Lead Removal from Aqueous Solutions Using Bean Shells – Equilibrium, Kinetics and Thermodynamic Studies

Lead ions removal from aqueous solutions onto bean shells is presented in this paper. The experiments were conducted in a batch system. The bean shells samples were rinsed with distilled water before the adsorption experiments. The analysis of the rinsed water showed that a significant amount of alkali and alkaline-earth metal ions are transferred from the adsorbent structure into the rinsed solution during the rinsing process. The COD analysis showed that these waters should be treated before being discharged into the surrounding watercourses. The influence of different process parameters (the pH value of the solution, the initial metal ions concentration, and the initial mass of the adsorbent) on the adsorption capacity was investigated. The adsorption capacity was higher at higher pH values of the solution. The adsorption capacity showed a decrease with the increase in the mass of the adsorbent. The increase in the initial metal ions concentration was shown to lead to an increase in the adsorption capacity until 0.8 g dm, after which a slight decrease was noted. Characterization of the adsorbent was performed by SEM-EDX, DTA-TGA, and FTIR analysis. The SEM-EDX analysis indicates a change in the morphology of the sample after the adsorption, as well as that K and Mg are possibly exchanged with lead ions during the adsorption process. The results obtained by the DTA-TGA analysis showed a weight loss of 77.8 % in the temperature range from 20C to 900C. The FTIR analysis indicated that the amide group is involved in the adsorption process. The pseudo-second order kinetic model was shown to be the best fit for the analyzed data, which led to the conclusion that chemisorption was a possible way of binding lead ions onto the surface of the bean shells. The Hill isotherm model was the best model for the analyzed adsorption equilibrium data. Obtained thermodynamic data indicated that the adsorption process was spontaneous, endothermic, and disordered, in which lead ions are bound to the surface of the bean shells by chemisorption. The maximum achieved adsorption capacity was 46.36 mg g.


Introduction
Water pollution is one of the biggest environmental issues today, as a result of an increase in the waste generated by the industrial activities, that are often discharged into the surrounding watercourses without prior treatment [1].
The continuous water pollution by heavy metals leads to an increased research activity focused on how to remove these pollutants from the environment. One of the most researched heavy metal, among others, is lead (Pb) [2].
Industrial wastewaters originate from factories and various industrial plants, as a result of clean water being used in the production process. These waters contain many toxic materials [3]. Inadequate treatment of industrial wastewater can lead to pollution of surrounding streams, rivers, lakes, etc. [4].
The effect of pH value on the adsorption capacity was determined by adjusting the pH values of the solutions with 0.1 M HNO3 and 0.1 M KOH solutions. The cation exchange capacity CEC was determined using the 1 M NH4Cl solution. All the chemicals were of p.a. quality.

Adsorbent
Bean shells, used as an adsorbent for the adsorption experiments, were collected on the fields in a village Rudna Glava, located near the town of Majdanpek (Eastern Serbia). Collected bean shells were ground, and sieved through a set of laboratory sieves. The -1 +0.4 fraction was used for further adsorption experiments. Figure 1 shows raw bean shells sample.

Characterization of bean shells
The chemical composition of bean shells The bean shells sample was burned and the ash was used to determine the chemical composition of the sample. Chemical analysis of the obtained ash was performed by ICP-AES and ICP-MS methods on the SPECTRO BLUE and AGILEND 7700 device, respectively.

Determination of COD in the rinsing water
The COD in the rinsing water was determined volumetrically by titrating the rinsing water with a 0.02 M KMnO4 solution. The COD was determined to be 39 mgO2/dm 3 . According to the ecree on limit values of pollutants in surface and groundwater in Serbia (Official Gazette of RS, No. 50/2012) [22], such waters belong to the fourth class of wastewater, which is why they must not be discharged into the surrounding watercourses without prior treatment.

The cation exchange capacity (CEC)
Cation exchange capacity (CEC) is the ability of a material to adsorb cations from water solutions. The CEC between the bean shells and the liquid phase was determined volumetrically, by the ion exchange method, using NH4Cl solution, during which the ammonium ions from the solution were exchanged with the alkali and alkaline earth metal ions from the structure of the bean shells [23].

SEM-EDS analysis
The morphology of the bean shells before and after Pb 2+ ions adsorption was examined by a SEM scanning electron microscope (VEGA 3 LMU TESCAN) with integrated energy-dispersive X-ray detector (X-act SDD 10 mm 2 , Oxford Instruments). Before the SEM analysis, the samples were steamed with thin layer of chromium in the sputter coater (QUORUM Q150T ES) to become conductive. These samples were then transferred into the microscope chamber and observed at a voltage of 8 kV.

DTA-TGA analysis
The DTA-TGA analysis of a bean shells sample was performed on a simultaneous DSC-TGA device (SDT Q600) under an inert nitrogen atmosphere. 3.5 mg of bean shells sample was heated at a constant rate of 10 o C/min at the temperature interval from 20 to 900 o C.

FTIR analysis
The infrared spectra of the analyzed samples was recorded by a Bomem MB-100 (Hartmann & Braun, Canada) FTIR spectrometer, in the range of 4000 -400 cm -1 , at resolution of 2 cm -1 with 16 scans. The samples were compressed in a potassium bromide (KBr) tablets prior to analysis. Obtained FTIR spectra were then analyzed by Win Bomem Easy software.

Adsorption experiments procedure
The adsorption experiments were performed in batch conditions, in laboratory beakers equipped with a magnetic stirrer, in order to keep the adsorbent in the suspension. Adsorption conditions, like temperature, pH value of the solution, initial lead ions concentration, initial mass of the adsorbent, were modified, depending on the experiment. After conducting the experiments, the suspension was filtered, and the filtrate analyzed for lead ion content. All experiments were carried out at ambient temperature, except for thermodynamic studies.
Adsorption capacity was calculated using the following equation [18]: where q(t)-is the adsorbent capacity defined as mass of the adsorbed metal per unit mass of the adsorbent (mg g −1 ) at time t; ci and c(t)-are the initial and actual concentrations of metal ions (g dm −3 ) at time t; V-is the volume of the solution used in the adsorption experiments (dm −3 ); and m-is the mass of the adsorbent (g) [18].

Adsorption kinetics
Kinetic models are used to determine the adsorption rate, mechanism of the adsorption process, its speed, and time needed to reach the equilibrium. Pseudo-first order kinetic model, pseudo-second order kinetic model, Elovich kinetic model, and the interparticle diffusion kinetic model were used in this paper to analyze the obtained experimental data, as the models most often used in literature [24][25][26].

Pseudo-first order kinetic model
Pseudo-first order kinetic model was determined by Lagergren. This model assumes that adsorption is a reversible process [27]. https://doi.org /10.37358/Rev.Chim.1949 Rev. Chim., 72 (4) The pseudo-first order model can be expressed as: where: q(t) -is the adsorbent capacity defined as the mass of the adsorbed metal per unit mass of the adsorbent (mg g -1 ) at time t; qe-is the adsorption capacity defined as mass of the adsorbed metal per unit mass of the adsorbent (mg g -1 ) at equilibrium; and k1 -is the adsorption rate constant for the pseudofirst order kinetic model (min -1 ). By integrating the equation (2), the following equation is obtained: Plotting log(qe-q(t)) vs. t gives a linear dependence, from which the pseudo-first order kinetic parameters can be determined.

Pseudo-second order kinetic model
This model assumes that on the surface of the adsorbent, adsorption and ion exchange takes place, whereby the adsorbate is bound to the adsorbent surface by chemisorption [28].
A nonlinear form of the pseudo-second order kinetic model can be expressed as [29]: where: q(t) -is the adsorbent capacity defined as the mass of the adsorbed metal per unit mass of the adsorbent (mg g -1 ) at time t; qe-is the adsorption capacity defined as mass of the adsorbed metal per unit mass of the adsorbent (mg g -1 ) at equilibrium; and k2-is the adsorption rate constant for the pseudosecond order kinetic model (g mg -1 min -1 ) [29]. Rearranging Eq. (4) to obtain its linear form, provides the following equation [29]: ( ) Plot t/q(t) vs. t is used to determine the pseudo-second order model kinetic parameters.

Intraparticle diffusion (Webber-Morris) kinetic model
The intraparticle diffusion kinetic model assumes that adsorption does not take place only on the surface of the adsorbent, but also includes diffusion and adsorption inside the adsorbent structure [30].
Linear form of this model is given by [31]: where: q(t) -is the adsorption capacity defined as the mass of the adsorbed metal per unit mass of the adsorbent (mg g -1 ) at time t; ki -is the internal particle diffusion rate constant (mg g -1 min -0,5 ); and Ci -is a constant that provides insight into the thickness of the boundary layer. If the Ci value is higher, the boundary layer effect is greater, so the effect of surface adsorption in controlling the process speed is greater (mg g -1 ). Elovich kinetic model Elovich kinetic model was primarily used for investigating gas chemisorption on solid adsorbents processes. However, this model was later successfully applied on adsorption of toxic materials from aqueous solutions [32]. It is often given in the following form: where: αis the starting adsorption rate (mg g -1 min -1 ); βis the parameter that expresses the degree of surface coverage and activation energy for chemisorption (g mg -1 ); and q(t) -is the adsorption capacity defined as the mass of the adsorbed metal per unit mass of the adsorbent (mg g -1 ) at time t.
Rearranging the equation (7) the following expression is obtained: From the graph q(t) = f(lnt) the Elovich kinetic model parameters can be determined.

Adsorption isotherms
Adsorption isotherms provide insight into the mechanism of adsorption of metal ions on a given adsorbent and can be used to determine the maximum adsorption capacity, i.e. the maximum amount of metal that a given adsorbent can adsorb under certain conditions.
In this paper, the non-linear Langmuir, Freundlich, Temkin, Redlich-Peterson and Hill isotherm models were used to describe the Pb 2+ adsorption process onto bean shells.

Langmuir isotherm model
This model was primarily developed to describe the gas-solid adsorption onto activated carbon. It also found its application in describing the possibilities of different adsorbents. This empirical model assumes that the adsorption occurs in a monolayer [33].

Freundlich isotherm model
This model represents the earliest known relationship that describes the non-ideal and reversible adsorption, which is not limited by the formation of a monolayer. It can be applied to multilayer adsorption with non-uniform distribution of the adsorption heat and affinities towards a heterogeneous surface [33]. where Ce -is the equilibrium concentration of metal ions in the solution (mg dm -3 ); qe -is the adsorbent capacity defined as mass of the adsorbed metal per unit mass of the adsorbent (mg g -1 ) at equilibrium; and KF -is the Freundlich equilibrium constant ((mg g -1 ) (dm 3 mg -1 ) 1/n ) [18].

Temkin isotherm model
This model is suitable for analysis of multilayer chemical adsorption based on strong electrostatic interactions between positive and negative charges [34].
This model is represented by the following equation [18]: where: B = RT/b -is the Temkin constant, which refers to the adsorption heat (J mol -1 ); b -is the variation of adsorption energy (J mol -1 ); R -is the universal gas constant (J mol -1 K -1 ); T-is the temperature (K); KT -is the Temkin equilibrium constant (dm 3 g -1 ); qe-is the adsorption capacity (mg g -1 ) at equilibrium; and Ce -is the equilibrium concentration of metal ions in the solution (mg dm -3 ) [18].

Hill isotherm model
This model assumes that adsorption is a cooperative phenomenon, according to which the ability to bind ligands at one site on a macromolecule can affect the appearance of other binding sites at the same macromolecule [35].
Hill isotherm model can be expressed as [35]: where: Ce -is the equilibrium concentration of metal ions in the solution (mg dm -3 ); qe-is the adsorbent capacity (mg g -1 ) at equilibrium; and KD, nH, and qsH are constants [35].

Redlich-Peterson isotherm model
This model represents a hybrid combination of the Langmuir and Freundlich models, by which the three parameters are incorporated into an empirical equation [35].
Redlich-Peterson isotherm model is represented by the following equation [35]: where: Ce -is the equilibrium concentration of metal ions in the solution (mg dm -3 ); qe -is the adsorbent capacity (mg g -1 ) at equilibrium; KR -is the Redlich-Peterson constant (L g -1 ); aR -is a constant (L mg -1 ) g ; and g -is an exponent with values between 0 and 1 [35].

Thermodynamics of the adsorption
Changes in activation energy (Ea), entropy (ΔS), enthalpy (ΔH), and Gibbs free energy (ΔG) can be determined by analyzing the change in equilibrium constants with temperature [35,36].

Chemical composition of bean shells
After determining the moisture and ash content of bean shells which was 7.83% and 7.24%, respectively, the remaining sample was analyzed for the chemical composition, and the results are given in Table 1. As can be seen from Table 1, the K2O content was dominant in comparison to other oxides.

Cation exchange capacity CEC of bean shells
CEC of bean shells was determined by bringing in contact 1 g of bean shells sample with 50 mL of 1 M NH4Cl solution, and stirring the suspension for 24 h. Obtained results are given in Table 2. The total cation exchange capacity was calculated, as a sum of all the cations exchanged in the experiment. From data given in Table 2, the CEC was found to be 1.57 mmol Me z+ g -1 , where Me z+ represented the alkali and alkaline earth metal ions exchanged in the adsorption process. It can also be seen that K + was the dominant ion with the highest cation exchange capacity of 1.15 mmol g -1 . This suggested that the K + ions present in the structure of the bean shells would probably be exchanged with Pb 2+ ions during the adsorption process.

SEM-EDS analysis of bean shells
The SEM-EDS analysis of bean shells was carried out before and after the biosorption of Pb 2+ ions. The SEM micrographs, along with the corresponding EDS spectra, before and after the adsorption are shown in Figure 2. Pores and cavities (honeycomb-like structure) could be spotted on the micrograph of https://doi.org /10.37358/Rev.Chim.1949 Rev. Chim., 72 (4) raw bean shells (Figure 2a), which facilitated the penetration of the aqueous phase into the adsorbent structure, where lead ions were adsorbed at the internal active sites [38,39]. After the adsorption of Pb 2+ ions (Figure 2c), quite a different morphology of the surface of bean shells was observed. The absence of cavities and channels was evident, with compact and uniformly ordered lamellar structure. The changes in the morphology of bean shells after the biosorption of lead ions occurred due to the incorporation and binding of lead ions to active sites in the adsorbent structure [40]. The EDS analysis was performed by recording several points on the surface of the bean shells, before and after the adsorption, to determine the eventual heterogeneous distribution of elements. The EDS analysis was performed in order to determine which chemical elements were present in the sample, as well as to estimate their relative abundance (quantitative analysis). The elements that appeared on the analyzed samples, as well as their content in wt%, are given in Tables attached to Figures 2b and Tables (spectrum 6 and 7) attached to Figures 2b and 2d.

DTA-TGA analysis of bean shells
Thermal analysis provided insight into thermal stability and degradation of a material. The obtained results from the DTA-TGA analysis of bean shells are shown in Figure 3. At TGA curve, a weight loss of 6.68 % in the temperature range from 20 o C to 184 o C was noticed, as a result of the loss of physically bonded (adsorbed) water within the sample. This process was followed by an endothermic peak on the DTA curve with a minimum at 64.86°C. With further heating, in the temperature range from 184 to 900°C, a weight loss of about 71.12 % was observed on the TGA curve, originating from the degradation of the lignocellulose components present in the bean shells, and the formation of volatile products such as CO, CO2, and other [41]. In the same temperature range, on DTA curve an exothermic peak was observed, with a maximum at 244.32ºC, corresponding to the decomposition of hemicellulose [42]. The total weight loss, in the temperature range from 20 o C to 900 o C, was 77.8 %.

Fourier transform infrared spectroscopy (FTIR) analysis of bean shells
The FTIR spectroscopy was performed in order to compare the obtained spectra of before and after the adsorption process, in order to determine which functional groups are responsible for the adsorption of metal ions. Detected adsorption bands in the range from 1300 to 1000 cm -1 correspond to -OH, C-OH, and -O-C-O vibrations of the glucoside cellulose groups. Bands in the range from 1000 to 700 cm -1 correspond to the deformation of CH groups [43][44][45][46].
Bands appearing in the range from 1730 to 1260 cm -1 are typical for hemicellulose, and correspond to -C-OH, CH, CH2 and C=O vibrations. Ones from 1600 cm -1 to 1260 cm -1 originate from C=C and OCH3 vibrations, and are typical for lignin. In the low-frequency area, from 600 to 400 cm -1 , bands from valent -O-M vibrations are expected, pointing to the metal interactions with the adsorbent [47]. Figure 4 shows the FTIR spectra of bean shells rinsed with distilled water (a), and after the adsorption of Pb 2+ ions (b). The adsorption bands could be observed on the given spectra, indicating the presence of cellulose, hemicellulose, and lignin. Bands in the range from 1646 to 1530 cm -1 (marked with arrows) corresponded to C=O functional groups and deformation of NH vibration (Amide I, and Amide II) of (NH)C=O protein groups, respectively [48].
Comparing these results to the FTIR spectra after the adsorption, it can be recorded that these bands disappeared, and in low-frequency areas, a weak band at around 525 to 480 cm -1 appeared, which corresponded to O-Pb valence vibrations [47]. These facts indicated that amide functional groups were involved in the binding of Pb 2+ ions onto bean shells during the adsorption process.

The effect of pH value change on the adsorption capacity
To determine the effect of pH value on the adsorption capacity, a number of experiments was performed, using a synthetic lead ions solution of different pH values, ranging from 2 to 5. The pH of the solutions was adjusted with 0.1 M HNO3 and 0.1 M KOH. The obtained results are presented in Figure 5.
The pH of the solution is shown to have a significant effect on the adsorption capacity ( Figure 5). As the pH of the solution increased, the adsorption capacity increased as well. At pH = 2 the adsorption capacity was 11.13 mg g -1 , while its maximum value of around 20 mg g -1 was achieved at a pH = 3. A further increase in the pH value did not significantly affect the adsorption capacity.
At low pH values, the concentration of H + ions is higher, and they occupy active sites in the structure of bean shells, and suppress the already adsorbed lead ions and reduce the adsorption capacity. At higher pH values, active sites on the surface of bean shells could exchange alkali and alkaline earth metals with lead ions from the solution, resulting in a higher adsorption capacity [49].

The influence of the change in the mass of the adsorbent on the adsorption capacity and the adsorption degree
The effect of mass change of the adsorbent on the adsorption capacity and the adsorption degree of lead ions onto bean shells was also investigated. The synthetic solution of lead ions with an initial concentration of 0.2 g dm -3 was brought into contact with bean shells samples of different initial mass, ranging from 0.1 to 1 g, at a constant stirring rate of 300 min -1 . The obtained results are shown in Figure  6.
As can be seen from Figure 6, the adsorption capacity increased with the increase of the initial mass of the adsorbent, due to the increase of specific surface area with an increase of the number of active sites within the structure of the adsorbent [35]. A sharp increase in the adsorption degree was observed with an increase in the mass of the adsorbent from 0.1 to 0.2 g, after which there was no significant change.
However, the adsorption capacity decreased with increasing the mass of the adsorbent. This occurred due to the fact that with the increase of the mass of the adsorbent over 0.2 g, a larger amount of the adsorbent was brought into contact with the solution than it was necessary to adsorb a given amount of lead ions present in the solution [50]. https://doi.org /10.37358/Rev.Chim.1949 Rev. Chim., 72 (4)

The influence of the initial lead ion concentration on the adsorption capacity
The influence of the initial Pb 2+ ions concentration on the adsorption capacity was determined by bringing into contact 0.5 g of bean shells with a synthetic solution of lead ions of different concentrations, in the range from 0.002 to 1 g dm -3 . The obtained results are shown in Figure 7.
As can be seen from Figure 7, the adsorption capacity increased with the increase of initial concentration of metal ions in solution, reaching a maximum value of 60.12 mg g -1 at the initial concentration of 0.8 g dm -3 of lead ions, after which it started to decrease. It was assumed that, as the initial concentration of lead ions increased, the probability of their contact with active sites in the adsorbent structure increased, indicating a higher adsorption capacity [51]. With a further increase in the initial concentration of lead ions, after 0.8 g dm -3 , a decrease in the adsorption capacity occurred, due to the saturation of the active sites with the lead ions in the molecular structure of the adsorbent.

Kinetic studies
The change in the adsorption capacity of lead ions with time was determined by a number of experiments, in which the solutions of lead ions with an initial concentration of 0.2 g dm -3 were brought into contact with 0.5 g of the sample for different contact time. The adsorption was terminated after 90 min, assuming that this time was long enough to establish the equilibrium in the system [52,47]. The change in the adsorption capacity of lead ions with time is shown in Figure 8a. It can be seen that at the beginning of the process (5-10 min) the adsorption capacity rapidly increased. It is assumed that this rapid increase in the adsorption capacity was a result of a large number of available active sites in the structure of the adsorbent occupied by lead ions [24].
In this paper, a pseudo-first order kinetic model, a pseudo-second order kinetic model, an intraparticle diffusion kinetic model, and an Elovich kinetic model were used to describe the kinetics of the adsorption of lead ions onto bean shells. Experimental data in Figure 8a were linearized using Eqs. (3, 5, 6, and 8).
The corresponding graphs are shown in Figure 8 b-e, allowing the determination of kinetics parameters which are given in Table 3.  The pseudo-second order kinetic model was determined to be the best fit for the analyzed data (R 2 = 1). The high functionality of the analyzed model was also confirmed by a negligible difference between the values of experimentally determined and calculated adsorption capacity qe,exp and qe,cal. Based on this model it can be concluded that the chemical binding of lead ions to the active sites within the structure of the adsorbent was a limiting factor for the adsorption of lead ions onto bean shells [26].

Isotherm studies
In order to obtain data for the adsorption isotherms the following experiment was performed: 0.5 g of the sample was brought into contact with 50 mL of a synthetic solution of lead ions of different initial concentrations (500 mg dm -3 , 300 mg dm -3 , 200 mg dm -3 , 100 mg dm -3 , 50 mg dm -3 , 30 mg dm -3 , 20 mg dm -3 , 10 mg dm -3 , and 5 mg dm -3 ). The suspension was stirred by a magnetic stirrer for 60 min, considering this time long enough to reach the equilibrium between phases [29]. The suspension was then filtered, and the filtrate analyzed on the residual amount of lead ions.
Langmuir, Freundlich, Temkin, Hill, and Redlich-Peterson non-linear isotherm models were used to analyze the experimental data, and the obtained results are shown in Figure 9.
The experimental data were fitted using Langmuir, Freundlich, Temkin, Hill, and Redlich-Peterson non-linear isotherm adsorption models, presented in Figure 9, using Eqs. (9) - (13). Observing the correlation coefficients for each model given in Figure 9, it can be concluded that all tested models satisfactorily described the analyzed data, with exception of Hill's model which showed the best Figure 9. The adsorption isotherm models agreement with the experimental data (R 2 = 0.978). According to this model the adsorption of lead ions onto bean shells was a cooperative phenomenon according to which the ability to bind ligands at one site on a macromolecule could affect the appearance of other binding sites on the same macromolecule [33].

Thermodynamics studies
In order to determine the thermodynamic parameters, 0.5 g of bean shells were brought into contact with 50 mL of a synthetic lead ions solution with a concentration of 200 mg dm -3 at three different temperatures (296.9, 303, and 313 K), and stirred for 90 min.
The graph ln Kd = f (1/T) is shown on Figure 10. Based on the experimental data, and the given dependence, thermodynamic parameters were calculated, and the results are given in Table 4.  It can be seen from Table 4 that ΔG 0 had a negative value at all temperatures, which indicated that the removal of Pb 2+ ions using bean shells was a spontaneous process.
The enthalpy values lower than 8.4 kJ mol -1 indicate that ion exchange is the dominant mechanism of the adsorption of metal ions [52]. The obtained positive value of enthalpy (ΔH 0 = 7.9 kJ mol -1 ) indicated the existence of an energy barrier and endothermicity of the process [53].
The obtained negative value of entropy (ΔS 0 = -19.34 J mol -1 K -1 ) indicated a decrease in the "disorder" in the liquid-solid intermediate region during the adsorption process. It is assumed that this results were a direct consequence of: (i) changes in adsorbent structure, (ii) increased mobility and depth https://doi.org /10. of solution penetration within the adsorbent structure, and (iii) predominance of activation energy and enhanced intraparticle diffusion [37].
The activation energy (Ea) of physical adsorption usually does not exceed 4.2 kJ mol -1 , while chemical adsorption is based on bonds much stronger than physical ones, and its activation energy is usually in the range of 8.4 to 83.7 kJ mol -1 . In this case, the obtained value of activation energy was (Ea = 65.986 kJ mol -1 ), meaning that the adsorption of lead ions onto bean shells occurred by chemisorption [35].

Conclusions
In this work, the characterization of the bean shells biomass was performed, as well as its possible use as an adsorbent for Pb 2+ ions adsorption analyzed.
After drying and burning the sample, it can be concluded that it contained 7.83 % moisture, 7.24 % ash, and 92.76 organic (combustible) matter.
The COD of the water used to rinse the adsorbent was determined to be 39 mgO2 dm -3 , which leads to the conclusion that these waters must not be discharged into the surrounding watercourses without prior treatment.
The CEC analysis showed a total cation exchange capacity 1.57 mmol Me z+ g -1 , with the conclusion that K + is the dominant ion in an exchangeable position.
The SEM-EDS analysis, performed before and after the adsorption process, showed that the surface morphology of the bean shells sample changed after the adsorption process, as well as that K and Mg could be exchanged with lead ions during the adsorption process.
The DTA-TGA analysis showed a weight loss of 6.68 % in the temperature range from 20 to 184 o C, as a result of the loss of physically bound water within the sample. Another weight loss of 71.12 % was noticed in the range from 184 to 900 o C, originating from the degradation of the lignocellulose components present in the bean shells sample, and the formation of volatile products, such as CO and CO2.
The performed FTIR analysis indicated that the amide group is involved in the process of Pb 2+ ions adsorption onto bean shells.
The point of zero charge (pHpzc) was determined to be 4.32 for the 0.1 M KNO3 solution and 5.13 for the 0.01 M KNO3 solution.
The pH value of the solution has a significant influence on the adsorption capacity. The adsorption capacity increases with an increase in the pH value of the solution, from 10 mg g -1 at pH 2 to about 20 mg g -1 at pH values 3 to 5.5.
The adsorption capacity increases with the increase of the concentration of lead ions in the solution until 0.8 g dm -3 , where it reaches its maximum value of almost 60 mg g -1 , after which it decreases with the further increase of the initial concentration of lead ions.
Kinetic parameters of the Pb 2+ adsorption process was analyzed using four adsorption kinetic models, the pseudo-first order kinetic model, pseudo-second order kinetic model, Webber-Morris kinetic model, and Elovich kinetic model. The obtained data indicated that the pseudo-second order kinetic model best described the adsorption process, which leads to the conclusion that chemisorption is a possible way of binding lead ions on the surface of bean shells.
The adsorption isotherm data was analyzed using five isotherm models such as: Langmuir model, Freundlich model, Temkin model, Hill model and Redlich-Peterson model. The Hill adsorption isotherm model showed the best agreement with the analyzed data, meaning that the adsorption of lead ions onto bean shells was a cooperative phenomenon, according to which the ability to bind ligands in one place on the macromolecule could affect the appearance of other binding sites at the same macromolecule. https://doi.org/10.37358/RC. 21.4.8462 The values of the calculated thermodynamic parameters (ΔG 0 ; ΔH 0 ; ΔS 0 ; and Ea) indicated that the adsorption of lead ions onto bean shells was a spontaneous, endothermic, disordered process, in which lead ions were bound to the surface of the bean shells by chemisorption.
The maximum adsorption capacity was determined to be 46.36 mg g -1 .