# Comparative Biosorption Studies of Hexavalent Chromium Ion onto Raw and Modified Palm Branches

Authors: Mona A. Shouman; Nady A. Fathy; Soheir A. Khedr; Amina A. Attia
Journal: Advances in Physical Chemistry
 (2013)

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## Abstract

The waste of palm branches (PB) was tested for its ability to remove chromium (VI) from aqueous solution by batch and column experiments. Palm branches chemically modified with an oxidizing agent (sulphuric acid) then coated with chitosan and surfactant (hexadecyl trimethyl ammonium bromide surfactant, HDTMA), respectively, were carried out to improve the removal performance of PB. The results of their Cr (VI) removal performances are pH dependent. The adsorption data could be well interpreted by the Langmuir, Freundlich, and Flory-Huggins isotherm models. The maximum adsorption capacity obtained from the Langmuir model for the chitosan coated oxidized palm branches is 55 mg/mg. The adsorption process could be described by pseudo-second-order kinetic model. The intraparticle diffusion study revealed that film diffusion might be involved. The biosorbents were successfully regenerated using 1 M HCL solution.


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## Body

## 1. Introduction

Increased industrialization and human activities have created impact on the environment through the disposal of waste containing heavy metals. Heavy metals are toxic even in extremely minute quantities on human health and to the fauna and flora of receiving water [1]. It is known that legal standards on environment control are becoming strict and, as a result, the discharge of heavy metals into aquatic bodies and sources of potable water is being rigorously controlled. Most heavy metals exist in a cationic form, but Cr (VI) combined with oxygen is anionic in natural aquatic system. The maximum permissible limit of chromium in natural water is only 0.05 mg/L by the US Environmental Protection Agency [2]. Acute exposure to Cr (VI) causes nausea, diarrhea, liver, and kidney damage, internal hemorrhage, and respiratory problems [3, 4]. Chromium compounds are widely used in electroplating, metal finishing, magnetic tapes, pigments, leather tanning, wood protection, chemical manufacturing, electrical and electronic equipment, and catalysis [5]. Several treatment technologies have been developed to remove chromium from aqueous phase. Common methods include biosorption [6], ion exchange [7], chemical reduction, ultrafiltration [8], and adsorption [9]. The adsorption process is an effective technique for the treatment of anionic metals. Activated carbon, a representative adsorbent, has been widely used to remove diverse pollutants, including organic pollutants, reactive dyes, cationic heavy metals, and anionic metals from the aqueous phase. Although it is a preferred adsorbent, and its widespread use, it is restricted due to its high cost production. In order to decrease the cost of treatment, attempts also have been made to find inexpensive alternative adsorbents [10]. Recently, there is a great interest to improve the adsorption capacity of low-cost adsorbents for remediation of wastewater [11–13]. Nowadays, surface modification technique of diverse biosorbents has been implied to increase the surface charge and to enhance their adsorption capacity for ionic pollutants [11, 13]. Surfactants are chemicals that have an amphipathic structure along with a hydrophobic tail and a hydrophilic head. The surfactants are adsorbed onto solid surface and change the surface properties of solids, especially at the interface. Up to date, the performance of surfactants in water and wastewater treatment has been regarded for the separation of metal ions and other toxic substance [14, 15]. Also, the use of chitosan over commercially available agricultural byproducts for the removal of heavy metals is becoming popular due to its low cost, abundant availability, non toxicity, potential for ion exchange, and it has been recently reported in some papers that chitosan adsorbent has high adsorption capacities towards many classes of heavy metals [16], dyes [17], and other ionic molecules [18]. Chitosan, a naturally occurring linear polymer of glucosamine, can be obtained from crustacean chitin on an industrial scale due to a large amount of crustacean’s exoskeleton is available as a byproduct of food processing industries.Palm branches (PB) are produced in great quantities in our country, especially in Siwa Oasis. Palm tree wastes include trunks, fronds, fruit waste, and empty fruit branches. The farmers trim about 12 branches yearly from each tree, so the authors have decided that a fraction of this wood is believed to be enough to supply the growing demand for preparing potential biosorbents. The palm tree branches have the characteristic of exchange sorption properties due to the presence of some functional groups such as carboxylic, hydroxyl, and lactone which have a high affinity for metal ions [19]. In the present investigation an attempt was made to explore the effect of both the chitosan and cationic surfactant modifying agents on the adsorption performance of palm branches for removing Cr (VI) ions by using batch and column systems. In order to better understand the adsorption characteristics, isotherms and kinetics were employed to evaluate the sorption process.


## 2. Material and Method

### 2.1. Adsorbate Solution

The stock solution of Cr (VI) was made by dissolving 0.2827 g K2Cr2O7 (AR grade from Merck, Germany) in 1 L double distilled water. The solutions of different concentrations used in various experiments were obtained by dilution of the stock solutions.


### 2.2. Adsorbent Materials

The palm tree branches (PB) were provided from the farm of our institute. The precursor was first washed to remove dirt from its surface and then dried in an oven overnight at 100°C. The dried PB were crushed and sieved to desired mesh size of 2-3 mm. Then the dried PB were treated with 2% H2SO4 (v/v) in an incubator at 100°C for 24 h and soaked with deionized water until the solution pH was stable. Afterwards, the adsorbent was soaked in 3% NaHCO3 (w/v) till any residual acid left was removed. Finally, the sample was dried overnight in an oven at 100°C, then cooled at room temperature, and stored in a desiccator [16]. Hereafter, it was called OPB (oxidized palm tree branches).Surface modification of OPB by the chitosan and cationic surfactant was carried out as follows. About 20 g of chitosan flakes (98% acetylation degree, Figure1(a)) were immersed in 10% of acetic acid (v/v) with constant stirring for 24 h to form a homogenized whitish viscous gel. Afterwards, OPB was dipped into the gel (dose ratio of chitosan to OPB is 1 : 5) and shaken overnight at 120 rpm. This sample was then washed with deionized water and dried at 100°C. It was called COPB. The process was repeated for four times to form a thick coating of chitosan onto oxidized palm branches. In case of modification with a cationic surfactant, a 10 g of OPB was added to a solution of hexadecyltrimethyl ammonium bromide (HDTMA) of 0.197 g/L (Figure 1(b)). This concentration corresponds to the 0.5 critical micelle concentration (CMC), which is a measure of surfactant efficiency. Above the CMC, a surfactant forms a micelle that is very stable and remains in the bulk zone. For that reason, 0.5 CMC was selected to maximize the amount of surfactant adsorbed onto the OPB. The mixture was agitated for 24 h and then filtered. The filtered OPB was dried at 100°C for 4 h. The surfactant modified OPB was called SOPB. The characteristics of the three raw PB, COPB, and SOPB biosorbents are summarized in Table 1.Table 1
Physicochemical properties of the biosorbents.
Properties
PB
COPB
SOPB
pHPZC
6
7.3
7.5
Surface area, m2/g
2.8
6.3
5.9
Total pore volume, cm3/g
0.004
0.012
0.012
Average pore diameter, Å
9.9
12.5
19.5Chemical structures of (a) chitosan and (b) hexadecyltrimethyl ammonium bromide surfactant.
(a)
(b)


### 2.3. Characterization of the Biosorbents

The determination of pHPZC for the biosorbents was performed in 50 mL of 0.01 M NaCL solution and placed in a closed Erlenmeyer flask. The pH was adjusted to a value between 2 and 12 by adding 0.1 M NaOH or HCl solutions. Then, 0.1 g of biosorbents was added and the final pH was measured after 48 h under agitation at 30°C. The pHPZC is the point where the curve pHfinal versus pHinitial crosses the line pHinitial=pHfinal [20].The morphology of biosorbents was analyzed by scanning electron microscope using JEOL 560LV SEM at 20 KeV with background subtraction and a summation of 60 scans. The functional surface groups present on the prepared biosorbents were determined by FTIR analysis. The FTIR spectra of the biosorbents were recorded using JASCO-410 FTIR instrument in the region 4000–400 cm−1. The specific surface area and total pore volume of the biosorbents were calculated from the adsorption data of nitrogen at 77 K by BET analyzer (Quantachrome AS1Win, Version 2.01, USA).


### 2.4. Batch Studies

All batch experiments were carried out with biosorbent samples in 250 mL conical flasks with 100 mL Cr (VI) aqueous solutions on a rotary shaker at 250 rpm/min. The concentration of chromium ion was determined spectrophotometrically at 540 nm using 1,5-diphenyl carbazide (Sigma-Aldrich, USA) as the a complexing agent [21]. The study of the pH (2–12) dependency of Cr (VI) adsorption onto PB, COPB, and SOPB was carried out in 100 mL of Cr (VI) solution with initial concentration of 100 mg/L at room temperature. The pH value was adjusted by 1 M NaOH or HCl solution.Isotherm studies were conducted by contacting 0.1 g of PB, COPB, and SOPB samples with 50 mL of Cr (VI) solution at different initial concentrations (20, 40, 60, 80, and 100 mg/L) shaking for 24 h at room temperature (30°C). The maximum removal of Cr (VI) solution was found at pH 2 for PB while for COPB and SOPB adsorbents at pH 6, respectively. The percentage dye adsorbed (%R) and the amount of residual metal ions adsorbed on biosorbents (qe, mg/g) was calculated by the following equations:
(1)%R=(C0-Ce)100C0,qe=(C0-Ce)VW,
where qe is the equilibrium adsorption capacity (mg/g) and C0 and Ce are the initial and equilibrium liquid-phase concentrations of solute in aqueous solution (mg/L), respectively. V is the liquid phase volume (L) and W is the mass of biosorbent used (g).To investigate the kinetic characteristics of the adsorption, 0.1 g of biosorbents was added to 100 mL of heavy metal solution with initial concentration of 100 mg/L and the samples were agitated for designated time periods (5, 10, 20, 30, 40, 50, and 60 min).


### 2.5. Column Studies

The fixed-bed column was made of Pyrex glass tube of 2 cm inner diameter and 30 cm height. The column performance of Cr (VI) adsorption onto biosorbents was studied at initial concentration of 100 mg/L, bed height 60 mm, and flow rate 2 mL//min. The Cr (VI) solution was pumped to the column in a down-flow direction by a peristaltic pump. Samples were collected at regular intervals at 30°C and the initial pH was 2.


### 2.6. Column Regeneration Studies

Regeneration studies help to elucidate the recycling nature of the spent biosorbent. The regeneration method was performed in the same column packed with 1 g of each of the three different biosorbents. Then the column was loaded with the 100 mg/L concentration of Cr (VI) solution at the flow rate of 2 mL/min. The column was operated until the effluent concentration matched the concentration of the loaded chromium (VI). After complete saturation of the adsorbent, distilled water was passed through the column to remove traces of undesorbed Cr (VI). Afterwards, the efficiency of 1 M HCl was tested as an eluent to desorb Cr (VI) from the three biosorbents.


## 2.1. Adsorbate Solution

The stock solution of Cr (VI) was made by dissolving 0.2827 g K2Cr2O7 (AR grade from Merck, Germany) in 1 L double distilled water. The solutions of different concentrations used in various experiments were obtained by dilution of the stock solutions.


## 2.2. Adsorbent Materials

The palm tree branches (PB) were provided from the farm of our institute. The precursor was first washed to remove dirt from its surface and then dried in an oven overnight at 100°C. The dried PB were crushed and sieved to desired mesh size of 2-3 mm. Then the dried PB were treated with 2% H2SO4 (v/v) in an incubator at 100°C for 24 h and soaked with deionized water until the solution pH was stable. Afterwards, the adsorbent was soaked in 3% NaHCO3 (w/v) till any residual acid left was removed. Finally, the sample was dried overnight in an oven at 100°C, then cooled at room temperature, and stored in a desiccator [16]. Hereafter, it was called OPB (oxidized palm tree branches).Surface modification of OPB by the chitosan and cationic surfactant was carried out as follows. About 20 g of chitosan flakes (98% acetylation degree, Figure1(a)) were immersed in 10% of acetic acid (v/v) with constant stirring for 24 h to form a homogenized whitish viscous gel. Afterwards, OPB was dipped into the gel (dose ratio of chitosan to OPB is 1 : 5) and shaken overnight at 120 rpm. This sample was then washed with deionized water and dried at 100°C. It was called COPB. The process was repeated for four times to form a thick coating of chitosan onto oxidized palm branches. In case of modification with a cationic surfactant, a 10 g of OPB was added to a solution of hexadecyltrimethyl ammonium bromide (HDTMA) of 0.197 g/L (Figure 1(b)). This concentration corresponds to the 0.5 critical micelle concentration (CMC), which is a measure of surfactant efficiency. Above the CMC, a surfactant forms a micelle that is very stable and remains in the bulk zone. For that reason, 0.5 CMC was selected to maximize the amount of surfactant adsorbed onto the OPB. The mixture was agitated for 24 h and then filtered. The filtered OPB was dried at 100°C for 4 h. The surfactant modified OPB was called SOPB. The characteristics of the three raw PB, COPB, and SOPB biosorbents are summarized in Table 1.Table 1
Physicochemical properties of the biosorbents.
Properties
PB
COPB
SOPB
pHPZC
6
7.3
7.5
Surface area, m2/g
2.8
6.3
5.9
Total pore volume, cm3/g
0.004
0.012
0.012
Average pore diameter, Å
9.9
12.5
19.5Chemical structures of (a) chitosan and (b) hexadecyltrimethyl ammonium bromide surfactant.
(a)
(b)


## 2.3. Characterization of the Biosorbents

The determination of pHPZC for the biosorbents was performed in 50 mL of 0.01 M NaCL solution and placed in a closed Erlenmeyer flask. The pH was adjusted to a value between 2 and 12 by adding 0.1 M NaOH or HCl solutions. Then, 0.1 g of biosorbents was added and the final pH was measured after 48 h under agitation at 30°C. The pHPZC is the point where the curve pHfinal versus pHinitial crosses the line pHinitial=pHfinal [20].The morphology of biosorbents was analyzed by scanning electron microscope using JEOL 560LV SEM at 20 KeV with background subtraction and a summation of 60 scans. The functional surface groups present on the prepared biosorbents were determined by FTIR analysis. The FTIR spectra of the biosorbents were recorded using JASCO-410 FTIR instrument in the region 4000–400 cm−1. The specific surface area and total pore volume of the biosorbents were calculated from the adsorption data of nitrogen at 77 K by BET analyzer (Quantachrome AS1Win, Version 2.01, USA).


## 2.4. Batch Studies

All batch experiments were carried out with biosorbent samples in 250 mL conical flasks with 100 mL Cr (VI) aqueous solutions on a rotary shaker at 250 rpm/min. The concentration of chromium ion was determined spectrophotometrically at 540 nm using 1,5-diphenyl carbazide (Sigma-Aldrich, USA) as the a complexing agent [21]. The study of the pH (2–12) dependency of Cr (VI) adsorption onto PB, COPB, and SOPB was carried out in 100 mL of Cr (VI) solution with initial concentration of 100 mg/L at room temperature. The pH value was adjusted by 1 M NaOH or HCl solution.Isotherm studies were conducted by contacting 0.1 g of PB, COPB, and SOPB samples with 50 mL of Cr (VI) solution at different initial concentrations (20, 40, 60, 80, and 100 mg/L) shaking for 24 h at room temperature (30°C). The maximum removal of Cr (VI) solution was found at pH 2 for PB while for COPB and SOPB adsorbents at pH 6, respectively. The percentage dye adsorbed (%R) and the amount of residual metal ions adsorbed on biosorbents (qe, mg/g) was calculated by the following equations:
(1)%R=(C0-Ce)100C0,qe=(C0-Ce)VW,
where qe is the equilibrium adsorption capacity (mg/g) and C0 and Ce are the initial and equilibrium liquid-phase concentrations of solute in aqueous solution (mg/L), respectively. V is the liquid phase volume (L) and W is the mass of biosorbent used (g).To investigate the kinetic characteristics of the adsorption, 0.1 g of biosorbents was added to 100 mL of heavy metal solution with initial concentration of 100 mg/L and the samples were agitated for designated time periods (5, 10, 20, 30, 40, 50, and 60 min).


## 2.5. Column Studies

The fixed-bed column was made of Pyrex glass tube of 2 cm inner diameter and 30 cm height. The column performance of Cr (VI) adsorption onto biosorbents was studied at initial concentration of 100 mg/L, bed height 60 mm, and flow rate 2 mL//min. The Cr (VI) solution was pumped to the column in a down-flow direction by a peristaltic pump. Samples were collected at regular intervals at 30°C and the initial pH was 2.


## 2.6. Column Regeneration Studies

Regeneration studies help to elucidate the recycling nature of the spent biosorbent. The regeneration method was performed in the same column packed with 1 g of each of the three different biosorbents. Then the column was loaded with the 100 mg/L concentration of Cr (VI) solution at the flow rate of 2 mL/min. The column was operated until the effluent concentration matched the concentration of the loaded chromium (VI). After complete saturation of the adsorbent, distilled water was passed through the column to remove traces of undesorbed Cr (VI). Afterwards, the efficiency of 1 M HCl was tested as an eluent to desorb Cr (VI) from the three biosorbents.


## 3. Results and Discussion

### 3.1. Surface Features of the Prepared Biosorbents

Figures2(a), 2(b), and 2(c) show the SEM micrographs for the raw palm branches (PB) and its modified biosorbents (COPB and SOPB). In Figure 2(a), the surface of PB was rough and dented, besides small pores are also evident. For COPB and SOPB biosorbents, Figures 2(b) and 2(c) show highly defined pores and cavities resulted in the modification process. This explains why the BET surface area value for PB was lower than that of COPB and SOPB (cf. Table 1).SEM images of (a) PB, (b) COPB, and (c) SOPB biosorbents.
(a)
(b)
(c)To confirm the type of functional groups, FTIR spectra of the three biosorbents are shown in Figure3. When comparing the FTIR spectra of palm tree branches precursor with the modified samples, significant shifts in absorption peaks were obtained which may be assigned to the effect of modification process on the surface of PB. As shown in Figure 3, characteristic peaks obtained for PB are 3452 cm−1 (stretching vibration of the –OH groups), 2926 and 2874 cm−1 (C–H sp3 asymmetric and symmetric stretching), 1633 cm−1 (C=O stretching of acetyl groups), 1446 cm−1 (O–H bending), and 1174 and 1090 cm−1 (alcoholic C–O) [22]. After modification, the peaks obtained for COPB biosorbent are found to be 3423 cm−1 (wide peak of the –NH groups stretching), 2926 cm−1 (C–H stretching of polymer backbone), 1628 cm−1 (N–H bending and C=O stretching of acetyl groups), 1320 cm−1 (C–N stretching), and 617–560 cm−1 (nitro compounds) [23]. This confirms that the chitosan copolymer is successfully loaded onto palm tree branches. For SOPB (Figure 3(c)), their absorption peaks were observed at 3751 cm−1 (N–H groups stretching of secondary amides). Depending on the configuration of surfactant molecules, the vibration bands of HDTMA micelles possess vibration at 2927 cm−1 that have been assigned to the symmetric and asymmetric stretching vibration of methyl and methylene groups. Bands at about 1600–1200 cm−1 have been ascribed to vibration of trimethyl ammonium quaternary group CN (CH)3+ [24].  This result indicates that the HDTMA modified the surface properties of palm tree branches.Figure 3
FTIR spectra of (a) PB, (b) COPB, and (c) SOPB biosorbents.


### 3.2. Effect of pH

The effect of initial solution pH on Cr (VI) removal by three biosorbents was performed over a range 2–12 and the studies were carried out for 24 h. Figure4 indicates that the Cr (VI) uptakes were much higher in acidic solution than those in alkaline conditions. Optimal metal removal efficiency occurs at pH 6 for COPB and SOPB while for PB the optimal is pH 2. It can be explained that the pH of the aqueous solution affects the speciation of chromium and the surface charge of the biosorbent [24, 25].Figure 4
Effect of pH on the removal of Cr (VI) by the biosorbents.Cr (VI) exists in different forms in aqueous solution and the stability of these forms is dependent on the pH of the system. In the pH range of 1 to 6 different forms of chromium ions such asCr2O72-, HCrO42-, Cr3O102-, and Cr4O132-, while HCrO42- predominates in the solution [25]. Thus Cr (VI) ions exist predominantly as HCrO42- in acidic medium and the amino groups (–NH2) of chitosan (COPB) would be in protonated cationic form (–NH3+) to a higher extent in acidic solution which cause an electrostatic interaction between the biosorbent and HCrO42- resulting in high chromium removal [26]. The same trend was also observed for SOPB as depending on the configuration of surfactant molecules, the surfactant head (trimethyl ammonium group) has a lot of positive charge in acidic medium [27]. With respect for PB, originally the palm branches have a lot of positively charged groups on the surface as depicted from FTIR, for this reason its adsorption capacity is relatively high in acidic medium.At higher pH, the decrease in removal of chromium may be explained by the dual competition of both anions [HCrO42- and OH−] to be adsorbed on the surface of the biosorbents of which OH− predominates in the solution [27, 28]. The pHPZC obtained for the three biosorbents were in the range 6–7.5. This is consistent with results reported in our study [27]. Accordingly, the biosorption of chromium ions performed at pH 2 for raw PB while for COPB and SOPB samples occurred at pH 6.


### 3.3. Adsorption Isotherms

Adsorption isotherm studies are important to determine the efficacy of adsorption. This provides a relationship between the concentration of Cr (VI) in the liquid phase and the amount of Cr (VI) adsorbed on the solid-phase when the two phases are at equilibrium. Langmuir, Freundlich, and Flory-Huggins adsorption isotherms were used in this study [29].The Langmuir model assumes that a monomolecular layer is formed when adsorption takes part without any interaction between the adsorbed molecules [30]. The Langmuir model can be represented as linear form:
(2)Ceqe=1KLqmax+1qmaxCe,
where qmax is the maximum monolayer adsorption capacity (mg/g), and KL is the Langmuir constant (g/L) related to the affinity of the binding sites. The experimental data were plotted as Ce/qe versus Ce (figure not shown). The values of Langmuir constants qmax and KL were obtained by linear regression method and are listed in Table 2.Table 2
Langmuir, Freundlich, and Flory-Huggins constants for the uptake of Cr (VI) on the biosorbents.
Adsorbents
Langmuir
Freundlich
Florry-Huggins
q
max (mg/g)
K
L (L/g)
R
2
K
F
n
R
2
K
FH
n
FH
Δ
G
0 (kJ/mol)
R
2
PB
25
0.023
0.982
0.230
1.0
0.883
0.018
3.4
−32.2
0.969
COPB
55
0.047
0.988
10
1.3
0.912
0.139
4.0
−31.8
0.978
SOPB
41.7
0.660
0.990
6.3
3.5
0.887
0.103
2.4
−31.7
0.982The biosorption isotherm of Cr (VI) on the three biosorbents is presented in Figure5. This figure shows that the biosorption of Cr (VI) within the three biosorbents corresponds to type-S adsorption isotherm in the Giles classification system [31]. In this type, the initial curvature shows that, as more sites in the adsorbent are filled, it becomes increasingly difficult for a bombarding solute molecule to find a vacant site available. This implies either that the adsorbed solute molecule is vertically oriented or that there is no strong competition from the solvent. The types of systems which give this curve do in fact fulfill these calculations. Thus they have one of the following characteristics: (a) are mono-functional, (b) have moderate intermolecular attraction, causing it to pack vertically in regular array in the adsorbed layer, and (c) meet strong competition for substrate sites from molecules of the solvent or of another species [31, 32]. The weakness of the adsorbate-adsorbent forces will cause the uptake at low concentrations to be small, but once a molecule has become adsorbed, the adsorbate-adsorbent forces will promote the adsorption of further molecules—a cooperative process. Therefore, the isotherm will become a convex to the concentration axis. A polar adsorbate of particular interest in this context is water because the dispersion contribution to its overall interaction energy is usually small compared with the polar contribution. Not surprisingly, water provided many examples of type-S isotherms. One of the palm branches is covered with a layer of adsorbed water; however, the adsorbate-adsorbent interaction would be virtually reduced to the weak dispersion energy of water with Cr (VI), so that a type-S isotherm is obtained. In a system that gives rise to a type-S isotherm, however, multiple layers are built upon some parts on the surface while a monomer is still incomplete on other parts.Figure 5
Adsorption isotherms of Cr (VI) on the biosorbents.The Freundlich isotherm is an empirical equation that assumes that the adsorption process takes place on heterogeneous surfaces and adsorption capacity is related to the concentration of Cr (VI) at equilibrium. This isotherm model is defined by the following linear form:(3)logqe=logKF+1nlogCe,
where KF is the Freundlich constant (mg/g)/(L/mg)1/n and also referred to adsorption capacity, while n is the heterogeneity factor and related to adsorption intensity. The value of n can be used also to describe the adsorption whether is linear (n=1) or a physical process (n>1) is favorable, or a chemical process (n<1). On the other hand, the value of 1/n<1 or 1/n>1 indicates a normal Langmuir isotherm and cooperative adsorption, respectively. A plot of lnqe versus lnCe gives a straight line, where the values of KF and 1/n are determined from the intercept and the slope, respectively (figure not shown). As depicted from Table 2, the Cr (VI) adsorption characteristics of the three adsorbents are better explained by the Langmuir isotherm model. Also, the adsorption capacity of Cr (VI) onto COPB shows a higher amount, and it can be implied that the amine groups (NH3+) of chitosan are largely responsible for the adsorption of chromium ions from the solution, and the interaction is chiefly electrostatic attraction in nature [33].The Florry-Huggins accounts for the degree of surface coverage characteristics of adsorbate on the biosorbent and the linear form of the equation is expressed as [34]
(4)log(θC0)=logKFH+nFHlog(1-θ),
where θ is the surface coverage of the adsorbent by adsorbate and KFH and nFH are the Florry-Huggins constants. These constants can be obtained from the plot of log (θ/C0) versus log(1-θ) (figure not shown). Table 2 showed that the apparent number nFH of metal ionoccupying sorption sites is greater for COPB than that of SOPB and PB biosorbents. The values of correlation coefficient (R2) were found to be ≤0.98. The value of KFH was used for the calculation of the spontaneity Gibbs free energy (ΔG0=-RTln(KFH), J/mol). It was found that the ΔG0 value was negative that pointed out the feasibility and spontaneity of the process.


### 3.4. Adsorption Kinetics

Figure6 shows the effect of contact time on the adsorbed amount of Cr (VI) by the raw PB, COPB, and SOPB samples, respectively. It can be seen that the kinetics of Cr (VI) removal by modified palm branches presents a shape characterized by a strong increase of the capacity of chromium removal during the first few minutes of contact between the solution and biosorbents, and followup of a slow increase until a state of equilibrium is reached.Figure 6
Effect of contact time on the adsorbed amount of Cr (VI) by the biosorbents.In order to examine the mechanism of adsorption process, pseudo-first-order (Lagergren) and pseudo-second-order kinetic models were used, respectively, as follows:(5)log(qe-qt)=logqe-k1t2.303,tqt=1k2qe2+tqe,
where qe and qt are the amounts of Cr (VI) adsorbed (mg/g) onto the three biosorbents at equilibrium and at time t (min), respectively. k1 (min−1) and k2 (g/mg·min) are the adsorption rate constants of both pseudo-first and pseudo-second order models. A plot of (5) gave straight lines with very promising correlation. However, data was more linked when examined according to pseudo-second-order model as depicted by high value correlation coefficient (R2) as shown in Table 3 and also Figure 7. In addition the calculated values from the pseudo-second order model (qe,cal) were also found to be very closed to experimental values (qe,exp). Thus experimental results support the assumption behind the model that the rate limiting step in adsorption of heavy metals is chemisorption involving valences forces through the sharing or exchange of electrons between biosorbent and metal ions. Some studies on the kinetic of Cr (VI) adsorption onto various adsorbents have also reported higher correlations for pseudo-second-order model [35].Table 3
Kinetic parameters for Cr (VI) adsorption on the biosorbents.
Adsorbents
q
e
,
exp (mg/g)
Pseudo-first order
Pseudo-second order
Intraparticlediffusion
k
1
q
e
,
cal (mg/g)
R
2
k
2
q
e
,
cal (mg/g)
R
2
K
p
C
PB
80.5
0.013
38
0.9251
6.4
×
10
-
3
82
0.997
7.2
19
COPB
96.2
0.017
79
0.7294
1.7
×
10
-
4
98
0.999
6.0
20
SOPB
89.5
0.014
76
0.9225
7.1
×
10
-
4
90
0.998
7.6
24Figure 7
Pseudo-second-order plots for the biosorbents.Further, the intraparticle diffusion model can be added to determine the rate-determining step in the adsorption process of Cr (VI) by the prepared adsorbents. The equation is given as follows:(6)qt=kpt1/2+C,
where kp, t1/2, and C are the intraparticle diffusion rate constant (mg/g·min1/2), square root of the time (min)1/2, and the intercept from the plot of qt versus t1/2 (not shown here), respectively. Intraparticle diffusion plots show multilinear, implying that more than one process affected the adsorption of chromium ions. The initial portion of the plot indicated an external mass transfer, whereas the second linear portion is due to the intraparticle or pore diffusion [36].


### 3.5. Column Adsorption Studies

The time for breakthrough appearance and the shape of the breakthrough curve very important characteristics for determining the operation and the dynamic response of an adsorption column [37–39]. The breakthrough curves show the loading behavior of metal to be removed from solution in a fixed bed and is usually expressed in terms of adsorbed metal concentration [Cad = inlet metal concentration (C0) − outlet metal concentration (Ct)] or normalized concentration defined as the ratio of effluent metal concentration to inlet metal concentration (Ct/C0) as a function of time or volume of effluent for a given bed height [37]. Effluent volume (Veff) can be calculated from the following equation:
(7)Veff=Qt,
where t and Qt are the total flow time (min) and volumetric flow rate (mL/min), respectively. The area under the breakthrough curve (A) is obtained by integrating the adsorbed concentration (Cad) versus time (t), and plot can be used to find the total adsorbed metal quantity (maximum column capacity). Total adsorbed metal quantity (qtot, mg/g) in the column for a given feed concentration and flow rate is calculated from the following equation:
(8)qtot=QtA1000=Qt1000∫t=0t=totalCaddt.The total amount of adsorbed metal ions sent to column (mtot) is calculated from the following equation [38]:
(9)mtot=C0Qt1000.Total removal is calculated from the following equation [36]:
(10)Totalremoval%=qtotmtot×100.Equilibrium metal uptake (qeq) in the column is defined by (11) as the total amount of metal adsorbed per gram of adsorbent (X) at the end of the total flow time:
(11)qeq=qtotX.The breakthrough curves of Cr (VI) adsorption by PB, COPB, and SOPB at flow rate (2 mL/min), fixed bed height of 6 cm, and initial Cr (VI) concentration of 100 mg/L are shown in Figure8. At the interval time of 80 min, the value of Ct/C0 reached 0.29, 0.39, and 0.42 for PB, SOPB and COPB, respectively. The breakthrough curve of COPB becomes steeper than the other sorbents. The probable reason behind this is the presence of deriving force which represents the amine groups, these groups are responsible for interaction between COPB and Cr (VI) ions, which leads to an increase in the total adsorbed quantities and maximum Cr (VI) uptakes and removal percent [39] (cf. Table 4).Table 4
Parameters of breakthrough curve for Cr (VI) adsorption on the biosorbents.
Adsorbents
Breakthrough parameters
C
o (mg/g)
t
tot (min)
m
tot
q
tot   (mg/g)
Total% removal
q
e (mg/g)
PB
100
355
50
24
62.4
12
COPB
100
288
58
31
55.1
19.5
SOPB
100
250
71
39
41
15.6Figure 8
Breakthrough curves for Cr (VI) adsorption onto the biosorbents (flow rate = 2mL/min, initial concentration = 100 mg/L, and pH = 2).


### 3.6. Desorption Column Studies

The possibility of adsorbent regeneration (desorption) and metal recovery was primarily studied based on the general assumption that the regeneration of adsorbent promotes economic adsorption treatment. The results showed 50% desorption for raw palm branches (PB), while 95% desorption for both COPB and SOPB with 1 M HCl solution. From this data it may be implied that the adsorbed chromium forms a film of Cr (OH)3 on the external surface of modified palm branches and this film can be easily dissolved in 1 M HCl [1].


## 3.1. Surface Features of the Prepared Biosorbents

Figures2(a), 2(b), and 2(c) show the SEM micrographs for the raw palm branches (PB) and its modified biosorbents (COPB and SOPB). In Figure 2(a), the surface of PB was rough and dented, besides small pores are also evident. For COPB and SOPB biosorbents, Figures 2(b) and 2(c) show highly defined pores and cavities resulted in the modification process. This explains why the BET surface area value for PB was lower than that of COPB and SOPB (cf. Table 1).SEM images of (a) PB, (b) COPB, and (c) SOPB biosorbents.
(a)
(b)
(c)To confirm the type of functional groups, FTIR spectra of the three biosorbents are shown in Figure3. When comparing the FTIR spectra of palm tree branches precursor with the modified samples, significant shifts in absorption peaks were obtained which may be assigned to the effect of modification process on the surface of PB. As shown in Figure 3, characteristic peaks obtained for PB are 3452 cm−1 (stretching vibration of the –OH groups), 2926 and 2874 cm−1 (C–H sp3 asymmetric and symmetric stretching), 1633 cm−1 (C=O stretching of acetyl groups), 1446 cm−1 (O–H bending), and 1174 and 1090 cm−1 (alcoholic C–O) [22]. After modification, the peaks obtained for COPB biosorbent are found to be 3423 cm−1 (wide peak of the –NH groups stretching), 2926 cm−1 (C–H stretching of polymer backbone), 1628 cm−1 (N–H bending and C=O stretching of acetyl groups), 1320 cm−1 (C–N stretching), and 617–560 cm−1 (nitro compounds) [23]. This confirms that the chitosan copolymer is successfully loaded onto palm tree branches. For SOPB (Figure 3(c)), their absorption peaks were observed at 3751 cm−1 (N–H groups stretching of secondary amides). Depending on the configuration of surfactant molecules, the vibration bands of HDTMA micelles possess vibration at 2927 cm−1 that have been assigned to the symmetric and asymmetric stretching vibration of methyl and methylene groups. Bands at about 1600–1200 cm−1 have been ascribed to vibration of trimethyl ammonium quaternary group CN (CH)3+ [24].  This result indicates that the HDTMA modified the surface properties of palm tree branches.Figure 3
FTIR spectra of (a) PB, (b) COPB, and (c) SOPB biosorbents.


## 3.2. Effect of pH

The effect of initial solution pH on Cr (VI) removal by three biosorbents was performed over a range 2–12 and the studies were carried out for 24 h. Figure4 indicates that the Cr (VI) uptakes were much higher in acidic solution than those in alkaline conditions. Optimal metal removal efficiency occurs at pH 6 for COPB and SOPB while for PB the optimal is pH 2. It can be explained that the pH of the aqueous solution affects the speciation of chromium and the surface charge of the biosorbent [24, 25].Figure 4
Effect of pH on the removal of Cr (VI) by the biosorbents.Cr (VI) exists in different forms in aqueous solution and the stability of these forms is dependent on the pH of the system. In the pH range of 1 to 6 different forms of chromium ions such asCr2O72-, HCrO42-, Cr3O102-, and Cr4O132-, while HCrO42- predominates in the solution [25]. Thus Cr (VI) ions exist predominantly as HCrO42- in acidic medium and the amino groups (–NH2) of chitosan (COPB) would be in protonated cationic form (–NH3+) to a higher extent in acidic solution which cause an electrostatic interaction between the biosorbent and HCrO42- resulting in high chromium removal [26]. The same trend was also observed for SOPB as depending on the configuration of surfactant molecules, the surfactant head (trimethyl ammonium group) has a lot of positive charge in acidic medium [27]. With respect for PB, originally the palm branches have a lot of positively charged groups on the surface as depicted from FTIR, for this reason its adsorption capacity is relatively high in acidic medium.At higher pH, the decrease in removal of chromium may be explained by the dual competition of both anions [HCrO42- and OH−] to be adsorbed on the surface of the biosorbents of which OH− predominates in the solution [27, 28]. The pHPZC obtained for the three biosorbents were in the range 6–7.5. This is consistent with results reported in our study [27]. Accordingly, the biosorption of chromium ions performed at pH 2 for raw PB while for COPB and SOPB samples occurred at pH 6.


## 3.3. Adsorption Isotherms

Adsorption isotherm studies are important to determine the efficacy of adsorption. This provides a relationship between the concentration of Cr (VI) in the liquid phase and the amount of Cr (VI) adsorbed on the solid-phase when the two phases are at equilibrium. Langmuir, Freundlich, and Flory-Huggins adsorption isotherms were used in this study [29].The Langmuir model assumes that a monomolecular layer is formed when adsorption takes part without any interaction between the adsorbed molecules [30]. The Langmuir model can be represented as linear form:
(2)Ceqe=1KLqmax+1qmaxCe,
where qmax is the maximum monolayer adsorption capacity (mg/g), and KL is the Langmuir constant (g/L) related to the affinity of the binding sites. The experimental data were plotted as Ce/qe versus Ce (figure not shown). The values of Langmuir constants qmax and KL were obtained by linear regression method and are listed in Table 2.Table 2
Langmuir, Freundlich, and Flory-Huggins constants for the uptake of Cr (VI) on the biosorbents.
Adsorbents
Langmuir
Freundlich
Florry-Huggins
q
max (mg/g)
K
L (L/g)
R
2
K
F
n
R
2
K
FH
n
FH
Δ
G
0 (kJ/mol)
R
2
PB
25
0.023
0.982
0.230
1.0
0.883
0.018
3.4
−32.2
0.969
COPB
55
0.047
0.988
10
1.3
0.912
0.139
4.0
−31.8
0.978
SOPB
41.7
0.660
0.990
6.3
3.5
0.887
0.103
2.4
−31.7
0.982The biosorption isotherm of Cr (VI) on the three biosorbents is presented in Figure5. This figure shows that the biosorption of Cr (VI) within the three biosorbents corresponds to type-S adsorption isotherm in the Giles classification system [31]. In this type, the initial curvature shows that, as more sites in the adsorbent are filled, it becomes increasingly difficult for a bombarding solute molecule to find a vacant site available. This implies either that the adsorbed solute molecule is vertically oriented or that there is no strong competition from the solvent. The types of systems which give this curve do in fact fulfill these calculations. Thus they have one of the following characteristics: (a) are mono-functional, (b) have moderate intermolecular attraction, causing it to pack vertically in regular array in the adsorbed layer, and (c) meet strong competition for substrate sites from molecules of the solvent or of another species [31, 32]. The weakness of the adsorbate-adsorbent forces will cause the uptake at low concentrations to be small, but once a molecule has become adsorbed, the adsorbate-adsorbent forces will promote the adsorption of further molecules—a cooperative process. Therefore, the isotherm will become a convex to the concentration axis. A polar adsorbate of particular interest in this context is water because the dispersion contribution to its overall interaction energy is usually small compared with the polar contribution. Not surprisingly, water provided many examples of type-S isotherms. One of the palm branches is covered with a layer of adsorbed water; however, the adsorbate-adsorbent interaction would be virtually reduced to the weak dispersion energy of water with Cr (VI), so that a type-S isotherm is obtained. In a system that gives rise to a type-S isotherm, however, multiple layers are built upon some parts on the surface while a monomer is still incomplete on other parts.Figure 5
Adsorption isotherms of Cr (VI) on the biosorbents.The Freundlich isotherm is an empirical equation that assumes that the adsorption process takes place on heterogeneous surfaces and adsorption capacity is related to the concentration of Cr (VI) at equilibrium. This isotherm model is defined by the following linear form:(3)logqe=logKF+1nlogCe,
where KF is the Freundlich constant (mg/g)/(L/mg)1/n and also referred to adsorption capacity, while n is the heterogeneity factor and related to adsorption intensity. The value of n can be used also to describe the adsorption whether is linear (n=1) or a physical process (n>1) is favorable, or a chemical process (n<1). On the other hand, the value of 1/n<1 or 1/n>1 indicates a normal Langmuir isotherm and cooperative adsorption, respectively. A plot of lnqe versus lnCe gives a straight line, where the values of KF and 1/n are determined from the intercept and the slope, respectively (figure not shown). As depicted from Table 2, the Cr (VI) adsorption characteristics of the three adsorbents are better explained by the Langmuir isotherm model. Also, the adsorption capacity of Cr (VI) onto COPB shows a higher amount, and it can be implied that the amine groups (NH3+) of chitosan are largely responsible for the adsorption of chromium ions from the solution, and the interaction is chiefly electrostatic attraction in nature [33].The Florry-Huggins accounts for the degree of surface coverage characteristics of adsorbate on the biosorbent and the linear form of the equation is expressed as [34]
(4)log(θC0)=logKFH+nFHlog(1-θ),
where θ is the surface coverage of the adsorbent by adsorbate and KFH and nFH are the Florry-Huggins constants. These constants can be obtained from the plot of log (θ/C0) versus log(1-θ) (figure not shown). Table 2 showed that the apparent number nFH of metal ionoccupying sorption sites is greater for COPB than that of SOPB and PB biosorbents. The values of correlation coefficient (R2) were found to be ≤0.98. The value of KFH was used for the calculation of the spontaneity Gibbs free energy (ΔG0=-RTln(KFH), J/mol). It was found that the ΔG0 value was negative that pointed out the feasibility and spontaneity of the process.


## 3.4. Adsorption Kinetics

Figure6 shows the effect of contact time on the adsorbed amount of Cr (VI) by the raw PB, COPB, and SOPB samples, respectively. It can be seen that the kinetics of Cr (VI) removal by modified palm branches presents a shape characterized by a strong increase of the capacity of chromium removal during the first few minutes of contact between the solution and biosorbents, and followup of a slow increase until a state of equilibrium is reached.Figure 6
Effect of contact time on the adsorbed amount of Cr (VI) by the biosorbents.In order to examine the mechanism of adsorption process, pseudo-first-order (Lagergren) and pseudo-second-order kinetic models were used, respectively, as follows:(5)log(qe-qt)=logqe-k1t2.303,tqt=1k2qe2+tqe,
where qe and qt are the amounts of Cr (VI) adsorbed (mg/g) onto the three biosorbents at equilibrium and at time t (min), respectively. k1 (min−1) and k2 (g/mg·min) are the adsorption rate constants of both pseudo-first and pseudo-second order models. A plot of (5) gave straight lines with very promising correlation. However, data was more linked when examined according to pseudo-second-order model as depicted by high value correlation coefficient (R2) as shown in Table 3 and also Figure 7. In addition the calculated values from the pseudo-second order model (qe,cal) were also found to be very closed to experimental values (qe,exp). Thus experimental results support the assumption behind the model that the rate limiting step in adsorption of heavy metals is chemisorption involving valences forces through the sharing or exchange of electrons between biosorbent and metal ions. Some studies on the kinetic of Cr (VI) adsorption onto various adsorbents have also reported higher correlations for pseudo-second-order model [35].Table 3
Kinetic parameters for Cr (VI) adsorption on the biosorbents.
Adsorbents
q
e
,
exp (mg/g)
Pseudo-first order
Pseudo-second order
Intraparticlediffusion
k
1
q
e
,
cal (mg/g)
R
2
k
2
q
e
,
cal (mg/g)
R
2
K
p
C
PB
80.5
0.013
38
0.9251
6.4
×
10
-
3
82
0.997
7.2
19
COPB
96.2
0.017
79
0.7294
1.7
×
10
-
4
98
0.999
6.0
20
SOPB
89.5
0.014
76
0.9225
7.1
×
10
-
4
90
0.998
7.6
24Figure 7
Pseudo-second-order plots for the biosorbents.Further, the intraparticle diffusion model can be added to determine the rate-determining step in the adsorption process of Cr (VI) by the prepared adsorbents. The equation is given as follows:(6)qt=kpt1/2+C,
where kp, t1/2, and C are the intraparticle diffusion rate constant (mg/g·min1/2), square root of the time (min)1/2, and the intercept from the plot of qt versus t1/2 (not shown here), respectively. Intraparticle diffusion plots show multilinear, implying that more than one process affected the adsorption of chromium ions. The initial portion of the plot indicated an external mass transfer, whereas the second linear portion is due to the intraparticle or pore diffusion [36].


## 3.5. Column Adsorption Studies

The time for breakthrough appearance and the shape of the breakthrough curve very important characteristics for determining the operation and the dynamic response of an adsorption column [37–39]. The breakthrough curves show the loading behavior of metal to be removed from solution in a fixed bed and is usually expressed in terms of adsorbed metal concentration [Cad = inlet metal concentration (C0) − outlet metal concentration (Ct)] or normalized concentration defined as the ratio of effluent metal concentration to inlet metal concentration (Ct/C0) as a function of time or volume of effluent for a given bed height [37]. Effluent volume (Veff) can be calculated from the following equation:
(7)Veff=Qt,
where t and Qt are the total flow time (min) and volumetric flow rate (mL/min), respectively. The area under the breakthrough curve (A) is obtained by integrating the adsorbed concentration (Cad) versus time (t), and plot can be used to find the total adsorbed metal quantity (maximum column capacity). Total adsorbed metal quantity (qtot, mg/g) in the column for a given feed concentration and flow rate is calculated from the following equation:
(8)qtot=QtA1000=Qt1000∫t=0t=totalCaddt.The total amount of adsorbed metal ions sent to column (mtot) is calculated from the following equation [38]:
(9)mtot=C0Qt1000.Total removal is calculated from the following equation [36]:
(10)Totalremoval%=qtotmtot×100.Equilibrium metal uptake (qeq) in the column is defined by (11) as the total amount of metal adsorbed per gram of adsorbent (X) at the end of the total flow time:
(11)qeq=qtotX.The breakthrough curves of Cr (VI) adsorption by PB, COPB, and SOPB at flow rate (2 mL/min), fixed bed height of 6 cm, and initial Cr (VI) concentration of 100 mg/L are shown in Figure8. At the interval time of 80 min, the value of Ct/C0 reached 0.29, 0.39, and 0.42 for PB, SOPB and COPB, respectively. The breakthrough curve of COPB becomes steeper than the other sorbents. The probable reason behind this is the presence of deriving force which represents the amine groups, these groups are responsible for interaction between COPB and Cr (VI) ions, which leads to an increase in the total adsorbed quantities and maximum Cr (VI) uptakes and removal percent [39] (cf. Table 4).Table 4
Parameters of breakthrough curve for Cr (VI) adsorption on the biosorbents.
Adsorbents
Breakthrough parameters
C
o (mg/g)
t
tot (min)
m
tot
q
tot   (mg/g)
Total% removal
q
e (mg/g)
PB
100
355
50
24
62.4
12
COPB
100
288
58
31
55.1
19.5
SOPB
100
250
71
39
41
15.6Figure 8
Breakthrough curves for Cr (VI) adsorption onto the biosorbents (flow rate = 2mL/min, initial concentration = 100 mg/L, and pH = 2).


## 3.6. Desorption Column Studies

The possibility of adsorbent regeneration (desorption) and metal recovery was primarily studied based on the general assumption that the regeneration of adsorbent promotes economic adsorption treatment. The results showed 50% desorption for raw palm branches (PB), while 95% desorption for both COPB and SOPB with 1 M HCl solution. From this data it may be implied that the adsorbed chromium forms a film of Cr (OH)3 on the external surface of modified palm branches and this film can be easily dissolved in 1 M HCl [1].


## 4. Conclusions

This study investigated the biosorption characteristics of Cr (VI) on raw and modified palm branches (PB). Adsorption of hexavalent chromium ion is found to be effective in the lower pH range. Equilibrium isotherm data were fitted using Langmuir and Flory-Huggins. Kinetic study showed that the pseudo-second-order kinetic model is appropriate and film diffusion might be involved in the sorption process. The chitosan coated palm branches (COPB) appear to be technically feasible and eco-friendly and with high efficacy compared to the other adsorbents. The adsorbents could be regenerated by using hydrochloric acid and therefore can be reused. These adsorbents could be good candidate for adsorption of Cr (VI) ions.



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*Source: 159712-2013-07-07.xml*