Citric acid-incorporated cellulose nanofibrous mats as food materials-based biosorbent for removal of hexavalent chromium from aqueous solutions

Journal Pre-proof Citric acid-incorporated cellulose nanofibrous mats as food materials-based biosorbent for removal of hexavalent chromium from aqueous solutions

Die Zhang, Wei Xu, Jie Cai, Shui-Yuan Cheng, Wen-Ping Ding PII:

S0141-8130(19)38175-9

DOI:

https://doi.org/10.1016/j.ijbiomac.2020.01.199

Reference:

BIOMAC 14503

To appear in:

International Journal of Biological Macromolecules

Received date:

10 October 2019

Revised date:

16 January 2020

Accepted date:

20 January 2020

Please cite this article as: D. Zhang, W. Xu, J. Cai, et al., Citric acid-incorporated cellulose nanofibrous mats as food materials-based biosorbent for removal of hexavalent chromium from aqueous solutions, International Journal of Biological Macromolecules(2020), https://doi.org/10.1016/j.ijbiomac.2020.01.199

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© 2020 Published by Elsevier.

Journal Pre-proof (Title Page)

Citric acid-incorporated cellulose nanofibrous mats as food materials-based biosorbent for removal of hexavalent chromium from aqueous solutions

National R&D Center for Se-rich Agricultural Products Processing, Hubei Engineering Research

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Die Zhang a,b#, Wei Xu a,b#, Jie Cai a,b*, Shui-Yuan Cheng a, Wen-Ping Ding a,b

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Center for Deep Processing of Green Se-rich Agricultural Products, School of Food Science and

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Engineering, Wuhan Polytechnic University, Wuhan 430023, P. R. China Key Laboratory for Deep Processing of Major Grain and Oil, Ministry of Education, Hubei Key

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Laboratory for Processing and Transformation of Agricultural Products, School of Food Science and

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Engineering, Wuhan Polytechnic University, Wuhan 430023, P. R. China 

*Corresponding author. [email protected]; [email protected] (J. Cai)

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Postal address: School of Food Science and Engineering, Wuhan Polytechnic University, Wuhan 430023, P. R. China

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These authors contributed equally to this work.

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Journal Pre-proof Abstract：Nanoscale biomass materials derived from food materials (e.g., polysaccharide, protein, organic acids) have shown great promises with regard to the removal of heavy metal in wastewater treatment. Herein, we have developed the functionalized cellulose nanofibrous mats as an environment-friendly biosorbent via electrospinning of cellulose acetate solution, followed by deacetylation and citric acid modification. The morphology, chemical, and structural characterizations of the cellulose nanofibrous mats were examined by SEM, FTIR, XRD, DSC, and

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TGA to follow each stage of the preparation process of them. The effect of the incorporation of citric acid in the cellulose molecule on the adsorption performance of the naofibrous mats was then

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studied by batch adsorption experiments. Consequently, citric acid-modified cellulose nanofibrous

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mats with reasonably high absorption selectivity for Cr(VI) can be readily prepared. Results from

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this study may provide a promising food materials-based biosorbent that can be used as an

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emerging material in wastewater treatment application.

adsorption

1. Introduction

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Keywords: food materials; cellulose nanofibers; citric acid modification; electrospinning;

Given the rapid growth of modern industry and the boom in the population, aqueous environmental contamination due to toxic heavy metals is one of the most debatable and serious global problems [1,2]. As harmful contaminants, heavy metals are not biodegradable, threatening the health of human and ecological systems [3]. As one of the extremely toxic heavy metal pollutant in aquatic environments, hexavalent chromium (Cr(VI)) has recently received considerable attention due to its carcinogenic, teratogenic, and mutagenic risk to living organisms when 2

Journal Pre-proof discharged arbitrarily [4-6]. Thus, the removal of Cr(VI) from wastewater with high efficiency, simplicity, and low cost has generally been an important environmental issue worldwide. Various technologies have been examined in removing heavy metals from aqueous environments, such as ion exchange [7], filtration [8], photodegradation [9], membrane technology [10], and adsorption [4]. Adsorptive removal is one of the most popular physicochemical treatment methods, based on its feasibility, high efficiency, reversibility, low cost, and low secondary

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pollution [11,12]. The adsorption performance of absorbents can be improved by selecting good

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matrix material. Several types of materials, such as metal oxides [13], active carbons [14], magnetic nanoparticles [15], biomaterials [16-18], and composites [11,19] have been widely used to eliminate

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metal ions. Environmentally friendly biosorbents have attracted increasing research interest, based

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on their renewability, biodegradability, and biocompatibility [20,21]. Natural food-based polymers,

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such as cellulose, lignin, chitin, chitosan, protein, are used to develop the functional materials [22-27]. They are new, effective biosorbents that have generated attention with regard to the

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removal of toxic ions from aquatic effluent.

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The interest in cellulose as a food raw materials centers primarily on its high abundance in nature. Generally, free reactive hydroxyl groups (-OH) in cellulose can serve as coordination sites to bind with various metallic ions and be readily modified to improve their adsorption of metals [23]. However, in general, the adsorption capacity of crude cellulosic materials is not satisfactory. To enhance their performance, adsorbents has been extensively modified, such as by introducing functional groups. But these methods should consider selecting effective and inexpensive treatments without impacting the environment. Citric acid, one of the most versatile organic acids, is cost-effective and widely used in many industrial fields, especially food, cosmetics, and pharmaceuticals. Further, it can easily dehydrate under heated conditions and produce a reactive anhydride that can react with hydroxyl groups (-OH) 3

Journal Pre-proof to generate an ester. The net negative charge increases after carboxyl groups (-COOH) are introduced into citric acid. As a result, the as-prepared absorber has strong adsorptive ability, due to its greater binding potential for cationic contaminants. Citric acid has been used to functionalize hydroxyl-containing materials to improve their adsorptive capacity—for example, Yu et al. [28]reported that the citric acid-modified magnetite removes phosphorus efficiently. The efficiency of an adsorbent depends on its chemical structure and surface area. Nanosized

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materials have recently been developed to remove contaminants, based on their large surface area

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and high porosity, which create many adsorption sites and improves their adsorptive capacity. For example, Kahraman [29] prepared chitosan/clay nanobiocomposite by solvent casting method,

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demonstrating high adsorptive and regenerative capacity. Polymer that are generated into fibrous

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membranes from powders and flakes by electrospinning has attracted attention, because this

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approach is an economical, easy and effective means of fabricate nano- and submicro-sized polymer fibers. Thus, electrospun nanostructured cellulose-based fibrous mats with many adsorption sites

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has tremendous potential in eliminating heavy metal ions in wastewater treatment. Our group has

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reported that electrospun renewable cellulose acetate precursors and in situ hydrolysis constitute a feasible method of producing cellulosic nanofibers [4,30-33]. The aim of this work was to examine the adsorptive capacity of cellulose-based biosorbent that has undergone simultaneous chemical modification and nanostructure construction in removing Cr(VI) from aqueous solutions and the mechanism by which it does so. Citric acid-incorporated cellulose nanofibrous mats were prepared by electrospinning of cellulose acetate, followed by NaOH deacetylation and modification of citric acid. Batch experiments were performed, altering the initial Cr(VI) concentration, pH, temperature, contact time, and adsorbent dosage, to examine the adsorptive and removal capacity of the citric acid-functionalized cellulose nanofibrous mats. The morphology, chemical, and structural characterizations of the materials were analyzed by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction 4

Journal Pre-proof (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) to follow each stage of the preparation process of samples. Our results suggest that citric acid-modified cellulose nanofibrous mats are promising absorbents that can be applied to fields in which heavy metal pollution is a critical and urgent matter. 2. Experimental

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2.1. Chemicals Cellulose acetate (CA) (Mn = ~ 300, 000) was purchased from Sigma-Aldrich (USA). All other

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chemicals and solvents, such as acetone, N, N-dimethylacetamide (DMAc), sodium hydroxide

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(NaOH), citric acid, and ethanol, were analytic-grade and supplied by Sinopharm Chemical Reagent

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Co., Ltd (China). Deionized water was used in all experiments.

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2.2. Preparation of the nanofibrous mats

Various concentrations of CA solution (10~18%) in acetone/DMAc (2: 1, v: v) were prepared on

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a magnetic stirrer at room temperature until a uniform solution was formed. The prepared solution was poured into a 10-mL plastic syringe to generate nanofibers by electrospinning. The spinning parameters were as follows: voltage 20 kV, receiving distance 20 cm, and flow rate 1 mL/h. CA nanofibrous mats (CANFMs) were harvested on the grounded aluminum foil collector. The collected fibrous mats were dried at room temperature to remove residual solvent for 24 h, and CA in the nanofibers was regenerated by being immersed in a solution of NaOH in ethanol (0.1M) for 24 h to change into cellulose [31,34]. The as-prepared cellulose nanofibrous mats (CNMs) were washed thoroughly with deionized water for further modification and characterization after being vacuum-dried.

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Journal Pre-proof 2.3. Modification of the nanofibrous mats with citric acid The CNMs were modified with citric acid solution. The mats were first immersed in N, N-dimethylcarbinol solution (80 vol.%) and mixed continuously on a magnetic stirrer at room temperature for 24 h. Then, CNMs were rinsed with deionized water until it was colorless and dried in a vacuum at 80℃. After preliminary treatment, the mats were re-immersed in aqueous NaOH solution (0.1 M), gently stirred for 1 h filtered, washed with deionized water until pH 7, and

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vacuum-dried. Finally, the mats blended with citric acid solution (1 M), and sodium hypophosphite

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was added as a catalyst; this mixture was stirred for 2 h at 80℃ and rinsed with deionized water to pH ~7. After being filtered and dried in a vacuum at 80℃, citric acid-modified cellulose nanofibrous

2.4. Characterization of the materials

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mats (MCNMs) were obtained and used for adsorption and characterization.

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The microscopic appearance of the electrospun nanofibrous mats was analyzed by scanning

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electron microscopy (SEM, Hitachi S-3000N). The diameter of nanofibers was measured using Image J 1.43. The variation in vibration frequency of the functional group in the prepared sample was determined by Fourier transform infrared (FTIR) spectroscopy (NEXUS670). X-ray diffraction (XRD) patterns of the various samples were obtained on a Bruker D8 Advance X-ray diffractometer with CuKa radiation. Differential scanning calorimetry (DSC) was performed on a TA Q2000 at a heating rate of 10℃/min. Thermo-gravimetric analysis (TGA) was performed using a METTLER TOLEDO TGA/DSC/1100SF in N2 atmosphere from 30℃ to 800℃ at a heating rate of 10℃/min.

2.5. Evaluation of absorption performance All absorption experiments were conducted in conical flasks. The desired concentrations of 6

Journal Pre-proof Cr(VI) were prepared by dissolving potassium dichromate (K2Cr2O7) in deionized water on a magnetic stirrer at room temperature; the pH value was regulated with 0.1 M HCl and 0.1 M NaOH. The adsorption experiments were performed by immersing equal masses of MCNMs in aqueous chromium solution with continuous magnetic stirring. The parameters of adsorption experiments were as follows: initial ion concentration 20~60 mg/mL, contact time 10~180 min, pH of the solution 2~10, adsorption temperature 25~65 ℃, and adsorbent dosage 10~60 mg. The Cr(VI) content in the filtrate was measured on were a UV-visible spectrophotometer as described [35]. The

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concentration of free Cr(VI) ions in the aqueous solution was calculated by establishing a

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calibration curve between ion concentration (C) and absorbency (A). All adsorption results were

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corrected against blank tests, and all experiments were conducted in duplicate. The adsorptive

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C0 - Ct 100% C0

(1)

(2)

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

(C0 - Ct ) Vs M

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Qe 

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capacity and removal efficiency of MCNMs were evaluated per Eqs. (1) and (2), respectively.

where C0 and Ct (mg/L) are the initial concentration and equilibrium uptake value of Cr(VI), respectively; Qe (mg/g) and η (%) are the adsorptive capacity and removal efficiency, respectively; Vs (L) is the volume of the Cr(VI) solutions; and M (g) is the mass of the soaked adsorbents.

3. Results and discussions 3.1. Morphology of the nanofiber CA was dissolved in acetone/DMAc solution for electrospinning of nanofibers, the process of which is shown in Fig. 1a. The microscopic morphology of the preapared CA nanofibers (CANFs) 7

Journal Pre-proof are shown in Fig. 1b. While, it shows the nanofibrous mats (i.e. CANFMs) in the macrostructure (Fig. 1c). The micromorphology of electrospun nanofibers were influenced by electrospinning parameters, including voltage, tip-collector distance, flow rate, and the concentration and surface tension of the solution.

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CA powders

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2 μm

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c

CANFs.

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Figure 1. (a) Schematic of electrospinning of nanofiber mats, SEM images (b), and macroscopic photo (c) of

In this work, uniform CANFs were prepared by optimizing the concentration of the spinning solution under the same electrospinning parameters. Fig. 2 shows the SEM images of the electrospun CANFs with various CA concentrations (10, 14, 16 and 18%). During the process, cruder nanofibers were produced in 18% of CA solutions due to the higher viscosity of the solution, whereas the jet broke into droplets with solution that was too dilute (10% of CA concentration); thus, uniform nanofibers could not be prepared at a lower concentration of spinning solution. This phenomenon can be explained by the viscosity of the solution rising with increasing concentration—its surface tension and viscoelastic force must be balanced with the applied electrostatic force to continuously prepare good nanofibers [36]. A concentration of 16% CA was 8

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suitable for obtaining ultrafine fibers with a diameter of 407±120 nm.

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Figure 2. SEM images of CANFs at various concentrations of CA (a-10%, b-14%, c-16%, d-18%).

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3.2. Deacetylation of the nanofibers

The SEM micrograph of the CANFs after NaOH treatment was shown in Fig. 3a. Hereby, it obtained cellulose nanofibers (CNFs) for preparing CNFMs, which had an almost same morphology as the CANFs in CANFMs. In the analysis of functional groups before and after NaOH treatment by FTIR spectroscopy (Fig. 3b), stretching vibrations of C=O from ester groups (~1755 cm-1) disappeared from the main adsorption bands in the CNFs, and C-O from carboxylic groups (~1238, ~1051 and ~902 cm-1) was weakened. Further, bending vibrations of -CH3 deformation for groups of acetate substituent was not observed at ~1371 cm-1 [37,38] —the CA in nanofibers were completely deacetylated. The XRD patterns of the CANFs and CNFs are shown in Fig. 3c. Two considerable broad, amorphous peaks were seen in the XRD pattern of the CANFs from 5~15° and 9

Journal Pre-proof 15~25°, whereas after deacetylation, the CNFs had 3 crystalline peaks at 12°, 20° and 22°, —

corresponding to (101), (101) and (002) planes of the cellulose crystallites, respectively, which is the typical crystalline structure of cellulose II [39]. These results confirm that nanofibers have a higher crystallinity after sufficient deacetylation. DSC measurements also confirmed the full deacetylation of the CA (Fig. 3d). Compared with CNFs, the CANFs had a melting peak at approximately 227℃ , which attributes to their amorphous characteristics [40]. This result is

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consistent with the XRD patterns and validates that the CA in nanofibers converted into cellulose.

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The thermal decomposition of the nanofibers was examined by TGA (Fig. 3e). Compared with CANFs, the loss of CNFs weight starts at a lower temperature, which can be explained by the

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melting temperature of the regenerated cellulose decreasing with deacetylation [37].

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a 2 μm

c

b Intensity

CNFs

(101)

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902 1371 1051

1755 3500

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1238 1500

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Weight (%)

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CNFs

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CANFS

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2θ (°)

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d

Heat Flow (W/g)

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CNFs

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Wavenumbers (cm-1)

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CANFs

(101) (002)

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CANFs

60 40 20

CANFs CNFs

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Temperature (℃ )

Temperature (°C)

Figure 3. SEM images of (a) CNFs and FTIR spectra (b), XRD (c), DSC (d), and TGA (e) of CANFs and CNFs.

3.3. Modification of the nanofiber mats with citric acid A schematic of citric acid modifying cellulose nanofibers mats is shown in Fig. 4a. Untreated CNMs and MCNMs were studied by spectroscopy to determine whether the citric acid treatment was successful (Fig. 4c). There were two major changes in the FTIR spectra of the MCNMs, based: a notable increase in the absorbance of the carbonyl stretching vibration at ~1915 cm-1 and a 11

Journal Pre-proof weakening of the hydroxyl stretching peak at ~3479 cm-1, reflecting the esterification of citric acid. Thus, these changes might be beneficial for the adsorption of metal ions with the introduction of carboxyl groups [41]. The destruction of the nanofibrous structure in MCNMs even did not occur

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(Fig. 4b).

MCNMs..

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Figure 4. (a) Schematic of citric acid treatment, (b) SEM images of MCNMs, and (c) FTIR spectra of CNMs and

3.4. Adsorption of Cr(VI) by MCNMs

To determine the optimal conditions for Cr(VI) adsorption from aqueous solution, the initial ion concentrations, pH, temperature, contact time and the mass of adsorbent were analyzed. Fig. 5f shows the SEM image of MCNMs after adsorption. Notably, the morphology of the nanofibers was not affected throughout the entire adsorption.

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Journal Pre-proof 3.4.1. Effect of initial Cr(VI) ion concentration on adsorption The initial metal ion concentration significantly influenced the adsorption by the adsorbent. Fig. 5a shows the adsorption capacity Qe(mg/g) and removal efficiency η(%) of Cr(VI) as a function of initial Cr(VI) concentration from 30~70 mg/L at pH 2, contact time 120 min, temperature 45℃, and adsorbent dose 40 mg. Qe and η increased rising initial Cr(VI) concentrations from 30 mg/L to 50 mg/L. A further increase in metal ion concentration to 70 mg/L sharply reduced the adsorptive

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capacity and removal efficiency. This result can be explained by the finding that at low Cr(VI)

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concentrations, the active sites of the adsorbent surface is high relative to the metal ion in solution, which was beneficial to the adsorption process. However, at the higher concentrations, more ions

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had a competitive effect due to the limitation of binding sites, resulting in the decrease of the

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capacity of adsorption and the rate of removal [42]. Thus, the optimized initial Cr(VI) ion

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concentration for the removal of Cr(VI) was 50 mg/L.

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3.4.2. Effect of pH on adsorption

The pH of a solution is one of the most important parameters affecting adsorption. In many cases, the dominant chemical species changes as the pH varies. To determine the effect of pH on the removal of Cr(VI) , adsorption experiments were conducted in from pH 2~10 in 50 mg/L of solution using 40 mg adsorbent and a contact time of 120 min at 45℃. As shown in Fig. 5b, the adsorptive capacity and removal efficiency of the MCNMs decreased as the pH climbed, indicating that the adsorption of Cr(VI) is strongly pH-dependent. Cr(VI) species can exist in various forms as a function of pH, including H2CrO4, HCrO4-, CrO42-, and Cr2O72- , and the speciation of Cr(VI) is affected by the pH of a solution, per the following equations [43]:

HCrO4-  CrO4 2-  H  , pKa  5.9

(3) 13

Journal Pre-proof H 2CrO4  HCrO4-  H  , pKa  0.26

(4)

Cr2O7 2-  H 2O  2 HCrO4- , pKa  2.2

(5)

The adsorption value peaks at pH 2, where Cr(VI) might exist as HCrO4- or Cr2O72-. These anion species tend to combine with the protonated active sites of the cellulose in solution at the optimal pH[44]. When pH > 6.5, Cr(VI) exists as CrO42-, and with increasing pH value, the surface

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becomes negatively charged and the adsorptive capacity declines due to electrostatic repulsion.

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Concurrently, chromium metal precipitates at pH > 6.0. Thus, further experiments were conducted

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3.4.3. Effect of temperature on adsorption

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in acidic solution at pH 2.

Temperature (25~65℃) affects the adsorption of Cr(VI), as depicted in Fig. 5c (initial

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concentration 50 mg/L, pH 2, contact time 120 min, adsorbent dose 40 mg). The adsorptive

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capacity and removal efficiency of the MCNMs increased as the temperature rose from 25℃ to 65℃, and further increases in adsorption temperature resulted in nearly constant uptake. Within a certain range, the rise in temperature favors adsorption, due primarily to the finding that adsorption is an endothermic reaction [45]. Nevertheless, the heavy metal on adsorbents will be released because of the poor thermal stability of MCNMs. Thus, 45°C was selected as the ideal temperature for subsequent experiments in capturing Cr(VI) ions.

3.4.4. Effect of contact time on adsorption The effect of contact time (10~180 min) on adsorption was determined at pH 2, 45℃, and 120 min of equilibrium (Fig. 5d). The adsorption improved with higher contact times and reached an 14

Journal Pre-proof equilibrium within 120 min. Specifically, most of the active sites bound to metal ions and reached the rapid adsorption in the first 60 min, after which the adsorption rate decreased and eventually remained constant after 120 min, indicating that the adsorption reached a saturation point. This phenomenon can be explained by the relationship between the metal ion concentration in solution and functional group sites on the surface of the adsorbent: the many adsorptive sites that were available at the beginning resulted in a faster initial rate of adsorption, and then, a slower absorption rate was observed at the end due to the saturation of active sites of the biosorbent, rendering the

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adsorption rate nearly unchanged after a long period of contact [46]. To ensure the equilibratory

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adsorption of metal ions on the MCNMs, we chose a contact time of 120 min throughout the

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3.4.5. Effect of adsorbent dose on adsorption

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adsorption.

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Under the optimized conditions—initial Cr(VI) ion concentration 50 mg/L, pH 2, temperature 45℃, and contact time 120 min—adsorption was examined at various doses of adsorbent, from 20

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mg to 80 mg. Fig. 5e shows the effect of adsorbent dose on the adsorptive capacity and removal of Cr(VI). As the absorbent dose rose from 20 mg to 40 mg, Qe and η increased linearly. The climb in sorption ratio with sorbent dose can be attributed to the greater amount of adsorbent, with its high surface area and abundant active sites [47], while the decrease in the amount of adsorption can be attributed to the aggregation and overlap of the adsorption sites, resulting in a reduction in the total adsorbent area with the further increase of sorbent dose [48]. Thus, the adsorption peaks with 40 mg of MCNMs.

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Figure 5. Effect of (a) initial Cr(VI) concentration, (b) solution pH, (c) contact time, (d) temperature, and (e)

4. Conclusions

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adsorbent dose on the adsorption of Cr(VI) ions onto MCNMs and (f) SEM image of MCNMs after adsorption.

Nanoscale biomass materials derived from food materials (e.g., polysaccharide, protein, organic acids) is recently expected to be used as an agent for the removal of heavy metal ions in wastewater. In this work, citric acid-functionalized cellulose nanofibrous mats are prepared by the electrospinning of cellulose acetate, followed by deacetylation and citric acid modification. SEM, FTIR, XRD, DSC, and TGA were used to characterize the preparation and functionalization of the nanofibrous mats. Further, the adsorptive capacity and removal efficiency of the nanofibrous mats for the removal of Cr(VI) depended on the initial metal concentration, pH, contact time, temperature, and amount of absorbent are systematically studied. Adsorption experiments indicated that the functionalized nanofibrous mats exhibited a reasonably high adsorption performance for the 16

Journal Pre-proof removal of Cr(VI) from aqueous solutions. This work provides a novel method for fabricating low-cost, good-efficiency biosorbents derived from food materials for heavy metal ion removal from wastewater.

Conflict of interest statement

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The authors declare that they have no conflict of interests.

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Acknowledgments

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Dr. Jie Cai thanks the Young Elite Scientists Sponsorship Program by CAST (2018QNRC001)

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and the Chutian Scholar Program of Hubei Provincial Government, China. This work was also partially funded by the Hubei Key Laboratory for Processing and Transformation of Agricultural

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Products (WHPU, China) (No.2018HBSQGDKFB01), the Talent Introduction Foundation of WHPU (No. 2016RZ22), the Research and Innovation Initiatives of WHPU (No. 2018J01), and the

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Province Key Laboratory of Cereal Resource Transformation and Utilization, Henan University of Technology (No. PL2018001).

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Graphical abstract

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Author statement

Die Zhang and Wei Xu performed the experiments and data analyses. Shui-Yuan Cheng and Wen-Ping Ding supported the experiment. Die Zhang, Wei Xu, and Jie Cai, wrote of the manuscript.

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Jie Cai conceived the overall project and finalized the manuscript.

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