Development of a γ-Al₂O₃-Based Heterogeneous Fenton-like Catalyst and Its Application in the Advanced Treatment of Maotai-Flavored Baijiu Wastewater

Abstract

Heterogeneous Fenton technology was employed for the advanced treatment of Maotai-flavored Baijiu wastewater. Novel catalysts were prepared by loading different active ingredients (Mn, Fe, and Cu) on γ-Al₂O₃ using an impregnation method. The effects of active ingredient, reaction time, initial pH, H₂O₂ dosage, catalyst dosage, and other factors on the reaction were examined. The properties of the new catalysts were analyzed using BET analysis, XPS, and SEM. Moreover, the mechanisms of Fenton-like oxidation and its reaction kinetics were explored through experiments and analyses including GC–MS and intermediate active species scavenging by tertiary butyl alcohol (TBA) and/or para-benzoquinone. The results revealed that the most effective removal of organic matter was achieved with a Mn-Fe/Al (2:1 wt%) catalyst dosage of 30 g/100 g water, pH of 5.0, H₂O₂ dosage of 0.3 g/L, and reaction time of 60 min; the effluent COD value was 12 ± 1 mg/L, and the degradation rate was 65.7 ± 3%, approximately 14% higher than that of the conventional Fenton catalyst under similar conditions; moreover, the catalytic efficacy remained high after seven cycles. Kinetic analysis indicated that the heterogeneous Fenton oxidation reaction followed a third-order kinetics model, with R² = 0.9923 and K = 0.0006 min⁻¹.

1. Introduction

The production of Maotai-flavored Baijiu industrial wastewater in China increased annually from 2010 to 2022 [1]. Brewing wastewater is divided into high-concentration organic wastewater (distillation effluent and residual liquid) and low-concentration organic wastewater (cooling water, bottle-washing water, and site-rinsing water) [2,3]. The pollutant composition varies with the brewing stage. High-concentration organic wastewater is rich in organic matter such as starch, protein, amino acids, and sugars. It has a high chemical oxygen demand (COD), biochemical oxygen demand (BOD), suspended solids (SS) content, sulfur concentration, ammoniacal nitrogen (NH3-N) concentration, total nitrogen (TN) concentration, and total phosphorus (TP) concentration [4,5]. Maotai-flavored Baijiu wastewater is treated by anaerobic, biochemical, coagulation and oxidation methods, membrane filtration technology, and other combinations of enhanced processes. Water quality indicators other than COD can be reduced to the ideal level, while COD concentrations in the range of 30~80 mg/L necessitate further advanced treatment.

Advanced oxidation processes are characterized by an oxidizing capacity that surpasses that of common oxidants or an oxidation potential approaching that of hydroxyl (·OH) radicals. ·OH radicals can initiate a series of free radical chain reactions with organic pollutants, destroying the structure of the pollutants and leading to their gradual degradation into harmless low-molecular-weight organics, ultimately converting them into CO₂, H₂O, and other mineral salts [6]. Advanced oxidation processes have become increasingly popular in recent years, with widespread application in laboratory pollutant degradation experiments and in practical engineering. This study presents a novel use of heterogeneous Fenton-like technology to degrade real wastewater with experimental investigations of heterogeneous Fenton-like oxidation under complex conditions.

Conventional Fenton technology uses H₂O₂ to generate ·OH under the catalytic effect of Fe²⁺ to oxidize organic pollutants and treat wastewater [7,8]. During the conventional Fenton reaction, the pH is adjusted to 2−3, and the reaction conditions are harsh. Before the effluent can be discharged, the pH needs to be adjusted to neutral after the completion of the reaction. Additionally, conventional Fenton technology generates a large amount of iron sludge, which needs to be treated and disposed of. The catalyst cannot be recycled, and thus, the disposal costs are high. These issues significantly limit the practical application of conventional Fenton technology, which has led to the emergence of heterogeneous Fenton technology.

The heterogeneous Fenton process, in which Fe²⁺ or active components are loaded onto a functional carrier, enables the Fenton reaction or the Fenton-like reaction to proceed in the solid–liquid state to degrade pollutants. The goal of this technique is to enhance treatment effectiveness while addressing the challenges in the traditional Fenton process, including the complexity of catalyst recovery, potential secondary contamination from catalyst deactivation by iron ions, and the need for a low pH [7]. Furthermore, in contrast to heterogeneous Fenton techniques, heterogeneous Fenton offers a broader pH range and greater H₂O₂ utilization and enables easy separation and recycling of the heterogeneous catalyst [9]. Moreover, the concentration of active ingredients dissolved in the catalyst can be controlled to promote their stability and efficient utilization and therefore limit the production of iron sludge.

In recent years, advanced treatment of wastewater has been the focus of several studies. These include the use of polyaluminum silicate sulfate (PASS) [10], Fenton-like reactions (modified ceramsite [11], Fe/Ce-RGO [12], and Fe⁰ [13]), and Wang’s preparation of novel PASS coagulant for effective advanced treatment of coking wastewater. The parameters used were a dosage of 7 mmol/L PASS, flocculation time of 30 min and pH of 7, which reduced the COD in cooking wastewater from 196.67 mg/L to 59.94 mg/L [10]. Wan utilized Fenton and heterogeneous Fenton-like technologies for advanced treatment of municipal secondary wastewater, with the SCOD degradation efficiency of the Fe/CeRGO heterogeneous Fenton-like reaction reaching 63.10% [12]. These novel advanced treatment methods are more economically feasible and simpler, and the heterogeneous Fenton technology is a particular focus of research compared to advanced treatment methods.

Various studies have verified the effectiveness of diverse substances as heterogeneous Fenton-like catalysts, including Fe₃O₄ [14], Fe₃O₄/MWCNTs [15], CoFe₂O₄ [16], CuFe₂O₄ [17], NiFe₂O₄ [18], MnFe₂O₄ [19], and Fe/γ-Al₂O₃ [20,21,22]. P. Bautista et al. employed the impregnation method to synthesize a custom Fe/γ-Al₂O₃ catalyst and studied its degradation impact on cosmetic wastewater. This Fe/γ-Al₂O₃ catalyst effectively degraded cosmetic wastewater with a COD concentration of 4560 mg/L using a catalyst dosage of 5 g/L and H₂O₂ dosage of 9050 mg/L, achieving an 85% degradation rate in 4 h [20]. The study concluded that this material can efficiently catalyze the heterogeneous Fenton-like reaction and is effective at degrading organic pollutants. In this study, γ-Al₂O₃ was chosen as the catalyst carrier to ensure catalyst stability and activity and in consideration of economic and practical aspects.

Aluminum oxide (Al₂O₃) has eight forms. γ-Al₂O₃ is one of the many forms of alumina that serve as a suitable carrier material for heterogeneous Fenton-like catalysts, exhibiting excellent physicochemical stability, a high specific surface area and porosity [23], and good potential for application.

In the present study, pretreated γ-Al₂O₃ was utilized as the catalyst carrier, and Mn, Fe, and Cu were loaded onto the carrier as active components using the impregnation method. The aims of this study were to assess the effectiveness of the catalyst in the advanced treatment of Maotai-flavored Baijiu wastewater, analyze its characteristics and performance, establish the optimal reaction conditions and kinetics, investigate the reaction mechanism, and explore the potential of heterogeneous Fenton-like technology for practical applications.

2. Results and Discussion

2.1. Catalyst Characterisation

BET, XPS, and SEM analyses were performed on two catalysts: Mn-Fe (2:1 wt%)–γ-Al₂O₃ and Mn-Fe (2:2 wt%)–γ-Al₂O₃.

The changes in the specific surface area, pore volume, and pore diameter of γ-Al₂O₃ before and after high-temperature calcination and loading of the active ingredient were assessed. The BET results in

Table 1. Analysis of catalyst pores and other structures.

Catalyst Performance	BET Specific Surface Area (m²/g)	Pore Volume (cm³/g)	Average Pore Diameter (nm)
γ-Al2O3	201.50	0.40	8.30
γ-Al2O3 (600 °C, 4 h)	236.10	0.68	12.80
Mn-Fe/Al (Mn:Fe = 2:1)	151.50	0.45	11.84
Mn-Fe/Al (Mn:Fe = 2:2)	151.70	0.43	11.46

indicate that these parameters increased after high-temperature calcination but decreased after loading of the active ingredient.

The SEM analysis in Figure 1 revealed that the surface of γ-Al₂O₃ after calcination exhibited more folding and had a larger surface area. However, after the loading process, the active ingredient filled the surface depressions, leading to a reduction in surface area. Additionally, the active ingredient occupied some of the pore space.

By comparing two different catalysts, it was observed that a small amount of Fe from the active ingredient was loaded onto the non-concave surface. As the Fe content in the active ingredient increased, the catalyst area slightly increased.

These findings suggest that the properties of the catalyst, such as surface area and pore characteristics, are influenced by both the calcination process and the loading of the active ingredient.

The XPS full spectra of the two catalysts, as depicted in Figure 2, displayed characteristic peaks corresponding to Al(2p) and O(1s) in γ-Al₂O₃ at 74.05 eV and 528.48 eV, respectively. The active components Fe(2p) and Mn(2p) exhibited characteristic peaks at 712.82 eV and 640.84 eV [24,25], respectively, while the characteristic peak of C(1s) was observed at 285.47 eV.

Upon comparing the full XPS spectra of the two catalysts, it was noted that there were no significant changes in elemental valence states. This indicates that there were no changes in the types of active components in the prepared catalysts, even when the amount of precursor solution was varied while keeping the precursor composition constant. Subsequently, XRD detection was performed on Mn-Fe/γ-Al₂O₃. According to the XRD results in Figure 3, the composition of the catalyst is consistent with our speculation.

The SEM images in Figure 3 illustrate significant changes in the morphology of γ-Al₂O₃ before and after calcination. In Figure 1a, the surface of γ-Al₂O₃ appears smooth with no evident loading, while Figure 1b shows an uneven granular morphology with particles of different sizes and both concave and convex regions. The numerous irregular pores between these particles enhance the contact area for loading active ingredients. The loading capacity was notably affected by the presence of ash on the carrier surface, which hindered active ingredient loading and caused the collapse of some pores; these effects can be mitigated through calcination to aid active ingredient loading.

Figure 1d,f show that the depressions of γ-Al₂O₃ were filled, indicating the successful loading of active ingredients onto the surface. In contrast, Figure 1c,e show active ingredients filling the depressions with a uniform distribution on the catalyst surface. The protrusions in Figure 1e resulted from the loading of a small amount of Fe into nonconcave areas on the catalyst surface when the Fe active ingredient content was increased. We speculate that the protruding active component Fe on the surface may suffer from leaching during the reaction process due to its prominent state, as observed in the experimental catalytic effect of different catalyst types. It is evident that even though it exhibits good catalytic performance in the initial stages of the reaction, its later performance is inferior to that of the Mn-Fe/γ-Al₂O₃ (2:1 wt%) catalyst. Based on this, we infer that its stability is also not as good as that of the Mn-Fe/γ-Al₂O₃ (2:1 wt%) catalyst.

2.2. Effect of Catalyst Type on the Removal of COD from Maotai-Flavored Baijiu Wastewater

To investigate the effects of different kinds and ratios of active ingredients loaded onto γ-Al₂O₃ as catalysts for the heterogeneous Fenton-like reaction on COD removal, 100 g of catalyst was added to 200 mL water samples, and 30% hydrogen peroxide was added at a concentration of 0.1 g/L. The reaction pH was adjusted to 5 to investigate the effects of different catalysts on the removal of COD from Maotai-flavored Baijiu wastewater.

As depicted in Figure 4, the catalysts prepared by loading various active ingredients onto γ-Al₂O₃ exhibited significantly different effects on the heterogeneous Fenton-like treatment of COD in Maotai-flavored Baijiu wastewater. Fourteen catalysts were prepared, including Mn-Fe, Mn-Cu, and Mn-Fe-Cu, with different weight ratios [26,27,28,29]. Among the catalysts, the Mn-Cu catalysts resulted in higher posttreatment COD levels exceeding 23 ± 1 mg/L. Conversely, the Mn-Fe and Mn-Fe-Cu catalysts demonstrated greater effectiveness, with effluent COD levels generally below 16 mg/L. Specifically, the Mn-Fe catalysts with weight ratios of 2:1 wt% and 2:2 wt% exhibited even lower COD levels at 13 ± 1 mg/L. Considering factors such as cost effectiveness and catalyst stability [30,31,32], the Mn-Fe catalyst with a weight ratio of 2:1 wt% was selected for further experiments based on its COD-removal performance.

2.3. Effect of Reaction Time on the Removal of COD from Maotai-Flavored Baijiu Wastewater

To investigate the impact of varying reaction times on the treatment efficiency of the heterogeneous Fenton-like reaction, different reaction durations (10 min, 20 min, 30 min, 40 min, 50 min, 60 min, and 80 min) were studied. The experiment involved the addition of 100 g of Mn-Fe (2:1 wt%) catalyst, a 30% hydrogen peroxide dose of 0.1 g/L, and a reaction pH of 5. The experiment aimed to assess the effect of the heterogeneous Fenton-like reaction on the removal of COD from Maotai-flavored Baijiu wastewater at different time intervals.

Figure 5 shows that the initial 30 min of the reaction was highly effective, with a noticeable reduction in COD over time. Beyond 30 min, the rate of COD reduction started to slow, and by 60 min, it began to stabilize. Notably, at the 60 min mark, the COD-removal rate peaked at 66 ± 3%, with the COD value stabilizing at 12 ± 1 mg/L.

The data suggest that prolonging the treatment time does not necessarily lead to a substantial improvement in treatment efficacy. Therefore, based on this analysis, the optimal heterogeneous Fenton-like treatment time was determined to be 60 min.

2.4. Effect of Initial pH on the Removal of COD from Maotai-Flavored Baijiu Wastewater

To explore whether this heterogeneous Fenton-like process requires similarly stringent pH conditions as conventional Fenton reactions [6], the initial pH levels for this experiment were set at 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0. The experiment involved the addition of 100 g of Mn-Fe (2:1 wt%) catalyst and a 30% hydrogen peroxide dose of 0.1 g/L, and the aim was to determine the optimal reaction pH.

Figure 6 shows that as the pH changed from acidic to neutral, the ability of the heterogeneous Fenton-like process to remove COD from Maotai-flavored Baijiu wastewater initially increased and then decreased. The greatest COD removal was observed at an initial pH of 5.0. This suggests that a relatively neutral pH is more suitable for this reaction than for conventional Fenton reactions, reducing the need for strongly acidic conditions.

2.5. Effect of H₂O₂ Dosage on the Removal of COD from Maotai-Flavored Baijiu Wastewater

To investigate the impact of varying initial H₂O₂ dosages on the removal of COD from Maotai-flavored Baijiu wastewater using heterogeneous Fenton-like reactions, an experiment was conducted under the following conditions: 100 g of Mn-Fe (2:1 wt%) catalyst, a reaction time of 60 min, and a reaction pH of 5. H₂O₂ (3%) was prepared by diluting 30% H₂O₂ tenfold, and dosages of 0.1 g/L, 0.2 g/L, 0.3 g/L, 0.4 g/L, 0.5 g/L, and 0.6 g/L were applied [23].

Observing from Figure 7, the COD-removal performance of Maotai-flavored Baijiu wastewater increased and then decreased with increasing H₂O₂ dosage. The treatment efficacy was strongest when the 3% H₂O₂ dosage was 0.3 g/L and decreased when the dosage exceeded or fell below this level. The analysis indicated that, similar to classical Fenton reactions, ·OH played a crucial role in determining the treatment efficacy. Lower H₂O₂ dosages resulted in insufficient ·OH, leading to inadequate oxidative strength, while higher dosages generated excessive ·OH, triggering a quenching reaction: Excessive ·OH converts Fe²⁺ to Fe³⁺, and ·OH further reacts with itself to produce H₂O₂ [33,34]. It is speculated that an excess of hydroxyl radicals could also lead to the conversion of Mn²⁺ to Mn³⁺. All of these can result in the ineffective decomposition of H₂O₂, which reduced the efficiency of the H₂O₂ reaction.

2.6. Effect of Catalyst Dosage on the Removal of COD from Maotai-Flavored Baijiu Wastewater

To determine the optimal catalyst dosage for the reaction, experiments were conducted under consistent conditions: a 3% hydrogen peroxide dose of 0.3 g/L, a pH of 5, a reaction time of 60 min, and varying amounts of Mn-Fe (2:1 wt%) catalyst (4 g, 10 g, 20 g, 60 g, 100 g, 140 g, and 180 g). The impact of these different catalyst dosages on the removal of COD from Maotai-flavored Baijiu wastewater through heterogeneous Fenton-like oxidation was examined.

Figure 8 shows that as the catalyst dosage increased, the effectiveness of the heterogeneous Fenton-like treatment continuously improved. The treatment efficacy peaked at a catalyst dosage of 60 g, with a reduction in effluent COD to 12 ± 1 mg/L and a removal rate of 64%. At catalyst dosages above 60 g, the treatment efficacy did not increase further; rather, it started to decrease, with the decline becoming more pronounced at dosages exceeding 100 g.

The above results indicate that the optimal catalyst dose for this reaction is approximately 60 g, where the treatment effect is maximized. Increasing the catalyst dosage beyond this point does not yield additional benefits and may even lead to a reduction in treatment efficiency.

Based on the data and reasoning provided, along with insights from the reaction mechanism (see Section 2.8 for details) [35,36], it is evident that when the catalyst dose exceeded 60 g, the treatment effect did not improve but rather decreased. This decline in treatment efficacy can be attributed to catalyst overdosage, which led to certain detrimental effects on the reaction process.

First, an excessive amount of dissolved active ingredients such as Fe²⁺ and Mn²⁺ in the water resulted in the accelerated cycling of Fe²⁺/Fe³⁺ species. This accelerated cycling led to the rapid and extensive consumption of H₂O₂, causing a significant increase in the production of free hydroxyl radicals (·OH) with a very short half-life (on the order of 10⁻⁹ s). These highly reactive ·OH radicals, in turn, reacted with each other to regenerate H₂O₂ rather than effectively degrading the pollutants in the water samples. This process hindered oxidation and reduced the treatment efficiency.

Second, the nonselective nature of hydroxyl radical oxidation reactions resulted in the formation of undesirable byproducts when ·OH reacted with active components in water. These byproducts interfered with the oxidative degradation of pollutants, further diminishing the overall effectiveness of the reaction mechanism.

Therefore, maintaining the catalyst dosage within an optimal range, such as the experimentally observed value of approximately 60 g, is crucial to ensure efficient pollutant degradation and prevent the negative impacts associated with catalyst overdosage.

2.7. Catalyst Stability Analysis

To assess the stability of the Mn-Fe catalyst under optimal parameters, a continuous reaction experiment was conducted using 60 g of catalyst, a 3% hydrogen peroxide dose of 0.3 g/L, a reaction pH of 5, and continuous aeration at the bottom of the reactor. The initial COD of the wastewater was measured to be 35 ± 1 mg/L.

After 60 min, the supernatant was collected, centrifuged, and filtered for COD measurement using the potassium dichromate method. After sampling, all the water was removed by filtration, the original catalyst was retained, and a new round of wastewater was reintroduced to continue the reaction.

By monitoring the effectiveness of the catalyst in removing COD from Maotai-flavored Baijiu wastewater over multiple reaction cycles, this study aimed to evaluate the catalyst’s stability and ability to maintain consistent pollutant degradation performance over time.

According to Figure 9, the treatment effect of the heterogeneous Fenton-like reaction gradually decreased with increasing number of cycles. However, the effluent COD remained at approximately 12 ± 1 mg/L within eight cycles. The results indicate that the catalyst exhibited good stability, supporting its potential application in the advanced treatment of Maotai-flavored Baijiu wastewater.

A comparison of this type of Fenton reaction with the research of other scholars is presented in

Table 2. Compare the results with the reported values in the literature.

Catalysts and Reagents	Wastewater Type	Results
CuFe2O4 + TA + H2O2 [37]	Methylene blue	The results showed that introducing TA enhanced MB decolorization rate from 52.0% to 92.1% within 80 min [37].
MnFe2O4/biochar + H2O2 [19]	Methylene blue	When using the MnFe2O4/BC composite as an ultrasound-assisted heterogeneous Fenton-like catalyst, 95% of MB (20 mg/L) was degraded at pH = 5 in the presence of 15 mmol/L H2O2 in 20 min [19].
Polyaluminum silicate sulfate [10]	Coking wastewater	A dosage of 7 mmol/L PASS, flocculation velocity of 75 r/min, flocculation time of 30 min, pH of 7, and temperature of 20 °C could decrease the COD concentration from 196.67 mg/L to 59.94 mg/L [10].
Fe/Ce-RGO + H2O2 [12]	Municipal secondary effluent	The removal efficiencies of DOC, SCOD, and SMT were 36.30%, 63.10%, and 35.00% using Fe/CeRGO as catalyst [12].
Fe0 + H2O2 [13]	Citric acid wastewater	When the initial pH was 3.0, H2O2/COD = 0.7 × Fe0/H2O2 = 3.29 (weight ratio), coagulation pH = 9.0, and the COD removal reached to over 35% in 2 h [13].
Modified ceramsite + H2O2 + MBFB (Fenton-like/MBFB) [11]	Synthetic dye wastewater	The optimal conditions were H2O2 concentration of 600 mg/L and HRT of 6 h, while the influent COD concentration was 90 mg/L, achieving COD-removal efficiency of 64.8%.
Mn-Fe/γ-Al2O3 + H2O2	Maotai-flavored Baijiu wastewater	The results revealed that the most effective removal of organic matter was achieved with a Mn-Fe/γ-Al2O3 catalyst dosage of 30 g/100 g water, pH of 5.0, H2O2 dosage of 0.3 g/L, and reaction time of 60 min, while the influent COD concentration is 35 mg/L; the effluent COD value was 12 ± 1 mg/L, and the degradation rate was 65.7 ± 3%.

.

2.8. Reaction Kinetics Studies

According to the above analyses, the best treatment effect of this heterogeneous Fenton-like reaction was obtained when the Mn-Fe/γ-Al₂O₃ (2:1 wt%) catalyst was used at pH 5.0, a catalyst dosage of 30 g (100 g water), a 3% H₂O₂ concentration of 0.3 g/L, and a reaction time of 60 min. The kinetics data were fitted with first-order (Equation (1)), second-order (Equation (2)), and third-order reaction kinetics equations (Equation (3)) to investigate the characteristics of this heterogeneous Fenton-like reaction for the removal of COD from Maotai-flavored Baijiu wastewater [38,39,40].

$$\ln \frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{T}}}={\mathrm{K}}_1\mathrm{t}$$

(1)

$${\mathrm{C}}_{\mathrm{t}}^{-1}-{\mathrm{C}}_0^{-1}={\mathrm{K}}_1\mathrm{t}$$

(2)

$$\frac{1}{2}\left({\mathrm{C}}_{\mathrm{t}}^{-2}-{\mathrm{C}}_0^{-2}\right)={\mathrm{K}}_1\mathrm{t}$$

(3)

In the above equations, “K₁” represents the degradation rate constant of an aqueous solution of Maotai-flavored Baijiu wastewater in units of min⁻¹. “C_t” denotes the concentration of the solution at time t in mg/L, while “C₀” signifies the initial concentration of the solution in mg/L, with “t” representing the reaction time in minutes.

In Figure 10, the heterogeneous Fenton-like reaction shows the highest fit to the third-order reaction kinetics equation, with a coefficient of determination of 0.99184. This coefficient of determination is significantly higher than those of the first-order and second-order equations. Therefore, the heterogeneous Fenton-like oxidation reaction involving the homemade Mn-Fe (2:1 wt%) catalyst is best modeled by third-order reaction kinetics:

$$\frac{1}{2}\left(\mathrm{C}{\mathrm{t}}^{-2}-{\mathrm{C}}^{-2}\right)=\left(5.56145\times {10}^{-5}\right)\mathrm{x}+4.06675\times {10}^{-4}$$

(4)

In the literature, it has been proposed that the main oxides present in the Fenton reaction are hydroxyl radicals (·OH) and superoxide anion radicals(·O²⁻) [7]. The oxidation of complex organic matter by hydroxyl radicals, which depends on the generation of hydroxyl radicals to stimulate a series of radical reactions, is typically conducted through the following pathway [1,7,35,36]:

Polyols and sugars such as starch and sucrose undergo dehydrogenation by hydroxyl radical reactions, where C-C bonds are broken and oxidized to CO₂.

Water-soluble macromolecules, along with vinyl compounds, undergo hydroxyl radical reactions with C=C breakage, resulting in the production of CO₂.

As hydroxyl radicals can only oxidize saturated aliphatic and saturated aliphatic carbonyl compounds to carboxylic acids, the contribution of these radicals to the removal rate of COD is low.

Aromatic rings in aromatic compounds are broken down by the action of hydroxyl radicals to form aliphatic compounds, thereby enhancing their biodegradability.

·OH decomposes chromogenic radicals in dyes, facilitating decolorization and COD removal.

The effluent of the heterogeneous Fenton-like reaction was analyzed using GC–MS with solid-phase microextraction to detect small molecules with melting and boiling points below 300 °C.

Comparison of the test results with the hydroxyl radical oxidation theory revealed that the heterogeneous Fenton-like reaction was consistent with this theory (

Table 3. Analysis of GC–MS detection results.

Organic Substance	Match Score	Percentage (%)	Estimated Concentration (mg/L)	Molecular Formula	C	H	O	Cl	S	N	P	I	Si	Br	Molecular Mass	COD Conversion Factor
Oxime-methoxyphenyl	82.8	0.88	0.130358919	C8H9NO2	8	9	2			1					151.2	1.957671958
Cycloheptasiloxane, tetramethyl	91.7	1.53	0.342745045	C14H42O7Si7	14	42	7						7		519.1	1.294548257
o-Hydroxybiphenyl	96.7	5.12	0.564091429	C12H10O	12	10	1								170.2	2.632197415
Hexadecamethylcyclooctasiloxane	96.6	0.52	0.116477292	C16H48O8Si8	16	48	8						8		593.2	1.29467296
1,2,4-Triazine-3,5(2H.)	51.2	0.98	0.365261591	C3H3N3O2	3	3	2			2					113.1	0.778072502
2-([1,1′-Biphenyl]-2-yloxy)	93.8	44.15	5.196572064	C14H14O2	14	14	2								214.3	2.463835744
3-Methyl-1-(trimethylsiloxy)	50.2	0.94	0.108335	C10H20OSi	10	20	1						1		184.4	2.51626898
2-(3-Iodopropyl)-1,3-dioxolane	50.5	1.01	0.285931815	C6H11IO2	6	11	2					1			242.1	1.024370095
N-(Methoxymethyl)-1,1-diphenyl-N-[(trimethylsilyl)methyl]methylamine	62.3	1.15	0.129396349	C19H27NOSi	19	27	1			1			1		313.5	2.577352472
o-chlorobromobenzene	50.6	1.12	0.353404545	C4H6BrC1	4	6		1						1	191.5	0.919060052
1-Acenaphthenol	60.4	35.01	3.857195491	C12H10O	12	10	1								170.2	2.632197415
Triphenylphosphine oxide	95.9	0.85	0.10088375	C18H15OP	18	15	1				1				278.3	2.443406396
3,5-Dimethylphenyl terephthalate	51.1	1.47	0.193849527	C18H18O4	18	18	4								298.3	2.199128394
Ethyl 1,3-dithiane-2-carboxylate	62.5	0.76	0.147162917	C7H12O2S2	7	12	2		2						192.3	1.497659906
look for a draw (chess)		95.49	11.89166573

). The reaction effectively treated sugars, water-soluble macromolecules, and aromatics in the wastewater; however, the presence of aliphatic compounds after the reaction indicated that the heterogeneous Fenton-like reaction was less effective in treating aliphatic and saturated aliphatic carbonyl compounds.

Although the results of this heterogeneous Fenton-like reaction are compatible with the theory of hydroxyl radical oxidation, numerous studies [33,34,35,36,41] have shown that the free radicals in the reaction exist for a short period due to their poor stability. Thus, further exploration is needed to determine whether hydroxyl radicals play a dominant role in the reaction. To investigate the catalytic mechanism of the reaction, the catalytic mechanism of this heterogeneous Fenton-like process was examined using tertiary butyl alcohol (TBA) as a quencher of the oxidative activity (Equation (5)) of ·OH and para-benzoquinone (BQ) as a quencher of ·O²⁻ (Equation (6)) [42,43].

·OH + C₄H₁₀O →·C₄H₉O + H₂O

(5)

·O²⁻ + C₆H₆O₂ → ·C₆H₅O₂+ O₂

(6)

The formula for the quenching rate of ·OH was as follows [30,31]:

·OH quenching rate (%) = [1 − Kapp (quenching)/Kapp (base)] × 100%

(7)

The quenching data were fitted to the third-order kinetic model of the heterogeneous Fenton-like reaction (

Table 4. Kinetic fits for tert-butanol reactions.

Dynamics (Math.)	Kinetic Model	R2	K Value/min−1
First order	y = −0.0008x + 0.0113	0.9837	0.0008
Second order	y = −2 × 10−5x + 0.0003	0.9705	0.00002
Third order	y = 8 × 10−5x + 0.0004	0.9935	0.00008

), and a ·OH quenching rate of 75% was obtained.

The experiments were conducted under the optimal reaction parameters with 10 mmol/L TBA and 10 mmol/L BQ. According to the results shown in Figure 11, the COD degradation rate was 66% without the addition of a quenching agent, 59 ± 3% with the addition of BQ, 32 ± 3% with the addition of TBA, and 25 ± 3% with the addition of both.

The results revealed that ·OH plays a dominant role in this heterogeneous Fenton-like reaction, while ·O^2- plays an auxiliary oxidation role. Additionally, the catalyst was found to have a certain adsorption effect on pollutants in the experiment.

On the basis of the XPS and GC–MS results as well as the results of the free radical capture experiments, the mechanism of the heterogeneous Fenton-like reaction involving this catalyst is presumed to be as follows (The process is illustrated in Figure 12): first, pollutants are adsorbed onto the surface of the catalyst; second, Fe³⁺ precipitated on the active sites on the surface of the catalyst reacts with Mn²⁺ in the following two pathways [28]: First, Fe³⁺ and Mn²⁺ react with hydrogen peroxide to produce superoxide radicals, hydroxyl radicals, and superoxide anion radicals.

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{2+}+·\mathrm{OOH}+{\mathrm{H}}^{+}$$

(8)

$${\mathrm{Mn}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Mn}}^{3+}+·{\mathrm{O}}^{2-}+·\mathrm{OH}$$

(9)

The Fe²⁺ generated from the above reaction reacts with Mn³⁺. Fe²⁺ continues to react with hydrogen peroxide to generate the hydroxyl radicals needed for this heterogeneous Fenton-like process.

$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+}+·\mathrm{OH}+{\mathrm{OH}}^{-}$$

(10)

$${\mathrm{Mn}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Mn}}^{2+}+{\mathrm{H}}^{+}+{\mathrm{O}}_2$$

(11)

Manganese and iron mix with each other to form manganese–iron dual active centers in the system, which accelerate the electron transfer process through charge rearrangement. Additionally, this dual-active center structure exhibits stronger catalytic synergy than a single-active component structure. The Mn²⁺ and Mn³⁺ generated by the reaction accelerate the cycling of Fe²⁺/Fe³⁺.

$$·\mathrm{OOH}\to {\mathrm{H}}^{+}+·\mathrm{OH}+{\mathrm{OH}}^{-}$$

(12)

$${\mathrm{Mn}}^{2+}+{\mathrm{Fe}}^{3+}\to {\mathrm{Mn}}^{3+}+{\mathrm{Fe}}^{2+}$$

(13)

The generated ·OH and ·O²⁻ radicals oxidize the organic matter adsorbed on the surface of the catalyst and decompose it to produce CO₂, H₂O, small-molecule inorganic acids and other products, reducing the COD in the wastewater.

3. Materials and Methods

3.1. Materials and Reagents

γ-Al₂O₃ was used in the form of a 2~4 mm spherical solid; ferric nitrate hydrate (Fe(NO₃)₃·9H₂O), manganese nitrate (Mn(NO₃)₂), and copper nitrate trihydrate (Cu(NO₃)₂-3H₂O) were obtained from Shanghai McLean Biochemical Technology (Shanghai, China). All chemicals were of analytical grade (AR) and did not need additional purification.

3.2. Synthesis of Catalysts

The preparation method is outlined in Figure 13. γ-Al₂O₃ was used as the carrier, and Mn(NO₃)₂, Fe(NO₃)₃·9H₂O, and Cu(NO₃)₂·3H₂O were used as the raw materials. The process involved washing 2~4 mm γ-Al₂O₃ with ultrapure water until the filtrate was clear, followed by calcination at 600 °C for 4 h in a muffle furnace with subsequent static cooling [23]. After cooling, 100 g of γ-Al₂O₃ was weighed into a beaker, soaked in ultrapure water for 12 h, and filtered, and the amount of absorbed water was calculated.

The precursors Mn(NO₃)₂, Fe(NO₃)₃·9H₂O, and Cu(NO₃)₂·3H₂O were loaded onto γ-Al₂O₃ using the equal volume impregnation method. The precursor solution was thoroughly mixed in a beaker, and γ-Al₂O₃ was placed in another beaker. The mixed solution was poured into the beaker containing γ-Al₂O₃ under continuous stirring. The reaction proceeded at room temperature for 12 h with constant stirring to ensure uniform impregnation. Subsequently, the impregnated material was dried in a constant temperature drying oven at 80 °C, followed by calcination at 600 °C for 4 h in a muffle furnace. After cooling, the material was cleaned with ultrapure water, dried, and stored for further use.

By controlling the addition amount of precursor solution and adjusting the type and content of active ingredients in the catalyst, 14 catalysts with different ratios were prepared, as detailed in

Table 5. Novel catalysts with different active components.

Active Ingredients	Proportional
Mn-Fe	1:1 wt%; 2:1 wt%; 3:1 wt%; 2:2 wt%; 3:3 wt%
Mn-Cu	1:1 wt%; 2:1 wt%; 3:1 wt%; 2:2 wt%; 3:3 wt%
Mn-Fe-Cu	1:1:1 wt%; 2:2:1 wt%; 3:2:1 wt%

. The experimental results (Section 2.2) indicated that the catalysts Mn-Fe (2:1 wt%) and Mn-Fe (2:2 wt%) exhibited the most effective catalytic performance, taking into account considerations such as cost effectiveness and catalyst stability [30,31,32]. Consequently, only these two catalysts were utilized for catalyst characterization.

3.3. Characterization of the Catalysts

A Brunauer–Emmett–Teller (BET) automatic surface area and porosity analyzer (Mac, ASAP2460, Micromeritics, Norcross, GA, USA) was used for BET microstructural analysis, and N₂ adsorption was used to determine the surface area and pore structure of the materials. The elemental composition and valence states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) on a Thermo K–alpha spectrometer (Norristown, PA, USA) with monochromated Al Kα radiation and a dual X-ray source. The baseline calibration data were obtained using C_1s (284.60 eV). X-ray diffraction analysis (XRD) was conducted using a Bruker D8 ADVANCE instrument (Billerica, MA, USA). Additionally, scanning electron microscopy(SEM; Gemini 300, Zeiss, Oberkochen, Germany) was utilized to characterize the morphology and microstructure of the samples.

3.4. Heterogeneous Fenton-like Experiment

Water samples (200 mL) were adjusted to the required pH before being poured into a 500 mL cylinder with an aeration device at the bottom for continuous aeration and stirring during the reaction. The water samples had a COD concentration of 35 ± 3 mg/L. An appropriate amount of H₂O₂ solution was added, followed by the introduction of a specific quantity of catalyst to initiate the reaction. The entire reaction took place at room temperature, following the parameters described in Section 2.

Immediately after the designated time elapsed, 50 mL of supernatant was extracted and centrifuged at 2000 r/min. The sample underwent wet digestion, and COD was measured using the potassium dichromate method [44,45,46]. To analyze the variations in organic pollutants in the water samples before and after the reaction, a gas chromatography–mass spectrometry (GC–MS) instrument (Agilent, 7890A GC-5975C MS, Santa Clara, CA, USA) was used to characterize the composition of the water samples.

4. Conclusions

A simple impregnation method for constructing heterogeneous Fenton-like catalysts is herein proposed. The results demonstrated that this catalyst could effectively remove COD from Maotai-flavored Baijiu wastewater and exhibited good stability for reuse. The reaction primarily depended on oxidation by hydroxyl radicals and was consistent with a third-order kinetic equation. The active components, Mn and Fe, loaded on the catalyst surface significantly enhanced the effectiveness of the heterogeneous Fenton-like reaction. They formed a synergistic catalytic dual-active center structure. These findings are highly significant for the practical application of heterogeneous Fenton-like technology, highlighting its potential in the development of advanced oxidation processes based on heterogeneous Fenton-like systems. This study represents an initial investigation of this reaction. Further optimization experiments are necessary for practical applications, such as reducing the catalyst dosage, indicating that there is some distance to go before practical implementation. Following the completion of the study, bio-toxicity testing was not conducted on the water samples.