RSM-BBD Optimization of Fenton-Like Degradation of 4-Nitrophenol Using Magnetite Impregnated Kaolin

Abstract

In this work, we have reported a low-cost and environmentally friendly Fe₃O₄-modified activated kaolin (AK-Fe₃O₄) composite for efficient Fenton-like degradation of 4-nitrophenol (4-NP) and optimization of the degradation variables. The AK-Fe₃O₄ composites were characterized by Fourier transform infrared spectroscopy, powder x-ray diffraction, scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). X-ray diffraction confirms the syntheses of pure phases of Fe₃O₄ and AK-Fe₃O₄. The SEM image of the AK-Fe₃O₄ composite reveals the formation of a highly porous surface. The room temperature VSM analysis describes the superparamagnetic nature of AK-Fe₃O₄ composites with 25 emu/g magnetization values. Response surface methodology coupled with Box-Behnken design was used to optimize the 4-NP degradation (%) variables such as contact time (10-90 minutes), 4-NP concentration (10-30 mg/L), and pH (3-8). The high regression value (R² = 0.9964 and adjusted R² = 0.9917) and analysis of variance (P < .0001) show that the quadratic model can sufficiently explain the 4-NP degradation (%). The optimum 4-NP degradation was found to be 96.01% ± 1.89% using 1 mg/mL of AK-Fe₃O₄, 20 mg/L of 4-NP, 97.9 mmol/L of H₂O₂, and pH of 3 at the end of 75 minutes of reaction time. Moreover, the catalyst shows good recyclability and stability after 5 successive degradations of 4-NP. In general, a low-cost and magnetically separable AK-Fe₃O₄ composite is an effective Fenton-like catalyst for the degradation of 4-NP.

Keywords

Kaolin 4-nitrophenol Fenton-like degradation ANOVA

Introduction

The unwarranted release of toxic effluents from domestic sewage, agricultural wastewater, and industrial wastes adversely contaminates the water bodies. Currently, wastewater discharge from industries such as textile, leather, cosmetics, plastics, pharmaceuticals, pesticides, food processing, and papers composed of numerous organic and inorganic water contaminants shows antagonistic effects in the water quality.^1,2 More importantly, colored organic compounds discharged into water bodies prevent the penetration of sunlight which affects the photosynthesis process in the aquatic environment.³ Moreover, such organic compounds are also toxic to humans and animals. 4-nitrophenol (4-NP) is a nitroaromatic compound, widely used as a precursor for the preparation of pharmaceutics and pesticides. Conversely, 4-NP is one of the hazardous chemicals affecting both human and animal health.^4,5 The Agency for Toxic Substances and Disease Registry (ATSDR) of the United States designated 4-NP as one of the 114 organic compounds listed, which poses a great danger to living organisms in the environment.⁶ Hence, 4-NP needs to be removed from wastewater before being discharged into the environment. There are various physical, chemical, and biological treatments such as photocatalytic degradation,⁷ flocculation,⁸ ion exchange,⁹ membrane filtration,¹⁰ adsorption,¹¹ and Fenton-like degradation¹² that have been investigated to remove these contaminants from wastewater. 4-nitrophenol owing to its extreme stability is unable to be easily removed from aqueous solutions using conventional approaches. There is considerable interest in Fenton-like technology due to low cost, simple procedures, and mild operating conditions for the degradation of organic pollutants.¹³ Fenton-like is an advanced oxidation process (AOP) that has tremendous potential for the degradation of a broad range of organic compounds. The hydroxyl radical (HO^•) generated in AOPs has strong redox potential (2.80 eV) and can react with almost all stable organic compounds.¹⁴ The conventional Fenton process (Fe²⁺/H₂O₂) owing to working in an extremely acid solution (pH <4) and due to the formation of ferric hydroxide sludge during wastewater treatment is not suitable for practical applications.^15,16 Hence, an alternative Fenton-like catalyst substitute Fe²⁺/H₂O₂ is desirable to overcome the aforementioned problems.

Recently, heterogeneous Fenton-like catalysts have been reported as an alternative for the efficient degradation of recalcitrant organic compounds. Fenton catalysts such as CuFe₂O₄,¹⁷ α-FeOOH,¹⁸ Fe₂O₃/Al₂O₃,¹⁹ and Fe₃O₄²⁰ have been widely used in combination with H₂O₂ for the formation of reactive HO^• radicals. In particular, Fe₃O₄ has received considerable attention in water treatment due to its excellent chemical stability, biocompatibility, and high magnetic saturation value.²¹ However, Fe₃O₄ nanoparticles due to their strong magnetic dipole-dipole interaction and large surface area are vulnerable to irreversible agglomeration, which in turn decreases the efficiency. The irreversible agglomeration of Fe₃O₄ nanoparticles (NPs) can be prevented by immobilization on different supporting materials such as metal-organic framework, graphene oxide, reduced graphene oxide, amino acids, polymer, cellulose, chitosan, and clay minerals.^22,23 A low-cost and naturally abundant kaolin clay due to high surface porosity shows excellent efficiency for adsorption of various guest materials including inorganic metal oxides.²⁴ Besides, kaolin can be used as a supporting material for various metal oxides to enhance their catalytic performances. For example, Guo et al²⁵ have reported an enhanced photo-Fenton degradation of rhodamine B by kaolin-Fe₂O₃ than Fe₂O₃ NPs. Similarly, Giri et al²⁶ reported a high-surface-area kaolin-supported Fe₃O₄ adsorbent for the removal of water pollutants. Kaolin-ZnO NPs and kaolin-TiO₂ NPs are also reported as efficient photocatalysts for the degradation of organic compounds.^27,28 Ayodele and Hameed also reported kaolinite-supported ferric oxalate for photo-Fenton degradation of 4-NP.²⁹ But there is no report on use of kaolin-Fe₃O₄ for Fenton-like degradation of 4-NP and detailed process optimizations.

Moreover, the efficiency of Fenton-like AOPs is varied with the interaction of different process parameters, such as contact time, pH of the solution, catalyst dose, and concentration of the substrate. Therefore, optimization of process variables using a systematic approach which requires fewer experiments and shows the interaction of process variables is highly desirable for practical application. The conventional optimization approaches are time-consuming, require a large number of runs, and are not able to clearly show the interaction of process variables.³⁰ The response surface methodology (RSM) has been widely applied to optimize and investigate the interaction of different variables during the experiment.^31,32 The RSM can effectively optimize experimental variables using a small number of experiments within a short period of time.³⁰ The RSM based on Box-Behnken design (BBD) optimization is the most recommended design approach for a 3-level incomplete factorial design. The BBD requires fewer experiments and estimates the factors in a quadratic model, which contains squared terms, products of 2 factors, linear terms, and an intercept.³³

Herein, we have reported a detailed RSM-BBD optimization of the Fenton-like degradation of 4-NP using kaolin-Fe₃O₄ composites. One-pot synthesis of Fe₃O₄ NPs in kaolin was used for the preparation of the composite. Powder x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM) techniques were used to investigate phase, functional group, morphology, and magnetic properties of the composite, respectively. The Fenton-like degradation process was optimized using RSM-BBD at various contact time (minutes), pH, and 4-NP concentrations (mg/L).

Experiment

Materials

Raw kaolin clay was obtained from Hadiya, Ethiopia. Iron (III) chloride hexahydrate (FeCl₃·6H₂O), iron (II) sulfate heptahydrate (FeSO₄·7H₂O), concentrated sulfuric acid (H₂SO₄, 18.4 mol/L), hydrochloric acid (HCl, 12 mol/L), ammonia (NH₃, 13.4 mol/L), and hydrogen peroxide (H₂O₂, 9.79 mol/L) were purchased from Merck, India, and used as received. Double distilled water was used throughout the whole experimental process.

Acid activation of kaolin

Kaolin was activated by H₂SO₄ based on a protocol reported in previous work.^34,35 Particularly, the raw kaolin sample was purified by washing with distilled water 3 times followed by drying in an oven at 105°C for 3 hours. The dried sample was crushed to obtain a fine powder that was able to pass through a 63-μm mesh size sieve. The sieved sample was further washed with distilled water and the dispersed clay separated by centrifugation. The washing process was continued 4 times to obtain pure kaolin. The wetted samples were dried in an oven at 105°C for 3 hours. Then, the powdered kaolin (6 g) was mixed with 180 mL of sulfuric acid (0.5 mol/L) and refluxed for 12 hours at 70°C for activation. The slurry was washed with distilled water 3 times till the supernatant became neutral with no trace amount of sulfate ion present. The slurry was separated and dried in an oven at 105°C for 3 hours. The final powder product (<63 μm size) is designated as activated kaolin (AK).

Synthesis of magnetite nanoparticles

The facile coprecipitation technique was used to synthesize Fe₃O₄ NPs. In particular, aqueous solutions of ferric and ferrous salts were mixed together in a molar ratio of 2:1, respectively. The mixture was heated with continuous stirring under reflux setup, and then NH₃ was added drop by drop to adjust the pH of the solution to 10. The solution was heated at 70°C for 45 minutes. Finally, the product was washed 3 times with distilled water to remove excess ammonium and dried under vacuum.

Synthesis of AK-Fe₃O₄ composite

The magnetically separable AK-Fe₃O₄ composite was synthesized by in situ coprecipitation of Fe₃O₄ NPs onto AK. The solution of FeCl₃·6H₂O and FeSO₄·7H₂O in a 2:1 molar ratio, respectively, was added drop by drop to 100 mL of AK solution. The mass ratio of iron salts to AK was fixed at 2:1, 4:1, and 6:1, respectively. After 30 minutes of continuous stirring, the ammonia solution was added to adjust the pH to 10. Then, the solution was refluxed for 8 hours at 70°C. The dark brown precipitate was washed 3 times with distilled water and dried under vacuum.

Characterization techniques

The functional groups present on the synthesized materials were investigated by FTIR spectroscopy. The FTIR was recorded using the Perkin Elmer FT-IR (spectrum RX-I) spectrometer in the range 400 to 4000 cm⁻¹ by mixing the samples with spectroscopy-grade KBr in 1:2 mass ratio. X-ray diffraction patterns of samples were recorded using SHIMADZU XRD, excellent in the science of XRD (model number XRD-7000; Shimadzu, Kyoto, Japan), which was operated at 40 kV and 45 mA, with a Cu-Kα (λ = 1.54056 Å) radiation. Scanning electron microscopy studies of the morphology of AK-Fe₃O₄ were analyzed by microstructure analysis with a Nova NanoSEM analyzer operated at an accelerating voltage of 10 keV. The samples were degassed at 150°C under vacuum. The magnetic property AK-Fe₃O₄ was investigated using a VSM (Lakeshore 665; Lake Shore Cryotronics, Inc., Westerville, OH, USA) at room temperature. The pH of point of zero charge (pH_PZC) of AK-Fe₃O₄ was determined to know the surface charge of AK-magnetite nanoparticles. For this purpose, 50 mL of sodium chloride (NaCl) solution was placed in a 100-mL Erlenmeyer flask. The pH was then adjusted to successive initial values between 3 and 9 using either sodium hydroxide or hydrogen chloride (0.1 mol/L), and 0.15 g of adsorbent (AK-Fe₃O₄) was added to the solution. After a contact time of 24 hours, the final pH was measured and plotted against the final pH. The pH at which the curve crosses the line pH (final-initial) is taken as pH_PZC.

RSM-BBD optimization of the Fenton-like degradation variables

The Fenton-like degradation of 4-NP was carried out in a batch reactor at varying concentrations of 4-NP. Specifically, 50 mg of the Fenton catalyst was added to 50 mL of 4-NP solution (10-30 mg/L) and stirred continuously. After adding the oxidation catalyst, 0.5 mL of H₂O₂ (9.79 mol/L) was added to the solution, which is considered as the initial (time t = 0) for the reaction. The pH of the solution was adjusted using NaOH (0.1 mol/L) and HCl (0.1 mol/L). The degradation (%) of 4-NP was analyzed by measuring the absorbance at λ_max = 400 nm using an ultraviolet-visible spectroscopy. The RSM-BBD optimization of 4-NP degradation mainly focused on 3 independent variables: pH, contact time (minutes), and concentration of 4-NP (mg/L). These 3 variables are selected because catalysis processes are highly dependent on them. The other remaining variables including 0.5 mL of 9.79 mol/L H₂O₂ (97.9 mmol/L), 50 mg of catalyst dose in 50 mL solution (1 mg/mL), and room temperature (298 K) were fixed throughout the whole reaction process. Design Expert 11 software program was used to compute the statistical analysis. The experimental levels of factors used for RSM-BBD are given in Table 1.

Table 1.

The experimental levels of factors used for RSM-BBD.

Factors	Levels
Factors	Low (−1.000)	Center (0.000)	High (1.000)
A = x₁: time (min)	10	50	90
B = x₂: pH	3	5.5	8
C = x₃: 4-nitrophenol (mg/L)	10	20	30

Abbreviation: RSM-BBD, response surface methodology coupled with Box-Behnken design.

Results and Discussion

Characterizations

FITR analysis

Figure 1 shows the FTIR spectra of AK, Fe₃O₄ NPs, and AK-Fe₃O₄ composite. Activated kaolin (Figure 1A) shows 2 clearly identifiable bands at 3690 and 3620 cm⁻¹. The peak at 3690 cm⁻¹ is due to stretching vibrations of surface hydroxyl groups, whereas the other peak at 3620 cm⁻¹ is ascribed to the vibrations of inner hydroxyl groups.³⁶ The absorption bands observed at 3420 cm⁻¹ could be assigned to the OH vibrational mode of water. The band associated with Si-O deformation is detailed to be 1040 cm^-1.³⁷ The band at 796 cm⁻¹ is assigned to the Si-O-Si symmetric stretch.³⁸ Besides, a strong peak at 544 cm^-1 indicates the presence of Si-O-Al stretching vibrations.³⁹ Figure 1B shows the FTIR spectrum obtained from pure magnetite nanoparticles. The 2 intense strong peaks observed at 557 and 637 cm⁻¹ are attributed to the symmetric stretching vibration mode associated with Fe-O bond in the crystalline lattice of Fe₃O₄.⁴⁰ The FTIR spectrum of AK-Fe₃O₄ composite (Figure 1C) shows the peak due to both AK and Fe₃O₄ NPs, which implies the formation of the composite. But the intensity of inner and outer hydroxyl (OH) of kaolin becomes weak in the composite, indicating interruption of the kaolin structure.⁴¹ This weak intensity is due to the exchange of Al³⁺ of kaolin structure with Fe³⁺ from Fe₃O₄.⁴¹

Figure 1.

Fourier transform infrared spectra of (A) activated kaolin (AK), (B) Fe₃O₄, and (C) AK-Fe₃O₄ composites.

XRD analysis

The powder XRD analysis identified the unknown clay to be kaolin, and crystallite size and lattice cell parameters of Fe₃O₄ NPs, AK, and AK-Fe₃O₄ composites. The raw clay obtained from Hadiya, Ethiopia, was found to be kaolin. As shown in Figure 2A, raw kaolin is exactly matched with kaolinite Al₂H₄O₉Si₂ (COD: 00-900-9230).⁴² The raw kaolin after chemical activation did not show a significant change in the XRD pattern compared with the raw kaolin (Supplemental Figure 1). Figure 2B shows the formation of spinel-type Fe₃O₄ NPs with 14 nm average crystallite size (D) calculated using Scherrer formula (equation (1)).⁴³ The characteristic plans are observed at (220), (311), (400), (422), (511), (440), and (553), which are in agreement with the inverse cubic spinel structure of Fe₃O₄ (JCPDS card no. 85-1436, space group Fd3m). Figure 2C presents the XRD patterns of the AK-Fe₃O₄ composite. The characteristic peaks due to Fe₃O₄ and AK were observed concurrently in the AK-Fe₃O₄ composite. The analysis illustrates that inverse spinel Fe₃O₄ NPs were successfully precipitated with activated clay to form the AK-Fe₃O₄ composite.⁴⁴ Besides, the average crystallite size of Fe₃O₄ NPs was found to be 11 nm which is less than naked Fe₃O₄ NPs. Moreover, the intercalation of Fe₃O₄ into AK layer is plausibly investigated by calculating the interplanar distance (d) before and after Fe₃O₄ NP treatment. As shown in Supplemental Figure 2, 2θ and d values at (100) of AK are different before and after Fe₃O₄ NP treatment. The shift of diffraction angle from 2θ = 12.3776° to 2θ = 12.0862° and expanding of the d value from 7.14531 to 7.31692 Å are attributed to the intercalation of Fe₃O₄ to the AK layer:⁴⁵

D = \frac{0.9 λ}{β \cos θ}

(1)

where λ is the wavelength of the x-ray source (λ = 0.15418 nm), β is the peak width of the diffraction peak profile at half maximum height resulting from small crystallite size in radians, and θ is the Bragg angle.

Figure 2.

The powder x-ray diffraction patterns of (A) activated kaolin (AK), (B) Fe₃O₄, and (C) AK-Fe₃O₄ composite.

SEM, VSM, and pH_PZC analyses

The rough texture of the AK-Fe₃O₄ composite was investigated by the SEM image (Figure 3A). AK-Fe₃O₄ due to the formation of an infinite number of pores on the surface increases the surface area for adsorption of 4-NP. The magnetization value of naked Fe₃O₄ and AK-Fe₃O₄ was recorded at 300 K using VSM to investigate their magnetic nature. As shown in Figure 3B, both naked Fe₃O₄ and AK-Fe₃O₄ composites show superparamagnetic nature with zero remanence and coercivity. The saturation magnetization (MS) was found to be 57 and 25 emu/g for the naked Fe₃O₄ NPs and AK-Fe₃O₄ composite, respectively. In this sense, in the AK-Fe₃O₄ composite, the non-magnetic kaolin is responsible for weakening the magnetization value. Besides, those magnetization values are significantly lower than bulk Fe₃O₄ (MS ~82 emu/g).⁴⁶ The plot of ΔpH versus initial pH of the solution of AK-Fe₃O₄ is presented in Supplemental Figure 3. It is revealed that the AK-Fe₃O₄ composite shows a positive charge below pH_PZC = 6 and shows a negative charge as the pH increases.

Figure 3.

(A) Representative scanning electron microscopy image and (B) vibrating sample magnetometer curve of AK-Fe₃O₄ composite. AK indicates activated kaolin; NPs, nanoparticles.

Optimization of 4-NP degradation using RSM-BBD

During catalysis reaction, optimizations of the process variables using statistical approaches are crucial for the economic utilization of time and resources. Moreover, the statistical analysis gives a clear understanding of the reaction process and minimizes the experimental errors. Herein, the RSM-BBD approach was applied to optimize the Fenton-like degradation of 4-NP using AK-Fe₃O₄ composite. Box-Behnken design is a second-order design (equation (2)) based on 3-level factorial designs.⁴⁷ The complete BBD matrix for the 3 independent variables (contact time (A), pH (B), and 4-NP concentration (C)) and the response of both coded and real values are presented in Table 2:

Y = b_{0} + \sum_{i = 1}^{n} b_{i} X_{i} + \sum_{i = 1}^{n - 1} \sum_{j = i + 1}^{n} b_{i j} X_{i} X_{j}

(2)

where Y is the predicted response (degradation, %) of 4-NP; b is a constant; and b_i, b_ii, and b_ij refer to the linear, quadratic, and interaction coefficients, respectively. X_i and X_j are lists of the coded values of factors.

Table 2.

The RSM-BBD design matrix of the 3 variables with coded and real values, and response with actual and predicted values.

	Coded values			Real values			Response
	A	B	C	A	B	C	Actual	Predicted
1	0.000	0.000	0.000	50	5.5	20	81	81.60
2	0.000	0.000	0.000	50	5.5	20	81	81.60
3	0.000	1.000	−1.000	50	8	10	65	65.50
4	0.000	0.000	0.000	50	5.5	20	79	81.60
5	0.000	0.000	0.000	50	5.5	20	82	81.60
6	−1.000	0.000	−1.000	10	5.5	10	47	47.63
7	0.000	−1.000	1.000	50	3	30	74	73.50
8	0.000	0.000	0.000	50	5.5	20	85	81.60
9	−1.000	0.000	1.000	10	5.5	30	23	23.88
10	1.000	1.000	0.000	90	8	20	71	71.38
11	1.000	0.000	1.000	90	5.5	30	74	73.38
12	1.000	0.000	−1.000	90	5.5	10	83	82.13
13	−1.000	−1.000	0.000	10	3	20	54	53.63
14	1.000	−1.000	0.000	90	3	20	93	94.13
15	−1.000	1.000	0.000	10	8	20	29	27.88
16	0.000	1.000	1.000	50	8	30	46	46.25
17	0.000	−1.000	−1.000	50	3	10	87	86.75

Abbreviation: RSM-BBD, response surface methodology coupled with Box-Behnken design.

The final equation (equation (3)) generated from 4-NP degradation using H₂O₂-AK-Fe₃O₄ based on BBD was found to be well fitted to the quadratic model:

\begin{array}{l} Degradation (y) = 81.6 + 21 A - 12.13 B - 8.13 C \\ + 0.75 A B + 3.75 A C - 1.50 B C \\ - 15.55 A^{2} - 4.30 B^{2} - 9.30 C^{2} \end{array}

(3)

Equation (3) shows how the 3 factors in a quadratic or interactive model affect the 4-NP degradation using the AK-Fe₃O₄ composite. As shown in equation (3), those with negative coefficients affect 4-NP degradation inversely, whereas those with positive coefficients enhance the degradation of 4-NP. Moreover, the adequacy of the quadratic model to sufficiently explain the 4-NP degradation process is validated via statistical parameters such as R² and analysis of variance (ANOVA). The detailed ANOVA results of 4-NP degradation using AK-Fe₃O₄ are presented in Table 3. As shown in Table 3, R² (0.9964) is much closer to unity, which implies that the data are well fitted to the quadratic model. In particular, 99.64% of the Fenton-like degradation of 4-NP can be described by the proposed quadratic model (equation (3)). Besides, Figure 4 shows that the predicted R² (0.9823) has a reasonable agreement with adjusted R² (0.9917), that is, the difference is less than 0.2. It is affirmed that the model can sufficiently explain the relationship between factors and 4-NP degradation (%).

Table 3.

The analysis of variance of 4-nitrophenol degradation based on quadratic model.

Source	Sum of squares	df	Mean square	F value	P value	Fit statistics
Model	6886.81	9	765.20	214.69	<.0001	SD = 1.89
Residual	24.95	7	3.56	–	–	CV(%) = 2.78
Lack of fit	5.75	3	1.92	0.3993	.7617	R² = 0.9964 Predicted R² = 0.9823
Pure error	19.20	4	4.80	–	–	Adjusted R² = 0.9917
Terms	Coefficients			SE coefficients	Mean square	F value	P value
Constant	+81.60
A	+21.00			0.3536	3528.00	989.82	<.0001
B	–12.13			0.3536	1176.12	329.97	<.0001
C	–8.13			0.3536	528.13	148.17	<.0001
AB	+0.7500			0.5000	2.25	0.6313	.4530
AC	+3.75			0.5000	56.25	15.78	.0054
BC	–1.50			0.5000	9.00	2.53	.1561
A²	–15.55			0.4873	1018.12	285.64	<.0001
B²	–4.30			0.4873	77.85	21.84	.0023
C²	–9.30			0.4873	364.17	102.17	<.0001

Abbreviation: CV, coefficient of variation.

Figure 4.

The comparison of actual (experimental) and model prediction values of 4-nitrophenol degradation (%).

The ANOVA (Table 3) of the model F value is 214.69, which implies that the quadratic model is significant and there is only a 0.01% chance that an F value this large could occur due to noise. The P value (<.0001) of the model also shows the significance of the model. Moreover, the significance of the model parameters is also validated by P values. The P value less than .05 (5%) indicates the model terms are significant. Herein, all the linear (A, B, and C), quadratic (A², B², and C²), and interaction (AC) terms are significant, whereas model terms greater than 0.1 are not significant; therefore, the AB and BC interaction model terms are not insignificant.

The lack of fit (F = 0.40) from ANOVA (Table 3) implies that the lack of fit is not significant compared with the pure error. There is a 76.17% chance that a lack of fit F value this large could occur due to noise. This implies that the Fenton-like degradation of 4-NP is well explained by our quadratic model with a 95% confidence level.

The 2-dimensional (2D) and 3-dimensional (3D) surface plots of the response as a function of process variables are useful for a clear understanding of the reaction process. The 2D-3D plots (Figure 5) show the 4-NP degradation (%) with varying contact time, pH, and 4-NP concentrations. The degradation (%) of 4-NP differs with varying interaction variables and is found to be maximum with AB interaction terms than with AC and BC at a fixed 1 mg/mL concentration of AK-Fe₃O₄. As shown in Figure 5A, the AB interaction optimized at a fixed 4-NP concentration (20 mg/L) is the most important interaction parameter in predicting the maximum degradation of 4-NP. Hence, the optimum 4-NP degradation in 50 mL solution was found to be 96.01% ± 1.89% using 1 mg/mL of AK-Fe₃O₄, 20 mg/L of 4-NP, 97.9 mmol/L of H₂O₂, and pH of 3 at the end of 75 minutes of reaction time. Moreover, the RSM-BBD optimization with other scenarios gives maximum degradation of 4-NP, and based on considering the available resource it is possible to adapt different combinations of contact time, pH, and 4-NP concentration.

Figure 5.

The 2-dimensional and 3-dimensional surface plots of the effect of (A) contact time and pH, (B) initial 4-NP concentration and contact time, and (C) 4-NP initial concentration and pH of solution on the degradation (%) efficiency of 4-NP by AK-Fe3O4 composite. AK indicates activated kaolin; 4-NP, 4-nitrophenol.

In general, the numerical optimization of the desired goal is expected to maximize, minimize, target, be within the range, and be none of the responses and adjust to a precise value for the factors.⁴⁸ Herein, the numerical optimization found a point that maximizes the desirability function in 4-NP degradation. For example, the criteria set for the 3 factors in the range including contact time (10-90 minutes), pH of the solution (3-8), and initial 4-NP concentration (10-30 mg/L) and 96.08% maximum degradation of 4-NP were found to be the maximum desirability. Figure 6 shows ramp desirability that was generated from 100 optimum points via numerical optimization.

Figure 6.

The desirability ramp for optimization of 3 factors (contact time, pH solution, and 4-NP concentration) for the maximum degradation (%) of 4-NP using AK-Fe₃O₄ composite. AK indicates activated kaolin; 4-NP, 4-nitrophenol.

Mechanism and kinetics of 4-NP degradation

Supplemental Figure 4(a) shows the removal of 4-NP with and without H₂O₂. AK, Fe₃O₄, AK-Fe₃O₄, and H₂O₂ show very less removal of 4-NP, whereas AK-Fe₃O₄ composite in combination with H₂O₂ shows a significant degradation of 4-NP. This is attributed to the removal of 4-NP, which was predominantly through Fenton-like degradation of 4-NP. Moreover, Supplemental Figure 4(b) shows the comparative degradation efficiency of AK/H₂O₂, Fe₃O₄/H₂O₂, and AK-Fe₃O₄/H₂O₂. Fe₃O₄/H₂O₂ and AK-Fe₃O₄/H₂O₂ due to the presence of Fe₃O₄, which activates H₂O₂, show greater degradation efficiency than AK/H₂O₂.²⁰ For Fenton-like reaction on the iron-based heterogeneous catalyst, the activation of H₂O₂ using Fe²⁺ and Fe³⁺ proceeds via adsorption of H₂O₂ on the catalyst surface, which in turn depends on the surface area of the catalyst.⁴⁹ Fe₃O₄ nanoparticles show a strong affinity toward H₂O₂, which facilitates the formation of active oxidizing agents (HO^• and $H O_{2}^{•}$ ).^25,49 The plausible mechanism for the degradation of 4-NP using the AK-Fe₃O₄/H₂O₂ system can be suggested as shown in Figure 7. Hence, H₂O₂ is initially adsorbed on the AK-Fe₃O₄ composite where it is activated by Fe₃O₄ for the formation of HO^• and HOO^•. HO^• is a powerful oxidant that can mineralize 4-NP into H₂O and CO₂.

Figure 7.

The plausible Fenton-like degradation of 4-NP using AK-Fe₃O₄ composite as a catalyst for the activation of H₂O₂. AK indicates activated kaolin; 4-NP, 4-nitrophenol.

The kinetics of the Fenton-like degradation of the 4-NP can be investigated using the pseudo-first-order equation (equation (4)):²⁹

\ln (\frac{C_{t}}{C_{o}}) = - k t

(4)

where C_o and C_t are the initial and final concentrations at time “t” of 4-NP. Therefore, a plot of ln(C_t/C_o) with respect to reaction time gives a straight line whose slope is k (Supplemental Figure 4(c)). The kinetic analysis was performed by varying the initial concentrations of 4-NP while maintaining others constant. As shown in Supplemental Figure 4(d), the rate constant (k) decreases with an increase in 4-NP concentrations. This implies that 4-NP degradation is relatively fast at low concentration. Moreover, Supplemental Table 1 shows the comparison of AK-Fe₃O₄ with other catalysts used for 4-NP Fenton-like degradations. AK-Fe₃O₄ shows faster 4-NP degradation efficiency than others that are working at room temperature (298 K).

Reusability and stability of AK-Fe₃O₄

Reusability and stability are very important criteria for evaluating the practical application of catalysts. Successive cycles of the Fenton degradation were carried out to evaluate the stability and reusability of our catalyst under the same reaction conditions. The optimum condition of degradation parameters in 50 mL solution such as 1 mg/mL of AK-Fe₃O₄, 20 mg/L of 4-NP, 97.9 mmol/L of H₂O₂, and pH of 3 at the end of 75 minutes of reaction time was used to investigate the stability and reusability of the catalyst. After each successive cycle, the AK-Fe₃O₄ was collected by centrifugation, washed at least 3 times, and dried in the oven at 100°C for 12 hours.⁵⁰ As shown in Figure 8, the catalyst could be successfully reused for 5 successive degradation cycles with more than 90% degradation efficiency, indicating that the AK-Fe₃O₄ Fenton-like catalyst is stable. Moreover, the stability of the AK-Fe₃O₄ before and after 5 times use was examined by XRD. As can be seen from Figure 9, the XRD patterns of AK-Fe₃O₄ do not change before and after degradation. This implies the stability of the catalyst crystal structure after use.⁵¹ But the peak intensity decreased after use, which is possibly due to catalyst loss during recovery.

Figure 8.

The reusability of AK-Fe₃O₄/H₂O₂ for the degradation (%) of 4-NP in 50 mL solution (1 mg/mL of AK-Fe₃O₄, 20 mg/L of 4-NP, 97.9 mmol/L of H₂O₂, pH of 3; contact time, 75 minutes). Error bar represents 5% error. AK indicates activated kaolin; 4-NP, 4-nitrophenol.

Figure 9.

The x-ray diffraction patterns of AK-Fe₃O₄ (A) before and (B) after Fenton-like degradation of 4-NP. AK indicates activated kaolin; 4-NP, 4-nitrophenol.

Conclusion

In conclusion, Fe₃O₄-modified AK was successfully prepared using a facile in situ coprecipitation of Fe₃O₄ in the presence of kaolin. The crystal structure, phase formation, and surface morphology were characterized by XRD, FTIR, SEM, and VSM. The AK-Fe₃O₄ composite is effective for Fenton-like degradation of 4-NP. The most important 4-NP degradation parameters (such as contact time, concentrations of 4-NP, and pH) were optimized using RSM-BBD. A 3-level quadratic model can effectively show the interaction of parameters. This was validated by both regression (R² = 0.9964 and adjusted R² = 0.9917) and ANOVA (P < .0001) analyses. Based on RSM-BBD, the optimum 4-NP degradation in 50 mL solution was found to be 96.01% ± 1.89% with the use of 1 mg/mL of AK-Fe₃O₄, 20 mg/L of 4-NP, 97.9 mmol/L of H₂O₂ and pH of 3 at the end of 75 minutes of reaction time. Moreover, the RSM-BBD can suggest other optimization scenarios that give maximum degradation of 4-NP based on the available resource. Thus, the AK-Fe₃O₄ composite and optimization using RSM-BBD can be applied for the Fenton-like degradation of water pollutants. Moreover, the catalyst shows good recyclability and stability after 5 successive degradations of 4-NP.

Supplemental Material

Supplemental_material – Supplemental material for RSM-BBD Optimization of Fenton-Like Degradation of 4-Nitrophenol Using Magnetite Impregnated Kaolin

Supplemental material, Supplemental_material for RSM-BBD Optimization of Fenton-Like Degradation of 4-Nitrophenol Using Magnetite Impregnated Kaolin by Neway Belachew, Redeat Fekadu and Amare Ayalew Abebe in Air, Soil and Water Research

Footnotes

Acknowledgements

The authors acknowledge Debre Berhan University for supporting the work. The authors also acknowledge Adama Science and Technology University for recording the x-ray diffraction samples.

Funding:

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Declaration of Conflicting Interests:

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Author Contributions

NB designed the study, contributed to data interpretation, and edited the manuscript. RF helped with the experiment and wrote the manuscript. AA helped with the experiment and contributed to data interpretation. All authors read and approved the final manuscript.

ORCID iD

Neway Belachew

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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RSM-BBD Optimization of Fenton-Like Degradation of 4-Nitrophenol Using Magnetite Impregnated Kaolin

Abstract

Keywords

Introduction

Experiment

Materials

Acid activation of kaolin

Synthesis of magnetite nanoparticles

Synthesis of AK-Fe3O4 composite

Characterization techniques

RSM-BBD optimization of the Fenton-like degradation variables

Results and Discussion

Characterizations

FITR analysis

XRD analysis

SEM, VSM, and pHPZC analyses

Optimization of 4-NP degradation using RSM-BBD

Mechanism and kinetics of 4-NP degradation

Reusability and stability of AK-Fe3O4

Conclusion

Supplemental Material

Supplemental_material – Supplemental material for RSM-BBD Optimization of Fenton-Like Degradation of 4-Nitrophenol Using Magnetite Impregnated Kaolin

Footnotes

Acknowledgements

Funding:

Declaration of Conflicting Interests:

Author Contributions

ORCID iD

Supplemental Material

References

Supplementary Material

Synthesis of AK-Fe₃O₄ composite

SEM, VSM, and pH_PZC analyses

Reusability and stability of AK-Fe₃O₄