The Chemically Modiﬁed Leaves of Pteris vittata as Efﬁcient Adsorbent for Zinc (II) Removal from Aqueous Solution

: High concentrations of zinc along with other metals are released by steel mills, and this has a number of negative effects on organism health; most notably, neurological symptoms have been recorded with a high risk of brain atrophy. In the current study, Zn (II) was eliminated from steel mill efﬂuent, utilizing chemically processed Pteris vittata plant leaves as a biosorbent. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and energy dispersive X-ray spectroscopy (EDX) were applied to characterize the chemically modiﬁed Pteris vittata leaves, from now onward abbreviated as CMPVL. In order to identify the ideal parameter, batch studies were conducted varying a single parameter affecting the biosorption process at a time, including variations in temperature (293–323 K), initial metal concentration (20–300 mg/L), and adsorbent doses (0.01–0.12 g), pH (2–8), as well as contact time (10–140 min). To describe the isothermal experimental results, a number of models were used including Freundlich, Langmuir, Temkin, Jovanovich, and Harkins–Jura. Among these models, the Langmuir model provided a signiﬁcant ﬁt to the isotherm data with an R 2 of 0.9738. The kinetics data were ﬁtted to the pseudo ﬁrst order, pseudo second order, power function, Natarajan–Khalaf, and intraparticle diffusion models. The highest R 2 (0.9976) value was recorded for the pseudo second order model. Using the Langmuir isotherm, the highest uptake ability (84.74 mg/g) of Zn was recorded. The thermodynamic investigation, carried out at various temperatures, led to the conclusion that the biosorption process was exothermic and spontaneous in nature. The CMPVL, thus, has the potential to function well as an alternative to existing carbon-based adsorbents in the effective elimination of zinc from aquatic environments.


Introduction
Heavy metals being released into the surroundings as a result of industrial activity is a serious global issue, which directly or indirectly affects human health. The heavy metals released can then be mobilized in soil and, in turn, taken up by plants that are consumed by human populations. Thus, the entry of heavy metals into both soil and water is a major threat towards the destabilization of the ecosystem and harmful to organisms including humans when it enters their bodies through the food chain. In numerous regions of the world, recent rapid industrialization and urbanization have resulted in causing havoc with

Preparation of Biosorbent
Zn (II) was found to be present in high levels in the effluent of steel mills operating in Dargai, Pakistan. The potential of several plants to perform phytoremediation was tested; these were taken from the drainage line's bank of the steel mills. Among these plants, because of its remarkable phytoremediation abilities, the Pteris vittata plant was chosen [15]. To make biosorbent, the chosen plant's leaves were gathered from the selected locality. These were first cleaned utilizing tap water, followed by distilled water to eliminate the dirt and dissolved pollutants and allowed to drying in the shade. The leaves were heated for a while in an oven set to 50 • C to remove moisture, followed by grinding into fine powder.

Chemical Modification of Biosorbent
Approximately 100 g mass of ground leaf powder was taken and immersed in a 0.1 M HNO 3 solution for 24 h until it was filtered with 42 Whatman filter paper as well as frequently rinsed in distilled water. The HNO 3 solution was utilized to remove previously trapped metals (Fe, Zn, Cr) from the CMPVL. For neutralization process, 0.1 M NaOH was used. Before being baked in an oven at 100 • C, the neutralized biosorbent was allowed to dry in shade. For activation of biosorbent, the whole acid-treated mass was added in the 0.1 M solution of calcium chloride. After this, the treated biosorbent was desiccated in an oven. The activation process was necessary in order to provide a suitable surface (uniform, cylindrical, shrunken, and ruptured edges) for adsorption [19].

Characterization of CMPVL
Functional groups in the biomass of CMPVL were determined, employing an FTIR spectrophotometer (PerkinElmer, Merlin model 2000). The range of wavelength was from 400 to 4000 cm −1 and spectra were recorded both before and after biosorption. The structural characterization of the active and loaded samples of CMPVL was inspected by employing scanning electron microscopy (model JSM 59, JEOL, Tokyo, Japan). The TGA analysis was carried out by employing a thermo gravimetric analyzer (Pyres Diamond Series TG, Perkin Elmer, Chicago, IL, USA), utilizing the temperature up to 800 • C to conclude the degradation of adsorbent with increase in temperature. The elemental compositions of the unloaded as well as loaded adsorbent were examined by EDX (JEOL USA JSM-5910). A Quantachrome surface area analyzer (NOVA, 2200e, Boynton Beach, FL, USA) was used to study surface area and pore parameters of the biosorbent.

Adsorption Experiment
A stock solution (1000 ppm) of Zn (II) was made to conduct the adsorption tests, and using the dilution formula, different concentrations of solutions ranging from 20-300 ppm were prepared from the mentioned stock solution, which was then used in the batch adsorption experiments. For the batch experiment, 0.1 g biomass of CMPVL was stirred with Zn (II) solution (50 mL) for 2 h. To determine the quantity of zinc (II) in the filtrate, the mixture was filtered and subjected to an atomic absorption spectrophotometer. The uptake capacity q e (mg/g) and percent elimination of Zn (II) was measured applying the given formulae.
Here C i (mg/L) shows initial Zn (II) concentration and C f (mg/L) final concentration after adsorption. The adsorbate volume (V) is measured in liters, and the adsorbent's mass (m) is quantified in grams.

Isotherm Analysis
Biosorption of Zn (II) on CMPVL was examined in solutions with concentrations ranging from 20-300 mg/L. Other parameters such as pH (6), volume of solution (50 mL), contact time (2 h), and adsorbent dose (0.1 g), were kept constant. Using equation (1), the q e values were determined and plotted against concentration. Different kinds of isothermal models such as Freundlich, Langmuir, Temkin, Jovanovich, and Harkins-Jura were applied to evaluate the isothermal data.

Kinetic Study
To study the kinetics of Zn (II) biosorption on CMPVL, a definite mass of adsorbent (0.1 g) was introduced to Zn (II) solution (100 mg/L) and stirred at 130 rpm for two hours. The kinetic data of biosorption was subjected to a variety of models such as pseudo first order, pseudo second order, power function, Natarajan-Khalaf models, and intraparticle diffusion.

Influence of pH and Adsorbent Dose
Batch adsorption experiments were conducted to examine the impact of pH and CMPVL dose in the reclamation of Zn (II) from aqueous solution (100 mg/L) keeping all the other conditions constant. The solution's pH was set between 2-8 for the batch experiments using NaOH (0.1 M) and HNO 3 (0.1 M). To determine the impact of adsorbent dosages on elimination of Zn (II), various doses (0.01-0.12 g) were used, keeping other parameters constant as mentioned before.

Thermodynamic Study
The influence of temperature on the biosorption of Zn (II) was investigated at varying temperatures (293 K, 303 K, 313 K, and 323 K) keeping other experimental conditions constant.

FTIR Spectra of Unloaded and Loaded Samples
Functional groups of both the Zn (II) loaded and unloaded biosorbent were visualized using FTIR spectroscopy. Figure 1A,B shows the spectra of treated and loaded biosorbent. In Figure 1A, the spectra of the treated biosorbent indicate that the NH 2 group is evident from the peak at 3200 to 3400 cm −1 , whereas the C-H stretch is evident from the peak at 3000 to 3100 cm −1 . Carbonyl group stretching can be seen between 1630 and 1680 cm −1 , while N-H group stretching at 600 cm −1 . The C-N group stretching is apparent in the spectra between 1100 and 1300 cm −1 [20]. The loaded biosorbent showed that N-H stretching is apparent at 3300 cm −1 and C-H stretching is there at 2920 cm −1 . The peak at 1630 cm −1 is for C=O stretching or for N-H bending as perhaps both may overlap to give a single peak at 1030 cm −1 that may also represent C-H stretching. The decreased band intensities in the spectra of the Zn loaded sample show that the functional groups of the treated biosorbent were occupied with Zn (II), thus demonstrating that Zn was adsorbed.

SEM Study
SEM was employed to examine the biosorbent at a 10,000× magnification. The che ically treated and Zn-loaded biosorbent SEM images are shown in Figure 1c,d. These i ages clarify the surface morphology and texture of the unloaded and Zn (II) loaded b sorbent. The SEM images depict porous structures with curved surfaces and bent edg that are ideal for use as a biosorbent [21]. The treated biosorbent was seen to have fir ness, cylinder, contraction, and shattering of the structure. As shown in the SEM imag after adsorption, there are very few changes as zinc ions are very small and it is diffic to observe the adsorbed atoms with SEM on the biosorbent surface.

EDX Study
The elemental study of the unloaded and loaded adsorbent is depicted in Figure 1 The chemically treated adsorbent presented more distinct peaks of oxygen and carbo whereas small peaks of Mg, K, Ca, Cl, Si, and P were also observable, which may ha appeared due to impurity. After biosorption of Zn (II) on CMPVL, the loaded biosorbe presented an evident peak of Zn, confirming the biosorption of Zn.

TGA Analysis
The TGA chart is presented in Figure 1g. In thermal gravimetric analysis, a treat adsorbent sample weighing 7.873 mg was used. As per the increase in temperature, t mass loss was observed in three stages. Water evaporation accounted for the first stage mass loss in relation to temperature, whereas cellulose breakdown contributed to the s ond mass loss. In the third step, the sample stabilized at a temperature of around 800 ° resulting in the formation of carbonaceous material [21]. Table 1 displays the CMPVL's surface area and pore volume. The data indicate th CMPVL is a more favorable biosorbent in terms of biosorption capability due to its grea surface area and better pores [21].

SEM Study
SEM was employed to examine the biosorbent at a 10,000× magnification. The chemically treated and Zn-loaded biosorbent SEM images are shown in Figure 1C,D. These images clarify the surface morphology and texture of the unloaded and Zn (II) loaded biosorbent. The SEM images depict porous structures with curved surfaces and bent edges that are ideal for use as a biosorbent [21]. The treated biosorbent was seen to have firmness, cylinder, contraction, and shattering of the structure. As shown in the SEM images, after adsorption, there are very few changes as zinc ions are very small and it is difficult to observe the adsorbed atoms with SEM on the biosorbent surface.

EDX Study
The elemental study of the unloaded and loaded adsorbent is depicted in Figure 1E,F. The chemically treated adsorbent presented more distinct peaks of oxygen and carbon, whereas small peaks of Mg, K, Ca, Cl, Si, and P were also observable, which may have appeared due to impurity. After biosorption of Zn (II) on CMPVL, the loaded biosorbent presented an evident peak of Zn, confirming the biosorption of Zn.

TGA Analysis
The TGA chart is presented in Figure 1G. In thermal gravimetric analysis, a treated adsorbent sample weighing 7.873 mg was used. As per the increase in temperature, the mass loss was observed in three stages. Water evaporation accounted for the first stage of mass loss in relation to temperature, whereas cellulose breakdown contributed to the second mass loss. In the third step, the sample stabilized at a temperature of around 800 • C, resulting in the formation of carbonaceous material [21]. Table 1 displays the CMPVL's surface area and pore volume. The data indicate that CMPVL is a more favorable biosorbent in terms of biosorption capability due to its greater surface area and better pores [21].

Adsorption Isothermal Investigation
The amount of Zn removed by the CMPVL increased with the initial concentration of Zn (II) until a specific point, which is considered the optimum concentration. Beyond that, no further increases in biosorption capacity were recorded since the biosorbent had attained saturation and all of the biosorption-accessible holes were occupied by Zn (II) [22]. In isothermal studies, the concentrations of adsorbate were varied, whereas the adsorbent amount was kept constant. Figure 2A demonstrates that when the initial Zn (II) concentration was raised (20-300 mg/L), a rapid increase in the rate of biosorption was noticed up to the point when the pores of the biosorbent were completely filled with adsorbate, attaining an equilibrium state. Numerous isothermal models as mentioned below were applied to describe the adsorption properties.

Adsorption Isothermal Investigation
The amount of Zn removed by the CMPVL increased with the initial concentration of Zn (II) until a specific point, which is considered the optimum concentration. Beyond that, no further increases in biosorption capacity were recorded since the biosorbent had attained saturation and all of the biosorption-accessible holes were occupied by Zn (II) [22]. In isothermal studies, the concentrations of adsorbate were varied, whereas the adsorbent amount was kept constant. Figure 2a demonstrates that when the initial Zn (II) concentration was raised (20-300 mg/L), a rapid increase in the rate of biosorption was noticed up to the point when the pores of the biosorbent were completely filled with adsorbate, attaining an equilibrium state. Numerous isothermal models as mentioned below were applied to describe the adsorption properties.

Freundlich Isotherm
This model is used when one is dealing with heterogeneous adsorption systems [23]. Mathematically, it can be given as: where n is an empirical constant related to adsorption strength and KF is the Freundlich constant referring to the adsorption potential of an adsorbent. Figure 2b shows the isotherm of lnCe vs. lnqe. The slopes and intercepts of the plot were used to derive the values of KF and n and are given in Table 2.

Freundlich Isotherm
This model is used when one is dealing with heterogeneous adsorption systems [23]. Mathematically, it can be given as: where n is an empirical constant related to adsorption strength and K F is the Freundlich constant referring to the adsorption potential of an adsorbent. Figure 2B shows the isotherm of lnC e vs. lnq e . The slopes and intercepts of the plot were used to derive the values of K F and n and are given in Table 2.

Langmuir Isotherm
This model illustrates the monolayer adsorption where it is assumed that the adsorbent has a limited number of surface-active sites of homogeneous distribution and that no interactions from further adsorbed molecules are anticipated after the adsorbent has interacted with particular pollutant molecules [24]. In equation form this model can be given as: Here q e shows the quantity of Zn (II) adsorbed at equilibrium, C e is the concentration of Zn (II) at equilibrium, q m represents the highest Zn (II) uptake capability by CMPVL, and K L is the Langmuir constant linked with biosorption energy. The graph of C e /q e vs. C e shown in Figure 2C was utilized to estimate the quantities of q m and K L , and these are listed in Table 2.

Temkin Adsorption Isotherm
This model suggests that surface coverage during the adsorption phase of the Zn (II) on the CMPVL was connected to the framework's free energy. The following mentioned equation can be utilized as the representative linear form of the model [25].
Here, β = RT/b where T = absolute temperature (K), b = adsorption heat constant, and R = general gas constant (8.314 J/mol.K). Using the slope and intercept of the graph shown in Figure 2D, the values of the constants were computed and are given in Table 2.

Jovanovic Isotherm Model
This model is used to represent the mechanical connections involved between the adsorbent and adsorbate. A mathematical representation of the model is given as below [26].
ln q e = ln q max − K J C e (6) Here, C e indicates the equilibrium adsorbate concentration, q e is the adsorbate amount adsorbed per unit of the adsorbent, and q max stands for the highest adsorption capacity of the adsorbent where K J is the Jovanovic constant. By plotting lnq e versus C e , the Jovanovic isotherm was obtained. Based on the slope and intercept of the curve, as given in Figure 2E, the computed data for K J and q max are listed in Table 2.

Harkins-Jura Isotherm
According to this isotherm, the adsorbent surface has a heterogeneous porous surface that allows multilayer adsorption. Mathematically, it is given as [27]: The plot of 1/q e 2 against log C e , as can be seen in Figure 2F, was used to check the isotherm's linearity, and the values of the constants A H and B H that are listed in Table 2.
The Langmuir model showed the highest R 2 = 0.9738 value among the different isothermal models employed, and thus it is the best model to accommodate the experimental data.

Kinetic Study
The kinetic investigation of Zn (II) adsorption utilizing CMPVL as a biosorbent was conducted at a concentration of 100 mg/L. For the first 30 min, a high rate of adsorption was seen continuing at a slow rate for 120 min. With time, the adsorption became consistent, and equilibrium was attained at 120 min. The adsorption process eventually slowed down after the rapid step, as initially more sites were available, which then reduced in number as most of them were occupied by Zn from the solution [28] ( Figure 3A). The adsorption kinetic parameters were computed using a number of kinetic models as described below.

Kinetic Study
The kinetic investigation of Zn (II) adsorption utilizing CMPVL as a biosorbent was conducted at a concentration of 100 mg/L. For the first 30 min, a high rate of adsorption was seen continuing at a slow rate for 120 min. With time, the adsorption became consistent, and equilibrium was attained at 120 min. The adsorption process eventually slowed down after the rapid step, as initially more sites were available, which then reduced in number as most of them were occupied by Zn from the solution [28] (Figure 3a). The adsorption kinetic parameters were computed using a number of kinetic models as described below.

Pseudo First Order Kinetic Model
The description of this isotherm can be mathematically given as follows [29]: Here K1 represents the first order rate constant, and qe and qt stand for the quantity of metal adsorbed at equilibrium and at time t, respectively. Table 3 shows the results of K1 and qe computed from the plot's slope and intercept as shown in Figure 3b.

Pseudo First Order Kinetic Model
The description of this isotherm can be mathematically given as follows [29]: ln (q e − q t ) = lnq e − K 1 t (8) Here K 1 represents the first order rate constant, and q e and q t stand for the quantity of metal adsorbed at equilibrium and at time t, respectively. Table 3 shows the results of K 1 and q e computed from the plot's slope and intercept as shown in Figure 3B.

Pseudo Second Order Kinetic Model
The expression of this model is illustrated as below [30].
where K 2 represents pseudo second order rate constant and its values were measured using the intercept of the plot given in Figure 3C, and their values are recorded in Table 3.

Power Function Kinetic Model
This isotherm expression can be given as follows [31]: The reaction rate constant values of b and a are given in Table 3, and were obtained using the slope and intercept of the graph between log q t and log t as illustrated in Figure 3D.

Natarajan and Khalaf Kinetic Model
The mathematical form of the model is given as [32]: where initial and final concentrations are represented by C o and C t , respectively. K N is a constant, which is deduced from the slope of the graph, as displayed in Figure 3E.

Intraparticle Kinetic Model
The model can be expressed as given below [33]: Here K diff is the rate constant of this model. The parameter C corresponding to the thickness of the boundary layer was computed from the intercept of q t vs. t 1/2 of the chart displayed in Figure 3F, and their values are mentioned in Table 3.

pH Study
To measure the ideal pH at which the biosorption occurred, the biosorption of Zn (II) on CMPVL was investigated in the pH limit of 2-8 as demonstrated in Figure 4. The highest remediation of Zn (II) was observed at pH 6. At low pH, the adsorption capacity of CMPVL was limited due to the competition of Zn (II) and H 3 O + ions. At lower pH, the concentration of H 3 O + was high in the solution and occupied the active sites of the adsorbent resulting in the decreased adsorption of Zn (II) on CMPVL. At improved pH, the concentration of H 3 O + steadily reduced and was eliminated from the material surface resulting in reduced competition between Zn (II) and H 3 O + , which allowed the metal ions to approach the active site of the biosorbent, resulting in increased biosorption of Zn(II) on CMPVL. Therefore, the optimum removal of 80.35% was seen at pH 6. When the pH was higher than 6, Zn (II) biosorption was low, and this was caused due to the precipitation of Zn (II) [34]. Water 2022, 14, x FOR PEER REVIEW 13 of 17 Figure 4. Effect of pH on biosorption process.

Impact of the Biosorbent Dosage
The effect of the biosorbent dosage (0.01-0.12 g) on the removal of Zn (II) from the solution was also investigated in this study. Figure 5 depicts the connection between the dosage of the adsorbent and the elimination of Zn (II). The elimination of Zn (II) was boosted by raising the dosage of the biosorbent from 0.01 to 0.12 g. At a higher biosorbent dose (0.1 g), the maximum remediation of Zn (II) was observed because of the increased surface area and the presence of more active sites [35,36]. Therefore, in the batch tests, 0.1 g was added to the solution of Zn (II), which was the optimum dose recorded with a removal efficiency of 67.18%.

Adsorption Thermodynamics
From an industrial point of view, it is important to determine whether the adsorption process is exothermic or endothermic; therefore, a thermodynamics investigation was conducted to find out the favorable conditions for the process under study. Here the adsorption tests were performed at various temperatures (293, 303, 313, and 323 K). The ΔH°

Impact of the Biosorbent Dosage
The effect of the biosorbent dosage (0.01-0.12 g) on the removal of Zn (II) from the solution was also investigated in this study. Figure 5 depicts the connection between the dosage of the adsorbent and the elimination of Zn (II). The elimination of Zn (II) was boosted by raising the dosage of the biosorbent from 0.01 to 0.12 g. At a higher biosorbent dose (0.1 g), the maximum remediation of Zn (II) was observed because of the increased surface area and the presence of more active sites [35,36]. Therefore, in the batch tests, 0.1 g was added to the solution of Zn (II), which was the optimum dose recorded with a removal efficiency of 67.18%.

Impact of the Biosorbent Dosage
The effect of the biosorbent dosage (0.01-0.12 g) on the removal of Zn (II) from the solution was also investigated in this study. Figure 5 depicts the connection between the dosage of the adsorbent and the elimination of Zn (II). The elimination of Zn (II) was boosted by raising the dosage of the biosorbent from 0.01 to 0.12 g. At a higher biosorbent dose (0.1 g), the maximum remediation of Zn (II) was observed because of the increased surface area and the presence of more active sites [35,36]. Therefore, in the batch tests, 0.1 g was added to the solution of Zn (II), which was the optimum dose recorded with a removal efficiency of 67.18%.

Adsorption Thermodynamics
From an industrial point of view, it is important to determine whether the adsorption process is exothermic or endothermic; therefore, a thermodynamics investigation was conducted to find out the favorable conditions for the process under study. Here the adsorption tests were performed at various temperatures (293, 303, 313, and 323 K). The ΔH°

Adsorption Thermodynamics
From an industrial point of view, it is important to determine whether the adsorption process is exothermic or endothermic; therefore, a thermodynamics investigation was conducted to find out the favorable conditions for the process under study. Here the adsorption tests were performed at various temperatures (293, 303, 313, and 323 K). The ∆H • and ∆S • , which represent the enthalpy and entropy change, respectively, were determined utilizing the Van 't Hoff plot as given in Figure 6, and its numerical data are presented in Table 3. The mathematical description of the Van 't Hoff equation is described as follows [37,38]: where q e , C e , R, and T are already mentioned above. ∆G • stands for the Gibbs energy change that was computed by applying the formula as shown below.
Water 2022, 14, x FOR PEER REVIEW 14 of 17 and ΔS°, which represent the enthalpy and entropy change, respectively, were determined utilizing the Van 't Hoff plot as given in Figure 6, and its numerical data are presented in Table 3. The mathematical description of the Van 't Hoff equation is described as follows [37,38]: where qe, Ce, R, and T are already mentioned above. ∆G° stands for the Gibbs energy change that was computed by applying the formula as shown below. ∆°= ∆°− ∆° (14) The calculated data of ∆G° are displayed in Table 4, highlighting the favorable and spontaneous aspect of the process under study. The negative and positive values of ΔH° and ΔS°, respectively, represents the exothermic and spontaneous nature of the process.

Comparative Study with the Literature
A comparative study was conducted between CMPVL and reported adsorbent adsorption capacities, and these are displayed in Table 5.  The calculated data of ∆G • are displayed in Table 4, highlighting the favorable and spontaneous aspect of the process under study. The negative and positive values of ∆H • and ∆S • , respectively, represents the exothermic and spontaneous nature of the process.

Comparative Study with the Literature
A comparative study was conducted between CMPVL and reported adsorbent adsorption capacities, and these are displayed in Table 5.

Conclusions
In this study, an efficient adsorbent was made from Pteris vittata leaves with natural affinity in the form of phytoremediation capability for the selected Zn metal. The leaves were chemically modified in order to enhance the biosorption process. By employing these modified leaves, Zn (II) was successfully removed from the steel mill effluents. The optimal experimental conditions established were: initial Zn (II) concentration = 100 mg/L, contact time = 2 h, pH = 6, biosorbent dosage = 0.1 g, and temperature = 30 • C. The most effective isotherm model among the tested ones was the Langmuir isotherm (R 2 = 0.9738). The pseudo second order kinetic model described the kinetics data well with an R 2 value of 0.9976, showing the chemical nature of the process. The thermodynamic investigation described that the Zn (II) biosorption on CMPVL was favorable and exothermic, as well as spontaneous. These findings suggest that CMPVL can be effectively used to eliminate Zn (II) from aqueous environments, and is a readily available, low-cost biosorbent. The CMPVL biosorbent utilized in this work needs some improvements in order to boost its capacity for biosorption further, and needs some other chemical treatments. It also needs to be tested for the biosorption of other pollutants. Data Availability Statement: All the associated data is presented in this paper.