Cationization of Cellulose Nanofibers for the Removal of Sulfate Ions

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Cationization of cellulose nanofibers for the removal of sulfate ions from aqueous solutions Muhammad Muqeet, Hammad Malik, Rasool Bux Mahar, Farooq Ahmed, Zeeshan Khatri, and Krista Carlson Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 05 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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Industrial & Engineering Chemistry Research

Cationization of cellulose nanofibers for the removal of sulfate ions from aqueous solutions

Muhammad Muqeet a, Hammad Malik a, Rasool Bux Mahar a, *, Farooq Ahmed b, Zeeshan Khatri b, Krista Carlson c

a

US - Pakistan Center for Advanced Studies - Water, Mehran University of Engineering and

Technology, jamshoro - 76060, Pakistan b

Nanomaterials Research Lab, Textile Engineering Department, Mehran University of

Engineering and Technology, Jamshoro - 76060, Pakistan c

Department of Metallurgical Engineering, University of Utah, Salt Lake City - UT - 84112,

United States

* Corresponding author: Prof. Rasool Bux Mahar, Dr. Eng. Address: US-Pakistan Center for Advanced Studies in Water, Mehran University of Engineering and Technology, Jamshoro-76060, Pakistan. Email: [email protected] Tel: +92-22-2772250 Ext: 6508 ORCID ID: 0000-0002-1966-0397

ABSTRACT

ACS Paragon Plus Environment

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In this study, adsorption properties of cationized cellulose nanofibers (c-CNF) were examined for the removal of sulfate (SO42-) ions from aqueous solutions under diverse experimental conditions. Nanofiber mats were fabricated through electrospinning and cationized with 3chloro-2-hydroxypropyl tri-methyl ammonium chloride (CHTAC). The resultant c-CNF with 0.134 mmol/g ammonium content showed the maximum adsorption capacity of 24.5 mg of SO42per gram of sorbent using a Langmuir isotherm model. A pseudo-second order (PSO) kinetic model was fitted to the adsorption rate data, showing a faster adsorption rate of 0.0022 mg•g1

min-1. The SEM (scanning electron microscope) micrographs revealed the average fiber

diameter 280±10 nm with BET surface area of 5.04 m2/g analyzed through BET surface area and porosity analyzer. Fourier transform infrared (FTIR) spectroscopy confirmed the conversion of cellulose acetate (CA) to cellulose, and its subsequent cationization. Furthermore, the consequences of cationization were evaluated by Zeta potential and Thermogravimetric analysis (TGA).

Keywords

Cellulose nanofiber; adsorption; electrospinning; cationization; sulfate removal

1. Introduction

Sulfate contamination in surface water is a major concern in both residential and industrial water supplies. When, sulfate contaminated water is used for residential applications (i.e., drinking water), ingestion can lead to dehydration, gastrointestinal irritation, and cathartic effects, if


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concentration becomes greater than permissible limit i.e., 250 mg/L.1 Increasing sulfate level in water provides a strong indicator for the acidification of water bodies owing to water pollution that also causes health concern.42 The use of sulfate laden water in industrial applications can produce hard scale in heat exchangers, which ultimately decreases the heat transfer rate. Additionally, SO42- ions are corrosive in nature, which can lead to material deterioration. The presence of sulfate ions in water is primarily a result of the chemical weathering, wide industrial application of sulfuric acid and acidic deposition due to the use of coal, and subsequent oxidation of minerals which contain sulfur.2,43

Current methods and materials for the removal of sulfate ions from water tend to be either limited due to different reasons or costly. For example, biological and ion exchange treatments are costly, while chemical precipitation methods produce sludge, which requires secondary treatment.3 Membrane filtration (mostly reverse osmosis) is highly effective, but has high energy consumption.4 In contrast, adsorption with an effective, biodegradable and inexpensive sorbent is considered a feasible solution for ions removal from aqueous solutions.5

Zirconium loaded sorbents are considered an effective choice for sulfate removal, but there are issues due to the cost of zirconium metal.3 Cellulose has been reported extensively as an aqueous pollutant sorbent due to its low-cost, as well as sustainable and eco-friendly nature.6 Past sulfate adsorption studies have reported different materials such as coir pitch carbon,7 modified raw straw,3 sugar cane bagasse cellulose,8 surfactant-modified palygorskite,2 carbon residue from biomass gasification,9 cationic cellulose nanofibers from waste pulp residues,10 and PVC-zeolite


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nanoparticle-surfactant anion exchanger membrane.43 These materials had limitations either in terms of extended adsorption time for sulfate ions removal, preparation method or material cost.

The use of electrospun c-CNF provides benefits over other fabrication methods, as the process is easy, scalable and cost effective.11,12 On the other hand, the remarkable performance of c-CNF within the shorter possible equilibrium time (i.e., 75 min) may become meaningful in industrial applications as it saves operations time compared to other well competent adsorbents.13 Additionally, the fabrication process produces mats which intrinsically immobilizes the fibers, which enables them to be reused. Immobilization is a critical challenge to the implementation of nanostructured materials in practical implementations, as any sorbent media needs to be easily separated from aqueous solutions after use. An immobilized matrix also enables the potential for reuse, which is important from an environmental and economic perspective.14,15

Several methods have been reported to fabricate polymeric nanofibers, such as drawing,16 template synthesis,17 self-assembly,18 phase inversion,19 and electrospinning.20 Among these, electrospinning is one such technique emerging as reliable, scalable and simple, as it produces smooth and continuous nanofibers with controllable physical properties from a variety of polymeric solutions.20,21

This work aimed to introduce an efficient, cost effective and eco-friendly material for the removal of sulfate ions via adsorption technique. Kinetic and isotherm modeling of experimental sorption data as a function of time and initial concentration, respectively were performed to predict the adsorption mechanism and capacity. On the other hand, several instrumental


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techniques were employed to characterize the sorbent such as, SEM, BET, FTIR, TGA, and Zeta-potential.

2. Experimental

2.1. Materials and reagents

Cellulose acetate (CA, 39.8 wt. % acetyl content and 30 kDa average molecular wt.) was obtained from Sigma Aldrich, Japan. N, N-Dimethyl formamide (DMF) and acetone were used as the solvent. The cationizing agent, 3-chloro-2-hydroxypropyl tri-methyl ammonium chloride (69% concentrated by wt. having average molecular wt. 188.1 g/mol) obtained from Sigma Aldrich, Japan. C.I acid blue 117 dye (molecular wt. 594.5; λmax = 580 nm) was received from Archroma Pakistan Ltd. To adjust the pH, 0.1M HCl or NaOH were used for adsorption experiments.

2.2. Preparation of c-CNF sorbent

CA solution (17% by wt.) was prepared in 2:1 acetone/DMF and filled in the plastic syringe as reported in literature.21 Briefly, a high voltage power supply (HV35P OV, FNM Co. Ltd) was used under an electric field of 14 kV to overcome surface tension forces with a 10cm tip to collector distance. Discharged nanofibers from the syringe tip having the internal diameter of 0.6 mm were collected on grounded rotating metallic collector (RMC) wrapped with aluminum foil.


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The speed of RMC was controlled to around 12 rpm. The jet was elongated and dried before it touched the collector, where it formed ca. 50-70 µm thick well-connected mat of nanofibers.

The fabricated cellulose acetate nanofiber (CANF) web was deacetylated in 0.05M NaOH aqueous solution for 30 hrs to convert into cellulose nanofiber (CNF). The sample was air dried and completely washed with deionized (D.I) water to remove unfixed -OH- group.21-24 To prepare the c-CNF, the dried CNF web was again treated with 0.35M NaOH (first bath) and 1M CHTAC (second bath) via two bath pad-batch process.22,23 The complete reaction occurred in two steps: 1) CHTAC instantly converted to EPTAC (epoxy tri-methyl ammonium chloride) and 2) subsequently reacted with cellulose to form cationic cellulose.23 Both sequential baths were 100% wet-on-wet. After the second bath, sample left in the air tight pot at room temperature for 24 hrs owing to accomplishment of ionic crosslinking and afterward thoroughly washed with DI water. A graphical depiction of this preparation process is illustrated in Figure 1.

Figure 1. Schematic showing the route for the synthesis of cationic cellulose nanofiber sorbent


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2.3. Characterization

Scanning electron microscopy (SEM) was employed to examine the morphology of electrospun nanofibers. Samples were sputter coated (BAL-TEC AG, Liechtenstein) with 10 nm gold and imaged using a SEM (Helios Nanolab TM) with an accelerating voltage of 3 kV and a working distance of 4.2 mm. Specific surface area of the CNF and c-CNF mats were analyzed using BET surface area and porosity analyzer (micromeritics ASAP - 2020) with the pore size distribution by nitrogen (N2) adsorption-desorption isotherm. Both samples were degassed individually before analysis for 6 hours at 90 oC to evacuate the sample tube from moisture.

Success of reaction and chemical modification was determined using Fourier transform infrared (FTIR) spectrometer (Nilcolet iS50, Thermo Scientific) at ATR (attenuated total reflectance) mode in the range of wavenumber 4000 - 400 cm-1. Another qualitative test was also employed in this study in which conversion of CANF to CNF was examined through DMF solubility test as discussed in literature.25 DMF is a good solvent for cellulose acetate but cellulose cannot be dissolved in this solvent.

The thermal stability of CNF and c-CNF was studied to determine the changes in chemical and physical properties as a function of increasing temperature at constant heating rate of 10 oC/min, before and after isothermal condition (i.e., 105 oC for 20 min). The data were obtained using SDT-Q600 thermogravimetric analyzer (TA instruments) under nitrogen flow rate of 50 ml/min from 30 oC to 500 oC. The degree of cationization was determined through an analytical method


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in which acid blue 117 dye (molecular wt. = 594.5 g/mol) having sulfonate group was adsorbed using CNF and c-CNF at equilibrium conditions (shown in Figure S1, Supporting Information). The residual dye concentration was determined using UV/Vis/NIR (UV-3600, Shimadzu) spectrophotometer at λmax = 580 nm. Zeta potential of CNF and c-CNF was performed at different pH values using a Zetasizer NanoZS (Malvern, UK). For sample analysis, suspension of ca. 0.1% by wt. was prepared in D.I water.

2.4. Adsorption studies

A series of preliminary experiments were performed for isotherm and kinetic modeling to predict adsorption mechanism and understanding of adsorption behavior. In all experiments, 20 ml of aqueous solution were shaken (Lab-Line Industries, Inc.) at 200 rpm at ca. 23˚C. Adsorption kinetic experiments were carried out by following the batch adsorption method for various contact times (15, 30, 45, 60, 75, 90, 105, 120, 135 and 150 min). The sulfate solution was synthesized in D.I water with pH adjustments performed through the addition of either 0.1 M NaOH or HCl.26 Adsorption studies were also carried out as a function of pH (ca. 5, 6, 7, 8 and 9), and initial concentration of SO42- ions (25, 50, 100, 150, 200, 250, and 300 mg/L).

Percent sulfate removal was determined by analyzing initial and residual SO42- ions concentration in water using a UV/Vis/NIR (UV-3600, Shimadzu) spectrophotometer to measure the absorbance of the aqueous solution (filtrate) at a fixed wavelength (λmax = 420 nm) followed by Turbidimetric method,2 In the Turbidimetric method, residual sulfate ions are precipitated in an acetic acid medium with barium chloride (BaCl2) to produce barium sulfate (BaSO4)


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crystals.27 Light absorbance of the barium sulfate suspension was measured and the sulfate concentration is determined by comparison of the turbidity reading with a stable calibration curve having R2 = 0.999. On the other hand, chloride concentration was critically investigated using silver nitrate (AgNO3) titration method to understand the ion exchange phenomenon.28

3. Results and Discussion

3.1. Sorbent characterization Physical examination of all the fibers is important to investigate the effect of process parameters and external conditions on the morphology during electrospinning and chemical modification. Figure 2 shows the SEM micrographs of CANF, CNF, and finally prepared c-CNF (a-f, respectively) in which the nanofibers appeared continuous and exhibit bead free morphology. The diameter of the originally fabricated cellulose acetate nanofibers (CANF) ranged from 75 to 650 nm (Av. diameter 280±10nm) and no significant change was observed in the fiber appearance or in the diameter of fiber after deacetylation and cationization. But slight changes appeared after deacetylation in the form of fiber diameter and surface roughness. Fiber diameter decreased slightly and micrographs showed rough surface but, again retained their surface smoothness after cationization. BET surface area of CNF and c-CNF are given in Table 1. A decrease in surface area can be seen after cationization of CNF. One potential reason for this decrease might be as a result of CHTAC agglomeration owing to interparticle hydrophobic interactions,29 which could result in pore blockage and hinder the accessibility of nitrogen during testing.6 Evidence for this blockage is


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also shown in the reduction of the cumulative pore volume from 0.0230 cm3/g for the CNF to 0.0017 cm3/g for the c-CNF.

Table 1. BET surface area, pore volume, zeta potential results at neutral pH and ammonium content in c-CNF

Sorbent

BET surface area (m2/g)

Pore volume (cm3/g)

Zeta potential (mV)

Ammonium content (mmol/g)

CNF

15.20

0.0230

-26.4

-

c-CNF

5.04

0.0017

39.1

0.134


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Figure 2. SEM micrographs and histograms: (a & b) CANF, (c & d) CNF and (e & f) c-CNF.

The DMF solubility test qualitatively confirmed the conversion of CANF to CNF, as the CNF did not dissolve upon contact with the DMF, indicating complete removal of acetyl group. This


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conversion was also confirmed through the ATR-FTIR (Figure 3) spectra, which also showed the subsequent cationization of the CNF to c-CNF. Figure 3a, shows three intense peaks matching the stretching vibrations of C=O, C-CH3 and C-O-C at 1748 cm-1, 1370 cm-1, and 1235 cm-1, respectively.11 After deacetylation of the CANF into CNF (Figure 3b), the carbonyl absorption peak at 1748 cm-1 disappeared and the O-H stretching peak at 3400 cm-1 broadened.30,31 According to Ahmed et al., 2017, this broader O-H stretching peak confirms the complete conversion of cellulose acetate into cellulose. After functionalization, the c-CNF spectra (Figure 3c) shows a noticeable new peak at around 1450 cm-1, indicative of C-N which is attributed to the presence of the quaternary ammonium groups.21

Figure 3. FT-IR spectra of (a) CANF, (b) CNF, and (c) c-CNF


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The TGA thermograms (Figure 4a &b) presented the evidence that the CNF and c-CNF starts to be degraded above 200 oC. The first weight loss in both samples was around 3 to 5 % up to isothermal condition (i.e. 105 oC for 20 min) owing to a release of significant amount of moisture present in both CNF and c-CNF.32 The CNF lost ca. 92 % wt. in the range 220 oC to 480 oC owing to molecular disintegration into great number of volatiles and char. Thermogram for CNF in Figure 4a, represented the main degradation peak at ca. 345 oC. This degradation peak was mainly designated with cellulose. Thermogram in Figure 4b, on the other hand represented other degradation peaks at ca. 220 oC and 280 oC which may be associated to the structural heterogeneity of c-CNF.10 Thermal degradation analysis represented that c-CNF can be used in temperature up to 220 oC.

The ammonium content (0.134 mmol/g) in c-CNF was determined through an analytical method (S1, Supporting information: 1). Moreover, surface charge information was also determined by zeta potential analysis. In Figure 4c, shift of zeta potential from negative to positive can be seen. Negative zeta potential shows negatively charged surface and vice versa. The CNF presented the negative zeta potential in the whole analysis with values ranging from -26.4 to -10 mV and the charge values are significantly dependent on the pH of the CNF suspension. The values of CNF zeta potential at a wide range of pH are attributed to the negatively surface charged group (i.e., hydroxyl group). On the other hand, results revealed the positive zeta potential of c-CNF with values ranging from 31.9 to 39.3 mV, as expected due the presence of quaternary ammonium functional groups.33 These results agreed with the presence of positively charged ammonium group on the surface of c-CNF.


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45 Zeta potential, m V

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30 15 2

4

0

6

8

10

pH

-15 -30

CNF c-CNF

Figure 4. TGA thermogram with differential curve (a) CNF, (b) c-CNF; and (c) zeta potential of CNF and c-CNF at different pH values

3.2. Comparative sorption study


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Before selecting a sorbent, different factors must be considered such as, effectiveness, capital costs, design simplicity, flexibility and ease of separation after adsorption. Adsorption data collected on the c-CNF was compared with previously reported data (Table. 2), which included different sorbent materials such as rice straw,3 palygorskite,2 zirconium loaded sugarcane bagasse cellulose,8 carbon residue from biomass gasification,9 coir pitch carbon,7 and c-CNF obtained from pulp residue.10 The mat of entangled fibers is advantageous from a use and re-use standpoint because in practical implementations it is difficult to separate free-standing nanostructured materials from the aqueous solution.41 Although several authors have reported higher adsorption capacities with other materials and/or preparation methods compared to the electrospun nanofibers developed in this work, there are several drawbacks to these materials that may limit their use in commercial settings such as, extended adsorption time and material cost.

Table 2. Adsorption data for the c-CNF membrane. Additional data from other studies is also included for comparison.

Material

Adsorption capacity

pH

(mg/g)

Equilibrium

Reference

time (min)

c-CNF

24.5

6.8

75

Present study

Cellulose nanofibers

52.0 (modified)

7.7

Overnight (ca.

10

from waste pulp Carbon residue from

720) 7.59 (activated carbon)

2


1440

9

15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

biomass gasification

19.5 (modified C.R)

4

Rice Straw

11.68 (raw)

6.4

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120

3

360

2

74.76 (modified) Palygorskite

0.324 (natural) 0.324 (acid activated)

-

3.240 (surfactant

4

modified) Sugar cane bagasse

57.5 (zirconium loaded)

10

5

8

0.06 (absence of ZnCl2)

-

30

7

4.9 (ZnCl2 activated)

4

cellulose Coir pitch carbon

3.3. Binding mechanism of sulfate ions

The adsorption behavior and binding mechanism of sulfate ions on the sorbent used in this work was explored by comparing the adsorption of chloride and sulfate ions using CNF and c-CNF. Under the constant sulfate ion concentration (50mg/L) and time (75 min), it was seen that electrospun CNF showed 18% removal of sulfate and 11% of chloride ions. Similarly, c-CNF showed 75% sulfate and 32% chloride ions removal under the same equilibrium conditions as shown in Figure 5a. This difference in adsorption was due to the cationic charge strength of ammonium group on the c-CNF and prepared c-CNF membrane was found more selective towards divalent sulfate ions.10


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b

Figure 5. (a) Comparative adsorption removal for sulfate and chloride (b) Schematic map showing the electrostatic relation between negatively charged sulfate ions and positively charged ammonium groups grafted on the surface of cationic cellulose nanofibers.


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Keeping in view the disposal problem of used adsorbent and minimizing the environmental pollution due to waste disposal; it was realized to recycle the saturated c-CNF after the primary attempt of adsorption test. For this purpose, saturated c-CNF was regenerated using 0.35 M NaOH (for sulfate desorption) along with the addition of ca.1-2 mg fresh c-CNF to makeup the weight lost during adsorption and fixed weight of nanofiber (i.e. 10 mg), beside this, stimulating the used materials with new active sites. The experiment observations showed that the regenerated cellulose nanofiber was capable to remove 55% of sulfate ions present in aqueous solution at optimized conditions (pH: 6.8, initial concentration: 50 mg/L, Contact time: 75 min).

3.4. Adsorption kinetics

It was perceived from the kinetic data that, the adsorption efficiency was rapid in the first 15 min and afterward progressed at the comparatively slower rate due to the enough vacant active sites available at initial times on the surface of sorbent.44 At 75 min equilibration time, the surface of the sorbent became saturate and subsequently the removal efficiency became almost constant. This equilibrium time was selected for further adsorption studies.

PFO (Figure 6b) and/or PSO (Figure 6c) kinetic models describe the adsorption rate and the factors that influence them in the attainment of equilibrium in a reasonable amount of time.34-35. The equations (models) can be expressed as Eq. 1 (PFO) and Eq. 2 (PSO):

q − q = log q −

1

2.303


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1 = + 2

Where qt and qe represent the adsorption at time t and at equilibrium time, respectively, k1 and k2 represent the rate coefficient for the PFO and PSO kinetic models.44 Commonly, solid-liquid adsorption generally involves film diffusion and IPD.34 Neither the PFO nor PSO kinetic models describe this diffusion phenomenon. Therefore, the IPD model (Figure 6d) was constructed to define the process of the diffusion. According to this equation, the plot against qt verses t1/2 must be linear and pass through the origin to IPD to be the rate limiting step. In the model developed by Poots, Morris and Mckay, the initial rate of IPD is calculated by linearization of Eq. 3.

= . + 3

Where Ki represents the IPD rate constant and C shows the intercept. The value of C (intercept) gives an idea of the thickness of the boundary layer, i.e., the larger the value of C, the greater the boundary layer effect.36 The value of the IPD rate constant evidently exhibited that the boundary layer has a significant effect on the diffusion mechanism of sulfate ions uptake by the c-CNF.


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Figure 6. (a) Plot for a sulfate uptake kinetics by c-CNF (pH: ca. 7, mass of sorbent: 10mg: initial concentration: 50 mg/L, shaking time range: 15-150 min), (b) pseudo-first order (PFO), (c) pseudo-second order (PSO), and (d) intra-particle diffusion (IPD) kinetic model.

Results illustrated in Table 3 represent the values of constants obtained via kinetic modeling. Results concluded that adsorption of sulfate ions followed PSO kinetics as adsorption data, indicating the co-occurrence of both physisorption and chemisorption.37,38 Figure 6d showed that the plot (t1/2 verses qt) is not linear over the whole range of time. Data-points revealed the trilinearity of three successive adsorption stages of mass transport with a decline rate. This type of


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tri-linearity attributed to; 1) the outer surface adsorption associated to the boundary layer diffusion; 2) The IPD states where this step is highly involved in the rate control of this mechanism and; 3) the ultimate equilibrium stage where the IPD starts to slow down due to the low sulfate ion concentration in the aqueous solutions.38

Table 3.

Kinetic parameters of pseudo-first order, pseudo-second order and intra-particle

diffusion rate equations

Pseudo-second order

Pseudo-first order K1

qe

(min-1)

(mg/g)

0.0285

22.98

R2

0.673

K2

qe

(mg/g/min)

(mg/g)

0.0022

62.11

R2

Intra-particle diffusion Ki

R2

C

0.76

40.64

mg/g/min1/2 0.995

1.68

3.4. Adsorption Isotherm

As the concentration of sulfate ions present in natural water varies depending on location, it is imperative to investigate the adsorption capacity of sorbent at different concentrations. Figure 7(a-d, respectively) represents the adsorption of sulfate ions as a function of initial concentration. Figure 7a shows the reduction in adsorption efficiency of c-CNF with increase in sulfate ions initial concentrations. This reduction may be attributed to the finite number of active sites available on c-CNF that were under possession of adsorbed sulfate ions subsequently leading to a decrease in sulfate adsorption.


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Experimentally derived batch adsorption data was also correlated with two most famous adsorption isotherm models, Langmuir and Freundlich, in order to model the adsorption mechanism.5 The equations for Langmuir (Eq. 4) and Freundlich (Eq. 5) can be illustrated as:

=

!

+

1 4

! "

1 lnq = ln %

+ ln& 5

Where qm is the maximum adsorption capacity, Ce is the equilibrium solution phase concentration, KL is associated to adsorption free energy and specifies the sorbent-sorbate affinity. KF is adsorption capacity at unit concentration and the value 1/n from Freundlich isotherm provides a comparative dissemination of active sites; the smaller the value of 1/n, the greater the availability of heterogeneous active sites and that the adsorption mechanism would preferably be physical in nature.13 The Langmuir adsorption isotherm model assumes adsorption of monolayer with no adjacent interaction between the adsorbed molecules on an identical surface sites And the same mechanism is involved for all adsorption.7,45 This contrasts with the Freundlich model, which is an empirical equation and linear form of this isotherm-model. This is an empirical relation between the solute (sulfate ion) concentrations on sorbent (c-CNF) to the concentration of the residual solute concentration in solution after adsorption. The essential characteristic term of the Langmuir equation can be expressed by RL. this factor can be expressed as: (" = 1/1 + "

*)


(6)

22

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Where RL designates the type of isotherm, whether it is linear (RL=1), irreversible (RL=0), favorable (0

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