Investigation of Efficient Adsorption of Toxic Heavy Metals (Chromium, Lead, Cadmium) from Aquatic Environment Using Orange Peel Cellulose as Adsorbent

Abstract

Heavy metals in the environment cause adverse effects on living organisms. Agro-wastes have the potential to remove heavy metals from aqueous solutions. In this study, the orange peel cellulose (OPC) beads were utilized as adsorbents to remove metals from wastewater. The surface of the adsorbent was studied by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (SEM-EDS). The concentrations of the metals before and after adsorption were measured using inductively coupled plasma mass spectrometry. The removal of the metal ions (i.e., Cr⁶⁺, Cd²⁺, and Pb²⁺) using the OPC was investigated by varying the pH, contact time, and adsorbent dosages parameters. The maximum removal efficiency obtained for the metal ions occurred at pHs 4–8. The use of the Langmuir isotherm and Freundlich isotherm models demonstrated the statistical significance of the heavy metal adsorption processes (R² > 0.96). At a neutral pH, the OPC adsorption order was Pb²⁺ > Cd²⁺ > Cr⁶⁺ with % removal values of 98.33, 93.91, and 33.50, respectively. The adsorption equilibrium for Cr⁶⁺ was reached after 36 h. For Cd²⁺ and Pb²⁺, equilibrium was reached after 8 and 12 h, respectively. The FT-IR and SEM-EDS confirmed the presence of many functional groups and elements on the adsorbent. The adsorption of heavy metals using the OPC is a low-cost, eco-friendly, and innovative method for the removal of metals in aquatic environments. The findings of this study will be highly significant for the public in the affected areas worldwide that have credible health concerns due to water contamination with heavy metals.

1. Introduction

Water is the most vital and valuable resource in human civilization. However, water sources such as lakes and rivers have been contaminated by agricultural, industrial, and domestic wastes [1]. Water pollution in developed and developing countries has increased due to anthropogenic activities, such as fertilizer preparation, electroplating, leather making, sugar milling, textile making, mining, metallurgical processing, and municipal waste processing [2,3,4,5]. Toxic metals are known to cause severe impairment to marine organisms, terrestrial plants and animals, and human beings; thus, their release into water systems due to such activities is of significant concern [6,7]. In industrial wastewater, lead, cadmium, arsenic, mercury, and chromium ions are the most commonly found toxic heavy metal ions [8,9,10]. The quality of food and vegetables grown on soil contaminated with metals is significantly impacted, and their consumption may have negative health effects on human nutrition levels [11]. Heavy metals are able to block specific cellular functions of certain biomolecules, such as proteins and enzymes [12]. They can also gather in living tissues and have been associated with many diseases (e.g., cancer) [13].

In particular, lead (Pb) causes several health issues, including mephitis, hypertension, stomach pain, constipation, vomiting, nausea, speaking difficulties, etc. [14]. Long-term cadmium exposure has been related to various types of cancer, including lung, kidney, breast, prostate, pancreas, etc. [15]. Additionally, the increased concentration of hexavalent chromium is poisonous, mutagenic, and carcinogenic to living things; as a result, it is categorized as a priority pollutant [16], whereas trivalent chromium is much less toxic. Therefore, it is essential to remove heavy metals from water bodies for safe water drinking and human activities [2,17,18].

Water bodies are particularly vulnerable to heavy metal contamination. The slow increase in metals in marine environments has become a significant health concern worldwide [19]. The monitoring and removal of metal ions using effective technologies are essential tasks for the safety assessment of the overall environment [20]. Various physicochemical approaches have been established to remove toxic substances from contaminated water bodies. These methods include electrochemical treatment, ion exchange, reverse osmosis, evaporation, precipitation, adsorption on activated coal, entrapment, encapsulation, microbial biomass, and so on [21,22,23]. However, most methods are expensive and inefficient, especially for metals at low concentrations in large solution volumes [18]. In addition, they often have poor filtration and adsorption capabilities.

Biological techniques that are less expensive and more environmentally friendly should be taken into consideration as alternative methods for the remediation of heavy metals [24]. Adsorption is one of the most promising approaches for the elimination of heavy metal ions in wastewater sources which has attracted the interest of chemists and ecologists [25]. However, the creation of more efficient adsorbents and their preparation to be safer and more ecologically friendly is a challenge [26]. Plant biomass-based activated carbons can be used for the sequestration of metal ions [27]. Chemically modified chitosan is a great adsorbent for heavy metals [28,29] as well as bacteria-based biomass which could be used for the removal of heavy metals [17,23]. Agricultural wastes and non-edible plant parts are rich in natural polymers (e.g., hemicelluloses, cellulose, pectin, and lignin), which are known to have an unusual strength and attractive mechanical properties [30,31,32]. Lignocellulosic materials are biobased and biodegradable; thus, their use and disposal as bioadsorbents contribute to the enrichment and isolation of environmental pollution. Lignocellulosic waste has been proposed as a bioadsorbent for the removal of heavy metal ions in wastewater [33,34,35]. Thus, it is hypothesized that beads formulated with agricultural lignocellulosic waste can bind sufficiently and eliminate heavy metal pollutants in environmental wastewater. Among the various bioadsorbents, fruit peels have received global attention as they are readily available. Meanwhile, orange peels are also adsorbents, which are resource saving and environmentally friendly. They are also available abundantly and inexpensively. They contain pectin, cellulose, hemicelluloses, chlorophyll pigments, lignin, and numerous low-molecular-weight hydrocarbons, which are appropriate adsorptive materials [36]. Thus, the adsorption of heavy metal ions by orange peel cellulose (OPC) is considered as an effective, low-cost, and innovative method to remove the heavy metal ions from aquatic environments.

Although adsorbents are environmentally friendly and low-cost, most raw bioadsorbents show a low metal ion sorption capacity because of the absence of suitable functional groups for effective adsorption. A lignocellulosic adsorbent has been modified by chemical treatment [37,38,39]. In the first step, the cellulose fiber from orange fruit peels is obtained using an acid (e.g., hydrochloric acid (HCl) or sulphuric acid (H₂SO₄)) or an alkaline solution (e.g., sodium hydroxide (NaOH)). The use of a NaOH solution is known as the soda process. A bleaching process (e.g., with sodium hypochlorite (NaClO) or sodium chlorite (NaClO₂)) is then performed to eradicate the residual lignin and to obtain a purified cellulose fiber. Extracted cellulose fibers contain multiple functional groups, such as hydroxyl (-OH), carbonyl (=CO), carboxyl (-COOH) [40], amidoxime [41], and amide [42]. They are used in various applications (e.g., as adsorbents) [43,44].

This work presents an eco-friendly, low-cost, and straightforward heavy metal cleanup technique. The adsorption capacity of the OPC was thoroughly examined to determine the best approach. This study aimed to use the agricultural waste material (i.e., OPC) as an excellent source of bioadsorbent for the removal of heavy metal ions and to apply the bioadsorbent for the elimination of contaminants from wastewater, thus contributing to the reduction in environmental pollution.

2. Materials and Methods

2.1. Reagents

The chemicals used in this study were obtained from Fujifilm Wako Pure Chemical Corporation (Osaka, Japan) unless otherwise mentioned. The chemicals were American Chemical Society grade and used as received without any purification procedure. Gum guar was obtained from Sigma (Steinheim, Germany). Whatman filter paper pore size 45 µm from Advantec Toyo (Tokyo, Japan) was used to filter the cellulose. Standard stock solutions of 1 M potassium chromate (K₂CrO₄), cadmium chloride (CdCl₂), and lead acetate Pb(C₂H₃O₂)₂ were prepared with Milli Q water and stored at 4 °C. The 100 and 1.0 mM working solutions were made by diluting the standard stock solution with purified water from a Milli Q system from Millipore (Merck, Darmstadt, Germany), with Ʊ set at 18.2. The pHs of the metal solutions were adjusted, adding with 0.1 M HCl or 0.1 M NaOH. All the adsorption experiments were performed at 28 °C.

2.2. Extraction of Cellulose from Orange Fruit Peels

Oranges were bought from local grocery stores in Saga, Japan. First, the fruits were thoroughly washed with tap water to remove any dust particles on the surface of the fruits. Then, the peels were removed and cut into ~0.25 cm² pieces. The cut peels were then washed with deionized water to remove extra particles. The washed peels were dried in an air oven at 70 °C for 24 h. The dried peels were then crushed to a fine powder by using a mechanical blending machine. The particles with sizes of <240 μm were separated using a mesh strainer. OPC fibers were extracted from the powdered sample by following a previous procedure [45]. Briefly, 30 g of powder was soaked in a mixture of 150 mL of ethyl alcohol (99.5%), 75 mL of 0.8 M NaOH, and 75 mL of 0.8 M calcium chloride (CaCl₂). The resulting mixture was incubated for 20 h at room temperature on the lab bench. The fibers were filtered and rinsed using deionized water until a neutral pH was obtained. The fibers were then dried in an air oven at 70 °C for 24 h. Finally, the obtained OPC fibers were used to prepare beads with gum guar, which is an exo-polysaccharide composed of galactose and mannose. Gum guar has stabilizing and thickening properties, which are advantageous in food, feed, and industrial applications [46]. A total of 5 g of OPC powder and 2.5 g gum guar were mixed into 5 mL of 2% acetic acid (CH₃COOH) to make a slurry. The slurry was heated and stirred in a water bath at 60 °C for 30 min. The round-shaped beads were prepared by hand, and they were then dried overnight at 50 °C. The beads prepared were approximately 3 mm in diameter (Figure 1). The obtained OPC beads were stored in a ziplocked bag at room temperature until further use.

2.3. Fourier Transform Infrared (FT-IR) Spectroscopy

The FT-IR spectroscopy thoroughly describes the detailed information on the functional groups existing in a sample. Therefore, the structural information of the bioadsorbents was assessed by FT-IR in order to discover the chemical functional groups and binding processes involved in the metal adsorption process using a model VERTEX 70v from Bruker (Osaka, Japan). FT-IR absorbance spectra of untreated and treated OPC samples with three metal ions (i.e., Cr⁶⁺, Cd²⁺, and Pb²⁺) were in the wavelength range of 400–4000 cm⁻¹.

2.4. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

The surface of the prepared OPC was investigated by SEM that was equipped with an EDS using a model 3400N from Hitachi (Tokyo, Japan). The OPC beads were exposed to a 1.0 mM cocktail of the three metal ions for 12 h at room temperature with continuous shaking at 160 rpm. The OPC beads were collected and washed carefully using deionized water. Before imaging, the samples were coated with platinum. An OPC bead sample for SEM was mounted on an aluminum stub with double-sided carbon tape and covered with 10 nm of gold-palladium with a vacuum magnetron sputtering equipment model MSP-1S Sputtering Targets Manufacturer™ (Ibaraki, Japan). The surface morphology was analyzed using the EDS at 15.0–25.0 kV. EDS investigations were accomplished to determine the existence of the ions on the adsorbent beads. Samples for EDS were coated with 15 nm of carbon using a model 500× carbon coater attachment on the sputter coater from Emitech (Mahwah, NJ, USA). High-resolution spectra were fitted and calculated using the AVANTAGE software provided by Thermo Fisher Scientific (XPS, Wilmington, DE, USA), where wt% is expressed as the amount of the element in terms of mass fraction of the element in the sample. The EDS analyses were performed by removing the other elements, except chromium, cadmium, and lead. However, oxygen and carbon were preserved as the signal elements. Our system provided information on the quantitative element percentages, including the atomic percentage and weight percentage of each element studied.

2.5. Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) Analyses

After the adsorption of the metal ions, using the prepared OPC beads, the concentration of the metal ions was measured by using an ICP-MS instrument model 7900 from Agilent Technologies (Santa Clara, CA, USA). The OPC beads were exposed to a mixture of the three metal ions for 12 h with continuous shaking at 160 rpm and a temperature of 28 °C. Control samples were shaken similarly but without the OPC beads. A cellulose-free supernatant was collected by centrifugation for 10 min at 12,298× g. The cellulose-free liquid was filtered using a 0.45 μm filter and syringe to remove all small particles. All the solutions were acidified with 2M HNO₃. Appropriate dilutions were performed before analysis. The samples were made to a constant volume before determining metal ion contents. Standard solutions of mixed metals of 0, 0.01, 0.1, 1, 10, 20, and 50 ppm were prepared. A standard curve was prepared before measuring the metal concentrations of the control and samples. All measurements were performed in triplicate and the results were expressed as the mean ± the standard error. The adsorption percentage was quantified using Equations (1) and (2).

C_ad = C₀ − C_t

(1)

$$\mathrm{Adsorption}\%=\frac{\mathrm{Cad}}{\mathrm{C}0}\times 100$$

(2)

where C₀, C_t, and C_ad (ppm) are the initial concentration, the concentration at a time (t), and the concentration adsorbed, respectively.

Equation (3) was used to calculate the adsorption capacity q_e (mg g⁻¹) after the equilibrium.

$${\mathrm{q}}_{\mathrm{e}}=\frac{\mathrm{Cad}}{\mathrm{W}\left(\mathrm{g}\right)}. \frac{\mathrm{V}\left(\mathrm{mL}\right)}{1000}$$

(3)

where W, V, and q_e are the amount (g) of OPC used, volume (mL) of the mixed metal solution, and adsorption when equilibrium was attained, respectively [29].

An adsorption isotherm describes useful information on the adsorption capacity, binding affinity, and surface characteristics of the adsorbent, which may help to know the binding mechanism of the adsorbate with the adsorbent [47]. In this study, the equilibrium adsorption properties of OPC beads for metals uptake were clarified using Langmuir adsorption isotherm. Equation (4) represents Langmuir’s isotherm.

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}$$

(4)

where q_max represents the highest adsorption capacity (mg g⁻¹), and K_L is Langmuir’s isotherm constant that illustrates the binding affinity between metals and test beads. The isotherm constants can be calculated from the intercepts and slopes of linear plots. Equation (5) represents the linear form of Langmuir’s isotherm.

$$\frac{1}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{q}}_{\max }}.\frac{1}{{\mathrm{C}}_{\mathrm{e}}}+\frac{1}{{\mathrm{q}}_{\max }}$$

(5)

The separation factor (R_L) was calculated using Equation (6).

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+{\mathrm{C}}_{\mathrm{i}}{\mathrm{K}}_{\mathrm{L}}}$$

(6)

where R_L indicates the adsorption opportunity is either favorable (0 < R_L > 1), unfavorable (R_L > 1), linear (R_L = 1), or irreversible (R_L = 0) [48].

Additionally, adsorption data from solutions are most frequently represented using the Freundlich isotherm. The linear logarithmic Equation (7) can be used to express the Freundlich model [29].

$${\ln \mathrm{q}}_{\mathrm{e}}=\frac{1}{\mathrm{n}}\ln {\mathrm{c}}_{\mathrm{e}}+\ln {\mathrm{K}}_{\mathrm{F}}$$

(7)

where K_F (mg g⁻¹) stands for the adsorption intensity-related Freundlich characteristic constants, and n stands for the adsorption intensity. Particularly, it denotes the favorable metal adsorption when 1/n is in the range of 0.1–1.0 [49].

2.6. Effect of pH on Adsorption

The adsorption by OPC was assessed at different pHs of a metal ion solution. In a 3 mL tube, 10 mg of OPC and 1.0 mL of a 1.0 mM metal ion solution were mixed. The pH was adjusted to 3.0, 4.0, 5.0, 6.0, 7.0, or 8.0 ± 0.1. Adsorption was accomplished at 28 °C for 24 h with nonstop shaking at 160 rpm. All experiments were performed in triplicate.

2.7. Effect of Time on Adsorption

To determine how contact time affects bioadsorption, five beads of OPC (~100 mg) were added to a 10 mL cocktail solution of the three ions, each at a concentration of 1.0 mM. The pH was maintained to 7.0 ± 0.2. Bioadsorption was performed at 28 °C for 2, 4, 8, 12, 24, 36, and 48 h with continuous shaking at 160 rpm.

2.8. Bioadsorbent Dosages

The quantity of adsorbent used plays a crucial role in bioadsorption. Therefore, a 10 mL cocktail solution of the three metal ions, each at a concentration of 1.0 mM, was incubated with an increasing amount of OPC beads 1, 2, 3, and 5 (~20–100 mg). A metal solution was adjusted to pH 7.0 ± 0.2 and incubated at 28 °C with shaking for 24 h at 160 rpm.

2.9. Statistical Analysis

The OriginPro 2021 software from OriginLab corporation (Northampton, MA, USA) was used to complete the statistical analysis. The results of three duplicate tests (n = 3) were reported as the mean value with error bars denoting the standard deviation. The coefficient of determination (R2) was used to evaluate how well the bioadsorption model fit the data. A linear fit correlation was performed to assess the relationship between metal concentration and adsorbent amount. The amount of adsorbent and metal concentration significantly correlated positively, R² = 0.999. The slope deviates significantly from zero at the 0.05 threshold. Mendeley software (Elsevier, Mendeley Ltd., London, UK) was used for reference management.

3. Results and Discussion

Orange fruit peel is a low-cost adsorbent widely available as waste material. Therefore, it can be an effective adsorbent for the removal and recovery of metal ions from wastewater. This study investigated the adsorption of selected metal ions onto the OPC in batch experiments. The adsorption was found to depend on the adsorbent dosage, exposure of time, and pH of the sample. After a comparison with similar published studies, the developed OPC showed a better adsorption capacity for the metal ions studied.

3.1. FT-IR Analysis

The FT-IR investigation was accomplished to determine the main functional groups present on the OPC surface that may be playing a significant role in the adsorption process on the surfaces of adsorbents [50]. The FT-IR spectra of the prepared adsorbents were obtained in the scanning range of 400–4000 cm^–1 and are presented in Figure 2. The broad and intense peak was observed at around 3371 cm⁻¹, which suggested the presence of carboxylic, ketone, and phenolic functional groups in the cellulose, hemicellulose, pectin, and lignin. These functional groups have been shown to assist in metal ion adsorption in aqueous solutions [51]. The most intense peak was observed at 1629 cm⁻¹, which indicated the strong presence of carboxylate anions (Figure 2). Another major peak at 2917 cm⁻¹ was attributed to the CH stretching vibrations of the methyl and methoxy substituent groups [52]. A simple peak was found at 2355 cm⁻¹ in both the control and metal-treated OPC due to the N-H or C=O functional group, whereas the vibrational peak at 2850 cm⁻¹ may be due to the C–H asymmetric stretching vibration [53]. The peaks at 1633 and 1423 cm⁻¹ indicated asymmetric and symmetric vibrations of ionic carboxylic groups (-COOA), respectively [51].

The peaks at 1100 and 1383 cm⁻¹ were referred to as the C−O−C and C-H groups, respectively (Figure 2). Deprotonated carboxyl and hydroxyl groups have been shown to coordinate with metal ions [54]. The peak of the OPC slightly shifted from 1612 to 1629 cm⁻¹ (C=O stretching vibration of carboxyl groups), thus revealing the possible involvement of metal ions adsorption. These changes have been related to carboxylate and hydroxylate ions, which contribute to the metal uptake [51]. The FT-IR results recommended that the increasing number of carboxylate ligands enhanced the metal-binding ability to the adsorbent [55]. Therefore, the FT-IR study revealed the possible involvement of the major functional groups in the adsorbent, such as hydroxyl and carboxyl, which contributed to the ion exchange with metal ions during the bioadsorption process. The oxygen-containing functional groups, including hydroxyl, carboxyl, phenolic, and carbonyl, played essential roles in the Cd(II) ion adsorption [56]. According to the FT-IR analysis, the carboxyl and hydroxyl functional groups were mostly responsible for the Pb(II) ion adsorption [57].

3.2. SEM-EDS Analysis

The morphology of the OPC surface was observed by using SEM-EDS. An analysis was completed before and after exposure to the cocktail solution of the three metals considered. This was performed to determine the metal ions distribution on the surface of the adsorbents (OPC) and the physical morphology of the adsorbents. A representative SEM of the rough surface of an OPC sample after bioadsorption is shown in Figure 3. At higher magnifications (i.e., 500×, 1000×, and 2000×), the OPC sample displayed an uneven surface, which is made of valleys and hills that formed dimples of different sizes (Figure 3a–c). The surface of the OPC sample was heterogeneous; therefore, the bioadsorption performance of this cellulose was likely better than smooth cellulose. After incubation for 12 h, the OPC appeared swollen and larger at the 500× magnification (Figure 1c).

The EDS results suggested that the surface of each cellulose sample consisted mainly of carbon and oxygen. The ions of interest were not found in the EDS spectrum of the OPC before the bioadsorption (control samples). On the other hand, after the bioadsorption, the ions were found throughout the surface of the sample (Figure 3e).

The mapping and overlapping of the images were accomplished to validate the distribution of the metal ions on the surface of the OPC. The image mapping was performed using 64 images of the corresponding metals. This approach detected the distribution of the metal ions on the surface of the prepared OPC. The results are presented in Figure 4, where the metal ions are depicted in different colors. The results suggested the uptake and accumulation of all three metal ions on the surface of the OPC, which were due to the exposure of the OPC to the metal solution.

The EDS intensity levels for all the metal ions continued persistently during the mapping. The image mapping further confirmed the results found in the ion imaging of the surface of the orange peel cellulose. The bioadsorption performances were compared based on the quantification by ICP-MS. Nevertheless, they were able to confirm the presence of the metal ions on the surface of the adsorbents. Similarly, the OPC was exposed to a solution without the metal ions. This was used as the control. Metal ions in the control samples were not detected.

3.3. ICP-MS Analyses

To further confirm the ability of the OPC to accumulate the selected metal ions, the ICP-MS analyses were performed. After the bioadsorption, the metal ions in the control and exposed samples were determined by the ICP-MS varying conditions, such as the pH, adsorbent dosages, and contact time.

3.4. Effect of pH

For the bioadsorption of metal ions using biological materials, pH is an important parameter that influences the protonation of functional groups and controls the metal chemistry of the material [50,58]. The bioadsorption was studied individually at varying pHs in the range of 3–8, and the removal of metals was investigated. The results are summarized in Figure 5.

The removal of the studied ions increased markedly in the pH range of 4–8. The maximum bioadsorption was observed at pH 3–5, 3–8, and 4 for Cr0₄²⁻, Cd²⁺, and Pb²⁺, respectively. The bioadsorption of Cr0₄²⁻ decreased with an increasing pH. At an acidic pH, the electronegative functional groups on the surface were protonated; thus, they were most suitable for binding with anions [35]. The bioadsorption difference was insignificant for the other metal ions at pH values between 4 and 8. A nearly identical pattern of metal ions accumulation was seen at various pHs. Similar observations have been reported in the removal of Cu²⁺, Pb²⁺, Cd²⁺, Ni²⁺, and Zn²⁺ using a modified orange peel in which the bioadsorption reached an equilibrium at pH values between 5.0 and 5.5 [59]. In a similar study by Nathan et al., bioadsorption using Kiwi beads showed that Cd²⁺, Cr⁶⁺, Cu²⁺, and Ni²⁺ were stable in the pH range studied [35]. Several studies have shown that the maximum bioadsorption of Cr⁶⁺ was achieved at various pH values of 2 [60,61], 3 [62], and 5 [63].

At an acidic pH, chromium ions occur in two forms, namely as chromic acid (H₂CrO₄) and hydrogen chromate ions (HCrO₄-) at pH ranges of 1–2 and 3–7, respectively [64]. The high concentration of H⁺ and H₃O⁺ protonates the carbonyl and hydroxyl groups, when the pH is low. There is little to no adsorption because of the competition between these ions and the aqueous heavy metal ions for the available binding sites in adsorbents. [65]. At a basic pH, some metals are precipitated [66]. According to the World Health Organization guideline, drinking water is neutral in pH. The OPC beads could be applied to purify drinking water and wastewater. Therefore, all the experiments were conducted at pH 7 in a cocktail solution of the metal ions.

3.5. Effect of Adsorbent Dosage

The dosage of the adsorbent significantly affected the adsorption process, removal efficiency, adsorption capacity, and other studied parameters. The adsorbent dosages of the OPC beads studied were 1, 2, 3, and 5 (~20–100 mg), containing a 1.0 mM concentration of each of the three metal ions. The results are shown in Figure 6. The adsorption of the metal ions increased with the increase in the adsorbent concentration (i.e., from 1 to 5 beads for all the metal ions).

As shown in Figure 6, there is a rising trend in the metal adsorption capacity of the OPC beads. However, due to their surface and strong affinity for metals, the OPC beads demonstrated an outstanding adsorption capacity. Meanwhile, several studies have shown that an increase in the adsorbent dosage improved the removal efficiency [35,62,63,67,68]. Additionally, more surfaces and functional groups were available on the adsorbent at higher adsorbent doses, which improved the overall adsorption of metals [69,70].

The unit adsorption of the metal ions was calculated based on the amount of adsorbent used. The Langmuir adsorption isotherm is the most used linear model for monolayer adsorption and is frequently used to calculate the adsorption parameters [71]. On heterogeneous surfaces, multilayer adsorption is modeled using the Freundlich isotherm [71]. The equilibrium values were well-fitted by the Langmuir and Freundlich isotherm models. Figure 7 displays the estimated model parameters together with the linear regression coefficient (R²) for the Langmuir and Freundlich isotherm models. The R² values are calculated for the experimental linear relationship to be statistically significant.

Table 1. Langmuir and Freundlich isotherm model parameters for the removal of metal ions by OPC.

Models	Parameters	Cr6+	Pb2+	Cd2+
Langmuir	qmax (mg g−1)	4.90	50.10	29.00
	KL(L/mg)	0.0509	0.2015	0.2204
	RL	0.6574	0.0013	0.0037
	R2	0.9879	0.9999	0.9999
Freundlich	KF (mg g−1)	4.18	47.46	25.79
	1/n	0.2892	0.3291	0.2896
	R2	0.9939	0.9662	0.9929

contains a list of the corresponding adsorption parameters.

The R² values of all the metal ions were close to 1, confirming the Langmuir model’s outstanding applicability to the adsorption processes [72]. The expressions of the straight lines were initiated by means of a mathematical transformation of the isotherm equation. The maximum adsorption capacities of the OPC beads for Pb, Cd, and Cr were observed to be 50.10, 29, and 4.9 mg g⁻¹, respectively. The detailed results are shown in Table 1, which illustrates the linear regression coefficient values, Freundlich and Langmuir’s constant and adsorption possibilities. The values of R_L were found to be between 0 and 1, which confirmed the favorable uptake of the heavy metal ions [71].

However, the unit adsorption decreased with the increase in the adsorbent dosages. For example, the unit adsorption of Pb was reduced from 50.10 to 10.94 mg g⁻¹ as the adsorbent dosage was increased from 1 to 5 beads (~20–100 mg/10 mL). Similarly, the unit adsorption values of Cd and Cr were reduced from 29 to 6.82 and 4.9 to 1.12 mg g⁻¹, respectively (Figure 8). The possible explanation for this may be due to the overlapping or aggregation of the adsorbent surface area, which was available to the metal ions in the solution. Therefore, when the amount of adsorbent is more, some surface areas of the adsorbent may be occupied with each other, and metals cannot be adsorbed on those sites of the adsorbent. A similar study was performed by Yang and Cui where the alkali-treated tea residue (ATTR) was used as an adsorbent. The adsorption of Pb was decreased from 2.09 mg g⁻¹ to 0.63 mg g⁻¹ with the increase in the ATTR dosage from 1 to 5 g L⁻¹ [57].

In addition, for a more concentrated solution or effluent, a given mass of the adsorbent is able to purify a smaller volume of the effluent [69]. Therefore, in this study, a 100 mg (5 beads) adsorption dosage was selected as suitable for conducting the adsorption procedure. It was noticed that the ratio of the adsorption rates was not equal to the adsorbent dosages. An increase in the adsorbent dosage reduces the available metal ions for the adsorption. However, at the optimal amount of adsorbent, the number of sites available were sufficient for an interaction with the metal ions in the solution. Therefore, an excess adsorbent is not suitable for bioadsorption. Several studies have selected an optimal dosage for different bioadsorbent materials in the removal of metal ions from contaminated waters [35,62,63,67,68]. However, the dosages differed from the results obtained in this work. This was because of the different conditions, such as the bioadsorbent source, metal ions studied, and concentration of metal ions.

3.6. Influence of Contact Time

The exposure time is the most crucial parameter in the development of surface charges on the bioadsorbent, which is used for the bioadsorption of metal ions. To study the effect of the exposure time on the bioadsorption, seven time points between 2 and 48 h were set. The results are presented in Figure 9. The bioadsorption of Cr⁶⁺ increased linearly in the first 24 h. It then slightly increased for Cr⁶⁺ at 36 h. The adsorption of Cr⁶⁺ was not significantly high. A similar study has shown that Cr⁶⁺ was not adsorbed significantly by apple beads [67]. For Cd²⁺, better bioadsorption was observed after 4 h. Then, the bioadsorption rate was slow until 36 h. After that, the rate was almost constant. Therefore, the maximum bioadsorption was 36 h for the Cd²⁺ ions. For Pb²⁺, the bioadsorption increased in the first 36 h and then the adsorption rate was constant.

In this study, the bioadsorption rate was faster during the initial stages. The faster initial removal rate followed by a slower rate was likely due to the availability of the binding sites on the OPC surface during the initial phases [31,73]. For Cr⁶⁺, the bioadsorption equilibrium was reached after 36 h (Figure 9). For Cd²⁺ and Pb²⁺, the equilibrium was reached after 8 and 12 h, respectively.

As anticipated, the maximum adsorption capacities were different for the individual metal ion solutions under the optimized circumstances. After 48 h of exposure, the percentage concentration of the Pb, Cd, and Cr ions decreased by 98.33, 93.91, and 33.50, respectively (Figure 10). However, the OPC adsorbents which contain cellulose, hemicellulose, and lignin would exhibit a distinct mechanism of adsorption with the removal of various metal ions in a different process. This process might be involved in the complex formation between phenolic, hydroxyl, and carboxylic groups with heavy metal ions as well as an electrostatic attraction. A similar study using orange peel activated carbon has shown the removal efficiency order trend of Pb²⁺ > Cr³⁺ > Cd²⁺, which disagrees with the current research, except for Pb²⁺ [69]. This was due to many circumstances, such as the bioadsorbent supply, the examined metal ions, and the concentration of metal ions. Another study using a kiwi peel bead has shown that the decreasing order of the bioadsorption was Cd > Pb > Cr, with approximately 92%, 67%, and 34%, and the simultaneous removal of ions, respectively [35]. The bioadsorption performance of Cd and Cr agreed with the current study.

The maximal adsorption capacities when using orange peel which are reported for the absorption of several metals are listed in

Table 2. Summary of the types of adsorbents, types of metal, and maximum metal removal percentage capacity.

Biosorbents	Cr %	Cd %	Pb %	Reference
Orange peel	66.8			[62]
Orange peel			95.1	[74]
Orange peel	89.6	77.2	80	[69]
Orange peel	80			[60]
Modified orange peel		90	99	[59]
Orange peel		44.42		[68]
Orange peel			85	[75]
Orange peel			64.3	[45]
Orange peel		91	98	[36]
Orange peel		48.4		[68]
Modified orange peel		91		[76]
Dried orange peel		97.75		[77]
Orange peel cellulose	33.50	93.91	98.33	This study

along with the appropriate references.

3.7. Future Research, Practice, and Policy

The adsorption of the chromate from the aqueous phase was observed by 33.50%, which is relatively low. Thus, these beads at the present formulation are likely less useful for the Cr removal from polluted water. However, the high removal efficiencies of these metals could be possible by changing the formulation and controlling the parameters, such as the mass of the adsorbent, pH of the reaction system, and contact time. It would also be possible to learn more about the type of ion attaching to the surface of the beads, the energy changes, and the viability of the reaction via a further investigation into bioadsorption isotherms and thermodynamics. More research is required to determine to what extent the metal ions may be reduced to 100% using the same adsorbent materials.

4. Conclusions

The present study developed low-cost, environmentally friendly, and greater adsorption-featured beads from orange peel cellulose (OPC). Considering the findings of this study, the OPC was confirmed as a great substance to adsorb heavy metals such as Pb, Cd, and Cr under experimental conditions. The SEM-EDS confirmed the presence and distribution of metal ions on the surface of the OPC exposed in a metal solution. Moreover, the FT-IR spectra of the treated OPC have revealed numerous functional groups in its surfaces that can efficiently adsorb metal ions. Therefore, compared to previously reported adsorbents, our orange peel-based adsorbents have shown effectiveness in a multi-ion solution, which is more like the water we drink. Overall, the findings from this study indicate that the developed OPC can be used in many broad-scale and alternative applications for cleaning wastewater. Nevertheless, we think this work is a significant advancement in the field and will be useful to a wide range of scientists working in the analytical and environmental sciences. Thus, the use of the material as a biosorbent may play an important role in reducing the pollution caused by direct anthropogenic activities and bring considerable economic benefits.