Removal of amoxicillin employing Fenton-type process using delaminated clay and layered double hydroxides impregnated with Fe or Cu as catalysts

doi:10.21203/rs.3.rs-5276828/v1

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Removal of amoxicillin employing Fenton-type process using delaminated clay and layered double hydroxides impregnated with Fe or Cu as catalysts

https://doi.org/10.21203/rs.3.rs-5276828/v1

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The presence of antibiotics in the environment has raised concerns due to their potential negative effects on ecosystems. Conventional water treatment methods are ineffective at removing antibiotics. This study aims to investigate the efficiency of Fenton-like processes catalyzed by delaminated clay and layered double hydroxides impregnated with Fe or Cu for the degradation of amoxicillin. The catalysts were obtained by synthesizing delaminated clay and layered double hydroxides and subsequently impregnating them with Fe or Cu. The characterization of catalysts involved X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), N₂ adsorption-desorption, and X-ray photoelectron spectroscopy (XPS). Catalytic activity was assessed by varying the concentration of hydrogen peroxide, the initial concentration of amoxicillin, and the amount of catalyst. The determination of byproducts was done by high-performance liquid chromatography (HPLC) with a quadrupole time-of-flight mass spectrometer (QqTof). The study found that layered double hydroxides impregnated with Fe or Cu were able to remove 100% of amoxicillin in just 20 min. The study identified 16 byproducts, indicating a degradation process. Under all of the studied conditions, the copper catalysts showed the highest percentage of amoxicillin removal.

Amoxicillin

Fenton – type process

layered double hydroxides Wastewater treatment

Emerging contaminants

Antibiotics have been used for the treatment of infectious diseases in humans and for disease prevention in animals[1], [2], [3]. According to the World Health Organization (WHO), Global consumption of these drugs has increased by 46% between 2000 and 2018, resulting in a worldwide daily intake of 40.5 billion doses in 2018 [4], [5]. Antibiotics present in human and animal excretions are a major source of environmental contamination; an estimated 54,000 tons of these compounds were excreted in 2013 [6]. These compounds are persistent and resistant to conventional treatment plant methods. As a result, surface waters in America, Europe, and Asia have recorded concentrations of 15 µg/L, 50 µg/L, and 450 µg/L, respectively[7].

Antibiotic-resistant genes rapidly spread due to the presence of antibiotics, posing a risk to human health by causing infections with low treatment efficacy. This can result in fatalities, making it a significant public health concern [3]. According to the World Health Organization, superbug infections cause at least 700,000 deaths annually, and the estimated cost of treating these infections could reach approximately $100 trillion by 2050 [4]. Over 65% of the antibiotics mentioned belong to the β-lactam family, with amoxicillin being the most widely used because of its broad spectrum and ease of absorption [8], [9]. The molecular formula of amoxicillin is C₁₆H₁₉N₃O₅S. They are highly soluble in water and inhibit cell wall synthesis in organisms [10], [11]. Urine excretes approximately 80% of the consumed amoxicillin without metabolization [12]. The persistence and bioaccumulation of antibiotics can disrupt the natural balance of ecosystems, affecting processes such as photosynthesis in algae in the short term [13].

The detection of amoxicillin concentrations of 1 ng/L in surface waters in China [14], 10 ng/L in groundwater [15], 75 ng/L in seawater [16], and 150 ng/L in wastewater [15]. Supports the increasing need to eliminate these compounds from different aquatic ecosystems. Researchers have found that conventional treatments like adsorption, coagulation, sedimentation, and air flotation are ineffective due to the low concentrations of antibiotics in water bodies, their high persistence, and poor biodegradability [17].

To tackle this challenge, researchers are investigating various physical, biological, and chemical techniques in the laboratory, including precipitation, filtration, coagulation, anaerobic digestion, aerated sludge, activated sludge, and advanced oxidation processes [18]. However, these treatments have some drawbacks. Physical methods involve transferring the contaminant from the liquid phase to the solid phase, but the contaminant persists in the matrix [19]. However, biological methods typically result in low or nonexistent mineralization of the compound for removal, leading to low removal efficiency percentages[20]. Sheng-Fu Yang's research demonstrated the low efficiency of antibiotic adsorption using activated sludge. The removal rates of sulfamethoxazole (SMX), sulfadimethoxine (SDM), and sulfamonomethoxine (SMM) were 6.5%, 19%, and 11%, respectively[21]. In addition, the removal of ciprofloxacin from anaerobic sulfate-reducing bacteria sludge systems achieved a removal rate of 28%[22]. Researchers have also employed artificial wetlands, achieving an 84% removal rate of oxytetracycline and difloxacin[23]. Another study evaluated the removal of sulfamethoxazole using coagulation with activated carbon and achieved a removal rate of 57.2% [24].

Because of these problems, chemical methods, especially advanced oxidation processes (AOPs), look like good options for breaking down pollutants that are hard to get rid of [25]. Researchers have extensively explored Fenton-type oxidation processes for the elimination of amoxicillin. Several studies presented in Table 1 on Fenton-type processes have focused on photocatalysis and electrolysis. Although these methods are highly efficient, they still have some disadvantages, such as high energy consumption, the use of specific reactors, large amounts of catalyst, and the toxicity of some synthetic metals. Furthermore, we still need to address the high costs associated with these processes[26].

In this context, Fenton-type processes are significant, particularly when using heterogeneous catalysts with transition metals supported on lamellar structures such as delaminated clay and double-layer hydroxides [27], [28]. These procedures address the limitations of traditional Fenton processes, such as a limited pH range and challenges in separating the catalyst from the aqueous solution. These catalysts offer economic benefits and avoid the use of environmentally harmful metals such as gold and platinum during extraction [29]. Clays have served as catalysts or supports for metallic species in Fenton-type reactions since approximately 2000, facilitating the degradation of compounds such as Remazol, Brilliant Orange 3C, azoic violet dyes, phenol, and hydrocarbons, among others[30]. Since 2017, clays mixed with TiO₂, ZnO, and MoS₂ have made photocatalytic reactions better, such as photo-Fenton [31]. Researchers have developed various methods in recent years to modify the clay structure, significantly improving the specific surface area and porosity, and enhancing clay properties [32]. Some of the changes that have been suggested to make it easier for clays to remove contaminants are thermal treatments, acid treatments, pillar implementation (also called "pillaring"), and layer exfoliation (also called "delamination")[33]. Delamination involves the formation of clay layer aggregates, either edge to edge or edge to face, resulting in solids with efficient pore distribution, excellent dispersion of active phases, and mesoporosity [34], [35]. The modification of clays through delamination confers or enhances mechanical, thermal, optical, chemical, and electrical properties, such as heat resistance, strength, and flexibility [36], [37]. Furthermore, double-layer hydroxides or anionic clays with a general molecular formula of [M^2 + _1-x Mx³⁺ (OH)₂]x + (An−)x/n × m H₂O are used as catalysts to enhance the reaction efficiency. The formula consists of divalent or trivalent ions represented by M and anions represented by An-. The ratio of divalent to trivalent ions typically ranges from 2 to 3. Advanced oxidation processes use these structures as catalysts, catalyst precursors, or catalytic supports to remove various organic pollutants[38].

However, the effectiveness of the Fenton-type process depends on three critical factors: the concentration of pollutants, the concentration of hydrogen peroxide, and the amount of catalyst [28]. The aim of this study was to evaluate various operating variables of a Fenton-type process using delaminated clay (DC) and layered double hydroxides (LDH) impregnated with Fe or Cu as catalysts (DC-Fe, DC-Cu, LDH-Fe, LDH-Cu) for the removal of amoxicillin.

Table 1

A summary of the studies that investigated the Fenton-type process for the removal of amoxicillin.
Article	OAPs	Catalysts	% removal	Ref.
Oxidation mechanisms of amoxicillin and paracetamol in the photo-Fenton solar process	Photo-Fenton solar process	Fe₂(SO₄)₃	90	[39]
Microwave-assisted Fenton’s oxidation of amoxicillin.	Microwave-assisted Fenton’s	FeSO₄	100	[2]
Photo-Fenton degradation of amoxicillin via magnetic TiO₂-graphene oxideFe₃O₄ composite with a submerged magnetic separation membrane photocatalytic reactor (SMSMPR)	Photo-Fenton	TiO₂-graphene oxide (GO)-Fe₃O	90	[3]
Degradation of the antibiotics amoxicillin, ampicillin and cloxacillin in aqueous solution by the photo-Fenton process	Photo-Fenton	FeSO₄·7H₂O	100	[4]
Pillared montmorillonite supported ferric oxalate as heterogeneous photo-Fenton catalyst for degradation of amoxicillin	Photo-Fenton	PMFeOx	99,65	[5]
Sub-stoichiometric titanium oxide as a new anode material for electro-Fenton process: Application to electrocatalytic destruction of antibiotic amoxicillin	Electro - Fenton	Ti₄O₇	100	[6]
Kinetic degradation of amoxicillin by using the electro-Fenton process in the presence of a graphite rods from used batteries	Electro - Fenton	Fe²⁺	95	[7]
Cu–Fe–FeC₃@nitrogen-doped biochar microsphere catalyst derived from CuFe₂O₄@chitosan for the efficient removal of amoxicillin through the heterogeneous electro-Fenton process	Electro - Fenton	Cu-Fe-FeC₃	99.3	[8]
Heterogeneous electro-Fenton process by Nano-Fe₃O₄ for catalytic degradation of amoxicillin: Process optimization using response surface methodology	electro-Fenton	Fe₃O₄	98,2	[9]
Treatment of amoxicillin by O3/Fenton process in a rotating packed bed	O₃/Fenton	Fe²⁺	65	[10]

Figure 1. illustrates that the research was conducted in three stages. The first stage involved the synthesis of support (delaminated clay or layered double hydroxides), which served as the catalytic support. The second stage involved synthesizing the catalyst by wet impregnation of either Fe or Cu metals. After obtaining the catalysts, we evaluated the catalytic activity by assessing the removal of amoxicillin, varying hydrogen peroxide concentrations, initial contaminant concentrations, and catalyst amounts. In addition, some degradation by-products were identified.

2.1 Synthesis of catalyst support

2.1.1 Synthesis of delaminated clay

The catalytic support was obtained by modifying clay through delamination of bentonite (Bentonitas de Colombia S.A). First, a homogenization process was performed using a NaCl solution for 24 hours with agitation (200rpm). Subsequently, the conductivity was adjusted to 100 µs and 20g of polyvinyl alcohol (PVA) were added. The mixture was refluxed for 24 hours. After this, it was subjected to a microwave process (35 min, 125°C, 1000W) and dried (70°C).

2.1.2 Synthesis of Layered double hydroxides

The support was synthesized using the coprecipitation method. An aqueous mixture of Na₂CO₃ was stirred at a constant speed of 200 rpm while adding 3 moles of Mg (NO₃)₂ and 1 mole of Al(NO₃)₃. After 1 h of stirring, the mixture underwent microwave treatment (Milestone Srl, Sorisole, Italy) at 180°C and 1000 W for 20 min and was allowed to age for 24 h. The mixture was then washed with distilled water until the conductivity reached 100 µS. The double-layer hydroxide was synthesized and, dried at 90°C for 24 h to obtain the support.

2.2 Catalyst synthesis

After synthesizing the delaminated clay (DC) and layered double hydroxides (LDH) support, it was impregnated with either Fe or Cu. The wet impregnation process involved the preparation of an aqueous solution containing the catalytic support (layered double hydroxides), Fe(NO₃)₃ * 9 H₂O or Cu(NO₃)₂ * 3 H₂O and water to obtain a weight percentage of 5% of the supported metal on the support. The solution was stirred constantly at 200 rpm for 24 h. The water was then evaporated in an oven at 120°C for 12 h. Finally, the catalyst was calcined at 400°C for 4 h. This process produced two catalysts: delaminated clay with Fe (DC-Fe), delaminated clay with Cu (DC-Cu), double layer hydroxides impregnated with Fe (LDH-Fe), and double - layer hydroxides impregnated with Cu (LDH-Cu). The impregnation percentages are consistent with values used by other authors in previous laboratory studies [40], [41].

2.3 Characterization

The support (LDH) and catalysts (LDH-Fe and LDH-Cu) were morphologically characterized using a scanning electron microscope (ZEISS MA15 VP-SEM, Jena, Germany) equipped with a thermionic cathode with a tungsten filament, a 35 kV acceleration potential, and a micro-EDS. Elemental composition was evaluated using X-ray fluorescence (Shimadzu XRF-1800, Kyoto, Japan) with a 4kW X-ray tube, thin window, and ultra-fast scanning (300°/min) over a total measurement time of 40,000 seconds. The synthesis of double-layer hydroxides and metal incorporation was evaluated using X-ray diffraction. The instrument used was a Shimadzu XRD-7000 in Kyoto, Japan. Cu-Kα (40 mA, 45 kV) was used with a step size of 0.05° and 300 s step time, within a 2θ range from 10 to 80 °. The catalysts' textural properties were analyzed using a high-performance porosity and surface area analyzer, specifically nitrogen adsorption-desorption analysis performed using the Micromeritics Tristar II instrument in Georgia, USA. The specific surface area and pore volume were determined via application of the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models. The oxidation state of the active phases was determined using the X-ray photoelectron spectroscopy (XPS) technique. Analysis was conducted with the LEYBOLD-HEREUS system (model LHS 10/20) using Al-Kα radiation (1486.6 eV) and a mono-chromator focused at 30 mA × 11 kV. The scan energy was maintained at 200 eV overall, but for targeted scans, a narrower energy band of 50 eV was used.

2.4 Amoxicillin removal: catalytic evaluation

The experiment to degrade amoxicillin was conducted in a semi-batch reactor with temperature control at 20°C. A solution of 250 ml of amoxicillin at a concentration of 10 ppm was added to 0.125 g of catalyst at a flow rate of 2 ml/min for air and 2 ml/h for hydrogen peroxide (0.1 M). The mixture was stirred constantly at 220 rpm and maintained at pH 8 and atmospheric pressure. To determine the degradation of the contaminant, aliquots were taken every 5 min, and the reaction was monitored. Possible degradation by-products were determined by Nexera Series X2 high-performance liquid chromatography (HPLC) (Shimadzu Scientific Instruments,Maryland, USA). The samples were analyzed using electrospray ionization (ESI) mass spectrometry in positive mode on a Shimadzu Scientific Instruments Quadrupole-time-of-flight (QqTOF) LCMS 9030 analyzer. The chromatographic conditions were maintained isocratically for 36 min with 80% mobile phase A and 20% mobile phase B at a constant flow rate of 0.15 ml/min and a temperature of 40°C. The mobile phase A was composed of 0.1% aqueous formic acid, while the mobile phase B was composed of acetonitrile. The Shim-pack XR-ODS II column (2.2 µm, dp X 2.0 mm, ID X 150 mm, L) was used for injecting the sample. The detector was set to positive acquisition mode with an acquisition range of 50–1000 m/z, an interface voltage of 5 kv, and an interface temperature. The experimental setup involved heating at 300°C with a desolvation line temperature of 250°C and a nebulizer gas flow of 3 L/min, along with a drying and heating gas flow of 10 L/min. Various reaction parameters were evaluated, including the amount of catalyst (0.06g, 0.12g, and 0.25g), peroxide concentrations (0.05M, 0.1M, and 0.2M), and initial pollutant concentrations (1 ppm, 10 ppm, and 20 ppm). The reactions were evaluated at three different pH levels (3, 7, and 10) using delaminated clays. The experimental conditions for the delaminated clays impregnated with Fe or Cu are shown in Table 2. The reactions for the reactions with double layer hydroxides, only pH 10 was evaluated due to the catalyst's pH buffering properties. The Table 3 shows the conditions of the reactions with double layer hydroxides. Each treatment had three replicates experimental.

Table 2

Experimental conditions for each of the treatments evaluated for amoxicillin removal when the catalytic support was delaminated clay
Active phase	Contaminat	pH	Contaminant concentration (ppm)	Hydrogen peroxide concentration (M)	Catalyst Quantity (g)
Fe	AMX	3	10	0.05	0.125
		7	10	0.05	0.125
		10	10	0.05	0.125
Cu	AMX	3	10	0.05	0.125
		7	10	0.05	0.125
		10	10	0.05	0.125
Fe	AMX	3	10	0.1	0.125
		7	10	0.1	0.125
		10	10	0.1	0.125
Cu	AMX	3	10	0.1	0.125
		7	10	0.1	0.125
		10	10	0.1	0.125
Fe	AMX	3	10	0.2	0.125
		7	10	0.2	0.125
		10	10	0.2	0.125
Cu	AMX	3	10	0.2	0.125
		7	10	0.2	0.125
		10	10	0.2	0.125
Fe	AMX	3	10	0.1	0.06
		7	10	0.1	0.06
		10	10	0.1	0.06
Cu	AMX	3	10	0.1	0.06
		7	10	0.1	0.06
		10	10	0.1	0.06
Fe	AMX	3	10	0.1	0.25
		7	10	0.1	0.25
		10	10	0.1	0.25
Cu	AMX	3	10	0.1	0.25
		7	10	0.1	0.25
		10	10	0.1	0.25
Fe	AMX	3	1	0.1	0.125
		7	1	0.1	0.125
		10	1	0.1	0.125
Cu	AMX	3	1	0.1	0.125
		7	1	0.1	0.125
		10	1	0.1	0.125
Fe	AMX	3	20	0.1	0.125
		7	20	0.1	0.125
		10	20	0.1	0.125
Cu	AMX	3	20	0.1	0.125
		7	20	0.1	0.125
		10	20	0.1	0.125

Table 3

Experimental conditions for each of the treatments evaluated for amoxicillin removal when the catalytic support was delaminated clay.
Active phase	Contaminat	pH	Contaminant concentration (ppm)	Hydrogen peroxide concentration (M)	Catalyst Quantity (g)
Fe	AMX	10	10	0.05	0.125
Cu	AMX	10	10	0.05	0.125
Fe	AMX	10	10	0.1	0.125
Cu	AMX	10	10	0.1	0.125
Fe	AMX	10	10	0.2	0.125
Cu	AMX	10	10	0.2	0.125
Fe	AMX	10	10	0.1	0.06
Cu	AMX	10	10	0.1	0.06
Fe	AMX	10	10	0.1	0.25
Cu	AMX	10	10	0.1	0.25
Fe	AMX	10	1	0.1	0.125
Cu	AMX	10	1	0.1	0.125
Fe	AMX	10	20	0.1	0.125
Cu	AMX	10	20	0.1	0.125

The catalyst's stability was evaluated under the same conditions. After the first cycle, the catalyst was washed with water and ethanol and, then dried at 105°C for 12 h. The experiment was repeated for four cycles, and the concentrations of metals, specifically iron and copper, remaining in the solution after the last cycle were analyzed to evaluate active phase leaching. The concentration of iron was determined using a modified TPTZ method, and the concentration of copper was measured using a modified EPA method. Analytical measurements were conducted using the Hanna 83399 multiparameter method.

2.5 Statistical Analysis

A one-way analysis of variance was used to examine significant differences in the treatments. A post hoc test was performed to determine the groups with the most notable differences. For the delaminated clay treatments, a two-way analysis of variance was performed to determine the correlation between the pH condition and the other operating parameters, such as hydrogen peroxide concentration, initial contaminant concentration, or amount of catalyst. All analyses were performed using GraphPad - Prism software.

3.1 Characterization of delaminated clay (DC)

3.1.1. Scanning Electron Microscopy (SEM)

Morphological features were evaluated using scanning electron microscopy (SEM). It was used scanning electron microscopy (SEM) to look at the shape of the catalytic support (DC) and the catalysts (DC-Fe and DC-Cu). Figure 2a reveals that the catalytic support does not have a completely smooth surface, which is different from the surfaces reported for clays[42]. Instead, the polymer residues used in the delamination process [43] Figs. 2b and 2c exhibit a spongy morphology, suggesting the incorporation of metals via the wet impregnation technique. Additionally, as previously reported [44], [45], a significant portion of the metals deposited on the surface of the delaminated clay were the catalytic support (DC) and the catalysts (DC-Fe and DC-Cu). Figure 2a shows that the catalytic support does not have a completely smooth surface, contrary to the surfaces typically observed for clays[42]. Instead, distinct flake-like features associated with the polymer residues used in the delamination process[43], are present. Figures 2b and 2c show a spongy morphology, indicating the incorporation of metals via the wet impregnation technique. Moreover, as reported [44], [45], a significant portion of the metals were deposited on the surface of the delaminated clay.

To determine the elemental composition and quantify the percentage of active phases, techniques such as energy-dispersive X-ray (EDX) spectroscopy and X-ray fluorescence (XRF) were employed. The support material, namely the clay, was found to contain 5.6% Fe, which falls within the reported range for bentonites from various global locations [11], [12] It was observed that approximately 5% of the active phases were integrated into the delaminated clay. The catalyst had a higher percentage of Fe, as revealed by both EDX and XRF analyses, which is attributed to the natural Fe content inherent in the bentonite (Table 4). In contrast, Cu was not detected in the natural composition of the support.

Table 4

Elemental composition of delaminated clay unimpregnated (DC) and impregnated with Fe and Cu (DC-Fe, DC-Cu) by X-ray fluorescence (XRF)
Element	DC	DC- Fe	DC-Cu
Si	64.05	64.05	64.05
Al	25.97	25.97	25.97
Mg	0.04	0.04	0.04
Na	2.40	2.40	2.40
Ca	1.30	1.30	1.30
P	0.07	0.07	0.07
Fe	4.96	10.02	5.23
Cu	0.02	0.02	5.82

3.2.2 X-ray Diffraction

X-ray diffraction can be used to analyze catalytic supports and catalysts and to monitor the structural changes occurring during delamination processes. Figure 3a shows the X-ray diffraction patterns of the starting material (Bentonite). Figure 3b shows the catalytic support DC (Delaminated Clay). Figure 3c delaminated clay with Fe (DC-Fe) and Fig. 3d delaminated clay with Cu (DC-Cu). Comparing the diffractograms of the natural and modified clays (Fig. 3a and 3b), we observe the disappearance of the signal at 2θ = 6.98°, corresponding to the d₀₀₁ plane. This indicates a reduction in interlayer spacing and a reorganization of the layers, confirming the success of the delamination modification process[34].

Also, the presence of peaks at 2θ = 19.9 and 36.2, which correspond to the montmorillonite phase, and peaks at 2θ = 21.71, 26.56, and 28.1, which correspond to quartz, show that the change caused the clay structure to delaminate instead of collapse [55, 56].

The diffractogram of the Fe catalyst (Fig. 3c) shows peaks at 2θ = 31.94, 35.82, and 51.12, which are associated with α-Fe₂O₃ [46]. The diffractogram of the sample shows peaks at 2θ = 41.32, 52.2, and 2θ = 36.12, 43.02 corresponding to FeO and Fe₃O₄, respectively[13], [14]. The Cu catalyst's diffractogram shows three peaks at 2θ = 36.3°, 48.9°, and 51.2°. These are for copper oxide (CuO) and two peaks at 2θ = 39.9° and 33.5° are for cuprous oxide (Cu₂O) [47], [48]. This indicates the successful incorporation of the metal into the delaminated clay. These results are consistent with those obtained using X-ray Photoelectron Spectroscopy (XPS), as shown in Fig. 4.

3.2.3 X-ray photoelectron spectrometry

Figure 4. presents the XPS spectra of catalysts supported on delaminated clay. The spectrum of catalysts impregnated with Fe (DC-Fe) is shown in Fig. 4a. which highlights two prominent peaks at 725.69 eV and 730.47 eV. These peaks are associated with Fe 2p_3/2 and Fe 2p_1/2, respectively. Following spectrum deconvolution, we identified additional peaks at 716.56 eV, 717.72 eV, and 722.65 eV, indicating the presence of Fe²⁺ and Fe³⁺ in the catalyst. Figure 4b. presents XPS spectra corresponding to Cu catalysts. X-ray photoelectron spectroscopy (XPS) analysis showed two clear peaks, one corresponding to Cu 2p_3/2 and the other to Cu 2p_1/2. These peaks had binding energies of 946.37 eV and 948.51 eV, respectively. The first peak's deconvolution at 936.91 eV and 938.96 eV reveals the presence of Cu⁺ and Cu²⁺ species, while the second peak's binding energy of 946.37 eV correlates with the presence of Cu²⁺. Shifts in binding energies are apparent, attributable to the surrounding chemical environment. The presence of oxygen and interactions with the support, in this case, delaminated clay primarily composed of silicon and aluminum, could explain the changes in binding energy.

3.2.4 Surface area - adsorption - desorption of N₂

The Nitrogen (N₂) adsorption-desorption isotherms were used to characterize the textural properties of the support and catalyst materials. Surface area was determined by processing Brunauer-Emmett-Teller (BET) data, and pore volume was obtained using the Barret-Joyner-Halenda (BJH) method. Figure 5a. shows the adsorption isotherms of the delaminated bentonite support (DC), the catalyst DC-Fe (Fig. 5b.), and the catalyst DC-Cu (Fig. 5c.). Table 5 records the main textural parameters that were determined. The delaminated bentonite support (DC) and the DC-Fe and DC-Cu catalysts exhibit a type IV isotherm with an H4 hysteresis loop, which is characteristic of mesoporous solids[49]. This mesoporosity type is typical of 2:1 clay minerals that have undergone successful delamination, confirming that the bentonite treatment with PVA and microwaves caused the loss of laminar ordering [34]. The delamination process's success is also evident from the increase in external surface area in all three materials, as reported in Table 4. According to Maggio et al. (2022), natural bentonite has a surface area of approximately 30 m²/g, while the delaminated support has a surface area of 89 m²/g. [60] These results confirm the XRD analyses, which showed the disappearance of the d001 signal after the delamination process. The active phase particles added during the wet impregnation process likely blocked some support pores, leading to the reduction in surface area of the DC-Fe (74 m²/g) and DC-Cu (71 m²/g) catalysts. Furthermore, the Scherrer equation was used to determine the crystallite size in the DC-Fe and DC-Cu materials. The average crystallite size was observed to be approximately 41.17 nm for DC-Fe and 38.84 nm for DC-Cu catalysts, indicating potential aggregate formation.

Table 5

Textural properties of the DC support and DC-Fe and DC-Cu catalysts
Catalyst	BET Surface Area (m²/g)	Pore Volume (cm³/g)	Crystallite Size (nm)
DC	89	0.104	-
DC-Fe	74	0.072	38.84
DC-Cu	71	0.084	41.17

3.3 Characterization of double layer hydroxide (LDH)

3.3.1 Scanning Electron Microscopy (SEM)

Figure 6 displays the micrographs of the support (LDH) and the catalysts (LDH-Fe and LDH-Cu). The double-layer hydroxide without an active phase has an amorphous surface without agglomerations. In contrast, the solids impregnated with Fe and Cu, as shown in Figs. 6B and 6C, exhibited agglomerations. Table 6 shows that Mg and Al are the predominant elements, as indicated by the energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF) analyses. The M(II)/M(III) ratio of these elements was determined to be 3, indicating an effective synthesis. The presence of copper and iron can be attributed to the metals incorporated by the wet impregnation method [50]. This finding aligns with previous literature reports[51] and suggests incorporating the active phase at a rate of approximately 5%.

Table 6

Elemental composition of unimpregnated and impregnated with Fe and Cu (LDH-Fe, LDH-Cu) determined by X-ray fluorescence (XRF).
Element	LDH %	LDH – Fe %	LDH-Cu %
Mg	36,87	34.46	34.17
Al	18.51	17.04	17.53
Fe	-	4.95	-
Cu	-	-	4,78

3.3.2 X-ray Diffraction

X-ray diffraction (XRD) profiles of the double layer hydroxides, the support (LDH), and the catalysts (LDH-Cu-LDH-Fe) are shown in Fig. 7. The diffraction patterns exhibit intense characteristic signals at low angles °2θ corresponding to the interlaminar distance. Signals at °2θ = 11.38°, 23.18°, and 34.70° correspond to the (003), (006), and (101) planes, respectively (Fig. 7a)[1], [2]. These peaks correspond to the JCPDS 00-0048-0601 pattern, which is characteristic of double-layer hydroxides structures [41]. This result demonstrates the effectiveness of the proposed material synthesis. The Fig. 7b shows the presence of copper oxide phases is evidenced by the appearance of signals at 2θ = 36.63°, 42.559°, and 61.76°, which correspond to planes (111), (200), (220), (311), and (222), respectively. In particular, peaks at 2θ = 36.63° and 61.76° are consistent with the JCPDS 01-078-0428 pattern indicative of cupric oxide[4].

On the other hand, although the signals corresponding to the crystalline phase of Cu₂O are not well-defined in the diffraction patterns, it is concluded that these signals may be overlapped at 2θ = 36.5, 42.4, and 61.5, where the (111), (200), and (220) planes of Cu₂O appear. The XPS results, which demonstrated the presence of both Cu²⁺ and Cu¹⁺, support and confirm this statement. Figure 7c. illustrates the X-ray diffraction profile for the iron catalyst, where peaks identified at 2θ = 36.92°, 42.05°, and 60.98° correspond to the (111), (200), and (220) planes of FeO, respectively. The identified peaks indicate the presence of ferrous oxide (FeO), and conform to the JCPDS 01-075-1550 pattern. Additionally, peaks at 2θ = 62.4°, 62.9°, and 64.7° indicate the presence of maghemite and magnetite, as reported by Rahman et al. (2021)[52]. The presence of signals at 2θ values 11.8°, 26.7°, 34.0°, 35.2°, 39.2°, 46.4°, and 64.4° corresponding to FeOOH suggests the presence of Fe³⁺. Both diffraction profiles in Fig. 7 show notable peaks at 2θ = 42.5° and 61.7°, matching the pattern of magnesium oxide (MgO) [43]. The correlation between Fig. 7b and Fig. 7a, along with the EDS and XRF findings, confirms the successful integration of the active iron and copper phases. Finally, a signal at 2θ = 29° is observed, which may correspond to some residues of NaNO₃ remaining after the material synthesis process.

3.3.3 X-ray photoelectron spectrometry

To confirm the chemical composition results obtained through X-ray fluorescence (XRF) and energy dispersive X-ray spectroscopy (EDS), and to determine the oxidation state of the active phases of the LDH-Fe and LDH-Cu catalysts, X-ray photoelectron spectroscopy was used. Figure 8a. and 8b display the XPS spectra of Fe 2p_1/2, Fe 2p_3/2, and Cu2p_3/2. Figure 8a. shows the LDH-Fe spectrum, which shows Fe²⁺ by having energy peaks at 710.32 eV and 723.87 eV, as well as satellite peaks at 717.98 eV. Additionally, the energy peaks at 711.40 eV and the satellite peak at 732.21eV indicate the presence of Fe³⁺ [53]. Figure 8b. shows energy peaks at 732.95 eV and 735.36 eV, representing Cu⁺ and Cu²⁺, respectively, along with satellite peaks at 941.25 eV and 942.79 eV [51], [54]. According to research papers, the Fenton-type process can help different kinds of reactions happen when iron (Fe) and copper (Cu) ions are present in the oxidation states of Fe²⁺, Fe³⁺, Cu⁺, and Cu²⁺. This is due to the possibility of valence cycling, which can lead to the formation of reactive oxygen species (ROS) [53], [55], [56]. However, understanding the catalytic mechanism remains a great challenge because of the difficulty in identifying the critical roles of coexisting Cu, Cu⁺, and Cu²⁺ species in technical catalysts[47]. The presence of Cu⁺ species in the LDH-Cu catalyst indicates that the primary reaction in these systems involves the interaction between Cu⁺ and H₂O₂, resulting in the production of hydroxyl radicals (OH), which are responsible for the degradation of the contaminant. This finding is consistent with Sun, Y., et al. 2019 [57]. In addition, it was observed that Cu²⁺ reacts with H₂O₂, producing O₂- [56]. Iron (Fe) significantly increases the presence of Fe³⁺, which under certain circumstances reduces to Fe²⁺. This study demonstrates the promotion of an Fe³⁺/Fe²⁺ cycle, resulting in an increase in hydroxyl radical production, as shown by Chen[58].

3.3.4 Surface area - adsorption - desorption of N₂

Figure 9a, 9b and 9c show that both the support (LDH) and the catalysts (LDH-Fe and LDH-Cu) exhibit type IV isotherms in accordance with the IUPAC classification. The results suggest that the material is predominantly mesoporous [49]. Furthermore, the three solids exhibited different hysteresis loops. The LDH support exhibited a H2(b)-type loop, indicating distinctive characteristics in the adsorption and desorption dynamics. The analysis of the catalysts shows that there are significant differences between them. Specifically, the LDH-Fe catalyst exhibits an H5-type hysteresis loop, whereas the LDH-Cu catalyst displays an H4-type hysteresis loop. These distinct hysteresis patterns are inherently linked to the unique textural properties of each material. The H5 loop in LDH-Fe indicates a mixture of open and blocked pores, expanding our understanding of the material's internal structure. The LDH-Fe and LDH-Cu catalysts exhibit different pore structures. On the other hand, the H4 loop in LDH-Cu indicates the co-existence of micro- and mesoporous-sized pores, adding complexity to its porous structure[49], [59] [59]. The surface area of double-layer hydroxides (LDHs) varies according to the literature and depends on the synthesis method used. Surface area values range from 4.0 m²/g to approximately 168 m²/g, according to the literature. It is worth noting that the surface areas of LDH, LDH-Fe, and LDH-Cu did not exceed 50 m²/g, as shown in Table 6. This result is consistent with previous investigations that used the coprecipitation method and a calcination temperature of 400°C [60], [61]. The determination of the crystallite size was performed using the Scherrer equation. For LDH-Fe, the peak at 2θ = 39.02 was employed, while for LDH-Cu, the peak at 2θ = 35.719 was used. LDH-Fe had an average crystallite size of approximately 56.14 nm, indicating the potential formation of crystalline aggregates. In contrast, LDH-Cu had a recorded crystallite size of 14.40 nm. The observed variation could be attributed to the reduction in surface area after Fe impregnation, which decreased from 27 m²/g to 12 m²/g, possibly due to metal agglomeration [62].

Table 7 shows that the LDH-Cu catalyst had the highest surface area and pore volume and achieved the best removal percentages in most of the evaluated treatments (Fig. 7). It was also found to be the most efficient, removing amoxicillin within 5 to 10 min in some cases. The catalyst's texture exhibits these characteristics because a larger specific area provides more active sites for catalytic activity. Furthermore, the catalyst's large pore volume can facilitate the transfer of reagents, which favors Fenton-type process reactions[50].

Table 7 Textural properties of the LDH support and LDH-Fe and LDH-Cu catalysts

Catalyst	Area BET (m²/g)	Pore volume (cm³/g)
LDH	27.	0,0907
LDH-Fe	12.	0,0412
LDH-Cu	44.	0,1280

3.4 Catalytic Activity

3.4.1 Reaction Blanks

To evaluate the relevance of the different components in the Fenton-type process, several reactions were performed under specific initial conditions. The initial conditions included a pollutant concentration of 10 ppm, a hydrogen peroxide concentration of 0.1M, and a catalyst amount of 0.12 g. 1. Absence of H₂O₂: The study examined the reaction in the absence of hydrogen peroxide, which is the primary promoter of the radicals that cause the degradation of pollutants. 2. Absence of air: The potential impact of air on the reactions was considered, as it may contribute to secondary reactions and the creation of reactive species. 3. Absence of light: Because amoxicillin is a photosensitive compound, we investigated whether the presence of light would increase the percentage of contaminant removal. 4. High purity standard of amoxicillin: We used a commercial amoxicillin product in the experiment, which contains several excipients. These excipients may also be vulnerable to radical action, which could impact degradation efficiency. To evaluate this, an amoxicillin standard with 98% purity was used. The purpose of these analyses was to isolate the individual contributions of the components of the Fenton system and gain a better understanding of their impact on the amoxicillin degradation process.

Figure 10 shows the significance of each component of the Fenton-like system. When the reaction took place without the presence of the oxidant hydrogen peroxide, the removal percentage of amoxicillin was approximately 10% with all catalysts. The absence of an oxidant in the medium, essential for catalyzing the oxidation-reduction reaction of the Fenton-like process and generating radicals conducive to contaminant decomposition, may explain the observed small reduction in contaminant concentration[63]. The small percentage removed may be attributed to an absorption process. However, reactions without air failed to remove more than 52%, whereas the presence of dissolved oxygen achieved 100% removal. The ability of dissolved oxygen to act as an electron acceptor and participate in secondary reactions that generate singlet oxygen (¹O₂), thereby contributing to the degradation of amoxicillin, explains this difference[63], [64]. Regarding reactions exposed to light, the elimination percentage did not show significant variations compared to the initial conditions. However, it was considered pertinent to carry out this experiment due to previous research demonstrating the feasibility of direct photolysis of amoxicillin [65]. Finally, degradation was evaluated using a high-purity amoxicillin standard. Comparing these results with reactions under initial conditions using commercial amoxicillin, it was confirmed that the Fenton-like process is highly effective. This is true even when excipients such as carbohydrates, polyols, and polysorbate are part of the formulation of commercial amoxicillin[66].

3.4.2 Catalytic Activity of Delaminated Clays

3.4.2.1 Effect of Hydrogen Peroxide Concentration

In Fenton-like systems, the concentration of hydrogen peroxide is a critical parameter because it is directly associated with the generation of reactive species responsible for the mineralization process of contaminants. Figures 11a, 11b, and 11c show the behavior of reactions with hydrogen peroxide concentrations of 0.05M, 0.1M, and 0.2M, respectively, using the DC-Fe catalytic system. Meanwhile, Figs. 11d, 11e, and 11f display reactions catalyzed with DC-Cu. When reacting with DC-Fe, increasing the peroxide concentration from 0.05M to 0.2M at an acidic pH resulted in complete (100%) elimination of amoxicillin within the first 20 min, compared to the 80 min required with a 0.05M concentration. Moreover, higher peroxide concentrations achieved contaminant removal at both acidic and neutral pH. At an alkaline pH, the removal percentage increased from 51–90%. Figures 11d, 11e, and 11f demonstrate the behavior of reactions catalyzed with DC-Cu. In contrast to those catalyzed with DC-Fe, the highest removal percentages of amoxicillin were observed under alkaline conditions, and in less time. Another difference was observed when increasing the concentration from 0.05M to 0.1M, complete removal was achieved in 20 min, but when increasing from 0.1M to 0.2M, an additional 20 min were required at alkaline pH for 100% removal. In reactions at acidic pH, an increase in peroxide concentration resulted in a 10% increase in removal when using a 0.2M concentration.

As the hydrogen peroxide concentration increased, the removal percentages approached 100% in most treatments. The efficiency of degrading organic contaminants depends on the number of active radicals generated from the interaction between hydrogen peroxide and the metal (Fe or Cu), whether hydroxyl (•OH) or superoxide (•O2-), as noted by Tian et al. (2023)[67]. However, an excessive amount of hydrogen peroxide can lead to radical annihilation, which decreases process efficiency. This was observed in processes catalyzed with DC-Cu when the concentration was increased from 0.1M to 0.2M at alkaline pH. In this case, it took 40 min to achieve 100% removal, compared to the 20 min required when using a 0.1M concentration of H₂O₂. Treatments catalyzed with DC-Fe behaved similarly to those in Tian et al.'s study. Increasing the peroxide concentration from 34 mg⋅L − 1 to 1700 mg⋅L − 1 resulted in an increase in the tetracycline removal rate from 55.4–95.9% using a Fenton-like process catalyzed with iron tailings. Furthermore, all treatments resulted in removal percentages exceeding 90% under all pH conditions and with both catalysts (DC-Fe, DC-Cu). This study's behavior aligns with Lin et al.'s (2023) report, which used a Fenton-like process to achieve high sulfamethazine removal percentages under pH conditions of 3, 5, 7, and 9 [68].

3.4.2.2 Effect of Contaminant Concentration

Figures 12a, 12b, and 12c show how the reactions changed as the amount of the contaminant AMX changed. These reactions were sped up by DC-Fe, and Figs. 12d, 12b, and 12c show reactions sped up by DC-Cu. At an acidic pH, we observed the highest percentage of removal for reactions with DC-Fe at 100%. As the contaminant concentration increased, the time to achieve 100% removal of amoxicillin decreased. For example, when the concentration was 1 ppm, it took 100 min for complete removal, which is twice the time required when the initial concentration of the contaminant was 20 ppm. In contrast, treatments with DC-Cu showed the best results for 100% removal at alkaline pH. Moreover, when the contaminant concentration was 20 ppm, it required an additional 40 min, compared to the 20 min needed for 100% removal when the concentration was 10 ppm. Treatments with Fe provide an explanation for the behavior of treatments when the initial concentration varies. This suggests that the amount of reactive species was sufficient to remove 100% of 20 ppm amoxicillin. Another possible reason for this behavior is that increasing the contaminant concentration will not reveal reactions where radicals recombine with each other, decreasing efficiency[69]. Instead, the current radicals interact directly with the contaminant. Li et al. (2024) conducted a study on the simultaneous removal of oxytetracycline and Cr (VI) using a photo-Fenton process[70]. The study found that as the concentration of contaminants increased, the antibiotic's removal percentage increased to 100%, while that of Cr (VI) increased to 82%. As the concentration of copper contaminants increases, a deficit of radicals may be generated, resulting in decreased efficiency [70]. Furthermore, many contaminants may hinder the interaction between hydrogen peroxide and the catalyst [71].

3.4.2.3 The Effect of the Amount of Catalyst

This study investigates the potential influence of catalyst quantity on amoxicillin degradation. Evaluations were conducted using quantities of 0.06, 0.1, and 0.24 g of catalyst, respectively. Figure 13 shows that as the amount of catalyst increased, the efficiency of breaking down amoxicillin also increased, as shown by the fact that the reaction time decreased noticeably. In reactions catalyzed with iron (Fe) at an acidic pH (as shown in Figs. 13a, 13b, and 13c), it was observed that increasing the amount of catalyst required more time to reach the maximum removal percentage. The maximum removal percentage was achieved at approximately 40 min with a catalyst quantity of 0.6 g, whereas reactions conducted with 0.12 g and 0.24 g required 80 and 120 min, respectively. Under optimal acidic pH conditions, the DC-Cu catalyst achieved 100% removal of amoxicillin across all pH conditions tested. Figures 13d, 13e, and 13f show that varying quantities of the DC-Cu catalyst resulted in different maximum removal percentages. Notably, total removal was achieved in 20 min under alkaline pH conditions, compared to the 100 min required with a higher catalyst quantity of 0.24 g. The treatments that achieved the highest removal percentage in the shortest time used the lowest evaluated catalyst quantity. This is because a large quantity of catalyst or an excess thereof can rapidly reduce the present peroxide, leading to excessive •OH formation. This can rapidly react with all active sites present, resulting in the rapid decrease of reactive species and the rapid formation of the hydroxide ion OH- [72]. Karaca et al.'s study revealed that optimal concentrations did not always yield the highest removal percentages. Using a heterogeneous sono-Fenton process, they evaluated doses of 0.1, 0.2, 0.25, 0.3, and 0.4 g/L of the MnFe₂O₄-rGO catalyst to remove the antibiotic tetracycline. At a catalyst load of 0.3 g/L, the removal was 92.85%, but at 0.4 g/L, it decreased to 82.67% [73].

3.4.2.4 Statistical Analysis for delaminated clay as catalyst

To examine potential significant differences in treatments when using various concentrations of peroxide (0.05M, 0.1M, and 0.2M), initial contaminant concentrations (1 ppm, 10 ppm, and 20 ppm), and different amounts of catalyst (0.06g, 0.12g, and 0.24g) at different pH levels (3, 7, and 10), a one-way analysis of variance (Kruskal-Wallis ANOVA) was used due to the non-normal distribution of the data. To accurately determine the most notable differences between groups, a Bonferroni post-hoc test was conducted. This analysis was performed after 20 min of reaction, revealing significant differences between the different pH levels in all evaluated treatments, with a p-value of 0.0036. The Bonferroni post hoc analysis revealed the most significant difference between pH 3 and 10 in DC-Fe treatments, and between pH 3 and 7 in DC-Cu treatments.

The results of the analysis indicate that both the concentration of hydrogen peroxide and the pH have a highly significant effect on the percentage of amoxicillin removal when using delaminated clay impregnated with iron. The pH accounts for a considerable 59.84% of the total variance, suggesting that it is the most influential factor in the removal of amoxicillin. In contrast, the H₂O₂ concentration, although also significant, represents only 6.33% of the total variance, indicating that its impact, although important, is less than that of the pH. Moreover, the interaction between these two factors accounts for 33.76% of the total variance, indicating that the effect of the H2O2 concentration on the percentage of removal varies significantly according to the different pH levels. About the catalytic treatments with clay delaminated impregnated with Cu, the results of the analysis indicate that both the concentration of H₂O₂ and the pH exert a highly significant influence on the percentage of amoxicillin removal in the Fenton-type process with Cu catalyst. The pH accounts for 54.35% of the total variance, indicating that it is the most influential factor in the efficiency of amoxicillin removal. The concentration of hydrogen peroxide, although also highly significant, represents 39.8% of the total variance, indicating that its impact is considerable but less than that of pH. Moreover, the interaction between these two factors accounts for 5.813% of the total variance. Although this percentage is relatively modest, the statistical significance of the data indicates that the impact of H₂O₂ concentration on amoxicillin removal varies according to different pH levels. This significant interaction suggests that both factors must be considered together in order to fully comprehend their impact on the removal of amoxicillin.

The catalytic effects of clay delamination impregnated with a specific catalyst, as well as the pH, have been observed to have a significant impact on the observed results after 20 minutes. The interaction between the catalyst quantity and pH is also highly significant, suggesting that the effect of one may depend on the level of the other. The quantity of catalyst accounts for 30.54% of the total variance, while the pH accounts for 47.72%. Both factors exert a substantial influence on the outcome, thereby underscoring their significance in the context of the experimental inquiry. In the Cu-catalyzed phase-active treatments, the quantity of catalyst is the predominant factor, accounting for the majority of the total variance (66.95%) and exerting a dominant influence on the percentage of amoxicillin removal. Although pH is a significant factor, it accounts for a smaller percentage of the total variance (28.93%). The interaction between the catalyst quantity and pH, although also significant, represents a smaller fraction of the total variance (4.08%). These findings underscore the necessity of optimizing the catalyst dosage to enhance the experimental outcomes, with due consideration of the pH level to achieve more precise and efficacious control.

The results of the analysis indicate that both the concentration of amoxicillin and the pH have a highly significant effect on the percentage of contamination removal in the Fenton-type process with delaminated clay impregnated with iron. The pH accounts for 75.39% of the total variance, indicating that it is the most influential factor in the efficiency of amoxicillin removal. The concentration of the contaminant, although also highly significant, represents only 3.735% of the total variance, indicating that its impact is considerably less than that of the pH. Moreover, the interaction between these two factors accounts for 20.78% of the total variance. Although this percentage is less than that of pH, the statistical significance indicates that the effect of contaminant concentration on amoxicillin removal varies according to different pH levels. This significant interaction suggests that both factors should be considered together in order to fully comprehend their impact on the removal of amoxicillin. Consequently, pH is the dominant factor in the variability of amoxicillin removal, followed by the interaction between pH and contaminant concentration. Although the concentration of contaminant is significant, its impact is less pronounced than that of pH. With regard to the treatments with delaminated clay impregnated with Cu, the analysis revealed that both pH and amoxicillin concentration exerted highly significant effects. The pH is the most influential factor, accounting for 68.05% of the total variance, indicating its predominant role in the efficiency of amoxicillin removal. The concentration of the contaminant, although also highly significant, represents only 15.16% of the total variance, suggesting that its impact is considerable but less pronounced than that of the pH. The interaction between pH and contaminant concentration explains 16.69% of the total variance, indicating that the combined effects of these factors are significant and should be considered when optimizing the process. The statistical significance of this interaction underscores the necessity of evaluating these factors in concert to fully comprehend their influence on the removal of amoxicillin. The pH is the most influential factor in the removal of amoxicillin when employing a Fenton-type process catalysed by delaminated clay impregnated with copper (Cu), followed by the interaction between the pH and the amoxicillin concentration, and then the contaminant concentration itself. These findings underscore the necessity of adjusting and controlling both factors to optimize the efficiency of the amoxicillin removal process.

3.4.3 Catalytic Activity of Layered Double Hydroxides: The Effect of Hydrogen Peroxide Concentration, Initial Pollutant Concentration, and Catalyst Amount

Figures <link rid="fig14">14</link>a and 14 show the effect of hydrogen peroxide concentration on the degradation of amoxicillin in the presence of Fe (LDH-Fe) and Cu (LDH-Cu). In both cases, an increase in the hydrogen peroxide concentration reduces the amoxicillin degradation time. As shown in Fig. 13a, the LDH-Fe catalyst can remove 100% of amoxicillin within 10 min under the examined hydrogen peroxide concentrations (0.05M, 0.1M, and 0.2M). In contrast, LDH-Cu catalyzed reactions achieve 100% removal in only 5 min at hydrogen peroxide concentrations of 0.1M and 0.2M, while at a lower concentration (0.05M), 10 min are required to achieve the same percentage of removal, as shown in Fig. 14d. Figures 14a and 14d demonstrate that the reactions under the 0.05M concentration took the longest time to achieve the highest percentages of amoxicillin removal in both systems. This pattern may be due to a scarcity of reactive species, because hydrogen peroxide is the primary source of radicals[74]. These results are similar to those found by Costa-Serge, who tested different concentrations of peroxide to get rid of the antibiotic sulfathiazole and discovered that higher concentrations of oxidant led to faster removal rates [75].

The study investigated the potential effect of the initial contaminant concentration on the effectiveness of the Fenton-type process catalyzed by double layer hydroxides (LDH). Three different concentrations of amoxicillin were examined: 20 ppm, 10 ppm, and 1 ppm. The results showed that LDH-Fe required an additional 5 min compared with LDH-Cu to reach a removal rate of 100% (Figs. 14b and 14e). Figure 14b shows that amoxicillin was almost completely removed in 5 min at concentrations of 10 ppm and 20 ppm, but not at a concentration of 1 ppm, which required 10 min for complete removal. The observed behavior can be attributed to the low concentration of contaminants, specifically 1 ppm. This lowers the probability of interaction with the reactive species, resulting in a decrease in the reaction rate. This phenomenon is related to collision theory, which states that the more reactants present, the more interactions occur, and the less interference there is in performing oxidation-reduction reactions [76]. According to Fig. 14e, the LDH-Cu catalyst can remove amoxicillin in just 5 min, regardless of the initial concentration. Therefore, it can be used to treat varying concentrations of amoxicillin.

The efficacy of the amoxicillin degradation process in Fenton-type chemical reactions directly depends on the amount of catalyst used. Figures 14c and 14f show the results of this analysis. The examination of LDH-Fe treatments showed that using 0.125 g of catalyst resulted in complete removal in just 5 min. This trend was also observed in the reactions catalyzed with LDH-Cu. The results differ significantly between the two active phases, which correspond to 0.06 g and 0.25 g of catalyst. Reactions with Fe, using either 0.06 g or 0.25 g of catalyst, require 20 min for the total elimination of 100%. Treatment with 0.06 g of catalyst in the presence of Cu is insufficient for the complete removal of amoxicillin in 20 min, whereas 0.25 g achieves complete removal in only 5 min. Based on Fig. 14, LDH-Fe achieved complete removal within 20 min when the catalyst amount was 0.06g under both systems With regard to the treatments with delaminated clay impregnated with Cu, the analysis revealed that both pH and amoxicillin concentration exerted highly significant effects. The pH is the most influential factor, accounting for 68.05% of the total variance, indicating its predominant role in the efficiency of amoxicillin removal. The concentration of the contaminant, although also highly significant, represents only 15.16% of the total variance, suggesting that its impact is considerable but less pronounced than that of the pH. The interaction between pH and contaminant concentration explains 16.69% of the total variance, indicating that the combined effects of these factors are significant and should be considered when optimizing the process. The statistical significance of this interaction underscores the necessity of evaluating these factors in concert to fully comprehend their influence on the removal of amoxicillin. The pH is the most influential factor in the removal of amoxicillin when employing a Fenton-type process catalysed by delaminated clay impregnated with copper (Cu), followed by the interaction between the pH and the amoxicillin concentration, and then the contaminant concentration itself. These findings underscore the necessity of adjusting and controlling both factors to optimize the efficiency of the amoxicillin removal process [77].

3.4.3.1 Statistical Analysis for double-layer hydroxides as catalyst

The analysis of variance (ANOVA) revealed no significant differences between peroxide concentrations when the reactions were catalyzed with LDH-Fe (p > 0.05). However, variations were noted concerning hydrogen peroxide concentrations evaluated with the LDH-Cu catalyst. This suggests that the concentration of hydrogen peroxide may influence the percentage degradation of amoxicillin, depending on the catalyst used. Further investigations are needed to elucidate the underlying mechanisms driving these differences and to optimize the conditions for enhanced degradation efficiency. Notably, in systems employing LDH-Cu, the most efficient treatment was observed at a concentration of 0.1M, as determined by Tukey's test.

The LDH-Fe treatments were significantly different in the analysis of variance (ANOVA) (p < 0.05, confidence interval), and Tukey's test showed that the treatment with an initial contaminant concentration of 10 ppm worked the best. In contrast, the LDH-Cu treatments did not show significant differences among the different contaminant concentrations (p > 0.05).

However, the analysis of variance (ANOVA) showed significant differences between treatments when varying the amount of LDH-Fe (p < 0.05). However, differences were identified in the treatments with LDH-Cu between the smallest amount (0.06 g) and the amounts of 0.12 g and 0.25 g, but no differences were found between the larger amounts (p < 0.05). The Tukey test indicates that the most effective treatment using LDH-Fe is the one with 0.125 g of catalyst. Conversely, the highest amounts of catalyst (0.25 g) yielded the best results in the LDH-Cu treatments.

3.5 Catalyst Stability

The stability and reusability of the catalysts were evaluated in the degradation of amoxicillin using a Fenton-type process over four consecutive cycles. Figure 15 shows that all catalysts were highly efficient in removing amoxicillin and exhibited great stability. The LDH-Cu catalyst was able to remove 100% of the contaminant during the first two cycles. In the third cycle, the removal rate decreased to 85%, and in the fourth cycle, it decreased further to 50%. In contrast, the LDH-Fe catalyst showed a decrease in removal efficiency starting from the second cycle, with a removal rate of 82%. By the fourth cycle, the removal rate had decreased to 53%. One of the causes of decreased activity is the absorption of organic acids generated during the oxidation process on the catalyst surface[78]. Figure 16 illustrates the generation of various degradation by-products from decarboxylation processes. Another possible reason for the decrease in activity is the loss of catalyst mass due to the treatments required to recover the catalyst after the reaction [79], [80].

3.6 Probable degradation pathway of Amoxicillin

Based on the catalytic results, the optimal catalyst was chosen due to its ability to achieve the highest percentage of removal in the shortest amount of time across most treatments. The double-layer hydroxides impregnated with Cu were selected. to determine the by-products. To establish a potential amoxicillin degradation pathway via Fenton-type processes catalyzed by double-layer hydroxides impregnated with iron and copper, various degradation byproducts were identified using mass spectrometry (Fig. 16). The identified byproducts included previously reported structures found in similar studies of amoxicillin oxidation[81]. Likewise, the authors propose new structures considering the high oxidation rates of nitrogen compounds in the direct products of aromatic ring oxidation [82]. Figure 16 illustrates the degradation process, which begins with the opening of the β-lactam ring and the oxidation processes of the amines or amides. The results suggest that amines are oxidized to nitro or nitroso groups, and amides are oxidized to hydroxamic acid, which exhibit higher molecular weights (383, 381, and 431 m/z). On the other hand, the amide hydrolysis processes generate a 234 m/z fragment, which then undergoes oxidative deamination, resulting in the production of a 217 m/z product. Furthermore, it is suggested that the 431 m/z product, which has the highest molecular weight, undergoes denitration and dehydration processes, resulting in compounds with m/z values of 366 and 349. Regarding the 383 m/z compound, two decarboxylations and one deamination are proposed, yielding compounds with m/z values of 339, 324, and 294. In addition, lower molecular weight products with m/z values of 230, 217, 189, and 159 were generated through hydrolysis of hydroxamic acid, denitrosation, and loss of oxalate. The degradation of amoxicillin can undergo various chemical processes, resulting in diverse degradation products that may have significant environmental implications. Lugo evaluated the toxicological impact of the effluent generated from the degradation of amoxicillin using a Fenton-type process catalyzed by lamellar structures, including layered double hydroxides impregnated with Fe or Cu, and found that the effluent had negative effects on L. sativa, S. capricornutum, and D. magna. Therefore, post-treatment should be conducted to ensure that the effluents are non-toxic [83].

A study looked at how well catalysts made of delaminated clay or double-layer hydroxides (LDHs) filled with iron (Fe) and copper (Cu) break down amoxicillin, which is becoming a bigger problem in drinking water. The results indicate that these catalysts show significant promise for eliminating antibiotics from aqueous systems through Fenton-like reactions. Experiments were conducted to demonstrate the exceptional performance of Cu-impregnated LDHs in terms of degradation rate and efficiency. Conditions such as catalyst amount, initial contaminant concentration, and hydrogen peroxide concentration were varied. Characterization techniques yielded valuable information on the structure and composition of LDHs, aiding in the understanding of their enhanced catalytic ability. The LDH-Cu catalysts' ability to completely remove amoxicillin in just 20 min is a highly promising outcome. It is also critical to identify the reaction by-products during the degradation process to fully understand the chemical transformation of amoxicillin. These by-products may have significant environmental effects and require further investigation to determine their toxicity and longevity. This research shows that Fe- and Cu-impregnated LDH-based catalysts might be able to get rid of amoxicillin and maybe even other pharmaceutical compounds that are in dirty water. These findings have significant implications for the development of advanced water treatment methods that address concerns regarding the presence of emerging contaminants and antibiotic resistance in aquatic environments.

Funding: This research and the publication of this article were funded by the Pontificia Universidad Javeriana, Bogotá, Colombia. No. 20078 -Support for research projects related to the Encyclical Laudato Sí (VRI14-19).

Author Contributions: Lorena Lugo (L.L): Conceptualization, Formal analysis, Investigation, Data curation, Writing original draft. John Diaz (J.D): Formal analysis, Data curation. Julian Contreras (J.C): Methodology, Formal analysis. Sergio Diaz (S.D): Data Curation. Miguel Centeno (M.C): Methodology, Resources. Juan Carlos Cortés García (J.C.C): Methodology. Sonia Moreno (S.M): Conceptualization. Alejandro Perez-Flórez (A.P-F): Conceptualization, Methodology, Validation, Formal analysis, Writing-review and editing. And Crispin Celis (C.C): Conceptualization, Formal analysis, Writing-review and editing, Supervision, Project administration, Funding acquisition.

Institutional Review Board Statement: Not applicable

Informed Consent Statement: Not applicable

Conflict of interest" and "Data Availability" Statement: The authors declare that they have no conflict of interest".

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Removal of amoxicillin employing Fenton-type process using delaminated clay and layered double hydroxides impregnated with Fe or Cu as catalysts

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Abstract

Figures

1. Introduction

2. Materials and Methods

2.1 Synthesis of catalyst support

2.1.1 Synthesis of delaminated clay

2.1.2 Synthesis of Layered double hydroxides

2.2 Catalyst synthesis

2.3 Characterization

2.4 Amoxicillin removal: catalytic evaluation

2.5 Statistical Analysis

3. Results and discussion

3.1 Characterization of delaminated clay (DC)

3.1.1. Scanning Electron Microscopy (SEM)

3.2.2 X-ray Diffraction

3.2.3 X-ray photoelectron spectrometry

3.2.4 Surface area - adsorption - desorption of N2

3.3 Characterization of double layer hydroxide (LDH)

3.3.1 Scanning Electron Microscopy (SEM)

3.3.2 X-ray Diffraction

3.3.3 X-ray photoelectron spectrometry

3.3.4 Surface area - adsorption - desorption of N2

3.4 Catalytic Activity

3.4.1 Reaction Blanks

3.4.2 Catalytic Activity of Delaminated Clays

3.4.2.1 Effect of Hydrogen Peroxide Concentration

3.4.2.2 Effect of Contaminant Concentration

3.4.2.3 The Effect of the Amount of Catalyst

3.4.2.4 Statistical Analysis for delaminated clay as catalyst

3.4.3.1 Statistical Analysis for double-layer hydroxides as catalyst

3.5 Catalyst Stability

3.6 Probable degradation pathway of Amoxicillin

4. Conclusions

Declarations

References

Additional Declarations

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3.2.4 Surface area - adsorption - desorption of N₂

3.3.4 Surface area - adsorption - desorption of N₂