Synthesis and adsorption properties of chitosan-CDTA-GO nanocomposite for removal of hexavalent chromium from aqueous solutions

Mohamed E.A. Ali

doi:10.1016/j.arabjc.2016.09.010

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Original article

11 (

); 1107-1116

doi:

10.1016/j.arabjc.2016.09.010

Synthesis and adsorption properties of chitosan-CDTA-GO nanocomposite for removal of hexavalent chromium from aqueous solutions

Mohamed E.A. Ali

Water Desalination & Treatment Unit, Hydrogeochemistry Dept., Desert Research Center, Cairo 11753, Egypt

Received: 2016-6-12, Accepted: 2016-9-15, Published: 2016-11-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

A novel adsorbent of chitosan-1,2-Cyclohexylenedinitrilotetraacetic acid – graphene oxide (Cs/CDTA/GO) nanocomposite was successfully prepared in the presence of glutaraldehyde as cross linker. The adsorption and removal efficiency of hexavalent chromium (Cr⁶⁺) using Cs/CDTA/GO adsorbent were investigated at different adsorption conditions. The prepared adsorbent was characterized by FTIR and SEM. The effects of adsorbent chemical composition, CDTA/GO concentration, adsorbent dose, pH, temperature, contact time and initial metal ion concentration on Cr⁶⁺ sorption were investigated. The results showed that the optimum adsorbent dose was 2 g/l at pH of 3.5 and equilibrium time of 60 min. In addition, the temperature and pH were shown with great influence of the adsorption process. The adsorption kinetics of Cr⁶⁺ onto Cs/CDTA/GO followed the pseudo-second order model and the adsorption isotherm was well fitted by the Langmuir model. The maximum adsorption capacity of the adsorbent was 166.98 mg/g, and the equilibrium parameter (R_L) at different concentrations was less than unity indicating the adsorption of Cr⁶⁺ ions onto Cs/CDTA/GO is favorable. Cs/CDTA/GO adsorbent could be regenerated more than three times based on its adsorption/desorption cycles.

Keywords

Chitosan

Graphene oxide

CDTA

Nanocomposites

Hexavalent chromium

Adsorption

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1 Introduction

Chromium has been used in many industries including electroplating, tannery industries, iron and steel industries. Therefore, it can be released to water resources, and as a result it harms the human because of its toxicity (Bhattacharyya and Sen Gupta, 2006; Goharshadi and Moghaddam, 2015). Chromium occurs in water with two predominant states: trivalent Cr³⁺ and hexavalent Cr⁶⁺ where the latter is more soluble than former one (Sarin and Pant, 2006). These both stats can contaminate the surface water and even groundwater when there is no control to treat the industrial wastewater before contact with water resources. Because of the toxicity of Cr⁶⁺ even with low concentrations (>0.05 mg/l) on the human health, it is important to remove it from water. There are many developed techniques for removing of Cr⁶⁺ ions from water: among them adsorption (Li et al., 2013; Baral et al., 2006; Gupta et al., 2010; Mohan et al., 2005 ), membranes (Kulkarni et al., 2007; Piedra et al., 2015), electrochemical precipitation (Golder et al., 2011; Fang et al., 2012), and ion exchange (Petruzzelli et al., 1995; Rafati et al., 2010). Compared to other methods, economically the adsorption technique is the common effective in removal of chromium due to the low cost, ease of operation and efficiency (Owlad et al., 2008; Ravikumar et al., 2005). The adsorption capacity mainly depends on the type of adsorbent that should be characterized by chemical stability and high surface area in addition to containing the functional groups that act as active sites for the adsorbate (Dula et al., 2014). Organic polymers, recently inorganic nanomaterials and hybrid of them have been used as adsorbents for various heavy metals (Samiey et al., 2014; Wang et al., 2012; Singh et al., 2013 ). Examples of the organic polymers are chitosan (Li and Bai, 2006; Sankararamakrishnan et al., 2006), sodium alginate (Park and Chae, 2004; Lee et al., 2013), and polyvinyl alcohol (Lv et al., 2013). Some examples of the inorganic adsorbents are activated carbon (Acharya et al., 2009), zeolite (Basaldella et al., 2007), magnetite (Chowdhury and Yanful, 2010) and recently graphene oxide (GO) Kumar et al., 2013. The natural biosorbents such as chitosan have had the great attention in the removal of both cations and anions. In the neutral solutions the cations can be adsorbed onto the amine groups (Huang et al., 2013). On the other hand the anion groups such as dichromate Cr₂O₇⁻² and chromate CrO₄⁻² (the two phases of chromium) can be removed by ion exchange in acidic solutions due to protonation of amine groups (Barakat, 2011). Recently, GO has been used as adsorbent because of its variety of functional groups: hydroxyl, carboxyl and epoxy (Najafabadi et al., 2015). These groups can be protonated in acidic solution and therefore can act as active sites for removal of chromate groups. But due to the difficulty of separation of GO from treated water it is hard to use in such purpose. To overcome this restricts, GO and chitosan can be prepared and composed together and used as nanocomposite (Li et al., 2013). Polyaminopolycarboxylic acids such as ethylenediamine tetraacetic acid (EDTA), 1,2-Cyclohexylenedinitrilotetraacetic acid (CDTA), and Diethylenetriaminepentaacetic acid (DTPA) are strong chelating agents and form stable chelates with different types of heavy metals in the aqueous solutions. As a result, the metal uptake capacity of the adsorbent can further be improved by adding the chelator compounds.

The main goal of this work was to prepare a new adsorbent of Cs/CDTA/GO nanocomposite. More specifically, this work relates to use GO that contains multifunctional groups in addition to a chelating agent of CDTA that captures and forms strong nanocomposite with the ions in the aqueous solutions. Moreover, a comparison of the removal efficiency of Cr⁶⁺ ions using different prepared adsorbents was investigated to select the better one in studying the kinetic, isotherms and its application in wastewater treatment. Moreover, regeneration and de-complexing the chelated chromium metal with Cs/CDTA/GO nanocomposite were estimated using diluted sulfuric acid solution at room temperature. The main text of the article should appear here with headings as appropriate.

2 Materials and methods

2.1

2.1 Materials

Natural graphite powder, potassium permanganate, sulfuric acid, nitric acid and hydrogen peroxide were used in preparation of graphene oxide; all the chemicals were supplied by Fisher Scientific, except for graphite powder which was supplied by Across Organics Company. 1,2-Cyclohexylenedinitrilotetraacetic acid (CDTA) was purchased from Sigma-Aldrich and used as received. Formic acid, isopropyl alcohol, sodium hydroxide, hydrochloric acid, sulfuric acid, 50% glutaraldehyde solution and potassium chromate were purchased from El-Nasr Chemicals Co, Egypt.

2.2

2.2 Preparation of the adsorbents

GO was prepared following a modified Hummers’ method as discussed elsewhere (Ali et al., 2016). To prepare the chitosan solution, 2 g of chitosan powder was dissolved in 100 ml of a 1% formic acid solution. After completing the dissolution process, 2 ml of 50% glutaraldehyde solution was added to form the cross-linked chitosan hydrogel. Subsequently, the hydrogel was kept in oven at 40 °C until constant weight of chitosan powder was produced. Finally, this powder was washed several times with distilled water followed by 0.1 M of NaOH solution. This washing was repeated until the pH of washing solution reaching 7. To prepare Cs/GO solution, 0.1 g of GO was dispersed in 13% formic acid and has been kept in ultrasonic for 20 min. Afterward this solution was mixed with 25 ml of 2% Cs. solution for 1 h. Cs/CDTA solution 0.5 g of CDTA was dissolved in 10 ml of isopropyl alcohol and then mixed with 25 ml of 2% Cs. solution. Cs/CDTA/GO adsorbent was prepared as follows: (1) dispersion of 0.1 g of GO in 5 ml of 13% formic acid with ultrasonic, (2) dissolving of 0.5 g of CDTA in 10 ml of isopropyl alcohol, and (3) mixing the two solutions with continuous stirring at 25 °C overnight. The solution was filtrated and the supernatant was decanted; the filtrate was washed with distilled water and dried in an oven at 50 °C. Then different concentrations (0.1, 0.5, 1, 1.5 and 2 g) of CDTA/GO were added to 25 ml of 2% chitosan solution with continuous stirring at 25 °C overnight. All Cs/GO, Cs/CDTA and Cs/CDTA/GO solutions were cross-linked and drying as described in pure chitosan preparation.

2.3

2.3 Characterization

Infrared spectroscopy was carried out using Genesis Unicam FT-IR spectrophotometer by incorporating the sample in a KBr disk. Scanning electron microscopy (SEM), Quanta FEG 250 microscope.

2.4

2.4 Batch adsorption experiment

The stock standard solution of 1000 mg/l of Cr⁶⁺ was prepared by dissolving 3.7307 g of potassium chromate in 1000 ml of DI water. Different concentrations (1, 5, 10, 20, 40, 50, 60, 80, 100, 250, 500, and 750 mg/l) were prepared by dilution of the stock solution with DI water. All batch adsorption experiments were performed on a mechanical shaker with a shaking speed of 160 rpm. To optimize the concentration of the adsorbent, different concentrations of Cs/CDTA/GO adsorbent (0.5, 1, 2, 5, 10, 15 and 20 g/l) were added to the above Cr⁶⁺ solution under mechanical agitation. To study the effect of adsorption time, 0.05 g of Cs/CDTA/GO was agitated with 50 ml of 25 mg/l of Cr⁶⁺ solution at interval times (10, 20, 30, 40, 50, 60, 70, 80, 90, 120, 150 and 180 min.). The initial pH values of the Cr⁶⁺ solutions were adjusted by adding 1:1 HCl and 1% NaOH solutions. To investigate the effect of the adsorbate solution temperature, 0.05 g of adsorbent was agitated with 10 ml of 100 mg/l Cr⁶⁺ solution at 30, 40, 50 and 60 °C. After each adsorption process, the solution was filtrated and the supernatant was immediately analyzed by inductive coupled plasma Mass Spectrometry (ICP-MS). The adsorption capacity was calculated according to the following equation: $q = \frac{(C_{i} - C_{f}) V}{m}$

The removal % of Cr(IV) was calculated as follows: $% = \frac{(C_{i} - C_{f})}{C_{i}} \times 100$ where “q” is the adsorption capacity (mg/g), C_i and C_f are the initial concentration and the final metal ion concentration (mg/L), V is the volume of the adsorbate solution (L), and m is the mass of adsorbent (g).

2.5

2.5 Reusability of the adsorbent

Reusability of the adsorbent was carried out by washing with 0.1% H₂SO₄ solution and then washed with DI water several times. Afterward, the adsorbent was used again and the adsorption and removal of chromium were calculated according to the previous equations.

3 Results and discussions

3.1

3.1 Effect of the adsorbent chemical composition

The removal efficiency of the heavy metals mainly depends on the chemical structure and number of the active sites of the adsorbent. Therefore, the removal % of different prepared adsorbents Cs, GO, Cs/GO, Cs/CDTA and Cs/CDTA/GO was estimated in the same process, Fig. 1. From the figure, it can be seen that the order of Cr⁶⁺ removal is Cs/CDTA/GO > Cs/GO > Cs/CDTA > Cs > GO. This increase in the removal % is due to the presence of the functional groups of GO, in addition to the chelation effect of CDTA with chromium ions. Actually, the crucial component is Cs, while CDTA and GO act as modifiers to increase the adsorption capacity of CS. Therefore, all adsorption, isotherm and kinetic studies were carried out using Cs/CDTA/GO nanocomposite.

Effect of adsorbent type on the removal % of chromium. Adsorbent dose, 1 g/l & adsorbate solution conc., 25 mg/l & contact time, 1 h and pH of 3.5.

3.2

3.2 Adsorbent characterization

Cs/CDTA/GO nanocomposite was prepared by solution mixing as described in the experimental section. The interaction between these components, Cs, CDTA and GO was due to containing of Cs onto the amino and hydroxyl groups and its polycationic nature an acid media, Scheme 1. Therefore, electrostatic attraction and hydrogen bonding between Cs, CDTA and GO are achievable and could induce homogeneous co-dispersion of them on the molecular scale and enhance the adhesion as well as mechanical performance of the nanocomposite (Han and Lu, 2009; Yang et al., 2010). The FT-IR spectra of cross-linked chitosan and Cs/CDTA/GO adsorbents are shown in Fig. 2. In the spectrum of GO, Fig. 2(c), the characteristic peaks that appeared at 3430, 1734, 1620 and 1052 cm⁻¹ correspond to hydroxyl, C⚌O stretch of the carboxylic group, deformations of the O—H bond in water and epoxide functional groups onto GO surface, respectively (Ali et al., 2016). In the spectrum of the pure cross-linked chitosan Fig. 2(a), the broad peaks appeared in the range between 3100–3650 cm⁻¹ and 1510–1690 cm⁻¹ corresponding to N—H and O—H stretching and the amide groups in the structure of cross-linked chitosan, respectively. Compared to FT-IR spectrum of cross-linked chitosan, the spectrum of Cs/CDTA/GO shows multiple descriptive peaks indicated the interaction between chitosan, CDTA and GO, where the peaks at 1050, 1600 and 2920 cm⁻¹ are the vibrational frequencies of C—N, C—H and C—O of CDTA, indicating that chitosan has been successfully modified with CDTA. These results are in agreement with previous work (Lü et al., 2015). The new major characteristic peaks of Cs/CDTA/GO (Fig. 2(b)) at 1090, 1150, 1350, and 1390 cm⁻¹ indicate the formation of Cs/CDTA/GO nanocomposite. The appearance of the characteristic peak around 1600 cm⁻¹ is attributed to N—H bending vibration and indicates the presence of the amine group of chitosan (Donia et al., 2007). Moreover, compared with pure GO, the peak at 1730 cm⁻¹ related to C⚌O stretch of the carboxylic group of GO disappears in the spectra of Cs/CDTA/GO nanocomposite. These reflect the formation of hydrogen bonding between Cs and the oxygenated functional groups in GO in addition to the electrostatic interaction between polycationic Cs and the negative charge on the surface of GO (Yang et al., 2010). SEM images of chitosan, Cs/CDTA, Cs/GO and Cs/CDTA/GO are shown in Fig. 3. Fig. 3(a) and (b) of the cross-linked chitosan showed smooth surface with the micro porous structure compared to non-smooth surface of non-cross-linked chitosan that indicates the strong interaction between chitosan molecules (Mohanasrinivasan et al., 2014). Compared to the pure cross-linked chitosan, there was a significant difference in the surface morphology when chitosan was composed with CDTA, GO and CDT/GO, Fig 3(c)–(h). On the hand, in Fig. 3(c) and (d) there was an irregular morphology when Cs was composed with CDTA. The rougher surfaces with porous structure appeared in Fig. 3(e) and (f) reveal the assembling of Cs on the surface of GO layers (Fan et al., 2013). In Fig. 3(g) and (h) of Cs/CDTA/GO adsorbent there are different features, where the surface is irregular and rough with increasing the microporous structure. In addition, the pore openings and cavities have been shown in Fig. 3(g) and (h) enhance the adsorption kinetics.

Schematic reaction mechanism of Cs/CDTA/GO nanocomposite and its adsorption for Cr6+ ions.

FT-IR spectra of (a) pure cross-linked chitosan, (b) Cs/CDTA/GO and (c) GO.

Surface morphology at two magnifications (10,000× left and 20,000× right) of (a and b) chitosan, (c and d) Cs/CDTA, (e and f) Cs/GO and (g and h) Cs/CDTA/GO.

3.3

3.3 Factors affecting removal of Cr⁶⁺ ions

The different factors affecting the removal efficiency of the adsorbent to chromium are shown in Fig. 4. Fig. 4(a) shows the effect of the amount of CDTA/GO into chitosan on the removal efficiency of Cr⁶⁺ ions. The initial concentration of chromium solution was 50 mg/l, adsorbent dosage of 1 g/l, and pH of 3.5 at 25 °C. This increase in removal efficiency was due to increasing the number of both oxygen functional groups of GO and carboxylic groups of CDTA that are protonated at low pH values and increased the active sites for chelating of the Cr⁶⁺ ions. The capability of the adsorbent for removal of heavy metals depends mainly on the adsorbent dose. The effect of the adsorbent dose (g/l) on the removal efficiency of Cr⁶⁺ is shown in Fig. 4(b). The adsorbent dose was varied between 0.5 and 15 g/l at constant initial Cr⁶⁺ concentration of 50 mg/l, pH of 3.5 and contact time of 1 h. The results indicate that the removal % and adsorption capacity are increased from 27.7 to 97.1% and 3.08 to 9.83 mg/g, respectively, as the adsorbent dose increased from 0.5 to 15 g/l. This trend of increases is due to the increase in the active sites and functional groups as a function of adsorbent dose increase. The figure shows no further increase in the removal % and adsorption capacity as adsorbent increased from 5 to 15 g/l. In the adsorption studies, the pH of the solution is an important factor affecting the efficiency of the adsorbent in removal efficiency of heavy metals due to protonation and de-protonation of the functional groups of the adsorbent (Mousavi et al., 2010). Fig. 4(c) shows the effect of pH on the removal % of Cr⁶⁺ ions using Cs/CDTA/GO adsorbent at constant contact time of 1 h, constant dose of 5 g/l at room temperature and 50 mg/l of chromium as initial concentration. From the figure, the maximum removal % is shown at low pH 2–3, and this is because Cr⁶⁺ ions in solution are present as negative groups of mono-chromate (CrO₄²⁻), dichromate (Cr₂O₇²⁻) and hydrogen chromate (HCrO₄⁻). These entire groups can adsorbed onto the protonated amine (NH₃⁺), hydroxyl (OH²⁺) and carboxylic (COOH) of chitosan, CDTA and GO via electrostatic attraction. On the other hand, at the high pH values (pH = 6) the adsorption capacity was shown to be decreased due to de-protonation of those functional groups and decreasing the negative charge densities (Debnath et al., 2014). The temperature of the adsorbate solution can effect the adsorption efficiency of the adsorbent. Fig. 4(d) shows the removal efficiency of Cs/CDTA/GO at different temperatures ranges between 20 and 60 °C. From the figure, it can be seen that the adsorption efficiency increases with the increase in temperature. The might be due to the strengthening of the adsorptive forces between the active sites of the adsorbents and adsorbate species (Naiya et al., 2009), in addition to increasing the electrostatic interaction between chromium ions and the adsorbent.

Effect of different adsorption parameters onto the chromium removal efficiency using Cs/CDTA/GO adsorbent; concentration of CDTA/GO (a), adsorbent dose (b), pH of the aqueous solution (c), and solution temperature (d).

3.4

3.4 Adsorption kinetic models

The effect of contact time on the adsorption of Cr⁶⁺ ions onto the surface of Cs/CDTA/GO is shown in Fig. 5(a). From the figure, it is observed that the maximum removal % was shown at 60 min. After that, no significant change was observed; therefore, the optimum contact time is 60 min. The pseudo-first-order kinetic model (Lagergren, 1898), pseudo-second-order kinetic model (Ho and McKay, 1999) and intra-particle diffusion (Figaro et al., 2009) kinetic models were employed to fit the experimental data and to understand the adsorption mechanism of Cr⁶⁺ ions using Cs/CDTA/GO adsorbent.

Removal of chromium as a function of contact time (a) & kinetic model plots for Cr6+ adsorption on Cs/CDTA/GO adsorbent at different initial chromium concentrations of pseudo-first-order (b), pseudo-second-order (c), and intra-particle diffusion model (d).

The equation of Pseudo-first-order kinetic model is as follows:

(1)

Log (q_{e} - q_{t}) = \log q_{e} - k_{1} t

where q_e and q_t (mg/g) are the adsorption capacities at equilibrium time and time t, respectively. k₁ (l/min) is the pseudo-first-order rate constant. From plotting log (q_e − q_t) versus t (Fig. 5(b)), K₁ and q_e can be obtained from the slope and intercept, respectively.

The equation of Pseudo-second-order kinetic model is as follows:

(2)

t / q_{t} = 1 / k_{2} q_{e}^{2} + {tq}_{e}

where k₂ [g/(mg min)] is the pseudo-second-order rate constant. From plotting t/q_t versus t (Fig. 5(c)), the values of q_e and k₂ can be obtained from the slope and intercept, respectively.

The equation of intra-particle diffusion kinetic model is as follows:

(3)

q_{t} = k_{di} t^{1 / 2} + C_{i}

where k_di (mg/g min^1/2) and C_i (mg/g) are the intra-particle diffusion rate constant and the intercept of the stage I of the adsorption process, respectively. The values of k_di and C_i can be obtained from the slope and intercept of q_t versus t^1/2, (Fig. 5(d)) respectively. Table 1 illustrates all the kinetic parameters of the three models at different concentrations of adsorbate solution. From the table, it was observed that the values of R² of the pseudo-second-order kinetic model were better than other two models. Therefore it’s more applicable to the kinetics adsorption of Cr⁶⁺ ions and therefore suggests a chemisorption process. In the intra-particle diffusion model (Table 1), the high values of diffusion rate constant (k_d₁) for the first sharper portion than that (k_d₂) for the second portion indicate that the rate of Cr⁶⁺ removal was higher in the beginning.

Table 1 Kinetic parameters of Cr⁶⁺ ions sorption onto Cs/CDTA/GO adsorbent.

Conc. mg/l	Pseudo-first-order			Pseudo-second-order				Intra-particle diffusion
Conc. mg/l	K₁	q_e	R²	K₂	R²	q_e (calc.)	q_e (exp.)	K_d1	K_d2	C₁	C₂	R²₁	R²₂
20	0.0041	3.52	0.86	0.014	0.999	18.18	18.66	0.66	0.113	12.16	16.34	0.91	0.92
40	0.0066	14.06	0.944	0.002	0.999	36.76	35.86	3.49	0.679	7.49	25.98	0.99	0.95
60	0.0049	13.98	0.985	0.0089	0.998	47.16	46.22	3.5	1.353	6.204	24.15	0.99	0.98

3.5

3.5 Effect of initial metal ion concentration and adsorption isotherm models

The adsorption capacity of Cs/CDTA/GO adsorbent is related to initial ion concentrations (Fig. 6), where the adsorption capacity was increased with increasing the initial ion concentration while the removal efficiency was decreased. However, to describe the adsorption mechanism and analyze the experimental data of Cr⁶⁺ adsorption onto Cs/CDTA/GO adsorbent, the adsorption isotherm models of Langmuir (1918) and Freundlich (1906) were used. For Langmuir model the linear form could be expressed from the following equation:

(4)

\frac{C_{e}}{q_{e}} = \frac{1}{K_{l} q_{m}} = \frac{C_{e}}{q_{m}}

Effect of initial concentration on the removal and adsorption capacity of Cs/CDTA/GO. Adsorbent dose, 5 g/l &, contact time, 1 h & pH, 3.5 and temperature of 25 °C.

The Freundlich model, the linear form could be expressed from the following equation:

(5)

Log q_{e} = \ln K_{F} + (1 / n) \log C_{e}

where C_e is the equilibrium concentration of Cr⁶⁺ in mg/l, q_e and q_m are the adsorption amount at equilibrium (mg/g) and adsorption capacity (mg/g), respectively, and K_L is the Langmuir constant (L/mg). The values of K_L and q_m can be obtained from the intercept and slope of C_e/q_e versus C_e (Fig. 7(a)). K_F is the empirical Freundlich constant (mg/g) and 1/n is the Freundlich exponent. The values of K_F and 1/n can be expressed from the intercept and slope of Log q_e versus Log q_c (Fig. 7(b)). By comparing the correlation coefficients of the three isotherm models, it was found that the Langmuir (R² = 0.987) isotherm model fitted better than Freundlich (R² = 0.956) isotherm model, suggesting that sorption of Cr⁶⁺ ions onto Cs/CDTA/GO is monolayer coverage. This might be attributed to the homogeneous distribution of the multi-functional oxygen groups of GO and the carboxylic groups of CDTA onto the surface of chitosan. The calculated maximum adsorption capacity from Langmuir isotherm model (q_m = 169.49 mg/l) is fitted with the experimental maximum adsorption capacity (q_m = 166.98 mg/l). The maximum adsorption capacity of Cs/CDTA/GO adsorbent for the removal Cr⁶⁺ ions is compared with other adsorbents reported in previous works, Table 2. From the table, it is obvious that Cs/CDTA/GO adsorbent has highest maximum adsorption capacity compared to other adsorbents. This is due to containing the adsorbent onto multi-functional groups of (1) chitosan (amino and hydroxyl groups), (2) GO (hydroxyl, carboxyl and epoxy groups), and CDTA (carboxylic groups) which act as chelation sites for chromium ions.

Langmuir (a) and Freundlich (b) isotherm models for the adsorption of Cr6+ onto Cs/CDTA/GO.

Table 2 Comparison of the maximum adsorption capacity of Cr⁶⁺ onto different adsorbents.

Adsorbent	Conditions	q_m (mg/g)	Ref.
Surface modified sand	pH, 3 & Temp, 25 °C& Time, 24 h	6.42	Lee et al. (2010)
Magnetic cyclodextrin–chitosan/graphene oxide	pH, 3 & Temp., 303 K	67.66	Li et al. (2013)
Magnetic chitosan/graphene oxide	pH, 3 ± 1 & Temp., 45 °C & time 24 h	101.6	Debnath et al. (2014)
Chitosan cross-linked with epichlorohydrin	pH, 5.5	52.3	Tianwei et al. (2001)
Chitosan, glucosamine biopolymer onto ceramic alumina	pH, 2 &	153.85	Boddu et al. (2003)
Poly(ethylene-co-vinyl alcohol) nanofiber	pH, 2 & Temp., 25 °C & time, 100 min.	90.74	Xu et al. (2015)
Chitosan, GO/EDTA	pH, 2 & Temp., 25 °C & time, 90 min.	86.17	Zhang et al. (2016)
Chitosan/CDTA/GO	pH, 3.5 & Temp., 25 °C & time, 60 min.	166.98	This study

To predict whether an adsorption system is favorable or unfavorable, the equilibrium parameter (R_L) was calculated according to the following equation:

(6)

R_{L} = \frac{1}{1 + {bC}_{f}}

where b is the Langmuir constant (l/mg) and C_f is the final Cr⁶⁺ concentration (mg/l) in the solution, if R_L > 1 the isotherm is unfavorable, whereas if R_L < 1 the isotherm is favorable. The calculated R_L values at different concentrations were less than unity indicating the adsorption of Cr⁶⁺ ions onto Cs/CDTA/GO is favorable. In addition, from the data of Freundlich isotherm model it was found that 1/n value smaller than 1 indicates favorable adsorption.

3.6

3.6 Reusability of Cs/CDTA/GO

Three cycles of chromium adsorption and desorption experiments were conducted to investigate the changes in Cs/CDTA/GO efficiency at equilibrium. It was found that the removal efficiency of chromium from the aqueous solutions was more than 95%.

4 Conclusion

In this work, the nanocomposite adsorbent of Cs/CDTA/GO was prepared and characterized by FTIR and SEM to confirm the functional groups and the morphological structure, respectively. The obtained results showed that the optimum adsorbent dose was 2 g/l at pH of 3.5 and equilibrium time of 60 min. The adsorption kinetics of Cr⁶⁺ onto Cs/CDTA/GO followed the pseudo-second order model and the adsorption isotherm was well fitted by the Langmuir model. The maximum adsorption capacity of the adsorbent was 166.98 mg/g, and the equilibrium parameter (R_L) at different concentrations was less than unity indicating the adsorption of Cr⁶⁺ ions onto Cs/CDTA/GO is favorable.

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Rafati L., Mahvi A., Asgari A., Hosseini S., . Removal of chromium (VI) from aqueous solutions using Lewatit FO36 nano ion exchange resin. Int. J. Environ. Sci. Technol.. 2010;7:147-156.
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Show Sections

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[17] Golder A.K., Chanda A.K., Samanta A.N., Ray S., . Removal of hexavalent chromium by electrochemical reduction–precipitation: Investigation of process performance and reaction stoichiometry. Sep. Purif. Technol.. 2011;76:345-350.
[Google Scholar]

[18] Gupta V.K., Rastogi A., Nayak A., . Adsorption studies on the removal of hexavalent chromium from aqueous solution using a low cost fertilizer industry waste material. J. Colloid Interface Sci.. 2010;342:135-141.
[Google Scholar]

[19] Han Y., Lu Y., . Characterization and electrical properties of conductive polymer/colloidal graphite oxide nanocomposites. Compos. Sci. Technol.. 2009;69:1231-1237.
[Google Scholar]

[20] Ho Y.S., McKay G., . Pseudo-second order model for sorption processes. Process Biochem.. 1999;34:451-465.
[Google Scholar]

[21] Huang R., Yang B., Liu Q., . Removal of chromium (VI) Ions from aqueous solutions with protonated crosslinked chitosan. J. Appl. Polym. Sci.. 2013;129:908-915.
[Google Scholar]

[22] Kulkarni P.S., Kalyani V., Mahajani V.V., . Removal of hexavalent chromium by membrane-based hybrid processes. Ind. Eng. Chem. Res.. 2007;46:8176-8182.
[Google Scholar]

[23] Kumar A.S.K., Kakan S.S., Rajesh N., . A novel amine impregnated graphene oxide adsorbent for the removal of hexavalent chromium. Chem. Eng. J.. 2013;230:328-337.
[Google Scholar]

[24] Lagergren S., . Zur Theorie der sogenannten Absorption gelöster Stoffe. PA Norstedt & söner; 1898.

[25] Langmuir I., . The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc.. 1918;40:1361-1403.
[Google Scholar]

[26] Lee S.-M., Kim W.-G., Laldawngliana C., Tiwari D., . Removal behavior of surface modified sand for Cd (II) and Cr (VI) from aqueous solutions. J. Chem. Eng. Data. 2010;55:3089-3094.
[Google Scholar]

[27] Lee I., Lee C.-G., Park J.-A., Kang J.-K., Yoon S.-Y., Kim S.-B., . Removal of Cr (VI) from aqueous solution using alginate/polyvinyl alcohol–hematite composite. Desalin. Water Treat.. 2013;51:3438-3444.
[Google Scholar]

[28] Li N., Bai R., . Development of chitosan-based granular adsorbents for enhanced and selective adsorption performance in heavy metal removal. Water Sci. Technol.. 2006;54:103-113.
[Google Scholar]

[29] Li L., Fan L., Sun M., Qiu H., Li X., Duan H., Luo C., . Adsorbent for chromium removal based on graphene oxide functionalized with magnetic cyclodextrin–chitosan. Colloids Surf. B. 2013;107:76-83.
[Google Scholar]

[30] Lü H., Wang X., Yang J., Xie Z., . One-step synthesis of CDTA coated magnetic nanoparticles for selective removal of Cu (II) from aqueous solution. Int. J. Biol. Macromol.. 2015;78:209-214.
[Google Scholar]

[31] Lv X., Jiang G., Xue X., Wu D., Sheng T., Sun C., Xu X., . Fe⁰-Fe₃O₄ nanocomposites embedded polyvinyl alcohol/sodium alginate beads for chromium (VI) removal. J. Hazard. Mater.. 2013;262:748-758.
[Google Scholar]

[32] Mohan D., Singh K.P., Singh V.K., . Removal of hexavalent chromium from aqueous solution using low-cost activated carbons derived from agricultural waste materials and activated carbon fabric cloth. Ind. Eng. Chem. Res.. 2005;44:1027-1042.
[Google Scholar]

[33] Mohanasrinivasan V., Mishra M., Paliwal J.S., Singh S.K., Selvarajan E., Suganthi V., Devi C.S., . Studies on heavy metal removal efficiency and antibacterial activity of chitosan prepared from shrimp shell waste, 3. Biotech. 2014;4:167-175.
[Google Scholar]

[34] Mousavi H., Hosseynifar A., Jahed V., Dehghani S., . Removal of lead from aqueous solution using waste tire rubber ash as an adsorbent. Braz. J. Chem. Eng.. 2010;27:79-87.
[Google Scholar]

[35] Naiya T.K., Bhattacharya A.K., Das S.K., . Adsorption of Cd (II) and Pb (II) from aqueous solutions on activated alumina. J. Colloid Interface Sci.. 2009;333:14-26.
[Google Scholar]

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[Google Scholar]

[38] Park H.G., Chae M.Y., . Novel type of alginate gel-based adsorbents for heavy metal removal. J. Chem. Technol. Biotechnol.. 2004;79:1080-1083.
[Google Scholar]

[39] Petruzzelli D., Passino R., Tiravanti G., . Ion exchange process for chromium removal and recovery from tannery wastes. Ind. Eng. Chem. Res.. 1995;34:2612-2617.
[Google Scholar]

[40] Piedra E., Álvarez J.R., Luque S., . Hexavalent chromium removal from chromium plating rinsing water with membrane technology. Desalin. Water Treat.. 2015;53:1431-1439.
[Google Scholar]

[41] Rafati L., Mahvi A., Asgari A., Hosseini S., . Removal of chromium (VI) from aqueous solutions using Lewatit FO36 nano ion exchange resin. Int. J. Environ. Sci. Technol.. 2010;7:147-156.
[Google Scholar]

[42] Ravikumar K., Deebika B., Balu K., . Decolourization of aqueous dye solutions by a novel adsorbent: application of statistical designs and surface plots for the optimization and regression analysis. J. Hazard. Mater.. 2005;122:75-83.
[Google Scholar]

[43] Samiey B., Cheng C.-H., Wu J., . Organic-inorganic hybrid polymers as adsorbents for removal of heavy metal ions from solutions: a review. Materials. 2014;7:673-726.
[Google Scholar]

[44] Sankararamakrishnan N., Dixit A., Iyengar L., Sanghi R., . Removal of hexavalent chromium using a novel cross linked xanthated chitosan. Bioresour. Technol.. 2006;97:2377-2382.
[Google Scholar]

[45] Sarin V., Pant K.K., . Removal of chromium from industrial waste by using eucalyptus bark. Bioresour. Technol.. 2006;97:15-20.
[Google Scholar]

[46] Singh S., Barick K., Bahadur D., . Functional oxide nanomaterials and nanocomposites for the removal of heavy metals and dyes. Nanomater. Nanotechnol.. 2013;3:3-20.
[Google Scholar]

[47] Tianwei T., Xiaojing H., Weixia D., . Adsorption behaviour of metal ions on imprinted chitosan resin. J. Chem. Technol. Biotechnol.. 2001;76:191-195.
[Google Scholar]

[48] Wang X., Guo Y., Yang L., Han M., Zhao J., Cheng X., . Nanomaterials as sorbents to remove heavy metal ions in wastewater treatment. J. Environ. Anal. Toxicol.. 2012;2012
[Google Scholar]

[49] Xu D., Zhu K., Zheng X., Xiao R., . Poly (ethylene-co-vinyl alcohol) functional nanofiber membranes for the removal of Cr (VI) from water. Ind. Eng. Chem. Res.. 2015;54:6836-6844.
[Google Scholar]

[50] Yang X., Tu Y., Li L., Shang S., Tao X.-M., . Well-dispersed chitosan/graphene oxide nanocomposites. ACS Appl. Mater. Interfaces. 2010;2:1707-1713.
[Google Scholar]

[51] Zhang L., Luo H., Liu P., Fang W., Geng J., . A novel modified graphene oxide/chitosan composite used as an adsorbent for Cr(VI) in aqueous solutions. Int. J. Biol. Macromol.. 2016;87:586-596.
[Google Scholar]

Synthesis and adsorption properties of chitosan-CDTA-GO nanocomposite for removal of hexavalent chromium from aqueous solutions

Abstract

Keywords

Chitosan

Graphene oxide

CDTA

Nanocomposites

Hexavalent chromium

Adsorption

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Preparation of the adsorbents

2.3 Characterization

2.4 Batch adsorption experiment

2.5 Reusability of the adsorbent

3 Results and discussions

3.1 Effect of the adsorbent chemical composition

3.2 Adsorbent characterization

3.3 Factors affecting removal of Cr6+ ions

3.4 Adsorption kinetic models

3.5 Effect of initial metal ion concentration and adsorption isotherm models

3.6 Reusability of Cs/CDTA/GO

4 Conclusion

References

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3.3 Factors affecting removal of Cr⁶⁺ ions