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Article

Experimental Design and Numerical Optimization of Photochemical Oxidation Removal of Tetracycline from Water Using Fe3O4-Supported Fruit Waste Activated Carbon

by
Manasik M. Nour
1,
Maha A. Tony
2,3,*,
Hossam A. Nabwey
1,* and
Shaaban M. Shaaban
4
1
College of Science and Humanities in Al-Kharj, Prince Sattam bin Abdulaziz University, Al-Kharj 11942, Saudi Arabia
2
Advanced Materials/Solar Energy and Environmental Sustainability (AMSEES) Laboratory, Basic Engineering Science Department, Faculty of Engineering, Menoufia University, Shebin El-Kom 32511, Egypt
3
Planning & Construction of Smart Cities Program, Faculty of Engineering, Menoufia National University, Birket el Sab 32651, Egypt
4
Center for Scientific Research and Entrepreneurship, Northern Border University, Arar 73213, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(4), 351; https://doi.org/10.3390/catal15040351
Submission received: 19 February 2025 / Revised: 1 April 2025 / Accepted: 2 April 2025 / Published: 3 April 2025
(This article belongs to the Special Issue Remediation of Natural Waters by Photocatalysis)
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Abstract

:
The ever-increasing importance of sustainable environmental remediation calls for academics’ contribution to satisfy such a need. The 3R’s criteria of recover, recycle and reuse is designed to sustain the waste stream to produce a valuable product. In this regard, the circular economy looks to deliver banana peel waste as a photocatalyst for pharmaceutical effluent oxidation, which we investigated in this study. Banana peel waste is treated thermally and chemically then augmented with magnetite nanoparticles and labeled as ACBP-Fe3O4. The mixture is characterized through Scanning Electron Microscopy (SEM) and the composition of the composite material is attained by energy dispersive X-ray spectroscopy (EDX), and then introduced as a Fenton catalyst. The notable oxidation of tetracycline (TC), evaluated by TC removal and chemical Oxygen Demand (COD) oxidation tenancy, is achieved. The effectiveness of the operational parameters is also assessed and the most influenced parameters are optimized through numerical optimization based on a Response Surface Methodology (RSM) tool. The effects of initial pH value, ACBP-Fe3O4 and H2O2 concentrations on the oxidation efficiency of the Tetracycline were optimized at pH 6.6 and 350 mg/L and 43 g/L for H2O2 and ACBP-Fe3O4, respectively. Thermodynamics and kinetics were also studied and the experimental and model data revealed the reaction is spontaneous and exothermic in nature and follows the first-order reaction kinetics. Also, the thermodynamic results the reaction proceeds at a low energy barrier of 34.33 kJ mol−1. Such a system introduces the role of engineers and academics for a sustainable world without a waste stream.

Graphical Abstract

1. Introduction

Today, one of the major concerns that the world is suffering from is the global water crisis, particularly experienced in most developing parts of the world [1]. Unskilled consumption and adaptation of natural water resources with a rapid rate of population and global industrialization increases the necessities of fresh water. But with the limited availability of fresh water resources, about one billion human beings are categorized as having access to unsafe drinking water [2,3,4,5]. Furthermore, such statistics might be due to the fact that the world population is predicted to rise by ten billion by the year 2050. The successful adoption of a water roadmap is dependent on the availability of a sustainable method of water conservation. Cost and eco-friendly water management are the key. In order to cut down the cost of treatment, an industrial ecology approach is essential [6,7,8,9].
Currently, reports declare that there are no regulatory protocols for chemical chiral pollutants, including some drug residues that threaten human health due to the carcinogenic effects and economic losses linked to them [10,11]. Their presence in minimal concentrations in water as low as 1.0 ng/L might affect fish reproduction capability [12]. Generally, when pharmaceutical contaminants are present in the aquatic system, it causes drugs resistance in creatures [7]. Also, some pharmaceuticals and drug contaminants are converted onto receptor-specific chiral metabolites that trigger severe health outcomes [13,14,15].
Massive amounts of wastewater are generated from pharmaceutical discharge. Among the most applied medications, antibacterial drugs such as tetracycline (TC) are regarded as the most generally used [16,17,18]. Thus, this indicates that wastewater is highly contaminated with this drug. But tetracycline has become a severe warning to the environment due to its overuse by individuals [8]. On the other hand, this material possesses a weak ability to degrade, thereby accumulating along the food chain and causing toxicity to water and imbalance in the ecosystem. Thus, treating such discharge is a must. Effective selection of wastewater treatment is essential since ineffective wastewater treatments are among the main causes for the augmented antibiotic concentrations in water resources [5,19,20,21,22].
Various researchers [8,10,23,24] dealing with chiral pollutant removal facilities have introduced numerous research attempts. Such attempts include physical [25], chemical [26,27] and biological [28] treatments, for instance, filtration, coagulation, sedimentation, membrane separation, evaporation, reverse osmosis, ion exchange and adsorption [29,30,31,32,33]. However, such techniques harnessed the pollutants only by transferring them from one phase to another with no further mineralization [9,34]. Thus, advanced oxidation process (AOP) and photo-catalysis are gaining scientists’ attention, due to their complete mineralization of such pollutants [35,36,37,38]. Fenton’s oxidation technique, as one of the advanced oxidation processes, is gaining special interest due to its considerable high yield at low treatment costs for mineralizing undesirable chemicals and hazardous discharge [39].
Fenton’s reaction is categorized by its unique photocatalytic activities and stable chemical structure [40,41,42]. However, such a reaction is not regarded as an ideal real applicable solution, since it possesses some limitations, such as the cost of the process due to the price of fresh chemical precursors as well as the essential nature of the reaction to work under acidic pH conditions, which requires excess treatment prior to the final disposal [14]. Thus, such issues make the application even more complicated [43,44]. Consequently, various advances have been outlined to overcome the drawbacks of the Fenton system reaction. The construction of a hetero-junction catalyst as a heterogeneous photocatalyst is required for adequate recovery facilities, and sustainable reuse is essential [45]. Nanoparticles, especially those that possess magnetic properties, are introduced as an excellent strategy for the option of recover, recycle and reuse facilities [46]. Thereby, the catalytic Fenton’s oxidation is gaining researchers’ interest.
On the other hand, with the major issues of solid waste, their elimination is gaining considerable attention [6]. Such waste material and their derivatives possess numerous functional groups that could be used for attacking the water contaminants and thereby eliminating them [6]. Biomass material waste valorization is most preferred recently due to its economic value and environmental friendliness [4,47,48,49,50]. In this regard, such substances have expanded the attention for pollutant adsorption from aqueous discharge [51]. Efficient bio-waste materials have been tested, including potato peels, corn cob, sugarcane bagasse, rice husk, and banana peel, just to mention a few [52,53,54]. But this method has not been applied so far, to the authors’ knowledge, as a photocatalyst in modified form.
The production of carbon from waste streams, namely activated carbon (AC)-based materials, has received great attention from researchers due the fact that it possesses reliable characteristics, including economic cost, large surface area and high catalytic activity. Furthermore, such materials become more reliable and economic when prepared from waste streams through chemical and physical treatments [55,56,57,58,59]. Additionally, their catalytic activity could be improved through modification and functionalization [57]. Amongst the chemical modifications, metal oxides are regarded as valuable materials due to their reliable efficiency in pollutants’ remediation. Different waste biomass materials have been introduced as a source of AC, such as wood waste and lignin, rice husk and fruit waste [56,57,58,59,60]. Chemical activation techniques are essential to promote a carbonaceous structure of high porosity and large surface area, with various functional groups for numerous applications, especially wastewater treatment [57].
On the other hand, to date, numerous research articles have introduced the improvement of the photocatalytic characteristics of metal oxides by doping of varied inorganic and organic materials through simple, cost-effective techniques. These include wet chemical reduction, solvothermal facilities, hydrothermal techniques, wet chemical reduction, the sol-gel method and chemical deposition [55,58]. Also, their photocatalytic performances could be explored under visible, ultraviolet irradiance and/or sunlight radiation against toxic chemicals. Such materials include numerous nanostructured materials and their composites, such as carbon-based materials [59,61]. Such augmentation of metal oxides with AC-based materials could improve the surface catalytic activity, enduring photostability, and remarkable advantageous optical features [62,63,64,65,66]. Furthermore, the use of iron-based materials is superior, since they exhibit improved separation efficiency, contributing to sustainable use [59,67,68,69]. It makes the catalyst a suitable candidate for efficient utilization. Thus, combining metals oxides with carbon materials as a photocatalyst is a research topic.
Herein, activated carbon (AC) derived from waste banana peels modified with magnetite (ACBP@Fe3O4) was used as a photocatalyst for tetracycline (TC) removal from wastewater. ACBP@Fe3O4 nanoparticles were selected due to their cost-efficiency, magnetic properties, as well as the inclusion of various mixed elements that might help in the Fenton’s oxidation. Also, it has the advantage of simple, reliable and cost-efficient preparation with a high yield. The optimum operating Fenton’s parameters have been evaluated through experimental design using response surface methodology (RSM) techniques. The modified Fenton’s reaction is a superior candidate to substitute the classical Fenton’s type with no restrictions on the acidic pH and a recyclable environmentally benign catalyst facility.

2. Materials and Methods

2.1. Synthesis of Photocatalyst

Initially, the banana peels (BPs) were collected from a local fruit shop and the waste material was used to prepare the activated carbon (AC). Chopped and dried banana peels were used to prepare activated carbon. To prepare this type of activated material, initially, the peels were kept overnight at 70 °C. Thereafter, the dried material was first soaked in concentrated phosphoric acid (60%) for 12 h. Then, the soaked material was semi-carbonized in an electric oven to 200 °C (4 h). The semi-carbonized substance was cooled to room temperature then subjected to further heating and carbonized at 500 °C (2 h) for activation. The obtained material is labeled as ACBP. Afterwards, the attained material was successively washed with distilled water to remove any excess acid until exposure to a neutral pH. Finally, the obtained material was finally washed with NaOH solution (0.1 M), followed by distilled washer washing. The produced powder was dried in an oven 105 °C (5 h) and ground in a mortar to obtain a fine powder.
On the other hand, a simple co-precipitation routine was applied for Fe3O4 nanoparticle preparation. The nanoparticles were prepared by mixing Fe2(SO4)3 and Fe(SO4) in stoichiometric ratios with distilled water. Afterwards, a sodium hydroxide solution was added dropwise until the pH reached the alkaline range (about 11.0); in this stage the Fe3O4 nanoparticles were produced. Next, the liquid was elevated to 80 °C under continuous stirring. Subsequently, the material under went washing with distilled water to remove Na2SO4 and the excess NaOH and Na2SO4 until it reached neutral pH. The obtained substance was filtered and subjected to oven drying at 60 °C.
The powder activated carbon banana peel (ACBP) substance and the magnetite nanoparticles (Fe3O4) were then mixed in a ratio of 2:1 (ACBP: Fe3O4), respectively. This ratio was used according to a preliminary study and this is regarded as a cost-effective ratio. Then, the resultant material was labeled as ACBP-Fe3O4. This material was used as the source of Fenton’s reaction as a modified Fenton’s type. The graphical representation of the preparation steps is displayed in Figure 1.

2.2. Characterization of the Photocatalyst

The structure of the prepared sample is characterized by X-ray diffraction (XRD) measurements performed on a Bruker-Nonius Kappa CCD diffractometer (Bruker, Billerica, MA, USA) with CuKα radiation source (λ = 1.5406 Å).
The morphology of the prepared ACBP-Fe3O4 substance was evaluated and imaged by field-emission scanning electron microscope (SEM) (FE-SEM, Quanta FEG 250, Madrid, Spain). Typical magnifications used were x8000 and x60,000. This measurement was accompanied by energy dispersive X-ray spectroscopy (EDX). The principal elements’ contents in the ACBP-Fe3O4 were examined by energy dispersive spectrum. Also, the BET surface area of the ACBP-Fe3O4 material was determined using a nitrogen gas sorption analyzer (Quantachrome, Model: TouchWin version 1.21, Sydeny, Austria).

2.3. Methodology

A lab-scale batch mode setup is applied as an experimental test using a 12 W UV lamp. The lamp is a glass sleeve inside the reactor, immersed in a glass container. Initially, ACBP-Fe3O4 is added to 100 mL of wastewater solution, where the pH is first adjusted when required. Afterwards, the reaction is initiated by adding H2O2 reagent at a certain concentrations. H2O2 (30% w/v) was used and applied in the reaction medium as the initiator of the modified Fenton oxidation reaction. The pH of the tetracycline antibiotic-containing aqueous solution was recorded at 6.5 and then was adjusted, if needed, to the required values using H2SO4 or NaOH (all supplied by Sigma-Aldrich, St. Louis, MO, USA).
The reagents are immersed in the aqueous solution, then the solution is poured into the photo-chemical reactor. After 10 min of reaction time treatment, the substrate of the treated wastewater samples is subjected to TC and COD removal analysis.
The graphical illustration of the treatment technique augmented with the material the preparation steps is exhibited in Figure 1.

2.4. Analysis

A UV-vis spectrophotometer (Unico UV-2100 spectrophotometer, Dayton, NJ, USA) was used to evaluate the residual TC concentration at set time intervals. Prior to measurements being made, the ACBPs augmented with Fe3O4 catalyst were exposed for separation through the use of a micro-filter (0.45 µm). The samples’ pH was adjusted when needed by using a digital pH meter (Model AD1030, Adwa instrument, Szeged, Hungary). Also, the measurement of its chemical oxygen demand (COD) following standard methods (APHA, 1998) was carried out.

2.5. Box–Behnken Factorial Design

An experimental design model and statistical data analysis are applied for estimating the optimum effect of the operating parameters for the applied modified ACBP-Fe3O4-based photo-Fenton system. Response surface methodological analysis (RSM) related to the experimental factorial design is mostly applied to optimize the multi-varied response. The Box–Behnken experimental factorial design model was selected to represent the data. A 3-factor design, codded as –1, 0 and 1, was projected for each selected most influenced operational parameter, including H2O2 dose, ACBP augmented with Fe3O4 catalyst dose and the starting pH value. Therefore, the total design matrix was proposed as fifteen runs through the application of SA (Statistical Analysis) Software (SAS 9.4). A mathematical model was established and then the link between the selected predicted and experimental response and the 3 independent parameters was projected, referring to the following second-order polynomial model equation:
f = β o + ∑ β i X i + ∑ β i i X i 2 + ∑ β i j X i X j
where f is the predicted values of the response of the TC oxidation efficacy (%); βo, βi, β2 and βii are the model coefficients and Xi and Xj are the independent variables (SAS 9.4).
Mathematica software (version V 5.2) was applied to evaluate the optimized values of the coded independent factors for the interactive variables.

2.6. Statistical Analysis

For the purpose of obtaining the statistical significance of the proposed model, the data produced from the RSM design are statistically analyzed using analyses of variance (ANOVA) to estimate the significant difference (p < 0.05) between the same experimental group at different times and between the different experimental groups in the same experimental period. The statistical analysis was performed using Statistical Analysis software SAS (1990, ver 9.4).

3. Results and Discussion

3.1. Characterization of ACBP-Fe3O4

Figure 2 shows the XRD diffractogram of the ACBP-Fe3O4 substance showing two wide peaks (at 24° and 43°), which correspond to the (002) and (100) reflections planes of graphite (Pattern Diffraction File database, PDF 41–1487), respectively [70]. Furthermore, this XRD pattern verified the coexistence of Fe3O4 and ACBP in the substance. This is verified with the characteristic peaks of the magnetite. The peaks at 2θ values of 30° (220), 35° (311), 43° (400), 57° (511), and 63° (440) are in agreement with the standard XRD data for the pure cubic spine crystal structure of magnetite (JCPDS No. 89-4319) [71,72].
The physical morphology of the prepared ACBP-Fe3O4 material particles is exhibited in Figure 3. The figure displays the SEM images, which reveal that ACBP-Fe3O4 possesses a heterogeneous structure with non-uniform shape mixture. Consequently, the ACBP-Fe3O4 material retains its uneven structure. Also, the porous structure of the prepared ACBP-Fe3O4 is verified by its BET surface area, which signifies 428 m2/g BET surface area.
Elemental analysis of ACBP-Fe3O4 material as determined by EDX is demonstrated in Table 1. The calcined banana peel powder augmented with Fe3O4 contains the main Fe and C components, along with Cu and Zn, with the presence of small amounts of S, Cr, N and Ni. The augmentation of magnetite with the calcined material is reflected in the composition of the catalyst produced. Also, carbon is reflected in the biomass waste pyrolysis. The presence of these elements, along with Cu and Zn, supports the material’s potential role as a photocatalyst.

3.2. Photochemical Oxidation

3.2.1. Effect of Oxidation Time and Different Oxidation Systems

Primarily, to examine the optimum introduced modified photo-Fenton’s oxidation time for both tetracycline (TC) and chemical oxygen demand (COD) removals (% from contaminated aqueous effluent for further following tests), experiments were executed at various oxidation times, and both values of TC and COD oxidation efficiency rates were evaluated. Also, for the object of comparison, the modified Fenton’s reaction system is compared with the solo oxidation systems of ACBP/UV, ACBP, ACBP-Fe3O4/UV or H2O2/UV. The modified Fenton’s reagents were selected as 40 mg/L for ACBP and 400 mg/L for hydrogen peroxide reagent (H2O2), respectively. The data exhibited in Figure 4A,B verified that the values of TC and COD for all studied systems declined as the reaction time increased. For the modified Fenton’s reaction system, a rapid oxidation rate was first detected within the initial 10 min of irradiance time. Then, a steady oxidation rate was observed for the next 20 min. The total TC and COD removal values reached 95% and 93%, respectively. However, for the solo and magnetite-free systems, the COD removal values reached only 25, 33 and 66%, for ACBP/UV, H2O2/UV and ACBP/H2O2/UV, respectively. However, for the dark ACBP, the oxidation reached only 13% of COD reduction and 18% for TC removal. Also, the TC removal values declined from 95% to 33, 42 and 78% for the magnetite-free systems (ACBP/UV, H2O2/UV and ACBP/H2O2/UV, respectively). Thus, the investigation confirms the role of the combined Fenton system in the treatment.
The oxidation rate detected via TC and COD deduction is attributed to the establishment of the most reactive oxidation species of hydroxyl radicals (·OH) in the case of the Fenton oxidation reaction. These hydroxyl OH radicals possess viable oxidation ability, thereby attacking the tetracycline in the contaminated water and mineralizing it. Still, the solo system exhibited some oxidation capability, but at a low reduction rate. This result confirms the existence of the Fenton’s reaction in oxidizing the pollutants. This might be linked to the extra ·OH radicals’ production rate in the case of the combination of hydrogen peroxide and catalyst.

3.2.2. Effect of Fenton’s Reagent Parameters

Effect of Reaction pH

In general, the Fenton oxidation system is too sensitive to the operating pH conditions due to the fact that this oxidation reaction system is particularly determined by the generation of ·OH radicals. As previously stated in the cited data, pH plays a significant role in the Fenton oxidation reaction, since the Fenton reaction has a preferred optimum pH value that sharply affects the OH radicals’ production. Generally, .OH radicals play the role of the workhorse of H2O2 augmented with the catalyst oxidation systems. Consequently, iron speciation and hydrogen peroxide decomposition are affected by the oxidation reaction. Furthermore, the oxidizing power of hydroxyl radicals declines under high-pH conditions.
In this regard, experiments have been conducted in various pH conditions, and the results are displayed in Figure 5A,B. The data indicate that decreasing the pH to acidic conditions is not favorable. The highest TC and COD removal values correspond to pH 6.5, which is the natural pH of the wastewater without further adjustment. The TC and COD are 98 and 94%, respectively. But a further increase in pH into alkaline conditions is not favorable and results in a decline in the oxidation rate. TC oxidation strongly reduces as ACBP catalyst complexes might precipitate in the aqueous mixture instead of generating the OH radicals’ species in the aqueous reaction medium for the oxidation reaction. Therefore, the reaction includes the organometallic complex. On the other hand, the production of hydroxyl (·OH) radicals under mild pH conditions was maximized. These data verify the upsurge in the TC removal at the natural pH value, and the most noteworthy oxidation rate for both TC and COD was attained at a pH value of 6.5. Also, it is worth mentioning that the acidic pH conditions also oxidize TC to 62%, which is not too high for the natural pH. It is essential to mention that this treatment overcomes the drawback of Fenton’s limitation to the acidic pH [12]. This could be attributed to the multi-element source of the Fenton’s catalyst, and as such, might work under various pH conditions. Also, the unsuitable pH further scavenges the hydroxyl radicals’ effect and precipitates the elements in the reaction rather than reacting with hydrogen peroxide to generate OH radicals [40]. Furthermore, the presence of activated carbon in this treatment further treats the wastewater. These data indicate that the modified Fenton reaction based on ACBP-Fe3O4 was a superior catalyst for modified Fenton’s reagent to treat pharmaceutical discharge contaminated with TC due to the fact that it delivers great oxidation and elimination of organics at low cost, as well as eradicating the fruit waste.

Effect of H2O2 Reagent

The existence of .OH radicals and oxygen in the reaction medium is mainly related to the occurrence of H2O2. From this point of view, the influence of the concentration of hydrogen peroxide on the modified ACBP-based Fenton’s reaction was investigated by altering the initial hydrogen peroxide dose from 100 to 800 mg/L at natural pH (6.5) and ACBP-Fe3O4 of 40 mg/L.
Figure 6A,B exemplify the TC and COD removal efficiency, respectively. The oxidation efficacy is improved by the elevation in the hydrogen peroxide reagent from 100 to 400 mg/L, improving the %COD removal (from 55 to 95%) and %TC reduction (from 47 to 98%). This could be related to the production of hydroxyl (·OH) radicals, which are chiefly responsible for the oxidation process. Nevertheless, the excess upsurge in H2O2 dose above the concentration limit results in a decline in both %COD and %TC elimination. The excess H2O2 leads to the auto-decomposition of the hydrogen peroxide reagent into O2 and H2O, which could illustrate this. Also, the resultant OH radicals in the reaction medium further recombine with each other. Hence, extra H2O2 beyond the optimal limit serves as a ·OH radical scavenger rather than a generator. Our examination verifies the role of an optimum dose of hydrogen peroxide in generating hydroxyl (·OH) radicals [12,44]. These data are in agreement with those previously reported in the cited literature regarding the elimination of pollutants from wastewater by the Fenton’s reaction oxidation system.

Effect of Catalyst “ACBP-Fe3O4” Reagent

Besides the initial dosing of H2O2 reagent, the initial catalyst dose limit is also important. The catalyst catalyzes the production of hydroxyl radicals from the hydrogen peroxide reagent. Hence, the production of (hydroxyl ·OH) radicals is exaggerated by the ACBP-Fe3O4 catalyst dose, as displayed in Figure 7A,B. ACBP catalyst addition into the reaction was conducted at different ACBP-Fe3O4 concentrations in the range from 10 to 80 mg/L. The pH of 6.5 and 400 mg/L of hydrogen peroxide dose were maintained at their constant optimal levels. As exhibited in Figure 7A,B, the increase in ACBP concentration has a favorable influence on both the TC and COD oxidation efficiency until a specific critical value limit of 40 mg/L. But the oxidation rate was produced with the elevation in the ACBP-Fe3O4 dose above this 40 mg/L concentration. Therefore, ACBP-Fe3O4 species with various elements are a limiting reagent in the modified Fenton reaction system and may exist at much lower concentrations than hydrogen peroxide reagent. This examination of the use of minimal catalyst concentration validates the advantage of the modified Fenton method in diminishing the final sludge generated after the oxidation of wastewater. Villegas-Guzmana et al. [14] reported this data trend in previous research in treating contaminated aqueous effluent with organics using the Fenton system.

3.3. Numerical Optimization

3.3.1. Model Establishment

Numerical optimization is applied to assess and optimize the ACBP-based Fenton’s reagent parameters based on Box–Behnken factorial design as a response surface methodology (RSM) tool. The design is applied to explore the influence of the 3 effective modified Fenton’s reaction operating variables, namely H2O2, ACBP-Fe3O4 concentrations and pH. According to the preliminary experiments, the range as well levels of the experimental domain design were determined, and these values are listed in Table 2, including their coded and corresponding natural parameter levels. The presentation of the modified Fenton system was assessed by the TC removal, in terms of %, as the predicted and experimental response.
A total of 15 experimental runs are displayed in Table 3, in agreement with the design model in terms of the predicted and natural (uncoded) values of their consistent predicted and experimental responses. The data exhibited in Table 4 are the triplicates of the experimental runs. Regarding the experimental data, a second-order polynomial equation of the predicted model was projected to examine the correspondence of the response function.
Consequently, the quadratic polynomial equation in the terminologies of the coded parameters levels is exhibited according to the following equation:
f = 44.33 − 24.87   X 1 + 7.54   X 2 + 3.62   X 3 + 0.45 X 1 2 − 7.13 X 1 X 2 + 0.75 X 1 X 3 + 3.79 X 2 2 − 1.52 X 2 X 3 − 4.54 X 3 2

3.3.2. ANOVA Test

ANOVA (analysis of variance) was selected to assess the appropriate fit of the experimental model (Table 4). In general, the model is coherent, since the probability (p-value) is small, whereas the large F (Fisher-test) consists of large values, with a correlation coefficient that should be >0.8. The data displayed in Table 4 reveal the fitness of the model, with a confidence level assessed by the regression coefficient value (R2 = 97%), with a minimal model probability value (p-value < 0.05) [14]. The proposed model is considered as highly suggestive with a low probability value.
Analysis of variance (ANOVA) data (Table 4) reveal that the proposed model possess a p-value of 0.0024, which confirms the model validity. Furthermore, the ‘lack-of-fit’ F-value for the response of the TC removal percent predicted by the proposed model was applied to check its validity. The F-value of 18.58 suggests the effectiveness of the model in fitting the data with good consistency. Moreover, the ANOVA of the data shows a high significance between the experimental and predicted TC removal, which verifies the goodness-of-fit of the model for the results.

3.3.3. Graphical Illustration

To further illustrate the design, consequently, a response 3-D surface and 2-D contour graphical illustrations, obtained based on the experimental results, were used to explain the 3 interacting parameters and their influences on the TC reduction efficacy using MATLAB R2017a software. Figure 8A,C,E shows the interaction outcome between the two independent variables and the response (% TC removal). The graphs demonstrate that TC elimination efficacy improved as both ACBP-Fe3O4 and H2O2 reagents increased up to a certain limit, as displayed in Figure 8. Nevertheless, the extra upsurge in the reagent concentrations beyond the optimum region resulted in a decrease in the TC removal proficiency. Afterwards, extra ·OH radicals affect the ACBP precipitation because they are converted to OH ions. Consequently, the precipitated ACBP-Fe3O4 contributes to a decrease in the concentration of ACBP in the system, leading to an overall decrease in the TC reduction efficacy.
Likewise, at constant H2O2 dose, a significant TC removal efficiency upsurge was obtained with the elevation of both ACBP-Fe3O4 dose and pH value. The plots in Figure 8E,F reveal that there is an optimum range for pH and ACBP-Fe3O4 dose. But beyond the suggested range, a decline in the TC removal effectiveness is noted. The pH of the aqueous solution had a terminal influence on the Fenton’s reaction system that affects both the production of OH radicals and the amount of catalyst in the mixture. These data might be due to the fact that at a higher pH value, the possibility of H2O2 decay is higher. Consequently, the results validate higher TC removal efficiency under acidic pH conditions. Furthermore, high pH values help ACBP ions to trigger the ferric-hydro-complex, and this produces the overall oxidation efficiency.
Further, the predicted and experimental values of responses are also compared according to Figure 9. The predicted values of response are obtained from a quadratic model using SAS software Ver 9.4. The response functions with the determined coefficients for TC removals (%) are investigated based on Equation (2). As displayed in Figure 9, a high correlation is achieved between the predicted and experimental values (R2: 98%).

3.3.4. Model Verification

Ultimately, for the purpose of verifying the predicted model and the proposed statistically estimated values (H2O2 350 mg/L, ACBP-Fe3O4 43 mg/L and pH 6.6), an additional three replicates were conducted and the experimental TC removal response values were compared with those projected and proposed by the suggested model (98% TC removal). After 30 min of irradiance time, the experimental TC removal reached 99%, which is close to the estimated value using the Box–Behnken design. Consequently, those results confirm the significance of the Box–Behnken design based on RSM for locating the optimum operating parameters for the oxidation of organics through the ACBP-based Fenton system.

3.4. Study of Temperature on Reaction Kinetics and Thermodynamics

For the purpose of evaluating the operating temperature effect on TC oxidation, the process is conducted at various operational temperatures and the results are displayed in Figure 7. Modified Fenton reagent doses are introduced to the jar test at constant values of ACBP-Fe3O4 (40 mg/L) and hydrogen peroxide (400 mg/L) at a natural pH value of 6.5.
As presented in Figure 10A,B, both TC and COD decline in their rates as the temperature increases from room temperature (28 °C) to 60 °C. Thus, a decline in the tetracycline oxidation is achieved with the elevation of temperature. TC removal decreased from 98 to 49% and COD removal also declined from 95 to 55% with the temperature increase from 28 to 60 °C. This could be attributed to the decay rate of the hydrogen peroxide reagent into O2 and H2O that happened at a high rate at elevated temperatures. Consequently, H2O2 acts as a free radical oxidation scavenger rather than a radical creator. Thus, the result is a decrease in the OH radical generation, which initiates tetracycline oxidation [43]. This oxidation tendency is clearly in agreement with that reported in the previously cited work in the literature for wastewater treatment with Fenton’s reaction [12,33,54].
To obtain a thorough understanding of the modified Fenton oxidation system based on ACBP-Fe3O4/H2O2 under mild pH conditions, kinetic modeling is applied to realize a practical real-life facility for application. These data act as a guide to further investigate the varied mechanisms of the oxidation process. To achieving appropriate kinetic modeling, the optimum operational variables, economic investigation, system control and also a scale-up design facility should be assessed. In this regard, a set of kinetic models is applied based on varied operating temperatures. According to the equations of the zero, pseudo-first- and second-order reaction rates, respectively, displayed in Table 5, which sets the relationship between different initial concentrations and time, the models are applied. The data tabulated in Table 5 represent the kinetic rate constants and the reaction half-life (t1/2) of the three applied models.
Notably, from Table 5, the assessment for all the proposed models is based on the regression coefficient (R2) values. According to the data of R2 values exhibited in Table 5, the first order is the most appropriate in terms of fitting the experimental data. Also, the half-reaction time (t1/2) is shown to be increased with the elevation of temperature for tetracycline oxidation. This trend might be linked to the fact that the overall amount of hydroxyl radicals is reduced with the increase in temperature. Furthermore, this could be explained by the fact that increasing the temperature of the aqueous solution hindered the catalytic activity of ACBP-magnetite to adsorb TC, since the catalyst surface deteriorates under elevated temperature. This is due to the desiccation of hydrogen peroxide into oxygen and water instead of producing hydroxyl radicals [61,62]. Various researchers [63,64] previously reported the experimental evidence for the dependence of catalytic oxidation on temperature. Thus, the overall oxidation reaction yield declines [45]. Therefore, the effect of temperature on tetracycline oxidation through the novel modified ACBP-Fe3O4/H2O2 composite-based Fenton catalyst follows first-order reaction kinetics. First-order reaction kinetics indicate that the rate of reaction is directly proportional to the concentration of reactants. Also, according to the values of the rate constant k1 displayed in Table 5, the rate constant depends on the reaction temperature, thereby affecting the rate of TC oxidation. To sum up, the half-life time is related to the rate constant k1.
This investigation is in agreement with other researchers [8,13,50] who explained the Fenton system by the first-order kinetic model. However, some investigators [59,60,61,62,63,64,65,66] indicated that the second-order rate equation fit the kinetic reaction of the photocatalytic oxidation processes.
Referring to the results of the different experimental data at various temperatures (28, 40, 50 and 60 °C), the thermodynamic functions are estimated. The functions include Gibbs free energy of activation (ΔG`), the activation enthalpy (ΔH`), and the activation entropy (ΔS`), which are essential to evaluate the temperature effect on TC oxidation by modified ACBP-Fe3O4-based Fenton illumination treatment system. The temperature dependence on the first-order reaction kinetic rate constant of the modified ACBP-Fe3O4 Fenton oxidation reaction was estimated through the application of the natural log of ( K = A e ( − E a R T ) ) the Arrhenius relation, while the activation energy (Ea) is achieved from the slope of the plot of Figure 11, where k1 is the kinetic rate constant; Ea is the activation energy (KJ.mol−1); R is the gas constant (8.314 J mol−1 K−1); T is temperature (K) and A is the Arrhenius factor [9,10].
To obtain the Gibbs free energy of activation (ΔG′), Eyring’s equation ( k 1 = k B T h e ( − ∆ G ⋕ R T ) ) is applied (where kB and h are Boltzmann and Planck’s constants). Additionally, the activation enthalpy (ΔH’) is projected from the equation of ∆ H ′ = E a − R T [19,44], and the entropy of activation (ΔS′) is recognized as ∆ S ′ = ( ∆ H ′ − ∆ G ′ ) / T .
Thermodynamic parameters are investigated and tabulated in Table 6. Negative values of entropy of activation indicate the reduction in the randomness and verify the exothermic tendency of the oxidation system. Further, according to the data displayed in Table 6, the positive Gibbs free energy values suggest that the modified oxidation system has a spontaneous nature. Also, the degree of spontaneity of the oxidation reaction decreases with the increase in temperature. This means the reaction could be induced without being driven by the addition of an external energy source. Since the reaction is exothermic, it releases energy. The overall process was determined to be an exothermic process. These data also support the notion that the proposed t reaction mechanism was energetically stable and that the rate of the Fenton oxidation reaction primarily decreased with the elevation in temperature, since high temperatures have an opposite influence on the rate of exothermic systems.
It is also worth mentioning that the reaction is guided at a low energy barrier of 34.33 kJ/mol. This activation energy value verifies that the reaction slows down with increasing temperature. Previous reports in the literature [9,10] also confirm these findings. Previous researchers reported that the activation energy for dye oxidation by the Fenton reaction is 53.96 kJ/mol [61]. Furthermore, the data tabulated in Table 6 also indicate that ∆S` exhibited negative values; this indicates that the randomness decreases as a result of the reaction. Randomness in the solution was higher at the beginning of the oxidation reaction compared to the end of the reaction because the reaction rapidly proceeded at the beginning and then slowed down over time.

3.5. Catalyst Sustainability

It is worth examining the sustainability of the catalyst. To this end, the ACBP-Fe3O4 catalyst’s recover and reuse utility is explored to evaluate its reusability by assessing its activity to ensure its oxidation affinity after use. Primarily, ACBP-Fe3O4 is regenerated after magnetic collection and then subsequently subjected to successive distilled water washing. Afterwards, the sample is exposed to oven drying (105 °C) for 1 h. Then, the regenerated catalyst is used as the source of Fenton oxidation and the experiments are conducted under the optimum conditions. The data exhibited in Figure 12 demonstrate the viable efficiency of the material in oxidation, reaching six cyclic uses with a feasible efficiency that decreased from 99% to 90% with successive use.
These results could be associated with the occupation of active sites by TC pollutant intermediates, which hinder the activity. However, it is worth mentioning that the ACBP- Fe3O4 material still possesses a high oxidation affinity and could be easily recovered.

4. Conclusions

Rapid growth of water contamination is common in current industrial societies and is a growing problem. The use of fruit waste as a precursor for treatment is an industrial ecology approach to substitute the iron source in the classical Fenton’s reaction system and could represent a key tool in highlighting the sustainable management of the environment. The removal of tetracycline (TC) as a simulated contaminant from aqueous pharmaceutical discharge is carried out using a modified Fenton reaction based on treated banana peels augmented with magnetite nanoparticles. The removal efficiency reached 98% and 95% for TC and COD removals, respectively. The optimal operational parameters are achieved through response surface numerical methodology, and the optimum oxidation conditions are recorded at H2O2 350 mg/L, ACBP-Fe3O4 43 mg/L and pH 6.6 with a high correlation coefficient (R2 0.97%). The temperature effect showed that the reaction is exothermic in nature. The kinetics and thermodynamics of the reaction are also assessed and the reaction follows the first-order reaction kinetics. Consequently, the novel Fenton reagent route from waste could be introduced as a promising green technology for advanced pharmaceutical aqueous effluent oxidation.

Author Contributions

Conceptualization, M.A.T.; Methodology, M.M.N.; Software, M.M.N., M.A.T., H.A.N. and S.M.S.; Formal analysis, M.M.N.; Investigation, H.A.N.; Resources, M.A.T. and S.M.S.; Writing—original draft, M.A.T. and H.A.N.; Writing—review and editing, M.M.N., H.A.N. and S.M.S. All authors have read and agreed to the published version of the manuscript.

Funding

The authors extend their appreciation to Prince Sattam bin Abdulaziz University for funding this research work through the project number (PSAU/2024/01/31504).

Data Availability Statement

Data available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The graphical illustration of material preparation steps and treatment technique.
Figure 1. The graphical illustration of material preparation steps and treatment technique.
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Figure 2. XRD pattern of the prepared ACBP-Fe3O4 catalyst.
Figure 2. XRD pattern of the prepared ACBP-Fe3O4 catalyst.
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Figure 3. SEM images of activated banana peel samples augmented with magnetite nanoparticles.
Figure 3. SEM images of activated banana peel samples augmented with magnetite nanoparticles.
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Figure 4. Comparison of reaction time effectiveness on different oxidation activity systems: (A) the change in TC with the reaction time; (B) the change in COD with the reaction time.
Figure 4. Comparison of reaction time effectiveness on different oxidation activity systems: (A) the change in TC with the reaction time; (B) the change in COD with the reaction time.
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Figure 5. Effect of pH on the (A) TC removal and (B) COD reduction for the modified Fenton oxidation system.
Figure 5. Effect of pH on the (A) TC removal and (B) COD reduction for the modified Fenton oxidation system.
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Figure 6. Effect of hydrogen peroxide dose on the (A) TC removal and (B) COD reduction for the modified Fenton oxidation system.
Figure 6. Effect of hydrogen peroxide dose on the (A) TC removal and (B) COD reduction for the modified Fenton oxidation system.
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Figure 7. Effect of ACBP-Fe3O4 catalyst dose on the (A) TC removal and (B) COD reduction for the modified Fenton oxidation system.
Figure 7. Effect of ACBP-Fe3O4 catalyst dose on the (A) TC removal and (B) COD reduction for the modified Fenton oxidation system.
Catalysts 15 00351 g007
Catalysts 15 00351 g008
Figure 8. Box–Behnken design graphical illustration for TC removal from aqueous solution using ACBP-Fe3O4 -based Fenton system: (A) 3-D surface and (B) 2-D contour plot of coded H2O2 and ACBP-Fe3O4, (C) 3-D surface and (D) 2-D contour plot of coded H2O2 and pH and (E) 3-D surface and (F) 2-D contour plot of coded ACBP-Fe3O4 and pH.
Figure 8. Box–Behnken design graphical illustration for TC removal from aqueous solution using ACBP-Fe3O4 -based Fenton system: (A) 3-D surface and (B) 2-D contour plot of coded H2O2 and ACBP-Fe3O4, (C) 3-D surface and (D) 2-D contour plot of coded H2O2 and pH and (E) 3-D surface and (F) 2-D contour plot of coded ACBP-Fe3O4 and pH.
Catalysts 15 00351 g008
Catalysts 15 00351 g009
Figure 9. Graphical illustration of Box–Behnken design for actual and predicated regression plot responses.
Figure 9. Graphical illustration of Box–Behnken design for actual and predicated regression plot responses.
Catalysts 15 00351 g009
Catalysts 15 00351 g010
Figure 10. Effect of temperature on the (A) TC removal and (B) COD reduction of modified Fenton oxidation system.
Figure 10. Effect of temperature on the (A) TC removal and (B) COD reduction of modified Fenton oxidation system.
Catalysts 15 00351 g010
Catalysts 15 00351 g011
Figure 11. Arrhenius plot of the 1st-order kinetic constants.
Figure 11. Arrhenius plot of the 1st-order kinetic constants.
Catalysts 15 00351 g011
Catalysts 15 00351 g012
Figure 12. TC oxidation in the Fenton-based ACBP-Fe3O4 system in consecutive cycles.
Figure 12. TC oxidation in the Fenton-based ACBP-Fe3O4 system in consecutive cycles.
Catalysts 15 00351 g012
Table 1. Chemical composition of ACBP-Fe3O4 inferred by EDX.
Table 1. Chemical composition of ACBP-Fe3O4 inferred by EDX.
ElementCNSCrFeNiCuZnTotal
Weight %10.560.550.040.0687.960.260.420.15100
Table 2. Design boundaries of the experimental domain with their spacing levels in their coded and uncoded values.
Table 2. Design boundaries of the experimental domain with their spacing levels in their coded and uncoded values.
VariableSymbols Range and Levels
NaturalCoded−101
H2O2 (mg/L)x1X1 350400450
ACBP-Fe3O4 (mg/L) x2X2354045
pHx3X36.06.57.0
Table 3. Coded and actual Box–Behnken design of experiments and the predicted and experimental response.
Table 3. Coded and actual Box–Behnken design of experiments and the predicted and experimental response.
Run
No.		Codified Variables Natural Variables Response (%TC Removal)
X1X2X3x1x2x3 Experimental Predicted
1−1−10 353506.5 6159
2−110354506.5 9488
31−10453506.5 1723
4110454506.5 2224
50−1−1403506.0 3231
60−11403507.0 4442
701−1404506.0 4649
8011404507.0 5253
9−10−1354006.0 5962
1010−1454006.0 1612
11−101354007.0 6368
12101454007.0 2320
13000404006.5 4444
14000404006.5 4444
15000404006.5 4544
Table 4. ANOVA test results for the response surface model for TC removal by modified ACBP-Fenton system.
Table 4. ANOVA test results for the response surface model for TC removal by modified ACBP-Fenton system.
SourceDegree of Freedom (DF)Sum of Squares (SS)Mean Squares (MS)Fisher
(F-Value)		Probability (p-Value)
Model95850.817650.090718.582870.002485
Linear35505.255505.25157.3677720.15941
Square3199.025641199.0256415.6891561.761538
Interaction3135.9359135.93593.8857331.115889
Error5174.916734.98333
Total146025.733
R2: 97%
Table 5. Kinetic parameters of TC pharmaceutical wastewater treatment by modified Fenton under various temperatures *.
Table 5. Kinetic parameters of TC pharmaceutical wastewater treatment by modified Fenton under various temperatures *.
T,
°C		Zero-Order Reaction Kinetics Model First-Order Reaction Kinetics Model Second-Order Reaction Kinetics Model
C t = C o − k o t C t = C o − e k 1 t 1 C t = 1 C 0 − k 2 t
ko,
min−1		R2t1/2, mink1,
min−1		R2t1/2, mink2,
L mg−1 min−1		R2t1/2, min
280.464(±0.121)8310.7758 0.047(±0.015)0.973.3804 0.241(±0.064)0.820.01085
400.344(±0.117)7414.5348 0.081(±0.017)0.928.5556 0.016(±0.002)0.910.16351
500.284(±0.102)7217.6056 0.064(±0.013)0.9110.8281 0.008(±0.002)0.840.32703
600.708(±0.091)707.0621 0.047(±0.012)0.8714.7446 0.005(±0.002)0.710.52326
* Co and Ct: initial TC concentration and TC concentration at time t (mg/L); t: time (min); k0, k1, k2: kinetic rate constants of zero-, first- and second-order reaction kinetic models.
Table 6. Thermodynamic parameters for TC oxidation by modified Fenton system.
Table 6. Thermodynamic parameters for TC oxidation by modified Fenton system.
Temperature, °CLn k1Ea,
kJ mol−1		∆ G′,
kJ mol−1		∆H′,
kJ mol−1		∆S′,
J mol−1
28−1.5834.3368 (±0.56)76.91431.85−151.17
40−2.5183.3231.73−164.82
50−2.7486.7031.65−170.43
60−3.0690.3331.56−176.45
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Nour, M.M.; Tony, M.A.; Nabwey, H.A.; Shaaban, S.M. Experimental Design and Numerical Optimization of Photochemical Oxidation Removal of Tetracycline from Water Using Fe3O4-Supported Fruit Waste Activated Carbon. Catalysts 2025, 15, 351. https://doi.org/10.3390/catal15040351

AMA Style

Nour MM, Tony MA, Nabwey HA, Shaaban SM. Experimental Design and Numerical Optimization of Photochemical Oxidation Removal of Tetracycline from Water Using Fe3O4-Supported Fruit Waste Activated Carbon. Catalysts. 2025; 15(4):351. https://doi.org/10.3390/catal15040351

Chicago/Turabian Style

Nour, Manasik M., Maha A. Tony, Hossam A. Nabwey, and Shaaban M. Shaaban. 2025. "Experimental Design and Numerical Optimization of Photochemical Oxidation Removal of Tetracycline from Water Using Fe3O4-Supported Fruit Waste Activated Carbon" Catalysts 15, no. 4: 351. https://doi.org/10.3390/catal15040351

APA Style

Nour, M. M., Tony, M. A., Nabwey, H. A., & Shaaban, S. M. (2025). Experimental Design and Numerical Optimization of Photochemical Oxidation Removal of Tetracycline from Water Using Fe3O4-Supported Fruit Waste Activated Carbon. Catalysts, 15(4), 351. https://doi.org/10.3390/catal15040351

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