Lead Ion Adsorption on Glutathione-Modified Carbon

Abstract

This study explores the adsorption of Pb(II) from aqueous solutions using glutathione-modified carbon powder at room temperature. The graphite powder was modified through oxidation followed by surface immobilization of glutathione. The Pb(II) concentration was measured using square wave anodic stripping voltammetry (SWASV). Experiments were conducted with the following varying initial Pb(II) ion concentrations: 20.72 mg L⁻¹, 41.44 mg L⁻¹, 62.16 mg L⁻¹, 82.88 mg L⁻¹, 103.60 mg L⁻¹, and 124.32 mg L⁻¹. The effect of varying the glutathione-modified carbon powder dosage (12.5 mg, 25.0 mg, 50.0 mg, 75.0 mg, and 100.0 mg) on Pb(II) uptake was studied. The adsorption data were modeled using the Freundlich isotherm, resulting in a regression coefficient (R²) of 0.96, which signifies a good fit. The Freundlich constants obtained were K_F = 3.54 × 10⁻⁵ (adsorption capacity) and n = 1.56 (adsorption intensity). At optimal conditions (10.0 mL of 20.72 mg L⁻¹ Pb(II) solution with 100.0 mg of glutathione-modified carbon powder), the adsorption efficiency was 96.3%. The glutathione-modified carbon powder exhibits a high capacity for adsorbing Pb(II) from aqueous solutions.

1. Introduction

Contamination by heavy metals is a major environmental issue with serious implications for human health and ecosystems [1,2,3]. The presence of heavy metals in wastewater and industrial effluents is a significant concern for both environmentalists and society as a whole [4,5,6]. Lead, a major industrial pollutant, enters the ecosystem through soil, air, and water [7,8,9]. Major sources of lead resulting from human activities include lead mining, smelting, the use of leaded gasoline, lead-based paints, and lead-containing water pipes [10,11,12,13,14,15]. Elevated levels of lead(II) ions (Pb²⁺) are highly toxic, leading to disabilities in children and contributing to a large amount of annual deaths in developing countries [16,17]. Lead accumulates primarily in bones, the brain, kidneys, and muscles, causing severe health issues such as anemia, kidney disease, and nervous disorders [18,19,20,21,22]. Consequently, the safe and effective removal of lead is essential to prevent its detrimental effects on the environment and public health.

A range of materials is used to remove heavy metals from aqueous systems, including modified substances, agricultural waste, industrial by-products, and natural materials [23,24,25,26]. Modified carbon materials are employed for heavy metal uptake, alongside their traditional applications in analysis, catalysis, and biological processes [27,28,29,30]. Carbon surface modification is achieved through three main approaches: covalent bonding via electrochemical oxidation of amines or electrochemical reduction of diazonium salts, physical adsorption, and the application of carbon paste electrodes [31,32,33]. Carbon surfaces are functionalized with organic groups including phenolic, carboxylic, carbonyl, quinoyl, and lactonyl groups. Oxidation of carbon surfaces introduces additional acidic groups [34,35,36]. Modification of graphite powder with glutathione is achieved through amide bonding between the acid chloride group (-COCl) from oxidized graphite powder and the amine group (-NH₂) in glutathione [37,38]. Lead is a significant industrial pollutant that can enter the ecosystem through soil, air, and water [11]. Key sources of human exposure to lead include leaded gasoline, industrial processes such as lead mining, smelting, and coal combustion, as well as lead-based paint and lead-containing pipes in water supply systems. The extraction of metals from wastewater is primarily achieved through various processes, including adsorption, precipitation, electroplating, chemical coagulation, ion exchange, membrane separation, and electrokinetics [39,40,41,42].

The glutathione-modified graphite powder can significantly enhance lead ion adsorption from aqueous solutions compared to non-modified graphite due to the increased availability of functional groups (-COOH, -SH, and -NH₂) that facilitate the interaction and binding of Pb²⁺ ions. This study demonstrates this enhancement through improved adsorption efficiency as measured by electrochemical techniques and X-ray photoelectron spectroscopy (XPS).

2. Experimental Procedures

2.1. Reagents and Equipment

Lead nitrate (Aldrich, St. Louis, MO, USA) was employed to create a stock solution of lead ions (5 mM). Synthetic graphite powder (Aldrich), with particle sizes smaller than 20 µm in diameter, was used directly in the experiments. pH measurements were taken using a PHS-W series microprocessor pH/mV meter, while electrochemical measurements were carried out with a computer-controlled Metrohm Autolab potentiostat (PGSTAT 302N, Metrohm AG Ionenstrasse CH-9100 Herisau, Switzerland). The three-electrode setup in the Autolab included a glassy carbon electrode as the working electrode, a saturated silver–silver chloride electrode as the reference electrode, and a platinum electrode as the counter electrode. Stirring was carried out using a magnetic stirrer (IKA C-MAG HS7). X-ray photoelectron spectroscopy (XPS) was conducted at the Indian Institute of Technology Madras using an Omicron ESCA Probe spectrometer with Al Kα radiation (hν = 1486.7 eV).

The processed spectra were visualized using the software’s plotting tools, which allowed for the generation of high-quality graphs and plots that were used for the presentation of results. The resulting spectra obtained from the experiments were analyzed using OriginPro 8 (OriginLab Corporation, Northampton, MA, USA). OriginPro 8 is a valuable tool for its robust data analysis capabilities and is particularly well suited for processing and interpreting spectral data.

2.2. Preparation of L-Glutathione-Modified Carbon

Covalent modification of graphite powder using L-glutathione was carried out in two steps:

1. Oxidation of graphite powder: This step involves treating the graphite powder to introduce acidic functional groups onto its surface (see Figure 1).

2. Modification of oxidized graphite powder with L-glutathione: In this step, L-glutathione is covalently bonded to the oxidized graphite powder, typically through an amide bond formation.

2.2.1. Oxidation of Graphite Powder

The oxidation of graphite particles (<20 µm in diameter) to introduce carboxylic groups onto the surface was carried out as follows:

One gram of graphite powder was stirred with 20 mL of an acid mixture, consisting of concentrated nitric acid and sulfuric acid in a 3:1 ratio [15 M HNO₃ and 18 M H₂SO₄] for 12 h [10]. After oxidation, the graphite powder was rinsed with distilled water until the washings were neutral. The oxidized graphite powder was then dried under suction and stored in a desiccator.

2.2.2. Modification of Oxidized Graphite Powder Using L-Glutathione

In our experiment, 2 grams of oxidized graphite powder was treated with 15 mL of thionyl chloride with gentle stirring for 30 min. The reaction mixture was subsequently transferred to a rotary evaporator to remove the excess thionyl chloride, and the resulting product was then dried thoroughly.

A total of 25 mL of dry oxane and 0.85 g (2.77 mmol) of L-glutathione were added to the well-dried oxidized graphite. Next, 1.7 mL (10 mmol) of dry ethyl diisopropylamine was introduced into the mixture. The reaction was stirred under nitrogen for 18 h. Following the reaction, the mixture was filtered and the solid was washed with dry oxane and deionized water. The resulting product was then dried thoroughly under a vacuum to obtain glutathione-modified carbon powder. Figure 1 illustrates the schematic diagram for the preparation of glutathione-modified carbon powder. The experimental procedure discussed in this section has been documented in a previous study [43].

2.3. Determination of Pb²⁺ Ions Using SWASV

The standard addition method was used to establish the linear range for Pb²⁺ detection utilizing a pH 4.5 buffer solution prepared by combining 0.5 M sodium acetate with 0.5 M acetic acid. The electrochemical cell was filled with 25.0 mL of this buffer solution. A glassy carbon electrode was maintained at a deposition potential of −1.1 V versus a saturated silver–silver chloride reference electrode for 120 s while stirring, followed by a 10 s equilibration period. Before each voltammetric measurement, the potential was swept from −1.1 to −0.3 V at a scan rate of 0.1 V/s. A conditioning potential of +0.8 V was applied for 180 s to remove any residual deposits from previous experiments.

SWASV yielded a linear detection range of 1–14 µM for Pb²⁺. Figure 2a shows the current versus potential graph for square wave anodic stripping voltammetry (SWASV) with standard additions of Pb²⁺ on the glassy carbon electrode in a 25.0 mL acetate buffer solution (pH 4.5). This was measured after a 120 s pre-concentration period at −1.1 V and a 10 s equilibrium time. Figure 2b shows the standard addition plot of the peak area (from Figure 2a) versus the concentration of added Pb²⁺ (ranging from 1 to 14 µM).

2.4. Optimal Concentration for the Adsorption of Pb²⁺ Ions

The optimal initial concentration was determined by stirring a fixed amount of oxidized graphite (100.0 mg) with 10.0 mL of Pb²⁺ solutions at concentrations of 50 µM, 100 µM, 200 µM, 300 µM, 400 µM, and 500 µM for 30 min, followed by an additional period of 6 h. The mixtures were then filtered by suction, and the remaining lead ions in the filtrates were quantified using the standard addition method.

2.5. Optimal Dosage of Oxidized Graphite for the Adsorption of Pb²⁺ Ions

The optimal dosage of oxidized graphite was determined by stirring 10.0 mL of a 100 µM Pb²⁺ solution with different amounts of oxidized graphite (100.0 mg, 200.0 mg, 300.0 mg, and 400.0 mg) for 30 min. The mixtures were then allowed to equilibrate for 6 h. Following equilibration, the mixtures were filtered by suction, and the remaining Pb²⁺ ions in the filtrates were measured.

2.6. Calculations of the Adsorption Percentage of Pb²⁺ Ions

The percentage of lead ions adsorbed from the aqueous solution was determined using the following equation [44]:

$$\mathrm{Percentage} \mathrm{of} {\mathrm{Pb}}^{2+} \mathrm{ions} \mathrm{adsorbed}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}}}{{\mathrm{C}}_{\mathrm{i}}}} \times 100\%$$

(1)

where ${\mathrm{C}}_{\mathrm{i}}$ and ${\mathrm{C}}_{\mathrm{f}}$ are the initial and final concentrations of Pb²⁺ ions, respectively.

The amount of lead ions adsorbed from an aqueous Pb²⁺ solution was calculated using the following equation [44]:

$$\mathrm{A}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d} \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{P}{\mathrm{b}}^{2+}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}}}{\mathrm{W}}}\times \mathrm{V} \mathrm{m}\mathrm{o}\mathrm{l} \mathrm{m}{\mathrm{g}}^{-1}$$

(2)

where W (mg) is the amount of oxidized graphite carbon powder and V (mL) is the volume of lead ion solution.

Every experiment has three replicates, and the average of these replicates was used in calculations and graphs.

3. Results and Discussion

3.1. X-ray Photoelectron Spectroscopy (XPS) Analysis

An increased surface area and a higher density of active functional groups improve the availability of adsorption sites on materials [45]. Various adsorbent materials with different attached chemical functional groups have been reported for the adsorption of lead ions [46]. In particular, the presence of acidic functional groups greatly enhances the adsorptive properties of these materials [47]. For example, the adsorption mechanisms of lead ions onto okra waste involve ion exchange or the formation of hydroxyl complexes. [48].

The rate of adsorption is affected by the structure and chemical properties of the adsorbent, as well as by factors such as the coordination number and initial concentration of the metal ions, the pH of the solution, the amount of adsorbent used, and the temperature [49].

Samples of both blank graphite powder and oxidized graphite powder were characterized using XPS to assess the extent of oxidation on the surface of the graphite. This analysis offered insights into the chemical changes and the presence of oxygen-containing functional groups on the oxidized graphite.

Figure 3a presents the wide XPS spectrum of blank graphite powder, showing two prominent peaks at binding energies of 284.6 eV and 532.6 eV. These peaks correspond to photoelectrons emitted from the C 1s and O 1s levels, respectively. Figure 3b displays the wide XPS spectrum of oxidized graphite powder. The observed peaks are at binding energies of 282.9 eV and 532.4 eV, which correspond to photoelectrons from the C 1s and O 1s levels, respectively.

A quantitative analysis of the XPS data for the blank graphite powder showed that the atomic percentage of oxygen (O 1s peak) on the surface was 6.13%, while the atomic percentage of carbon (C 1s peak) was 93.87%. For the oxidized graphite powder, the atomic percentages of the O 1s and C 1s peaks were 15.97% and 84.03%, respectively. In the oxidation process, graphite undergoes chemical modification where oxygen atoms are incorporated into the carbon matrix. This typically results in the formation of various oxygen functional groups such as hydroxyl (-OH), carbonyl (C=O), and carboxyl (-COOH) groups on the graphite surface.

XPS analysis was conducted on the glutathione-modified carbon powder before and after exposure to Pb²⁺ ion solutions (100 µM) for 6.5 h (30 min of stirring followed by 6 h of equilibrium).

Figure 4a shows the wide scan XPS spectrum of the glutathione-modified carbon powder before exposure to Pb²⁺. The spectrum covers the range of 0–1300 eV and primarily shows peaks corresponding to C 1s and O 1s. Figure 4b illustrates the wide scan XPS spectrum of the glutathione-modified carbon powder after exposure to Pb²⁺. This spectrum, also covering the 0–1300 eV range, displays additional peaks beyond the C 1s and O 1s peaks. Notably, a new peak appears in the range of 136–146 eV.

Figure 5a confirms that this peak was not present in the XPS spectrum of the glutathione-modified carbon powder before exposure, indicating its presence due to Pb²⁺ adsorption. Figure 5b provides a detailed scan of the XPS spectrum in the 134–152 eV range, showing two distinct peaks at 139.1 eV and 143.95 eV. These peaks correspond to Pb 4f₇/₂ and Pb 4f₅/₂ binding energies, respectively.

3.2. Impact of Pb²⁺ Ion Concentration on Adsorption

To investigate the effect of Pb²⁺ concentration on its adsorption by glutathione-modified carbon powder, solutions with different Pb²⁺ concentrations were prepared. The tested concentrations were 100 µM, 200 µM, 300 µM, 400 µM, 500 µM, and 600 µM, obtained by diluting a 5 mM stock solution of Pb²⁺ ions. For each concentration, 100.0 mg of glutathione-modified carbon powder was added to 10.0 mL of the corresponding Pb²⁺ solution. The mixtures were stirred for 30 min and allowed to equilibrate for 6 h. After this period, the concentration of Pb²⁺ remaining in the solution was determined using SWASV. Figure 6 shows the average moles of Pb²⁺ adsorbed by 100.0 mg of glutathione-modified carbon powder as a function of the initial Pb²⁺ concentration, which ranges from 100 µM to 600 µM.

The adsorption of Pb²⁺ ions onto glutathione-modified carbon powder was evaluated across a range of initial Pb²⁺ concentrations from 100 µM to 600 µM. The moles of Pb²⁺ adsorbed by 100.0 mg of the modified carbon powder were calculated and found to range from 0.96 × 10⁻⁶ moles to 2.47 × 10⁻⁶ moles as the initial Pb²⁺ concentration increased from 100 µM to 600 µM. Notably, the adsorption capacity did not significantly increase beyond an initial Pb²⁺ concentration of 400 µM. This indicates that the optimal initial concentration of Pb²⁺ for effective adsorption by the glutathione-modified carbon powder is 400 µM.

3.3. Impact of the Amount of Glutathione-Modified Carbon Powder on Adsorption

The adsorption of Pb²⁺ ions was investigated by using different amounts of glutathione-modified carbon powder. Specifically, 12.5 mg, 25.0 mg, 50.0 mg, 75.0 mg, and 100.0 mg of the modified carbon powder were added to separate 10.0 mL samples of a 100 µM Pb²⁺ ion solution. The remaining concentrations of Pb²⁺ in the solution were measured using SWASV. Figure 7 illustrates the average moles of Pb²⁺ adsorbed as a function of the glutathione-modified carbon powder dosage (12.5 mg–100.0 mg).

The adsorption of Pb²⁺ increased as the amount of glutathione-modified carbon powder was raised; however, beyond 75.0 mg, there was no further significant increase in adsorption. Therefore, the optimal dosage of glutathione-modified carbon powder was determined to be 75.0 mg.

3.4. Thermodynamic Assessment

Adsorption isotherms characterize the capacity of an adsorbent and illustrate the equilibrium relationship between the adsorbent and adsorbate. They show the ratio of the amount of adsorbate adsorbed to the quantity remaining in the solution at equilibrium for a specific temperature. The adsorption–isotherm relationship can be described by the Freundlich isotherm model [50].

The Freundlich isotherm is an empirical model used to describe adsorption and is expressed by the following linearized equation:

$$\mathbf{log}{\mathbf{N}}_{\mathbf{a}\mathbf{d}\mathbf{s}}=\mathbf{log}{\mathbf{K}}^{\mathbf{\prime }}+{\displaystyle \frac{1}{\mathbf{n}}} \mathbf{log}{\mathbf{C}}_{\mathbf{b}\mathbf{u}\mathbf{l}\mathbf{k}}$$

(3)

where ${\mathbf{N}}_{\mathbf{a}\mathbf{d}\mathbf{s}}$ represents the number of moles of metal ions adsorbed per milligram of modified carbon powder, ${\mathbf{C}}_{\mathbf{b}\mathbf{u}\mathbf{l}\mathbf{k}}$ is the concentration of metal ions remaining in the solution at equilibrium (in mol/dm³), and ${\mathbf{K}}^{\mathbf{\prime }}$ and n are Freundlich constants that relate to the adsorption capacity and intensity, respectively.

Table 1. The number of moles of lead ions adsorbed per milligram of glutathione-modified carbon powder and the equilibrium concentration were determined from an initial Pb²⁺ ion concentration of 100 µM.

Equilibrium Concentration of Lead(II) Ion in µM [Cbulk]Na Local Hop	Mass of Glutathione-Modified Carbon in mg	Adsorbed Moles of Pb2+ in mol mg−1(×10−8) [Nads]	Log[Cbulk]	Log[Nads]
33.4	12.5	5.33	−4.48	−7.27
20.5	25	3.18	−4.69	−7.49
8.1	50	1.84	−5.09	−7.74
3.7	75	1.28	−5.43	−7.89
33.4	12.5	5.33	−4.48	−7.27

shows the number of moles of lead ions adsorbed per milligram of glutathione-modified carbon powder and the equilibrium concentration from an initial concentration of 100 µM of Pb²⁺. Figure 8 shows the Freundlich plot for the uptake of Pb(II) by glutathione-modified carbon powder.

The Freundlich isotherm was validated for the adsorption of Pb(II) ions onto glutathione-modified carbon powder using 100 µM Pb(II) solutions and the following varying amounts of modified carbon powder: 12.5, 25.0, 50.0, and 75.0 mg. The adsorption data were analyzed using the Freundlich isotherm model, which proved to be a good fit to the experimental results (linear least squares regression coefficient, R² = 0.96). The values of ${\mathbf{K}}^{\mathit{\prime }}$ and n were determined to be 3.54 × 10⁻⁵ and 1.56, respectively. A higher ${\mathbf{K}}^{\mathit{\prime }}$, a higher n value, and a lower n value indicate a greater affinity for the adsorbate, confirming that Pb(II) ions effectively bind to glutathione-modified carbon powder in this study.

3.5. Comparison of Pb²⁺ Ion Adsorption by Various Materials

When comparing the adsorption capacities of various materials for the uptake of lead ions, glutathione-modified carbon demonstrates a significantly higher efficiency than several other commonly used adsorbents. Graphite powder achieved an uptake percentage of 41.18% [51]. Graphite is a widely studied material due to its stability and surface properties, but its unmodified form lacks the necessary functional groups to effectively capture lead ions, resulting in lower adsorption efficiency. Oxidized graphite powder improves on raw graphite, with an uptake percentage of 73.3% [51]. The oxidation process introduces oxygen-containing functional groups, such as carboxyl, hydroxyl, and carbonyl groups, which enhance the affinity of material for lead ions. However, it still falls short compared to more specialized adsorbents. Clay powder has a lead ion uptake percentage of 69% [51]. While clay offers a large surface area and is readily available, its natural adsorption capacity is limited. The surface chemistry of clay does not interact as strongly with lead ions, making it less effective without further modification. Clay-brick powder has a slightly higher uptake percentage of 77.3%. Clay-brick powder is more effective than raw clay, likely due to thermal treatment during brick production, which alters the surface properties and possibly increases the availability of active sites for lead ion binding [51]. Glutathione-modified carbon achieves a remarkable uptake percentage of 96.3%, significantly outperforming the other adsorbents. The modification of carbon with glutathione introduces -SH groups, which have a high affinity for lead ions. This strong chemical interaction results in a much higher adsorption capacity, making glutathione-modified carbon an exceptionally effective material for lead ion removal. The comparison clearly shows that glutathione-modified carbon is far superior in adsorbing lead ions compared to other tested materials. The primary reasons for this are the chemical interactions between the thiol groups in glutathione and lead ions, which are much stronger than the physical adsorption mechanisms predominantly seen in other materials like clay or unmodified graphite.

4. Conclusions

In conclusion, glutathione-modified carbon proved to be an effective adsorbent for Pb²⁺ ions from aqueous solutions. The adsorption efficiency reached 96.3% when 10.0 mL of a 100 µM lead ion solution was treated with 100.0 mg of the modified carbon powder, highlighting its strong affinity for Pb²⁺ ions. Furthermore, the adsorption capacity increased significantly, handling concentrations up to 400 µM with the same amount of adsorbent. The optimal dosage for adsorbing Pb²⁺ from a 100 µM solution (10.0 mL) was determined to be 75.0 mg. These findings suggest that glutathione-modified carbon powder holds substantial promise for the removal of Pb²⁺ from aqueous solutions. The experimental data also aligned well with the Freundlich isotherm model, supporting the practical applicability of this adsorbent in environmental remediation efforts.