Functionalized chitosan nanocomposites for removal of toxic Cr (VI) from aqueous solution

Journal Pre-proof Functionalized chitosan nanocomposites for removal of toxic Cr (VI) from aqueous solution

Tarek E. Khalil, Amel F. Elhusseiny, Ali El-dissouky, Nagwa M. Ibrahim PII:

S1381-5148(19)30970-8

DOI:

https://doi.org/10.1016/j.reactfunctpolym.2019.104407

Reference:

REACT 104407

To appear in:

Reactive and Functional Polymers

Received date:

14 September 2019

Revised date:

14 October 2019

Accepted date:

5 November 2019

Please cite this article as: T.E. Khalil, A.F. Elhusseiny, A. El-dissouky, et al., Functionalized chitosan nanocomposites for removal of toxic Cr (VI) from aqueous solution, Reactive and Functional Polymers (2018), https://doi.org/10.1016/ j.reactfunctpolym.2019.104407

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© 2018 Published by Elsevier.

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Functionalized chitosan nanocomposites for removal of toxic Cr (VI) from aqueous solution Tarek E. Khalil*, Amel F. Elhusseiny, Ali El-dissouky, Nagwa M. Ibrahim Chemistry Department, Faculty of Science, Alexandria University, P.O. box 426, Ibrahimia, Alexandria 21321, Egypt

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Abstract

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Starting from the biopolymer chitosan, two new functionalized chitosan nanocomposities, (CTS-Cin) and (Fe3O4@CTS-Cin) were synthesised and explored for the removal of toxic

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chromium from aqueous solution. The two chitosan based adsorbents were prepared by coprecipitation method under N2 conditions and fully characterised by means of different

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analytical techniques, FT-IR, EDS, XRD, SEM, HR-TEM and VSM. Adsorption mechanism

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of toxic Cr (VI) was performed by batch experiments as a function of pH, adsorbent dosage, contact time and initial hexavalent chromium concentration.

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The adsorption isotherm and kinetics were fitted well by Langmuir and pseudo-second–order model indicating that the adsorption process is monolayer adsorption. The maximum

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adsorption capacity for CTS-Cin adsorbent at 298 K is 61.35 mg/g at pH=2 and it achieved adsorption equilibrium within 35 minutes. However, Fe3O4@CTS-Cin adsorbent achieved adsorption equilibrium within 80 min and its maximum adsorption capacity is 58.14 mg/g at pH=3. These results indicate that the two modified adsorbents represent promising adsorbents that would have a practical impact on wastewater treatment applications.

Keywords: Functionalized chitosan nanocomposities; Magnetite nanocomposites; Adsorption; Cr(VI); Kinetic; Equilibrium; Thermodynamics

*Corresponding author Tel.: +201227294198. E-mail address: [email protected]

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1. Introduction Recently, water pollution has become a serious environmental problem that has attracted global concern. Pollutants as heavy metals [1], phosphates, nitrates, fluoride, hazardous and toxic chemicals, dyes, biodegradable waste, heat, sediment, radioactive pollutants, pharmaceuticals and personal care products are subjects of main concern [2]. Heavy metals are one of the most severe environmental problems since they are highly toxic at low concentrations, carcinogenic, not biodegradable, and tend to accumulate in living organisms

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causing several disorders and diseases [3]. One of the heavy metal which causes environmental threats is chromium, that arises from

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chromate manufacturing, electroplating, leather tanning [4] metal polishing, electroplating [5], pigments and dyeing industries and alloy and steel manufacturing [6]. Chromium mainly exits

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in the aquatic environment in two states Cr(III) and Cr(VI) [1]. The Cr(VI) is very toxic, and

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has high carcinogenic effect on biological system, so it is essential to get rid of Cr(VI) from

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wastewater former to its discharge into water forms [7]. Numerous methods have been applied for the removal of heavy metals from wastewater [8, 9] but unfortunately, the methods are limited for high operational cost and inefficient at trace

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level concentrations [10]. One of the most effective and economic physicochemical method

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for the removal of heavy metal from wastewater is the adsorption process [1]. Zeolites, clay, biopolymers such as starch, chitin and chitosan are classified as low-cost adsorbents [11]. Among the available biopolymers; chitosan has the highest adsorption ability towards heavy metal ions [12] due to its chemical and biological properties. Unfortunately, chitosan tend to agglomerate forming gels in aqueous solution, this problem can be modified by developing the reactivity of different functional groups present in chitosan. Different types of modifications of chitosan have been reported to increase the adsorption capacity and adsorption selectivity of metal ions [11, 13-17]. The insertion of functional groups in the chitosan matrix may improve its capacity of interaction with metallic ions by complexation. In this sense the modification of chitosan with aldehydes to produce Schiff bases may result in a potentially complexing material for metallic species with potential analytical and environmental applications [17].

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The introducing of engineered nanoparticles into drinking water treatment technologies against the removal of heavy metals seems as a very dynamic branch of nanotechnology [1821]. Magnetic nanoparticles have a specific concern in separation technique because they can simply be isolated by an external magnetic field placed outside of the extraction container [21-23]. Coating the magnetic particles provides them with a functionalizable surface [21], protects them from oxidation, reduces toxicity and aggregation and sequentially increases their physical stability which extended their storage life [2]. The ability of chitosan, modified cross-linked chitosan and modified magnetic chitosan as adsorbents for Cr(VI) ions in

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aqueous solution had been the emphasis of many researchers world-wide [24-34].

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The objectives of the present study were to: (a) synthesize and fully characterize two new adsorbents: modified chitosan cinnamaldehyde CTS-Cin and magnetite modified chitosan

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cinnamaldehyde Fe3O4@CTS-Cin; (b) examine their potential use in the aqueous removal of toxic Cr(VI); (c) investigate the parameters affecting the Cr(VI) uptake using batch methods

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as a function of the pH, the point of zero, contact time, initial metal ion concentration and the

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adsorbent dosage; (d) evaluate the adsorption properties in the batch experiments by applying different equilibrium isotherm models (e) estimate the kinetics of the adsorption process utilizing different kinetic models; (f) compare the adsorptive capacity of the magnetite and

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non-magnetite modified chitosan cinnamaldehyde with chitosan and other modified chitosan

2.

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adsorbents.

Materials and methods

2.1. Materials

Ferrous Sulphate heptahydrated FeSO4.7H2O, ferric chloride anhydrous FeCl3, sodium hydroxide, silver nitrate were purchased from Nice Chemicals, India. Chitosan, Cinnamaldehyde C9H8O, Potassium dichromate, 1,4-Dioxan C4H8O2, absolute ethanol, hydrochloric acid, sulphuric acid, phosphoric acid and acetone were purchased from SigmaAldrich Chemical Company, Germany as analytical grade. 1,5-diphenylcarbazide C13H14N4O and acetic acid glacial were purchased from LOBA Chemie, India. Deionized water obtained from aqua safe reverse osmosis system, United States. All chemicals were used as received without further purification.

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2.2. Instrumentation Metal analysis was determined using a double beam UV-Vis spectrophotometer (T80 + UV/Vis spectrometer, PG Instruments Ltd, UK) at a 542 nm wavelength at central laboratory unit, Faculty of Science, Alexandria University. pH meter (HANNA Instruments pH-211 Microprocessor pH-meter) was used for measurement of pH. FT-IR spectra (FT-IR, KBr Tablets; three millimeter thickness) were measured on a Perkin-Elmer FT-IR Spectrophotometer (FT-IR 1650). The spectra are recorded as KBr discs in the range varying from 350 – 4000 cm-1. Chemical compositions of metal complexes were determined by

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energy-dispersive X-ray spectroscopy (EDS) FEI (QUANTA-250) at the National institute of

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oceanography and fisheries (NIOF). X-ray powder diffraction (XRD) analysis was performed using (XRD-7000, Shimadzu, Japan) Cukα radiation (λ =1.54060 Å) at the scientific research

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and technology application city (Alexandria – Egypt). Scanning electron microscopy (SEM) analysis was performed using (JEOL-JSM5300) at the E-Microscope Unit, Faculty of

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Science- Alexandria University. High resolution transmission electron microscopy (HR-TEM)

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analysis was performed using A Japan-JEOL (JEM-2100) at Egyptian Petroleum Research Institute. The vibrating sample magnetometer (VSM) (Lake Shore -7400) was used for

Institute.

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magnetization analysis that performed at Central Metallurgical Research and Development

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2.3. Preparation of modified chitosan (CTS) 2.3.1. Synthesis of chitosan-cinnamaldehyde (CTS-Cin) A solution of cinnamaldehyde (6.93 g, 52.4 mmol) in 75.0 ml absolute ethanol was added to a suspension of chitosan (3.0 g in 75.0 ml absolute ethanol). The solution mixture was stirred and refluxed for 36 hours at 80 ̊C. With time, the colour of chitosan powder change from white to dark yellow. The formed dark yellow precipitate was filtered off, then washed with absolute ethanol and desiccated at room temperature [25, 35]. 2.3.2. Preparation of magnetic chitosan nanoparticles (Fe3O4@CTS) Magnetic chitosan nanoparticles (Fe3O4@CTS) were prepared by chemical co-precipitation of Fe(II) and Fe(III) ions in the presence of chitosan by using NaOH followed by treatment under hydrothermal conditions as described in literature [12]. All the prepared solutions were

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bubbled with nitrogen gas to remove dissolved oxygen. Chitosan 2.0 g was dissolved in 100mL (20%) acetic acid and mixed with FeSO4.7H2O (3.31 g, 11.9 mmol) and FeCl3 (4.34 g, 26.8 mmol) using 1:2 molar ratio under nitrogen atmosphere. The formed solution was precipitated at 40 °C using 1.0 M NaOH solution at controlled pH (10.0–10.4). The suspension was heated at 90 °C for 2 h under stirring and finally recovered by decantation and magnetic separation. The precipitate was washed several time with distilled water until complete removal of chloride ion followed by absolute ethanol, filtrated and dried in vacuum oven for 72 hours at 60°C.

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2.3.3. Preparation of magnetic chitosan-cinnamaldehyde (Fe3O4@CTS-Cin).

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A solution of cinnamaldehyde (5.18 g, 4.9 mmol) in 75.0 ml of 1,4-dioxan has been added to a suspension of Fe3O4@CTS 3.0 g in 75.0 ml 1,4-dioxan. The resulted mixture was

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alkalinized to pH (9.5–10.0) using a 1.0 M NaOH solution, stirred and refluxed for 36 hours

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in vacuum oven for 48 hours at 60°C [12].

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at 80°C. The product was washed with distilled water followed by ethanol, filtrated and dried

2.4. Batch adsorption experiments of Cr(VI)

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Batch experiments were carried out to study the effect of the prepared adsorbents CTS-Cin, Fe3O4@CTS-Cin on the adsorption of Cr(VI) as a function of pH, contact time, mass of

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adsorbent, initial metal ion concentration and adsorption temperature. Potassium dichromate (K2Cr2O7) was used for preparing the stock solutions of Cr(VI). The concentrations of remaining Cr(VI) were determined by measuring the absorbance of the purple complex of Cr(VI) with 1,5-diphenylcarbohydrazide method using UV-Vis spectrophotometer at wavelength 542 nm.

The stock solution of Cr(VI) 1000 ppm, was prepared by dissolving 2.83 g of K2Cr2O7 in deionized water in 1000 mL measuring flask. Solutions of different Cr(VI) concentrations were obtained by diluting the stock solution with deionized water, HCl 1.0 M and NaOH 1.0 M solutions were used to adjust pH of the solution using pH-meter. The experiments were carried out in 125 ml stoppered glass bottle containing 50 mL Cr(VI) solution and agitated on orbit shaker at a shaking rate 250 rpm.

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2.4.1. Effect of pH The pH of Cr(VI) solution was adjusted in the range of 1.0 to 8.0 by adding 1.0 M hydrochloric acid or 1.0 M sodium hydroxide. A weight of 0.15 g of the adsorbents were added to a 50.0 mL of Cr(VI) solution (50.0 mg L-1) and shaken at a constant shaking rate of 250 rpm at room temperature for predetermined time [36]. The removal percentage (% R) was calculated using Eq. (1). % R = (C o - Ce)/ Co  100

(1)

Effect of contact time

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2.4.2.

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respectively in the solution after adsorption [36].

equilibrium Cr(VI) concentration,

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Where Co and Ce (mg L-1) represent the initial and

The metal sorption capacity was determined at the optimum pH value for the Cr(VI). A

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weight of 0.15 g of the adsorbents were added to 50.0 mL of Cr(VI) solution

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(50.0 mg L-1). The whole system was shaken at a constant shaking rate of 250 rpm at room temperature and at appropriate intervals in the range of (2 to 360) min [37]. The adsorption

qt = (C o – Ct) V/ m

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capacity (the amount of adsorption at time t), was calculated by Eq. (2). (2)

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Where qt (mg g-1) is the amount of adsorbed metal ion at time t (min), C0 and Ct (mg L -1) are the initial concentration and the metal ion concentration remaining in aqueous solution at time t (min), respectively, V (L) is the volume of the solution, and m (g) is the adsorbent mass [37].

2.4.3. Effect of initial Cr(VI) concentration A 50.0 mL of Cr(VI) solutions with initial metal ion concentrations C0 varying from 5.0 to 300.0 mg L-1 were placed in stoppered glass bottle (125 mL). Equal mass of 0.15 g of the adsorbents were added. The system was shaken at a constant shaking rate of 250 rpm at room temperature for predetermined time [37]. The amount of adsorption at equilibrium (mg g-1), was calculated by using Eq. (2).

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2.4.4. Effect of adsorbent dosage Different amounts of the adsorbents varying from 0.025 to 0.400 g were added to a 50.0 mL of Cr(VI) solution (50.0 mg L-1). The whole system was shaken at a constant shaking rate of 250 rpm at room temperature for predetermined time [37]. 2.4.5. The point of zero charge (pHPZC) The point of zero charge was determined by preparing a series of 125 ml stoppered glass bottle containing 0.1 N NaCl adjusted at different pH (1.0 to 10.0) using 1.0 M HCl and 1.0

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M NaOH. Equivalent mass of 0.15 g of the adsorbents were added to every flask and the

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mixture was shaken for 24 and 48 hours. The final pH of each solution was recorded. The difference between the initial pH (pH0) and final pH (pHf) values was calculated using Eq. (3)

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and plotted versus the pH0. The point of intersection at which ΔpH equal zero, gave the pHPZC

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[36, 37].

3.

Results and discussion

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3.1. Synthesis and characterization

(3)

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ΔpH = pH0 – pHf

CTS-Cin and Fe3O4@CTS-Cin, were prepared as shown in, Scheme 1. The prepared adsorbents are stable in air and were fully characterized using different analytical tools.

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The FT-IR spectral data of CTS, CTS-Cin, Fe3O4 and Fe3O4@CTS-Cin are listed in Table 1. The infrared spectrum of chitosan was in agreement with what reported in literatures [12, 24, 25, 35]. The FT-IR spectrum of CTS-Cin showed strong absorption bands at 1634 cm-1 due to ѵ (C=N) of azomethine. No characteristic bands for the free aldehyde groups were observed in the spectra of CTS-Cin. The bands at 1567 correspond to ѵ (C=C) while the band at 753 assigned to the δ (C–H) in the aromatic ring for CTS-Cin [25, 35]. The FT-IR spectrum of Fe3O4@CTS displayed the same characteristic absorption bands of chitosan, in addition to new bands at 582 and 405 cm-1 related to Fe–O group [6]. The bands of ѵ (O–H), ѵ (N–H), ѵ (C=O) and δ (N–H) were shifted to lower wave number, confirming successful coating of CTS to the Fe3O4. Also, the spectrum of Fe3O4@CTS-Cin lack any adsorption due to ѵ (C=O) and δ (N–H) and showed strong absorption band at 1630 cm-1, 1541 and 749 cm-1 that belong to ѵ (C=N) of azomethine, ѵ (C=C) and δ (C–H), respectively [25, 35].

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Scheme 1- Synthesis of CTS-Cin and Fe3O4@CTS-Cin adsorbents

Compounds

(NH),

( C-H)

3439

CTS-Cin

3431

Fe3O4

2921, 2880

1652

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CTS

(C=O)

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(OH)

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Table 1- FT-IR spectral data of CTS, CTS-Cin, Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin.

3434

 (C=C)

antis

δ (C-H)

(imine)

aromatic ring

bridge C-O-C

aromatic ring

1586

----

----

1155

----

----

δ (N-H)

 (C=N)

 (Fe-O)

2921, 2881

----

----

1634

1567

1145

753

----

----

----

----

----

----

----

----

587, 414

Fe3O4@CTS

3363

2927, 2885

1637

1562

----

----

1150

----

582, 405

Fe3O4@CTS-Cin

3374

2926, 2884

----

----

1630

1541

1149

749, 796

582, 408

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The Energy dispersive X-Ray spectrum (EDS), of CTS-Cin, Fig. 1(a), showed C, N, O peaks which is the major constituent of the modified chitosan. The EDS spectrum of Fe3O4@CTS-Cin,

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Fig. 1(b) displayed a new peak, corresponding to Fe [9, 23].

Fig. 1- EDS spectra of (a) CTS-Cin and (b) Fe3O4@CTS-Cin

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The X-ray diffraction (XRD) patterns of Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin nanoparticles are shown in Fig. 2. The six characteristic diffraction peaks For Fe3O4 at 2θ

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=30.2, 35.6, 43.3, 53.6, 57.3 and 62.8 correspond to the Bragg diffractions of the crystal

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planes (220), (311), (400), (422), (511) and (440), respectively were observed in the two adsorbent [9, 12, 23], indicating that the Fe3O4@CTS maintained the crystalline structure of Fe3O4 and the full width at half maximum (FWHM) of the peak at 2θ= 35.5 (0.81) is higher

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than at 2θ= 35.6 of Fe3O4 (0.77). The XRD patterns of Fe3O4@CTS-Cin were shifted slightly 62.6.

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to lower angles and their intensity were decreased at 2θ = 29.9, 35.4, 43.1, 53.3, 57.0 and

The crystallite size of Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin were calculated by Debye Scherer's Eq. (4). D = 0.9  / Cos 

(4)

Where D is the crystallite size in nm, λ is the wave length (0.154060 nm), β is the line broadening in radian acquired from the (FWHM) of diffraction peak and θ is the diffraction angle. The calculations were performed on the major peak corresponding to index (311) [12]. The crystallite size of Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin nanoparticles was found to be 10.36, 9.84 and 8.3 nm, respectively. It is clear that the crystallite size decreases upon coating Fe3O4 and the coating is affected by the type of modifier used.

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The scanning electron microscopy (SEM) photographs, Fig. 3, show that chitosan (CTS) has more regular surface and large particle size than CTS-Cin. The CTS-Cin is porous irregular structure with holes that may increase the contact area and hence increase the metal ion adsorption [25]. The high resolution transmission electron microscopy (HR-TEM) images of the Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin nanoparticles are given in Fig. 4. The mean diameter μ for the nominated nanoparticles are 9.49, 12.11 and 9.10, respectively. Fe3O4 and Fe3O4@CTS have roughly spherical morphology and are homogeneously distributed and quite

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agglomerate. The image of Fe3O4@CTS-Cin showed that the coating shell have quasi-sphere

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and tubes shapes. The agglomeration of crystals is attributed to the dipole-dipole magnetic attraction of nanoparticles [12]. Moreover, TEM images showed different contrast of

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Fe3O4@CTS and Fe3O4@CTS-Cin, the dark areas represent crystalline Fe3O4 while the bright ones are assigned for amorphous unmodified and modified chitosan. The observed data

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revealed that the average diameter increased from 9.49 nm (Fe3O4) to 12.11 nm

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(Fe3O4@CTS) due to the incorporation of iron to the chitosan matrix. However, a decreased in the average diameter was observed upon coating magnetite with modified chitosan Fe3O4@CTS-Cin (9.10 nm) reflecting the increasing in the specific surface area of the

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magnetic modified chitosan. The data agreed with the crystallite size obtained by XRD.

Fig. 2- X-ray diffraction pattern of Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin

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Fig. 3 - SEM image of (a) CTS, (b) CTS-Cin

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Fig. 4- HR-TEM images of (a) Fe3O4, (b) Fe3O4@CTS and (c) Fe3O4@CTS-Cin Vibrating sample magnetometer (VSM) was used to examine the magnetic properties of

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Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin at room temperature. The magnetization curves are presented in Fig. 5. The values of remanence magnetization (Mr) and coercivity field (Hc) are nearly zero, suggesting super paramagnetic properties of the adsorbents. The magnitudes of

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the saturation magnetization (Ms) (when 1/H ≈ 0), were found to be 46.974, 25.717 and 20.343 emu/g, for Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin, respectively [38], suggesting that, the coating of Fe3O4 with chitosan or modified chitosan decreases the saturation magnetization of Fe3O4 [9, 12].

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Fig. 5 - Magnetic hysteresis curves of Fe3O4, Fe3O4@CTS and Fe3O4@CTS-Cin

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3.2.1. The effect of point of zero charge (pHpzc)

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3.2. Cr(VI) Uptake studies

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The point of zero charge (pHPZC) of the adsorbents was determined by plotting the difference between the initial pH (pH0) and final pH (pHf) values (ΔpH= pH0 − pHf) versus the pH0 after

ΔpH

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1.5 CTS-Cin… 1 CTS-Cin… 0.5 0 -0.5 0 1 2 3 4 5 6 7 8 9 10 11 -1 -1.5 -2 -2.5 pH0

5

Fe3O4@CTS-Cin (24 h)

4

Fe3O4@CTS-Cin (48 h)

3 ΔpH

and Fe3O4@CTS-Cin, respectively.

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24 and 48 hours as shown in Fig. 6; [36]. The value was found to be 5.6 and 4.9 for CTS-Cin

2 1 0 -1 0 -2

1

2

3

4

5

6

7

8

9 10 11

pH0

Fig. 6- Point of zero charge of (a) CTS-Cin (b) Fe3O4@CTS-Cin 3.2.2. Effect of pH The batch equilibrium studies at different pH values were carried out in the range of 1–8. The % removal of Cr (VI) and the adsorption capacity onto both adsorbents as a function of pH are shown in Fig. 7. The % removal of Cr(VI) reached maximum at pH 3.0 and 2.0 for CTSCin and Fe3O4@CTS-Cin respectively, then decreased sharply with increasing pH to 8.0. Based on the point of zero charge of the adsorbents, the observed decrease in the uptake

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capacity value at pH <2 and pH <3 for CTS-Cin and Fe3O4@CTS-Cin adsorbents respectively may attributed to the predomination of H2CrO4 and strong competition for adsorption sites between H2CrO4 and protons. However, as the pH increased to 8.0, the adsorption capacity decrease, this may revealed to the competition of CrO42- and OH- for adsorption [39]. So an optimized pH of 3.0 and 2.0 for CTS-Cin, and Fe3O4@CTS-Cin, respectively, were chosen for further adsorption studies. Taking into account, the CTS-Cin adsorbent exhibited higher capacity for Cr(VI) than Fe3O4@CTS-Cin, this may attributed to the higher availability of active groups of the inserted cinnamaldehyde moieties which are able to chelate the metal

CTS-Cin

CTS-Cin Fe3O4@CTS-Cin

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-p

%R

qe (mg/g)

Fe3O4@CTS-Cin

18 16 14 12 10 8 6 4 2 0

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100 90 80 70 60 50 40 30 20 10 0

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cation.

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

pH

pH

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0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Fig. 7- Effect of pH on the % removal and adsorption capacity of Cr(VI) onto

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CTS-Cin and Fe3O4@CTS-Cin 3.2.3. Effect of contact time

The adsorption of Cr(VI) onto both adsorbents was explored with increasing contact time from 0 to 300 min and displayed in Fig. 8. The data revealed that the adsorption capacity increased as the contact time increase. The equilibrium adsorption capacity occurred at about 35, 80 min with a % removal of 93 %, and 95 % for CTS-Cin and Fe3O4@CTS-Cin, respectively. Within the first 10 minute the % removal are 90 %. The rapid uptake of Cr(VI) at the early stage of the adsorption may attributed to the availability of positively charged sites on the adsorbent surface and the rapid chelation of Cr(VI) ions to the adsorbents used. As the time increases, these sites got occupied and the remaining adsorbate ions would compete for the remaining unoccupied sites, resulting in slower adsorption rate [40]. The Fe3O4@CTS-Cin adsorbent showed a good efficiency and high adsorption capacity to Cr(VI) rather than CTS-

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Cin. Due to the increase porosity of surface area accompanied by the decrease in the particles

16.2 16 15.8 15.6 15.4 15.2 15 14.8 14.6 14.4 14.2

CTS-Cin Fe3O4@CTS-Cin

0

40

100 98 96 94 92 90 88 86 84

%R

qt (mg/g)

size of Fe3O4@CTS-Cin as observed from XRD data [29].

CTS-Cin Fe3O4@CTS-Cin

0

80 120 160 200 240 280 320

40

80

120 160 200 240 280 320

Time (min)

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Time (min)

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Fig. 8 - Effect of contact time on Cr(VI) adsorption capacity onto CTS-Cin and

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Fe3O4@CTS-Cin.

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3.2.4. Effect of initial concentration

As shown in Fig. 9, as the initial concentration of Cr(VI) increased from 5.0 mg/L to 300.0

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mg/L the adsorption capacity for CTS-Cin adsorbent, increased from 3.32 mg/g to 59.74 mg/g and the percentage removal decreased from 100% to 59.74%, respectively. However, for

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Fe3O4@CTS-Cin, increasing the initial concentration of Cr(VI) from 5.0 mg/L to 300.0 mg/L, the adsorption capacity increased from 1.63 mg/g to 56.92 mg/g and the percentage removal

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decreased from 97.71% to 56.92%, respectively. As the initial concentration of Cr(VI) increased, the adsorption capacity increased owing to the increase occupation of the available binding sites and the increase of contact between chromium ion and adsorbent particles. The initial concentration affords a significant driving force to overcome all mass resistance of all molecules between the aqueous solution and adsorbents. 3.2.5. Effect of Adsorbent Dosage The % removal of Cr(VI) and its adsorption capacity onto the prepared adsorbents as a function of adsorbents dosage are represented in Fig. 10. The percentage removal of Cr(VI) by both adsorbents increased as the mass of the adsorbents increased up to a certain limit "optimum dosage" then it remains almost constant. The optimum dosage for such adsorption process is 0.25 g for both adsorbents. Conversely, the adsorption capacity decreases with increasing the dosage of the adsorbent. Thus increasing the dosage of the adsorbents increases the surface area and allows more available adsorption sites and in turn increases the removal efficiency.

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Increasing CTS-Cin adsorbent dose from 0.025 g to 0.4 g, the percentage removal increased from 64% to 92.2%, while the adsorption capacity decreased from 64.04 to 5.76 mg/g, respectively. When Fe3O4@CTS-Cin dose increase from 0.025 g to 0.4 g the percentage removal increased from 34.2 % to 100%, while the adsorption capacity decreased from 34.19 mg/g to 6.25 mg/g. The high adsorption capacity observed for 0.025 g of the adsorbents was attributed to easy accessibility of free active sites on the surface of the adsorbents. By increasing the adsorbent dose, the number of active sites increased but ratios of Cr(VI) to active sites decreased causing a gradual decrease of the adsorption capacity. However, for Fe3O4@CTS-Cin the % removal of aqueous Cr(VI) ion reaches 100% upon using 0.3 g which

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may be attributed to the increase in the surface area accompanied by the decrease in particles

60

120

50

100

40

80

30

60

20

40

20

10

20

0

0

60 30 40

lP

20 10 0 40

80 120 160 200 240 280 320

0 0

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0

-p

40

%R

80

qe (mg/g)

50

qe (mg/g)

60 100

%R

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40

80 120 160 200 240 280 320

Co (mg/L)

Co (mg/L) qe

%R

120

CTS-Cin

re

70

ro

size of Fe3O4@CTS-Cin as observed from TEM data [29].

qe

%R

Fig. 9 - Effect of initial concentration of Cr(VI) onto CTS-Cin and Fe3O4@CTS-Cin

qe (mg/g)

50

80

40

CTS-Cin

30

60 40

20

20

10 0

100

0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

40 35 30 25 20 15 10 5 0

120 100 80 60 40 20 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

m (g)

m (g) qe

qe

%R

%R

Fig. 10 - Effect of adsorbents dosage on Cr(VI) uptake onto CTS-Cin and Fe3O4@CTS-Cin

%R

60

120

%R qe (mg/g)

70

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3.3. Characterization of the adsorbents after Cr(VI) uptake As shown in Fig. 11, the EDS spectra of the adsorbents after Cr(VI) adsorption displayed a new peak appearing at 5.4 KeV associated to Cr element, and providing the evidence for Cr(VI) adsorption onto the adsorbents surface [9, 23]. The surface morphology of CTS-Cin after adsorption of Cr(VI), Fig. 12(a), was observed to have much asperity and coarsely surface. The HR-TEM image of the Fe3O4@CTS-Cin after Cr(VI) adsorption is given in Fig. 12(b). It showed that the coating shell has tubes shape with

lP

re

-p

ro

of

an average size of 24.42 nm.

Jo ur

na

Fig. 11- EDS spectra after adsorption of Cr(VI) onto (a) CTS-Cin and (b) Fe3O4@CTS-Cin

Fig. 12- (a) SEM image after adsorption of Cr(VI) onto CTS-Cin (b) HR-TEM image after adsorption of Cr(VI) onto Fe3O4@CTS-Cin

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3.4. Cr (VI) uptake kinetics In order to investigate the mechanism of Cr(VI) adsorption onto the two prepared adsorbents, pseudo-first-order, pseudo-second-order, intra-particle diffusion, Elovich and Boyd kinetic models were used. The linear forms of these models were utilized and expressed using Eqs. (5) – (9). The pseudo-first order: The pseudo-second order:

ln (qe – qt) = ln qe – k1 t t/q = 1/h + t /qe and h =

(5) k2 qe2

0.5

(6)

qt = Ci + kid t

(7)

Elovich model:

qt = (1/β) ln α β + (1/β)

(8)

Bt = - 0.4977- ln (1-F) and F = qt /qe

(9)

Boyed model: -1

of

Intra particle diffusion:

-1

ro

Where qt (mg g ) and qe (mg g ) are the quantity of metal ion adsorbed on adsorbent at time t (min) and at equilibrium, respectively. k1 (min-1) and k2 (g mg-1 min-1) are the rate constant of

-p

the pseudo-first-order and the pseudo-second-order adsorption, respectively. α (mg g-1 min-1) is the initial sorption rate and β (g mg-1) is correlated to the degree of surface coverage and

re

activation energy for chemisorption. The value of (1/β) is revealing to the number of available sites for adsorption while (1/β) ln α β is the adsorption magnitude when ln t is equal to zero

lP

[37, 41]. kid (mg g–1 min–0.5) is the intra-particle diffusion rate constant, which is estimated from the slope of the straight line of qt versus t0.5 and Ci is intercept of straight line which

na

gives an information about the thickness of boundary layer (mg g-1). Bt is the mathematical function of F which represents the fraction of solute adsorbed at time t (min) [37]. The

Jo ur

validity of each model is checked by the fitness of the straight line (R2), (Figs. 13a-e) as well as the consistence between experimental and calculated values of q e. The Kinetic data for Cr(VI) adsorption onto the two prepared adsorbents calculated from the related plots are summarized in Table 2.

From Fig. 13(a), it is clear that, the difference between the calculated and experimental adsorption capacity indicated by Δqe= qe (calc.) – qe (exp.) is high. Moreover, the values of R2 are relatively far from the linearity, indicating a poor pseudo first order fit to the experimental data. The R2 values for different models indicated that the pseudo-second-order kinetic model fits best since its highest value were close to 1 (R2 = 0.999). The calculated values qe, (cal) was very close to obtained qe, (exp); Fig.13b. Hence, the adsorption of Cr(VI) onto both adsorbents could obey the pseudo-second-order kinetic model ,suggesting that the

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predominant process here is chemisorption, which involves a sharing of electrons between the adsorbate and the surface of the prepared adsorbents [36, 37]. Elovich model was examined for describing adsorption on heterogeneous surface. It is shown in Fig. 13(c) that the Elovich equation matches the experimental data well with high correlation coefficient (R2) values indicating the heterogeneity of adsorbent surfaces [37, 42]. The values of Elovich parameters α and β for CTS-Cin are higher than that of Fe3O4@CTSCin demonstrating that, the number of sites available for adsorption in CTS-Cin, are higher than that in Fe3O4@CTS-Cin adsorbent.

of

The intra-particle diffusion model has been applied to determine the rate limiting step.

ro

Adsorption diffusion models are always constructed on the basis of four consecutive steps: (i) diffusion of the adsorbate from the bulk of solution to the boundary layer around the

-p

adsorbent particles; (ii) diffusion across the boundary layer; (iii) diffusion in the liquid contained in the pores (intraparticle diffusion); and (iv) adsorption and desorption between the

re

adsorbate and active sites. The limiting step could be the film diffusion, or the intraparticle

lP

diffusion or a combination of the steps [36, 43]. The plots qt versus t0.5 for the adsorption of Cr(VI) ions onto the prepared adsorbents did not follow Weber and Morris equation [37], Fig. 13(d), proposing that even though the adsorption process involved intra-particle diffusion, it is

na

not the only rate-controlling step. The low Ci values Table 2 indicated low mass transfer resistance [36], however, the two different mechanisms intra-particle diffusion and film

Jo ur

diffusion are involved at different intervals. Boyd model is tested to differentiate between the pore and film diffusion [6, 36]. As displayed in Fig. 13(e) the linear plot of Bt versus t for the adsorption of Cr(VI) ions onto the prepared adsorbents does not pass through the origin, confirming, film diffusion or chemisorption is the rate limiting step [36].

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1 0.5 0 -0.5 0 -1 -1.5 -2 -2.5 -3 -3.5 -4

20 18 16 14 12 10 8 6 4 2 0

(a) Pseudo-First-Order 60

80

t/qt (min g/mg)

40

ln (qe-qt)

20

(b) Pseudo-Second-Order

0

40

80

120 160 200 240 280 320

Time (min)

16.5

Time (min) CTS-Cin

Fe3O4@CTS-Cin

16.5

(c) Elovich Model

16 15.5

qt (mg/g)

15.5 15

15

-p

qt (mg/g)

(d) Intra particle Diffusion

ro

16

Fe3O4@CTS-Cin

of

CTS-Cin

14.5

re

14.5 14 2

3

ln t

CTS-Cin

4

5

6

14 0

2

lP

1

Fe3O4@CTS-Cin

na

0

6

6

CTS-Cin

8 10 12 t0.5 (min0.5)

14

16

18

Fe3O4@CTS-Cin

(e) Boyd Model

Jo ur

5

4

Bt

4 3 2 1 0

0

10

20

30

40

50

60

70

80

Time (min) CTS-Cin

Fe3O4@CTS-Cin

Fig. 13- Different adsorption kinetics models for Cr(VI) adsorption onto CTS-Cin and Fe3O4@CTS-Cin

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Table 2- Adsorption kinetics parameters for the removal of Cr(VI) by the prepared adsorbents Pseudo first order qe exp. (mg/g)

qe Calc. (mg/g)

Δq

K1 (min-1)

R2

CTS-Cin

15.568

1.251

- 14.317

0.1115

0.9595

Fe3O4@ CTS-Cin

15.867

1.309

- 14.558

0.041

0.9824

Pseudo second order (min-1)

R2

0.18

0.148

of

36.704

0.9999

0.21

ro

K2

h= K2 qe2 (mg/(g min))

0.064

16.542

0.9999

qe exp. (mg/g)

qe Calc. (mg/g)

Δq

CTS-Cin

15.568

15.748

Fe3O4@ CTS-Cin

15.867

16.077 1/β ln αβ

CTS-Cin

0.2097

14.68

Fe3O4@ CTS-Cin

0.3461

14.249

α (mg/(g min))

R2

4.769

5.30*1029

0.9118

2.889

2.625*1017

0.9627

β (g/mg)

lP

re

1/β

-p

Elovich Eq.

Intra particle diffusion

Kip

Fe3O4@ CTS-Cin

Ci

Stage 2 R2

Kip

2-35 min

Jo ur

CTS-Cin

na

Stage 1

0.1918

14.493

14.383

0.9787

0.0147

15.501

0.9595

0.8942

120-300 min 0.897

0.0317

15.54

0.9097

Boyd model

R2 CTS-Cin

R2

50-300 min

2-80 min

0.1845

Ci

R2 Fe3O4@ CTS-Cin

0.9824

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3.5. Adsorption isotherm models The adsorption isotherms models reflect the surface characteristics of the adsorbent, the possibility of the interaction between adsorbate and adsorbent and the homogeneity or heterogeneity of the solid surface of the adsorbent. Furthermore, it donates physicochemical information on how the adsorption occurs and how the reaction between adsorbate and adsorbent surface proceed [18]. The adsorption data of both adsorbents were analyzed using five isotherm models. The linear form of these models is given in Table 3 [37]. The linear plotting of these five adsorption models are shown in Fig. 14(a-e) and the parameters of

of

isotherm models are displayed in Table 4.

Linear form

Parameter

-p

Isotherm model Langmuir

ro

Table 3 - Equations describing adsorption isotherm models

re

Ce / qe = 1/ (qm KL) + Ce / qm

Temkin

DubininRadushkevich

log qt = log Kf + 1/n( log Ce)

Jo ur

Freundlich

na

lP

RL = 1/(1+KLCo)

qe = ( RT/bTe) lnKTe + (RT/bTe) lnCe

ln qe = ln qm – B ε2

ε = RT ln [1+ 1/Ce] E = 1/ (2B)0.5 Flory-Huggins

log θ/Co = log kFH + nFH log (1-θ) θ = (1-Ce/Co) KFH = exp (– ΔGo/RT)

Ce equilibrium concentration in solution (mg L-1), qe amount of adsorption at equilibrium (mg g-1), qm maximum adsorption capacity (mg g-1), KL Langmuir isotherm constant (L mg-1), RL separation factor (0 < RL < 1), Co initial adsorbate concentration (mg L-1). Kf measure of adsorption capacity (mg g-1), 1/n adsorption intensity KTe Temkin isotherm constant (L g-1), bTe Temkin constant related to heat of sorption (J mol-1), R gas constant (8.314 J mol-1 K-1), T absolute temperature (K) B constant related to the mean free energy of adsorption per mole of adsorbate, ε Polanyi potential, E mean free energy of sorption (KJ mol-1). θ the degree of surface coverage, kFH the Flory-Huggins equilibrium constant, nFH model exponent. ΔGo the standard free energy change.

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3.5.1. Langmuir isotherm model A linear plot of Ce/qe versus Ce, Fig. 14(a), is employed for the prepared adsorbents to determine the value of qm and KL. As presented in Table 3, the correlation coefficients (R2) of the linear form for Langmuir model were 0.9919, 0.9935 for CTS-Cin and Fe3O4@CTS-Cin respectively, indicating that Langmuir isotherm correlated well with the equilibrium data for the adsorption of Cr(VI) onto both adsorbents. The values of K L indicated high adsorption affinity of Cr(VI). The monolayer saturations capacity, qm, is shown to be 61.349 and 58.139 mg g-1 for CTS-Cin and Fe3O4@CTS-Cin, respectively. The values of RL for both adsorbents

of

decreased with increasing the initial concentration from 5.0 to 300.0 mg L-1 and it lies in the

ro

range (0 < RL < 1) suggesting the suitability of the prepared adsorbents for the adsorption process of Cr(VI) from aqueous solutions.

-p

3.5.2. Freundlich isotherm model

re

From the slope and intercept of the linear plot of log q e versus log Ce for the adsorption of

lP

Cr(VI) onto the prepared adsorbents Fig. 14(b), the values of 1/n and kF can be obtained, The variation of 1/n values from 0.395 to 0.465 demonstrated the heterogeneous surfaces of the adsorbents and the high favourability of Cr(VI) adsorption onto the prepared adsorbents. The

na

obtained linear regression lines for the adsorption of Cr(VI) onto CTS-Cin have correlation coefficients (R2 = 0.971) higher than that for Fe3O4@CTS-Cin (R2 = 0.94). The compiled data

Jo ur

in table 3 imply that the Langmuir model is more suitable and best fit for the adsorption of Cr(VI) onto the adsorbents than Freundlich model. 3.5.3. Temkin isotherm model

The experimental equilibrium data for the adsorption of Cr(VI) on the prepared adsorbents are fitted with the Temkin isotherm model and presented in Fig. 14(c). From the plot of qe versus ln Ce, the isotherm constants bTe and KTe were determined from the slop and the intercept. The variation values of the adsorption energy term bTe are positive, indicating exothermic adsorption reaction [36]. Based on R2 values, Temkin isotherm appears to provide a good fit with the equilibrium data for the adsorption of Cr(VI) onto Fe3O4@CTS-Cin rather than CTS-Cin.

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3.5.4. Dubinin-Radushkevich isotherm model Analysis of the adsorption data using D-R model is displayed in Fig. 14(d). The mean free energy of sorption (E) derived from D-R equation was confirmed using correlation coefficient values (R2). The E values derived from D-R model are 12.13 and 11.32 KJ mol-1 for CTS-Cin and Fe3O4@CTS-Cin, respectively, which being more than 8 and less than 16, suggesting ion exchange adsorption mechanism and that the Cr(VI) has more or less the same affinity for adsorption onto the adsorbents. These results are in agreement with those obtained from Temkin isotherm model, in which the values of bTe are high and more or less the same for the

of

two adsorbents demonstrating strong interaction between sorbate and sorbent supporting ion

ro

exchange or chemical adsorption.

-p

3.5.5. Flory-Huggins isotherm model

The kinetic coefficient (Flory-Huggins equilibrium constant) was determined from the linear

re

representation of the function log θ/Co versus log (1-θ) Fig. 14(e). The results of the experimental model are best described by Flory-Huggins model where the values of R2 are

lP

higher than the values obtained in case of Temkin model. The positive ΔGo values showed

na

that the adsorption process is nonspontaneous in nature. Comparing the correlation coefficient values obtained upon applying the different isotherm

Jo ur

models, Table 4, it is concluded that Langmuir model fitted well the adsorption data and that the other models were applicable. 3.6. Adsorption thermodynamic

Generally , the temperature has a significant effect on the adsorption process, whereas the temperature of solution increase, the viscosity of the aqueous solution containing chromium ions will decrease and hence the rate of adsorbate diffusion across the external boundary layer and in the internal pores of the adsorbent increases. Temperature may also affect the equilibrium of the adsorbate depending on the nature of the adsorption process; whether it is endothermic or exothermic. Furthermore, it has an effect on the stability of the metal ion species initially placed in solution [39].

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2

2

log qe

Ce/qe (g/L)

(b) Freundlich Model

(a) Langmuir Model

2.5

1.5

1.5 1

1

0.5

0.5 0 20

40

60

80

100

120

0

140

-1.5

Ce (mg/L) CTS-Cin

-1

Fe3O4@CTS-Cin

0 -0.5 -20

2.5

3.5

4.5

ln Ce Fe3O4@CTS-Cin

ro

ln qe

qe (mg/g) 1.5

200

400

600

800

1000

5.5

ε*ε CTS-Cin

Fe3O4@CTS-Cin

na

(e) Flory-Huggins

-2

0 -1.5

-1

Jo ur

-2.5

2

Fe3O4@CTS-Cin

lP

CTS-Cin

0.5

1.5

-p

-1.5

0 -2 0 -4 -6 -8 -10 -12

re

-2.5

1

(d) Dubinin-Radushkevich Model

log (θ/C0)

-3.5

0.5

CTS-Cin

60

20

0

log Ce

(c) Temkin Model 40

-0.5

of

0

-0.5

-0.5

0

-1 -1.5 -2 -2.5

log (1-θ) CTS-Cin

-3

Fe3O4@CTS-Cin

Fig. 14- Different adsorption isotherm models for Cr(VI) adsorption onto CTS-Cin and Fe3O4@CTS-Cin

1200

2.5

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Parameters

CTS-Cin

Fe3O4@CTS-Cin

Freundlich

Kf (mg/(g(mg/L)1/n)) N 1/n R2

10.4232 2.5336 0.3947 0.971

8.3176 2.1501 0.4651 0.9372

qm (mg/g)

61.3497

58.1395

KL (L/mg) R2

0.1271 0.9919

0.1565 0.9935

Temkin

KTe (L/mg) bTe (J/mol) R2

5.9578 302.6052 0.8733

4.8068 304.7337 0.9661

DubininRadushkevich (D-R)

qm (mg/g) B E (KJ/mol) R2

2.3098*10-3 0.0034 12.1268 0.9599

3.2673*10-3 0.0039 11.3228 0.9737

Flory-Huggins

KFH (L/g) nFH º ΔG (KJ/mol) R2

1.0940*10-3 -0.9307 +16.6086 0.9405

6.6527*10-4 -1.2018 +18.4283 0.906

-p

re

lP

na

Jo ur

Langmuir

of

Isotherm Model

ro

Table 4 - Adsorption Isotherm parameters for removal Cr(VI) onto the prepared adsorbents

To evaluate the thermodynamic parameters of Cr(VI) adsorption on both adsorbents , the adsorption experiments were performed at three different temperatures (303, 313 and 323 K). The experimental data obtained at different temperatures were used in calculating the thermodynamic parameters such as Gibbs free energy (ΔGº), enthalpy (ΔH) and entropy (ΔS) according to Eqs. (10) – (11). ΔGº = - R T ln KL ln KL = [ΔSº/R] - [ΔHº/R]

(10) Van' t Hoff Eq.

(11)

Where KL (L mg-1) is the Langmuir adsorption constant, R is the universal gas constant (8.314 Jmol-1K-1) and T is the absolute temperature in Kelvin. Both ∆Hº and ∆Sº can be determined

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from Van’t Hoff equation from the slope and intercept of the line obtained by plotting ln K L against 1/T, respectively [37]. The obtained data are collected in Table 5. As presented in Table 5 the temperature increased from 303 to 323 K, the adsorption capacity of the CTS-Cin, adsorbent increased from 49.53 mg g-1 to 50.67 mg g-1, reflecting the endothermic nature of the adsorption process. On the other hand, the adsorption capacity of Fe3O4@CTS-Cin adsorbent decreases from 53.89 mg g-1 to 50.77 mg g-1 upon increasing temperature from 303 to 323 K, indicating that the adsorption process is exothermic in nature.

of

The estimated thermodynamics parameters ∆Gº, ∆Hº and ∆Sº, endorse the above conclusion in which, the positive ∆Hº values for the adsorption of Cr(VI) onto the CTS-Cin proved the

ro

endothermic nature of adsorption and governs the possibility of chemical adsorption which supported by the increase in adsorption capacity with increasing temperature. The positive

-p

∆Sº values indicate the increased of disorder and randomness during the adsorption process

re

and reflects the affinity of the adsorbent for the Cr(VI) ions. On the other hand, the negative values for ∆Hº and ∆Sº for the adsorption of Cr(VI) onto the Fe3O4@CTS-Cin adsorbent

lP

confirm the exothermic nature of the adsorption process and the decrease of randomness at the solid solution interface. Moreover, the positive ∆Gº values suggested the non-spontaneous

na

nature of Cr(VI) adsorption .

CTS-Cin

Jo ur

Table 5 - Thermodynamic parameters for the adsorption of Cr(VI) onto the prepared adsorbents T (K)

ΔGº (kJ/mol)

303 313

+5.190 +7.013

323

+3.080

303

+5.911

313

+6.742

323

+12.589

Fe3O4@CTS-Cin

ΔHº (kJ/mol)

ΔSº (J/(mol*K))

R2

+36.204

99.394

0.3271

-94.422

-328.553

0.813

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3.7. Comparison with other chitosan derived adsorbents To assess the adsorption performance of CTS-Cin and Fe3O4@CTS-Cin toward Cr(VI), the adsorption capacity of the prepared adsorbents was compared with that of chitosan and other chitosan derived adsorbents ,Table 6. It is clear from Table 6, that almost chitosan derived adsorbents have been applied to remove Cr(VI) in acidic solution as amino groups could be protonated at low pH, leading to an electrostatic interaction between positive charge on the surface of chitosan derived adsorbents and Cr(VI) [43, 44].

of

Compared with some bioadsorbents, the adsorption capacity of our prepared adsorbents is higher indicating that the modification of biosorbent improves the removal of hexavalent

ro

chromium [43]. The data obtained in this work, are compatible with the finding of other researchers, whereas the adsorption capacity of our prepared adsorbents to remove Cr(VI)

-p

from aqueous solutions, was not the best, but was higher or in the same order of magnitude as

re

other derived modified magnetic chitosan adsorbents [44] Table 6. Owing to the aforementioned results, CTS-Cin and Fe3O4@ CTS-Cin adsorbents are regarded as promising

Jo ur

na

lP

adsorbents and good candidates for the removal of Cr(VI) from wastewater.

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Table 6 - Comparison of Cr(VI) adsorption by CTS-Cin and Fe3O4@CTS-Cin with other chitosan derived adsorbents. pH

qm (mg/g)

Compost Olive stones Walnut shells Hazelnut shells Almond shells

Bioadsorbents 2.0 6.25 3.0 1.48 3.5 8.01 3.5 8.28 3.2 3.40

Rice bran Wheat bran

1.0 1.0

58.89 40.80

Ref. [43] [45] [43] [43]

of

Adsorbents

5.0

50

[27]

76.92

[28]

17.15

[46]

2.0

52.7

[47]

5-6

28.88

[48]

3.0

55.8

[49]

2.0

58.48

[3]

2.0.

51.813

[50]

3.0

82.14

[51]

2.0

86.17

[52]

3.0 2.0

61.35 58.14

The present work

3.0

lP

3.5

Jo ur

CTS-Cin Fe3O4@CTS-Cin

[27]

-p

78

re

5.0

na

CTS chitosan CTS-ECH chitosan cross-linked with pichlorohydrine CTSB chitosan beads n-butylacrylate grafted chitosan adsorbent Ethylamine modified chitosan carbonized rice husk composite beads Chitosan crosslinked modified silicon materials Magnetic chitosan nanoparticles Crosslinked magnetic chitosan antranilic acid glutaraldehyde Ehylenediamine-modified cross-linked magnetic chitosan resin (EMCMCR) Magnetic chitosan-GO nanocomposite (EDTA-2Na-GO-CTS) graphene oxide/chitosan composite with disodium ethylene-diaminetetraacetate

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Modified chitosan and magnetic modified chitosan

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4.

Conclusion

Modified chitosan, CTS-Cin, and magnetite modified chitosan Fe3O4@CTS-Cin have been prepared and fully characterized by means of analytical techniques (FT-IR, SEM, HR-TEM, XRD, VSM, EDS).The prepared adsorbents were used to investigate the adsorption properties of Cr(VI) metal ion in an aqueous solution using batch sorption technique. The Cr(VI) uptake is pH dependent and the optimal removal efficiency was observed at pH 3.0 for CTS-Cin and pH 2.0 for Fe3O4@CTS-Cin. Metal adsorption reached equilibrium after 35 and 80 min for CTS-Cin and Fe3O4@CTS-Cin, respectively. Kinetic studies proved that the pseudo-second–

of

order model illustrate the best description for the adsorption process of Cr(VI) onto the

ro

adsorbents with correlation coefficient R2 = 0.999. Equilibrium isotherm data were fitted using different five-parameter models. Among these models, Langmuir model is in good

-p

agreement with the experimental data with high R2 for the adsorption of Cr(VI) onto the CTSCin and Fe3O4@CTS-Cin with maximum adsorption capacity 61.35 mg/g and 58.14 mg/g,

re

respectively. The heat of adsorption (bTe) calculated from Temkin model and ΔG calculated from Flory model are positive values indicatingthat the the adsorption process is

lP

nonspontaneous and exothermic in nature. The positive ΔG°values revealed that the adsorption process is non-spontaneous in nature. The positive ΔH° value for CTS-Cin

na

indicates that the adsorption process is endothermic in nature while the negative value for

5.

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Fe3O4@CTS-Cin indicates the exothermic nature.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

6.

Data Availability

The raw data required to reproduce these findings are available to download from [INSERT PERMANENT WEB LINK(s)]. The processed data required to reproduce these findings are available to download from [INSERT PERMANENT WEB LINK(s)].

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References [1] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: A review, J. Environ. Manage., 92 (2011) 407-418. https://doi.org/10.1016/j.jenvman.2010.11.011. [2] D.H.K. Reddy, S.-M. Lee, Application of magnetic chitosan composites for the removal of toxic metal and dyes from aqueous solutions, Adv. Colloid Interface Sci., 201-202 (2013) 68-93. https://doi.org/10.1016/j.cis.2013.10.002. [3]

Y.G. Abou El-Reash, M. Otto, I.M. Kenawy, A.M. Ouf, Adsorption of Cr(VI) and As(V) ions by modified magnetic chitosan chelating resin, Int. J. Biol. Macromol, 49

[4]

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(2011) 513-522. https://doi.org/10.1016/j.ijbiomac.2011.06.001. L. Li, L. Fan, M. Sun, H. Qiu, X. Li, H. Duan, C. Luo, Adsorbent for chromium removal

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based on graphene oxide functionalized with magnetic cyclodextrin–chitosan, Colloids Surf., B, 107 (2013) 76-83. https://doi.org/10.1016/j.colsurfb.2013.01.074. Y. Zhou, B. Gao, A.R. Zimmerman, J. Fang, Y. Sun, X. Cao, Sorption of heavy metals

-p

[5]

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on chitosan-modified biochars and its biological effects, Chem. Eng. J., 231 (2013) 512-518. https://doi.org/10.1016/j.cej.2013.07.036. V. Sureshkumar, S.C.G. Kiruba Daniel, K. Ruckmani, M. Sivakumar, Fabrication of

lP

[6]

chitosan–magnetite nanocomposite strip for chromium removal, Appl. Nanosci., 6

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights 

Two new modified functionalized chitosan adsorbents, chitosan cinnamaldehyde (CTS-Cin) and magnetite modified chitosan Fe3O4@CTS-Cin has been prepared and characterized.



The prepared adsorbents were used to investigate the adsorption properties of Cr(VI) metal ion in an aqueous solution.



The sorption of Cr (VI) ions fit well with the second order reaction and Langmuir adsorption isotherm model.

of

The maximum adsorption capacity of Cr(VI) onto the CTS-Cin and Fe3O4@CTS-Cin

na

lP

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was 61.35 mg/g and 58.14 mg/g, respectively.

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

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