Fe₃O₄-Carbon-Based Composite Derived from the Charge-Transfer Reaction Using Waste Tea Leaves as the Carbon Precursor for Enhanced Removing of Azocarmine G2, Methyl Violet 2B, Eosin B, and Toluidine Blue from Aqueous Solution

Abstract

Ferric chloride salt (FeCl₃) is a typical vacant orbital acceptor that accepts electrons from urea molecules. This donor-acceptor interaction yields a Fe₃O₄ oxide, which can be used for preparing novel adsorbent material. Activated carbon (AC) materials, derived from biomass precursors and used for environmental remediation, have gained increasing attention owing to their sustainability, eco-friendly nature, ease of fabrication, and cost-effectiveness. Globally, a large amount of consumed tea leaf materials is discarded as solid waste. The present study aimed to reuse consumed tea leaves to generate biomass-derived-ACs and coupling AC with Fe₃O₄ oxide to generate Fe₃O₄-carbon-based composite for environmental remediation. The synthesized Fe₃O₄-carbon-based composite was examined to remove four common organic pollutant dye models from an aqueous solution. The effects of several parameters on the adsorption capacity of the synthesized composite were analyzed, and equilibrium data were examined. We found that the synthesized composite displayed promising adsorption activity against the investigated dyes.

1. Introduction

A chemical interaction called charge transfer (termed as CT) or electron donor-acceptor (termed EDA) has attracted great attention from researchers owing to its unique chemical, biological, and physical properties of the resulted products. This reaction occurs between a donor molecule (possessing different donating sites) and an acceptor molecule (vacant orbital, π-, or σ-electron acceptor). The interaction is significantly beneficial to academia (biochemistry, biology, physics, and chemistry) in addition to non-academia (medicine, technology, engineering, material science, and pharmacology) [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15].

Carbon-based nanomaterials such as carbon nanotubes, carbon nanofibers, hollow carbon spheres, fullerenes, activated carbon (AC), graphene, and graphene oxide are some of the ideal and useful materials for environmental remediation, water decontamination, and the recovery and removal of pollutants [16]. AC is one of the carbon-based nanomaterials that offers many services to achieve environmental sustainability. Its use is versatile owing to its low-cost and non-toxic material for wastewater treatment. Tea is one of the most popular beverages around the world, with approximately 20 billion cups of tea consumed daily. However, a significant amount of tea leaf waste is discharged daily owing to this huge tea consumption. Tea leaves are composed of structural proteins, condensed tannins, lignin, cellulose, hemicelluloses, and polyphenol [17]. These constituents are basic components of ACs, rich in oxygen, and functionally grouped in a hierarchically layered structure. Due to this unique structure and its many outstanding physicochemical properties, tea leaf waste is a potentially well-suited precursor to generate ACs, serving as a method to remove inorganic and organic pollutants from the environment [18]. Saudi Arabia consumes large amounts of tea (e.g., black tea, green tea, Chinese tea), which results in a large amount of tea leaf waste. Ideally, this tea leaf waste could be converted into a useful material instead of being discharged into the environment.

The purpose of this study is to develop and optimize a cost-effective and environmentally friendly method of using the CT interaction and tea leaf waste to create a Fe₃O₄-carbon-based composite and remove several organic dyes from aqueous solutions. First, raw tea leaf waste was carbonized at high temperatures (600 °C). This was followed by chemical activation that involved the impregnation of the carbon precursor with potassium hydroxide (KOH) and carbonization under inert atmospheric conditions at 600 °C. Next, the iron oxide (Fe₃O₄), prepared by the co-precipitation of FeCl₃ salt with urea, was ground with the biomass-derived ACs to obtain the Fe₃O₄-carbon-based composite. Finally, the adsorptive properties of the obtained composite were evaluated using four organic dyes through a batch adsorption technique under different experimental conditions (e.g., adsorbent mass, pH, contact time). A series of instrumentation techniques were applied to characterize the prepared composite; this includes scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectral analyses.

2. Experimentation

2.1. General

The tea leaf waste, collected from several tea shops in Taif University, Saudi Arabia, was used as the precursor for the biomass-derived ACs. Urea (NH₂CONH₂; 60 g/mol; purity ≥99.5%), potassium hydroxide (KOH, 56.11 g/mol, purity ≥99.95%), iron (III) chloride (FeCl₃, 162.20 g/mol, purity ≥99.99%), and all other reagent grade analytical chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Table 1. Name, abbreviation, dye content, molecular weight, and chemical formula of investigated dyes.

Dye Name	Abbreviation	Chemical Formula	MW (g/mol)	Dye Content
Azocarmine G2	Dye 1	C28H21N3	399.48	≥75.0%
Methyl violet 2B	Dye 2	C23H26N3Cl	379.90	≥75.0%
Eosin B	Dye 3	C20H8Br2N2O9	580.10	95.0%
Toluidine blue	Dye 4	C15H16ClN3S·0.5ZnCl2	373.97	90.0%

presents information about the organic dyes. The biomass-derived ACs were prepared from the collected tea leaf waste according to the methods described by [19,20,21,22].

Briefly, 100 g of tea leaf waste were collected, cleaned, ground, and dried. The dried tea leaf waste was used to prepare the biomass-derived ACs by pre-carbonizing 5 g of tea leaf waste at 600 °C, and carbonizing the resultant black carbon material with KOH at 600 °C. After that, the Fe₃O₄ oxide was prepared according to a previously published protocol [19,20,21,22] by the co-precipitation of urea with FeCl₃ metal salt.

2.2. Composite Material

The following steps were conducted to synthesis the Fe₃O₄-carbon-based composite material:

(i)

In a dry clean porcelain mortar, a porcelain pestle was used to mix 0.2 g of Fe₃O₄ oxide with 2.0 g of biomass-derived ACs.

(ii)

A few drops of methanol solvent were added to the mixture, and the mixing was continued for ~15 min until the mixture was completely uniform.

(iii)

The resulting product was collected from the mortar, dried in a vacuum desiccator containing CaCl₂ (anhydrous), and then structurally characterized.

2.3. Batch Adsorption Technique

Batch adsorption experiments were conducted at room temperature. Aqueous Dye 1, Dye 2, Dye 3, and Dye 4 solutions were prepared at a concentration of 100 mg/L. Next, 100 mL of each aqueous dye solution was transferred into a 250-mL conical flask. After adjusting the pH of the solution to the desired value (using HCl or NaOH; 0.01 N), the appropriate amount (0.05–0.20 g) of the composite material was added to the solution. A benchtop shaker was used to agitate the conical flask at 250 rpm at room temperature. Approximately 5 mL aliquots were pipetted from the flask at predetermined time intervals (10, 20, 30, …… min) and then centrifuged for 10 min to remove the composite. The remaining dye content was spectrophotometrically analyzed using an ultraviolet-visible (UV/Vis) spectrophotometer. The absorbance of the sample solution was measured at 550 nm for Dye 1, at 588 nm for Dye 2, at 518 nm for Dye 3, and at 586 nm for Dye 4. The equation: R% = [(A_o − A_t)/A_o] × 100 was used to calculate the investigated dye removal efficiency (R%) using the composite. In this equation, A_o and A_t are the absorptances of the dye solutions at baseline and time “t”, respectively [23].

2.4. Instrumental

2.4.1. SEM Analysis

An FEI Quanta FEG 250 scanning electron microscope (FEI Company, Hillsboro, OR, USA) was used to collect the SEM images. These images give information about the shape and size of the adsorbent material particles, surface structure, and morphology of the material. The SEM instrument was operated at 20 kV as electron acceleration voltage.

2.4.2. XRD Analysis

An X’Pert PRO PANalytical X-ray diffractometer (Malvern Panalytical, Tokyo, Japan) was used to determine the crystalline phase and structural properties of the composite material. The instrument was equipped with an X-ray source Cu Kα₁ (λ = 0.1540 nm) and a secondary monochromator. The samples were scanned at room temperature with a wide range of 2θ from 5° to 70°.

2.4.3. XPS Analysis

The surface analysis was performed on a Thermo Fisher Scientific XPS System (model K-Alpha) (Thermo Fisher Scientific, Waltham, MA, USA). The instrument was equipped with an 80° double-focusing hemispherical analyzer and a micro-focused, monochromated, low-power Al Kα X-ray source with variable spot size.

2.4.4. BET Analysis

A surface area and pore size distribution analyzer (model BELPREP MAX II; MICROTRAC MRB) (MICROTRAC MRB, Montgomeryville, PA, USA) was used to perform the nitrogen adsorption-desorption isotherm measurements to obtain the BET surface area and average pore diameter.

2.4.5. Spectrophotometers

The UV/Vis and FT-Raman spectra were collected at room temperature with an adsorption spectrum between 200 to 800 nm and 50 to 3600 cm⁻¹, respectively, on the Lambda 25 PerkinElmer UV/Vis spectrophotometer (Perkin Elmer, Waltham, MA, USA), and MultiRAM Stand Alone Bruker FT-Raman Spectrophotometer (Bruker, Berlin, Germany), respectively.

3. Results and Discussion

3.1. Carbon Precursor and Composite Material

Waste tea leaf is a potentially well-suited precursor to generate AC owing to its outstanding physicochemical properties and its unique structure. Figure 1 presents photos that summarize the process of preparing biomass-derived-ACs from the collected waste tea leaf. The process is in two steps:

Five grams of waste tea leaf were pre-carbonized at 600 °C for 3 h.

The obtained carbons were activated with potassium hydroxide with a ratio (3:1 wt KOH to carbon). Next, the mixture was carbonized at 600 °C for 3 h.

In this process, the carbons from the pre-carbonizing of waste tea leaf were activated using the KOH treatment. This activation proceeded by the reaction of the activator with a carbon atom in two steps [24]:

Step 1: 2C + 6KOH→2K₂CO₃ + 3H₂ + 2K

Step 2: K₂CO₃ + C→CO + CO₂ + 2K

During the activation and carbonization processes, carbon monoxide, carbon dioxide, hydrogen, and potassium vapors were generated and intercalated into the surface of the biomass-derived-ACs, resulting in the development of an abundant porosity by opening and expanding the pores on the surface of the biomass-derived-ACs. The co-precipitation of 6 mol of urea with one mole of FeCl₃ at 80 °C generated the Fe₂O₃ oxide which generated Fe₃O₄ at 800 °C according to the following equations:

$${\mathrm{FeCl}}_3+6{\mathrm{NH}}_2{\mathrm{CONH}}_2\stackrel{80°\mathrm{C}}{\to }{\mathrm{Fe}\left({\mathrm{NH}}_2{\mathrm{CONH}}_2\right)}_6{\mathrm{Cl}}_3$$

$$2\left[\mathrm{Fe}\right({\mathrm{NH}}_2{\mathrm{CONH}}_2{)}_6]{\mathrm{Cl}}_3\stackrel{80°\mathrm{C}}{\to }4{\mathrm{NH}}_3+9\mathrm{CO}+3{\mathrm{CH}}_4+7{\mathrm{N}}_2+6{\mathrm{NH}}_4\mathrm{Cl}+{\mathrm{Fe}}_2{\mathrm{O}}_3$$

$$3{\mathrm{Fe}}_2{\mathrm{O}}_3\stackrel{800°\mathrm{C}}{\to }2{\mathrm{Fe}}_2{\mathrm{O}}_4+½{\mathrm{O}}_2$$

In the presence of a few drops of methanol, the co-grinding of 10 g of biomass-derived-ACs with one gram of Fe₃O₄ oxide for ~15 min generated the Fe₃O₄-carbon-based composite.

3.2. Structural Characterizations

3.2.1. SEM Images

Figure 2 contains the SEM images of waste tea leaf, carbon material resulting from the pre-carbonization of waste tea leaf, biomass-derived-ACs material, and the Fe₃O₄-carbon-based composite. SEM images of waste tea leaf indicated its stem shape or tree root shape with rough surfaces. The pre-carbonizing of waste tea leaves at 600 °C destroyed its characteristic of the tree root morphology. The resulting carbon material had a rough surface with a coral reef-like shaped morphology. The biomass-derived-ACs material had a hierarchical porous structure, indicating that the KOH-induced chemical activation increased the porosity of the carbon precursor. The SEM images of the composite material showed that this material had a spongy-like texture with holes, covering the soft surface of adsorbent material.

3.2.2. XRD Diffractometry

Figure 3 presents the observed XRD patterns of the waste tea leaf, biomass-derived-ACs, and the Fe₃O₄-carbon-based composite. The XRD profile of the waste tea leaf sample displayed a prominent broad peak located at a 2θ of approximately 21.446°. A sample of biomass-derived-ACs displayed a very strong peak at 29.258° and three medium peaks at 43.083°, 47.442°, and 48.390°. The composite material displayed two characteristic peaks that appeared at 29.280° and 48.528°. The peak at 29.280° corresponded to the reflection plane (002), while that at 48.528° corresponded to (100) crystalline planes of disordered carbon material (JCPDS No. 41-1487) [25]. Peaks appeared at 62.432°, 56.928°, 53.392°, 39.360°, 35.392°, and 32.720° in the XRD diffractogram of the composite material, associated with the Bragg’s reflections (440), (511), (422), (400), (311), and (220), respectively, for typical Fe₃O₄ [26]. The XRD diffractograms of the composite material suggested that during the carbonization process at 600 °C, the waste tea leaf converted from imperfect aromatic sheets (solid carbonaceous product) at lower temperatures into a random ordering resulted in the formation of turbostratic structures (disordered carbons) at higher temperatures.

3.2.3. XPS Measurements

The surface chemical compositions of the waste tea leaf, biomass-derived-ACs, and the Fe₃O₄-carbon-based composite were examined by XPS, and the collected spectra were shown in Figure 4. The XPS spectrum of the C 1s core level of the waste tea leaf sample displayed peaks at 285.0, 286.6, 287.2, and 288.5 eV corresponding to the C=C, C–OH, C=O, and O–C=O bonds, respectively. The biomass-derived-ACs sample showed peaks corresponding to C=C (284.8 eV), C–OH (286.6 eV), C=O (287.9 eV), and O–C=O (289.8 eV) bonds. The XPS spectrum of the C 1s core level of the composite sample exhibited peaks at 284.3, 285.7, 286.4, 287.5, and 289.8 eV corresponding to the C=C (sp² hybridized carbon), C–C (sp³ hybridized carbon), C–OH, C=O, and O–C=O bonds, respectively [27]. In all samples, the most intense and highest peak appeared at 284–285 eV, corresponding to the C=C bond. The peaks centered at 532.7, 531.2, and 532.0 eV in the XPS spectrum of the O 1s core level of the waste tea leaf, biomass-derived-ACs, and the composite samples, respectively, related to the C=O bond [28]. The XPS survey spectrum of the waste tea leaf sample contained carbon, oxygen, nitrogen, sulfur, and phosphorus elements. Carbon, oxygen, and nitrogen elements were detected in the XPS survey spectra of the biomass-derived-ACs and through the composite samples. In addition, the iron element was observed in the spectrum of the composite.

Table 2. Elemental composition of the waste tea leaf, biomass-derived-ACs, and the Fe₃O₄-carbon-based composite as measured by XPS.

Sample	Chemical Composition (%)
	C	O	N	K	S	P	Fe
Waste tea leaf	74.44	22.56	1.96	-	0.56	0.48	-
Biomass-derived-ACs	43.97	52.59	1.11	2.33	-	-	-
Composite	43.91	47.20	1.69	1.95	-	-	5.25

lists the elemental analysis in percentage (%) of waste tea leaf, biomass-derived-ACs, and the Fe₃O₄-carbon-based composite as measured by XPS. The data presented in Table 2 indicates that the chemical activation achieved through alkali treatment increased the oxygen content from 22.5% in the waste tea leaf sample to ~47% in the composite. The composite sample contained approximately 1.95% potassium, which suggested that not all the potassium content evaporated when the sample was calcinated at 600 °C. The XPS analysis supports the generation of high-purity Fe₃O₄-carbon-based composite from tea leaf waste.

3.2.4. BET Surface Area Analysis

Nitrogen gas adsorption-desorption analysis was performed to determine the surface area and pore volume of the prepared materials. Figure 5 contains the N₂ gas adsorption-desorption isotherms of biomass-derived-ACs, and the composite material recorded at 77.36 K. The shape and type of the isotherms offer insight into the porous or non-porous nature of the sample according to the IUPAC classification system. The biomass-derived-ACs, and the composite material samples all exhibited type II isotherm properties. According to the IUPAC classification, type II describes adsorption on adsorbents that have micropore structures and strong adsorbate-adsorbent interactions. Type II isotherm adsorbents also have more complex pore structures and/or a disordered structure without a well-defined pore shape or size [29].

The BET surface areas of the biomass-derived-ACs and the composites were 630 and 784 m²/g, respectively (

Table 3. Surface area, pore size, and volume of the biomass-derived-ACs and the Fe₃O₄-carbon-based composite as measured by BET analysis.

Sample	Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
Biomass-derived-ACs	630	0.2753	2.40
Composite	784	0.2939	2.64

). The corresponding total pore volume of these samples was 0.2753, and 0.2939 cm³/g, respectively. These values indicate that Fe₃O₄ increased the surface area of the prepared material. According to the XPS analysis, the biomass-derived-ACs and the composite contain a large amount of oxygen (~47–52%), resulting from the KOH activation step. Oxygen groups prevent nitrogen from accessing the micropores for adsorption, thereby reducing the surface area and pore volume. These two materials have a low surface area and pore volume values.

3.2.5. Raman Measurements

Figure 6 contains the Raman spectra of biomass-derived-ACs, and the composite material. The two samples possessed two distinct bands that belonged to the first-order spectral Raman lines (D and G bands). These two bands were centered at ~1585 cm⁻¹ (indexed as G band) and at ~1354 cm⁻¹ (indexed as D band). The D-band originates from the disordered carbons and represents the presence of structural defects. However, the G-band corresponds to the graphitized carbons and represents the stretching vibration of the sp² (C–C) bonded carbon atoms. The intensity ratio of the D to G bands (I_D/I_G) estimates the graphitization of the sample [30]. A smaller I_D/I_G ratio reflects a higher degree of graphitization in the carbon material. However, a higher I_D/I_G ratio reflects more defects presented in the carbon material. The two samples exhibited a ratio of ~1.0. The broad bands appearing at ~2870–3050 cm⁻¹ belonged to the second-order spectral Raman lines (2D band). Generally, the 2D band occurs in the 3250–2400 cm⁻¹ region and is related to the disorder in the stacking of carbon layers.

3.3. Batch Mode Adsorption

3.3.1. Dye Solutions

The adsorption efficiency of the Fe₃O₄-carbon-based composite was examined using four organic dyes (Dye 1–Dye 4) as dye pollutant models. Aqueous solutions (10 mg/100 mL) of Dye 1, Dye 2, Dye 3, and Dye 4 at pH 7 were pink, violet, orange, and deep blue, respectively, as indicated in Figure 7. Their UV-visible spectra were illustrated in Figure 8. Dye 1 displayed two characteristic absorption bands. A strong, wide band had two heads at 550 nm and 516 nm, and a medium intensity band had two heads at 334 nm and 293 nm. The two heads of the medium intensity band have approximately the same intensity. However, those of the strong, wideband are not. The intensity of the head located at 550 nm is a little lower than that at 516 nm. Dye 2 absorbs across a wide range from 200 nm to 660 nm. Through this range, it displayed two characteristic absorption bands: a strong, wideband accumulated at 588 nm; a weak, narrow band appeared at 302 nm. Dye 3 absorbs in the range 280–575 nm, showing two characteristic absorption bands: a very strong, wide band with λ_max at 518 nm and a narrow, medium-intensity band with λ_max at 299 nm. Dye 4 absorbs across a wide range from 280 nm to 725 nm. Through this range, it displayed two characteristic absorption bands: a very strong, wideband accumulated at 586 nm and a narrow; medium-intensity band located at 296 nm. The adsorption experiments were performed under the following conditions:

(i)

Temperature: Room temperature

(ii)

Time: 10–60 min

(iii)

Solution pH: 2–12

(iv)

Dye concentration: 100 mg/L

(v)

Composite dose: 0.05–0.20 g

(vi)

Detected band: 550 nm for Dye 1, 588 nm for Dye 2, 518 nm for Dye 3, and 586 nm for Dye 4

3.3.2. Factors Affecting Dye Removal

The effect of pH on the adsorption of Dye 1, Dye 2, Dye 3, and Dye 4 onto 0.1 g composite was investigated within the pH range of 2–12. An acid (HCl) and a base (NaOH) solution at a concentration of 0.01 N were used to adjust the pH of the dye solution to the desired value. The results showed that pH = 7 is the optimal value for the removal of the investigated dyes by the composite because after this value, the R% no longer increases (Figure 9). This is important economic point because at pH = 7, there is no need to consume more chemicals (acid or base) to adjust the optimum pH for dye removal, which represent an economic feasibility.

To investigate the influence of the composite’s dose on the adsorption of the investigated dyes, 100-mL of each dye solution (100-mg/L, pH 7) was incubated at room temperature with different composite doses (0.05–0.2 g). At equilibrium, the results showed that an increase in the composite’s dose led to an increase of the R%. The increasing R% is due to the additional surface area that contains more active sites on the available composite for adsorption. R% reached a saturation point at the point where all the active sites in the composite were occupied by the dye molecule. The minimum composite dose to obtain a maximum R% for Dye 1, Dye 2, Dye 3, and Dye 4 removal was 0.12, 0.15, 0.09, and 0.10 g, respectively. To investigate the influence of contact time on the adsorption of the investigated dyes, 100-mL of each dye solution (100-mg/L, pH 7) was incubated at room temperature with a fixed amount of the composite (0.12 g for Dye 1, 0.15 g for Dye 2, 0.09 g for Dye 3, and 0.10 g for Dye 4). The results showed that the maximum R% for Dye 3 (97.5%) and Dye 4 (98.1%) was achieved during the first 40 min; the maximum R% for Dye 1 (96.5%) was achieved in the first 45 min. However, the maximum R% for Dye 2 (95.3%) was achieved in the first 55 min. Figure 10 contains photograph of the Dye 2 solution before and after adsorption onto Fe₃O₄-carbon-based composite, as an example.

3.4. Adsorption Isotherms

To identify the maximum removal capacity of the composite for the investigated dyes (Dye 1, Dye 2, Dye 3, and Dye 4) and the adsorption type, the equilibrium data was modeled using Langmuir and Freundlich isotherm equations. For this purpose, the equilibrium test was performed by contacting a fixed amount of composite with 100 mL of Dye 1, Dye 2, Dye 3, and Dye 4 solutions at different initial concentrations at pH 7 at room temperature. The equation: q_e = [(C_o – C_e)V/W] × 100 was used to calculate the adsorbed amount onto the adsorbent at equilibrium (mg/g). In this equation, W is the amount of composite per gram, V is the volume of the solution in liters, C_o and C_e are the adsorbate concentration (mg/L) initially and at equilibrium, respectively. The model’s parameters are calculated from the slopes and intercepts of the graphs in Figure 11 and Figure 12 are listed in

Table 4. Langmuir and Freundlich isotherm parameters for dye adsorption onto composite at pH 7.0 and 298 K.

Adsorbent	Langmuir Isotherm			Freundlich Isotherm
	qm (mg/g)	KL (L/mg)	R2	KF	n (L/mg)	R2
Dye 1	238.6	0.093	0.9991	29.64	2.05	0.9817
Dye 2	192.7	0.122	0.9987	30.00	2.23	0.9689
Dye 3	304.0	0.182	0.9986	65.72	2.63	0.9414
Dye 4	286.5	0.062	0.9950	24.85	1.81	0.9890

. The correlation coefficient (R²) value is essential for deciding the appropriateness of an isotherm model for the adsorption process. Higher R² values indicate that the model reflects the adsorption process more precisely. From Table 4, the R² of the Langmuir isotherm was much closer to 1.0 (R² > 0.99) than the Freundlich isotherm (R² = 0.989–0.941) suggesting that dye adsorption onto composite follows the Langmuir isotherm model. The best fit for the equilibrium data in the Langmuir isotherm model suggests monolayer coverage of the investigated dye onto the homogenous surface of the composite particles. The maximum adsorption capacity of the investigated dyes by composite was and 238.6, 192.7, 304.0, 286.5 mg/g for Dye 1, Dye 2, Dye 3, and Dye 4, respectively. These adsorption capacity values indicated that composite was the best adsorbent for the removal of these dyes from aqueous media.

3.5. Suggested Mechanism of Adsorption

The Fe₃O₄-carbon-based composite has high surface-to-volume ratios with pores, pits, holes, and surface defects. This composite can adsorb the investigated dyes into the spaces/defects between the carbon nanoclusters through physical or/and chemical adsorption. Since KOH was used as an activation agent, the most likely physical adsorption is through hydrogen bond-based interactions. Dyes are likely adsorbed onto the surface of the composite by the formation of surface hydrogen bonds between the N/O-containing groups of the dyes and the hydroxyl groups on the composite surface [31]. Other physical adsorptions could exist (e.g., pore diffusion, Van der Waals force, π-π interaction, steric interaction, hydrophobic-hydrophobic interaction). In addition, chemical adsorption could exist that involves sharing electrons between dyes molecules and the surface of the composite.

3.6. Regeneration and Reusability

The desorption efficiency of composite was examined using eight desorbing eluents: 0.1 M H₂SO₄, 0.1 M HNO₃, 0.2 M HCl, 0.1 M HCl, 0.1 M KOH, 0.2 M NaOH, 0.1 M NaOH, and DI water. These eluting agents were tested for the remove adsorbed dyes from composite material using the following conditions: composite dosage, 0.12 g (for Dye 1), 0.15 g (for Dye 2), 0.09 g (for Dye 3), and 0.10 g (for Dye 4); dye concentration, 100 mg/L; pH 7; contact time, 40 min (Dye 3 and Dye 4), 45 min (Dye 1), and 55 min (Dye 2); temperature, ambient. The equation: Desorption (%) = [C_d/C_a] × 100 was used to calculate the desorption yield (%). In this equation, C_d and C_a are the desorption and adsorption equilibrium adsorbate concentrations (mg/L) [32]. NaOH (0.2 M) exhibits the maximum desorption efficiency among all these eluents. The desorption yields (%) using this eluent were 97.5, 97.3, 97.7, and 97.5% for Dye 1, Dye 2, Dye 3, and Dye 4, respectively. Therefore, NaOH at a concentration of 0.2 M was selected as the eluting agent to recover most efficiently the pesticides from composite. To evaluate the reusability performance of composite, seven successive adsorption-desorption cycling experiments were run under the aforementioned conditions using 0.2 M NaOH as the eluting solution. After each cycle, the composite adsorbent was filtered off from the solution, washed with deionized water, dried in an oven at 60 °C, and recycled. The reusability yield (%) for each cycle is illustrated in Figure 13. After the first four cycles, only about a 10%-reduction was observed in adsorption yield, and this reached about 18–19% after the seventh cycle. The desorption yield remained almost constant. The reduction in the adsorption capacity of composite could be partially due to the deactivation of its adsorption sites during the cycling treatment. These results suggested that the composite material can be reused at least seven times in adsorption-desorption cycles after the successful regeneration of a loaded adsorbate without a major reduction in adsorption capacity.

4. Conclusions

Saudi Arabia, similar to many Middle Eastern and Asian countries, consumes a large amount of tea, which results in a bulk of discarded tea leaf waste. Since tea is the third-most consumed beverage in Saudi Arabia after water and Arabian coffee, the large amount of tea leaf waste discarded daily represents sources of environmentally friendly, sustainable, renewable, and cost-free carbon materials. Re-utilization of inexpensive, abundant, and renewable tea leaf waste biomass into useful AC can be considered an eco-friendly approach to solid waste management. In this study, the iron oxide (Fe₃O₄) was synthesized from the co-precipitation of FeCl₃ salt with urea. Next, the oxide was combined with the biomass-derived-ACs by solid-solid interaction. The biomass-derived-ACs were prepared from tea leaf waste using a straightforward protocol that involved pre-carbonization and followed by potassium hydroxide-based activation. The potential of the synthesized Fe₃O₄-carbon-based composite for the adsorptive removal of the four dyes from aqueous solutions was examined under varying experimental conditions (pH, composite dosage, and contact time). The synthesized composite exhibited reasonable recyclability after the seventh cycle of reuse. The results suggested that Fe₃O₄-carbon-based composite was an eco-friendly, cost-effective, and promising adsorbent with reasonable reuse performance and a high potential to remove organic dyes.