Removal of Chromium (VI) from the Steel Mill Efﬂuents Using the Chemically Modiﬁed Leaves of Pteris vittata as Adsorbent

: Chromium (Cr), a metal that is released in appreciable amounts from the steel industry into water bodies, is not only the main causative agent of lung cancer in human but also negatively affects the metabolic activities of plants. Keeping in view the hazardous effects of Cr(VI), the present study was aimed to eliminate it from industrial efﬂuents of steel mills installed in Dargai District Malakand, Pakistan, using chemically modiﬁed Pteris vittata plant leaves as an adsorbent. The instrumental techniques such as FTIR, surface area analysis, SEM, TGA and EDX were used to evaluate surface functionality, morphology, thermal stability and elemental composition of the modiﬁed leaves. To identify the ideal conditions for the biosorption process, batch adsorption tests were carried out under varied conditions of pH, contact time, initial metal concentration, biosorbent dose, as well as temperature. Various models, such as those of Freundlich, Jovanovich, Temkin, Langmuir, and Harkins–Jura, were utilized to explain the isothermal experimental data. The high value of R 2 (0.991) was exhibited by the Langmuir model. Pseudo-ﬁrst-order, power function, pseudo-second-order, intraparticle diffusion, and Natarajan–Khalaf models were employed to obtain an insight into kinetics of the process. The highest R 2 value, close to unity was recorded with pseudo-second order. At pH = 2, the best elimination of Cr was observed with maximum uptake capacity q max (66.6 mg/g) as calculated from the Langmuir isotherm. The thermodynamic analysis, which was conducted at different temperatures, showed that the nature of this sorption process was exothermic and spontaneous. The modiﬁed leaves-based biosorbent could be used as an alternative adsorbent for effective Cr elimination from water, and its use could be extended to other heavy metals and organic pollutants as well, and further experimentation are needed in this regard.


Introduction
Heavy metal contamination has grown to be a significant issue in the present scenario because of its direct effects on living things, including humans and other animals as well as on plants. They can induce a variety of acute and long-term negative consequences if present in concentrations exceeding permissible limits, such as shortness of breath, coughing, and asthma. They also result in digestive complications such as abdominal pain, vomiting, bleeding and neurological complications [1]. In the heavy metals list, Cr(VI) is considered to be of prime concern due to its toxicity [2]. Most of the chromium as well as its compounds found in sewage come from different industries such as pigment manufacturing, metal processing, water cooling, leather tanning and mining [3]. Cr may exist in various oxidation states, but Cr +3 and Cr +6 are the most stable. As a strong oxidant, Cr(VI) is utilized mostly in the manufacturing of polymers, pigments, paints, and other dyes. In addition to being utilized in the manufacturing of steel, Cr(VI) is also employed in electroplating, the tanning of leather, and the restoration of wood. Cr(III) is not more

Preparation of Adsorbent
In Dargai Malakand, Pakistan, leaves of Pteris vittata were gathered from the bank of steel mills drainage lines, cleaned twice with distilled water and then allowed to dry in the shade before being put in an electric oven, at 50 • C. The crunchy leaves were ground into powder and processed through 44-mesh sieve. The powdered samples were cleaned in distilled water to eliminate impurities and dust, present if any. To dry the samples, they were baked in an oven. The dried samples were stored for subsequent processing in airtight vials.

Chemical Modification of Biosorbent
About 100 g of the powdered were steeped in 2 L of HNO 3 (Sigma Aldrich, Taufkirchen, Germany) solution for 24 h before being filtered through 41 Whatman filter paper and repeatedly rinsed using distilled water. The HNO 3 treatment was conducted to eliminate metals that had previously been affixed to the biomass feedstock. The sodium hydroxide (0.1 M; Sigma-Aldrich, Taufkirchen, Germany) was utilized to neutralize the unused acid. Before being heated at 100 • C in an electric oven, the neutralized adsorbent was dried at room temperature, and its 50 g was processed after being activated in 1 L of CaCl 2 solution (0.1 M). The CaCl 2 treatment (Sigma-Aldrich, Taufkirchen, Germany) was utilized to insert a specific group into biosorbent, which will improve the process of adsorption. From here on, it will be abbreviated as CMPVL (chemically modified Pteris vittata leaves) whereas its schematically its preparation is shown in Figure 1.
unused acid. Before being heated at 100 °C in an electric oven, the neutralized adsorbent was dried at room temperature, and its 50 g was processed after being activated in 1 L of CaCl2 solution (0.1 M). The CaCl2 treatment (Sigma-Aldrich, Taufkirchen, Germany) was utilized to insert a specific group into biosorbent, which will improve the process of adsorption. From here on, it will be abbreviated as CMPVL (chemically modified Pteris vittata leaves) whereas its schematically its preparation is shown in Figure 1.

Characterization of Biosorbent
To visualize the surface functionality, Fourier-transform infrared spectroscopy (PerkinElmer, USA) was used where the CMPVL spectra were recorded in the range of 400-4000 cm −1 for metal loaded and unloaded sample. A surface area analyzer was used to study the Brunauer-Emmett-Teller (BET) surface area and pore volume. Scanning electron microscopic analysis was conducted to see the morphological structure of both the loaded and unloaded samples of CMPVL. The thermo-gravimetric analysis (TGA) was conducted to check the stability of adsorbent. The elemental composition of the metal loaded and unloaded samples were evaluated by EDX (INCA200/Oxford Instruments, Abingdon, UK) analysis.

Batch Adsorption Experiment
For the adsorption tests, a stock solution of Cr(VI) in 1 L of distilled water was made and solutions of 20-300 mg/L (working dilutions) were subsequently obtained by diluting the stock solution. As chromium source K2Cr2O7 (Sigma-Aldrich, Germany) have been used. In batch tests 50 mL of solution of each dilution were taken and added with 0.12 g of biomass then the mixture was shaken at a speed of 130 rpm for 120 min. After filtration, the amount of Cr(VI) was estimated using atomic absorption spectrophotometer with the help of equations given:

Characterization of Biosorbent
To visualize the surface functionality, Fourier-transform infrared spectroscopy (PerkinElmer, Waltham, MA, USA) was used where the CMPVL spectra were recorded in the range of 400-4000 cm −1 for metal loaded and unloaded sample. A surface area analyzer was used to study the Brunauer-Emmett-Teller (BET) surface area and pore volume. Scanning electron microscopic analysis was conducted to see the morphological structure of both the loaded and unloaded samples of CMPVL. The thermo-gravimetric analysis (TGA) was conducted to check the stability of adsorbent. The elemental composition of the metal loaded and unloaded samples were evaluated by EDX (INCA200/Oxford Instruments, Abingdon, UK) analysis.

Batch Adsorption Experiment
For the adsorption tests, a stock solution of Cr(VI) in 1 L of distilled water was made and solutions of 20-300 mg/L (working dilutions) were subsequently obtained by diluting the stock solution. As chromium source K 2 Cr 2 O 7 (Sigma-Aldrich, Germany) have been used. In batch tests 50 mL of solution of each dilution were taken and added with 0.12 g of biomass then the mixture was shaken at a speed of 130 rpm for 120 min. After filtration, the amount of Cr(VI) was estimated using atomic absorption spectrophotometer with the help of equations given: where C i, shows initial Cr(VI) concentration and C f shows final concentration after adsorption. The V is the volume taken in liter for the batch test and m is the mass in g of adsorbent. The other parameters are: V = volume, m = mass of sorbent, q e = amount of metal adsorbed, whereas %R is percent removal of metal. Schematically, the batch adsorption experiments have been shown in Figure 2.
where Ci, shows initial Cr(VI) concentration and Cf shows final concentration after a sorption. The V is the volume taken in liter for the batch test and m is the mass in g adsorbent. The other parameters are: V = volume, m = mass of sorbent, qe = amount metal adsorbed, whereas %R is percent removal of metal. Schematically, the batch adsor tion experiments have been shown in Figure 2. Different isotherm models such as Langmuir, Freundlich, Temkin, Harkin-Jura an Jovanovic were employed to evaluate the sorption parameters of prepared biosorbent.

Effect of Operating Physicochemical Parameters on the Sorption Process
In addition to isothermal studies, the effect of parameters such as biosorbent ma pH of the solution, contact time, and temperature were also evaluated, where in each ca 50 mL metal solutions were contacted with a given quantity of sorbent, with the on changes in parameter under study. In the case of pH effect evaluation study, the soluti pH was adjusted using NaOH (0.1 M) and HNO3 (0.1 M), whereas in thermodynam effect study the experiment were conducted at 293 K, 303 K, 313 K, 323 K and 333 K.

Kinetic Study
To investigate the kinetic characteristics of Cr(VI) biosorption on CMPVL, a set qua tity of absorbent (0.12 g) was introduced into a series of flasks containing 50 mL of Cr(V solution (100 mg/L) which were then shaken at 130 rpm for different interval of time. T adsorption data obtained was fed into pseudo-first-order, pseudo-second-order, intr particle diffusion model, Natarajan-Khalaf model and power function model to obtain insight into kinetics of the process.  Different isotherm models such as Langmuir, Freundlich, Temkin, Harkin-Jura and Jovanovic were employed to evaluate the sorption parameters of prepared biosorbent.

Effect of Operating Physicochemical Parameters on the Sorption Process
In addition to isothermal studies, the effect of parameters such as biosorbent mass, pH of the solution, contact time, and temperature were also evaluated, where in each case, 50 mL metal solutions were contacted with a given quantity of sorbent, with the only changes in parameter under study. In the case of pH effect evaluation study, the solution pH was adjusted using NaOH (0.1 M) and HNO 3 (0.1 M), whereas in thermodynamics effect study the experiment were conducted at 293 K, 303 K, 313 K, 323 K and 333 K.

Kinetic Study
To investigate the kinetic characteristics of Cr(VI) biosorption on CMPVL, a set quantity of absorbent (0.12 g) was introduced into a series of flasks containing 50 mL of Cr(VI) solution (100 mg/L) which were then shaken at 130 rpm for different interval of time. The adsorption data obtained was fed into pseudo-first-order, pseudo-second-order, intraparticle diffusion model, Natarajan-Khalaf model and power function model to obtain an insight into kinetics of the process.

Result and Discussion
3.1. Characterization of CMPVL 3.1.1. FTIR Spectra of Unloaded and Loaded Cr (VI) CMPVL FTIR spectroscopy was utilized to examine the functional group in the unloaded biosorbent and the interaction between functional group of loaded biosorbent and metal ions. The unloaded and loaded spectra of CMPVL have been displayed in Figure 3A,B. The NH 2 group was deduced from the peak at 3200 cm −1 to 3400 cm −1 , and the peak at 3000 to 3100 cm −1 indicates the C-H stretch. The peak displayed between 1630-1680 cm −1 is because of carbonyl group stretching and at 600 cm −1 due to N-H group stretching.
The spectra at 1100-1300 cm −1 show the C-N group stretching [20]. The loaded spectra Figure 3B of CMPVL represents that at 3300 cm −1 O-H stretching and at 2930 cm −1 C-H stretch was reported. The peak at 1630 is for C=C stretching and at 1030 cm −1 for C-O stretch. A slight shift in the peak of loaded adsorbent has been reported.
Water 2022, 14, 2599 5 of 17 The NH2 group was deduced from the peak at 3200 cm −1 to 3400 cm −1 , and the peak at 3000 to 3100 cm −1 indicates the C-H stretch. The peak displayed between 1630-1680 cm −1 is because of carbonyl group stretching and at 600 cm −1 due to N-H group stretching. The spectra at 1100-1300 cm −1 show the C-N group stretching [20]. The loaded spectra Figure 3B of CMPVL represents that at 3300 cm −1 O-H stretching and at 2930 cm −1 C-H stretch was reported. The peak at 1630 is for C=C stretching and at 1030 cm −1 for C-O stretch. A slight shift in the peak of loaded adsorbent has been reported.

Surface Area and Pore Volume
The BET surface area and pore volume of CMPVL are all presented in Table 1. According to this, CMPVL has a larger surface area and greater pores, making it a good biosorbent in terms of biosorption capacity [20]. Table 1. Surface area, pore volume and diameter of CMPVL.

Surface Area and Pore Volume
The BET surface area and pore volume of CMPVL are all presented in Table 1. According to this, CMPVL has a larger surface area and greater pores, making it a good biosorbent in terms of biosorption capacity [20]. Table 1. Surface area, pore volume and diameter of CMPVL.

SEM Analysis
SEM images of treated and Cr-loaded biosorbent are displayed in Figure 3C,D, which illustrate the surface morphology and roughness of treated biosorbent. The pictures display a clear, porous structure with curves and bent edges that could be an appropriate surface for adsorption. The SEM images of loaded adsorbent showed that the surface was occupied by Cr(VI).

EDX Analysis
The elemental analysis of unloaded and Cr (VI)-loaded biosorbent is displayed in Figure 3E,F. In comparison with other tiny peaks of Ca, Si, K, S, P, Cl, and Mg that appeared as impurities, the oxygen and carbon peaks are more prominent in the unloaded biosorbent. The loaded biosorbent displayed an apparent peak of Cr, which represents the biosorption of Cr(VI) on CMPVL.

Thermal Gravimetric Analysis
The TGA analysis is displayed in Figure 3G. The sample of treated biosorbent examined for the thermal gravimetric investigation was 7.873 mg. The mass loss occurred with respect to temperature in three stages. In the first stage, the mass loss occurred with respect to temperature due to water evaporation, followed by the second stage, which was because of the breakdown of cellulose. In the third stage, the mass loss occurred due to the formation of carbonaceous material, and after that, stability of the sample was seen at round 800 • C.

Adsorption Isothermal Studies
Up to a certain point, the amount of metal removed by the adsorbent rises with an increase in starting concentration; after that, no further increase is reported in adsorption capacity as the adsorbent has reached the point of its saturation and the pores available for the adsorption were fully occupied [21]. Figure 4A shows that when the starting concentration of Cr(VI) was raised, a high speed in the biosorption process were reported until the biosorbent pores were fully occupied by Cr(VI) metal ion, and after that, a decline in the biosorption process was noticed. A number of isothermal models such as Freundlich, Langmuir, Temkin, Harkins-Jura and Jovanovic isotherm were utilized to describe the parameters of adsorption.  Figure 3C,D, which illustrate the surface morphology and roughness of treated biosorbent. The pictures display a clear, porous structure with curves and bent edges that could be an appropriate surface for adsorption. The SEM images of loaded adsorbent showed that the surface was occupied by Cr(VI).

EDX Analysis
The elemental analysis of unloaded and Cr (VI)-loaded biosorbent is displayed in Figure 3E,F. In comparison with other tiny peaks of Ca, Si, K, S, P, Cl, and Mg that appeared as impurities, the oxygen and carbon peaks are more prominent in the unloaded biosorbent. The loaded biosorbent displayed an apparent peak of Cr, which represents the biosorption of Cr(VI) on CMPVL.

Thermal Gravimetric Analysis
The TGA analysis is displayed in Figure 3G. The sample of treated biosorbent examined for the thermal gravimetric investigation was 7.873 mg. The mass loss occurred with respect to temperature in three stages. In the first stage, the mass loss occurred with respect to temperature due to water evaporation, followed by the second stage, which was because of the breakdown of cellulose. In the third stage, the mass loss occurred due to the formation of carbonaceous material, and after that, stability of the sample was seen at round 800 °C.

Adsorption Isothermal Studies
Up to a certain point, the amount of metal removed by the adsorbent rises with an increase in starting concentration; after that, no further increase is reported in adsorption capacity as the adsorbent has reached the point of its saturation and the pores available for the adsorption were fully occupied [21]. Figure 4A shows that when the starting concentration of Cr(VI) was raised, a high speed in the biosorption process were reported until the biosorbent pores were fully occupied by Cr(VI) metal ion, and after that, a decline in the biosorption process was noticed. A number of isothermal models such as Freundlich, Langmuir, Temkin, Harkins-Jura and Jovanovic isotherm were utilized to describe the parameters of adsorption.

Freundlich Adsorption Isotherm
The adsorption is multilayer with binding sites possessing various energies are described by this model [22]. This model's mathematical expression can be as given: where the equilibrium concentrations represented by Ce in the solution and qe (mg/g) is the quantity of Cr(VI) adsorbed per unit mass of CMPVL. The slope and intercept of the lnqe vs. Ce plot, displayed in Figure 4B, can be used to calculate the Freundlich constant (KF) and the adsorption coefficient (n), whose values are listed in Table 2.

Freundlich Adsorption Isotherm
The adsorption is multilayer with binding sites possessing various energies are described by this model [22]. This model's mathematical expression can be as given: where the equilibrium concentrations represented by Ce in the solution and q e (mg/g) is the quantity of Cr(VI) adsorbed per unit mass of CMPVL. The slope and intercept of the lnq e vs. C e plot, displayed in Figure 4B, can be used to calculate the Freundlich constant (K F ) and the adsorption coefficient (n), whose values are listed in Table 2.

Langmuir Adsorption Isotherm
This model can be expressed in the following linear form [23].
The maximum adsorption capacity is indicated by q m while the C e and q e as aforementioned. The Langmuir constant, which is concerned with free energy as well as binding strength, is referred as "b". When C e vs. C e /q e were plotted, Figure 4C shows a linear curve with b and q max as the intercept and slope, respectively. Higher R 2 (0.991) value indicates that experimental data can be well managed by the model. Thus the parameters of this model were calculated and expressed in Table 2.

Temkin Adsorption Isotherm
Adsorption heat and surface coverage are related by Temkin adsorption isotherm. The mathematical form of this model is given below [24,25]. q e = β ln α + βlnC e (5) where β = RT/b, R has a value of 8.314 J/mol K, and is called ideal gas constant, b represents a constant which is related to the heat of adsorption and T indicates the absolute temperature. The plot drawn between q e vs. lnCe is displayed in Figure 4D and the β, b and R 2 values calculated from the graph were listed in Table 2.

Jovanovic Adsorption Isotherm
The mechanical link between adsorbent and adsorbate is described by this model. The mathematical form of the isotherm can be expressed in the given form [26].
C e and q max can be inferred from the graph intercept. The slope and intercept of the lnq e vs. C e plot, as seen in Figure 4E, were used to calculate the K J and q max values that are enlisted in Table 2.

Harkins-Jura Adsorption Isotherm
This model reflects the probability of multilayer adsorption on an adsorbent surface having a different porous arrangement [27].
The intercept and slope of the graph depicted in Figure 4F were used to compute the B H and A H constants, and their values are given in Table 2. Figure 5A shows the impact of contact time on the biosorption of Cr (VI) on CMPVL. The increased accessibility of empty holes for the adsorbate is the cause of the increased adsorption of Cr (VI) on CMPVL with a time up to 20 min. Following the rapid stage, the biosorption process gradually slows down, representing a consistent rate of sorption, which eventually caused the saturation of binding sites. After 120 min, equilibrium was reached. The kinetic parameters of Cr (VI) biosorption on CMPVL could be calculated by applying the pseudo-first-order model, pseudo-second-order model, Natarajan-Khalaf models, intraparticle and power function to the experimental kinetic data.

Harkins-Jura Adsorption Isotherm
This model reflects the probability of multilayer adsorption on an adsorbent surface having a different porous arrangement [27].
The intercept and slope of the graph depicted in Figure 4F were used to compute the BH and AH constants, and their values are given in Table 2. Figure 5A shows the impact of contact time on the biosorption of Cr (VI) on CMPVL. The increased accessibility of empty holes for the adsorbate is the cause of the increased adsorption of Cr (VI) on CMPVL with a time up to 20 min. Following the rapid stage, the biosorption process gradually slows down, representing a consistent rate of sorption, which eventually caused the saturation of binding sites. After 120 min, equilibrium was reached. The kinetic parameters of Cr (VI) biosorption on CMPVL could be calculated by applying the pseudo-first-order model, pseudo-second-order model, Natarajan-Khalaf models, intraparticle and power function to the experimental kinetic data.

Pseudo-First-Order Kinetic Model
The mathematical form of this model is given below [28].
The qe (mg/g) and qt (mg/g) represent the amounts of metal adsorbed at equilibrium and at time t, while K1 is the rate constant. Table 3 displays the values of K1 and qe as computed from the slope and intercept of the ln (qe − qt) vs. t plot, as displayed in Figure  5B. The values of kinetics constants are given in Table 3.

Pseudo-Second-Order Kinetic Model
Following is the mathematical form of the model [29].

Pseudo-First-Order Kinetic Model
The mathematical form of this model is given below [28].
The q e (mg/g) and q t (mg/g) represent the amounts of metal adsorbed at equilibrium and at time t, while K 1 is the rate constant. Table 3 displays the values of K 1 and q e as computed from the slope and intercept of the ln (q e − q t ) vs. t plot, as displayed in Figure 5B. The values of kinetics constants are given in Table 3. Table 3. Kinetic parameters for Cr (VI) biosorption on CMPVL.

Kinetic Model Parameters Values
Pseudo-first-order

Pseudo-Second-Order Kinetic Model
Following is the mathematical form of the model [29].
t/q t = 1/K 2 q e 2 + t/q e (9) All other parameters in equation are specified above, except K 2 that is a pseudosecond-order constant, which was determined from the intercept of the plot, as shown in Figure 5C and as enlisted in Table 3.

Power Function Kinetic Model
Following is the linear form of this model [30].
In this equation, constants a and b represent the initial rate of adsorption and the reaction rate, respectively. Their values could be computed using the intercept and slope of the log q t vs. log t plot, as given in Figure 5D, and Table 3 displayed their values.

Intraparticle Kinetic Model
The representation of this model is described below [31,32].
where K diff (mg g −1 min 1/2 ) is the rate constant for this model. The intercept of the q t vs. t 1/2 graph can be used to calculate the constant C (mgg −1 ), which is related to the boundary layer's thickness. Table 3 provides a list of their values. The intraparticle diffusion curve was produced by plotting qt vs. t 1/2 is shown in Figure 5E.

Natarajan-Khalaf Kinetic Model
Following is the mathematical form of this model [33].
According to this equation, C o (mg/L) and C t (mg/L) stands for the initial and final concentration at time t, respectively, and K N (mg/L) for a constant that can be measured from the slope of graph, as shown in Figure 5F.

pH Analysis
The effect of pH on Cr (VI) biosorption was investigated in the pH ranging 2-7, as shown in Figure 6. The various dissociation states of Cr should be distributed differentially in aqueous solution depending on pH. Chromium exists in the form of Cr 2 O 7 −2 , HCrO 4 − and CrO 4 −2 in aqueous solution depending on pH. In the acidic pH range, Cr exists in the Cr 2 O 7 −2 and HCrO 4 − , and above pH 7, the stable species is CrO 4 −2 . The maximal removal of Cr(VI) was reported at pH 2, while at high Ph, the uptake capacities were decreased. At pH = 2, the adsorbent's surface become positively charged and HCrO 4 − is the main form which exists at this pH. Thus, at low pH, the attraction between adsorbent and the anion of Cr results in increased adsorption of Cr (VI). However, at pH = 5 or higher, the HCrO 4 − anions of Cr(VI) changes into CrO 4 −2 form and so the competition between these species and OH − result in the decline of Cr(VI) adsorption [34,35]. It was reported that at lower pH, the system attains equilibrium faster, and also, the percentage of chromium adsorption was increased. Therefore, the optimum pH = 2 was obtained for the batch adsorption tests.

Biosorbent Dose Effect
The impact of biosorbent dose (0.01-0.13 g) on the elimination of Cr(VI) was analyzed in the batch adsorption tests. Figure 7 displays the relation between the adsorbent dose and Cr(VI) removal. The elimination of Cr(VI) was enhanced by increasing biosorbent dose from 0.01 to 0.12 g. The highest elimination of Cr(VI) at higher biosorbent dose (0.12 g) was because of the risen surface area and presence of more active sites which made it possible [36,37]. Therefore, the optimum dose of 0.12 g was taken for the batch tests.

Biosorbent Dose Effect
The impact of biosorbent dose (0.01-0.13 g) on the elimination of Cr(VI) was analyzed in the batch adsorption tests. Figure 7 displays the relation between the adsorbent dose and Cr(VI) removal. The elimination of Cr(VI) was enhanced by increasing biosorbent dose from 0.01 to 0.12 g. The highest elimination of Cr(VI) at higher biosorbent dose (0.12 g) was because of the risen surface area and presence of more active sites which made it possible [36,37]. Therefore, the optimum dose of 0.12 g was taken for the batch tests.

Adsorption Thermodynamics
The study of thermodynamics is crucial, and it determines the nature and the effectiveness of the given biosorption process. By knowing the nature of the biosorption process, namely, whether it is exothermic or endothermic, is crucial from the industrial perspective. The batch adsorption experiments were carried out at different temperatures (293, 303, 313, 323, 333 K) to find out the thermodynamic parameters. The mechanism driving the increase in adsorption with temperature could be the low kinetic energy of Biosorbent dose (g) Figure 6. Effect of pH on Cr (VI) biosorption by CMPVL. Figure 6. Effect of pH on Cr (VI) biosorption by CMPVL.

Biosorbent Dose Effect
The impact of biosorbent dose (0.01-0.13 g) on the elimination of Cr(VI) was analyzed in the batch adsorption tests. Figure 7 displays the relation between the adsorbent dose and Cr(VI) removal. The elimination of Cr(VI) was enhanced by increasing biosorbent dose from 0.01 to 0.12 g. The highest elimination of Cr(VI) at higher biosorbent dose (0.12 g) was because of the risen surface area and presence of more active sites which made it possible [36,37]. Therefore, the optimum dose of 0.12 g was taken for the batch tests.

Adsorption Thermodynamics
The study of thermodynamics is crucial, and it determines the nature and the effectiveness of the given biosorption process. By knowing the nature of the biosorption process, namely, whether it is exothermic or endothermic, is crucial from the industrial perspective. The batch adsorption experiments were carried out at different temperatures (293, 303, 313, 323, 333 K) to find out the thermodynamic parameters. The mechanism driving the increase in adsorption with temperature could be the low kinetic energy of

Adsorption Thermodynamics
The study of thermodynamics is crucial, and it determines the nature and the effectiveness of the given biosorption process. By knowing the nature of the biosorption process, namely, whether it is exothermic or endothermic, is crucial from the industrial perspective. The batch adsorption experiments were carried out at different temperatures (293, 303, 313, 323, 333 K) to find out the thermodynamic parameters. The mechanism driving the increase in adsorption with temperature could be the low kinetic energy of Cr 2 O 7 −2 at lower temperature and the insufficient interaction between Cr 2 O 7 −2 and the active site. The kinetic energy increases with increasing temperature, which result in the maximum binding capacity. By using Van 't Hoff plot ( Figure 8) the enthalpy change (∆H • ) and entropy change (∆S • ) were computed, and their values were listed in Table 4. The positive value of ∆S • and negative value of ∆H • displayed that these processes were spontaneous and exothermic. Following is the presentation of the Van 't Hoff equation [38]: In the above equation qe (mg/g), Ce, R and T are as stated above. The following equation was used to compute Gibbs energy change ∆G° [39]. ∆G° = ∆H° − T. ∆S° (14) The values of ∆G° were shown in Table 3, which shows the spontaneous and favorable aspect of the biosorption process.

Regenration of CMPVL Biosorbent
The CMPVL biosorbent was regenerated and reused for five cycles, where the fall in Cr (VI) removal efficiency was 27%, which indicates that regenerated CMPVL biosorbent could be efficiently reused repeatedly. Figure 9 shows the decline in % removal of Cr(VI) with an increase in regeneration.  In the above equation q e (mg/g), C e , R and T are as stated above. The following equation was used to compute Gibbs energy change ∆G • [39].
The values of ∆G • were shown in Table 3, which shows the spontaneous and favorable aspect of the biosorption process.

Regenration of CMPVL Biosorbent
The CMPVL biosorbent was regenerated and reused for five cycles, where the fall in Cr (VI) removal efficiency was 27%, which indicates that regenerated CMPVL biosorbent could be efficiently reused repeatedly. Figure 9 shows the decline in % removal of Cr(VI) with an increase in regeneration.

Comparison of Adsorption Capacities of CMPVL with the Literature
A comparison of CMPVL with reported adsorbent capacities has been shown in Table 5. The adsorption capacities of CMPVL are reasonably high as compared to the one cited in the table.

Comparison of Adsorption Capacities of CMPVL with the Literature
A comparison of CMPVL with reported adsorbent capacities has been shown in Table 5. The adsorption capacities of CMPVL are reasonably high as compared to the one cited in the table.

Conclusions
In this study, the intrinsic affinity of a plant, the phytoremediation capacity was utilized in designing an efficient adsorbent. The CMPVL a low-cost and environmentally friendly adsorbent was used to reduce Cr(VI) concentration in the industrial effluents. The effect of initial pH, contact time, biosorbent dose, initial metal concentration, and temperature on adsorption process were evaluated in order to optimize the sorption process. The optimum parameters recorded were pH = 2, biosorbent dose = 0.12 g, contact period = 120 min, initial Cr(VI) concentration = 100 mg/L, and temperature = 30 °C. Langmuir isotherm with an R 2 value of 0.991 was found to be the most suitable isotherm model among tested ones. According to a kinetic investigation of Cr (VI) the biosorption follow pseudo-second-order kinetics pointing towards the chemical nature of the process. From values and signs of the thermodynamic parameters estimated, it was inferred that the sorption process investigated herein is favorable, spontaneous, and exothermic. FTIR, SEM, EDX and TGA analysis were also performed to explain the chemical and physical basis of the sorption process. In a nutshell, we conclude that the elimination of Cr(VI) from water might be accomplished successfully in an environmentally friendly way through CMPVL.

Conclusions
In this study, the intrinsic affinity of a plant, the phytoremediation capacity was utilized in designing an efficient adsorbent. The CMPVL a low-cost and environmentally friendly adsorbent was used to reduce Cr(VI) concentration in the industrial effluents. The effect of initial pH, contact time, biosorbent dose, initial metal concentration, and temperature on adsorption process were evaluated in order to optimize the sorption process. The optimum parameters recorded were pH = 2, biosorbent dose = 0.12 g, contact period = 120 min, initial Cr(VI) concentration = 100 mg/L, and temperature = 30 • C. Langmuir isotherm with an R 2 value of 0.991 was found to be the most suitable isotherm model among tested ones. According to a kinetic investigation of Cr (VI) the biosorption follow pseudo-second-order kinetics pointing towards the chemical nature of the process. From values and signs of the thermodynamic parameters estimated, it was inferred that the sorption process investigated herein is favorable, spontaneous, and exothermic. FTIR, SEM, EDX and TGA analysis were also performed to explain the chemical and physical basis of the sorption process. In a nutshell, we conclude that the elimination of Cr(VI) from water might be accomplished successfully in an environmentally friendly way through CMPVL.