphology of CSAS, and Fe₂O₃ was uniformly loaded on the surface of CSAS. In the reaction system, the degradation rate of wastewater was higher and a large amount of heat was released; therefore, it could be applied to the energy recovery from wastewater source heat pump technology. The degradation rate of 300 mL of o-phenylenediamine solution with a concentration of 0.04 mol·L⁻¹ was 89.0% under 0.25 mol·L⁻¹ H₂O₂, 532 g·L⁻¹ Fe₂O₃@CSAC, pH 7.1, and an initial reaction temperature of 30 °C, elevated to 7.9 °C. These findings clearly demonstrate the degradation performance and exothermic laws of the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system.

1. Introduction

Human activities and industrial processes produce a variety of wastes or emissions, and energy recovery is an important strategy for minimizing waste and the adverse impact on the environment. Various technologies, such as waste incineration for power generation, biogas fermentation, and flue gas recovery, are commonly used; however, the energy converted during wastewater treatment has rarely been studied and utilized. Wastewater source heat pump technology is a green energy technology that utilizes a small amount of high-grade electrical energy to enhance the low-grade heat energy in wastewater that cannot be directly utilized for enterprise production or winter heating [1,2]. The temperature drop of wastewater after heat extraction is generally controlled within 5 °C due to the wastewater treatment process. Therefore, analyses of the wastewater treatment process focused on wastewater temperature increases to enhance the external heat supply of wastewater source heat pumps and improve energy utilization are of great significance.

o-Phenylenediamine (OPD) is a raw material for the preparation of pesticides, fungicides, medical drugs, and various chemical products, and is easily discharged into the environment with industrial wastewater, agricultural runoff water, and domestic wastewater; it is potentially hazardous to aquatic organisms and human health [3,4]. Advanced oxidation technology is currently used to treat difficult-to-degrade wastewater. This technology mainly involves Fenton- [5], ozone- [6], electrochemical- [7] and microwave-assisted [8], as well as photocatalytic, oxidations [9]. Of these, the Fenton process is the most commonly utilized wastewater oxidation technology owing to its convenient dosing and high oxidizing capacity [10,11]. The Fenton reaction is a Fenton reagent composed of ferrous ions and oxidant H₂O₂, which produces a short but extremely active substance, namely hydroxyl radical (·OH), which has a significant oxidizing ability to convert pollutants into non-toxic products, including CO₂ and H₂O. However, a homogeneous Fenton reaction is only feasible when the pH value is lower than 4. When the pH value exceeds 4, iron ions are converted into ferric hydroxide sludge, and some catalysts are lost, thus decreasing the efficiency of the Fenton reaction; furthermore, the treatment and disposal of solid sludge also produces additional costs, which limits its application to a certain extent [12,13,14]. Multiphase Fenton technology is an advanced oxidation technology that replaces the iron ions in traditional Fenton technology with multiphase Fenton catalysts to catalyze the production of free radicals in hydrogen peroxide; it reduces some process limitations, such as sludge production and chemical inputs, thereby reducing costs [12]. For example, Pelalak et al. investigated the reaction mechanism of multimetallic catalysts in the Fenton process, describing effective methods to enhance the activity, electron transfer rate, and generation of hydroxyl radicals [15]. Dong et al. investigated Fe/Al₂O₃-loaded catalysts for the degradation of wastewater and proposed an efficient and low-cost solution for Fenton treatment with loaded catalysts [16]. Liu et al. investigated the performance of elemental iron-loaded catalysts for the degradation and treatment of sulfamethoxazole wastewater and measured the contributions of adsorption and Fenton oxidation to the degradation rate [17]. Zhang et al. prepared a non-homogeneous Fenton catalyst containing iron resin, which showed high degradation rates for organic wastewater with multiple reuses [18]. The selection of catalyst materials that overcome the disadvantages of existing materials, such as high costs and complicated preparation processes, has become a major area of research.

The application of biomass feedstock as a sustainable resource in wastewater treatment processes has increased worldwide. Coconut shell activated carbon (CSAC) is a biomass activated carbon prepared from coconut shell, an agricultural waste and sustainable resource. CSAC is a commonly used catalyst due to its large surface area and porosity [19]. For example, Li et al. synthesized CuFe₂O₄@CSAC for the degradation of tetrabromobisphenol A (TBBPA) and demonstrated that the surface active sites are the key factors affecting the degradation performance [20]. Pang et al. prepared coconut shell composites with high photocatalytic properties for methyl orange [21]; the metal oxide particles were uniformly loaded onto the CSAC, which improved the thermal stability and specific surface area of the material. Sun et al. developed a CSAC catalyst for the synthesis of H₂/CO, obtaining a conversion rate greater than 90%, and showed that CH₄/CO₂ content has a great influence on the synthesis of H₂/CO [22].

Previous studies have mainly been concerned with the degradation of wastewater using catalysts and conversion of organic matter, and studies on energy utilization in wastewater degradation processes are lacking. In this study, a CSAC-loaded Fe₂O₃ multiphase Fenton-like catalyst was prepared using 2–4 mm industrial-grade granular CSAC as the carrier, and the degradation performance of the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system and exothermic pattern of the degradation of OPD simulated wastewater were investigated. The newly developed catalyst had good degradation performance for OPD wastewater, and the degradation of wastewater increased the temperature of the system so that it contained more recoverable thermal energy, providing a theoretical basis for the energy utilization of wastewater. The study schematic is shown in Figure 1.

2. Materials and Methods

2.1. Instruments

Analytically pure C₆H₈N₂(OPD), 30% H₂O₂, Fe₂(SO₄)₃, H₂SO₄, NaOH, Ag₂SO₄, (NH₄)₂Fe(SO₄)₂·6H₂O, and K₂Cr₂O₇ were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China), and coconut shell activated carbon (CSAC) was obtained from Zhengzhou Wastewater Treatment Materials Co. (Zhengzhou, China). Additional instruments included a tube furnace (XL-R; Yangzhou Xingliu Electric Co., Ltd., Yangzhou, China), drying oven (DZF; Beijing Kewei Yongxing Instrument Co., Ltd., Beijing, China) infrared spectrometer (ALPHAII; Bruker), field emission scanning electron microscope (Tescan Mira4; Regulus 8100, Hitachi, Tokyo, Japan), and an X-ray diffractometer (D8 Advance; Bruker, Billerica, MA, USA).

2.2. Preparation of Fe₂O₃@CSAC

CSAC particles with a particle size of 2–4 mm were selected and soaked in 1.5 mol·L⁻¹ NaOH solution and 1.0 mol·L⁻¹ H₂SO₄ solution for 24 h, respectively. After drying at 200 °C for 12 h, the modified CSAC was obtained. In order to ensure that the ratio between Fe³⁺ and CSAC in the preparation process was 1.2 mol·L⁻¹: 500 g, 500 g modified CSAC was placed in 500 mL Fe₂(SO₄)₃ solution with a concentration of 0.6 mol·L⁻¹ and fully stirred for 24 h. The filtered samples were immersed in 500 mL of 0.6 mol·L⁻¹ NaOH solution for 2 h, and then filtered and washed with deionized water until the solution was neutral. Fe₂O₃@CSAC was obtained by heating the filtered and washed material in a tube furnace at 500 °C for 2 h.

2.3. Degradation Experiment

A certain mass of OPD agent was accurately weighed with a precision balance and used to prepare a certain concentration of OPD simulated wastewater in a 2000 mL volumetric flask with deionized water. The pH of the simulated wastewater was adjusted with 0.05 mol·L⁻¹ H₂SO₄ and 0.1 mol·L⁻¹ NaOH.

The reaction vessel was a 500 mL round-bottomed flask with a temperature probe and a stirring rotary head inside the vessel. The temperature probe was connected to an external temperature recorder, the stirring rotary head was connected to an external mixer, and the reaction vessel was wrapped with a heat preservation layer. The water temperature of the constant temperature water bath was set at 10 °C above the reaction temperature at the beginning of the experiment for heating the simulated wastewater backup. The weighed Fe₂O₃@CSAC catalyst and 300 mL of heated OPD solution were poured into the round-bottomed flask successively, followed by stirring at 600 r·min⁻¹. When the temperatures of the OPD solution and the Fe₂O₃@CSAC catalyst in the flask dropped to the set initial reaction temperature, the H₂O₂ solution was injected into the OPD solution, and a temperature recorder was activated at the same time. The sample solution was pipetted at 10 min intervals after the start of the reaction, and the reaction solution was filtered with a 102-type medium-speed qualitative filter paper and then used for COD (Chemical Oxygen Demand) testing.

2.4. Data Processing

The default reaction conditions were as follows: The temperature at the beginning of the reaction was 30 °C. The concentration of OPD was 0.04 mol·L⁻¹, and H₂O₂ was added at 0.25 mol·L⁻¹. Fe₂O₃@CSAC was added at 532 g·L⁻¹, and the pH was adjusted to neutral. The number of experimental repetitions was 3, and the data were averaged across all experiments. The COD degradation rate was expressed as η. The amount of degradation was expressed as ΔC, and the amount of solution temperature increase during the reaction process was expressed as ΔT, calculated as shown in Equations (1)–(3):

η (%) = ΔC/C₀ × 100

(1)

ΔC = C₀ − C

(2)

ΔT = T − T₀

(3)

where C₀ is the COD value at the beginning of the reaction, C is the COD value during the reaction process, T is the temperature of the solution after the reaction, and T₀ is the temperature of the reaction at the beginning of the reaction.

3. Results and Discussion

3.1. X-ray Diffraction (XRD) Analysis

Figure 2 presents the XRD results of the modified CSAC and Fe₂O₃@CSAC. New characteristic peaks appeared for Fe₂O₃@CSAC, and the diffraction peaks at 30.2°, 35.6°, 57.3° and 63.0° corresponded to the (220), (311), (511), and (440) crystal planes, respectively, which is in general agreement with the standard card of maghemite γ-Fe₂O₃ (PDF#39-1346) [23]. These results indicate that Fe₂O₃ is the main active form and was successfully loaded onto the surface of the activated carbon. The Fe₂O₃@CSAC diffraction peaks were clear and sharp, suggesting that the backbone structure of CSAC did not change after loading Fe₂O₃.

3.2. Fourier Transform Infrared Spectroscopy (FTIR) Analysis

Figure 3 presents the FTIR results for Fe₂O₃@CSAC. The modified CSAC particles contained a large number of functional groups that are easily adsorbed and are good carrier materials for Fenton catalysts. The peaks near 3436.528 cm⁻¹ were mainly due to the stretching vibration of O-H bonds in hydroxyl and carboxyl groups [24,25]. The absorption peaks at 1734.656 cm⁻¹ and 1560.612 cm⁻¹ were generated by the stretching vibration of C=O and aryl ring skeleton [24,26]. The peaks at 1091.030 cm⁻¹ corresponded to C-O telescopic vibration peaks [24]. The 792.118 cm⁻¹ absorption band indicates the presence of aromatic C-H bonds in CSAS [21,26]. Two telescopic vibration peaks, 546.720 cm⁻¹ and 461.386 cm⁻¹, were also present on Fe₂O₃@CSAC, consistent with Fe-O vibrations in γ-Fe₂O₃ [27], further demonstrating that Fe₂O₃ was successfully loaded onto the modified CSAC.

3.3. Scanning Electron Microscope (SEM) Analysis

Figure 4 presents the SEM images of CSAC, modified CSAC (20 μm), modified CSAC (2 μm), and Fe₂O₃@CSAC. Modified CSAC pores were more abundant than those for CSAC. The porous structure of modified CSAC has a relatively large specific surface area that can be used for adsorption sites, making it an effective adsorbent. Adsorption performance is one of the main factors affecting the degradation performance of a heterogeneous Fenton system [12]. As shown in Figure 4c,d, Fe₂O₃ nanoparticles were uniformly dispersed on the modified CSAC without agglomeration, indicating that the Fe₂O₃ nanoparticles can expose more active sites, thereby increasing the catalytic activity of Fe₂O₃@CSAC.

3.4. Effect of Catalyst Dosage

The effects of various concentrations of Fe₂O₃@CSAC in the reaction are summarized in Figure 5.

As shown in Figure 5a, η increased rapidly in the first 20 min of the reaction. After the reaction proceeded for 20 min, η showed an initial increase, followed by a decrease in the rate of increase. Fe₂O₃@CSAC increased and eventually reached a plateau at 665 g·L⁻¹. This is because as Fe₂O₃@CSAC increased, the active points increased and the number of radicals, such as ·OH and HO₂·, produced from catalyzed H₂O₂ per unit time increased, prompting a rise in η. Further increasing Fe₂O₃@CSAC led to the rapid decomposition of H₂O₂ according to Reaction (4) below, followed by rapid decomposition via Reaction (5), while accelerating Reactions (6)–(9) and depleting free radicals, such as ·OH, were used to degrade OPD molecules [28]:

Fe³⁺ + H₂O₂ → Fe²⁺ + HO₂· + H⁺

(4)

Fe²⁺ + H₂O₂ → Fe³⁺ + ·OH + HO⁻

(5)

Fe²⁺ + ·OH →Fe³⁺ + HO⁻

(6)

HO₂· → O₂^— + H⁺

(7)

·OH + HO₂· → H₂O + O₂

(8)

Fe³⁺ + O₂⁻ → Fe²⁺ + O₂

(9)

Furthermore, the growth rates of η for different Fe₂O₃@CSAC concentrations all slowed gradually as the reaction proceeded. This is because as the reaction continues and H₂O₂ is consumed, the amount of radicals, such as ·OH, decomposed by H₂O₂ decreases and the degradation rate slows down.

As shown in Figure 5b, the solution temperature rose gradually as the reaction proceeded, indicating that heat was released during the reaction, and the more Fe₂O₃@CSAC was added, the larger ΔT reached. At each time point, the increase in ΔT after the addition of Fe₂O₃@CSAC over 399 g·L⁻¹ was small. This is due to the fact that the total exothermic capacity in the multiphase Fenton-like system is related to the total reaction volume in the system, mainly derived from the amount of free radicals, such as ·OH, for the catalytic decomposition of H₂O₂, which is affected by two factors, the amount of active points of Fe₂O₃@CSAC and the amount of H₂O₂ adsorbed. A gradual increase in the amount of Fe₂O₃@CSAC increased the amount of active points and adsorbed H₂O₂ in the system, and more H₂O₂ was catalyzed to decompose ·OH and other radicals, resulting in an increase in the total reaction volume and the total exothermic reaction, as well as a larger increase in ΔT. Continuing to increase the addition of Fe₂O₃@CSAC, the amount of active points in the system increased; however, limited by the amount of H₂O₂ in the system, the amount of H₂O₂ adsorbed and the amount of catalytic decomposition of radicals, such as ·OH, did not increase further, and the increase in ΔT slowed. As shown in Figure 5b, the reaction proceeded to a certain point, and as Fe₂O₃@CSAC increased, the ΔT increased faster. This is because the active points in the system rise with the addition of Fe₂O₃@CSAC, and a greater number of radicals, such as ·OH, can be catalytically decomposed per unit time to react with OPD.

As seen in Figure 5a,b, when Fe₂O₃@CSAC is added at 665 g·L⁻¹, η decreases while ΔT increases, compared to the addition of 532 g·L⁻¹. This is because there are several reactions in the system, such as free radical production from H₂O₂ catalyzed by Fe₂O₃@CSAC, free radical degradation of OPD, free radical self-consumption, and the decomposition of H₂O₂ itself. The free radical degradation of OPD and free radical self-consumption are exothermic, and the exothermic degradation reaction only accounts for a part of the total exothermic reactions [29]. When the addition of Fe₂O₃@CSAC increases, the number of free radicals, such as ·OH, produced by catalytic H₂O₂ increases; accordingly, the total reaction volume increases, resulting in a continued increase in ΔT.

3.5. Effect of the H₂O₂ Concentration

The results obtained when various amounts of H₂O₂ were added to the reaction conditions are shown in Figure 6.

As shown in Figure 6a, the growth rate of η decreased gradually as the reaction proceeded. When H₂O₂ addition was increased from 0.05 to 0.25 mol·L⁻¹, η increased, and from 0.25 to 0.3 mol·L⁻¹, η decreased. This pattern of η increasing and then decreasing with the increase of H₂O₂ is similar to the results of Forouzesh et al. [30]. This is due to the fact that the number of free radicals, such as ·OH and HO₂, in the reaction system increases with increasing H₂O₂ concentrations, which promotes the degradation of organic matter and leads to an increase in η. When increasing the amount of H₂O₂ added, the excess H₂O₂ may convert the highly reactive ·OH to other low-activity and highly selective radicals [31], as in Equation (10):

·OH + H₂O₂ → H₂O + HO₂

(10)

The depletion of radicals increases and the frequency of collisions between radicals and OPD molecules decreases, resulting in a decrease in η of the OPD solution [32,33]. As the reaction proceeds, the depletion of ·OH in the system results in a reduced rate of η growth. Additionally, η was found to be lower when the amount of H₂O₂ added was 0.05 mol·L⁻¹, and still tended to continue to rise when the reaction reached 180 min; this is because when the concentration of H₂O₂ was too low, the surface of Fe₂O₃@CSAC was unable to adsorb a sufficient amount of H₂O₂, and in turn, the catalytic decomposition of free radicals such as ·OH was also low, which delayed the degradation of OPD.

As shown in Figure 6b, in the initiation phase of the reaction, as more H₂O₂ was added, ΔT rose faster. This is because the greater the amount of H₂O₂ added, the more easily H₂O₂ can be adsorbed by Fe₂O₃@CSAC and decompose ·OH, thereby increasing the total reaction volume and heat release in the system. The rate of increase of ΔT decreased as the reaction proceeded because as the H₂O₂ in the system was consumed, both the adsorption and decomposition of H₂O₂ slowed down, resulting in a reduction in the total reaction volume and exothermic amount. At any time point, ΔT showed a similar increase to that for the H₂O₂ addition, indicating that the surface of Fe₂O₃@CSAC is very rich in active sites with superior catalytic performance. As a result, the catalytic decomposition of a higher concentration of H₂O₂ is still efficient, and an increase in H₂O₂ can effectively increase the exothermic reaction in the system.

3.6. Influence of the Initial Reaction Temperature

The results for various starting temperatures (T₀) of the solution are shown in Figure 7.

As shown in Figure 7a, when T₀ is greater than 30 °C, at the initial stage of the reaction, η increases with T₀, consistent with the reports of Wang et al. [34] and Liu et al. [35]. This is due to the fact that an increase in T₀ in the initial stage of the reaction improves molecular activity, accelerates the adsorption and catalytic decomposition of H₂O₂, increases the production of free radicals, such as ·OH, per unit time, and increases degraded organic matter. When T₀ is greater than 30 °C and the reaction proceeds to 180 min, η is very close. This is due to the fact that when the amount of H₂O₂ and OPD in the system is constant, the total amount of ·OH catalyzed by H₂O₂ as well as the number of OPD molecules degraded are nearly constant, even though the temperature increases the rate of the reaction. Although the temperature increases the reaction rate, it has a small effect on the percentage of collisions between radicals and OPD molecules or between the radicals, and therefore, the higher T₀ is, the faster the rate of degradation. When the reaction time is sufficiently long, the η of the solution is almost constant. When T₀ values were 10 °C and 20 °C, η was lower and the growth rate was slower, and η tended to increase when the reaction was carried out to 180 min. This is because the low T₀ affects the speed of Reactions (4)–(9); when the temperature is less than 20 °C, only some of the active H₂O₂ molecules have gained enough activation energy for the reaction, and as the reaction proceeds, the temperature of the system gradually increases and the H₂O₂ molecules gradually react, so that η and ΔT rise more gradually below 20 °C.

As shown in Figure 7b, the higher T₀ was at the initial stage of the reaction, the faster ΔT increased as the reaction proceeded. When T₀ was raised to 60 °C from 30 °C with 180 min of reaction, ΔT increased slightly. A higher T₀ is associated with faster adsorption of Fe₂O₃@CSAC, which improves the rates of catalytic decomposition and degradation, and more heat is generated per unit time of the reaction, resulting in a higher temperature on the surface of Fe₂O₃@CSAC. Because higher temperatures lead to faster diffusion of heat, ΔT rises faster in the initiation phase of the reaction, and the reaction continues to the same moment at a higher ΔT. When T₀ is 10 °C or 20 °C, ΔT is smaller and ΔT continues to increase as the reaction proceeds. This is because the low temperature inhibits the adsorption and catalytic decomposition of H₂O₂ and reduces the reaction rate, resulting in a lower rate of heat production in the system; the heat output is not sufficient to rapidly increase molecular activity in the system, and thus, the reaction rate and ΔT are maintained at a low level. Therefore, for values of 10 °C and 20 °C, T₀ has a substantial influence on the exothermic rate of the reaction; when T₀ is greater than 30 °C, it has weaker effect on the total exothermic reaction.

3.7. Effect of Solution pH

The results obtained when the initial pH of the reaction solution was adjusted are shown in Figure 8.

As shown in Figure 8a, η tended to decrease with an increase in the initial pH from 3.1 to 11.1. η was very similar within the range of pH 3.1 to 7.1, decreasing slightly when the pH was increased to 8.9, and decreasing drastically when the pH was increased to 11.1. This is because Equations (4)–(5) are the key processes in the formation of ·OH in the multiphase Fenton-like system [36]. Since Fe³⁺ undergoes hydrolysis reaction to generate Fe(OH)₃ precipitate at pH greater than 3 [37,38], Fe³⁺ has a strong binding ability with -OH (hydroxide ions). When the reaction system is in an alkaline environment, the concentration of -OH in the system is high, and the force between Fe³⁺ and -OH exposed in the active site of Fe₂O₃@CSAC influences the collisional binding between Fe³⁺ and H₂O₂; it also reduces the catalytic efficiency of H₂O₂ and fails to produce a sufficient amount of -OH, resulting in a lower η. Under the alkaline environment of pH 8.9, the rate of η increase slowed down as the reaction proceeded; however, there was still a gradual increase after 180 min of the reaction, indicating that Fe₂O₃@CSAC still had catalytic activity under the weak alkaline environment. When the pH was 11.1, the concentration of -OH in the solution was higher and the active sites of Fe₂O₃@CSAC were largely inhibited, resulting in a lower η than those for other pH values and little increase in η after 180 min of reaction.

As shown in Figure 8b, in the pre-reaction period, both ΔT and its rate of increase decreased slightly as the initial pH increased when the reaction proceeded. This is due to the fact that -OH binds to Fe³⁺ and inhibits the activity at the Fe₂O₃@CSAC active site. As pH increases, the higher the concentration of -OH, the more easily the Fe³⁺ of the Fe₂O₃@CSAC active site is seized by -OH, resulting in a lower efficiency of catalytic decomposition of H₂O₂ and a decrease in the exothermic reaction. An alkaline environment had a weaker effect than those of other pH values on ΔT of the system. Although the active site of Fe₂O₃@CSAC is inhibited by -OH, some of the H₂O₂ molecules are still catalytically decomposed when adsorbed onto the surface of Fe₂O₃@CSAC; however, the amount of free radicals catalyzed is small as radical quenching [39] occurs before diffusion. Due to the quenching reaction, free radicals themselves also release heat [40], resulting in a local temperature increase on the Fe₂O₃@CSAC surface and the pores within, and this increase improves the molecular activity and diffusion rate. Although a higher -OH concentration reduces the probability of collision between H₂O₂ molecules and the active site, the increase in molecular activity can also increase the number of collisions between the active site and H₂O₂ molecules per unit of time. The ΔT of the system is induced by the quenching reaction rather than the degradation reaction, resulting in a high ΔT.

3.8. Effect of the Initial Concentration of OPD

The results obtained for various OPD concentrations in the reaction system are shown in Figure 9.

As shown in Figure 9a, ΔC increases with the increase in OPD concentration; when the concentration of OPD is greater than 0.04 mol·L⁻¹ and the reaction time is greater than 60 min, the difference between the ΔC values decreases. For example, when the reaction was carried out for 180 min, the ΔC difference between 0.03 and 0.04 mol·L⁻¹ was 2014.362 mg·L⁻¹, and the ΔC difference between 0.04 and 0.05 mol·L⁻¹ was 477.288 mg·L⁻¹. As shown in Figure 9a, at any time point, ΔC increased with an increase in the OPD concentration, and the increase in ΔC became smaller when the addition of OPD exceeded 0.04 mol·L⁻¹. This is due to the fact that increasing OPD concentration results in increasing the chances of effective collision between OPD molecules and free radicals in the solution as well as an increase in the number of degraded OPD molecules. As the number of OPD molecules increases, limited by the amount of Fe₂O₃@CSAC and H₂O₂ in the system, H₂O₂ is rapidly consumed and a sufficient amount of ·OH cannot be produced, resulting in a decrease in the rate of ∆C increase. Figure 9b shows the η of the reaction for 180 min under different OPD concentrations, revealing that η first increased and then decreased. Limited by the Fe₂O₃@CSAC and H₂O₂ injection, when the OPD concentration is lower than 0.04 mol·L⁻¹, the excess catalyst and H₂O₂ in the solution can produce enough free radicals to degrade OPD, and when the addition of OPD exceeds 0.04 mol·L⁻¹, the increase in OPD exceeds the degradation of OPD and η decreases.

As shown in Figure 9c, ΔT was still greater than 0 at an OPD concentration of 0 mol·L⁻¹, indicating that the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system itself generates heat. The increase in ΔT is greater with increasing OPD concentration, which is due to the fact that free radicals, such as -OH, react with OPD molecules in a more exothermic manner than quenching themselves [29]. Therefore, in the degradation of OPD by the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system, increasing the concentration of OPD can improve the exothermic reaction and rate of temperature rise.

3.9. Kinetics

The solution ΔT is very closely related to the degradation of OPD; accordingly, analyzing the kinetics of the degradation process can provide insight into the exothermic process of the system. The first-order kinetic equation is shown in Equation (11):

$$\frac{d{C}_t}{dt}=-kt$$

(11)

where t denotes the reaction time, C_t is the amount of reactants at time t, and k is the first-order reaction rate constant. The solution of the kinetic equation expressed in terms of η is as described in Equation (12):

$$\eta =1-e^{-kt}$$

(12)

The kinetics of η under the fitted default reaction conditions are shown in Figure 10a.

As shown in Figure 10a, the R² value obtained after η fitting was 0.97002, and k was 0.0632 min⁻¹, indicating a good linear correlation and conformation to the characteristics of the proposed first-order kinetics. Because ΔT was mainly caused by the degradation of OPD and the quenching of free radicals, such as OH, ΔT and η were expected to have a similar influence on the change rule. Equation (12) was modified to obtain Equation (13), and an attempt was made to model the law of ΔT versus reaction time using Equation (13).

$$∆T=1-e^{-kt}$$

(13)

The results of the simulation of ΔT with the first-order kinetic equation are shown in Figure 10b. The R² value obtained after fitting was 0.99736 and the value of k was 0.03352 min⁻¹, which shows that the pattern of ΔT versus reaction time is similar to the first-order kinetic characteristics. This is because the heat released from the degradation of OPD is higher than that of the quenching reaction [28], and ΔT is more closely related to the degradation of OPD; accordingly, ΔT has a kinetic law similar to that of η. Since the kinetic equation mainly represents the reaction rate relationship, the law of ΔT and reaction time expressed in Equation (13) only suggests a simple prediction of ΔT in actual industrial production.

4. Conclusions

Through analyses of the degradation performance and exothermic law of the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system, the following conclusions were drawn.

(1)

Fe₂O₃ was uniformly dispersed on the CSAC carrier and had a better catalytic effect in the Fe₂O₃@CSAC/H2O₂ multiphase-like Fenton system. The OPD degradation process releases a large amount of heat, which is in line with the characteristics of a first-order reaction and can be used as a theoretical basis for the use of energy in wastewater treatment. A 300 mL solution of 0.04 mol·L⁻¹ OPD had an η of 89.0% and a ΔT of 7.9 °C under an H₂O₂ concentration of 0.25 mol·L⁻¹, Fe₂O₃@CSAC concentration of 532g·L⁻¹, pH of 7.1, and T₀ of 30 °C.

(2)

In the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system, the additions of H₂O₂ and Fe₂O₃@CSAC had a large influence on the exothermic reaction and degradation rate. Increases in H₂O₂ and Fe₂O₃@CSAC within a certain range could accelerate the reaction rate, exothermic reaction, and degradation rate of the system.

(3)

The Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system had a wide initial pH adaptability and good degradation performance in the range of pH 3.1–8.9. Variation in the initial pH had almost no effect on the exothermic reaction.

(4)

The higher the temperature at the beginning of the reaction, the faster the degradation of pollutants and the faster the temperature rise of the system. Under the same conditions, the exothermic reaction is enhanced by a concentration increase.

This research studied the degradation performance and exothermic rules of the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system, which has important implications for energy recovery during wastewater treatment. In practical industrial applications, the amount of H₂O₂ and Fe₂O₃@CSAC added can be adjusted according to various factors including energy recovery and cost of chemicals to obtain higher economic benefits. Further, utilizing the Fe₂O₃@CSAC/H₂O₂ multiphase Fenton-like system to treat wastewater while controlling the pH of wastewater in the range of 3.1–8.9 can improve degradation; concentrating the wastewater and increasing the initial temperature can further improve the degradation of pollutants and the speed of heat recovery. However, analyses of the exothermic reaction in each process in the system are still lacking and this will be the focus of subsequent research.