The Behavior of Divalent Metals in Double-Layered Hydroxides as a Fenton Bimetallic Catalyst for Dye Decoloration: Kinetics and Experimental Design

Abstract

This study investigates the influence of divalent metals—(Mg(II), Co(II), and Ni(II)) in layered double hydroxides (LDHs), with a constant trivalent Fe(III) component—on the decoloration of crystal violet and methyl blue dyes via a Fenton-type oxidation reaction. The catalysts, synthesized by co-precipitation and hydrothermal treatment, were tested in both hydroxide and oxide forms under varying agitation conditions (0 and 280 rpm). A 2² × 3 factorial design was used to analyze the effect of the divalent metal type, catalyst phase, and stirring. The Mg/Fe oxide, with the highest BET surface area (144 m²/g) and crystallite size (59.7 nm), showed superior performance—achieving up to 98% decoloration of crystal violet and 97% of methyl blue within 1 h. The kinetic analysis revealed pseudo-second-order and pseudo-first-order fits for crystal violet and methyl blue, respectively. These findings suggest that LDH-based catalysts provide a fast, low-cost, and effective option for dye removal in aqueous systems.

1. Introduction

Despite current scientific efforts to develop and implement technologies to remove organic species from water bodies, such as pharmaceuticals [1] and dyes [2], the problem persists and affects diverse ecosystems. It is estimated that dye pollution is one of the most persistent problems due to the structural complexity of the different dyes, as well as the vast variety of these compounds. Dyes directly affect photosynthesis in plants and aquatic life, as they negatively interfere with the passage of light in water bodies, leading to a decrease in oxygen consumption [3]. This form of pollution is closely associated with the textile industry, which is responsible for approximately 10,000 tons of the 70 million tons of synthetic dyes produced globally [4]. Synthetic dyes are well known for their complex chemical structures, which make them highly resistant to degradation under a wide range of environmental conditions, including high temperatures, variable pH, and exposure to other chemical agents [5]. For this reason, these compounds often persist in aquatic environments, posing serious risks to ecosystems and requiring advanced treatment methods to ensure their removal.

The two dyes of the greatest industrial interest are crystal violet (Figure 1) and methyl blue (Figure 2). Crystal violet (CV) is widely used in the textile industry. Its presence in the environment has been reported to cause toxic effects in humans, including increased heart rate, unwell feelings, yellowing of the skin and eyes, and in severe cases, it can induce respiratory and renal failure [4,6].

On the other hand, methyl blue is a dye that has gained popularity and is also used in the textile industry. An example of the demand for this dye is that to produce 1 kg of pigmented cotton, approximately 200 L of methyl blue solution is required, and it is estimated that between 10% and 50% is discharged by industries as effluent [7]. Another application of this dye is in generating a neutral dye for medical purposes by combining it with a cationic dye. Once the dyes, methyl blue (anionic) and the cationic dye, are mixed, the solution is used to stain the cytoplasm and extracellular substances such as mucin, extracellular matrix, and collagen [8].

A remarkable aspect of methyl blue, whose chemical structure is complex, is that there are few studies on removing it.

Various methodologies have been employed to address the problem of dye contamination, including physical, chemical, and biological processes. Examples of these methods include removal via microbial community-based sludge [9], dye adsorption using materials with adsorbent capacities [10] photocatalysis [11], and advanced oxidation processes (AOPs) [12].

Advanced oxidation processes (AOPs) have shown promising results for dye decoloration without the need to employ specific radiation or specialized reactors, as is the case in photocatalysis [12]. Therefore, optimizing an advanced oxidation process, such as the Fenton reaction, is of particular interest. This reaction is usually catalyzed by Fe²⁺ and Fe³⁺ species, which participate in a cycle of valence state changes by reacting with hydrogen peroxide, generating the hydroxyl radical (●OH) or hydroxyl ion (-OH) (Equation (1)). The activated ●OH species is highly oxidizing (2.8 eV) and favors the decoloration of organic compounds [13]. This radical reacts with pollutants by interacting with the hydrogen atoms present in their molecules, thus initiating their decoloration (Equation (2)). In addition, the hydroxyl group can react and incorporate into the contaminant molecule, degrading it to a less harmful species or to simple products such as CO₂ and H₂O [14] (Equation (3)).

Fe(II) + H₂O₂ → Fe(III) + OH− + •OH

(1)

RH + •OH → H₂O + R• → degraded products

(2)

R(organic contaminants) + •OH → •ROH → degraded products

(3)

Although the Fenton reaction has been studied for several decades, it remains a viable option for addressing pollution from organic species in water bodies due to its simplicity and ease of application, which only requires a transition metal such as Fe. However, it is also possible to employ other metals, such as Co²⁺/Co³⁺, Ni²⁺/Ni³⁺, and Cu⁺/Cu²⁺. This reaction with transition metals other than Fe is referred to as a Fenton-type reaction [15]. This type of reaction is of particular interest, since it allows the application of the advanced oxidation process in a wider pH range compared to the traditional Fenton process, which tends to form iron hydroxides at pH values below 4, limiting the oxidation process when catalyzed exclusively by the iron species [16]. A comparative overview of various Fenton-based processes, including that employed in this study, is presented in

Table 1. A comparative overview of Fenton and Fenton-like oxidation processes.

Process	OH Radical Source	Optimal pH	Requires Light	H2O2 Generation	Operating Cost	Material/ Catalyst	Ref
Classical Fenton	H2O2 + Fe2+	~3	No	External	Low	Fe3O4 (magnetite)	[17]
Fenton-like	H2O2 + transition metal	3–6	No	External	Low– medium	γ-Cu2(OH)3Cl/ Cu/Al-LDH	[18]
Photo- Fenton	H2O2 + Fe2+ + light	~2.8–3.5	Yes (UV/solar)	External	Medium	MnCuFe-LDH	[19]
Electro-Fenton	O2 (cathode) + Fe2+	~3	No	In situ	Medium–high	CoFe LDH/carbon felt (CF) cathodes	[20]

. The process developed here is advantageous because it does not require light irradiation, specialized equipment, or high energy input, making it more practical and energy-efficient than other alternatives.

Therefore, there is a need to implement different systems based on transition metals as catalysts for this type of reaction. Among the most promising materials are layered double hydroxides (LDHs), which are sheet-structured nanomaterials formed by a pair of metals—one divalent (M²⁺) and one trivalent (M³⁺), in a specific molar ratio (x)—according to the general formula [M(1 − x)²⁺ M_X³⁺(OH)₂] [Aⁿ-]_X/n − yH₂O. Both metal cations are coordinated in octahedral structures surrounded by hydroxyl (OH−) groups, forming positively charged sheets that require interlayered anions (Aⁿ-) to maintain structural stability. This interlayered region also accommodates water molecules (y) [21].

These nanomaterials have been successfully used to remove various contaminants present in water, including metal ions [22], oxyanions [23], dyes [24], and pharmaceuticals [25]. Also, LDHs have shown utility as catalysts and photocatalysts in different reactions. Notable examples include their use as a support for carbon nanotubes in the decolorization of sulfamethoxazole [26]; in the oxygen evolution reaction (OER), where an LDH-NiFe with a specific morphology act as a catalyst [27]; and in the production of biohydrogen, using LDH-MgAl modified with humic acid as a catalyst [28].

These materials, which contain two different metal ions, can exhibit enhanced catalytic activity due to synergistic effects, particularly when both are transition metals, as in Co/Fe or Ni/Fe systems. This synergism has been studied by several researchers, showing improved performance under specific conditions [14].

In this context, in the present research work, three-layered double hydroxides (LDH) were synthesized, incorporating different divalent metals in their structure: Mg/Fe, Co/Fe, and Ni/Fe, all with the same molar ratio. These nanomaterials were characterized by X-ray diffraction (XRD), nitrogen physisorption (N₂), and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS). Subsequently, their performance in the decoloration of two model dyes, crystal violet and methyl blue, was evaluated using Fenton and traditional Fenton-type reactions. For this study, a 2² × 3 factorial experiment was designed to analyze the effect of three variables: the type of divalent metal, the form of the catalyst (hydroxide or its corresponding mixed oxide), and the agitation condition during the reaction.

2. Results and Discussion

2.1. Material Characterization

2.1.1. X-Ray Diffraction (XRD)

The diffractograms of the synthesized layered double hydroxides (LDHs)—Mg/Fe, Co/Fe, and Ni/Fe—are shown in Figure 3. In all cases, a characteristic diffraction pattern of layered double hydroxides was observed, which is in agreement with previous reports [29].

Although the materials obtained with different divalent ions present slight variations in the 2θ values (see

Table 2. The diffraction data of dry LDH.

	Mg/Fe	Co/Fe	Ni/Fe
Peak No.		Pos. [°2θ]
1	11.54	11.82	11.62
2	23.06	23.55	23.17
3	34.19	34.36	34.66
4	38.39	38.93	39.13
5	45.56	46.57	46.93
6	59.63	59.52	60.16
7	60.94	60.85	61.40
8	64.77	64.80	65.26
9	73.22	70.17	71.16

), they all show the same number of diffraction peaks and a very similar pattern. This indicates that regardless of the divalent cation incorporated, the combinations studied maintained a well-defined layered structure.

Upon subjecting the materials to the heating treatment, the mixed oxides obtained from the layered double hydroxides showed significantly different diffraction patterns, as presented in Figure 4 and

Table 3. The diffraction data of heated LDH.

	MgFeO	CoFeO	NiFeO
Peak No.		Pos. [°2 θ]
1	36.49	18.67	18.50
2	43.24	30.63	30.54
3	62.37	35.88	35.81
4		36.63	37.37
5		42.68	43.46
6		57.54	53.89
7		61.76	57.55
8		63.16	63.01
9		64.56	75.47
10		74.00

. In the case of the Mg/Fe material, only two main peaks were observed at 2θ positions of 43° and 62.5°, which were attributed to the formation of the MgO compound [30].

On the other hand, transition metal-based materials Co and Ni exhibited a larger number of diffraction signals than Mg/Fe, and their patterns differed from each other. The Co/Fe material showed ten diffraction peaks, which were assigned to the presence of two crystalline-phase spinels—Co₃O₄ (37%) [31] and the compound CoFe₂O₄ (63%) [32]. In the case of the Ni/Fe material, nine diffraction peaks were identified, which were associated with the formation of a mixture of oxides similar to that of the Co/Fe system, specifically NiO (76.4%) and NiFe₂O₄ (23.6%) [33].

Table 4 presents the average crystallite sizes, which were calculated using the Scherrer equation (Equation (4)). The results showed a change in the average crystallite size, with an increase of approximately 10 nm for Mg- and Ni-derived oxides compared to the materials in their hydroxide form. In contrast, for Co oxide, a decrease in crystallite size was observed, approximately half the size of the material in its hydroxide form. Notably, the Co-based material also presented a diffraction pattern with the lowest intensity in its peaks, which could be related to its smaller crystallite size and a potentially greater dispersion or lower crystalline order.

$$D={\displaystyle \frac{K\lambda }{\beta \cos \theta }}$$

(4)

where

• D = crystallite size (nm);
• K = 0.9 Scherrer constant;
• λ = 0.15406 nm (Cu2α);
• Β = FWHM;
• θ = diffraction peak angle.

Table 4. The average crystallite size.

Material	Average Crystallite Size [nm]
Mg/Fe	48.47
MgFeO	59.70
Co/Fe	52.97
CoFeO	27.27
Ni/Fe	30.90
NiFeO	39.79

Table 4. The average crystallite size.

Material	Average Crystallite Size [nm]
Mg/Fe	48.47
MgFeO	59.70
Co/Fe	52.97
CoFeO	27.27
Ni/Fe	30.90
NiFeO	39.79

2.1.2. Morphological and Elemental Characterization (SEM-EDS)

The comparative micrographs of the materials synthesized in hydroxide and oxide forms are shown in Figure 5, Figure 6 and Figure 7, where morphological changes in the materials when heated to generate their respective oxides can be observed. In the hydroxide form, agglomerated lamellae were observed in three cases (Mg, Co, and Ni). However, in the derived oxides, this layered structure disappeared, giving rise to a more irregular morphology without a repeated or defined shape. In addition, the identification of each divalent metal used in the materials via EDS analysis is presented (see

Table 5. The EDS composition of double-layered oxide materials.

Sample	%wt
	C	O	Mg	Co	Ni	Fe
Mg/Fe	4.28	52.44	16.84	-	-	16.84
Mg/FeO	4.81	38.04	22.38	-	-	34.77
Co/Fe	12.83	39.20	-	36.53	-	11.45
Co/FeO	9.26	23.56	-	53.02	-	16.03
Ni/Fe	7.77	43.49	-	-	36.29	12.45
Ni/FeO	4.74	22.21	-	-	55.81	17.25

).

2.1.3. BET Surface Area and Textural Analysis of Catalyst Precursors

Figure 8 shows the adsorption–desorption isotherms of N₂, in which the type III isotherm shape was identified according to the IUPAC classification [34]. All materials show an H3-type hysteresis loop, indicating capillary condensation during the process.

Table 6. The textural properties of the studied materials.

Material	BET Surface Area [m2/g]	Pore Volume [cm3/g]
Mg/Fe	35.1	0.27
MgFeO	144	0.53
Co/Fe	47.6	0.51
CoFeO	22.8	0.17
Ni/Fe	89.4	0.41
NiFeO	33.6	0.34

summarizes the BET area and pore volume values obtained. In this study, materials in oxide form containing two different types of oxides, such as the spinel phase (CoFe₂O₄ and NiFe₂O₄) and the respective oxides of divalent metal (Co₃O₄ and NiO), presented a decrease in surface area and pore volume. This behavior supports what has been reported in the literature, which suggests that the sintering of the oxides to form the spinel phase causes such a reduction [35]. However, the Mg-based material exhibited different behaviors. The heated derivative increased both the pore area and pore volume. This material did not present the spinel phase (MgFe₂O₄), which influences the surface area of the obtained material, a phenomenon that has also been reported in previous studies.

2.2. Decoloration of Crystal Violet and Methyl Blue Using LDH-Based Catalysts

The catalytic activity of the synthesized LDH-based materials was evaluated through the decoloration of two model dyes: crystal violet (CV) and methyl blue (MB). Figure 9 presents the UV–Vis spectra of CV decoloration over time. Among all tested systems, the Mg/Fe catalysts—particularly in their oxide form—showed the most effective dye removal, as evidenced by an apparent reduction in the absorption band associated with CV. In contrast, Co/Fe and Ni/Fe materials exhibited lower activity, although the oxide forms performed better than their hydroxide counterparts in most cases.

The decoloration percentage achieved after 60 min is summarized in Figure 10. The Mg/Fe oxide catalyst reached up to 98% CV removal, while the hydroxide form achieved 93% (Table 6). Agitation enhanced decoloration efficiency, particularly during the first 30 min of the reaction. For MB, similar trends were observed (Figure 11 and

Table 7. A summary of the results of the decoloration of crystal violet using the studied materials.

			% Decoloration
			Without Stirring
t [min]	Mg/Fe	Co/Fe	Ni/Fe	MgFeO	CoFeO	NiFeO
10	8.43	6.87	1.31	61.24	6.49	27.84
20	31.75	16.01	5.45	94.93	10.45	45.42
30	58.28	30.97	12.16	95.55	17.96	54.80
40	77.95	41.83	18.69	95.72	26.57	62.11
50	87.91	53.51	25.45	95.77	32.61	66.82
60	92.50	62.24	31.64	95.90	39.20	69.90
			With Stirring (280 rpm)
t [min]	Mg/Fe	Co/Fe	Ni/Fe	MgFeO	CoFeO	NiFeO
10	65.15	18.56	15.07	94.18	15.07	30.82
20	89.03	52.49	23.13	95.42	20.17	49.50
30	91.31	70.72	33.84	96.84	24.85	61.47
40	91.83	79.15	45.45	97.41	32.54	70.07
50	92.05	82.89	55.67	98.18	42.69	75.37
60	93.10	86.67	64.63	98.21	49.95	78.73

): Mg/Fe and Co/Fe catalysts were more effective than Ni/Fe, with the oxide forms again showing slightly higher performance. In the case of Ni/Fe, heating treatment and agitation produced a more noticeable improvement, increasing decoloration by approximately 10%. All catalysts were evaluated both with and without stirring, as shown in Table 7 and

Table 8. A summary of the results of the decoloration of methyl blue using the studied materials.

			Decoloration (%)
			Without Stirring
t [min]	Mg/Fe	Co/Fe	Ni/Fe	MgFeO	CoFeO	NiFeO
10	21.92	36.05	12.50	87.14	49.18	12.86
20	62.32	44.57	23.19	92.93	58.42	26.81
30	75.54	51.09	38.22	96.56	67.75	50.36
40	86.41	79.89	51.27	96.56	78.62	54.35
50	88.77	91.67	58.70	96.56	83.51	58.15
60	90.22	94.57	67.93	96.56	88.04	66.49
			With Stirring (280 rpm)
t [min]	Mg/Fe	Co/Fe	Ni/Fe	MgFeO	CoFeO	NiFeO
10	85.87	64.67	53.08	96.56	50.00	28.44
20	88.95	76.63	66.85	97.46	61.78	51.81
30	90.76	82.07	73.19	97.64	73.73	59.96
40	93.66	87.68	78.08	97.64	74.46	64.31
50	94.02	91.49	80.80	97.64	83.88	66.12
60	95.47	94.02	83.88	97.64	90.76	70.29

.

The decoloration efficiency was calculated using Equation (5):

$$Decoloration efficiency (\%)={\displaystyle \frac{A_0-A_t}{A_0}}\ast 100$$

(5)

• $A_0=\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \mathrm{a}\mathrm{t} {\lambda }_{max}$;
• $A_t=\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \mathrm{a}\mathrm{t} \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e} t \mathrm{m}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{d} \mathrm{a}\mathrm{t} {\lambda }_{max}$.

A more detailed comparison reveals that while Mg/Fe and Ni/Fe catalysts improved upon heating treatment, Co/Fe behaved differently. The Co/Fe hydroxide form outperformed its oxide counterpart by 36%, suggesting stronger Fenton-like activity in the hydroxide state. Agitation consistently improved dye removal for all materials, particularly those containing transition metals.

These results highlight that Mg/Fe systems are highly effective in promoting Fenton-like oxidation, especially in their heated form. While Mg does not participate directly in redox cycling, it has been proposed to facilitate H₂O₂ formation through water oxidation, as shown in Equation (6) [36]:

Mg + 2O₂ + 2H⁺ → H₂O₂ + Mg²⁺

(6)

In contrast, Co/Fe and Ni/Fe systems may provide dual activation through both Fe and the transition metal. However, the presence of two redox-active metals may also introduce competition or interference, potentially reducing overall efficiency and increasing the reaction time. Although the exact reaction pathway was not determined in this study, the results suggest that the Mg/Fe system facilitates a more direct and efficient oxidation process.

2.3. Decoloration Kinetics of Crystal Violet and Methyl Blue

The kinetic behavior of the Fenton-type decoloration reaction was evaluated for each catalyst using pseudo-first-order and pseudo-second-order models.

Table 9. Pseudo-first- and pseudo-second-order models.

Kinetic Model	Equation	Lineal Adjusted
Pseudo-First Order	$C_t=C_0{e}^{-k_{1}t}$	$ln({\displaystyle \raisebox{1ex}{$C_t$}\!\left/ \!\raisebox{-1ex}{$C_0$}\right.})$ vs. t
Pseudo-Second Order	${\displaystyle \frac{1}{C_t}}-{\displaystyle \frac{1}{C_0}}=k_{2}t$	${\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$C_t$}\right.}$ vs. t

summarizes the linearized forms of both kinetic equations, from which the rate constants k₁ and k₂ (min⁻¹) were determined as the slope of the corresponding plots. This approach follows the methodology outlined in [37].

The kinetic constants for the decoloration of crystal violet and methyl blue, summarized in

Table 10. A calculated summary of kinetic constants and the reaction order.

		Crystal Violet
	PFO		PSO
Material	k1	R2	k2	R2
Mg/Fe	0.0458	0.7623	0.2275	0.8557
MgFeO	0.0253	0.9678	0.6395	0.9631
Co/Fe	0.0356	0.9632	0.0973	0.9940
CoFeO	0.0173	0.9707	0.0291	0.9972
Ni/Fe	0.0178	0.9837	0.0244	0.9425
NiFeO	0.0107	0.9620	0.0122	0.9344
		Methyl Blue
	PFO		PSO
Material	k1	R2	k2	R2
Mg/Fe	0.0226	0.9811	1.5993	0.9630
MgFeO	0.0190	0.8898	3.6437	0.9091
Co/Fe	0.0351	0.9980	1.4662	0.9339
CoFeO	0.0316	0.9514	0.8538	0.8215
Ni/Fe	0.0205	0.9736	0.4311	0.9975
NiFeO	0.0159	0.8928	0.1984	0.9558

, reveal distinct trends depending on the type of divalent metal used. For crystal violet, the highest rate constants were obtained with Mg-based catalysts, which correlates with the greater decrease in absorbance observed in the UV-Vis spectra. These were followed by Co-based materials and, lastly, Ni-based catalysts. Across all cases, higher rate constants were observed for the hydroxide forms than their heated counterparts, regardless of the kinetic model applied.

Regarding kinetic model fitting, the pseudo-second-order model provided the best correlation (R²) for Mg- and Co-based catalysts, while the Ni-based material was better described by the pseudo-first-order model (Figure 12). This suggests that chemical adsorption or surface interaction might dominate in the former, whereas diffusion or physisorption may play a more significant role in the latter.

In the case of methyl blue, an inverse behavior was observed. For all materials, the pseudo-first-order model offered a better fit. The rate constants again followed the trend Mg > Co > Ni, with hydroxide forms performing better than oxides (Figure 13). These results are consistent with the pattern observed for crystal violet, indicating that the surface chemistry of the hydroxide phase contributes positively to catalytic efficiency.

This behavior is consistent with heterogeneous Fenton-like reactions, where the active sites on the catalyst contribute both to oxidant activation and subsequent degradation of the dye molecules.

While complete mineralization was not evaluated in this study, and the reaction pathway was not elucidated in detail, the generation of hydroxyl radicals (·OH) due to H₂O₂ activation by the Co, Ni, and Fe sites is presumed to play a central role in the decoloration mechanism (Figure 14). Therefore, the overall process can be interpreted as a surface-catalyzed radical reaction, consistent with previous literature on non-iron-based Fenton-like systems [38,39].

2.4. Experimental Design Analysis

Statistically significant models were obtained in the experimental design of crystal violet and methyl blue dyes. For crystal violet, after 10 min of treatment, a p-value of 0.026 was obtained with an R² of 0.9941, indicating that the studied factors adequately explained the variability in response regarding the decoloration percentage.

% Decoloration = (p = 0.011) Metal, (p = 0.022) Form, (p = 0.020) Stirring.

For the model generated for methyl blue, data collected after 20 min of treatment were used. The p-value (p = 0.009) and R² of 0.9979 indicated that the model was suitable for explaining the response in methyl blue decoloration.

% Decoloration = (p = 0.003) Metal, (p = 0.004) Stirring.

The generated models are expressed as follows:

$$Y={\beta }_0+{\beta }_1{X}_1+{\beta }_2{X}_2+\cdots +{\beta }_k{X}_k+\epsilon$$

For crystal violet, a model was obtained in which two-way interactions, such as Metal*Form and Metal*Stirring interactions, played an important role in the response. This indicates that the heating process had a significant impact on the decoloration of crystal violet.

Decoloration (%) = 29.25 + 28*Mg − 17.51*Co − 10.02*OH − 10.56*without stirring − 10.44*Mg*Mg-OH + 10.99*Co*Co-OH − 11.86*Mg*without stirring

For methyl blue, a simpler model was obtained than that of crystal violet, in which the Metal*for interaction was shown to be equally important for both dyes.

Decoloration (%) = 62.563 + 22.77*Mg − 11.27*without stirring − 7.56*Mg-OH − 5.788*OH-without stirring

3. Materials and Methods

3.1. Material Synthesis

The layered double hydroxides Mg/Fe, Co/Fe, and Ni/Fe were synthesized by a co-precipitation method assisted by hydrothermal treatment, maintaining an M(II):M(III) molar ratio of 3:1. In a general procedure, stoichiometric amounts of Mg(NO₃)₂.6H₂O (Sigma-Aldrich, St. Louis, MO, USA, 98%), Co(NO₃)₂.6H₂O (Meyer, Mexico City, Mexico, 98%), or Ni(NO₃)₂.6H₂O (Meyer, Mexico City, Mexico, 98%) were dissolved together with Fe(NO₃)₃-9H₂O (Meyer, Mexico City, Mexico, 98%) in 100 mL of deionized water. This precursor solution was added dropwise to an alkaline solution containing the appropriate amounts of NaOH (Macron, Fine Chemicals, Center Valley, PA, USA, 97%) and Na₂CO₃ (Mallinckrodt, St. Louis, MO, USA, 100%) in order to induce LDH co-precipitation.

Subsequently, the pH of the mixture was adjusted to 10 by adding the required volume of 3 M NaOH solution. The mixture was then transferred to a Teflon beaker with a screw cap and subjected to hydrothermal treatment at 80 °C for 24 h. The resulting solid was recovered by filtration, washed with distilled water, and oven-dried at 100 °C overnight.

To obtain the corresponding mixed oxides, LDH Mg/Fe, Co/Fe, and Ni/Fe were heated at 600 °C for 6 h using a heating ramp of 2 °C/min in a nitrogen atmosphere (N₂).

3.2. Characterization of Materials

The hydroxide and oxide forms of the materials were characterized by X-ray diffraction (D2 Phaser, Bruker, Billerica, MA, USA), SEM-EDS (SUPRA 55 VP, Zeiss with a secondary electron detector, Oberkochen, Germany), and N₂ Physisorption (Micromeritics ASAP 2020, Norcross, GA, USA).

3.3. Catalytic Evaluation

The catalytic activities of the synthesized materials were evaluated by decoloration assays using two model dyes: crystal violet (CV) and methyl blue (MB). For each experiment, individual 50 mL solutions were prepared with an initial concentration of 10 mg/L of the corresponding dye. To each solution, 1 mL of hydrogen peroxide (H₂O₂, J.T. Baker, 30%, Phillipsburg, NJ, USA) was added as an oxidizing agent, and 10 mg of the catalyst, either in its hydroxide or heated (mixed oxide) form, was incorporated.

The reactions were performed under two conditions: with stirring (280 rpm) and without stirring, for a total time of 60 min. The process was monitored by taking reaction aliquots every 10 min. The collected samples were centrifuged for 40 s to separate the solids and analyzed using UV-Vis spectrophotometry (Genesys 10S Vis, Thermo Scientific, Waltham, MA, USA) with the wavelengths of maximum absorption corresponding to each dye: 583 nm for crystal violet and 589 nm for methyl blue.

All experiments were conducted in triplicate, and the average values were used for analysis and graphical representation.

3.4. Experiment Design

To evaluate the effect of different variables on catalytic performance in the decoloration of crystal violet (CV) and methyl blue (MB) dyes, a general factorial experiment was designed (the variables are summarized in

Table 11. The factors and levels involved in the experimental design.

Factor	Level	Values (−1)	(+1)	(0)
Metal (II)	3	Mg	Co	Ni
Form	2	Hydroxide	Oxide
Stirring (rpm)	2	0	280

). The design considers a total of 12 experimental runs per dye, considering combinations of the following factors: the type of divalent metal in the LDH, the catalyst form (hydroxide or oxide), and the agitation conditions (present or absent).

The response variable was the percentage of decoloration, evaluated at specific times for each dye. For crystal violet, the percentage of decoloration at 10 min of treatment was taken as a reference. In contrast, for methyl blue, the value at 20 min was considered, and times were selected based on the kinetics observed experimentally.

The factorial experimental design and statistical analysis were generated using MINITAB^® Statistical Software (version 21, Minitab LLC, State College, PA, USA).

4. Conclusions

In this study, three-layered double hydroxides containing Mg, Co, and Ni were successfully synthesized and characterized in both hydroxide and oxide forms. All the hydroxide materials exhibited very similar X-ray diffraction patterns, with slight shifts in peak positions. This indicates that the synthesis method produced materials with the same crystalline structures despite the variation in the divalent metal. All the materials had crystallite sizes below 100 nm, confirming the formation of lamellar nanomaterials. BET surface area analysis showed similar values for the hydroxide forms; however, for the corresponding oxides, an increase in surface area was observed when the material contained only one type of oxide. The heated Mg/Fe material exhibited the highest surface area (144 m²/g) as determined using BET analysis. In contrast, the Co and Ni materials, which contained two types of oxides, showed a decrease in both surface area and pore volume.

When these materials were evaluated as catalysts for decoloring crystal violet and methyl blue via the Fenton reaction, the results showed that all materials achieved acceptable decoloration values within one hour of treatment. In the best-case scenario, up to 98% decoloration was achieved for both dyes using the Mg-based material in its oxide form. In contrast, at least 60% decoloration was obtained with the Ni-based material in both of the studied forms.

For both materials, it can be concluded that stirring at 280 rpm and heating enhance dye decoloration, accelerating the reaction.

In the kinetic study, no clear trend was observed between the divalent metal and the associated reaction order. However, the rate constant was generally higher for the Mg-based material, followed by Co and Ni.

The experimental design showed that for treatment times of 10 min for crystal violet and 20 min for methyl blue, the generated models exhibited a good fit, with R² values of 0.9941 and 0.9979 for crystal violet and methyl blue, respectively. Both models had p-values indicating that they were suitable for describing changes in the dye decoloration percentage. The models showed that the studied factors—metal, form, and agitation—were not equally significant based on their associated p-values. While all three factors were significant for crystal violet, the catalyst form was not significant for methyl blue, resulting in a simpler model for the latter dye.