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Article

Sustainable Processes of Biosorption of Pb(II) Ions from Synthetic Wastewater Using Waste Biomass from Mullein Leaves

by
Žaklina Tasić
1,
Maja Nujkić
1,*,
Ivana Savić Gajić
2,
Dragana Medić
1 and
Snežana Milić
1
1
Department of Chemical Technology, Technical Faculty Bor, University of Belgrade, Vojske Jugoslavije 12, 19210 Bor, Serbia
2
Faculty of Technology in Leskovac, University of Niš, Bulevar oslobođenja 124, 16000 Leskovac, Serbia
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(14), 5982; https://doi.org/10.3390/su16145982
Submission received: 10 May 2024 / Revised: 3 July 2024 / Accepted: 8 July 2024 / Published: 12 July 2024
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Abstract

:
The aim of this study is to evaluate mullein (Verbascum thapsus) as a cost-effective and sustainable adsorbent for the biosorption of Pb(II) ions from synthetic wastewater samples. Biosorption of mullein was investigated as a function of initial Pb(II) concentration (25–400 mg L−1), biosorbent dosage (2–20 g L−1), solution pH (3–7), and contact time (10–120 min). Mullein as a material with a high affinity for Pb(II) ions had a biosorption efficiency of 98.56% under the optimal conditions: pH 6, initial concentration of Pb(II) at 100 mg L−1, contact time of 90 min, and biosorbent dosage of 20 g L−1. The FTIR spectra of mullein leaves showed that oxygen-containing functional groups on the surface are potentially active sites for the biosorption of Pb(II) ions. EDS analysis and the pHPZC value confirmed the adsorption of Pb(II) ions at the active sites of the mullein. Kinetic and isotherm data enabled insights into the modes of Pb(II) biosorption on the mullein surface which were best explained with the pseudo-second-order kinetic model and the Freundlich adsorption isotherm. Biosorption occurs on the mullein surface via multilayer adsorption. The reusability of mullein showed that the native biosorbents can be reused five times, showing the economic and sustainable benefit of this low-cost biosorbent material.

Graphical Abstract

1. Introduction

Heavy metals are pollutants that cannot be degraded and can accumulate throughout the food chain. The pollution of the environment with heavy metals has been accelerated by rapid industrialization and urbanization [1]. They are transported primarily through aquatic environments such as rivers and groundwater [2,3]. Therefore, treatment of contaminated water is required before it reaches other water bodies. Various conventional methods for removing toxic chemical from wastewater have been studied including coagulation [4], chemical precipitation [5], adsorption [6], membrane separation [7], ion exchange [8], and solvent extraction [9]. Most conventional methods are economically unfavorable, especially when concentrations of heavy metals are under 100 mg/L. The biosorption method is a promising and sustainable technique that uses natural materials such as algae, fungi, bacteria, and plants to eliminate even low concentrations of heavy metals from polluted water. Biosorption is effective and efficient, making it an attractive option for wastewater treatment.
Industrial activities such as the glass and ceramics industries, the production of acid batteries, the production of tetraethyl lead, and metal coating and finishing are important sources of Pb(II) compounds from untreated wastewater [10]. The Environmental Protection Agency (EPA) specifies a maximum permissible concentration limit of 0.05 mg L−1 for Pb(II) in industrial wastewater, but Pb(II) concentrations in many wastewaters are found to be in the range of 200–500 mg L−1. It is therefore necessary to minimize Pb(II) concentrations to a range of 0.05–0.10 mg L−1 prior to discharge into smaller or larger watercourses [11].
Exposure to Pb(II) ions in water can pose a significant risk to the environment and consequently to human health. Lead ions and compounds are important pollutants that are accountable for the pollution of soil, water, and the atmosphere and can be harmful to aquatic life and humans even in low concentrations. Exposure of individuals, especially sensitive groups such as children and pregnant women, to high concentrations of lead can cause a number of health problems. This can include cardiovascular and reproductive problems, as well as reduced fetal growth in pregnant women. In children, effects can include hearing problems, anemia, and hyperactivity [12].
Plant biomass such as coffee waste (41.15 mg g−1) [13], tomato waste (152 mg g−1) [14] and sugarcane bagasse (116.7 mg g−1) [15] have shown a high adsorption capacity for lead ions. Other materials such as chitosan–alginate beads (60.97 mg g−1), clinoptilolite/magnetite/chitosan nanocomposite (124.95 mg g−1), and magnetic nanoparticles (53.11 mg g−1) have also been used to remove lead ions [16]. The efficiency of these biosorbents depends on the morphological structure of the biosorbent, the concentration of Pb(II) ions, and the pH value of the solution under investigation. Several authors [16,17] have proposed optimal conditions (0.1–0.5 g/L; 25–100 mg/L; and pH = 4–6) for the removal of lead ions using biosorbents, which can vary greatly and must be determined experimentally for each specific case.
Mullein leaves (Verbascum thapsus), which also have medicinal properties, were chosen as a potential biosorbent because, after the expiration date, this material is considered waste. The presence of biologically active compounds in mullein [18] that could affect water quality should also be considered. The use of this low-cost and sustainable material as a biosorbent would have an impact on reduced waste disposal, as well as on wastewater treatment. As far as we are aware, there are only a limited number of publications on mullein as a biosorbent and not a single one on the removal of Pb(II) ions with mullein. Shah et al. [19] experimentally demonstrate that mullein is a good biosorbent for the biosorption of Cu(II) and Ni(II) ions from synthetic solutions. Our previous work [18] exhibited multilayer adsorption of Cu(II) ions and a maximum removal of 84.51% from synthetic wastewater samples. The aim of this study is therefore to investigate mullein leaves as a potential biosorbent capable of adsorbing Pb(II) ions from test samples. For this reason, biosorption experiments were carried out and the influence of different parameters (contact time, biosorbent dosage, initial Pb(II) ion concentration, and solution pH), biosorption kinetics, and adsorption isotherm of mullein leaves were investigated. To reduce costs and emphasize the need for sustainable processes, the possibility of regeneration and reuse of mullein was also examined.

2. Materials and Methods

2.1. Preparation of Biosorbent and Sample Solution

The expired mullein leaf tea used in this study was supplied by a local tea supplier in Adonis, Serbia. Before the biosorption tests, the mullein biomass was thoroughly washed and dried. In the next step, the mullein was crushed and sieved to achieve a particle size of less than 2 mm. The prepared biosorbent samples were then stored in sealed paper bags until the biosorption experiments were carried out.
Lead (II) nitrate (Pb(NO3)2) with an analytical purity of 99% (Merck) and distilled water were combined to obtain the sample solution (1 g L−1). The dilution solutions 25 mg L−1, 50 mg L−1, 100 mg L−1, 200 mg L−1, and 400 mg L−1 of Pb(II) ions were then made from the working stock (i.e., the synthetic solutions) used in the biosorption experiments.

2.2. Biosorbent Characterization

The surface of the mullein was characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). FTIR (Bomem Hartmann & Braun MB-series, Quebec, QC, Canada; wavelength range 400–4000 cm−1) was used to determine the functional groups on the mullein surface as possible biosorption sites for Pb(II) ions. SEM-EDS (Tescan VEGA 3 LMU, Brno, Czech Republic) was used to record the morphology of the mullein surface before and after the biosorption experiment.
The point zero charge (pHPZC) of the mullein was carried out using the salt addition technique [20]. Measurements of pHPZC were determined using a pH meter (pH Meter 700 EUTECH instruments, Thermo Fisher Scientific, Waltham, MA, USA). The pH values of the NaNO3 solutions were fitted to obtain a pH range of 3, 4, 5, 6, and 7 by adding a certain solution (0.02 M HNO3 or KOH). The mullein sample of 20 g L−1 was placed in a glass beaker containing 50 mL of 0.1 M NaNO3. Samples with adjusted pH were mixed with a magnetic stirrer (Rotamik SHP-10, Tehtnica, Zelezniki, Slovenia) at 200 rpm for 24 h. After the precipitation of mullein, the pH values of aqueous solutions in each beaker were determined. Each set of experiments was performed in triplicate.

2.3. Biosorption Experiment

This study examined the influence of contact time (10–120 min), biosorbent dosage (2–20 g L−1), solution pH (3–7), and initial Pb(II) concentration (25–400 mg L−1) on the biosorption process. All experiments were performed with 50 mL working solutions at room temperature. The pH value of the working solution was corrected with 0.02 M HNO3 or KOH, which was checked with a pH meter. The biosorption samples were homogenized with a magnetic stirrer at a stirring speed of 600 rpm for a certain time. After biosorption, the solutions were filtered using a Buchner funnel and were stored for subsequent analysis in a volumetric flask. An optical emission spectrometer with inductively coupled plasma (ICP-OES Optima 8300; Perkin Elmer, Waltham, MA, USA) was used to evaluate the concentration of Pb(II) ions in the synthetic solutions before and in the working samples after biosorption. Explanations related to the adsorption capacity (qe, mg g−1) and biosorption efficiency (E, %) of the investigated biosorbent and its equations are provided in our previous work [18].

2.4. Desorption Experiments

Desorption tests were performed with a goal to examine the reusability of mullein as a biosorbent. Mullein was shaken in 50 mL 0.1 M HCl with a magnetic stirrer at 600 rpm for 90 min. Subsequently, the solution was filtered, and the filtrates were collected in volumetric flasks for further analysis using ICP-OES. The biomass was rinsed and dried at 80 °C. The dried mullein was reused in the adsorption process. The adsorption–desorption cycles of mullein were carried out five times under optimal experimental conditions. The desorption efficiency (ED, %) was calculated following Equation (1) [21]:
E D % = concentration of metal ion desorbed concentration of metal ion adsorbed × 100
After use, the mullein waste is collected in one place and annealed at a temperature of 550 °C. The ash obtained in this way can be used as fertilizer for agricultural land, which underlines the importance of recycling the biosorbent used and thus contributes to environmental protection and sustainable development.

3. Results and Discussion

3.1. Characterization of the Mullein

3.1.1. FTIR Analysis

FTIR spectra of the surface properties of unused and original mullein leaves and mullein used for biosorption of Pb(II) ions from the synthetic solutions were determined (Figure 1a,b). In the spectrum for mullein (Figure 1a), a broad intense peak with an absorption maximum at 3422 cm−1 is associated with the O-H stretching vibrations of the hydroxyl groups, which originate from the carboxylic acids [22]. The weak adsorption peaks at 2925 and 2857 cm−1 indicate aliphatic C–H bonds with the asymmetric vibration. The peaks at 1736 and 1638 cm−1 are attributed to the C=O functional group in carboxylic acids, aldehydes, and ketones present in the structure of the plant material. The valence vibration of the C-O bond showed a peak at 1050 cm−1.
After biosorption of Pb(II) ions, the spectrum of the mullein samples revealed a slight shift of the peak C=O groups at 1740 cm−1 (Figure 1b). The slight shift in the position of the C=O can be explained by the possible complexation of carbonyl groups due to the binding of Pb(II) ions to functional groups on the mullein surface [15]. Therefore, the oxygen-containing functional groups on the mullein surface are the main players in the biosorption of Pb(II) ions.

3.1.2. SEM and EDS Analysis

The morphology of the surface was examined with SEM before soaking the mullein and after biosorption, which is shown in Figure 2a,b with the corresponding EDS spectrum (Figure 2c,d). The SEM micrographs of pristine mullein showed various morphological structures characterized by different surface areas and crevices (Figure 2a). These structures favored the passage of the Pb(II) ions containing solutions and Pb(II) ions adsorption onto the mullein surfaces. The SEM micrographs after biosorption (Figure 2b), show no obvious changes in the morphology of the mullein surface structures, except for the appearance of white particles, possibly due to the presence of Pb(II)-based compounds formed on the mullein surface [14,21]. The EDS spectra of mullein samples before and mullein samples after biosorption (Figure 2c,d) contain peaks of O and K, while peaks of O, Ca, and Pb are visible after biosorption (Figure 2d). The elements K and O are normally present on the surface of the mullein, while Ca, S, and Cl were detected after biosorption, most likely due to the diffusion of these elements through the pores to the surface of the biomass under the influence of the pH of the synthetic solutions during biosorption. The detected peaks of O, K, Ca, S, and Cl could therefore originate from the cellulosic material of the mullein itself, which is built up by the plants’ mechanisms to accumulate macro- and micronutrients for successful plant growth and reproduction. Based on the FTIR spectra and SEM/EDS analysis, the mullein showed the potential to remove Pb(II) ions, which was further analyzed through experiments to determine the biosorption capacity and efficiency.

3.2. Influence of the pH Value on Biosorption

The effect of pH on the removal efficiency of Pb(II) ions was tested in the pH range of 3–7, while other experimental parameters were initial Pb(II) concentration of 100 mg L−1, the biosorbent dosage of 20 g L−1, and the contact time of 90 min. The results of the pH influence on biosorption are shown in Figure 3.
It is noted that the biosorption efficiency of mullein was high at the lowest pH tested (~95%), followed by a further increase in efficiency with pH. The mullein showed a maximum removal efficiency of 98.56% at pH = 6.0. This result indicates a high potential of mullein for the removal of Pb(II) in the experimental pH range. Within the investigated pH range, the efficiency of removing Pb(II) ions was higher than 90%. At pH 7.0, the removal efficiency significantly decreased due to the onset of the formation of Pb insoluble hydroxides thus lowering the availability of Pb(II) ions for biosorption on the active sites of mullein [23]. When the pH value increases, the metal binding sites are deprotonated, which enhances the number of the negatively charged ions on the surface of the biosorbent and thus improves the adsorption of toxic elements [23,24]. Several studies have reported a decrease in sorption capacity at lower pH values, which is related to the protonation/hydroxylation of surface charges that depend on the pH of the solution [25,26]. The positively charged surfaces of the biosorbent and the alternating displacement of hydrogen ions and metal ions may result in electrostatic repulsion and reduced sorption capacity at pH < 5.0 [27].
In addition, the impact of pH on Pb(II) removal can also be clarified by the point of zero charge (pHPZC) of the biosorbent [28,29]. If the pH value is below the pHPZC, the surface of the biosorbent may be positively charged, which leads to electrostatic repulsion of Pb(II) ions [30]. Conversely, the surface of the biosorbent is negatively charged at a pH value above the pHPZC, which can lead to a stronger attraction between the surface of the biosorbent and the Pb(II) ions [30]. The pHPZC value for mullein was determined to be 5.35, indicating a negatively charged mullein surface that has the potential to attract lead cations at pH values above the pHPZC value. These results are consistent with the excellent efficiency and capacity of mullein in the pH range of 3–6 (Figure 3) and several studies showing good biosorption of most metals by various biosorbents at pH = 4–6 [21,31].
According to Wang [32] and our FTIR analysis of mullein, deprotonation of the carboxyl group at higher pH values results in an increase in negatively charged points where positive Pb(II) ions can be adsorbed. Based on this result, pH 6.0 was chosen as the optimum condition for the biosorption of Pb(II) ions in the subsequent analysis.

3.3. The Influence of Biosorbent Dosage on the Biosorption of Pb(II) Ions

To examine the impact of mullein dosage on the biosorption of Pb(II) ions, the biosorbent was added in a dosage range from 2 g L−1 to 20 g L−1 while the other parameters remained the same: pH = 6.0, initial Pb(II) concentration of the working solution was 100 mg L−1, and contact time was 90 min. The results, depicted in Figure 4, indicate that the efficiency of biosorption increases with increasing dosage of the biosorbent. The highest efficiency of 95.03% was observed at 20 g L−1 of mullein. Nevertheless, the adsorption capacity of mullein declined when the dosage of the biosorbent was increased (Figure 4). Similar findings were observed in the paper by Wang [33]. An increase in biosorbent dosage could lead to an increase in the availability of active sites and improve biosorption efficiency, while the occurrence of aggregation of the biosorbent at high dosages could be the result of a decrease in adsorption capacity [18]. Having this in mind, 20 g L−1 of mullein was the optimum value for the following experiments.

3.4. The Influence of Contact Time

The contact time plays a role in the kinetics of biosorption and, therefore, the biosorbent efficiency. At the beginning of biosorption, the bound metal ions show high binding to the available area with functional groups on the surface of the biosorbent. Different biosorbents require different times to achieve maximum biosorption and optimal contact times, depending on the morphological structure of the biosorbent used, the metal ion, and the way they are combined [1]. Accordingly, the impact of contact time was observed for 10, 40, 60, 90, and 120 min at room temperature, an initial Pb(II) concentration of 100 mg L−1, a pH of 6, and a biosorbent dosage of 20 g L−1. The analysis of the results introduced in Figure 5 shows that a high efficiency (97.6%) is achieved in the first 10 min in relation to the available area with functional groups on the mullein surface [34], which then increases slightly up to 90 min of contact time (98.3%). A further extension of the contact time (t > 90 min) leads to a deterioration in the degree of sorption of Pb(II) ions. The adsorption equilibrium was achieved after approximately 90 min. The obtained results indicate that mullein, as a biosorbent, has a good affinity towards lead ions [35]. Furthermore, Kumar [36] noted that the enhancement in removal efficiency with extended contact times may be related to a reduction in the boundary layer around the sorbent particles.

3.5. Influence of Initial Pb(II) Concentration

As can be seen from Figure 6, the initial concentration of Pb(II) ions also influences the capacity of the mullein and the biosorption efficiency. The concentration of the working solutions was varied in the range from 25 mg L−1 to 400 mg L−1, with the other factors remaining constant: contact time of 90 min, pH = 6, and biosorbent dosage of 20 g L−1. The sorption capacity of mullein increased with the increase in Pb(II) concentration while the biosorption efficiency decreased. When the initial Pb(II) ions varied in the range of 25 mg L−1 to 100 mg L−1, an increase in the sorption percentage (97.23–98.29%) of these ions was observed. However, a further increase in Pb(II) ions led to a decrease in biosorption efficiency, although the sorption capacity increased continuously. The greater uptake of Pb(II) ions at lower concentrations could be due to the greater number of functional groups available on the surface [37]. Conversely, the lower sorption of Pb(II) at higher concentrations could be due to a scarcity of accessible functional groups on the surface of the mullein [38]. Similar observations have been made by other research groups on the uptake of Pb(II) ions with various materials as biosorbents [39].

3.6. Biosorption Kinetics

The determination of the potential rate-controlling mechanism was evaluated using three kinetic models: the pseudo-first-order kinetic model, the pseudo-second-order kinetic model, and the intraparticle diffusion model. The equations describing these three kinetic models can be found in our previous work [18]. The pseudo-first model (Figure S1) indicates physical adsorption which is one of the rate-controlling mechanisms. The pseudo-second model shows the control of the mechanism by chemical adsorption, and the intraparticle diffusion model represents the diffusion mechanism [40]. The data of the kinetic study were obtained with observed experimental parameters: an initial concentration of Pb(II) of 100 mg L−1, pH 6, a biosorbent dosage of 20 g L−1, and contact times of 10 to 120 min. The sorption of Pb(II) ions from the synthetic sample is provided in Figure S2 using the linear forms of the aforementioned kinetic models [18]. Table 1 presents a summary of the corresponding parameters.
A comparison of the regression coefficients (R2) was summarized in Table 1 and shows that the results for the pseudo second-order model are close to 1. This indicates that the processes of biosorption of Pb(II) ions proceed according to the pseudo-second-order kinetic model. The pseudo-second-order kinetic model is applied to fit some biosorption processes that take a long time to occupy biosorption sites. For this reason, the biosorption rate depends on the interaction of the biosorption sites with the adsorbent, which takes place during the entire biosorption process [41]. Moreover, the experimentally determined (qexp) and calculated (qcal) adsorption capacities exhibit related values, providing further confirmation of the viability of the kinetic model. Conversely, the pseudo-first order model shows no approximation between the experimental values (qe) and the calculated values (qe). Also, the regression coefficient of the pseudo-first order model is very low, suggesting that biosorption does not occur at a single site for each ion [21]. As depicted in Figure S3, the linearized curve does not intersect the origin, so the dominant rate-controlling step does not follow the diffusion model [42]. Therefore, it is likely that the biosorption process involves a chemical reaction [43]. These results indicate that the overall mechanism controlling the biosorption rate was controlled by chemisorption, whereby valence forces between the binding sites of the mullein and the metal ions are generated by electron exchange or sharing [44]. The comparison of the calculated kinetic rate constant (k2) of Pb(II) and the kinetic rate constant (k2) of Cu(II) from previous studies [18] indicated that the biosorption of Pb(II) ions is faster than the sorption of Cu(II) ions. Similar results were found by [40].

3.7. Adsorption Isotherm Study

Four non-linear models were tested in this analysis, including Temkin, Langmuir, Redlich–Peterson, and Freundlich, and the factors obtained are shown in Table 2 and Figure 7. The analysis of the data in Table 2 shows that the results of the Freundlich non-linear model have the highest R2 value and lower chi-square (Χ2) value, which emphasizes its superiority and provides a better suitability for describing the sorption of Pb(II) ions on mullein. If the factor n falls within the range of 1 to 10, according to the work [45,46], then biosorption is a favorable process. The factor 1/n is often used to predict the linearity between biosorption and lead ion concentration and is explained as a function of adsorption strength. If the factor 1/n is equal to 1, the adsorption process behaves like a linear isotherm; if the factor 1/n is less than 1, it denotes a chemical process; and if the factor 1/n is greater than 1, it represents a physical process. Analysis of the factors in Table 2 shows that the Freundlich parameter 1/n indicates a favorable biosorption of Pb(II) ions under the optimal conditions [47]. According to the determined value of Kf, mullein leaves have a good adsorption capacity for Pb (II) ions [48]. This indicates a multilayered process taking place on the surface of the mullein, which is heterogeneous according to the SEM analysis [32].

3.8. Comparative Studies

The sorption capacity of mullein was estimated with other unmodified biomass materials listed in Table 3. Comparing the biosorption capacity values, mullein showed better performance in removing Pb(II) ions than several recently reported biosorbents. Therefore, mullein leaves can be regarded as a suitable, low-cost biosorbent for lead ions from synthetic samples.

3.9. Reusability of Mullein

To investigate the possibility of reusing mullein, desorption was carried out under optimal parameters using an HCl solution (0.1 M) as the desorbent. According to the results provided in Figure 8, the desorption efficiency and the percentage of Pb(II) ion uptake were highest in the first cycle. The biosorption efficiency shows a slight decrease, which can be explained by the change in sorption sites available on the mullein surface and the reduction in the biosorbent dosage [54]. The high percentage desorption performance when using 0.1 M HCl solution could also be explained by the protonation of the mullein surface, which contributes to the desorption of lead cations [55]. Considering the modest decrease in the sorption of lead ions, one dose of mullein can be used effectively many times. Additionally, since the desorption process was successful using a strong acid, it can be concluded that the mechanism of Pb(II) adsorption on the mullein surface involves chemical bonding, which is in agreement with the proposed biosorption isotherm model and kinetic model [33].

4. Conclusions

In this work, mullein showed good ability for the biosorption of Pb(II) ions from synthetic samples. The optimal parameters for the Pb(II) maximum biosorption efficiency were the following: contact time of 90 min, pH 6, biosorbent dosage of 20 g L−1, and initial Pb(II) ion concentration of 100 mg L−1 at a constant temperature. The surface morphology of mullein displayed a porous structure with macro and micropores which favors the adsorption. After biosorption, the SEM micrographs and the EDS spectrum displayed the particles of Pb(II) ions in the pores and reduced porosity. FTIR analysis revealed C=O bonds on the mullein surface, which may be important for the uptake of Pb(II) ions from synthetic samples. The maximum mullein biosorption capacity of 4.92 mg g−1 was determined at a pH value of 6.0. The experimental data agree with the Freundlich adsorption isotherm, which proves to be more precise in the interpretation of Pb(II) biosorption. The biosorption of Pb(II) ions on the biomass of mullein leaves is consistent with the pseudo-second-order kinetic model, suggesting that the overall mechanism controlling the biosorption rate is through chemisorption. Overall, the biosorption of Pb(II) ions is a multilayered process that takes place on the mullein surface, which is a heterogeneous biosorbent. In addition, mullein could be effectively reused for five cycles, with over 90% regeneration. The results of this study show that mullein leaves are a green and economical biosorbent with a very good potential to remove Pb(II) from synthetic samples. In the future, the biosorption capacity and efficacy of modified mullein could be examined.
Despite the considerable adsorption potential of mullein, the presence of biologically active compounds that could affect water quality must be investigated in the future.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su16145982/s1, Figure S1: The pseudo-first order kinetic model for the biosorption of Pb(II) ions; Figure S2: The pseudo-second order kinetic model for the biosorption of Pb(II) ions; Figure S3: Intraparticle diffusion kinetic model for the biosorption of Pb(II) ions.

Author Contributions

Ž.T.: Conceptualization; Methodology; Software; Data curation; Formal analysis; Writing—original draft. M.N.: Investigation; Conceptualization; Methodology; Data curation; Writing—review and editing. I.S.G.: Methodology; Investigation; Data curation. D.M.: Investigation; Conceptualization; Methodology; Data curation; Validation. S.M.: Project administration; Supervision; Validation. All authors have read and agreed to the published version of the manuscript.

Funding

The research presented in this paper was financially supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia, within the funding of the scientific research work at the University of Belgrade, Technical Faculty in Bor, and University of Niš, Faculty of Technology in Leskovac, according to the contract with registration number 451-03-65/2024-03/200131 and 451-03-65/2024-03/200133.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are true and reliable. Available on the following link: https://repozitorijum.tfbor.bg.ac.rs/handle/123456789/5835.

Conflicts of Interest

We wish to confirm that there are no known conflicts of interest associated with this publication. All authors of this manuscript declare they have no financial interests.

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Figure 1. FTIR spectra: (a) mullein samples before and (b) mullein samples after biosorption of Pb(II) ions.
Figure 1. FTIR spectra: (a) mullein samples before and (b) mullein samples after biosorption of Pb(II) ions.
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Figure 2. SEM/EDS micrograph of mullein: (a,c) before and (b,d) after Pb(II) biosorption.
Figure 2. SEM/EDS micrograph of mullein: (a,c) before and (b,d) after Pb(II) biosorption.
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Figure 3. Biosorption efficiency and capacity of Pb(II) ions on the mullein in solution with varied pH value (3–7).
Figure 3. Biosorption efficiency and capacity of Pb(II) ions on the mullein in solution with varied pH value (3–7).
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Figure 4. Biosorption efficiency and capacity of Pb(II) ions as a function of biosorbent dosage (2 g L−1–20 g L−1) of mullein.
Figure 4. Biosorption efficiency and capacity of Pb(II) ions as a function of biosorbent dosage (2 g L−1–20 g L−1) of mullein.
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Figure 5. Biosorption efficiency and capacity of Pb(II) ions on mullein by contact time (10–120 min).
Figure 5. Biosorption efficiency and capacity of Pb(II) ions on mullein by contact time (10–120 min).
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Figure 6. Biosorption efficiency and capacity of Pb(II) ions on mullein as a function of the initial Pb(II) concentration (25–400 mg L−1).
Figure 6. Biosorption efficiency and capacity of Pb(II) ions on mullein as a function of the initial Pb(II) concentration (25–400 mg L−1).
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Figure 7. Non-linear models of adsorption isotherms.
Figure 7. Non-linear models of adsorption isotherms.
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Figure 8. The biosorption efficiency and desorption efficiency of Pb(II) ions from mullein.
Figure 8. The biosorption efficiency and desorption efficiency of Pb(II) ions from mullein.
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Table 1. Calculated factors for the removal of Pb(II) ions with mullein.
Table 1. Calculated factors for the removal of Pb(II) ions with mullein.
Kinetic ModelFactorsqexp (mg g−1)
Pseudo-first orderqcal (mg g−1)
0.245		k1 (min−1)
0.0033		R2
0.46071		4.92
Pseudo-second orderqcal (mg g−1)
4.79		k2 (g mg−1·min−1)
2.0		R2
0.9998
Intraparticle diffusionKi (mg·min0.5 g−1)
−0.0073		C
4.8812		R2
0.1616
Table 2. Equations of tested non-linear isotherm models and corresponding calculated factors for the biosorption of Pb(II) ions on mullein.
Table 2. Equations of tested non-linear isotherm models and corresponding calculated factors for the biosorption of Pb(II) ions on mullein.
Isotherm ModelEquationFactorsValues
Freundlich non-linear model q e = K f · C e 1 / n Kf (mg g−1)·(L mg−1)1/n2.44
1/n 0.58
n1.7
R20.9985
Χ21.053
Langmuir non-linear model q e = q m · K L · C e 1 + K L · C e qm (mg g−1)28.57
KL (L g−1)0.05374
R20.9405
Χ22.910
Temkin non-linear model q e = B l n ( K T C e ) KT (L g−1)1.70
B (J mol−1)4.13
R20.9144
Χ24.186
Redlich–Peterson non-linear model q e = K R · C e 1 + a R · C e g KR (L g−1)0.216
a R (L mg−1)g2.37
g 0.46
R20.9702
Χ21.456
qe represents sorption capacity at equilibrium (mg g−1), Kf defines the Freundlich equilibrium constant ((mg g−1)·(L mg−1)1/n), Ce is the equilibrium concentration of Pb(II) ions in the liquid phase (mg L−1), n represents adsorption intensity; qm is the maximum biosorption capacity (mg g−1), KL denotes the Langmuir equilibrium constant (L g−1); B = RT/b—Temkin constant (J mol−1); KT is the Temkin equilibrium constant (L g−1); KR denotes the Redlich–Peterson constant (L g−1); a R is constant (L mg−1)g; and g defines an exponent in the range from 0 to 1.
Table 3. Comparative estimation of Pb(II) adsorption capacity with unmodified biomass materials.
Table 3. Comparative estimation of Pb(II) adsorption capacity with unmodified biomass materials.
BiosorbentAdsorption Capacity, qe (mg g−1)Reference
Broadleaf Cattail1.76[49]
Water Hyacinth2.90[49]
Streptomyces sp.4.56[50]
Sugarcane bagasse0.26[51]
Lantana camara3.5[52]
Java plum seed3.95[53]
Jackfruit4.93[53]
Verbascum thapsus4.92This work
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Tasić, Ž.; Nujkić, M.; Savić Gajić, I.; Medić, D.; Milić, S. Sustainable Processes of Biosorption of Pb(II) Ions from Synthetic Wastewater Using Waste Biomass from Mullein Leaves. Sustainability 2024, 16, 5982. https://doi.org/10.3390/su16145982

AMA Style

Tasić Ž, Nujkić M, Savić Gajić I, Medić D, Milić S. Sustainable Processes of Biosorption of Pb(II) Ions from Synthetic Wastewater Using Waste Biomass from Mullein Leaves. Sustainability. 2024; 16(14):5982. https://doi.org/10.3390/su16145982

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Tasić, Žaklina, Maja Nujkić, Ivana Savić Gajić, Dragana Medić, and Snežana Milić. 2024. "Sustainable Processes of Biosorption of Pb(II) Ions from Synthetic Wastewater Using Waste Biomass from Mullein Leaves" Sustainability 16, no. 14: 5982. https://doi.org/10.3390/su16145982

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