Nanodiamond-filled chitosan as an efficient adsorbent for anionic dye removal from aqueous solutions

Accepted Manuscript Title: Nanodiamond-filled chitosan as an efficient adsorbent for anionic dye removal from aqueous solutions Authors: M. Raeiszadeh, A. Hakimian, A. Shojaei, H. Molavi PII: DOI: Reference:

S2213-3437(18)30250-1 https://doi.org/10.1016/j.jece.2018.05.005 JECE 2368

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

23-1-2018 6-3-2018 1-5-2018

Please cite this article as: M.Raeiszadeh, A.Hakimian, A.Shojaei, H.Molavi, Nanodiamond-filled chitosan as an efficient adsorbent for anionic dye removal from aqueous solutions, Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2018.05.005 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.

Nanodiamond-filled chitosan as an efficient adsorbent for anionic dye removal from aqueous solutions

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M. Raeiszadeh1, A. Hakimian1, A. Shojaei1, 2* [email protected] , H. Molavi2

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Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran

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Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, P.O. Box 111558639, Tehran, Iran

author. Tel./fax: +98-21-66166432.

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Corresponding

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[email protected] (Milad Raeiszadeh), [email protected] (Alireza Hakimian), [email protected] com (Hossein Molavi).

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ABSTRACT

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A series of novel chitosan/nanodiamond (CTS/ND) composites containing NDs with variable surface carboxyl groups and various concentrations were prepared using solution casting method. Powdery CTS/ND composites were employed as adsorbent of a model anionic dye (methyl orange, MO). Experimental results showed that the incorporation of NDs with high carboxylic content (ND-H) in to CTS increased substantially the maximum adsorption capacity of neat CTS from 167 mg/g to 454 mg/g. The remarkable adsorption capacity of dye on CTS/ND composites was associated to the oxygen-containing groups on the outer surface of NDs which would be beneficial to interact with the dye molecules through hydrogen bonding and electrostatic interactions. NDs with higher carboxylic content could improve the degree of dispersion in the CTS which in turn improved the accessible surface area of NDs and adsorption affinity of CTS/ND. The adsorption kinetics of CTS/ND adsorbent was well described by pseudo-second-order and Langmuir isotherm model showed very good fit with adsorption data. Keywords: Chitosan; Nanodiamond; Nanocomposite; Adsorption; Methyl orange

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Introduction

Over the last century, water pollution has become a serious global concern. The extensive use of dyes, organic compounds, microorganisms, heavy metals, etc. has led to the spreading of hazardous materials in environment which are threatening human beings. Among all, dyes are of a serious threat which should be eliminated effectively from wastewater. This is because of the fact that many modern

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industries such as textile, plastic, rubber, paper and cosmetic are generating plenty of colored wastewaters [1, 2] which can pollute water resources, even in small amounts [3].

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Azo dye, either anionic or cationic, compounds are large class of dyes containing one or several azoic bonds (N=N) in their chemical structure [4]. Most of the azo dyes are toxic and can cause serious diseases such as genetic mutation, allergic problems, vomiting and cyanosis. Moreover, due to their complex structure, they are stable toward light, heat and aerobic digestion making azo dye removal a serious issue, so it has been the subject of many research activities in scientific communities [3, 5, 6]. Overall, numerous physico-chemical and biological methods, e.g. coagulation techniques [7], biological degradation [8], photo-oxidation [9], membrane processes [10], and adsorption, [11] have been utilized for wastewater treatment. Among various methods, adsorption using suitable solid adsorbents has found great attractions due to its simplicity and nontoxicity.

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Conventional adsorbents like activated carbon [12] as well as newly introduced nanoparticles like graphene oxide (GO) [13] and carbon nanotube (CNTs) [3] have known as efficient solid adsorbents, thanks to their high specific surface area, surface reactivity and insensitivity to toxic pollutants. However, these adsorbents are often expensive and their regeneration from aqueous solution is difficult [14].

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Nevertheless, environmental issues have attracted the attention of researchers for employing natural based polymeric adsorbents. In this context, chitosan (CTS) as biosorbent is increasingly receiving much attention in research communities for wastewater treatment. CTS is a plentiful natural biopolymer containing many functional groups such as amino, acetamido, primary and secondary hydroxyl groups in its molecular structure which offer great potential for the adsorption of anionic dyes through the electrostatic interaction and coordination sites [5]. Such unique molecular structure of CTS along with biodegradability, biocompatibility, nontoxicity and cost effectiveness has made CTS an ideal biosorbent for wastewater treatment [15-17].

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Recent investigations have shown that CTS nanocomposites, i.e. CTS incorporated with nanoparticles, present better adsorption capability against dyes and show tolerable resistance to acidic environment [18]. In this context, CTS nanocomposites with various nanoparticles such as GO, CNT, montmorillonite, bentonite and magnetite materials were examined for dye removal from wastewater [6, 19]. Accordingly, CTS nanocomposites have become an emerging area in the field of wastewater treatment.

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In spite of great achievements in this field, CTS nanocomposites suffer from several drawbacks such as complicated fabricating and oxidizing procedure in the case of GO [16, 20, 21], highly-cost synthesis and regeneration of CNT [14, 22, 23], relatively low improvement caused by graphite oxide [18], and the need for high loading of some nanoparticles such as silica, bentonite and montmorillonite to enhance the adsorption capacity of neat CTS to a desirable level [6, 14, 24, 25]. Moreover, nanoparticles are often expensive which can restrict their practical applications in large scale. Therefore, much effort is still required for improvement of CTS nanocomposites to become well accepted materials for wastewater treatments. Nanodiamond (ND) is kind of carbon based nanoparticle which has shown many interesting properties such as high specific surface area (greater than 200 m2/g), large amounts of oxygen-containing functional groups at outer surface, excellent mechanical and thermal properties, and biocompatibility [26-28]. Consequently, ND provides very high surface reactivity thanks to its rich surface chemistry [29].

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This offers a great opportunity to tailor readily the surface chemistry of ND by various chemicals [27, 30, 31] or even by simple thermal oxidation at high temperatures [32, 33]. Indeed, thermal oxidation at ~ 450 ˚C introduces more carboxyl groups primarily through the hydrolysis of anhydrides by water.

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Current investigations on polymer/ND composites have dealt mainly with various aspects such as mechanical properties, morphological characteristics, cure kinetics behavior and biocompatibility [28, 30, 34-36]. However, performance of polymer/ND composites in removal of water pollutants has not been reported in literature so far, to the best of the authors’ knowledge. There are only a few reports on the role of ND particles to remove pollutants from wastewater like fluorescence dye [26], azo dyes [22] and metal ions [37].

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Based on the above mentioned perspective, the present research is aimed to incorporate ND as a stateof-the-art nanoparticle into CTS matrix at very low concentrations which is expected to enhance significantly the adsorption capacity of CTS for anionic dyes and improve its mechanical properties considerably. Such expectation is based on the rich and easily tailorable surface chemistry as well as exceptional mechanical properties of ND. ND functional groups can act as active sites for adsorption of dyes as well as contribution on the chitosan-ND interaction leading to improvement of ND dispersion and enhancement of mechanical properties.

Materials and Methods Materials

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Moreover, ND shows essential biocompatibility [29] which is able to retain the biocompatibility and biodegradability of the hosting polymers as well [34]. Availability of economically viable mass production technique like detonation process [38] and capability of ND to improve the mechanical properties of CTS at low concentrations [39] are other important aspects that can make CTS/ND composite as a promising biosorbent.

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CTS with 75-85% degree of deacetylation and molecular weight of 180000-280000 Da was purchased from Sigma-Aldrich. ND powder (purity 98–99%, average single particle diameter of 4–6 nm, surface area of 282 m2/g and density of 3.05–3.3 g/cm3) was acquired from NaBond Technologies Co., China. Methyl orange (MO, C14H14N3NaO3S, 327.33 g/mol, dye content 85%, Sigma Aldrich), which is one of the most common azo dye compounds reported in literature [40, 41], was used as a model anionic dye in the present investigation. All other agents used in this study were analytical grade and all solutions were prepared with double distilled water.

Preparation of CTS/ND composites

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CTS/ND composites with four different concentrations of NDs, e.g., 0.1, 0.5, 1, 1.5 wt%, were prepared using solution casting method under acidic environment, see more details in our previous work [39]. In addition to as-received ND, referred to as UND taken from untreated ND, NDs oxidized at 425 ̊ C under atmospheric condition were also incorporated in to CTS at various concentrations. It is well known that thermal oxidation promotes the carboxylic group content of ND [33], and the extent of increment in carboxylic content is dependent on the oxidation time [31]. Accordingly, ND-L and ND-H which was air oxidized for 1.5 h and 4 h, respectively, was incorporated in to CTS as well. The carboxylic group content

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of ND-L and ND-H was measured to be 1.66 mmol/g and 2.7 mmol/g, respectively, by Boehm titration which were higher than that of 0.3 mmol/g for UND [31]. Solution casting method was resulted in thin films which were then ground to powder form and sieved to 200 mesh sizes. The uniform sized powders of neat CTS and CTS/ND composites were used for adsorption experiments.

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Batch adsorption experiments

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Firstly, MO solutions were prepared by direct dissolution of MO with known weight in deionized water. All batch adsorption experiments were performed in 250 ml beakers, where a certain mass of adsorbent powder, either neat CTS or CTS/ND composites, was added into 100 ml of MO aqueous solutions and allowed to be adsorbed at room temperature (25 ̊ C). The adsorption process was performed on a thermostat stirrer with a shaking rate of 1000 rpm until the system reached to equilibrium. At given time intervals, the adsorbents were separated and the MO concentration in the colored solutions was measured. After the completion of shaking, the adsorbent particles were allowed to settle down and then the sediments were separated from the solutions. Due to large amount of water uptake of the adsorbents, i.e. CTS and CTS/ND as reported earlier [39], sedimentation took several minutes. To assist the sedimentation, the suspensions were centrifuged at 4000 rpm for 10 min.

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MO concentration after adsorption process was determined by inspecting the absorbance at 464 nm in ultraviolet-visible (UV-Vis) spectrum using Philips PU9400 atomic adsorption spectrophotometer. First, by using standard solutions with known concentrations of MO, adsorption versus concentration calibration curve was drawn and the linearity was ensured, then it was used for calculating the unknown concentrations. To avoid any error caused by possible not-removed adsorbent, solutions with known concentration of adsorbent in water were used as the blank sample in this method.

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For all adsorption tests, the initial pH values of the solution were adjusted with 0.1 mol/L HCl and 0.1 mol/L NaOH solutions using pH-meter. To study the influence of pH on the removal of MO, the initial pH values of the solution were adjusted to a wide range from 2.0 to 11.0. To examine the influence of NDs concentrations of CTS/ND composites, adsorption experiments were performed using four composites containing 0.1, 0.5, 1 and 1.5 wt% of NDs loading. These experiments were carried out using all treated and untreated NDs, i.e. UND, ND-L and ND-H. The adsorption capacity (Q) and removal percentage were calculated according to the following expressions [42]: (C0 - Ct )V W

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Qt =

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Removal (%) =

(1)

(C0 - Ce ) C0

× 100

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where C0 and Ce are the concentrations of MO in the solution (in mg/L) at initial and equilibrium stages, respectively, Ct is MO concentration at any time t, V is the volume of MO solution, in liters, and W is the mass of the absorbent used, in grams. In the adsorption experiments, Qt (mg/g) reached to equilibrium adsorption capacity Qe after a long period. Each adsorption experiment was repeated until to obtain repeatability of data within ±5%. In many cases, the repeatability criterion was ensured by performing the adsorption experiments twice.

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2.4

Characterization

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The morphology of ND particles as well as CTS/NDs was investigated by using a field emission scanning electron microscope (FESEM, LEO model 1455VP, UK) at an accelerating voltage of 30 kV on the fractured surfaces coated with gold. Moreover, TEM image of ND particles was recorded on a Philips CM 120 transmission electron microscopy at an accelerating voltage of 120 kV. Fourier transform infrared (FTIR) spectra was recorded using 100-FT-IR Spectrometer, Perkin-Elmer, with a wavenumber resolution of 4 cm-1 to characterize CTS and its composites, and to illustrate the interactions between MO and adsorbents. Atomic force microscopy (AFM) imaging was carried out sing a Nanoscope (DualScopeTM (DS 95) Scanner).

Results and discussion

3.1

Characterization of the adsorbents

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The morphological characteristics of the CTS/ND composites used for adsorption experiments were fully characterized in our previous work [39]. Briefly, it was shown that NDs loading did not alter the crystallinity of neat CTS; however, NDs could interact efficiently with CTS through the interaction between carboxyl groups of NDs and amine groups of CTS. Such interaction caused a strong interphase in CTS/ND composites. It was also shown that the thickness and strength of the interphase was greatly dependent on the degree of carboxylic content of NDs. Accordingly, ND-H with highest carboxylic content showed the strongest and thickest interphase, amongst CTS/ND composites investigated.

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In the present investigation, morphology of ND particles is investigated by using FESEM and TEM micrographs. According to the micrographs shown in Fig. 1, ND particles have a spherical crystal structure with a characteristic size of about 5-10 nm. In addition, some agglomeration centers with maximum sizes of about 500 nm are also observed. To confirm the nanostructured surface of the adsorbents, AFM images of neat CTS with CTS composites at two different loadings are compared in Fig. 2. The three dimensional morphology of CTS/ND obtained by AFM illustrates the existence of ND nanoparticles on the surface, as the quite homogeneous surface of CTS has been altered by the addition of a few loading of ND. As indicated in the scale bar of the AFM images, the average size of nanoparticles of the surface was measured to be about 50 nm. However, since smaller ND particles (5-10 nm) are wellincorporated into the CTS matrix, they are hardly seen on the surface images. Meanwhile, dark regions in the topographical images identify the pores in the CTS/ND and interestingly the depth of pores (<25nm) appears to be related to the dimensions of smaller NDs. AFM images also indicate the well dispersion of ND-H at 1 wt% loading; however, at higher loadings, i.e. 1.5 wt%, ND-H starts to be agglomerated which well agrees with the results obtained from FESEM images in our previous work [39]. FTIR spectra are employed to investigate the functional groups of neat CTS and CTS/ND composites before and after the dye adsorption process (see Fig. 3). The strong and broad absorption band observed at about 3200-3500 cm-1 can be attributed to (i) O-H and N-H stretching vibrations and (ii) the formation of hydrogen bond between hydroxyl and carboxylic acid groups of ND and sulfonate groups of MO. Moreover, the absorption bands around 2850-2950 cm-1, which can be associated to the C-H stretching vibration, show several changes indicating that MO is successfully adsorbed by CTS/ND. These changes include a relative intensity increment observed throughout this range, a new noticeable peak

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formed at 2916 cm-1 for CTS/ND, and 2991 cm-1 peak shifted to 2949 cm-1 for both adsorbents, i.e. CTS and CTS/ND composites.

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Additionally, it is well-known that MO contains aromatic rings (benzene) which result in vibration peaks at 1600-1700 cm-1 in the FTIR spectrum. These peaks are mostly covered in FTIR spectrum of MOadsorbed samples in this work due to the high density of peaks in this range caused by functional groups of adsorbents such as various oxygen-containing, acetyl, and amide groups. Nevertheless, after MO adsorption, some noticeable changes are observed in this range, such as shifting 1664 cm-1 peak to a higher wavelength (1693 cm-1) in both CTS and CTS/ND caused by the adsorbed aromatic rings of MO.

3.2 Effect of ND loading on the adsorption capacity of CTS/ND

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Fig. 4-A displays MO equilibrium adsorption capacity of CTS/ND-H composite at various ND-H concentrations. The adsorption capacity of neat CTS is found to be 162.4 mg/g which is in the range of 10-1100 mg/g reported in literature for unmodified CTS against anionic dyes [5]. As deduced from Fig. 4A, it is revealed that the adsorption capacity of CTS/ND composites is more than two-fold with respect to neat CTS even by incorporating very small amount ND-H, i.e. 0.1 wt%. Such a great improvement in adsorption capacity of CTS by adding few percent ND-H suggests the great tendency of anionic dyes to interact with ND-H which is believed to be established through the hydrogen bonding in which sulfonate and hydroxyl groups from the dye form hydrogen bonds with oxygen-containing groups on the ND-H surface [22]. Indeed, ND-H contains many carboxyl and other oxygen containing groups which could facilitate its interaction with MO in our case.

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As can be inferred from Fig. 4-A, MO adsorption capacity of CTS/ND-H increases slightly by increasing ND-H concentration up to 1 wt% which is followed by a relative decrease at 1.5 wt%. Morphological studies presented in previous work [39] and AFM analysis illustrated in Fig. 2 have shown that the agglomeration of ND in CTS matrix is promoted at 1.5 wt% which is resulted in lower accessible specific surface area. Therefore, this causes comparatively a fewer active sites for interaction with MO reflecting in a relative decrease in the adsorption capacity.

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Fig. 4-B compares the equilibrium adsorption capacity of CTS incorporated with UND, ND-L and ND-H at various concentrations. As inferred, the adsorption capacity increases by increasing the carboxyl group content of NDs at any concentrations. Accordingly, ND-H having the higher carboxyl content exhibits the highest adsorption capacity in the composite.

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It was shown [39] that the carboxyl groups of NDs are essential to provide interaction with CTS through hydrogen bonding. As suggested in literature [43], established bonds between polymer and reinforcement can also contribute on the electrostatic attraction with anionic dyes at acidic conditions. Therefore, it is believed that bipolar hydrogen bonding in our case can involve in interaction with dyes through electrostatic interaction as well. Moreover, it can be speculated that carboxyl groups are able to provide more efficient hydrogen bonding with anionic dyes compared with ketones and ether groups leading to higher adsorption capacity. Additionally, the state of dispersion is finer with NDs having higher carboxyl content as can be deduced from FESEM microphotographs shown in Fig. 5 (A and B). Finer dispersion offers greater accessible surface area for CTS/ND composites which promotes the adsorption process.

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Despite relative decrease of MO adsorption in CTS/ND composites at 1.5 wt% loading compared with lower ND loadings, the difference in MO adsorption of CTS composites with different NDs, i.e. UND, NDL and ND-H, at concentration of 1.5 wt% is more pronounced, as deduced in Fig. 4-B. As the promotion of agglomeration was considered as dominant factor in adsorption capacity at 1.5 wt% loading, the difference can be associated to the extent of agglomeration for NDs with various carboxyl contents. Morphological investigation reported earlier [39] exhibited that the NDs with higher carboxyl group (NDH) provide less agglomerations compared with UND (see Fig. 5 (A and B) for the sake of comparison as well) which can lead to considerable difference in larger interfacial region and higher adsorption capacity between these NDs.

Effect of adsorbent dosage on MO adsorption

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Although ND-H increased considerably the adsorption capacity of CTS even at very low concentrations, e.g. 0.1 wt%, we examine only CTS/ND-H composite at 1 wt% concentration in the adsorption experiments at the rest of the present work. This is because of the fact that this composition has shown slightly highest dye adsorption capacity and indicated the maximum improvement in mechanical properties of CTS [39] which can be beneficial to obtain CTS adsorbent with improved adsorption and enhanced mechanical properties. Henceforth, this composite is designated by CTS/ND-H(1).

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The influence of adsorbent dosage of both CTS and CTS/ND-H(1) on the adsorption of MO was investigated at an initial MO concentration of 100 mg/L. Fig. 6 displays the influences of CTS and CTS/ND-H(1) dosage on the removal percentage and the equilibrium adsorption of MO.

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For neat CTS, it is evident that by increasing adsorbent dosage from 0.05 to 1 g/L, the removal percentage increases from 8.2% to 53.3%. Obviously, the overall surface area of the adsorbent increases by increasing the amount of the adsorbent which provides more active binding sites [43] leading to higher removal percentage. Interestingly, CTS/ND-H(1) shows greater removal at any adsorbent concentration with respect to neat CTS indicating the higher aptitude of ND-H in adsorption of dyes. So, the removal percentage of CTS/ND-H(1) increases from 25% to 88% at the same range of adsorbent dosage used for neat CTS, i.e. 0.05-1 g/L.

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Contrarily, the equilibrium adsorption capacity (Qe) exhibits a significant decline in absorbent dosage of 0.05g/L to 1 g/L for both CTS and CTS/ND-H(1). In the low adsorbent dosages, the active adsorption sites are almost fully covered by MO molecules while by increasing adsorbent dosage the sites could not be covered completely leading to a decrease in the adsorption capacity [44].

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In practice, the least amount of adsorbent that is able to meet the dye removal needs should be selected. Therefore, by taking into account the concentrations less than 100 mg/L of MO, the mass of adsorbent was set in 0.1 g/L for the rest of the experiments.

3.4

Effect of initial pH

The chemical characteristics of an adsorbent and adsorbate can significantly be influenced by the pH level of the adsorption medium. Therefore, an investigation on the pH level can provide deep insight into adsorption process and shed light on the interaction between the adsorbent and adsorbate. Literature shows that the lower pH values (acidic media) are much favorable for dye adsorption capacity

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of unmodified CTS; however, the acidic condition increases the risk of dissolution of CTS in the adsorption system. Despite such drawback, dye removal performance of unmodified chitosan has been examined by researchers in a wide range of pH values, i.e. pH of 2-11, even in strong acidic conditions [45-47].

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Fig. 7-A compares the adsorption behavior of CTS and CTS/ND-H(1) at various pH levels. Consistent with literature [48, 49], the maximum MO removal for neat CTS is observed in some acidic conditions with the pH value near 4. It is generally known that anionic dyes such as MO, which contains the sulfonate groups in its chemical structure, are ionized in water resulting in sulfonate anions as follows [20]:

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In acidic situations, amine groups of chitosan are protonated by adsorbing the dissolved protons in the solution, according to Fig. 7-B. [31]. Therefore, at pH values below pKa of chitosan, having a value close to 6.5 [50], the amino groups are positively charged. The electrostatic attractions between cations (adsorbent) and anions (adsorbate) promote the adsorption process; see Fig. 7-C. Accordingly, the acidic media can be suitable condition for the adsorption of anionic dyes by CTS.

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The relative decrease in the dye removal capacity of CTS at alkaline conditions; i.e. pH values higher than 6, could be attributed to the electrostatic repulsion caused by the promotion of negatively charged sites on the adsorbent [51]. Moreover, presence of excess OH- ions in alkaline condition competes with anionic dye for the adsorption on adsorbent.

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As can be deduced from Fig. 7-A, the trend of variation of MO removal by CTS/ND-H(1) with pH is interestingly similar with neat CTS, so that the maximum adsorption occurs at pH=4. This behavior suggests that the MO adsorption on the CTS/ND-H(1) proceeds with the same mechanism as the neat CTS, which is mostly the physisorption. However, it is obvious that the extent of MO removal by CTS/NDH(1) is considerably higher than that of the neat CTS. This offers that ND-H provides greater adsorption sites in the CTS composite which can in turn enhance the adsorption capacity of this adsorbent. Such behavior can be attributed to the presence of plenty of oxygen-containing groups, particularly carboxyl groups, on the surface of ND-H. As reported earlier [39], carboxyl groups of ND-H is interacted with amino groups of CTS through hydrogen bonding which is speculated to contribute on the electrostatic interactions with sulfonate anion due to bipolar nature of hydrogen bonding, as described in literature as well [43]. Moreover, acid dyes can interact with the oxygen-containing groups of ND-H through the strong hydrogen bonding [22]. Similar with neat CTS, by increasing pH at alkaline conditions, the adsorption of MO on CTS/ND-H(1) decreases which could be attributed to the electrostatic repulsion between the adsorbent and adsorbate [22]. The effect of increased electrostatic repulsion tends to counteract with the strong hydrogen bond between MO and ND-H, which leads to the decrease of the removal. Similar pH dependence was also observed for the adsorption of anionic dyes of this kind in the previous studies [25, 40, 52].

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3.5

Adsorption isotherms

Qe =

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To get deep insight into the adsorption mechanisms of MO on CTS/ND and predict the equilibrium data, Langmuir and Freundlich isotherm models were utilized. Langmuir’s equation is commonly used for isotherm modeling, which is derived from simple mass kinetics assuming adsorption, that can reduce to Henry’s law at lower initial concentrations. Alternatively at higher concentrations, it predicts a monolayer sorption capacity. The two-parametric non-linear form of the Langmuir isotherm is expressed as [52]: Q m K L Ce 1 + K L Ce

(3)

where Ce is the equilibrium concentration of the adsorbent (MO) in solution (mg/L), Qe is the adsorbed amount of MO at equilibrium (mg/g) and Qm is the maximum adsorption capacity (mg/g). Parameter KL represents the Langmuir binding constant which is related to the energy of adsorption and should vary with temperature.

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The Freundlich model is an empirical model that considers heterogeneous adsorptive energies on the adsorbent surface. This power-function-formed model can be expressed as [41]: 1

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Q e = K F Ce n

(4)

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where Qe (mg/g) is the adsorbed amount of the adsorbent (MO) at equilibrium, Ce (mg/L) is the equilibrium concentration of MO in solution, and Kf and n are Freundlich constants, indicating the adsorption capacity and heterogeneity factor, respectively.

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According to Table 1, it is obvious that Freundlich adsorption model does not fit the experimental data well (in terms of low correlation coefficient of R2 ≤ 0.9), whereas Langmuir adsorption isotherm describes better the adsorption of MO on both CTS and CTS/ND-H(1). Therefore, the parameters of Langmuir adsorption isotherm model extracted from the curve fitting are listed in Table 1. Meanwhile, only Langmuir adsorption isotherm model fitted with experimental data for MO removal is depicted in Fig. 8A.

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In order to verify the validity of the Langmuir isotherm model, additional error analysis is provided employing Chi-Square (χ2) and average prediction errors (APE) which are calculated by using Eqs. 5 and 6, respectively. Practically, χ2 or APE exhibits the fit between the experimental and modeled values of adsorption capacity in which large value of χ2 or APE indicates that the trend estimation is rather unlikely and data from the model differs from the experimental data [41, 53]. 𝑁

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(𝑄𝑒,𝑒𝑥𝑝 − 𝑄𝑒,𝑐𝑎𝑙 ) 𝐶ℎ𝑖 − 𝑆𝑞𝑢𝑎𝑟𝑒 = 𝜒 = ∑ 𝑄𝑒,𝑒𝑥𝑝

𝐴𝑃𝐸 =

2

(5)

1

∑𝑁 1

|𝑄𝑒,𝑒𝑥𝑝 − 𝑄𝑒,𝑐𝑎𝑙 | 𝑄𝑒,𝑒𝑥𝑝 × 100 𝑁

(6)

It is clear from Table 1 that the χ2 and APE have very much lower values for Langmuir isotherm than that for Freundlich for both adsorbents. Thus, it indicates that the Langmuir model is able to describe equilibrium data precisely. As inferred, in the case of non-linear isotherm model, the error remains

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constant. Hence, it is appropriate to use the R2 values for comparing the best-fitting nonlinear isotherm models.

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Langmuir adsorption isotherm is based on the assumption that the adsorption process occurs not only in a homogeneous manner but also independent from the circumstance of adjacent sites, whether filled or not. This means that all active sites of both CTS and CTS/ND-H exhibit equal adsorption affinity, the dye molecules form a monolayer on their surfaces, and interactions between adsorbed molecules can be ignored. As given in Table 1, the maximum adsorption capacity (Qm) of MO adsorption on CTS/ND-H(1) was calculated to be 454.5 mg/g which is much higher than that of the neat CST with Qm of 166.7 mg/g. Such Qm value for CTS/ND-H(1) is higher than many adsorbents of MO reported in literature (see Table 2). This behavior could be due to effective role of NDs which provide more efficient and active binding sites for adsorption of MO besides the abundant amino and hydroxyl groups of CTS. The favorability of the adsorption process can be determined using dimensionless equilibrium parameter (RL) which is given as follows [20]:

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1 1 + (𝐾𝐿 𝐶0 )

(7)

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RL=

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where C0 (mg/L) is the initial concentration of MO and KL (L/mg) is the Langmuir constant. The RL value indicates whether the isotherm is favorable, when 0 < RL< 1, or not [54]. In this study, the value of RL was found to be 0.233 which is in the range of 0-1. This suggests that the uptake of MO onto the CTS/NDH(1) is favorable.

Effect of contact time (adsorption kinetics) To evaluate the mechanism of adsorption and kinetics controlling the adsorption process, pseudo-firstorder and pseudo-second-order kinetics as two generally used models are utilized for the analysis of experimental data. The pseudo-first-order kinetics is employed in the form proposed as [52]:

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Qt = Qe (1-𝑒 −k1𝑡 )

(8)

where Qe and Qt (mg/g) represent the amounts of MO adsorbed by the adsorbent at equilibrium and any time t (min), respectively, and k1 stands for the rate of adsorption (min-1). The values of

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k1 and Qe were calculated from the model fitting with the experimental data. Accordingly, k1 and Qe for CTS/ND-H(1) were estimated to be 0.251 min-1 and 75.39 mg/g, respectively. The low regression coefficient (R2 = 0.8158) along with the significant difference between the calculated (75.3921 mg/g) and experimental (90.05 mg/g) Qe values illustrates that the uptake of MO onto CTS/ND-H(1) does not well-match with pseudo-first-order kinetics model.

k 2 𝑄𝑒 2 𝑡 1 + k2 Qe𝑡

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Pseudo-second-order model is commonly expressed nonlinearly as follows [52]: (9)

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where k2 exhibits the rate constant of pseudo-second-order adsorption, Qe and Qt have already been defined.

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From the curve-fitting procedure depicted in Fig. 8-B, k2 (0.0046 g/mg.min) and Qe (91.74 mg/g) values were calculated for CTS/ND-H(1). Unlike the pseudo-first-order model, in the pseudo-second-order model the calculated Qe (91.74 mg/g) agrees very well with the experimental one (91.05 mg/g) and the determination coefficient has a reasonable value (0.9981). This suggests that the pseudo-second-order kinetics model predicts the mechanism of the adsorption accurately, which is the same as the dominant kinetics for adsorption on CTS.

CC

EP

Error analysis is also conducted for adsorption kinetic similar to one discussed in section 3.5. Resultant values for χ2 and APE in the case of pseudo-second-order model are dramatically low which verifies the well prediction of pseudo-second-order kinetic model for MO adsorption on CTS and CTS/ND. The kinetic model constants, their respective correlation coefficients (R2) and the error values for both adsorbents are all shown in Table 3.

A

Pseudo-second-order model is predominant in a wide range of adsorption reactions, indicating the fact that the total amount of MO dye adsorbed on the adsorbent is rate-controlled by the chemical processes, through the electron-sharing or by covalent forces (hydrogen bindings) through the electronexchanging between the adsorbate and adsorbent [59]. In addition, as illustrated in Fig. 8-B, the large part of the adsorption of the dye takes place within 15 min which is an acceptable adsorption time for fast removal of a dye.

11

3.6

Effect of ionic strength

SC RI PT

It is well known that the industrial effluents are rich in different types of salts and surfactants, which may affect the efficiency of the applied wastewater treatment process. Salinity, often sodium chloride, can retard or accelerate the dye adsorption process based on two opposite effects: (i) screening the electrostatic attractive forces between oppositely charged adsorbent and dye molecules, lowering the amount of dye uptake and (ii) conversely, enhancing dye dissociation which may result in an increase in dye removal [50]. Herein, the effect of additional salts on the adsorption of MO on CTS/ND-H(1) is studied at pH=4 with different ionic strength adjusted by NaCl and KCl.

Desorption study

N

3.7

U

The results shown in Fig. 9 reveal that the presence of both salts decreases the dye adsorption. Meanwhile, the extent of dye adsorption decreases further by increasing the concentration of both salts. This result is due to the Cl- ions which may screen the positively charged sites of CTS/ND-H(1), leading to the reduction of electrostatic attractive force between MO and adsorbent, and accordingly, the MO adsorbed amount declines.

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A

The reusability of the CTS/ND-H(1) composite was determined by studying the adsorption ability of the regenerated composites in consecutive adsorption/desorption cycles. Alkaline condition seems to be more efficient for desorption study as reported in literature. However, to avoid alkali induced alterations on CTS/ND-H(1) active carboxylated surface and easier going out of the pores due to low boiling temperature in the re-activating step, ethanol was used rather than an alkali [60, 61]. To do this, in any cycle, the MO-loaded CTS/ND-H(1) were collected, thoroughly rinsed with distilled water to remove any unadsorbed dyes, and dried in a vacuum. Then it was put into an ethanol solution and stirred for 12 h. Finally, the recovered adsorbents were washed with distilled water, vacuum-dried in 80 ˚C overnight, and reused for another adsorption in succeeding cycles.

A

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EP

The results of five consecutive adsorption–desorption cycles are shown in Fig. 10. The MO removal decreased almost linearly with subsequent repeated uses up to four cycles and then it levels off. In comparison with the first adsorption, the second dye removal decreased almost 3%, and the fourth (and fifth) decreases by about 15%. Such a reduction in adsorption capacity can be associated with a decrease in the number of binding sites after each desorption cycle [62]. However, almost 70% removal after the last step still presents good reusability of the composite.

Conclusion

Novel biosorbent based on CTS/ND composites prepared by solution casting method were utilized to remove an anionic dye (MO) from aqueous solution. As indicated by the experimental data, NDs could enhance greatly the adsorption capacity of neat CTS without changing the adsorption mechanisms. The adsorption capacity of CTS/ND composites was found to be dependent on the ND loading in CTS and the degree of carboxylic content of NDs. For CTS filled with 1 wt% of ND-H (having higher carboxyl content), the maximum adsorption capacity increased from 167 mg/g for neat CTS to 454 mg/g for CTS/ND-H(1) composite. Such a great adsorption behavior of CTS/ND composites could be due to rich various oxygen-

12

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containing groups on the ND surface which made possible some strong interactions as hydrogen bonding with dye molecules and interaction with amino groups of CTS molecules which in turn dominated the degree of dispersion of ND in CTS and increased the accessible surface area of the ND particles in composites. The adsorption kinetics of CTS/ND composites was more accurately described by the pseudo-second-order model and equilibrium data could also be well-matched with the Langmuir isotherm model. Due to the considerable effect of ND on the improvement of mechanical properties of CTS, as suggested by our previous investigation, and on the enhancement of dye adsorption properties of CTS, CTS/ND composites could be considered as versatile, efficient and cost-effective biosorbent for anionic dyes from aqueous solutions where high mechanical properties, like column adsorption processes, are required. Acknowledgements

The authors acknowledge the financial support received from Sharif University of Technology. References

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chitosan, Arabian Journal of Chemistry 10(1) (2017) 24-32. [58] V.K. Gupta, R. Kumar, A. Nayak, T.A. Saleh, M.A. Barakat, Adsorptive removal of dyes from aqueous solution onto carbon nanotubes: A review, Advances in Colloid and Interface Science 193–194 (2013) 24-34. [59] D.J. Malik, V. Strelko, M. Streat, A.M. Puziy, Characterisation of novel modified active carbons and marine algal biomass for the selective adsorption of lead, Water Research 36(6) (2002) 1527-1538. [60] P. Banerjee, S.R. Barman, A. Mukhopadhayay, P. Das, Ultrasound assisted mixed azo dye adsorption by chitosan–graphene oxide nanocomposite, Chemical Engineering Research and Design 117 (2017) 43-56. [61] J. Abdi, M. Vossoughi, N.M. Mahmoodi, I. Alemzadeh, Synthesis of metal-organic framework hybrid nanocomposites based on GO and CNT with high adsorption capacity for dye removal, Chemical Engineering Journal 326 (2017) 1145-1158. [62] R. Machado, J.R. Carvalho, M. Joana Neiva Correia, Removal of trivalent chromium(III) from solution by biosorption in cork powder, Journal of Chemical Technology & Biotechnology 77(12) (2002) 1340-1348.

17

(B)

(C)

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(A)

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Fig. 1. FESEM microphotographs (A) untreated ND (UND) and (B) ND-H along with (E) TEM image of NDH.

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(B)

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(A)

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(C)

Fig. 2. AFM phase images of (A) neat CTS along with the CTS composites of (B) CTS/ND-H(1) and (C) CTS/ND-H(1.5).

19

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Fig. 3. FTIR spectra of CTS and CTS/ND-H(1) before and after adsorption of MO.

(A)

(B)

ND-L

Qe (mg/g)

EP

300

UND

390

CC

Qe (mg/g)

400

420

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360

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330

100

300

0 0

0.5 1 ND-H content (wt%)

0.1

1.5

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1 1.5 Mass fraction of ND (wt%)

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Fig. 4. Equilibrium adsorption capacity of CTS/ND composites for MO, (A) CTS/ND-H composites with different ND-H loadings and (B) the composites with different NDs at various concentrations, (conditions: the adsorbent dosage of 0.1 g/L, the initial MO concentration of 100 mg/L and pH of 5.5).

M

Fig. 5. FESEM microphotographs of (A) CTS/ND-H(1) and (B) CTS/UND(1).

500

(B)

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(A)

100

400

80

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CTS/ND-H(1)

Removal (%)

EP

300

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CC

Qe (mg/g)

CTS

100

A

0.2

40 CTS/ND-H(1)

20

CTS

0

0

60

0 0.4

0.6

0.8

1

0

0.2

0.4

0.6

Dosage (g/L)

0.8

1

Dosage (g/L) Fig. 6. Effect of CTS and CTS/ND-H(1) dosage on (A) the equilibrium adsorption capacity and (B) the removal percentage of MO, (conditions: MO initial concentration 100 mg/L and pH 5.5).

21

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CTS/ND-H(1) 80

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0 4

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8

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pH

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Removal (%)

CTS

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(B)

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(C)

A

Fig. 7. (A) Effect of solution pH on MO removal using neat CTS and CTS/ND-H(1), (B) protonation of CTS in acidic medium and promotion of cationic amino groups and (C) development of electrostatic attractions between CTS, (conditions: initial MO concentration of 10 mg/L and adsorbent dosage of 0.1 g/L).

23

500

(A) 400

300

CTS

SC RI PT

Qe (mg/g)

CTS/ND-H(1)

Langmuir Isotherm 200

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Ce (mg/l)

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80

CTS/ND-H(1)

A

20

CTS Pseudo-second-order

0 0

20

40

60

80

100

120

Time (min) Fig. 8. Model fits of (A) Langmuir isotherm and (B) pseudo-second-order kinetics for the adsorption of MO on CTS and CTS/ND composites. (Conditions: the adsorbent dosage of 0.1 g/L, and pH of 5.5). Inset: optical

24

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images of dye solution samples from the adsorption by CTS/ND-H(1) in 15 min intervals from t = 0 (right) to t = 105 min (left).

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Removal (%)

80

KCl

60

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0 0

0.01

0.1

0.5

Salt Concentration (mol/L)

Fig. 9. Effect of ionic strength on adsorption of MO onto CTS/ND-H(1) (conditions: initial MO concentration of 10 mg/L, adsorbent dosage of 0.1 g/L, and pH=4). 25

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Cycle times

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Fig. 10. Recyclability of CTS/ND-H(1) for the adsorption of MO.

27

Table 1 Parameters of Langmuir and Freundlich isotherm models determined based on curve fitting of experimental data for both CTS and CTS/ND-H(1). Freundlich Isotherm

Langmuir Isotherm

CTS/ND-H(1) CTS

KL

Qm (mg/g)

R2

APE

Χ2

R2

APE

Χ2

0.393 0.185

454.5 166.7

0.9925 0.9902

0.2092 0.0247

0.1998 0.012

0.8590 0.8916

1.0776 1.6042

5.3033 5.0290

SC RI PT

Adsorbent

U

Table 2 Comparison of maximum adsorption capacities (Qm) for the adsorption of MO on various adsorbents reported in literature with the present work.

Qm (mg/g)

Reference

29.41 60.5-66.1 224.8 398.08

[41] [55] [24] [54]

34.29

[56]

34.83 180.2 7 66.90 416.7

[48] [57] [5] [58] [15]

Magnetic chitosan/maghemite composite

779

[40]

Porous chitosan graphene oxide aerogel CTS/ND-H CTS

686 454.5 166.7

[21] Present work Present work

A

ƴ-Fe2O3/SiO2/chitosan composite

M

ƴ-Fe2O3/chitosan composite films ƴ-Fe2O3/MWCNTs/chitosan Chitosan/bentonite composite Magnetic chitosan–graphene oxide composite

N

Adsorbent

CC

EP

TE

D

Chitosan Protonated cross-linked chitosan GLA cross-linked chitosan Chitosan/Carbon nanotube Poly HEMA–chitosan-MWCNT nanocomposite

A

Table 3. Parameters of pseudo-first-order and pseudo-second-order kinetic models determined based on curve fitting of experimental data for both CTS and CTS/ND-H(1).

Pseudo-second-order kinetic Adsorbent K2 CTS/ND-H(1) CTS

0.0046 0.0115

Qe (mg/g) 91.741 37.037

Pseudo-first-order kinetic

R2

APE

Χ2

R2

APE

Χ2

0.9981 0.9975

0.0074 0.0569

4.9e-05 0.0014

0.8158 0.7313

1.6273 1.2840

2.3844 0.7293

28

29

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CC

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