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Adsorption of Chromium from an Aqueous Solution onto Chitosan Beads Modified with Sodium Dodecyl Sulfate (SDS)

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Naoki Kano, Zou Ming, David Eva Vanessa Anak and Muhammad Nabil Md Sari

Submitted: February 15th, 2022Reviewed: March 1st, 2022Published: April 17th, 2022

DOI: 10.5772/intechopen.104093

IntechOpen
Sorption - From Fundamentals to ApplicationsEdited by George Kyzas

From the Edited Volume

Sorption - From Fundamentals to Applications [Working Title]

Prof. George Z. Kyzas

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Abstract

The goal of this research is to make chitosan beads that have been treated with sodium dodecyl sulfate (SDS) to remove chromium (Cr) from an aqueous solution effectively. The successful synthesis of the SDS-chitosan was proven through characterization, which were carried out using by scanning electron microscopy–energy dispersive X–ray spectroscopy (SEM-EDS), Fourier transform-infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The adsorption of Cr on the SDS material was investigated by varying experimental conditions such as pH, contact time and adsorbent dosage. The maximum adsorption capacity of SDS-chitosan for Cr(III) was estimated to be 3.42 mg·g−1 and 3.23 mg·g−1 for Cr(VI). Based on the results of adsorption kinetics and isothermal models, the adsorption process conform to the pseudo-second-order and Langmuir isotherm models. This indicates that the adsorption of Cr on SDS-chitosan is mainly dominated by chemical adsorption and monolayer reaction. In addition, according to thermodynamic analyses, the adsorption of Cr is an endothermic reaction. These results show that the new adsorbent has obvious application prospect for removing Cr.

Keywords

  • chitosan beads
  • sodium dodecyl sulfate (SDS)
  • chromium
  • adsorption
  • adsorption isotherms
  • adsorption kinetics

1. Introduction

Due to the rapidly growing number of manufacturing industries, toxic metal contamination in aquatic environments has gotten a lot of attention. Among the contaminants, heavy metals are targeted for major environmental concern because they are non-biodegradable, and they cannot be decomposed or metabolized [1]. Several metals cause serious health and environmental problems, and chromium (Cr) compounds are one of the most toxic contaminants in wastewater due to their high solubility and toxicity, as well as their free transferability [2].

Cr has been widely applied in a variety of industrial activities due to its excellent properties, including electroplating, leather tanning, nuclear power plants, and textile industries [3, 4]. Furthermore, it can be used for anodizing, corrosion control, and chemical manufacturing [5, 6, 7]. In a natural environment, Cr usually exists in two stable oxidation states: trivalent Cr(III) and hexavalent Cr(VI). Meanwhile, other oxidation states are not stable in aerated aqueous media [8]. Specifically for Cr(VI), it may exist in the form of CrO42− or HCrO4 in a natural aqueous environment, whereas Cr(III) is inclined to form [Cr(H2O)6]3+, Cr(H2O)5(OH)2+, Cr(H2O)4(OH)2+, or Cr(III) organic complexes.

Given its considerable risk to biological systems, many studies have focused on the removal of Cr(VI). Cr(VI) is highly toxic, carcinogenic, and mutagenic [8, 9]. Adverse health effects have been linked to Cr(VI) exposure, such as bronchitis, liver damage, kidney damage, brain damage, and even lung cancer. On the other hand, Cr(III) is the most stable form in reducing conditions and it exists as cationic species Cr(OH)2+ and Cr(OH)2+, with the first or second hydrolysis products dominating at pH values ranging from 4 to 8. Although Cr(III) is an essential microelement for the effective maintenance of mammal’s glucose, lipid, and protein metabolism [10], high doses of Cr(III) may cause negative consequences to the environment. Moreover, there are currently just a few articles on the adsorption of Cr(III). Therefore, the development of a recovery method for this metal (both Cr(III) and Cr(VI)) is significant from an environmental aspect.

Ion exchange, precipitation, ultrafiltration, reverse osmosis, and electro dialysis are one of the physical and chemical technologies that have been reported for the removal of heavy metals [11]. However, these procedures have some drawbacks, such as a high consumption of reagents and energy, low selectivity, high operational costs, and difficult further treatment due to toxic sludge production [12]. Adsorption is an effective method for removing metallic ions from aqueous solutions [10, 13]; and biological adsorption (biosorption) is one of the most environmentally friendly, cost-effective, recyclable, and technically simple methods [14, 15].

Among the many biosorbents available, chitosan can be an excellent biosorbent for metals because its amine (-NH2) and hydroxyl (-OH) groups may serve as coordination sites to form complexes with various heavy metal ions [16]. Chitosan, whose full chemical name is (1,4)-2-amino-2- deoxy-β-D-glucose, has been proven to be particularly effective as a biosorbent for the recovery of several toxic metals, including mercury (Hg), uranium (U), molybdenum (Mo), vanadium (V), and platinum (Pt) [17, 18, 19]. It can be employed as an environmentally friendly adsorbent because it is cost effective, and it does not result in secondary pollution. Chitosan is a polymer that is made via alkaline deacetylation of chitin, which comes from cellulose, the most abundant biopolymer. It can be acquired from the shells of seafood, such as prawns, crabs, and lobsters [20]. The biopolymer has a high nitrogen content, which is present in the form of amine groups, free amino groups, and hydroxyl groups, all of which are responsible for metal ion binding through chelation mechanisms [21].

Despite the uses of chitosan in the removal of various pollutants have been adequately reviewed [22], on the other hand, it has some defects, including notable swelling in aqueous media and nonporous structures, resulting in a very low surface area [23]. Therefore, a variety of chemical modifications can be used to produce chitosan derivatives that improve the removal efficiency of the heavy metal [24]. For example, to increase the number of exposed active sites, several chemical or physical modifications can be adopted [25, 26]. Moreover, since silicon dioxide has numerous properties, including rigid configuration, porosity, and large surface area, it can be employed to counteract the defects of chitosan. In addition, modified silicon dioxide has been produced through a graft between silanol groups and ligands [27, 28, 29]. In prior work, we synthesized a hybrid membrane of carboxymethyl chitosan and silicon dioxide as adsorbents for the removal of Cr(VI) [30]. Furthermore, we used epichlorohydrin (EP) and glutaraldehyde (GA) as cross-linked agents in an adsorption experiment of chromate ions onto cross- linked chitosan [31].

In this work, we evaluated the adsorption of chitosan modified with sodium dodecyl sulfate (SDS) as part of the adsorption study of Cr using modified chitosan. SDS-modified chitosan beads have been reported to be effective for removing cationic dyes [32]. Adsorption on surfaces is enabled by the metal ion strength and the presence of key functional groups on the polymer chain [33, 34, 35]. The particle aggregation via a bridging structure can be described as a two-step pathway: (1) initial chain adsorption and bridging, followed by (2) floc maturation/ reconfiguration. Before the interparticle connection occurs, the chain of SDS must be adsorbed on a chitosan surface [36]. Furthermore, chitosan modified with SDS has recently been used for the removal of heavy metals, such as cadmium [37, 38]. However, the use of SDS-modified chitosan as a Cr adsorbent, using different initial concentrations of SDS to optimize the adsorbent, has rarely been investigated. The objective of the present research is to determine the efficacy of SDS-modified chitosan beads as a sorbent for Cr(III) and Cr(VI) for future practical applications, as well as to understand the adsorption mechanism. After the characterization of SDS-chitosan by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and Fourier transform-infrared spectroscopy (FT-IR), batch experiments with SDS-modified chitosan beads were carried out to optimize the parameters and to obtain the maximum removal of Cr(III) and Cr(VI). In this study, we examined the effects of several parameters, including solution pH, contact time, adsorbent dosage, and initial concentration.

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2. Materials and methods

Chemical reagents, such as chitosan and sodium dodecyl sulfate (SDS; M.W.: 288.372 g/mol), were purchased from Tokyo Chemical Industry Co., Inc. Acetic acid, NaOH, HNO3, NaSO4, ethylenediaminetetraacetic acid disodium salt dihydrate, and toluene, were purchased from Kanto Chemical Industry Co., Inc. All reagents used were of analytical grade. During the whole working process in this study, the water (>18.2MΩ) treated by the ultrapure water system (RFU 424TA, Advantech Aquarius) was employed. K2CrO7 standard solution (1000 mg·L−1 from Kanto Chemical Co., Inc.) was diluted and used to prepare the Cr standard solution for calibration.

2.1 Synthesis of the adsorbent

In this study, chitosan powder with a molecular weight (50–190 kDa) and degree of deacetylation (> 80%) was used. After drying, the viscosity of chitosan was 20 to 100 mPa·s (in 0.5%Acetic acid soln., 20°C). Firstly, 1.5 g of chitosan was placed in acetic acid solution (2.0%), and the solution was mixed for 24 h. The chitosan-gel was prepared by dropping the above chitosan solution into 200 mL of 0.20 mol·L−1 NaOH. Consequently, the obtained gel was rinsed with ultrapure water until its pH reached 7 after stirring for 24 h. Secondly, 200 tablets of chitosan-gel beads were placed in 100 mL of SDS solution (including the fixed concentration of SDS), and then left for five days. Thus, chitosan-gel beads modified with SDS were obtained, and finally, they were dried at 60°C overnight for use as an adsorbent. The synthesis procedure for the SDS-chitosan beads is demonstrated in Figure 1. It is considered that the prepared adsorbent has a bilayer of SDS over the surface of pure chitosan beads. This bilayer can have a higher ion capturing capacity [37].

Figure 1.

Procedure employed for the synthesis of sodium dodecyl sulfate (SDS)-chitosan beads.

2.2 Characterization of the adsorbent

The diameter of chitosan beads was measured to be about 0.5–2 mm (judging from 200 beads as representative chitosan beads). After weighing 200 hydrogel beads, the dry weight per chitosan bead was estimated to be 3.7 × 10−4 g, which suggests that the adsorbent contained 98% moisture. In order to determine the physicochemical properties of pristine and modified chitosan, several characterization methods have been employed. FT-IR spectra of the samples were recorded in the range of 4000–500 cm−1 with a JASCO Japan FTIR-4200 spectrophotometer using KBr pellet pressing method. The surface morphology and element distribution of the chitosan beads before and after the adsorption of Cr were observed using SEM-EDS (JEOL Japan: JCM-6000 with JED-2300). X-ray Photoelectron Spectroscopy (XPS, Thermo Scientific Center: K-Alpha) was also used to assess the surface chemistry properties of SDS-modified chitosan beads.

2.3 Adsorption experiments

The beads were put into contact with 50 mL of an aqueous solution containing Cr(III) or Cr (VI) ion with a known initial concentration. The pH of each solution was adjusted using 0.1 mol/L−1 NaOH and 0.1 mol/L−1 HNO3. The flask was then placed into an automatic shaker. In the adsorption experiments, the parameters were varied such as the pH range of 17, contact time of 172 h, SDS initial concentration of 109000 mgL−1, adsorbent dosage of 0.010.06 gL−3, temperature of 288318 K, and initial Cr(III) or Cr(VI) concentration of 0.13.0 mgL−1. The concentrations of Cr in the filtrate were measured by ICP-MS (Thermo Scientific Center: X-series II). The adsorption capacities of the chitosan beads modified with SDS were calculated by the following Eq. [9]:

qe=CiCe×V/mE1

where qerepresents the adsorption capacity at the equilibrium (mgg−1), Ciand Ceare the initial and equilibrium concentrations of Cr in the batch system, respectively (mgL−1), Vis the volume of the solution (L), and mis the weight of the adsorbents (g).

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3. Result and discussion

3.1 Characterization of materials

3.1.1 SEM-EDS micrographs

Figure 2 shows the SEM-EDS images of the chitosan beads and SDS-chitosan. These images showed the unevenness of the surface. This was most likely due to the release of water from the adsorbent during the drying process of chitosan. In addition to the irregularities, SDS-chitosan showed a mesh-like pattern. This was thought to be due to SDS modified on the chitosan surface. In particular, the modified chitosan beads with a high SDS concentration stood out compared to the other beads. As the irregularities on the adsorbent’s surface were considered, the adsorption proceeded in two ways: physical and chemical adsorption. From the mapping images, it was proven that Cr ions were adsorbed onto the adsorbent surface.

Figure 2.

Scanning electron microscopy (SEM) images of chitosan beads (a) and SDS-chitosan (b), and mapping images of SDS-chitosan after the adsorption of Cr(III).

3.1.2 FT-IR and XPS spectra

The surface functional groups and the chemical compositions of the modified SDS-chitosan beads were identified by FTIR and XPS analysis, respectively. The FT-IR results of chitosan and cross-linked chitosan are shown in our previous work [39]. Figure 3 shows the FT-IR results of SDS-chitosan and the chitosan beads in this study, with the peaks of SDS-chitosan and the chitosan beads can clearly visible. It is apparent from this figure that the main peaks common to each adsorbent were due to the -OH group at 3400–3500 cm−1 and the aliphatic methylene group at 2871 cm−1. The amine and ether groups are shown by wide peaks at 1560–1640 cm−1 and 1110 cm−1, respectively. For SDS-chitosan, the peak at 1248 cm−1 was characteristic of the asymmetrical vibration of the C-O-S group, confirming that the prepared adsorbent was a composite of SDS and chitosan.

Figure 3.

Fourier transform-infrared spectroscopy (FT-IR) spectra of chitosan and SDS-chitosan.

The chitosan beads with varying initial loading concentrations of SDS were analyzed using XPS, as shown in Figure 4. The C1s spectra of these samples displayed peaks at 284.5, 286.5, and 288.5 eV, corresponding to C-C, C-O, and C=O bonds, respectively. The S2p spectra of SDS600 and 6000-chitosan displayed peaks at 169 eV. The S that seemed to be derived from SDS was also not detected at SDS concentrations of 0 and 100 mg/L but was detected at concentrations of 600 and 6000 mg/L.

Figure 4.

X-ray photoelectron spectroscopy (XPS) spectra of chitosan (a), SDS100-chitosan (b), SDS600-chitosan (c), and SDS6000-chitosan (d) beads.

3.2 Adsorption experiment

3.2.1 Effect of the initial SDS concentration

To determine the optimum initial loading concentration of SDS for Cr(III) removal, the chitosan beads were modified with SDS solutions ranging from 10 to 9000 mg/L. The adsorption experiments were performed under the following conditions: an initial concentration of Cr(III) of 1 mg/L, a contact time of 2 days, and adsorbent dosage of 0.05 g, a pH of 4, and a temperature of 25°C. In the meantime, initial SDS concentrations were varied from 10 to 6000 mg/L for Cr(VI) removal. The experiment was performed under the following conditions: adsorbent dose of 0.05 g, initial concentration of Cr(VI) of 1 mg/L, pH of 4, and contact time of three days.

The results of Cr (III) adsorption are shown in Figure 5. The adsorption capacity continuously increased within 6000 mg/L as the SDS concentration increases, but the adsorption amount was almost constant at further higher concentrations. Thus, 6000 mg/L was the optimum initial loading SDS concentration for Cr(III). On the other hand, the adsorption capacity of SDS for the removal of Cr(VI) increased with an increase of the SDS concentration from 10 to 40 mg/L, and after that it decreased as shown in Figure 6. The maximum capacity for Cr(VI) was obtained at the initial SDS concentration of 40 mg/L.

Figure 5.

Effect of the initial SDS concentrations on the adsorption of Cr(III).

Figure 6.

Adsorption of Cr(VI) for different initial SDS concentrations (10–6000 mg/L). (a) Effect of initial SDS concentrations (10–6000 mg/L) on the adsorption of chromium (Cr) (VI). (b) Effect of initial SDS concentrations (10–1000 mg/L) on the adsorption of Cr(VI).

3.2.2 Effect of pH

The effect of pH on Cr(III) adsorption by SDS-chitosan was investigated in the pH range of 4–7. Other parameters were set as the following: the contact time was 24 h, the temperature was 25°C, the adsorbent dosage was 0.4 mg/L, and the initial Cr(III) concentration was 1 mg/L. The results are shown in Figure 7. As shown in Figure 7, the adsorption capacity of Cr(III) increased with the increase of pH from 4 to 7. However, it was found that Cr (III) precipitated as Cr (OH)3 at the pH value in 6 and 7. Thus, a pH of 6 and 7 were not suitable for the adsorption experiments and optimized pH value was 4.

Figure 7.

Effect of pH on the adsorption of Cr(III) by SDS-chitosan.

As for the adsorption experiment of Cr(VI), pH was set in the range of 4 to 10 (Figure 8). In this experiment, the shaking time was 24 h, the temperature was 25°C, and the dose of adsorbent was 0.02 g/L. As shown in Figure 8, pH affects Cr(VI) adsorption on SDS-modified chitosan beads. The acidity of the solution brought a significant effect on the adsorption of Cr(VI) on SDS-modified chitosan beads where the amino groups of the chitosan were protonated. Cr(VI) ions were most effectively adsorbed at pH 4–5, which may be related to changes in surface charge on the adsorbent. As pH increased above pH 5, the uptake decreased.

Figure 8.

Effect of pH on the adsorption of Cr(VI) onto the SDS-modified chitosan beads.

3.2.3 Effect of contact time

The influence of contact time on the adsorption of Cr(III) by SDS-chitosan was investigated. The experiment was conducted under the following conditions: a pH of 4, a temperature of 25°C, an adsorbent dosage of 0.05 g, and an initial concentration of Cr(III) of 1 mg/L. Figure 9 shows that the adsorption capacity of SDS-chitosan for Cr (III) increased sharply within the first 24 h, and continued until the contact time reached 48 h. Thus, 48 h was selected as the optimized contact time.

Figure 9.

Effect of contact time on the adsorption of Cr(III) by SDS-chitosan.

We have also studied how the contact time affects the adsorption capacities of SDS-modified chitosan beads towards Cr(VI) with varying contact times from 1 to 96 h (Figure 10). In this experiment, the concentration of Cr(VI) was set as 1 mg/L with the dose of 0.05 g at a temperature of 25°C adjusted pH to 4.

Figure 10.

Effect of the contact time on the adsorption of Cr(VI) on the SDS-modified chitosan beads.

The adsorption capacity of SDS-chitosan beads for Cr(VI) increased sharply within the first 24 h, which may be attributable to the availability of the sites on the surface of the adsorbent. It is suggested that a concentration gradient is present in both the adsorbent and adsorbate in the solution [40]. Then, adsorption reached equilibrium at 72 h, and afterwards, there was no appreciable increase (Figure 10). Hence for further studies, the optimized contact time was taken as 72 h.

3.2.4 Effect of the adsorbent dosage

The adsorbent dosage is an important factor that affects the adsorption capacity. To study the effect of adsorbent dosage on the adsorption of Cr(III), adsorption experiments were conducted at a pH of 4, a temperature of 25°C, a contact time of 24 h, and an initial concentration of Cr(III) of 1 mg/L. The results are shown in Figure 11. The adsorption rate increased as the adsorbent dosage increased, reaching approximately 70% at 0.8 mg/L, after that, there was no appreciable increase. Thus, 1 mg/L was selected as the optimized adsorbent dosage.

Figure 11.

Effect of the adsorbent dosage on the adsorption of Cr(III) by SDS-chitosan.

Meanwhile for the adsorption experiment of Cr (VI), 0.8 mg/L was regarded as the optimum dosage. The experiments were performed by varying the dosage (from 0.4 to 1.0 mg/L) and keeping all other parameters constant (temperature: 25°C; pH: 4; contact time: 24 h; initial concentration: 1.0 mg/L). The results are shown in Figure 12.

Figure 12.

Effect of the adsorbent dosage on the adsorption of Cr(VI) on the SDS-modified chitosan beads.

3.2.5 Effect of competitive ions

In this study, adsorption experiments of Cr(III) were conducted in the presence of several competitive ions with different concentrations (0, 50, 100, and 200 mg/L) of Na+, Ca2+, Ba2+, K+, Mg2+, and Sr2+. The initial concentration of Cr(III) was set to 1.0 mg/L, the pH was 4, the temperature was 25°C, the contact time was 24 h, and the adsorbent dosage was 1.0 mg/L. The effect of competitive ions on the adsorption of Cr(III) is shown in Figure 13. It was confirmed that the adsorption capacity of Cr(III) did not decrease at all even in the presence of other metal ions.

Figure 13.

The effect of competitive ions on the adsorption of Cr(III).

Conversely, it was found that Cr(VI) adsorption was inhibited when the concentrations of coexisting ions were high. The effect of competitive anions on the adsorption of Cr(VI) is shown in Figure 14. In this study, the initial concentration of Cr(VI) was fixed to 1 mg·L−1. These counter ions were tested collectively, and all the ions were included at 50, 100, or 200 mg/L in solution. From this figure, the removal of Cr(VI) was remarkably reduced under the presence of common ions at above 50 mg·L−1 (i.e., 50 times the Cr(VI) concentration or more), although no substantial decrease was observed when the concentration of each common ion was less than 10 mg·L−1 in our previous preliminary experiments.

Figure 14.

Effect of competitive anions on the adsorption of Cr(VI) on the SDS-modified chitosan beads.

3.3 Adsorption isotherms

Adsorption isotherms describe the interactive process between the adsorbents and adsorbates in aqueous medium at the attained saturation point. Adsorption isotherms of Cr(III) and Cr(VI) on SDS-chitosan were identified with different initial concentrations from 0.01 to 2 mg/L under optimized conditions for the pH (4), contact time (48 h for Cr(III), 72 h for Cr(VI)), and dosage of the adsorbent (1 mg/L for Cr(III), 0.8 mg/L for Cr(VI)). The adsorption of Cr on SDS-chitosan was evaluated using typical adsorption isotherms, the Langmuir and Freundlich models (Figure 15 (a) and (b)).

Figure 15.

(a).Adsorption isotherms of Cr(III) using SDS-chitosan.(b).Adsorption isotherms of Cr(VI) using SDS-chitosan.

The adsorption data obtained for Cr(III) using SDS-chitosan were analyzed by Langmuir and Freundlich equations (Figure 16 (a) and (b)).

Figure 16.

(a).Langmuir isotherm of Cr(III) adsorption onto SDS-chitosan.(b).Langmuir isotherm of Cr(VI) adsorption onto SDS-chitosan.

Langmuir equation:

Ceqe=Ceqmax+1KLqmaxE2

Freundlich equation:

lgqe=lgKF+1/nlgCeE3

where Ceand qeare the concentration of Cr at the equilibrium (mgL−1) and the amount of adsorption of Cr(III) at the equilibrium (mgg−1), respectively, qmaxis the maximum adsorption capacity on the surface of the chitosan bead (mgg−1), KLis the Langmuir constant related to the adsorption strength or intensity (Lmg−1), KFis the Freundlich constant and 1/nindicates the adsorption intensity of the system [41, 42].

The Langmuir isotherm model assumes that a monolayer adsorption occurs on the surface of an adsorbent. The slope of the linearized Langmuir isotherm can be used to explain the type of sorption using the Hall separation factor (RL), which is favorable (0<RL<1), unfavorable (RL<0), linear (RL=1) or irreversible (RL=0) [40]. The Freundlich isotherm model assumes a multilayer adsorption process without considering the adsorption saturation, which occurs on the multilayer heterogeneous surface. The value of the adsorption intensity of the system1/ncan be used to judge the difficulty of the adsorption process: irreversible 1/n=0, favorable 0<1/n<1, or unfavorable 1/n>1[43]. Table 1 displays the correlation coefficient (R2) of these isotherms for Cr(III) on SDS-chitosan along with other relevant parameters.

Metal
T(K)
Langmuir IsothermFreundlich Isotherm
Qmax(mg/g)RLR2KF(mg/g)1/nR2
Cr(III)2983.421.15 × 10−30.8421.411.210.820
Cr(VI)2983.230.31 × 10−40.9603.010.7000.921

Table 1.

Isotherm parameters for Cr(III) and Cr(VI) adsorption onto SDS-chitosan.

As shown in Figure 16 (a), (b), and Table 1, it was revealed that the R2values for Cr(III) and Cr(VI) were relatively large and a favorable adsorption of Cr(III) and Cr(VI) on SDS-chitosan was presented. Compared to the Freundlich isotherm, the Langmuir isotherm had a larger R2value, which suggests that the adsorption of Cr(III) and Cr(VI) on SDS-chitosan mainly occurred by a monolayer reaction. The maximum adsorption capacity (qmax) calculated from the Langmuir model was 3.42 mg/g for Cr(III) and 3.23 mg/g for Cr(VI).

3.4 Kinetic studies

The rate-controlling steps of the adsorption system are essential to survey the mechanism of Cr(III) and Cr(VI). Kinetic studies were conducted to explain the adsorption mechanism of Cr(III) and Cr(VI) ions onto the SDS-chitosan beads. The effects of contact time on the kinetics of Cr(III) and Cr(VI) adsorption by SDS-chitosan adsorbent are displayed in Figure 17 (a) and (b). According to Figure 17 (a), the removal of Cr(III) from SDS-chitosan increased sharply in the initial 6 h, indicating that the uptake of Cr(III) was primarily caused by chemical sorption. As shown in Figure 17 (b), the adsorption of Cr(VI) by SDS-chitosan beads increased significantly within 96 hours, although the increase continued at a slower rate for 48 h. The rapid adsorption within the initial 24 h indicated that Cr(VI) uptake was mainly dominated by chemical sorption or surface complexation.

Figure 17.

(a).Pseudo-first-order and pseudo-second-order Cr(III) adsorption on SDS-chitosan.(b).Pseudo-first-order and pseudo-second-order Cr(VI) adsorption on SDS-chitosan.

To investigate the mechanism of adsorption of Cr(III) and Cr(VI) on SDS-chitosan, fitting was determined according to the pseudo-first-order (Figure 18 (a) and (b)), pseudo-second-order (Figure 19 (a) and (b)) and intraparticle diffusion kinetic models (Figure 20 (a) and (b)). The equations of these models are given by:

Figure 18.

(a).Pseudo-first-order linear kinetic model of Cr(III) adsorption by SDS-chitosan.(b).Pseudo-first-order linear kinetic model of Cr(VI) adsorption by SDS-chitosan.

Figure 19.

(a).Pseudo-second-order linear kinetic model of Cr(III) adsorption by SDS-chitosan.(b).Pseudo-second-order linear kinetic model of Cr(VI)adsorption by SDS-chitosan.

Figure 20.

(a).Intraparticle diffusion kinetic model of Cr(III) absorption by SDS-chitosan.(b).Intraparticle diffusion kinetic model of Cr(VI) absorption by SDS-chitosan.

Pseudo-first-order model:

ln=lnqek1tE4

Pseudo-second-order model:

tqt=1k2qe2+tqeE5

Intraparticle diffusion model:

qt=Kidt1/2E6

where qeand qtare the adsorption capacities of Cr using the SDS-chitosan beads at the equilibrium and time, t, respectively (mgg−1), k1is the rate constant of the pseudo-first-order adsorption (h−1), k2is the rate constant of the pseudo-second-order adsorption (gmg−1h−1) and Kidis the rate constant of intraparticle diffusion (mgg−1h−1/2). [44, 45, 46].

The sorption kinetic parameters including k1,k2,Kid,qeand the correlation coefficients R2are presented in Table 2. From Table 2, the correlation coefficient R2for the pseudo-second-order model was higher than that for the pseudo-first-order and intraparticle diffusion models. Based on these results, the sorption kinetics of the Cr(III) ion on SDS-chitosan can be described by the pseudo-second-order model, indicating that the chemical adsorption may be the rate-limiting step of the process. Assuming adsorbent particle is sphere of radius ‘a’, the diffusion follows Fick’s law that was applied to confirm it. In the law, the relationship between adsorption capacity and time given by the following Equations [44].

Metalqe(mg/g)Pseudo-first-order modelPseudo-second-order modelIntraparticle Diffusion
qe(mg/g)k1(h−1)R2qe(mg/g)k2(g·mg−1·h−1)R2KidR2
Cr(III)0.691.050.220.881.260.070.980.170.90
Cr(VI)1.091.040.060.990.880.130.990.090.99

Table 2.

Kinetic parameters of Cr(III) and Cr(VI) absorption onto SDS-chitosan.

qtqe=6Dta21/2π1/2+2n=1ierfcnaDt3Dta2E7

After Dis replaced with D1, at a short time, Eq. (7) becomes as:

qtqe=6D1πa21/2t1/2E8

The film diffusion coefficient D1is calculated from the slope of the plot of qt/qeversus t 1/2. For moderate and large times, the diffusion equation given by

qtqe=16π2n=11n2expDn2π2ta2E9

If Dis replaced with D2, at the large time the Eq. (9) becomes as:

ln1qtqe=ln6π2D2π2a2tE10

The pore diffusion coefficient D2is obtained from the slope of the plot of ln(1 − qt /qe)versus t. The values of D1and D2are presented in Table 3.

С0(mg/L)D1·10−8 (cm2 /s)D2·10−12 (cm2 /s)
1.007.041.51

Table 3.

The film and pore diffusion coefficients of the adsorption.

It is also known that if the adsorption is controlled by the film diffusion, the values of the film diffusion coefficient may be in the range of 10−6  10−8 cm2 /s. While if the adsorption mechanism is controlled by the pore diffusion mechanism, the value of the pore diffusion coefficient belongs to the range of 10−1110−13 cm2/s [45]. As can be seen in Table 3, The D1 and D2 values in the model based on the Fick’s law were bigger than that above mentioned ranges. It is confirmed that the sorption kinetics of the Cr(III) ion on SDS-chitosan could be described by the pseudo-second-order model.

3.5 Thermodynamic study

To explore the effect of temperature on Cr(III) adsorption by SDS-chitosan, adsorption experiments were performed at temperatures ranging from 298 to 318 K. The results are displayed in Figures 21 and 22. In these temperature ranges, the amount of Cr(III) adsorbed on SDS-chitosan increased with the increase of temperature.

Figure 21.

Effect of temperature on the adsorption of Cr(III) onto SDS-chitosan.

Figure 22.

Effect of temperature on the adsorption of Cr(VI) on the SDS-modified chitosan beads.

Furthermore, based on the experimental results, the thermodynamic parameters of the adsorption, such as standard Gibb’s free energy change G0, a change in standard enthalpy H0and standard entropy S0, were determined from Van ‘t Hoff Equation [46, 47, 48].

G0=RTlnKcE11
lnKc=H0RT+S0RE12
G0=H0TS0E13

where Ris the universal gas constant (8.314 J/mol−1K−1) and Tis the temperature (K). The slope and intercept of the plot of lnKcversus 1/Twere used to determine the H0and S0.

Table 4 and Table 5 indicate the thermodynamic parameters for Cr(III) and Cr(VI), respectively. In the temperature range of 298318 K, the values of G>0were obtained. The values of Gat various temperatures indicated the feasibility of the process. This suggested that the adsorption process was more spontaneous at higher temperatures. Positive Hand Svalues also suggest that the sorption process is endothermic, and that the disorderliness of the sorption may occur at the solid-liquid interface. It was then proposed that the adsorption of Cr(III) on SDS-chitosan could be mainly dominated by chemisorption.

T(K)H(kJ/mol)S(J/mol)G(kJ/mol)
2980.855
30812.037.40.481
3180.107

Table 4.

Thermodynamic parameters for the adsorption of Cr(III) on SDS-chitosan.

T(K)ΔH(kJ/mol)ΔS(J/mol)ΔG(kJ/mol)
28880.70288.18−2.34
298−5.22
308−8.10
318−10.98

Table 5.

Thermodynamic parameters for the adsorption of Cr(VI) on SDS-modified chitosan beads.

3.6 Desorption study

Repeated use of adsorbent and recovery of the adsorbed metal ions are important indicators for evaluating economic efficiency. In this study, regeneration experiments were carried out using the SDS-chitosan beads after adsorption of Cr(III) and Cr(VI). After adsorption, the spent adsorbent was treated with 50 ml of 0.1 mol/L NaOH solution and ultrapure water as desorption agent in desorption experiment, and then filtered. The content of Cr(III) and Cr(VI) in the filtrate was determined by ICP-MS [49]. Figure 23, Cr(III) by using NaOH or pure water, respectively. As can be seen in Figure 23, the desorption efficiency of Cr(III) was calculated as 57% when using 0.1 mol/L NaOH. The desorption of NaOH was larger than ultrapure water. This indicated that NaOH could be a desorption agent for Cr(III). As shows in Figure 24, the desorption efficiency of Cr(VI) was found to be 50% when 0.1 mol/L NaOH was used, and the desorption was considerably lower with H2O than NaOH. It is suggested that NaOH can be used as a desorption agent for Cr(VI), although a further investigation is needed for the effective recovery and recycling of Cr(VI).

Figure 23.

Desorption efficiency of Cr(III) using NaOH or pure water.

3.7 Comparison with other adsorbents

The comparison of the maximum adsorption capacity of Cr(III) by SDS-chitosan in the present study with those of other adsorbents in the literature is presented in Table 6. As shown in Table 6, the adsorption capacity of SDS-chitosan for Cr(III) in this work was not necessarily high compared to other adsorbents. Meanwhile, Table 7 summarizes the comparison of maximum adsorption capacity of Cr(VI) on SDS-chitosan beads with other adsorbents. It can also be noted that the adsorption capacity is not so high compared to other adsorbents. However, it could be regarded as a potential adsorbent for treating Cr(III) and Cr(VI) from wastewater for practical usage because the synthesis method of the adsorbent is relatively simple.

AdsorbentAdsorption capacity (mg·g1)References
Vesicular basalt0.98[50]
Chitosan1.67[51]
Attapulgite11.0[51]
Chitosan/attapulgite27.0[51]
Crystalline hydrous titanium(IV) oxide14.0[52]
Natural sepiolite0.53[53]
Borax sludge4.00[54]
SDS-chitosan3.42This study

Table 6.

Comparison of adsorption capacity for Cr(III) by different adsorbents.

AdsorbentsAdsorption Capacity (mg·g−1)Reference
Magnetic Chitosan69.4[14]
Carboxymethyl Chitosan-Silicon Dioxide80.7[30]
Chitosan-g-poly/silica55.7 × 10−3[55]
Crosslinked chitosan bentonite composite89.1[56]
Graphene oxide/chitosan86.2[57]
Ethylenediamine-magnetic chitosan51.8[55]
SDS-chitosan3.23This study

Table 7.

The comparison of adsorption properties of several adsorbents.

Figure 24.

Desorption efficiency of Cr(VI) using NaOH or pure water.

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4. Conclusions

In this study, sodium dodecyl sulfate (SDS) was used to chemically modify chitosan to enhance its adsorption capacity for the removal of chromium. SEM-EDS, FT-IR, and XPS were used to characterize the SDS-chitosan beads. The effect of important operating parameters, such as the loading amounts of SDS, solution pH, contact time, adsorbent dose, temperature, and initial Cr(III) or Cr(VI) concentration, on the adsorption performance was examined in a batch system. The experimental data were found to be fit best using Langmuir isotherm and pseudo-first-order kinetic models. At pH 4–5 and higher temperatures, the adsorption process performed admirably. The maximum adsorption capacity of Cr(III) and Cr(VI) on SDS modified chitosan beads were estimated to 3.42 mg·g−1 and 3.23 mg·g−1, respectively in this work. Finally, the SDS-chitosan beads synthesized in this work can be effectively utilized to remove chromium successfully.

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Acknowledgments

The authors are grateful to Mr. H. Morohashi of the Industrial Research Institute of Niigata Prefecture for the measurement of XPS and useful advice. The authors also thank Mr. M. Ohizumi of the Office for Environment and Safety, and Mr. N. Miyamoto, Dr. M. Teraguchi, Mr. T. Nomoto, and Prof. T. Tanaka, of the Facility of Engineering in Niigata University for permitting the use of ICP-AES, ICP-MS, FT-IR, and SEM-EDS.

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Conflicts of interest

The authors declare no conflict of interest.

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Funding

The present work was partially supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Research Program (C), No. 21 K12290).

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Written By

Naoki Kano, Zou Ming, David Eva Vanessa Anak and Muhammad Nabil Md Sari

Submitted: February 15th, 2022Reviewed: March 1st, 2022Published: April 17th, 2022