Environ Eng Res > Volume 25(3); 2020 > Article
Altun and Ecevit: Cr(VI) removal using Fe2O3-chitosan-cherry kernel shell pyrolytic charcoal composite beads

### 1. Introduction

Heavy metal pollution in nature has natural causes as well as human causes. Volcanic activities, decomposition and erosion of minerals and rocks are some of the natural causes. Industrial activities such as mining, casting, electroplating and painting are the most important human factors. Nowadays, due to the increase in industrialization, the amount of heavy metals detrimental to the living organisms also increased. If heavy metals such as chromium, cobalt, lead, etc. are present in quantities above certain limits, they can cause adverse effects on the health of living organisms. For this reason, lowering the heavy metal concentrations below the certain limits is important to protect the health of humans and all other organisms [1].
Among these heavy metals, chromium exists in two forms as Cr(III) and Cr(VI). Cr(VI) is a much more toxic and carcinogenic species than Cr(III) form. Cr(VI) from soil and water environments can easily penetrate through the skin [2]. Maximum Cr(VI) concentration in potable water should be 0.05 mg/L according to the World Health Organization regulations [3].
Methods for removing heavy metals from aqueous solutions include adsorption, ion exchange, membrane filtration, chemical precipitation methods. Among these methods, chemical precipitation is the most commonly used method. But chemical precipitation does not completely remove metals and produces a large amount of toxic sludge that needs additional treatment processes. Other methods except adsorption have high operational costs [4]. So, adsorption is an attractive method for the removal of Cr(VI) due to its efficiency and simplicity of operation [5]. Besides, if low cost adsorbents are used, adsorption can be more economical than other methods [6].
Recently, scientific studies have been intensified on the use of low-cost adsorbents to remove heavy metals from water. Waste products that can be obtained abundantly from agricultural, industrial or food production processes can be used as low cost adsorbents [7]. Some waste products can be used directly as adsorbents owing to their high adsorption performance. However, some waste products need to be processed to improve their adsorption performance. Some of these processes may be to form charcoal, activated carbon or composites from these materials.
The cherry kernel constitutes 14.6% of the weight of the cherry fruit and as a result of food processes, is produced in large quantities as waste [8]. Pyrolytic charcoal was obtained from the cherry kernel shell which is an agricultural waste and obtained pyrolytic charcoal (CKSC) was used as adsorbent for Cr(VI) adsorption, in this study.
Pyrolytic charcoal can be obtained from substances of biological origin such as cherry kernel. In contrast to the chemical or physical activation used in the production of activated carbon, raw materials used in the production of pyrolytic charcoal are not modified with any chemical additives. The raw materials are only treated with pyrolysis in nitrogen atmosphere and at 400–1,000°C. In this way, the volatiles in the raw material are removed; the carbon ratio, surface area and porosity are increased. This also can improve the adsorption performance of the material [9, 10].
Chitosan is obtained by deacetylation of chitin. Chitosan has a high capacity of heavy metal adsorption due to the reactive hydroxyl (–OH) and amino (–NH2) groups present in its structure. However, chitosan has some disadvantages such as low-acid stability, deficient mechanical strength and low thermal stability. Therefore, to overcome these disadvantages, chitosan and composites may be formed with different materials [11].
Several studies have been carried out on magnetic adsorbents. Lingamdinne et al. [12] synthesized the biogenic magnetic inverse spinel iron oxide nanoparticles using seed extract of Cnidiummonnieri (L.) Cuss (CLC) as a precursor for the removal of Pb(II) and Cr(III) from aqueous solutions. Additionally Lingamdinne et al. [13] synthesized low-cost magnetized Lonicera japonica flower biomass for the sorption of Pb(II), Co(II) and Cu(II) from aqueous solutions. Jiang et al. [5] prepared magnetically separable millimeter-sized chitosan beads containing nanosized γ-Fe2O3 for the removal of Cr(VI). Karaer and Kaya [14] synthesized magnetic chitosan/activated carbon composite using Fe3O4 for the removal of methylene blue and reactive blue4. Recently, there has been a growing interest in the use of magnetic materials as adsorbents. The high performance of magnetic particles in the adsorption of dyes and heavy metals is the reason for this interest. An advantage of these materials is that the magnetic adsorbents can be easily recovered by means of an external magnetic field. Among these magnetic particles, Fe2O3 nanoparticles show high performance in the adsorption of many heavy metals and dyes [5, 15].
Although there are studies carried out on composite adsorbents of chitosan and magnetic adsorbents, no studies have been found on Fe-C-CKSC adsorbent for Cr(VI) removal in the literature. In this study, Cr(VI) adsorption efficiencies of the pyrolytic charcoal obtained from the cherry kernel shell (CKSC) and composite beads of this pyrolytic charcoal made of chitosan and Fe2O3 nanoparticle (Fe-C-CKSC) were investigated in synthetic solutions. The effect of various parameters such as pH, contact time, initial metal concentration was investigated to determine the optimum values of this adsorption process.

### 2.1. Materials

In order to synthesize pyrolytic charcoal, cherry kernel was broken and the oily core was separated. Then, the shells that had been separated were subjected to pyrolysis process at 500°C pyrolysis temperature at 10°C/min heating speed. The pyrolytic charcoal obtained from cherry kernel shell (CKSC) in this way was used in laboratory work.
Chitosan, which was used as matrix to form composite beads with pyrolytic charcoal and Fe2O3, and glutaraldehyde, which was used for cross-linking chitosan, were purchased from Sigma Aldrich. Potassium dichromate, acetic acid, sodium hydroxide, hydrochloric acid and ethanol were purchased from Merck; Fe2O3 nanoparticles were purchased from Inframat.

Cherry kernel shell pyrolytic charcoal (CKSC), which was used alone and in composite beads as an adsorbent, was ground with Retsch RM 100 grinder, sieved with Retsch AS 200 sieve shaker and its particle size was reduced to less than 125 μm.

Washed beads were stirred in 30 mL ethanol and 0.3 mL glutaraldehyde containing-solution at 70°C for 5 h and thus chitosan was cross-linked with glutaraldehyde. The beads were filtered and washed with ethanol to remove unreacted glutaraldehyde. Then the beads were washed until pH was 7 [11].

The characterizations of CKSC and Fe-C-CKSC adsorbents were made with elemental analysis (LECO CHNS-932), Fourier Transform Infrared Spectroscopy (FT-IR) (Bruker Vertex 70), Scanning Electron Microscope/Energy Dispersive X-Ray (SEM/EDX) (Hitachi – SU 1510) and Brunauer, Emmett and Teller (BET) surface area analysis (Quantachrome – Quadrasorb Evo 4) [9, 14].

The amount of Cr(VI) removed per unit adsorbent mass and the percentage of adsorption were calculated using Eq. (1) and Eq. (2), respectively.
##### (1)
$qe=(C0-Ce) Vw$
##### (2)
$% adsorption=C0-CeC0×100$
Where, qe is the amount of the adsorbed substance (mg/g), C0 is the initial Cr(VI) concentration of solution (mg/L), Ce is the Cr(VI) concentration at the end of the contact time (mg/L), V is the solution volume (L) and w is the adsorbent amount (g).

The adsorption kinetics study gives information about the adsorption mechanism and the adsorption rate of the solute. The pseudo-first-order and pseudo-second-order kinetic models were studied [3].

The enthalpy change (ΔH ), the entropy change (ΔS ) and Gibbs free energy change (ΔG ) during the adsorption were calculated using the data obtained from the study of determination of the effect of temperature on adsorption. Eq. (3) and (4) were used to calculate these thermodynamic parameters [3].
##### (3)
$log KD=0.434RΔS-0.434RTΔH$
##### (4)
$ΔG=ΔH-TΔS$
In these equations, KD is the thermodynamic equilibrium constant and is calculated as in Eq. (5). In addition, R (8.314 J mol−1K−1) is universal gas constant and T (K) is temperature.
##### (5)
$KD=qe(mmol)Ce(mmol)$

### 3.1. Characterizations of the Adsorbents

The FTIR spectra were obtained and are displayed in Fig. 1. For both adsorbents, the band in the range of 3,200–3,750 cm−1 is caused by O-H and N-H stretching vibrations [22, 23]. The band in the range of 2,850–3,021 cm−1 is due to the aliphatic C-H stretching [24]. The band in the range of 2,000–2,200 cm−1 is due to the triple bonds of alkynes and 1540–1570 cm1 is due to the C=O and C=C aromatic vibrations in ring [25]. The band seen at 1396–1403 cm−1 indicates deformation stretching of the −CH, −CH2 and −CH3 functional groups [23]. The band at 740–875 cm−1 is due to C–H and CH=CH2 [26].
The band seen at the Fe-C-CKSC spectrum at 1,617 cm−1 is due to N-H bending of aromatic ring structures [17]. The band at 1,361 cm−1 is due to −CH vibrations in the alcohol group [27]. The characteristic band of chitosan at 1,126 cm−1 is attributed to the stretching vibration of the glycosidic (C-O-C) bonds [28]. The band at 1,041 cm−1 is due to the stretching vibrations of CH-OH bonds in the chitosan structure [22]. The band at 1,022 cm−1 is assigned to the stretching vibrations of C-N bonds of amino groups [29]. The band at 622 cm−1 is due to the stretching vibrations of Fe-O bonds [14, 30]. These results show that after the formation of the composite of the CKSC with chitosan and Fe2O3, there is an increase in the functional groups such as hydroxyl and amine.
According to the FTIR spectrum of Fe-C-CKSC after Cr(VI) adsorption, the band appeared at 1,361 cm−1 before adsorption shifted to 1,373 cm−1. The band at 1,396 cm−1 shifted to 1,385 cm−1 by increasing its intensity. The band at 872 cm−1 transformed to band of 912 cm−1 and its intensity increased. The band at 622 cm−1 shifted to 646 cm−1 by decreasing its intensity. According to the elemental analysis results, C%, H%, N% and O% (+*Fe% − for Fe-C-CKSC) in the CKSC structure are 85.20%, 2.93%, 1.42% and 10.45%, respectively, and in the Fe-C-CKSC structure are 37.41%, 3.26%, 2.42% and *56.91%, respectively (*The value of 56.91% indicates the total amount of oxygen and iron in the structure). After the formation of composite with pyrolytic charcoal, chitosan and Fe2O3, carbon content in the structure decreased. In addition, nitrogen and hydrogen contents also increased. Due to the oxygen and iron in chitosan and Fe2O3 structures, the amount of oxygen and iron in the structure increased after the formation of Fe-C-CKSC composite. These data indicate that functional groups such as hydroxyl, amine are increased in the structure after composite formation with chitosan and Fe2O3.
SEM images of CKSC and Fe-C-CKSC adsorbents before and after adsorption are shown in Fig. 2.
When Fig. 2: (a), (b) and (c) are examined, it can be seen that CKSC surface, which was flatter before adsorption, was filled after Cr(VI) adsorption and the particulate structure formed on the surface.
The results of the EDX analysis of CKSC and Fe-C-CKSC adsorbents are given in Fig. 3.
With the EDX analysis, the presence of carbon and oxygen in the structure of both adsorbents and the presence of iron in the structure of Fe-C-CKSC adsorbent were confirmed. These results are also corroborating elemental analysis results. As a result of the analysis of the samples after adsorption, chromium was found in the structures and this showed that the chromium was retained by the adsorbents. It can also be seen from the map data that chromium was distributed homogeneously to the adsorbent surface.
As a result of BET surface area analysis, surface area of CKSC was determined as 224.148 m2/g and surface area of Fe-C-CKSC was determined as 31.286 m2/g.

#### 3.2.1. Effect of adsorbent amount

Adsorbent dose in adsorption experiments were kept between 1.0–20.0 g/L for CKSC and 1.0–6.0 g/L for Fe-C-CKSC. In order to determine the optimum amount of adsorbent for adsorption, 55 mg/L of the Cr(VI) solution adjusted to pH 2.0 was added to different amounts of adsorbents and the mixtures were stirred for 2 h. Effect of the amount of adsorbent on the percentage of Cr(VI) removal and adsorption capacity is shown in Fig. 4(a) and (b).
As a result of the experiment, optimum adsorbent amounts were determined as 5 g/L for CKSC and 3 g/L for Fe-C-CKSC. According to these values, for the adsorption of Cr(VI) ions, the efficiency of Fe-C-CKSC adsorbent appears to be higher [24].

The adsorption of Cr(VI) at different concentrations plays an important role in determining the adsorption capacity of the adsorbent. Cr(VI) solutions adjusted to pH 2.0 at different concentrations (8, 25, 58, 80, 98, 131, 158, 171, 191 mg/L) were added to the optimum amounts of adsorbents and the mixtures were stirred for 2 h. The equilibrium relationships between the adsorbents and Cr(VI) were evaluated using Freundlich, Langmuir, Scatchard, Dubinin-Radushkevich (D-R) and Temkin isotherm models [16, 1821]. Adsorption isotherms and effect of the concentration on the percentage of Cr(VI) removal are depicted in the Fig. 4(c) and (d).
The linearized forms of isotherms and the results of Freundlich, Langmuir, Scatchard, Dubinin-Radushkevich (D-R) and Temkin isotherm analyses calculated for the adsorption of Cr(VI) on CKSC and Fe-C-CKSC from aqueous solutions are shown in Table 1. Isotherm graphics are shown in Fig. 5.
The highest values of R2 were obtained when the experimental data were fitted into Langmuir model for both adsorbents. Therefore, it can be said that the adsorption mechanism is generally single layer and adsorbent surface is homogeneous [17, 18, 31]. The R2 values for Freundlich isotherm were found to be higher than 0.94. This indicates that physical adsorption also took place in the process. The R2 values for Scatchard isotherm were found to be higher than 0.91. This data supports the conclusion that the adsorption is compatible with the Langmuir isotherm. The R2 values for Dubinin-Radushkevich (D-R) isotherm were found to be higher than 0.97. This indicates that the mechanisms that are effective in adsorption can be determined by this model. Adsorption energy (E) values calculated with D-R isotherm indicate that adsorption mechanism is chemical. The R2 values for Temkin isotherm were found to be higher than 0.96. This indicates that the heat of adsorption of all molecules in the layer decreases linearly [21, 31]. Pearson’s r correlation coefficients of isotherm models are given in Table 1.
The maximum adsorption capacities of Cr(VI) according to the Langmuir isotherm model were 14.455 mg/g (0.278 mmol/g) for CKSC and 47.576 mg/g (0.915 mmol/g) for Fe-C-CKSC. As can be seen from these values, the adsorption capacity of Cr(VI) ions of Fe-C-CKSC adsorbent is 3 times more than the adsorption capacity of CKSC. In addition, maximum adsorption capacities (Qs) calculated from the Scatchard model supporting the conformity of the Langmuir model were found to be close to these values. Similar results are also available in the literature [12, 22]
Adsorption energies (E), which are one of the parameters of D-R isotherm, were found to be 15.430 kJ/mol for CKSC and 12.700 kJ/mol for Fe-C-CKSC. These values in the range of 8–16 kJ/mol indicate that complex formation and ion exchange are more effective in the adsorption mechanism. The D-R model is usually used to explain adsorption on the heterogeneous surface based on the pore filling mechanism with Gaussian energy distribution. [31, 32].
Values of b constant related to adsorption potential calculated from Temkin isotherm are 85.77 and 16.94 kJ/mol for CKSC and Fe-C-CKSC, respectively. These values greater than 8 indicate that there is strong cohesive forces between adsorbents and Cr(VI) and that chemical adsorption mechanism is the dominant mechanism in the adsorption process [21, 31]. According to these results, it can be said that homogeneous distribution of functional groups on the surface of the adsorbents causes a homogeneous binding energy distribution.
When the correlation coefficients (R2) of the adsorption isotherms were compared, all of the R2 values were found to be greater than 0.9. Therefore, it is thought that more than one mechanism is effective on adsorption. When the results of all the isotherm models were evaluated, it was seen that complex formation, ion exchange and electrostatic attraction were the predominant mechanisms on the adsorption of Cr(VI) with CKSC and Fe-C-CKSC. In addition, it was also seen that physical adsorption was occurred.
Table 2 shows the maximum adsorption capacities of CKSC, Fe-C-CKSC and some adsorbents that are recommended for Cr(VI) adsorption in the literature. It can be seen that when chitosan is used alone, its Cr(VI) adsorption capacity is 35.6 mg/g [33]. This value is lower than the Cr(VI) capacity calculated for Fe-C-CKSC. The interest of Fe-C-CKSC to Cr(VI) ions was found to be higher than CKSC. Fe-C-CKSC can be used as an effective adsorbent in the removal of Cr(VI).

#### 3.2.3. Testing of kinetic adsorption models

55 mg/L Cr(VI) solutions adjusted to pH 2.0 were added to the optimum amounts of adsorbents and the mixtures were stirred during different contact times. Effect of the contact time on percentage of Cr(VI) adsorption is given in Fig. 6(a).
As a result of the experiments, since the adsorbent surface was completely filled, it was seen that the adsorption reached equilibrium within 120 min. Cr(VI) adsorption percentages at the end of 120 min of contact time were found to be 80.5% for CKSC and 97.9% for Fe-C-CKSC [41].
In order to determine rate and rate constants of Cr(VI) adsorption with CKSC and Fe-C-CKSC, pseudo-first-order and pseudo-second-order kinetic model equations were used. Kinetic models were tested to investigate adsorption mechanisms such as mass transport and chemical reaction processes. Calculated kinetic model constants, adsorption capacities and correlation coefficients are given in Table 1 [31].
Table 1 shows the calculated adsorption capacity (q) values. At the end of 120 min of contact time, the experimentally determined adsorption capacities (qe) were found to be 0.192 mmol/g for CKSC and 0.388 mmol/g for Fe-C-CKSC.
When the correlation coefficients of kinetic models are compared, it is seen that the correlation coefficient of the pseudo-second-order kinetic model is greater than the correlation coefficient of the first order kinetic model. In addition, the adsorption capacities calculated from the pseudo-second-order-kinetic model equation is closer to the experimentally determined adsorption capacities. Therefore, the pseudo-second-order model was accepted as a useful model for the kinetic studies. This indicates that the chemical adsorption is more effective in the adsorption mechanism [31, 42]. The values of Pearson’s r calculated for the pseudo second-order model, has been found higher than that of pseudo first-order model (Table 1).

#### 3.2.4. Effect of solution pH on adsorption

55 mg/L Cr(VI) solutions adjusted to different pHs were added to the optimum amounts of adsorbents and the mixtures were stirred for 120 min. Effect of pH on percentage of Cr(VI) adsorption is given in Fig. 6(b).
As a result of the experiments, it is seen that the adsorption percentages were the highest at the pH values of 1.56 for CKSC and 2.00 for Fe-C-CKSC. Adsorption percentages at these pH values are 86% for CKSC and 95% for Fe-C-CKSC. When the pH increased to 2.97, the percentage of adsorption decreased to 27% for CKSC, and to 60% for Fe-C-CKSC, and the percentage of adsorption decreased as the pH increased [35].
The reason why adsorption is efficient at low pH is the protonation of the functional groups such as −NH2 and −OH on the adsorbent surface at acidic pHs. The Cr(VI) ions are retained by the positively charged adsorbent surface due to the electrostatic attractions, because Cr(VI) present in the solution as anionic components such as HCr2O7, HCrO4, CrO42− and Cr2O72− [41]. In addition, the reduction of Cr(VI) to Cr(III) which is present in solution as cationic form is also very low at low pH. Therefore, the total amount of chromium in the solution is considered to be equal to the amount of Cr(VI). However, the increase of pH also increases the reduction of Cr(VI) to the positively charged Cr(III). Adsorption of Cr(III) species onto protonated CKSC and Fe-C-CKSC surface decreased due to the fact that the opposite charges repel each other [43].
The mechanisms of removal of Cr(VI) by Fe-C-CKSC include mainly electrostatic attraction between protonated adsorbent surface (−OH2+ and −NH3+ groups) and Cr(VI) species, ligand exchange of Cr(VI) species and re-sorption of Cr(III) reduced from Cr(VI) via chelation. −OH groups of chitosan can supply electrons for Cr(VI) reduction. Hence, a part of reduced Cr(III) can interact with −NH2 groups via chelation. Reactions of electrostatic attractions can be written as Eq. (6) and (7), reactions of ligand exchange and chelation mechanisms can be written as Eq. (8) and (9), respectively [44]. The scheme of adsorption mechanism was given in Additional file 1: Fig. S2.
##### (6)
$FeO-OH2++HCrO4-→Fe-OH2+⋯HCrO4-$
##### (7)
$-NH3++HCrO4-→-NH3+⋯HCrO4-$
##### (8)
$FeO-OH+HCrO4-→Fe-HCrO4+OH-$
##### (9)
$-NH2+Cr3+→NH2-Cr3+$

#### 3.2.5. Thermodynamic interpretation

55 mg/L Cr(VI) solutions adjusted to pH 2 were added to the optimum amounts of adsorbents and the mixtures were stirred for 120 min at different temperatures. Effect of the temperature on Cr(VI) adsorption is given in Fig. 6(c) and (d).
It can be seen in Fig. 6(c) and (d) that when the temperature was increased, the percentage of adsorption increased. This indicates that the adsorption is endothermic. The increase in adsorption with temperature may indicate chemical interactions between the adsorbent and the adsorbate.
The increase in temperature increased the percentage of Cr(VI) adsorption, but this increase is negligible. Since operating the adsorption at high temperatures would cause high costs, 25°C was chosen as the optimum temperature.
The thermodynamic parameters were found with the slopes and intercepts of the plots of logKd versus 1/T using Eq. (3) and (4) and these parameters were shown in Table 1.
The positive value of the enthalpy change (ΔH°) confirms that the adsorption is endothermic [40]. The positive value of the entropy change (ΔS°) indicates that during the adsorption process the irregularity on the adsorbent and solution interface increased [45]. The fact that the Gibbs free energy (ΔG°) is negative indicates that adsorption is spontaneous at higher temperatures than −78.9°C for CKSC and −183.3°C for Fe-C-CKSC [46].

### 4. Conclusions

In this study, cherry kernel shell pyrolytic charcoal (CKSC) and composite beads (Fe-C-CKSC) were synthesized and Cr(VI) adsorption from aqueous solutions with synthesized Fe-C-CKSC composite beads and CKSC adsorbents was studied comparatively. Fe-C-CKSC composite beads consisted of CKSC, chitosan and Fe2O3.
As a result of isotherm studies, kinetic calculations and thermodynamic calculations, it can be concluded that the chemical adsorption was more dominant in the adsorption mechanism.
FTIR, SEM and EDX analyzes for the characterization of adsorbents showed that Cr(VI) was loaded onto the surfaces of adsorbents. As a result of BET surface area analysis, surface area of CKSC was determined as 224.148 m2/g and surface area of Fe-C-CKSC was determined as 31.286 m2/g.
This study showed that cherry kernel shell pyrolytic charcoal-chitosan-Fe2O3 composite can provide high adsorption efficiency for removal of Cr(VI) from aqueous solutions.

### Acknowledgments

We express our thanks to the Konya Technical University Scientific Research Foundation, which has financed the project (18101015/2018), a part of which is presented in this study. We would like to thank our colleague Prof. Dr. Yakup KAR, from Iskenderun Technical University, for his support in the preparation of pyrolytic charcoal in this study.

#### Nomenclature

ΔG

Gibbs free energy change (J mol−1)

E

A dsorption energy (KJ mol−1)

Qs

ΔH

Enthalpy change (J mol−1)

Fe-C-CKSC

Fe2O3-chitosan-cherry kernel shell pyrolytic charcoal composite

q

ΔS

Entropy change (J mol−1)

K

qe

Amount of the adsorbed substance (experimental) (mmol g−1)

ɛ

Polanyi potential

Kb

qt

Amount of adsorbed substance per unit adsorbent mass at any time t (mmol g−1)

As

KD

Thermodynamic equilibrium constant

R

Universal gas constant (J mol−1k−1)

AT

Temkin isotherm constant (L g−1)

KF

T

Temperature (K)

BT

Temkin constant related to adsorption temperature

Ks

Scatchard isotherm binding constant

t

Time (min)

C0

Initial Cr(VI) concentration of solution (mmol)

Pseudo first order kinetic model rate constant

V

Solution volume (L)

Ce

Cr(VI) concentration at the end of the contact time (mmol)

k2

Pseudo second order kinetic model rate constant

w

CKSC

Cherry kernel shell pyrolytic charcoal

n

Isotherm constant, expressed as the heterogeneity factor

Xm

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##### Fig. 3
EDX analysis results: (a) EDX spectrum of CKSC before adsorption, (b) EDX spectrum of CKSC after adsorption, (c) distribution map of elements on surface of CKSC after adsorption, (d) distribution map of chromium on surface of CKSC after adsorption, (e) EDX spectrum of Fe-C-CKSC before adsorption, (f) EDX spectrum of Fe-C-CKSC after adsorption, (g) distribution map of elements on surface of Fe-C-CKSC after adsorption, (h) distribution map of chromium on surface of Fe-C-CKSC after adsorption.
##### Fig. 4
(a) Effect of adsorbent amount on the adsorption for CKSC, (b) Effect of adsorbent amount on the adsorption for Fe-C-CKSC (adsorption conditions for adsorbent amount study: concentration of Cr(VI) 55 mg/L, solution pH 2, contact time 120 min, temperature 25°C) (c) Adsorption isotherms and change of Cr(VI) removal percentage for CKSC (d) Adsorption isotherms and change of Cr(VI) removal percentage for Fe-C-CKSC (Adsorption conditions for isotherm study: Adsorbent amount 5 g/L for CKSC and 3 g/L for Fe-C-CKSC, solution pH 2, contact time 120 min, temperature 25°C).
##### Fig. 5
Freundlich, Langmuir, Scatchard, Dubinin-Radushkevich and Temkin isotherms plot for the adsorption of Cr(VI) onto CKSC and Fe-C-CKSC.
##### Fig. 6
(a) Effect of contact time on the adsorption (Adsorption conditions: adsorbent amount: 5 g/L for CKSC and 3 g/L for Fe-C-CKSC, concentration of Cr(VI): 55 mg/L, solution pH: 2, temperature: 25°C) (b) Effect of pH on the adsorption (Adsorption conditions: adsorbent amount: 5 g/L for CKSC and 3 g/L for Fe-C-CKSC, concentration of Cr(VI): 55 mg/L, contact time: 120 min, temperature: 25°C) (c) Effect of the temperature on the adsorption for CKSC, (d) Effect of the temperature on the adsorption for Fe-C-CKSC (Adsorption conditions for temperature study: adsorbent amount: 5 g/L for CKSC and 3 g/L for Fe-C-CKSC, concentration of Cr(VI): 55 mg/L, pH: 2, contact time: 120 min).
##### Table 1
Isotherm, Kinetic and Thermodynamic Data for the Adsorption of Cr(VI) by CKSC and Fe-C-CKSC
Model Equation Parameter CKSC Fe-C-CKSC
Freundlich $log qe=log KF+1nlog Ce$ KF 0.443 2.930
n 7.981 4.117
R2 0.942 0.956
Pearson’s r 0.970 0.978

Langmuir $Ceqe=CeAs+1AsKb$ Kb 1,199.000 2,185.800
As (mmol/g) 0.278 0.915
R2 0.999 0.999
Pearson’s r 0.999 0.999

Scatchard $qeCe=QsKs-qeKs$ Ks 1,315.700 2,417.800
Qs (mmol/g) 0.276 0.888
R2 0.993 0.913
Pearson’s r −0.991 −0.955

Dubinin-Radushkevich $ln qe=ln Xm-Kɛ2E=(2K)-1/2$ Xm 0.329 1.395
K 0.002 0.003
E (kJ/mol) 15.430 12.700
R2 0.971 0.979
Pearson’s r −0.986 −0.989

Temkin $qe=BTln AT+BTln CeBT=RTb$ b (J/mol) 85,772.287 16,943.398
AT (L/g) 5732.914 515.628
R2 0.961 0.972
Pearson’s r 0.980 0.986

Pseudo-first-order kinetic model $log(q-qt)=log q-(kadt2.303)$ kad 0.050 0.039
q (mmol/g) 0.145 0.204
R2 0.980 0.956
Pearson’s r −0.989 −0.978

Pseudo-second-order kinetic model $tqt=1k2q2+(1q)t$ k2 0.071 0.435
q (mmol/g) 0.200 0.397
R2 0.999 0.999
Pearson’s r 0.997 0.998

Thermodynamic parameters $ΔH∘ (J/mol) ΔS∘ (J/K·mol)ΔG=ΔH-TΔS ΔG∘ (J/mol)$ 6,801.600 4,471.000
35.000 49.800
T = 298 K −3,638.500 −10,370.800
T = 308 K −3,988.600 −10,868.600
T = 318 K −4,338.800 −11,366.400
T = 328 K −4,689.000 −11,864.300
##### Table 2
Cherry kernel shell pyrolytic charcoal 14.46 mg/g This study
Fe2O3-chitosan-cherry kernel shell pyrolytic charcoal composite 47.58 mg/g

Coconut charcoal 5.26 mg/g [34]

Chitosan 35.60 mg/g [33]

γ-Fe2O3 nanocrystals 12.50 mg/g [35]

Magnetic magnetite (Fe3O4) nanoparticles 34.87 mg/g [36]

Bamboo charcoal chemically modified by iron 35.70 mg/g [37]

MnFe2O4/chitosan nanocomposites 35.20 mg/g [38]

Bamboo charcoal grafted by Cu2+-N-aminopropylsilane complexes 17.94 mg/g [39]

Fe3O4/Alginate-Ce3+ magnetic composite beads 14.29 mg/g [40]
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