Nanostructured oxide stabilized by chitosan: Hybrid composite as an adsorbent for the removal of chromium (VI)

Accepted Manuscript Title: NANOSTRUCTURED OXIDE STABILIZED BY CHITOSAN: HYBRID COMPOSITE AS AN ADSORBENT FOR THE REMOVAL OF CHROMIUM (VI) Authors: Pricila Maria Batista Chagas, Lucas Braganc¸a de Carvalho, Aline Aparecida Caetano, Francisco Guilherme Esteves Nogueira, Angelita Duarte Corrˆea, Iara do Ros´ario Guimar˜aes PII: DOI: Reference:

S2213-3437(18)30026-5 https://doi.org/10.1016/j.jece.2018.01.026 JECE 2146

To appear in: Received date: Revised date: Accepted date:

28-10-2017 26-12-2017 11-1-2018

Please cite this article as: Pricila Maria Batista Chagas, Lucas Braganc¸a de Carvalho, Aline Aparecida Caetano, Francisco Guilherme Esteves Nogueira, Angelita Duarte Corrˆea, Iara do Ros´ario Guimar˜aes, NANOSTRUCTURED OXIDE STABILIZED BY CHITOSAN: HYBRID COMPOSITE AS AN ADSORBENT FOR THE REMOVAL OF CHROMIUM (VI), Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2018.01.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

NANOSTRUCTURED OXIDE STABILIZED BY CHITOSAN: HYBRID COMPOSITE AS AN ADSORBENT FOR THE REMOVAL OF CHROMIUM (VI)

Pricila Maria Batista Chagasa,b, Lucas Bragança de Carvalhoa, Aline Aparecida Caetanob, Francisco

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Guilherme Esteves Nogueirac, Angelita Duarte Corrêaa, Iara do Rosário Guimarãesb*

Laboratório de Bioquímica, Departamento de Química, Universidade Federal de Lavras, CEP 37200-

Laboratório de Catálise Ambiental e Novos Materiais, Departamento de Química, Universidade

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000, Lavras, MG, Brazil.

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Federal de Lavras, CEP 37200-000, Lavras, MG, Brazil.

Laboratório de Engenharia Química, Departamento de Engenharia Química, Universidade Federal de

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São Carlos, São Carlos, SP, 13565-905, Brasil.

*Corresponding author Iara do Rosário Guimarães

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Laboratório de Catálise e Novos Materiais, Departamento de Química, Universidade Federal de Lavras CEP 37200-000, Lavras, MG, Brazil. Phone: +(55) 35 3829 1626 E-mail: [email protected]\

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GRAPHICAL ABSTRACT

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Abstract – The surface of iron oxides can be modified by treatments using organic components, which is an alternative for the production of new materials. Chitosan (CT) is a polymer that has been widely used as a protective and stabilizing agent, which can functionalize and improve adsorbent properties of

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iron oxides. In this study, the synthesis of chitosan-stabilized nanostructured iron oxide was carried

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out for application and the optimization of Cr(VI) removal. The material was synthesized by the direct

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incorporation of Fe(II) into the chitosan gel, producing CT-Fe beads. The chemical, morphological

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and structural characterizations of the materials were performed using SEM, XRD, FTIR, TGA and DSC. The magnetic iron oxide produced together with chitosan was identified as magnetite (Fe 3O4). In

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the formation of the CT-Fe composite, chitosan chains became less ordered, providing the formation of low-crystalline magnetic iron oxide capable of increasing the adsorption capacity. CT-Fe composite

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showed higher capacity for the removal of Cr(VI), relative) when compared to pure magnetite. Kinetic studies showed that chromium adsorption follows the pseudo-second order model, indicating chemical

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adsorption. The removal of Cr(VI) was pH-dependent and the highest removal was obtained in acid medium, in which the groups present on the surface of the materials are fully protonated, facilitating the electrostatic attraction of HCrO4-. Furthermore, the reduction of Cr(VI) by Fe(II) can cause Cr(III)

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to return to the aqueous medium and being readsorbed. The CT-Fe composite has promising adsorption capacity with remarkable reuse for the removal of chromium from solution. Keywords:

hybrid biocomposite; iron oxide; hexavalent chromium removal; reduction-chelation

mechanism.

1. Introduction 2

Chromium is one of the most toxic heavy metals and has been discharged in the environment by several industrial effluents, causing serious health problems [1, 2]. It can occur in different oxidation states, which ranges from -2 to +6. However, only Cr(III) and Cr(VI) are stable in the environment. Chromium (VI) anions are highly toxic and mutagenic for living organisms and can be released from

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anthropogenic sources, such as industrial effluents from tannery, galvanization, painting, and textile industries [3-5]. Therefore, the efficient removal of Cr(VI) from a contaminated aqueous medium is of

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great importance.

Among the methods to remove pollutants from water bodies, adsorption is generally recognized as an efficient and economical way due to its simplicity, reliability and safety [6, 7]. The

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removal mechanisms of Cr(VI) can be divided into two categories: 1) direct adsorption of Cr(VI)

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anions and 2) reduction of Cr(VI) to Cr(III) surface, followed by adsorption of Cr(III) [2, 8-10].

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In recent years, many reducing agents, such as modified carbon, Fe0, iron oxide (Fe3O4), and

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ascorbic acid are being used to remove Cr(VI) from different medium [11-13]. Iron oxides, such as Fe3O4 are commonly used in the preparation of magnetic materials due to their low cost, low toxicity,

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rapid kinetics and high reactivity for the degradation/removal of many chemical pollutants [14]. However, magnetic particles are often unstable. Nanoparticles of Fe3O4 can form suspended

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aggregates and are easily oxidized in open air, which limits their applications [15]. The surface of iron oxides can be modified with physical or chemical treatments to increase the adsorption capacity [16].

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In order to enhance the dispersion stability of the oxides in suspension medium and to

functionalize the surface, oxides coated with polymers or inorganic compounds have been gaining increasing attention [3, 17]. The stabilizing agents ought to be of low cost, widely available and able to

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disperse well, stimulating the reaction on their surface. The preparation of chitosan-based hybrid adsorbents has received attention due to its chemical stability and high adsorption capacity, with prospects for environmental applications [18, 19]. In addition, chitosan can function as a stabilizing and dispersing agent, improving the activity of iron oxides [20, 21]. The direct incorporation of iron oxides on chitosan does not prevent crystal growth, which leads, in the case of magnetite, to the formation of particles with a high crystallite size due to the 3

natural coalescence of this oxide. Several studies deal with the incorporation of this ready phase in the polymer [22, 23], which reduces its activity by increasing particle size or crystallinity. On the other hand, some in situ oxide syntheses require conditions of constant nitrogen gas flow before, during and after the formation of the hybrid composite, in order to prevent an oxidizing environment, which prevents the formation of inverted spinel that is typical of magnetite [24, 25]. On a bench scale, the

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need for nitrogen can be understood, however, it becomes technically unfeasible to large-scale systems. In this work, the direct incorporation of FeCl2 into CT was carried out without N2 flow,

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considering the oxidizing medium in which part of the Fe (II) ions were oxidized to Fe (III). In addition to the economic aspects resulting from the absence of controlled environment demands, the direct incorporation of Fe (II) prioritizes the nucleation stage in detriment to the growth of the crystals,

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which forms particles with smaller sizes of crystallites and a larger exposure of the active metal phases

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[26].

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Therefore, in this study, the synthesis of a new material based on a structure of nano-

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microcrystalline iron oxide stabilized by chitosan (CT) was performed. The CT-Fe composite was formed by direct incorporation of Fe(II) into the chitosan soluble gel. The polymer structure was

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regenerated for the production of nanostructured magnetic iron oxides CT-Fe in spherical form. Chitosan beads with different proportions of iron oxide were tested for the removal of Cr(VI) in

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aqueous medium, and the parameters pH, ionic strength and regeneration were assessed. The kinetic profiles and the adsorption isotherms were also studied. With the experimental data, a Cr(VI) removal

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mechanism was proposed.

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2. Material and methods

2. 1. Synthesis of chitosan beads and nanostructured iron oxide stabilized by chitosan (CT-Fe)

The chitosan solution was prepared by solubilizing 3.5 g of chitosan (CT) (low molecular weight chitosan- degree of deacetylation 90, SIGMA ALDRICH) in 100 mL of acetic acid solution 4

(2% v/v) under stirring for 1 h [27]. Different ratios of Fe(II) in relation to the chitosan mass were incorporated in the solution: a) 0.35 g of Fe(II) (CT-Fe 10% w/w); b) 0.70 g of Fe(II) (CT-Fe 20% w/w); c) 1.40 g of Fe(II) (CT-Fe 40% w/w) using FeCl2.4H2O (SIGMA ALDRICH). The mixture remained under stirring until complete solubilization of the reagent. For the production of the beads, the above mixture was dripped into a 2 mol L-1 NaOH

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solution, immediately generating gel coagulation in a spherical format (video). The chitosan solution was also dripped in order to obtain pure chitosan beads. The beads remained for 16 h in 2 mol L-1

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NaOH solution and were washed with distilled water until the neutralization of solution. Subsequently, the beads were oven dried at 60 oC.

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2.2. Preparation of pure magnetite

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The magnetite particles were synthesized according to the classical precipitation method

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proposed by Cornell and Schwertmann [28] and was used as standard for the evaluation of structural and morphological gains of CT-Fe composite. Briefly, FeCl3.6H2O (6.36 g) (SIGMA ALDRICH)

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and FeCl2. The mixture of 4H2O (2.34 g) was dissolved in a molar ratio of 2:1 in 200 mL of degassed distilled water and kept under mechanical stirring and bubbling N2 for 25 min. Ammonium hydroxide

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was added to raise the pH and the solution went from brown to black. Subsequently, the black

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particles were magnetically separated, washed with distilled water and oven dried at 60 °C.

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2.3. Characterization of materials

The morphology of the material was obtained using a SEM LEO 440 equipment with an

OXFORD detector operating with a 15 kV electron beam. Thermogravimetric analysis (TGA) were performed under air atmosphere at a heating rate of 10 oC min-1 up to 900 oC, using a Mettler Toledo TGA/DSC1 equipment. Differential scanning calorimetry (DSC) measurements were performed at 20550 °C using a DSC 60 (Shimadzu Co) equipment at a heating rate of 10 °C min -1 under air 5

atmosphere. The X-ray diffraction (XRD) measurements were carried out on a Rigaku Geigerflex Xray diffractometer equipped with a graphite monochromator and CuKα radiation (1.5406 Å), at current of 40 mA and voltage of 45 kV. The functional groups of the material were determined by Fouriertransform infrared spectroscopy (IV) (Varian-660 IR, Pike) coupled to ATR with 16 scans and 4 cm-1

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resolution in the spectral range of 400-4000 cm-1. The procedure for calculating the zero charge point (pzc) consisted of adding 10 mg of the adsorbent in 10 mL of 0.1 mol L-1 KCl under 12 different initial pH conditions (1-12), which were

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adjusted with 0.1 mol L-1 HCl or NaOH. The solutions were kept in a thermostatic bath at 25 ° C under mechanical agitation, and after 24 h of contact, when equilibrium was reached, the solutions were filtered and the final pH was recorded. The pzc corresponds to the pH range in which the surface

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behaved as a buffer [29].

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2.4. Cr(VI) removal tests

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Experiments to select the most suitable Fe ratio for Cr(VI) removal in the chitosan beads were performed in flasks containing 10 mL of Cr(VI) 5 mg L-1 solution (solution prepared with potassium

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dichromate (K2Cr2O7)) and 10 mg of CT/Fe adsorbents, kept under stirring for 24 h at room temperature. After adsorption, the content of residual chromium in the solution was determined by the

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colorimetric method, using 1,5-diphenylcarbazide at 540 nm [30]. Total chromium contents were determined after Cr oxidation in acidic medium with potassium permanganate at high temperature. Cr(III) was calculated by the difference between Cr(total) and Cr(VI) in the solution. The percent

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removal of Cr(VI) was calculated by Eq. (1).

R(%) 

C0  Ce  100 C0

(1)

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where: R = removal percentage, C0 (mg L-1) = initial content of Cr(VI) in the solution, and Ce (mg L-1) = content of the metal at equilibrium.

The effect of pH (ranging from 3 to 10, and adjusted using hydrochloric acid or sodium

removal. All experiments were also performed with pure chitosan and Fe3O4.

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hydroxide) in the adsorption of Cr(VI) was performed in materials that showed the highest Cr(VI)

The adsorption kinetics (time 30-1,440 min, 10 mg of material were added to 10 mL of Cr(VI)

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5 mg L-1 solution) and the adsorption isotherms (10 mg of material were added to 10 mL of solutions with different contents of Cr(VI) varying from 2.5 to 750 mg L-1) were performed in all treatments.

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C0  Ce V m

(2)

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qe (mg g 1 ) 

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(2) represents the calculation of the adsorption capacity.

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The adsorption capacity, qe (mg g-1), is defined as the mass of adsorbate bound/adsorbent mass. Eq.

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where, qe = adsorption capacity, C0 and Ce = described in Eq. (1), V (L) = volume of the solution, and m (g) = adsorbent mass.

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The reuse process was performed in an number of cycles by using 30 mL of 5 mg L-1 Cr(VI) solution and 30 mg of the CT-Fe composite under 24-h stirring. Then, the material was filtered,

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washed and placed again under the same reaction conditions, being performed the numbers of cycles. Removal of Cr(III) tests were performed to elucidate the adsorption mechanism. A solution of

5 mg L-1 Cr(III) was prepared using the reagent (Cr(NO3)3 9H2O); 10 mg of material (CT and CT-Fe

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40% w/w) was added in 10 mL of the solution, under 24-h stirring. The remaining chromium content was assessed by oxidation with potassium permanganate by the aforementioned colorimetric method.

3. Results

3.1. Characterization 7

The morphology of the beads was observed by scanning electron microscopy (Fig. 1). In the upper right of Fig. 1a is shown the photographic image of dried CT-Fe (40% w/w) after Cr adsorption.

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a) CT-Fe-Cr

c) CT-Fe

d) CT-Cr

e) CT-Fe-Cr

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b) CT

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Fig. 1 Scanning electron microscopy (SEM). a) Chitosan-Fe (CT-Fe) 40% w/w after removal of Cr(VI) (100 μm) and (photographic image insertion) dry bead size 1 mm; b) CT (10 μm); c) CT-Fe (40% w/w) (10 μm); d) pure CT after removal of Cr(VI) (10 μm); e) CT-Fe (40% w/w) after removal of Cr(VI) (10 μm).

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The CT-Fe granule (40% w/w) showed a dark brown color because the iron oxide was introduced into the polymer structure, whereas the pure chitosan beads were beige. By increasing the iron oxide content, there was a proportional increase in color intensity; for instance, sample CT-Fe

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(40% w/w) was black, which is typical of magnetite. Sample CT-Fe (40% w/w) has a spherical shape with a millimeter size distribution. Diameter distributions of all materials are based on chitosan beads (CT-Fe 40% w/w for pure chitosan) with an average diameter of nearly 1.0 mm. In the enlarged image of the surface of pure CT (Fig. 1b), a smooth and homogeneous structure was observed due to the well-ordered arrangement of chains of this polymer. The presence of reactive groups in the chitosan

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structure, such as -NH2 and -OH, provided a structural regularity of the polymer chain due to strong inter- and intramolecular hydrogen bonding interactions. In Fig. 1c is shown the composite material formed by the iron oxide dispersed in the chitosan chain (CT-Fe). Unlike the beads formed by pure CT, the addition of iron had a very pronounced effect on the composite morphology, forming a highly rough and irregular surface. This morphological

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alteration observed in the material was possibly due to the disorganization in the chitosan chains that, in the presence of iron, used the -NH2 and -OH groups to form complexes with the metal. Clearly, the

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increase in the disorder degree observed in the morphology of CT-Fe positively contributed to the optimization of the adsorption process.

The perception that the incorporation of the metallic element was responsible for disordering

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the chitosan morphology by the removal of chains strongly bound by hydrogen bonds was

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strengthened when the surface profiles of the materials were observed in aqueous medium after Cr(VI)

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adsorption (Fig. 1d and 1e).

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According to images obtained by SEM, after adsorption of Cr(VI) from aqueous solution (Fig. 1d and 1e), there was a significant modification in the morphology in samples after Cr adsorption,, and

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the surface showed an exfoliated appearance. The new texture formed after adsorption resulted in surface irregularities and fractured regions due to the formation of new covalent bonds between the

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biopolymer and the metal. These bonds may occur according to the Lewis theory of acid-base reactions between the groups -NH2 and -OH and Cr in the same chain or in more than one polymer

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chain.

The crystalline structure of the materials was analyzed by X-ray diffraction and the results are

shown in Fig. 2a. The diffractogram of pure CT shows a reflection signal at 2θ = 20o characteristic of

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semicrystalline polymers. The amplification of this signal can be attributed to a structure with low crystallinity degree. The semicrystalline profile of chitosan, shown in Fig. 2a in 2 = 20o, is directly related to strong intra- and intermolecular interactions characterized by hydrogen bonds formed between amino, hydroxyl, amide, and other functional groups present in the chitosan molecule [23]. In the XRD profile of chitosan after interaction with aqueous Cr(VI) and formation of CT-Cr complexes, 9

the typical reflection signal of the polymer had a reduction in crystallinity, thus representing a more amorphous structure. On the other hand, no new signal related to the metal presence can be visualized. As observed by Wang and colleagues [31], this behavior emphasizes the formation of a new organization in carbohydrate structures, in which the diffraction signals decrease in intensity after interaction with the metal ion. Since the hydrogen bonds, which previously held together the chitosan

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chains, were broken by the metal and chelates were formed with -NH2 or -OH, causing distortions in

CT

(a)

Mt

Mt

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the crystal structure (Fig. 2c).

(b)

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Intensity (a. u.)

CT-Fe-Cr

CT-Fe

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JCPDS 65-3107

ca

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CT-Fe

200 20

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60

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(c)

CT

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CT-Cr

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KCu 2 (degrees)

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Fig 2. X-ray diffraction (XRD). a) Chitosan (CT) beads, CT-Fe (40% w/w); and beads after the removal of Cr(VI) CT-Cr and CT-Fe-Cr; b) CT-Fe and Fe3O4 reference data (JCPDS 65-3107); c) illustrative scheme adapted from Wang and colleagues [31] to elucidate the decrease in CT crystallinity caused by the metal element.

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In Fig. 2c is shown a representative scheme of the molecular organization of chitosan in the

presence of the metal ion. This scheme also corroborates with the results from SEM analysis. The pure carbohydrate chains were organized in an almost linear structure, which refers to the smooth surface of the image (Fig. 1b). On the other hand, in order to satisfy the metal valence, the CT chains fold over each other, involving the Cr atom (Fig. 2c) and converting into the irregular structure evidenced by images 1c and 1e. 10

With respect to the XRD profile of the CT-Fe composites, it can be observed that the incorporation of Fe in the chitosan beads, similarly to that observed by the polymer in the presence of Cr, significantly reduced the semicrystalline nature of the polymer, with a decrease in the reflection intensity observed at 2θ = 20o. This behavior refers to the introduction of Fe in the CT matrix and consequent perturbation of the crystalline environment, especially by the loss of the hydrogen bonds

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[32]. In this material, after the incorporation of Fe, there were new diffraction signals at 2θ = 33.8, 35.3, 36.9, 41.1, 54.2, 59.0, 60.9, and 63.4o, which according to the indexed database, are

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characteristic of Fe3O4 cubic structure (JCPDS 65-3107, Fig. 2 b). Signals showed a small displacement when compared with pure Fe3O4 materials, which is related to the presence of Fe interactions with chitosan [8]. The magnetite phase found by the crystalline profile of the hybrid

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material suggested that Fe(II) was partially oxidized to Fe(III) during the synthesis. Primary

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crystallites or Fe3O4 nanocrystals are formed at the beginning of nucleation under alkaline conditions

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as those defined in the experimental part of this study, and can be represented by Eq. (3).

(3)

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2Fe3+ + Fe2+ + 8OH- → 2Fe(OH)3Fe(OH)2 → Fe3O4(s) + 4H2O

For the CT-Fe-Cr material, as the hydrogen interactions that confer semi-crystallinity to the

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material have already been altered by the presence of iron, little or no additional modification in the XRD profile was observed after adsorption of Cr(VI) in aqueous medium.

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The chitosan CT, CT-Cr, CT-Fe (40% w/w) and CT-Fe-Cr Fourier-transform infrared

spectroscopy (FTIR) spectra are shown in Fig. 3. For CT, the band at 3,000 to 3,600 cm-1 can be attributed to O-H and N-H functions present in the structure of the polysaccharide and from adsorbed

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water molecules [23]. C-H stretching vibrations were observed at 2,862 cm-1 and two bands at 1650 and 1590 cm-1 were attributed to amide groups (N-H stretching) present in chitosan. The vibrations between 1,420 and 1,300 cm-1 are related to C-N axial deformation of the amide and amine functions and bands between 1,153 and 897 cm-1 are characteristic of polysaccharide structures (C-O, C-N).

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C-O

C-N

N-H

O-H NH2

C-H

C-H

Fe-O

CT

Fe-O

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Absorbance (a. u.)

O-H

CT-Cr

CT-Fe-Cr 4000

3500

3000

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CT-Fe

1500 -1

500

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Wavenumber (cm )

1000

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Fig. 3 Infrared spectroscopy of chitosan (CT) beads and CT-Fe (40% w/w) and after removal of Cr(VI) (CT-Cr and CT-Fe-Cr).

For CT-Metal materials (Fig. 3), changes in band intensity (3,000 and 3,600 cm-1) related to

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H-O and N-H vibrations were observed after incorporation of Fe or Cr when compared to pure chitosan. The decrease in the intensity of these bands is directly related to the rupture of the hydrogen

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bonds and corroborate directly with the results from XRD analysis. The attenuation of this region may still be correlated to the loss of adsorbed water molecules in the chitosan structure, whose connections

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contributed to increase the relative band intensity observed in the FTIR spectrum at high frequencies. The formation of the complex between Fe and the -NH2 or -OH groups of chitosan reduced the amount of available hydrophilic groups and are responsible for the formation of strong H interactions between

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water molecules and the polymer [33,34]. Since they are no longer available, there is a consequent decrease in the influx and diffusion of H2O molecules into the chitosan bead, which contributed to signal decrease in FTIR analysis. Furthermore, two new bands appeared at 590 and 789 cm-1 in low frequency regions. According to Cornell and Schwertmann [28], these bands are compatible with Fe-O vibrations, which confirmed the formation of the complex oxide structure to the chitosan matrix [23]. 12

In Fig. 4, the results of TGA, DTG and DSC for CT, CT-Cr, CT-Fe (40% w/w) and CT-Fe-Cr are presented. The thermal degradation of the polysaccharide structure is characterized by depolymerization and thermal decomposition reactions initiated by a random break of glycosidic bonds, followed by the decomposition of the residual polymer chain at temperatures above 400 oC.

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Above this temperature, sample carbonization occurs [35].

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Fig. 4 TGA (a), DTG (b), and DSC (c) of chitosan beads (CT), CT-Fe (40% w/w) and after removal of Cr(VI) (CT-Cr and CT-Fe-Cr) under air atmosphere at a heating rate of 10 oC min-1.

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The thermodecomposition of CT can be categorized into three regions: the first was attributed

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to chitosan water loss that occurred at temperatures up to 100 oC approximately (Fig. 4a and b). The higher weight loss occurred at 294 °C (Fig.4 b) and reached loss of 80%, which is related to the first

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modifications of polymeric chains, resulting from the depolymerization and thermal decomposition reactions with the formation of volatile products of low molecular weight, such as carbon monoxide,

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carbon dioxide, and ammonia. The last mass loss occurred at 545 oC relative to 10% mass, which is characteristic of the decomposition and oxidation of carbonized residue [36, 37]. In relation to the degradation curve of CT-Fe beads, three regions of mass loss were also

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observed. The first one is related to the adsorbed water and the beginning of the polymer chain degradation, reaching mass loss of 14% until 200 oC. There were losses of 21 and 28% referring to two consecutive thermal events at 298 oC and 380 oC, which refer to the decomposition of the polymer chain into volatile products. Possibly, the Fe incorporated in chitosan caused a greater stability of the material structure and part of the chain decomposition occurred in higher temperatures of 380 oC.

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Above this temperature, the sample carbonization occurred with a residual mass of 37% relative to the remaining metal oxides in the system, such as Fe2O3. The thermal behavior of materials after the removal of aqueous Cr(VI), CT-Cr and CT-Fe-Cr followed a degradation profile similar to CT-Fe, showing the two thermal events related to the decomposition of the chitosan chain. After the material carbonization, the CT-Cr curve had a final

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residue of 6.7%, which may refer to the formation of chromium oxides. The CT-Fe-Cr degradation curve showed a final residue of approximately 40%, with a mass increase of 3% in relation to the CT-

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Fe curve, which refers to the formation of iron and chromium metal oxides.

In all DSC curves (Fig. 4c), two thermal events were observed: the first endothermic before 100 oC corresponding to the dehydration process [38] and the second signal near 300 oC, exothermic,

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corresponding to the degradation of chitosan polymer chains. In CT, the endothermic event occurred

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up to 100 oC and may be associated with the evaporation of absorbed residual water due to the

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hydrophilic nature of its functional groups. Regarding the other materials, this event occurred at a

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higher temperature, indicating less easiness in dehydration. Regarding the exothermic signal at 300 oC for the CT-Fe material, there was a decrease in the signal area. The baseline deviation in the DSC

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curves (CT-Cr, CT-Fe and CT-Fe-Cr) above 350 oC is due to a change in sample compaction caused by the thermal decomposition of metals with the removal of iron and chromium oxides, changing the

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thermal conductivity of the sample in relation to the reference. Thus, the presence of metals induced reorganization of polymer chains in beads, affecting the

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thermal degradation process. This effect can be attributed to the variation in the amine groups of chitosan, in which the connections for the formation of metal/chitosan complex occur [39]. In the CT-Cr and CT-Fe-Cr curves, the two events occurred in smaller amplitude in relation to

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chitosan, besides a shift of these signals to higher temperatures. This indicated less easiness of these materials in initiating degradation of the polymer chain. The increase in temperature may be due to the Cr(III) atoms being cross-linked in the chitosan chains due to the new chemical bonds. The increase in degradation temperature after removal of Cr(VI) also indicated the formation of new complexes with greater stability [40]. Thermal events are consistent with those observed in TGA and DTG curves.

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3. 2. Application of CT-Fe beads in the removal of Cr(VI)

In Fig. 5 is shown the ability to remove Cr(VI) for CT and CT beads with different Fe

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contents. It was observed that as the ratio of Fe(II) in relation to CT mass increased, an increase in aqueous Cr(VI) removal was obtained. The CT beads without iron showed a removal of 24.83% ±

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0.27, whereas the CT-Fe beads at 10%, 20%, and 40% w/w showed a greater removal capacity of 39.78% ± 1.70; 70.40% ± 1.28, and 73.58 ± 2.12, respectively, indicating that higher iron content incorporated into CT resulted in greater removal of chromium. Removal of aqueous Cr(VI) by pure

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magnetite was lower than all materials (21.83% ± 1.24).

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30

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40

39.78

24.83

21.83

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Removal of Cr(VI) %

60 50

73.58

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70.40

70

20 10

Fe3 O4

m/ m) CT -Fe (40 %

m/ m) CT -Fe (20 %

CT

m/ m) CT -Fe (10 %

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0

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Fig. 5 Effect of different ratios of Fe(II) on chitosan (CT) beads; CT-Fe - 10%, 20%, and 40% w/w in the removal of Cr(VI) (10 mL of 5 mg L-1 solution, pH 5.5, 10 mg of adsorbent, temperature of 25 oC, contact time of 24 h).

The obtained results can be attributed to the fact that during the synthesis of the CT-Fe beads,

the high viscosity of the medium containing the polysaccharide favors the nucleation process and delays the growth of Fe3O4 crystals, aiding in the formation of smaller particles of magnetite and a less ordered structure. The chemical surface of magnetite plays an important role in the removal of metal ions. Iron oxides in aqueous medium have active sites on the Fe-OH surface. The surface chemical 15

structure (O-H adsorption and coordination sites) depends on the oxide morphology and the crystal structure. The results of the removal of Cr(VI) suggested that the incorporation of iron in the CT structure potentiates the removal of Cr(VI). According to literature data, the structure of CT-Fe complex consists of Fe ions coordinated with chitosan molecules, in which Fe is complexed with O

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and N atoms of the polymer [9]. The -NH2 and -OH groups on the chitosan surface may be mono- or bidentate binders to coordinate with most transition metals, such as Fe(III) and Fe (II) [41]. The

SC R

creation of metal active sites on the chitosan surface facilitated the reduction of Cr(VI) in Cr(III) (Eq. 4) and increased the adsorption with a gradient proportional to the iron content. Once reduced to trivalent form, Cr3+ ions are acid Lewis species that covalently interact with electron-pair donors of the

N

U

CT structure (-NH or -OH), keeping the reduced metal adsorbed on the hybrid surface Eq. (5)

(4)

(5)

ED

Cr3+ + NH≡(s) → Cr3+-NH≡(s)

M

A

HCrO4- + 3Fe2+ + 7H+ → 3Fe3+ + Cr3+ + 4H2O

In particular, electroplating effluents and leather tannery facilities contain chromium in

PT

contents ranging from tenths to hundreds of milligrams per liter [42]. Since a number of agencies including the World Health Organization (WHO) [43] and the United States Environmental Protection

CC E

Agency (USEPA) [44] have given a tolerable limit of 0.05 mg L-1 in drinking water, all studied materials can be considered as promising adsorbents in the removal of Cr(VI), since the initial content

A

of Cr(VI) in adsorption tests was 100 times higher (5 mg L-1).

16

3. 3. Effect of pH in the removal of Cr(VI)

In order to optimize some parameters such as the adsorbent mass and pH variation, tests for metal removal have been performed with pure CT, Fe3O4, and CT-Fe (40% w/w), which was the

IP T

composite with the highest removal capacity of aqueous Cr(VI). The change in pH is an important parameter in the evaluation of adsorbent capacity, because it

SC R

affects both the metal ion speciation in the solution and the adsorbent nature, especially with the protonation/deprotonation of the functional groups present on the surface.

The influence of pH on aqueous Cr(VI) removal by the CT, CT-Fe (40% w/w) and Fe3O4

U

beads is shown in Fig. 6. The materials showed the same profile: a decrease in the adsorption occurred

N

with increased pH values. The CT-Fe beads showed higher Cr(VI) removal capacity at all pH values,

A

except at pH 3, where CT reached the same adsorption as CT-Fe.

M

5

CT-Fe

3

Fe3O4

CT

CC E

PT

-1

qe (mg g )

ED

4

2

1

0 3

4

5

6

7

8

9

10

pH

A

Fig. 6 Effect of pH on Cr(VI) removal (10 mL of 5 mg L-1 solution, 10 mg of adsorbent, 25 oC temperature, 24 h contact time). Total Cr = Cr(VI).

The pH affects both the chromium stability in relation to its speciation (Fig. A.1, Appendix A) and the surface charge of the adsorbents (CT, CT-Fe and Fe3O4). At pH 1, chromium exists as chromic acid (H2CrO4), whereas in a range of pH 1-6 different forms of chromium coexist, such as: dichromate 17

(Cr2O72-) and hydrogen chromate (HCrO4-), being HCrO4- predominant and, in values greater than 6, to chromate forms (CrO42-). The Cr2O72- ions occur when the content of Cr(VI) goes beyond 1 g L-1. The speciation of Cr(VI) is dependent on pH of the solution, following the reactions of Eq. (6, 7 and 8) [45, 46] (Fig. A.1, Appendix A):

HCrO4- ⇌ CrO4-2 + H+

IP T

(6)

SC R

H2CrO4 ⇌ HCrO4- + H+

(8)

U

Cr2O7-2 + H2O ⇌ 2HCrO4-

(7)

N

Cr(VI) exists predominantly as HCrO4 - in aqueous solution below pH 6. For chitosan whose

A

pKa amine groups range from 6.3 to 7.2 [47], in aqueous solution with a pH lower than 6, the groups

M

(-NH2 and -OH) present in their structure are completely protonated (-NH3+ and -OH2+), resulting in a surface positive charge that exerts strong attraction with negatively charged Cr(VI) ions present in the

ED

solution as HCrO4-. The electrostatic interactions between the adsorbent and the HCrO4- ions contributed to the high removal of chromium in strongly acid media. However, the decrease in the

PT

adsorption capacity at higher pH can be explained both due to the competition of the HCrO 4- and OH-

CC E

anions available in the medium to be adsorbed on the solid surfaces [48,49], as well as the decrease in number of sites with positive charge by increasing the pH. Magnetite contains Fe(II) and Fe(III) ions. Thus, electrostatic forces between metal ion species

A

and surface charges are responsible for adsorption. The high adsorption of Cr(VI) occurred in an acidic medium, in which the magnetite surface has a positive net charge due to Fe-OH2+ surface groups. The increase in pH resulted in a decrease in the removal of Cr(VI) and as the surface active sites of FeOH2+ were converted to Fe-O-, there was a decrease of electrostatic attractions among the chromium ionic species and repulsions became predominant. The Fe(II) and Fe(III) magnetite sites may be involved and the adsorption (HCrO4-) represented by the bond formed by two sites [50]. 18

To analyze the charge of the functional groups present in the QT-Fe composite, the point of zero charge (pzc) was calculated for pure CT and CT-Fe. The value of pzc for CT and CT-Fe were similar and close to 7.0 (Fig. A.2, Appendix A). Thus, it can be inferred that at pH below the pzc, both the functional groups present in the QT structure and the iron oxide formed in its matrix have a positive net charge. The typical -NH2 and -OH groups of the polymer will be completely protonated (-

IP T

NH3+ and -OH2+), as well as the FeOH2+ surface sites will exert strong attraction to the negatively charged Cr(VI) ions present in the solution as HCrO4- , resulting from the greater removal of Cr (VI)

SC R

in acid medium.

In the CT-Fe beads, the mechanism of Cr(VI) removal appears to occur in two steps: initially occurs Cr(VI) to Cr(III) reduction, which can be interpreted as the result of the reducing ends present

U

in chitosan under acid conditions. Furthermore, according to Eq. (4), H + ions are required and

N

consumed during the reaction process [22, 51]. Both H+ and Fe(II) may favor reduction from Cr(VI) to

A

Cr(III) [13]. This suggests that, besides Fe-O-Cr bonds, NH2 groups of chitosan are also involved in

M

the reduction of Cr(VI), probably resulting in the formation of NH 2-Cr(III) covalent bond, where Cr shares electrons with the N atom in the NH2 groups.

ED

The adsorption of chromium in the adsorbents of CT-Fe was higher in the studied pH range. However, the removal of chromium in the pH range from 5 to 6 by CT-Fe becomes relevant because it

PT

is the natural pH of the chromium solution and there is no need to adjust this parameter, which makes the process technically and economically viable.

CC E

In this scenario, kinetic studies and construction of adsorption isotherms were performed in

A

Cr solution without pH adjustments (pH of solution 5.5).

3.4. Kinetic study and proposed mechanism on the removal of Cr(VI)

Evaluating the kinetic parameters is one of the most important factors to describe the efficiency of an adsorption process. The kinetic model cannot only estimate the adsorption rate as it can also provide clues on possible mechanisms involved in the process [52]. In this study, the 19

experimental data were applied to two equation types in order to describe the process controlled by the adsorption reaction of the contaminant in aqueous solution at the interface with the solid adsorbent: (i) pseudo-first order and (ii) pseudo-second order [53]. The non-linear form of pseudo-first order kinetics is shown in Eq. (9)

qt  qe (1  e  k1t )

IP T

(9)

SC R

where k1 (min-1) is the pseudo-first order constant; qe (mg g-1) and qt (mg g-1) denote the amount of adsorbent used in the adsorption at equilibrium and at any time, respectively, and t (min).

U

The non-linear form of pseudo-second order kinetics is shown in Eq. (10).

N

2

A

k 2 qe t 1 k 2 qe t

(10)

M

qt 

ED

where k2 (g (mg min)-1) is the constant of the pseudo-second order adsorption model. The meanings for qe and qt are equivalent to those of the pseudo-first order.

PT

The experimental results for adsorption kinetics of Cr(VI) by the materials are shown in Fig. 7 and indicated a quick adsorption process in which the highest adsorption rate occurred in the first 60

CC E

min of adsorbent-adsorbate contact and the adsorption equilibrium between the beads and Cr(VI) ions

A

was reached within 300 min.

20

CT-Fe-40%

3,5

CT-Fe -20%

3,0

CT-Fe -10%

2,0 1,5

Fe3O4

1,0

IP T

-1

qe (mg g )

2,5

CT

SC R

0,5 0,0 0

300

600

900

1200

Time (min)

1500

M

A

N

U

Fig. 7 Effect of contact time in the removal of Cr(VI) in chitosan (CT) beads, CT-Fe (10, 20, and 40% w/w), and Fe3O4 (10 mg of adsorbent; 10 mL of Cr(VI) 5 mg L-1 solution at an initial pH of 5.5, temperature of 25 °C). The curves represent the experimental data adjusted by the pseudo-second order model.

The kinetic models were applied and evaluated by regression (Table 1).

CC E

PT

ED

Table 1 Kinetic adsorption parameters calculated by pseudo-first order and pseudo-second order kinetic models: non-linear regression analysis. Models Parameters CT CT-Fe 10% CT-Fe 20% CT-Fe 40% Fe3O4 k1 (min-1) 1.06 x10-3 8.36 x10-3 3.17 x10-3 5.47 x10-3 3.00 x10-2 Pseudoqe (mg g-1) ± SD 1.23 ± 0.131 1.74 ± 0.061 2.96 ± 0.104 3.20 ± 0.085 1.11 ± 0.206 first order R12 0.985 0.963 0.985 0.985 0.977 X12 0.002 0.014 0.018 0.021 0.003 SSE(%) 1.50 11.5 14.9 17.0 2.30

A

Pseudosecond order

k2 (g mg-1 min-1) qe (mg g-1) ± SD R22 X22 SSE(%)

3.55 x10-4 1.95 ± 0.305 0.983 0.002 1.70

5.49 x10-4 1.96 ± 0.047 0.989 0.004 3.30

3.17 x10-4 3.68 ± 0.154 0.990 0.012 10.0

1.71 x10-3 3.73 ± 0.077 0.995 0.007 5.60

6.00 x10-2 1.16 ± 0.012 0.995 6.12 x10-4 0.50

Data close to equilibrium were not included in the treatment, only data corresponding to the accumulation rates of approximately 85% were considered [54]. The best adjustment of the experimental data was expressed by the comparison of the coefficient of determination (R2), standard deviation (SD), the sum of squared errors of prediction (SSE), and chi-squared (X2). All these 21

statistical parameters were calculated using the software OriginPro 8.0 (graphing and data analysis software). The best adjustments of the kinetic models were those that obtained the highest R 2 value and lower values in relation to SD, SSE, and X2. The correlation coefficients resulting from the pseudo-second order R2 model were higher than the pseudo-first order values, as well as the lower values of SD, SSE and X2, except for CT, indicating

IP T

that the adsorption process is better adjusted to the pseudo-second order mechanism.

In the kinetic parameters of the pseudo-second order model, it was observed that the velocity

SC R

constant k2 is higher for pure magnetite. The data also showed that there is a tendency of increasing this constant when the iron content increases in the material, which allows inferring that the adsorption kinetics was favored with the iron oxide formation in the beads. The adsorption capacity at

U

equilibrium qe (mg g-1) also increased with increasing iron contents. Considering these data, sample

N

CT-Fe (40% w/w) showed to be the material with the highest adsorption kinetic efficiency. It is worth

M

integrity of the beads was compromised.

A

noting that the higher ratios of iron incorporated in chitosan were not tested because the physical

Since the studied system was properly described by the pseudo-second order model, a

ED

mechanism for adsorption could be described. According to results, chromium atoms must bind to the CT-Fe surface by chemical bonds, involving donation or exchange of electrons between adsorbate and

PT

adsorbent as covalent and ion exchange forces. In adsorptions of this nature, the chemical species are attracted to the active centers and bind to form a single layer. This points to the fact that the removal

CC E

of Cr(VI) occurs possibly in two stages: (1) in a first moment, the positive net charge of the CT-Fe surface materials electrostatically attract the hexavalent chromium ions that are in the form of HCrO 4in solution; (2) on the surface, the Cr(VI) ions are reduced to Cr(III) by the Fe (II) species of the

A

magnetite or chitosan reducing ends. In the trivalent form, Cr(III) is probably adsorbed by the formation of the covalent bond -H2N-Cr(III) [55]. In order to clarify the removal mechanism of hexavalent chromium by the CT-Fe beads, Cr(III) (5 mg L-1) was directly removed from the CT, CT-Fe, Fe3O4 materials (Fig. A.3, Appendix A). The results indicated a higher contribution of the pure chitosan beads, with removal capacity of approximately 95.5% while the CT-Fe beads removed 83.5% and the pure magnetite 27.7%. These 22

results suggest that, for pure CT, the reduced Cr is being adsorbed, and this would explain that in all tests for quantification of Cr (total) of this study, the presence of remaining Cr(III) in the supernatant was not detected. The reduction of Cr would result in the formation of the covalent bond -H2N-Cr(III) by the shared electrons and/or the formation of Fe-O-Cr mixed oxides [8]. Still with respect on the validation of the proposed mechanism, the effect of the ionic strength

IP T

change on the adsorption behavior was studied. This approach may provide insights into the adsorption mechanism, since it is known that ions that form outer sphere-type complexes, in which the

SC R

adsorbate is fixed only by electrostatic forces, such as the counter ion show a significant decrease in adsorption with increasing ionic strength. Conversely, for adsorbates that are adhered to the adsorbent species directly coordinated to the active sites, forming typical inner-sphere complexes. They show an

U

increase or absence of adsorption effect with the increase of ionic strength [56].

N

For the effect evaluation of the ionic strength in the adsorption of Cr(VI) and Cr(III) ions,

A

Cr(VI) and Cr(III) solutions were prepared with 1 mol L-1 KCl solution and the data are shown in Figs.

M

B.1 and B.2 of appendix B. In the removal of Cr(VI) (Fig. B.1, appendix B), it was observed that the increase of the ionic strength decreased the removal of the aqueous Cr(VI) by the CT-Fe and Fe3O4

ED

materials. Changes in ionic strength may indirectly affect the distribution of charges on solid surfaces, altering the attraction and repulsion phenomenon between ions and adsorbent particles. The adsorption

PT

decreases due to the higher repulsive interaction among the negative charges of solution ions (HCrO 4and Cl-), in which the electrolytic ions can form a barrier on the charged surface with high salt

CC E

concentration, preventing the reduction of Cr(VI ) and its adsorption. In this case, there is a reduction in the electrostatic attraction between adsorbent and adsorbate, showing that their interactions are

A

weak.

Regarding the removal of Cr(III) by the materials (Fig. B.2, appendix B), there was little

difference caused by the effect of the ionic strength. When adsorption is specific, it involves stable interactions, governed by the formation of high-energy chemical bonds between functional groups of the solid surface and the species under solution [57]. As a result, there is the formation of inner-sphere complexes. This type of bonding is often unaffected by ionic strength of the solution, characterized by high irreversibility. 23

It is possible to infer that the mechanism of Cr(VI) removal by the CT-Fe beads involves firstly the electrostatic attraction of the anionic forms of Cr(VI) by the CT-Fe beads, and then Cr(VI) is reduced by Fe(II) and later Cr(III) adsorption occurs by strong interactions, possibly through the formation of the covalent bond -H2N-Cr(III) by chitosan.

SC R

IP T

2. 5. Adsorption isotherms

The adsorption isotherm is important for the description of how the adsorbate ions interact with the active sites on the adsorbent surface. Experimental adsorption data were analyzed using non-

U

linear forms of Langmuir, Freundlich, and Sips models. The Langmuir isotherm is valid for monolayer

N

adsorption on an energetically homogeneous surface [58, 59]. The non-linear form of Langmuir

qm K L Ce 1  K L Ce

(11)

ED

qe 

M

A

isotherm is shown in Eq. (11).

PT

where qe is the adsorption capacity at equilibrium (mg g-1), Ce is the solution concentration (mg L-1), KL is the Langmuir constant, and qm represents the maximum adsorption capacity (mg g-1).

CC E

In contrast to Langmuir, the Freundlich isotherm describes well the multilayer adsorption on

an energetically heterogeneous surface. The non-linear form of Freundlich isotherm is shown in Eq.

A

(12).

qe  K F Ce1 / nF

(12)

24

where qe and Ce are the adsorbed amount (mg g-1) and the equilibrium concentration of the adsorbate (mg L-1), respectively; KF n and n are the constants related to Freundlich's capacity and adsorption intensity, respectively. The Sips model [60] incorporates the characteristics from Langmuir and Freundlich models into a single equation and is represented in Eq. (13).

IP T

qm( K s Ce ) ns qe  1  ( K s C e ) ns

SC R

(13)

where qe is the amount adsorbed at equilibrium (mg g-1), Ce is the equilibrium concentration of adsorbate (mg L-1), qm is the maximum adsorption capacity of Sips (mg g-1), KS is the equilibrium

U

constant of Sips (L mg-1)n and ns is the exponent of the Sips model. At low concentrations of

N

adsorbates, the Sips isotherm approaches the Freundlich equation, while at high concentration, the

A

Sips model predicts a monolayer adsorption capacity, characteristic of the Langmuir isotherm [61].

M

The calculated Langmuir, Freundlich and Sips isotherm parameters are summarized in Table 2. The correlation coefficients for the Langmuir and Sips isotherms are higher (R 2> 0.98) than those

ED

applied to the model described by Freundlich, which indicates that isotherm models are more adequate to the proposed adsorption system. For the pure magnetite, the isotherm models did not fit well,

PT

showing correlation coefficient R2< 0.970. This may be due to its compact and non-porous characteristics, with lower Cr(VI) adsorption capacity at high concentrations.

CC E

Table 2 Calculations of parameters of the Langmuir, Freundlich and Sips isothermal models. Parameters CT CT-Fe 10% CT-Fe 20% CT-Fe Fe3O4 40% 1.63 2.46 3.16 4.29 1.17 Freundlich KF (L g-1) Models

A

nF

R2

1.44 0.963

1.63 0.965

1.78 0.960

2.06 0.976

2.41 0.960

Langmuir

KL (L mg-1) qm (mg g-1) ± SD R2 RL

0.002 237 ± 27.3 0.984 0.678

0.003 175 ± 11.1 0.991 0.656

0.004 151 ± 11.5 0.983 0.623

0.006 112 ± 4.51 0.992 0.598

0.008 19.3 ± 1.40 0.954 0.866

Sips

Ks (L mg-1) qm (mg g-1) ± SD ns R2

0.004 166 ± 18.6 1.37 0.989

0.005 151 ± 14.9 1.16 0.992

0.005 145 ± 22.6 1.05 0.981

0.004 130 ± 13.9 0.846 0.993

0.002 29.8 ± 10.5 0.642 0.969 25

For the Sips isothermal model, the correlation coefficients were practically the same as the Langmuir model, however, lower standard deviation were obtained for the Langmuir model. Based on Langmuir equation, it is possible to evaluate how favorable is the adsorption process by analyzing the

1 1  K L C0

(14)

SC R

RL 

IP T

parameter RL, which also indicates the adsorption nature, described by Eq. (14):

where KL refers to the Langmuir constant and C0 as the initial adsorbed concentration (mg L-1).

U

Dimensioning these values, the parameter indicates that isotherm is not favorable if R L >1, linear if RL

A

is a favorable adsorption process for all materials.

N

= 1, favorable if 0
M

The adequacy of the adsorption equilibrium data with the Langmuir isotherm implies that all active adsorption sites were virtually equivalent and the surface roughly uniform. Therefore, Cr(VI) is

ED

reduced by Fe(II), and adsorbed Cr(III) does not interact or compete with each other and equilibrium will be established when a monolayer of the metal forms covalent bonds with nitrogenous or

PT

oxygenated groups of chitosan. This model is very useful to describe adsorption mechanisms of chemical nature [5], thus agreeing with the kinetic study adjusted for the pseudo-second order model.

CC E

It was observed that the adsorption capacity of beads increased progressively with increasing

content of Cr(VI) ions and finally reached the saturation states (Fig. 8a). The maximum adsorption capacity of all materials was calculated using the Langmuir

A

isothermal equation (Table 2). The construction of the isothermal curves for the proposed system still showed a certain peculiarity. The pH of the solution was dependent on the content of the initial chromium solution. In Fig. 8b is shown the expansion of adsorption curves in the range from 0 to 30 mg L-1. In this region, whose variation in chromium content did not significantly interfere in the pH of the medium, it was possible to observe that the incorporation of Fe in the beads improved Cr(VI) removal capacity. 26

140

(a)

120

CT CT-Fe 10% CT-Fe 20% CT-Fe 40% Fe3O4

(b)

20

60

10

40

20

0

10

0 0

100

200

300

20

30

SC R

0

IP T

80

-1

qe (mg g )

100

400

500

-1

Ce (mg L )

600

A

N

U

Fig. 8 Effect of chromium content on adsorption (2.5 - 750 mg L-1); (10, 20, and 40% w/w) and Fe3O4 (reaction volume 10 mL, temperature 25 °C, contact time 24 h, 10 mg adsorbent). a) the curves represent the experimental data from the Langmuir isothermal model; b) expansion of the concentration range of 0-30 mg L-1.

M

The opposite was observed at high contents of chromium, where the Cr(VI) removal capacity decreased in the CT-Fe beads in relation to CT. This can be explained because Cr solutions were

ED

prepared without pH adjustments.

For initial concentrations greater than 50 mg L-1, there is a conversion from the form HCrO4-

PT

to Cr2O72- with the release of H+. Thus, since Cr speciation occurred with the [H+] variation, the pH of

CC E

solutions decreased as the Cr content increased (2.5 mg L-1 - pH 5.7 to 750 mg L-1 - pH 4.2), causing protonation of the CT functional groups, which led to a higher removal capacity (qm = 237 mg g-1) in relation to other materials. It was also observed that the increase of the iron in the beads decreased the

A

qm in the high contents of chromium. Thus, it was evident in this study that the CT and CT-Fe beads have a great potential for the

treatment of Cr(VI) from waste water due to the high qm values, indicating the maximum adsorption capacity of the materials.

27

3. 6. Reuse of beads in the removal of Cr(VI)

The reuse of adsorbents is of great importance as a cost-effective process in the water treatment. The incorporation of Fe to CT increased the operational viability of the reuse process of

IP T

CT-Fe beads, since they are magnetic. Sample CT-Fe exhibited 70% removal efficiency of Cr(VI) in the first cycle (Fig. 9). After

SC R

five cycles, the Cr(VI) removal capacity was halved from its initial value and to 25% after 14 consecutive cycles from its initial value. The bonds/interactions of Cr ions occurred until the functional sites of the surface were fully occupied. Insofar as the number of cycles increased, the

U

active adsorption sites decreased, causing a smaller amount of Cr(VI) to be adsorbed. In addition,

N

Fe(II) could oxidize and the potential for reduction from Cr(VI) to Cr(III) decreased at every cycle.

A

80

M

CT-Fe 40% (w/w)

60

ED

50 40 30

PT

Removal of Cr(VI) %

70

20

CC E

10

0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Number of cycles

A

Fig. 9 Number of cycles of CT-Fe beads (40% w/w) in the removal of Cr(VI).

4. Conclusions

A composite CT-Fe can be obtained with the CT dissolution and the coordination of Fe ions. Interaction with iron decreases the intra- and intermolecular hydrogen bonds between the CT units. In 28

the formation of the CT-Fe complex, the CT chains become less ordered, providing the formation of low-crystalline magnetic iron oxide capable of increasing the adsorption capacity. The characterization of the materials allowed the identification of the nano-microcrystalline iron oxide obtained as Fe3O4. The morphology of CT-Fe beads shows a rough appearance responsible for a large increase in the surface area of the material. CT may have provided a structure or even a

CT has a higher capacity to remove aqueous Cr(VI) than pure magnetite.

IP T

model for the growth of smaller magnetite particles. Nano-microcrystalline iron oxide stabilized by

SC R

The kinetic adsorption studies show that Cr adsorbed on CT-Fe beads follows the pseudosecond order model, suggesting that the overall process is controlled by chemisorption. This process happens in stages: firstly, occurs the approach of the anionic forms of Cr(VI) (HCrO4-, predominant

U

form in pH 5.5) to the adsorbent surface. This approach can occur by electrostatic attraction

N

(protonated -NH2 groups present in CT) and/or by Fe(II) and Fe(III) atoms. The acidic medium

A

contributes to the increase in electrostatic attraction and in the reduction of Cr(VI) by Fe(II). In the

M

study pH (5.5), the groups present in CT are not fully protonated and Fe becomes responsible for a large part of Cr removal. Besides Fe (II), the NH2 and OH groups may contribute for the reduction

ED

from Cr(VI) to Cr(III). At this time, the Cr(III) may have been reduced on the own adsorbent surface and/or the reduced form returns to the medium and is re-adsorbed.

PT

The synthesized composite CT-Fe (40% w/w) shows the capacity to remove Cr(VI) in up to five consecutive cycles, besides the easiness of reuse, since beads become magnetic with the

CC E

incorporation of Fe. In this way, these materials containing iron are promising in the treatment of effluents due to the high removal capacity of aqueous Cr(VI).

A

Acknowledgements: CAPES, FAPEMIG, CNPq, Programa de Pós-Graduação em Agroquímica e Programa de Pós-Graduação Multicêntrico em Química/UFLA.

29

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IP T

2782-2799.

SC R

[2] C.G. Lee, S. Lee, J.A. Park, C. Park, S.J. Lee, S.B. Kim, B. An, S.T. Yun, S.H. Lee, J.W. Choi, Removal of copper, nickel and chromium mixtures from metal plating wastewater by adsorption with

N

U

modified carbon foam, Chemosphere, 166 (2017) 203-211.

A

[3] L. Sun, Z.G. Yuan, W.B. Gong, L.D. Zhang, Z.L. Xu, G.B. Su, D.G. Han, The mechanism study of

M

trace Cr(VI) removal from water using Fe-0 nanorods modified with chitosan in porous anodic

ED

alumina, Appl. Surf. Sci. 328 (2015) 606-613.

PT

[4] M.H. Farzana, S. Meenakshi, Photocatalytic aptitude of titanium dioxide impregnated chitosan

CC E

beads for the reduction of Cr(VI), Int. J. Biol. Macromol. 72 (2015) 1265-1271.

A

[5] L. Liu, C. Luo, J. Xiong, Z.X. Yang, Y.B. Zhang, Y.X. Cai, H.S. Gu, Reduced graphene oxide (rGO) decorated TiO2 microspheres for visible-light photocatalytic reduction of Cr(VI), J. Alloys Compd. 690 (2017) 771-776.

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[7] A.W. Chen, C. Shang, J.H. Shao, Y.Q. Lin, S. Luo, J.C. Zhang, H.L. Huang, M. Lei, Q.R. Zeng, Carbon disulfide-modified magnetic ion-imprinted chitosan-Fe(III): A novel adsorbent for

SC R

simultaneous removal of tetracycline and cadmium, Carbohydr. Polym. 155 (2017) 19-27.

U

[8] Z.H. Yu, X.D. Zhang, Y.M. Huang, Magnetic Chitosan-Iron(III) Hydrogel as a Fast and Reusable

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