Synthesis of Low-Cost, Bio-Based Novel Adsorbent Material Using Charge-Transfer Interaction for Water Treatment from Several Pollutants: Waste to Worth

Abstract

Tea is the third most consumed beverage in Saudi Arabia (a country in the Middle East) after water and Arabian coffee. Hence, a large amount of consumed tea leaves is discarded as solid waste. Waste tea leaves (WTLs) have no commercial value and could be considered as an environmentally sustainable costless material. This work aimed to manufacture an adsorbent material from the discarded WTLs and charge-transfer (CT) interaction and use this adsorbent material effectively for the removal of different kinds of pollutants from water. The adsorbent material was manufactured in three steps. First, a CrFeO₃ metal composite was synthesized from the CT interaction between FeCl₃ and CrCl₃ with urea. Second, activated carbons were prepared from consumed WTLs using facile and clean treatments of pre-carbonization, and a simple potassium hydroxide (KOH) activation treatment. Finally, the adsorbent material was fabricated by grounding CrFeO₃ composite with the activated carbons in a 1:10 molar ratio (metal composite to activated carbons). The prepared materials were characterized spectroscopically and morphologically using FT-IR, XRD, SEM/EDX, and TEM analysis. The synthesized absorbent material was used to adsorb two organic dyes (Azocarmine G2; M1, and Methyl violet 2B; M2), and two commercial pesticides (Tiller 480SL; M3, and Acochem 25% WP; M4) from aqueous solution, and it showed promising adsorption efficacy. The minimum adsorbent material’s dosage to obtain a maximum removal efficiency (R%) for M1, M2, M3, and M4 removal from 100 mL solution (100 mg/L) was 0.11, 0.14, 0.13, and 0.12 g, respectively. The max R% for M1 (96.8%) was achieved in the first 45 min, the max R% for M2, 95.5%, was achieved during the first 55 min, and the max R% for M3 (96.4%) was achieved in the first 35 min, while the max R% for M4, 98.6%, was achieved during the first 35 min.

1. Introduction

1.1. Background

Pollution is a term that refers to the presence of undesirable chemical substances causing adverse effects on the environment and living organisms or preventing the natural process [1]. The rapid progress and development of industrialization, the fast development of human society and economy, and the immense increase in global population led to a significant amount of environmental pollution and the depletion of sustainable resources, such as water. Industries release a large amount of wastewater polluted by different kinds of pollutants (i.e., industrial dyes, heavy metal ions, pesticides, and other pollutants). If the wastewater is discharged into the environment and natural water resources, it causes serious environmental problems to marine ecosystems, aquatic ecology, human health, plant and animal life, and environmental homeostasis [2]. Many pollutants present in wastewater are highly toxic, carcinogenic in nature, and resistant to biodegradation, so they possess many harmful effects on human beings, such as allergies, irritation, mutations, and cancer. Removing these pollutants from wastewater, contaminated water, and industrial effluents before discharging them into environments is an urgent and a major environmental concern [1,2].

The methods used for removing pollutants from contaminated wastewater can be biological, physical, or chemical methods [3]. Adsorption over porous materials is one of the most adaptable and feasible treatment approaches widely employed to recover and remove pollutants from water/wastewater [3]. It is beneficial, owing to its: (i) safety, (ii) high removal efficiency, (iii) powerful performance, (iv) low energy requirements, (v) low cost, (vi) insensitivity to pollutants, (vii) environmentally friendly, (viii) simple design, (ix) resilience against toxic substances and pollutants, (x) ease of implementation, handling, and operation, (xi) relatively less harmful byproducts, and (xii) reuse potential of the material involved. Numerous porous materials have been reported as significant adsorbents for the treatment of wastewater that is contaminated by different kinds of pollutants, which includes clays, zeolites, metal oxides, silica, metal-organic frameworks, polymer resins, alumina, polymer resins, metal oxides, and carbon-based nanomaterials (i.e., carbon nanotubes and carbon nanofibers, graphene and graphene oxide, activated carbons, and fullerenes) [4].

Activated carbons are carbon-based nanomaterials that are widely used in recovering and removing pollutants, water decontamination, and other environmental remediation applications. A large variety of sources can be used to obtain activated carbons on a large scale such as industrial products, paper waste, coal, wood, and agricultural wastes and residues. In addition to these sources, activated carbons can be produced from biomass precursors, and activated carbons generated from these precursors have gained increased research attention due to their cost effectiveness, ease of fabrication, eco-friendly nature, and sustainability. Biomass precursors used to drive activated carbons include rice husks, sugarcane bagasse, corn husks, groundnut shell, jute stick, olive stones, palm shells, kenaf core fiber, peach stones, banana stems, date stones, coconut shells, bamboo, corn grain, and straw [5,6,7,8,9,10,11].

Charge-transfer (CT) interaction is a kind of chemical reaction that involves the transfer of electron/electrons from an electron-donor molecule to an electron-accepter molecule. Researchers found that the resulting products via the CT interactions possess unique chemical, biological, and physical properties. The CT complexations are significantly beneficial to non-academic fields (pharmacology, material science, engineering, technology, and medicine) in addition to academia (chemistry, physics, biology, and biochemistry) [12,13,14,15,16,17,18,19,20,21,22,23,24].

1.2. Aim of the Work

People in Saudi Arabia (a country in the Middle East) consume large amounts of tea in different types (i.e., black tea, green tea, Chinese tea), which results in a large number of consumed WTLs discarded as solid waste. It is better to transform these waste tea leaves into useful materials instead of letting them discharge into the environment. We aim to perform an environmentally friendly method utilizing WTLs into activated carbons for the environmental redemption and removal of different kinds of pollutants from water. We selected WTLs as precursors for activated carbons for the following reasons:

(i)

Easy availability and costlessness: WTLs are widely available because tea is the third most consumed beverage in Saudi Arabia after water and Arabian coffee. Moreover, WTLs are a cost-effective material.

(ii)

Sustainability and renewability: WTLs are daily biomass waste residues, which can be considered sustainable and have high regeneration ability carbon resources.

(iii)

Ease of preparation: the easy preparation of activated carbons from WTLs with minima; the use of chemicals and hazardous materials makes the process eco-friendly and sustainable.

(iv)

Waste management: instead of throwing out WTLs as solid wastes in dump yards, it is better to manage these wastes by converting them to worth carbon precursors.

The current work consists of four parts:

(i)

Utilization of a CT reaction

FeCl₃ and CrCl₃ are classified as vacant orbital acceptors which can accept electrons from the electron-donor molecule urea. This CT reaction was utilized to obtain the metal composite (CrFeO₃).

(ii)

Management of solid WTLs

Consumed WTLs were used as the precursor to creating activated carbons. The collected WTLs were pre-carbonized at a high temperature (600 °C) and chemically activated via impregnation with KOH to crate activated carbons.

(iii)

Designing the adsorbent material

The adsorbent material was generated by crushing the CrFeO₃ metal composite with the activated carbons in a 1:10 molar ratio (metal composite to activated carbons) in the presence of a few drops of MeOH solvent.

(iv)

Application

The efficiency of the designed adsorbent material for removing pollutants from an aqueous solution was tested through the batch adsorption technique against different kinds of pollutants.

2. Experimental Section

2.1. Chemicals

All chemicals and materials used in this work were analytical-grade products and were obtained from Sigma-Aldrich (St. Louis, MO, USA) and Merck (KGaA, Germany) Chemical Companies. Chemicals used for the preparation of materials were urea (NH₂CONH₂; 60 g/mol; purity ≥ 99.5%), chromium (III) chloride (CrCl₃; 158.36 g/mol; purity 99.99%), iron(III) chloride (FeCl₃, 162.20 g/mol, purity ≥ 99.99%), and potassium hydroxide (KOH, 56.11 g/mol, purity ≥ 99.95%). Aqueous solutions were prepared using deionized water (DI) from A Milli-Q system (Millipore Co., Bedford, MA, USA). The examined model pollutants were two organic dyes and two commercial pesticides.

The two organic dyes were: Azocarmine G2 (labeled as M1) (C₂₈H₂₁N₃; 399.48 g/mol; dye content ≥ 75.0%), and Methyl violet 2B (labeled as M2) (C₂₃H₂₆N₃Cl; 379.9 g/mol; dye content ≥ 75.0%). The two pesticides were: Tiller 480SL (labeled as M3) (herbicide, active ingredient: Glyphosate IPA 48% w/v), and Acochem 25% WP (labeled as M4) (insecticide, active ingredient: imidacloprid).

2.2. Characterization Techniques

The manufactured materials were characterized using a series of instrumentation techniques. These instruments included the following: a Lambda 25 PerkinElmer UV/Vis spectrophotometer (USA) for collecting ultraviolet-visible (UV-Vis) spectra in the wavelength range of 200–800; an Alpha Bruker compact Fourier-transform infrared (FT-IR) spectrophotometer (Germany) for collecting FT-IR spectra in the wavenumber range of 400 and 4000 cm⁻¹; a Bruker FT-Raman Spectrophotometer (Germany) for collecting FT-Raman spectra in the wavenumber range of 50 and 3600 cm⁻¹; and an A X’Pert PRO PANalytical X-ray diffractometer (Japan) for scanning the XRD profiles in a wide range 2θ from 5° to 70°. The diffractometer used a secondary monochromator and a Cu Kα₁ (λ = 0.1540 nm) as the X-ray source. A FEI Quanta FEG 250 Scanning Electron Microscope (SEM) (USA), integrated with an Energy Dispersive Analysis X-ray (EDAX) detector for collecting SEM/EDX profiles, and a JEM-2010 JEOL Transmission Electron Microscope (TEM) (Japan) were used for capturing TEM photos. The SEM images were captured with an electron acceleration voltage of 20 kV, whereas the TEM images were captured with an electron accelerating voltage of 200 kV.

2.3. Fabrication of the Adsorbent Material

2.3.1. Step One

In the first step, the CrFeO₃ composite was prepared. Six mmol of urea, 1 mmol of CrCl₃, and 1 mmol of FeCl₃ were dissolved in a 100 mL binary solvent mixture (H₂O:MeOH) (1:1). Stirring the resultant mixture for 24 h at 80 °C formed mixed precipitates of [Cr (NH₂CONH₂)₆] Cl₃ and Fe (NH₂CONH₂)₆] Cl₃, which were separated by filtration and washed several times with hot water. The purified mixed precipitates were thermally decomposed in an air oxygen atmosphere at 800 °C for 3 h to generate the CrFeO₃ composite [22,23,24,25,26].

2.3.2. Step Two

In the second step, the activated carbons from WTLs were prepared. The WTLs were collected from the local tea shops. Approximately 100 g of collected WTLs were rinsed with DI water to eliminate all solid impurities and dust, then completely crushed on a porcelain mortar with a pestle. Around 5 g of the resulting powder were washed with boiled DI water to remove all colored ingredients and then dried in a hot air oven for 4 h at 80 °C. The purified, dried WTLs were carbonized for 3 h at 600 °C in an inert atmosphere furnace to generate pre-activated carbon material. This carbon material was chemically activated by mixing it with KOH via a 1:3 wt ratio (2 g of carbon material with 6 g of KOH) in a porcelain mortar and crushing the two components into the finest possible powder with a porcelain pestle. The resultant fine mixture was carbonized for 3 h at 600 °C in a tubular furnace in a nitrogen atmosphere which generated the desired activated carbons, which were then cleaned with DI water to remove the potassium compounds and electrical oven dried at 80 °C for 3 h [27,28,29,30,31].

2.3.3. Step Three

In the third step, the adsorbent material was fabricated. Bulk solids of CrFeO₃ composite (0.1 g) and activated carbons (10 g) were ground with a pestle on a dry, clean porcelain motor with a few drops of MeOH solvent at room temperature via a 1:10 molar ratio (composite to activated carbons). The adsorbent material was removed from the mortar, electrical oven, dried at 60 °C for 1 h, and used for the removal of the examined pollutants from the water.

2.4. Model Pollutants Adsorption Technique

The adsorption performance of the fabricated adsorbent material was examined towards the removal of several kinds of model pollutants: organic dyes (Azocarmine G2; M1, and Methyl violet 2B; M2), and commercial pesticides (Tiller 480SL; M3, and Acochem 25% WP; M4) from aqueous solution.

The adsorption performance was tested using the batch adsorption technique at room temperature according to the following steps:

(i)

Prepare standard aqueous solutions of M1 (100 mg/L), M2 (100 mg/L), M3 (100 mg/L), and M4 (100 mg/L).

(ii)

Transfer 100 mL of each aqueous standard solution into a 100 mL glass bottle.

(iii)

Adjust the pH of the solution to the desired value using 0.01 N HCl or 0.01 N NaOH.

(iv)

Add an appropriate amount of the fabricated adsorbent material to the solution.

(v)

Shake the bottle mechanically using a benchtop shaker at room temperature.

(vi)

Pipetting 5 mL aliquots from the solution after per-defined time intervals (5, 10, 15, 20, …… min).

(vii)

Centrifuge the pipetted aliquots for 10 min to remove the solid adsorbent material.

(viii)

Analyze the concentrations of the non-adsorbed dyes and pesticides (M1, M2, M3, or M4) in their solutions using an ultraviolet-visible (UV/Vis) spectrophotometer. The detected UV/Vis bands were 516 nm for M1, 588 nm for M2, 234 nm for M3, and 296 nm for M4.

(ix)

Calculate the removal efficiency (R%) using the following equations:

R% = [(A_o − A_t)/A_o] × 100

In this equation, A_o was the initial absorbance of the dye (or pesticide). A_t was the absorbance of the dye (or pesticide) at time t [32].

3. Results and Discussion

3.1. Fabrication of the Adsorbent Material

The adsorbent material used in this work for the environmental redamation was fabricated via three steps:

3.1.1. Step One: CrFeO₃

In the first step, the CrFeO₃ composite was prepared from the CT interaction between urea as an electron-donor molecule, with FeCl₃ and CrCl₃ as vacant orbital acceptors decomposing the generated product from this CT reaction at 800 °C. Figure 1 presents several characterization results of the dried solid product generated from the reaction of 12 mmol of urea with 1 mmol of CrCl₃ and 1 mmol of FeCl₃ in a binary solvent mixture (H₂O: MeOH) (1:1) at 80 °C. These results include the color and shape of the product, its IR spectrum, SEM image, and EDX profile. The solid product has a brown color and contains a [Cr (NH₂CONH₂)₆] Cl₃ and [Fe (NH₂CONH₂)₆] Cl₃ mixture, which is formed by the following chemical reaction:

$${\mathrm{FeCl}}_3+{\mathrm{CrCl}}_3+12{\mathrm{NH}}_2{\mathrm{CONH}}_2\stackrel{80℃}{\to }\left[\mathrm{Cr}{\left({\mathrm{NH}}_2{\mathrm{CONH}}_2\right)}_6\right]{\mathrm{Cl}}_3+\left[\mathrm{Fe}{\left({\mathrm{NH}}_2{\mathrm{CONH}}_2\right)}_6\right]{\mathrm{Cl}}_3$$

(1)

The IR spectrum of the solid product shows bands at 640, 830, 1440, 1632, and 3340 cm⁻¹, which could be the result of the δ_rock (NH₂), δ_twist (NH₂), δ_def (NH₂), ν (C=O), ν (NH₂) vibrations, respectively. The SEM image of the solid product, captured at 2000× magnification, indicated that it had stone-like shaped particles, and these particles were small and had similar sizes. The EDX spectrum of the product confirmed the presence of iron, chromium, carbon, oxygen, nitrogen, and chlorine elements.

A dark red homogenate solid was obtained when the purified [Cr (NH₂CONH₂)₆] Cl₃ and [Fe (NH₂CONH₂)₆] Cl₃ mixture was thermally decomposed in an air oxygen atmosphere at 800 °C for 3 h. This dark red solid is a metal composite formulated as CrFeO₃ and resulted from the following two chemical equations:

$$2\left[\mathrm{Cr}{\left({\mathrm{NH}}_2{\mathrm{CONH}}_2\right)}_6\right]{\mathrm{Cl}}_3+2\left[\mathrm{Fe}{\left({\mathrm{NH}}_2{\mathrm{CONH}}_2\right)}_6\right]{\mathrm{Cl}}_3\stackrel{800℃}{\to }{\mathrm{Cr}}_2{\mathrm{O}}_3+{\mathrm{Fe}}_2{\mathrm{O}}_3+12{\mathrm{NH}}_4\mathrm{Cl}+18\mathrm{CO}+6{\mathrm{CH}}_4+8{\mathrm{NH}}_3+14{\mathrm{N}}_2$$

(2)

$${\mathrm{Cr}}_2{\mathrm{O}}_3+{\mathrm{Fe}}_2{\mathrm{O}}_3\stackrel{800℃}{\to }2{\mathrm{CrFeO}}_3$$

(3)

Burning the solid mixture ([Cr (NH₂CONH₂)₆] Cl₃ and [Fe (NH₂CONH₂)₆] Cl₃) at 800 °C removed all nitrogen atoms in gaseous form, so the characteristic bands resulting from the vibrations of –NH₂ bonds [i.e., δ_twist (NH₂), δ_def (NH₂), ν (NH₂)] were no longer observed in the IR spectrum of CrFeO₃ composite (Figure 2). The IR spectrum of CrFeO₃ contains only two bands at 450 and 547 cm⁻¹, which could be revered to ν(Cr–O) and ν(Fe–O) vibrations, respectively. The SEM micrograph of the CrFeO₃ composite, presented in Figure 2, indicated that the composite had a cotton-like shaped morphology with a highly homogenized and uniform structure compared to the solid mixture. The EDX spectrum evidenced the presence of chromium (33.25%), iron (35.66%), and oxygen (30.72%) in the composite. The elemental composition of CrFeO₃ composite was determined by EDX analysis, which indicated that the presence of chromium (33.25%), iron (35.66%), and oxygen (30.72%) were in good agreement with the proposed chemical structure for the composite (CrFeO₃). The composite was polluted with around 4% carbon element, as indicated by its EDX spectrum. These carbons remained as residuals from the combustion process.

3.1.2. Step Two: Activated Carbons

In the second step, the activated carbons were generated from the WTLs. WTLs are composed of cellulose, condensed tannins, polyphenol, structure proteins, lignin, and hemicelluloses. These components provide the activated carbons generated from the hierarchically layered structure of WTLs, with a surface rich in oxygen-containing functional groups. This unique hierarchically layered structure which is rich in oxygen atoms makes the activated carbons from WTLs potentially well-suited and suitable material to remove a variety of environmental pollutants. The collected WTLs from the local tea shops were converted into activated carbons via four steps (Figure 3):

(i)

Cleaning and crushing the WTLs.

(ii)

Pre-carbonizing the purified WTLs at 600 °C for 3 h.

(iii)

Chemical activation of the pre-carbonized WTLs by grinding it with KOH as activating agent with a ratio (3:1 wt KOH to WTLs).

(iv)

Carbonizing the WTLs-KOH mixture at 600 °C for 3 h.

The activation and carbonization of WTLs using KOH released several gases (hydrogen, carbon dioxide, and carbon monoxide) as potassium vapors, as shown in the following chemical equations:

$$6\mathrm{KOH}+2\mathrm{C}\to 2\mathrm{K}+3{\mathrm{H}}_2+2{\mathrm{K}}_2{\mathrm{CO}}_3$$

(4)

$$\mathrm{C}+{\mathrm{K}}_2{\mathrm{CO}}_3\to 2\mathrm{K}+{\mathrm{CO}}_2+\mathrm{CO}$$

(5)

These gases intercalated or inserted on the surface of the activated carbons, which expanded and opened the surface’s pores of the activated carbons. The generated activated carbons from WTLs had a hierarchical porous structure, as indicated by the SEM micrographs presented in Figure 4.

3.1.3. Step Three: Combination CrFeO₃ with Activated Carbons

In the third step, the adsorbent material was fabricated by combining the CrFeO₃ composite with the generated activated carbons. Simply, bulk solids of the activated carbons and CrFeO₃ composite were crushed at room temperature via a 1:10 molar ratio (composite to activated carbons). A few drops of MeOH solvent were added through the crushing process to facilitate grinding the two components. The dried adsorbent material was scanned by an FT-IR instrument, and the resulting IR spectrum is shown in Figure 5. This represents only one broad absorption band located at 500 cm⁻¹ due to the M–O stretching vibrations (M: Cr or Fe). The adsorbent material was also scanned by Raman spectroscopy and the obtained spectrum is illustrated in Figure 5. The spectrum shows two absorption bands at 1344 cm⁻¹ (indexed as D band) and at 1589 cm⁻¹ (indexed as G band). This indicated that the material possessed two distinct bands that belonged to the first-order spectral Raman lines (G and D bands). The G-band and D-band originate from the graphitized carbons and the disordered carbons, respectively. The G-band represents the stretching vibration of the sp² (C–C) bonded carbon atoms, whereas the D-band represents the presence of structural defects. The graphitization of the material can be estimated from the intensity ratio of the D to G bands (ID/IG) [33]. The fabricated adsorbent material exhibited a ratio of 1.02, which indicates a higher degree of graphitization in the carbon material. The broad band centered at 2915 cm⁻¹ in the Raman spectrum of the adsorbent material belonged to the second-order spectral Raman lines (2D band) [34]. The EDX profile of the adsorbent material, shown in Figure 5, evidenced the presence of C, O, Cr, and Fe elements. Figure 6 contains the SEM and TEM images of the adsorbent material. Most of the adsorbent material particles have a stone-like shaped morphology with a rough surface. Several of the adsorbent particles clumped into large agglomerates with different irregular features, sizes, and shapes. The TEM images, captured at 134,000× magnification, revealed that most of the adsorbent particles tended to aggregate into big clusters with diameters ranging from 40 to 60 nm. The synthesized absorbent material was used to adsorb two kinds of pollutants; organic dyes and commercial pesticides from an aqueous solution. The composites based on carbon and polymers active in the formation of CT complexes can be used not only as adsorbents but also for other applications such as strength-enhancing fillers and the enhancement of the strength of composite materials [35,36].

3.2. Applications

The fabricated adsorbent material was tested to remove several kinds of model pollutants, including organic dyes (M1 and M2) and commercial pesticides (M3 and M4). These were removed from the aqueous solution using the batch adsorption technique.

3.2.1. Adsorption of Organic Dyes

Two model dyes (M1 and M2) were examined to reveal the adsorption efficiency of the fabricated adsorbent material. An aqueous solution of dye M1, with a concentration of 10 mg/100 mL at pH = 7, had a pink color. This pink solution displayed two absorption bands when scanned by a UV-visible instrument (Figure 7). A strong, broad absorption band had two heads at 550 nm and 516 nm, and a medium-intensity absorption band also had two heads at 334 and 293 nm. An aqueous solution of dye M2, with a concentration of 10 mg/100 mL at pH = 7, had a violet color. Scanning this violet color solution with a UV-visible instrument gave two absorption bands: at 588 nm, which is strong and broadband, and at 302 nm, which is a narrow and weak band (Figure 7). Based on Figure 7, the detected band for M1 was 516 nm, and for M2 was 588 nm. Three factors that affected the adsorption of M1 and M2 were investigated. These factors were:

(a)

Solution pH

Aqueous solutions of M1 and M2, at a concentration of (100 mg/L), were prepared at different pH values from 2 to 12 using drops of HCl or NaOH solutions (0.01 N). The effect of pH values on the adsorption of M1 and M2 onto 0.10 g of the adsorbent material was presented in Figure 8. This figure showed that the removal efficiency (R%) reached its maximum in the range of pH = 7~8, and after pH = 7.0, the R% did not significantly increase.

(b)

Adsorbent material’s dose

100 mL of an aqueous solution of M1 and M2 (100 mg/L) were contacted with different amounts of the fabricated adsorbent material (0.05–0.20 g) at pH = 7.0. It was found that, as the adsorbent material’s dosage increase, the value of R% increased and R% reached a saturation value where all the active sites in the adsorbent material were occupied by the dye molecule. The minimum adsorbent material’s dosage to obtain a maximum R% for M1 and M2 removal from 100 mL solution (100 mg/L) was 0.11, and 0.14 g, respectively.

(c)

Contact time

The effect of contact time on the absorption of M1 and M2 was investigated at room temperature under the following conditions:

−

Dye’s concentration was 100 mg/L

−

pH’s solution was 7.0

−

Adsorbent material’s dose was 0.11 g for M1, and 0.14 g for M2

−

Contact time was 5–75 min.

Figure 8 indicates that the max R% for M1 (96.8%) was achieved in the first 45 min, while the max R% for M2, 95.5%, was achieved during the first 55 min. Figure 9 represents photographs of the adsorption of M1 and M2 dyes by the fabricated adsorbent material.

3.2.2. Adsorption of Pesticides

Two commercial pesticides (M3 and M4) were examined to reveal the adsorption efficiency of the fabricated adsorbent material. An aqueous solution of M3, with a concentration of 100 mg/L at pH = 7, exhibited two absorption bands when scanned by a UV-Visible instrument (Figure 10). A strong and intense band at 234 nm and a less intense band at 276 nm. An aqueous solution of M4, with a concentration of 100 mg/L at pH = 7, gives an absorption band at 296 nm when scanned by a UV-Visible instrument (Figure 10). This characteristic UV-Vis band of M4, coupled with a long tail, ranged from 350 to 800 nm. Based on Figure 10, the detected band for M3 is 234 nm, and for M4 is 296 nm. Three factors that affected the adsorption of M3 and M4 were investigated. These factors were:

(a)

Solution pH

The effect of pH values on the adsorption of M3 and M4 onto 0.10 g of the adsorbent material was investigated using aqueous solutions of M3 and M4 at a concentration of (100 mg/L) prepared at different pH values from 2.0 to 12.0. The results are illustrated in Figure 11, which indicates that when the pH value of the solution increased from 2.0 to 8.0, R% value increased from ~52% (for M3) and ~44% (for M4) to ~98%. This proposed that the pH of the solution greatly influenced the adsorption of M3 and M4 onto the fabricated adsorbent material. Figure 11 also shows that R% reached its maximum at pH = 8.0, and after pH = 8.0, the R% no longer increased.

(b)

Adsorbent material’s dose

Approximately 100 mL of an aqueous solution of M3 and M4 (100 mg/L; pH 7) were contacted with varying amounts of the fabricated adsorbent material (0.05–0.20 g). It was found that, as the adsorbent material’s dosage increased, the value of R% increased and R% reached a saturation point where all the active sites in the adsorbent material were occupied by the adsorbate. The minimum adsorbent material’s dosage to obtain a maximum R% for M3 and M4 removal from 100 mL solution (100 mg/L) was 0.13, and 0.12 g, respectively.

(c)

Contact time

The effect of contact time on the absorption of M3 and M4 was investigated at room temperature under the following conditions:

−

Pesticide’s concentration was 100 mg/L

−

pH’s solution was 8.0

−

Adsorbent material’s dose was 0.13 g for M3, and 0.12 g for M4

−

Contact time was 5–75 min.

Figure 11 supports the idea that the max R% for M3 (96.4%) was achieved in the first 35 min, while the max R% for M4, 98.6%, was achieved during the first 35 min.

3.3. Regeneration and Reusability

Five desorbing eluents were used to examine the desorption efficiency of the adsorbent material. The tested eluents were DI water, 0.2M KOH, 0.2M NaOH, 0.2M HCl, and 0.2M HNO₃. We selected NaOH (0.2 M) as the desorbing eluent to recover the pollutants (M1, M2, M3, and M4) from the adsorbent material because it exhibits the maximum desorption efficiency among all eluents tested. The desorption yields (%) using NaOH (0.2 M) were 97.6, 97.5, 97.8, and 97.5% for M1, M2, M3, and M4, respectively. Seven successive adsorption–desorption cycling tests were run using NaOH (0.2 M) as an eluent to examine the reusability performance of the adsorbent material. After each cycle, the adsorbent material was removed from the solution, washed, oven-dried, and recycled. We found that, after the first four cycles, only about a 12%-reduction was observed in adsorption yield, and this reached about 20% after the seventh cycle. These results suggested that the adsorbent material can be reused at least seven times in adsorption–desorption cycles after the successful regeneration of a loaded adsorbate.

4. Conclusions

Three major factors led to a significant amount of environmental water pollution: rapid progress and development of industrial technology, the fast growth of human society, and the rapid increase in global population. The need for freshwater is growing year after year, and proving freshwater in the future is of the utmost importance. Among water resource pollutants, organic dyes and agricultural pesticides are an important class of pollutants. One of the important methods for removing organic dyes and agricultural pesticides from polluted water and wastewater is adsorption over porous materials, such as carbon-based materials. The adsorption performance of carbon-based materials can be improved by combining these materials with metal oxides. Since people in Saudi Arabia consume large amounts of tea (tea is the third most consumed beverage in Saudi Arabia after water and Arabian coffee), a large amount of daily discarded waste tea leaves (WTLs) is suitable for use as an environmentally friendly, effective, sustainable, renewable, and cost-less resource for carbon materials. In this work, we synthesized a low-cost, bio-based adsorbent material through a solid–solid interaction by crushing the chemically activated carbons from WTLs with a CrFeO₃ composite in a 1:10 molar ratio (metal composite to activated carbons). The synthesized absorbent material was used to adsorb two kinds of model pollutants: organic dyes (Azocarmine G2; M1, and Methyl violet 2B; M2), and commercial pesticides (Tiller 480SL; M3, and Acochem 25% WP; M4) from aqueous solution, and it showed promising adsorption efficacy. The promising results from this work has motivated us to plan future works on investigating other sources of carbon materials, such as Arabian coffee, and to apply the synthesized adsorbent material to absorb other pollutants, such as heavy metal ions.