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Reusable green aerogels from crosslinked hairy nanocrystalline cellulose and modified chitosan for dye removal Han Yang, Amir Sheikhi, and Theo G.M. van de Ven Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03084 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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Reusable green aerogels from crosslinked hairy nanocrystalline cellulose and modified chitosan for dye removal Han Yang, Amir Sheikhi, Theo van de Ven* Department of Chemistry, Centre for Self-Assembled Chemical Structures, Pulp and Paper Research Centre, McGill University, Montreal, H3A 2A7, QC, Canada

*Email: [email protected] Tel: 514-398-6177 Fax: 514-398-8254

ABSTRACT A novel biopolymer-based aerogel was developed by freeze-drying a hydrogel, synthesized from crosslinking bifunctional hairy nanocrystalline cellulose and carboxymethylated chitosan through a Schiff base reaction. The nanocelluloses, bearing aldehyde and carboxylic acid groups, facilitated the crosslinking with chitosan through imine bond formation while providing negatively-charged functional groups, and chitosan was modified to accommodate carboxylic acid. The potential of this bio-aerogel in environmental remediation was examined in a model system comprising methylene blue, a cationic dye. Electrostatic complexation between acidic groups on the anionic aerogel with the dye resulted in time-dependent dye adsorption, with longtime equilibrium dye concentration fitting well to the Langmuir isotherm, yielding a maximum adsorption capacity of ~ 785 mg g-1 and equilibrium constant K ~ 0.0089 at room temperature. Dynamics of adsorption was modelled by numerically solving the unsteady state diffusionadsorption mass balance in a 1D spherical coordinate, which attested to a diffusion-controlled process with a Langmuir adsorption time constant τads ~ 7.6 s. To the best of our knowledge, this bio-aerogel exhibits the highest removal capacity as yet for any reusable adsorbents prepared from biopolymers. Successful adsorption-regeneration cycles proved an excellent reusability, and the adsorption capacity remained constant over a wide pH range (e.g., pH > 7). This work may pave the way towards ultralight green functional materials.

KEYWORDS: aerogel, hairy nanocrystalline cellulose, chitosan, dye removal, environmental remediation 1 ACS Paragon Plus Environment

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INTRODUCTION Synthetic dyes have complex structures, are difficult to eliminate from contaminated water, and have harmful impacts, including teratogenetic, carcinogenic, and mutagenic effects on human health.1 Dyes are widely used in many industries, such as textile, leather, paper, plastics, printing and cosmetics.2 The rapid development of global industrialization has further increased the dye pollution problems in water, which is the most important and necessary natural resource for human beings and other living creatures. Thus it is an important and challenging task to eliminate dyes in industrial effluents before they are discharged into the environment.3

Various methods have been used for removing dyes from industrial wastewater, including photocatalytic oxidation, electrochemical destruction, membrane filtration, and adsorption, among which adsorption has been recognized as an economic treatment method as a result of its easy operation and relatively low cost.4 Adsorption is a physicochemical process in which molecules are attached to the surface of an adsorbent by physical forces (e.g., van der Waals forces and/or an electrostatic attraction between oppositely charged adsorbate molecules and an adsorbent surface) or chemical reactions (e.g. covalent bonding).

To achieve an efficient adsorption, it is important to select a suitable adsorbent according to the charge or functional groups carried by target dyes. Besides efficient adsorption, reducing the toxicity of the adsorbent is also important to avoid secondary pollution. Thus, an environmentally friendly and efficient adsorbent is always highly desired. Accordingly, many biopolymer-based adsorbents have been developed from agricultural waste and plants, such as rice husk,5 jute fiber carbon,6 wheat bran,7 sunflower seed shells,8 sugarcane bagasse,9 chitin,10 chitosan 11 and modified cellulose.12

Among naturally available biopolymers, cellulose is the most abundant, environmentally friendly, renewable, and biodegradable material on earth, which suggest cellulose as one of the potential resources for fabricating green adsorbents.

Recently, nanomaterials, benefiting from high

specific surface area and active site density for interaction with target molecules, have secured an important place among adsorbents, owing to their high adsorption capacity. Nanocrystalline

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cellulose (NCC), also know as cellulose nanocrystals (CNC), is a major type of bionanomaterial prepared from cellulose.

NCC can be produced from wood pulp by sulfuric acid hydrolysis (introducing sulfate half-ester groups),13 ammonium persulfate treatment (introducing carboxyl groups),14 or periodate-chlorite oxidation (yielding electrosterically stabilized15 NCC (ENCC) with a high content of carboxyl groups).16 NCC has negatively charged functional groups, which can interact with cationic dyes by electrostatic attraction.17 The content of carboxyl groups on NCC can be increased through 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical mediated oxidation to improve the removal capacity of NCC.18 NCC can also be modified with primary amines for adsorption of anionic dyes.19 ENCC is a recent novel type of NCC,20 which has not yet been used in the study of dye removal; however, it has a significant capacity in sequestering of heavy metal ions, such as copper ion removal from aqueous media.21 However, at low ion concentration, NCC or ENCC adsorbents maintain colloidal stability and often suffer from separation and regeneration difficulties.

Chitosan is also a biodegradable and renewable biopolymer, derived from deacetylation of the biopolymer chitin, which is the third most abundant polysaccharide in the world (after cellulose and hemicellulose).22 Traditionally, when chitosan is directly used in water purification, it is mostly effective in adsorbing negatively charged dyes 23 and heavy anions 24 in acidic conditions as a result of protonated amine groups. In this work, we use chitosan as a green crosslinker, by taking advantage of its amine groups as functional moieties participating in the formation of covalent bonds.

Aerogels are highly porous solids made from wet gels in which the liquid phase in the gels has been replaced by a gas (usually air).25 Aerogels exhibit many unique properties, such as low density, high porosity, large surface area, ultralow thermal conductivity, ultralow refractive index, and ultralow dielectric constant.26 The most investigated aerogels are traditionally prepared from silica (such as silica dioxide) and several kinds of non-silica inorganic oxides (e.g., titanium, tin, and aluminum). Recently, aerogels prepared from natural polymers (e.g., starch, chitosan, and cellulose) have been proposed due to their renewability, biodegradability and 3 ACS Paragon Plus Environment

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biocompatibility.27 Cellulose derivatives, especially nanocellulose, including nanofibrillar cellulose (NFC)28-30 and nanocrystalline cellulose (NCC),31-32 show particular promise in the preparation of flexible and environmentally friendly aerogels.

In this work, we aim at preparing nanocrystalline cellulose with bifunctional moieties (carboxyl and aldehyde groups) by sequential periodate and partial chlorite oxidations followed by a hot water treatment. We refer to these NCCs as bifunctional hairy nanocrystalline cellulose or simply bifunctional NCC (BNCC). Chitosan (CT) was modified in advance to carboxymethylated CT (CMCT), increasing the carboxyl group content. The amine groups on chitosan and aldehyde groups on NCC are able to form covalent imine bonds through a Schiff base reaction, crosslinking the BNCC particles and yielding a hydrogel network. The adsorbent was readily obtained by freeze-drying the hydrogel to yield an all-natural aerogel (BNCC-CMCT). Characterization of BNCC-CMCT was done by solid carbon-13 NMR, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Methylene blue (MB), a cationic dye (structure shown in Figure 1, molar mass 319.5 g mol-1), is used as a model target molecule in this dye adsorption study. The adsorption isotherm and dynamics of MB removal by BNCCCMCT, the pH effect, and aerogel reusability were investigated in this work.

Figure 1. Molecular structure of methylene Blue (MB).

EXPERIMENTAL SECTION Materials Softwood kraft pulp sheets (provided by FPinnovations) were used as the starting cellulose material. Chemicals for reactions, including sodium (meta) periodate, ethylene glycol, hydroxylamine hydrochloride, sodium chlorite, chloroacetic acid, chitosan (from crab shell, brookfield viscosity > 200), and methylene blue hydrate were purchased form Sigma-Aldrich, 4 ACS Paragon Plus Environment

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and propanol was provided by Fisher, hydrogen chloride (0.1 M and 1 M), sodium hydroxide (10 mM, 0.1 M and 1M) from Fluka, anhydrous ethyl alcohol from commercial Alcohols, and sodium chloride from ACP chemicals Inc. All chemicals were used as received. Milli-Q water (18.2 MΩ cm, Millipore Milli-Q Purification System) was used in all experiments.

Preparation of bifunctional NCC (BNCC) One gram of softwood kraft pulp was soaked in water and well dispersed by a disintegrator (Noram Quality Control and Research Equipment Limited), and then filtered to remove extra water from the pulp. Next, 0.98 g NaIO4 and the wet pulp were added to 67 mL water, including the moisture from the wet pulp. The reaction beaker was wrapped with aluminum foil to block light. The pulp was stirred at room temperature for 96 hours, then 1 mL ethylene glycol was added to this mixture to stop the reaction by quenching the residual periodate. The dialdehyde modified cellulose (DAMC) was washed thoroughly with water by filtration. The aldehyde content of DAMC was determined by the hydroxylamine hydrochloride method previously reported.33 A visual demonstration of this procedure may be found elsewhere.34

BNCC was prepared by converting part of aldehyde groups on DAMC to carboxyl groups by chlorite oxidation, followed by a hot water treatment.35-36 Briefly, one gram never-dried DAMC and 0.54 g NaClO2, 2.9 g NaCl, and 0.54 g H2O2 were dispersed in 50 mL 0.5 M acetic buffer solution (pH=5), and the slurry was stirred for 24 hours. Subsequently, the oxidized pulp was rinsed with water, followed by four times washing with 70 % ethanol, and then dried in an oven at 50 °C. The bifunctionally modified cellulose fiber (0.2 g) and 20 mL water were added to a 50 mL flask, and the suspension was stirred at 80 °C in an oil bath for one hour and subsequently centrifuged at 8000 rpm for 10 minutes (Aventi J-E centrifuge from Beckman Coulter) to remove the unfibrillated fibers (a negligible amount). The supernatant (containing BNCC) was precipitated by adding propanol (the weight of propanol is 1.5 times of supernatant), collected, and storied at 4 °C for further usage.

Preparation of carboxymethylated chitosan For the carboxymethylation of chitosan, we follow a previous method, with some modifications.37 Sodium hydroxide (1.35 g) was first dissolved in a propanol/water mixture with 5 ACS Paragon Plus Environment

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a volume ratio of 8:2. One gram chitosan was added to this alkaline solution, which was stirred and allowed to swell at room temperature for one hour. Subsequently, 1.5 g chloroacetic acid was dissolved in 2 mL propanol and added to the chitosan slurry in five equal portions in a period of 30 min. The mixture continued to react for another four hours at room temperature. The reaction was stopped by adding 50 mL 70% ethanol and filtered through a nylon cloth. The white solid was washed with 80% ethanol for four times and then with anhydrous ethanol. Finally, the powder was dried in an oven at 50 °C to obtain carboxymethylated chitosan (CMCT).

Preparation of aerogel (BNCC-CMCT) A BNCC suspension (1% wt) and a CMCT solution (1% wt) were heated by stirring in an oil bath at 60 °C for one hour. Then, these two sets of samples were mixed by a homogenizer (Polytron PT 2500E) at 8000 rpm for 1 minute to form a hydrogel. The hydrogel was frozen at 80 °C for 12 hours and then freeze-dried by a freeze dryer (Thermo ModulyoD).

Dye adsorption The MB adsorption process was conducted in batch experiments. MB solutions with desired concentrations were prepared by successive dilution of a stock MB solution with water. To obtain the calibration curve of MB, the absorbance of MB solutions with predetermined concentrations at λmax=664 nm was detected by a UV-Vis spectrophotometer (Cary 5000 UVVis-NIR Spectrophotometer). This calibration curve was used to determine the concentration of MB solution after absorption in the ongoing adsorption experiments.

Equilibrium experiments Batch adsorption measurements were performed to obtain the maximum adsorption of MB by the BNCC-CMCT aerogel. Five millilitres of MB solutions with known initial concentrations and 1 mg of the adsorbent were placed in a 10 mL vial and agitated using a magnetic stirrer at 120 rpm for 24 h to ensure the adsorption process has reached equilibrium. The equilibrium concentration (Ce) of MB was then determined using an UV-Vis spectrometer at 664 nm. Equation 1 was used to calculate the adsorbed amount of dye per gram of adsorbents (mg g-1) at equilibrium.


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Γ =

− ,

(1)

where Γe (mg g-1) is the equilibrium amount of dye adsorbed by one gram of adsorbent, C0 (mg L-1) and Ce (mg L-1) are the initial and equilibrium concentrations of MB, V (L) is the volume of MB solution, and mads (g) is the adsorbent weight. The volume of aerogel expands about ten times.

Kinetic experiments For kinetic adsorption studies, one milligram of BNCC-CMCT adsorbent was mixed with 5 mL of MB solution (240 mg L-1) and stirred at 120 rpm. The experiments were repeated for various desired times. The bulk MB concentration (C) at various times was measured by a UV-Vis spectrophotometer. The adsorbed dye amounts were calculated from Equation 2. Γ=

− ,

(2)

where Γ (mg g-1) is the amount of dye adsorbed for one gram of adsorbent at time t, C0 (mg L-1) and C (mg L-1) are the initial concentration and concentration of MB at time t.

Effect of pH on adsorption Various MB solutions with initial pH ~ 2 - 12 were investigated to determine the influence of pH on the efficiency of MB removal by BNCC-CMCT. The pH of a MB solution was adjusted by adding 1 mol L-1 HCl or 1 mol L-1 NaOH solutions. The initial MB concentration is 100 mg L-1, and the experiments were performed for 4 hours with stirring at 120 rpm, then the concentration of MB was measured with a UV-Vis spectrophotometer. The removed amount of MB at each pH value was calculated using Equation 1.

Reusability of adsorbent The reusability of adsorbent was also investigated. One milligram of BNCC-CMCT was placed in a 5 mL MB (50 mg L-1) solution and stirred at 120 rpm for one hour, then the concentration of MB was measured by UV-Vis. The dye on BNCC-CMCT was desorbed by soaking the aerogel 7 ACS Paragon Plus Environment

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in 10 mL 0.1 M HCl and agitating at 80 rpm with a shaker for 10 minutes, followed by washing with 0.1 M NaOH, rinsing with water, and finally rinsing with ethanol and drying under air. The adsorption-desorption procedure was repeated six times. The MB removal at each step was calculated using Equation 1.

Characterizations Conductometric titration The carboxyl group content of BNCC and CMCT were determined with a Metrohm 836 Titrando instrument according to a previously reported method.38 A certain amount of sample (with a solid content of ~ 20 mg) and 2 mL NaCl solution (20 mmol L-1) were added to 140 mL milli-Q water, and 0.1 M HCl was added to adjust the pH to ~ 3. Then, the suspension was titrated by a 10 mM NaOH solution at a rate of 0.1 mL min-1 until pH ~ 11. The part of the titration curve which represents a weak acid provides the carboxyl content.

Solid-state carbon-13 NMR measurements Solid-state carbon-13 NMR spectra were acquired on a Varian VNMRS400 NMR spectrometer operating at 100.5 MHz. Cross polarization spectra were obtained with a contact time of 1.5 ms and a recycle delay of 2 s. The sample was spun in a 7.5 mm rotor at 5000 Hz, and spinning sidebands were suppressed by the TOSS sequence.39 Typically, 8000 scans were acquired.

Atomic force microscopy (AFM) imaging The morphology of BNCC particles was obtained by AFM (Nanoscope IIIa MultiMode with Extender (Veeco Metrology Group, Santa Barbara, CA)). A drop of BNCC suspension was placed on a freshly cleaved mica surface for ten minutes, followed by rinsing off the excess liquid. The experiments were conducted in tapping mode using silicon cantilevers (ACTA model, AppNano) with a nominal spring constant ~ 37 N m-1, nominal resonant frequency ~ 300 kHz, and nominal tip radius ~ 6 nm. Nanoscope Analysis 1.4 was used to process the AFM images.

Scanning electron microscopy (SEM) imaging The aerogel sample was mounted on a specimen pin by a double sided carbon tape and coated with a 3 nm thick layer of Pt by a high vacuum coater (Leica EM ACE600). The aerogel 8 ACS Paragon Plus Environment

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microstructure was observed by SEM imaging (FEI Inspect F-50 FE-SEM). The images were taken at an accelerating voltage of 10 kV.

Porosity measurement The porosity of BNCC-CMCT aerogel was determined by the ethanol displacement method.40 Ethanol is able to penetrate into the pores easily and is not expected to change the geometrical volume of the aerogel. A piece of BNCC-CMCT (w1) was immersed in 40 mL anhydrous ethanol (ρ = 0.789 g mL-1) and then placed in a desiccator under a reduced pressure for 8 min to remove air bubbles inside the aerogel. The aerogel was taken out and the ethanol on its surface was gently removed by a piece of filter paper. The aerogel was re-weighed (w2) immediately. The porosity ɛ was calculated by the following equation:

=

( − )/

(3)

where Vads is volume of the aerogel calculated from its geometrical dimensions. An average value was taken from three replicates.

RESULTS AND DISCUSSION Characterizations of BNCC and aerogel The morphology of BNCC by AFM is shown in Figure 2. BNCC is a rod-like nanoparticle, obtained from a hot water treatment of periodate and partially chlorite oxidized cellulose fibers. The major advantage of this process is that it does not require a strong acid hydrolysis and/or extensive mechanical treatment (which usually involves a high energy consumption or specifically designed instruments), and no additional post-purification is required after the formation of the nanocellulose particles. Furthermore, in this way, one is able to easily obtain nanocellulose particles with desired charge content by adjusting the chlorite oxidation level on aldehyde groups prior to a hot water treatment. As shown in Figure 2, BNCC from this work has a length of about 110 -150 nm, and width of about 8 nm. The yield of BNCC in this study is about 40%.


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BNCC particles have two types of functional groups, namely aldehyde and carboxyl groups besides the original hydroxyl groups. These two functional groups on one particle make BNCC potentially a more versatile nanocellulose than traditional NCC. The charge contents of BNCC and CMCT are ~ 3 mmol g-1 and ~ 3.4 mmol g-1, respectively, measured by conductometric titration.

Mixing a BNCC suspension with a CMCT solution, a transparent hydrogel is formed instantly, which remains stable even when the vial is upside down (Figure 3a). The content of carboxyl groups in the hydrogel is about 3.2 mmol g-1. Crosslinking has occurred between the amine groups on CMCT and the aldehyde groups on BNCC, by imine bond formation, as shown in Figure 4a. This crosslinking reaction occurs without adding any other chemicals, no hazardous by-products are formed in this reaction, and there is no requirement for any post-purification treatment. All these advantages make this crosslinking process an environmentally-friendly process.

The cartoon in Figure 4b shows the formation of the hydrogel network. BNCC works as the supporting nanomaterial, the flexible CMCT polymer chains connect BNCC nano-rods, forming a porous network. Furthermore, BNCC and CMCT not only function as a hydrogel construction material, but also provide negatively-charged carboxyl groups, which can increase the available sites for binding cationic dye molecules by electrostatic attraction, improving the capability of dye adsorption. (The characterization of chemical groups on BNCC and CMCT by solid-state C13 NMR is shown in Supporting Information). The hydrogel was freeze-dried to form an aerogel, which was able to easily stand on the tips of the fine awns of a green foxtail without bending them (Figure 3b). This biopolymer-based aerogel is highly porous and lightweight; the porosity of the aerogel determined by solvent exchange was 98.8 ± 0.3 % (since ethanol may not completely fill the pores, the porosity is likely underestimated). The microstructure of dry aerogel was investigated by SEM. The image shows that the aerogel has an open porous geometry with pore sizes in the range of 35 - 70 µm (Figure 3c), which are separated by “walls”, sheet-like and ultrathin structures revealed by the enlargement shown in the inset (Figure 3c). These large pores are beneficial for easy and quick mass penetration during adsorption.


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Figure 2. AFM image of BNCC nanoparticles.

Figure 3. (a) Photograph of a transparent BNCC-CMCT hydrogel in an upside down vial, (b) photograph of a piece of aerogel standing on the tips of the fine awns of a green foxtail, and (c) SEM image of the aerogel, with the inset showing an enlargement of the “walls” formed in the aerogel.


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Figure 4. (a) Schematic of crosslinking reaction between BNCC and CMCT and (b) cartoon for the hydrogel formation.

Adsorption isotherm An adsorption isotherm is an important tool for the description of how adsorbate molecules interact with an adsorbent surface. To investigate the relationship between the aerogel and MB molecules at equilibrium, and to obtain the maximum adsorption capacity of the aerogel, Langmuir and Freundlich isotherms were fitted to the equilibrium adsorption data. For Langmuir isotherms, it is assumed that each adsorbate molecule adsorbing onto the adsorbent surface has the same adsorption activation energy, thus the adsorption process results in a monolayer coverage over a homogeneous adsorbent surface (provided there is sufficient adsorbate). Also, no adsorbate migrates after adsorption, and the adsorption is reversible. The Langmuir isotherm can be expressed by the following equation:41-42

1 1 = , Γ Γ

with the equilibrium constant K given by =

,

(4)

(5)

where Γe is the adsorption capacity at equilibrium, Ce, the equilibrium concentration of MB in solution and Γm, the maximum adsorption capacity. τads and τdes represent the characteristic times 12 ACS Paragon Plus Environment

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of adsorption and desorption, which equal the reciprocals of their corresponding rate constants. Langmuir plots for adsorption of MB in zero salt and 0.1 M NaCl are shown in Figure 5. Γm and K were calculated from the intercept and slope of the linear fitting, respectively. Increasing the ionic strength by NaCl addition may result in dye aggregation and a consequent diffusion coefficient reduction,43 increasing τads and thus increasing K. Moreover, the salt-mediated aggregation of MB may result in colloid formation and deviation from MB ionic behavior, decreasing the maximum adsorption capacity. Also, elevated ionic strengths decrease the swelling ratio of the anionic aerogel, affecting the gel pore size.

Figure 5. Langmuir isotherm plots for MB adsorption by the BNCC-CMCT aerogel in solutions with zero salt or 0.1 M NaCl (pH ~ 7.5 and T ~ 22 °C).

Table 1. Isotherm parameters for MB adsorption in solutions with 0 M and 0.1 M NaCl at 22 °C. Langmuir model NaCl (M)

Γm

K

R2

Freundlich model KF

n

R2

(mg g-1) 13 ACS Paragon Plus Environment

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0

784.8

0.0089

0.986

493.7

9.1

0.796

0.1

272.2

0.046

0.990

130.9

5.2

0.917

A Freundlich isotherm, which can describe heterogeneous adsorption systems, and which is not restricted to the formation of monolayer coverage, can be represented as:44 /

Γ = ,

(6)

where Γe is the adsorption capacity at equilibrium, Ce is the equilibrium concentration of MB in solution, and KF and n are constants. The Freundlich adsorption isotherm fit to the equilibrium data in the absence or presence of NaCl is shown in Figure 6.

Figure 6. Experimental data and the Freundlich isotherm fit for MB adsorption by the BNCCCMCT aerogel in solutions with zero salt or 0.1 M NaCl at pH ~ 7.5 and T ~ 22 °C.

Compared with the Freundlich adsorption isotherm, the Langmuir adsorption isotherm can describe this adsorption process more accurately, since the Langmuir isotherm fit has a much higher correlation coefficient (Table 1). The negatively-charged carboxyl groups on the aerogel, are mainly responsible for binding MB to the aerogel through electrostatic attraction. The 14 ACS Paragon Plus Environment

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maximum adsorption capacity of BNCC-CMCT is 785 mg g-1, which is much higher than that of adsorbents made from other nature-based materials (listed in Table 2). NCC, modified by TEMPO-mediated oxidation, shows a capacity close to that of BNCC-CMCT,18 but this NCC adsorbent is difficult to recycle and regenerate. The adsorption of BNCC-CMCT is also comparable to commercial activated carbon (980.3 mg g-1),45 although its capacity is about 20% lower, but activated carbon is very costly to prepare and regenerate. The maximum adsorption (in the absence of salt) is about 86% of the amount (909 mg g-1) calculated from charge stoichiometry (the content of carboxyl groups is 3.2 mmol g-1 and the molar mass of a MB+ ion is 284 g mol-1).

The reason that Γm is less than the stoichiometric value can be attributed to the solution pH of 7.5, slightly lower than the second pKa of dicarboxylic acid groups (~ 8.0, obtained from a pH titration). Almost half of the aerogel negative charge originates form the C6-conjugated carboxylic acid on modified chitosan, which is all deprotonated at pH ~ 7.5. At this pH, BNCC provides two adjacent carboxylic acid groups on C2 and C3, one of which being almost fully deprotonated (pKa ~ 4.6) while for the other one (pKa ~ 8.0) 32% carboxylic acid groups are deprotonated. In total, BNCC-CMCT provide ~ 50% (CMCT) + 25% (fully dissociated carboxyl groups on BNCC) + 8% = 83 % of maximum possible COO-, a value close to the experimental maximum removal capacity. Note that the steric hindrance of an adsorbed MB molecule on one of the two adjacent carboxyl groups may be unfavorable for an otherwise approaching MB, decreasing the probability of one-to-one MB-COO- binding.

Table 2. Comparison of the maximum MB adsorption by various adsorbents. Adsorbent

pH

Γm (mg g-1)

Rice husk5

7

312.0

Sugarcane bagasse9

7

99.6

NCC17

7.5

101.2

6.5

769.0

Commercial activated carbon

7.4

980.3

Cellulose nanofibrils46

9

122.2

Cellulose nanofibrils aerogel47

--

3.70

NCC modified by TEMPO reaction18 45


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Banana pith carbon48

4

233.4

Chitosan/bentonite composite49

5.1

142.9

BNCC-CMCT aerogel (This work)

7.5

785

Theoretical considerations of adsorption dynamics Here, we theoretically consider the dynamic diffusion-adsorption of dye molecules from a wellstirred bulk fluid to the swollen aerogel, shown in Figure 7.

Figure 7. Schematic of a spherical aerogel, equally discretized to m points in the radial direction, simplifying the governing adsorption-diffusion partial differential equation (Eqn. 8) and corresponding boundary conditions to m ordinary differential equations (Eqns. 11-13), which were solved numerically. The aerogel porosity is denoted by ε.

The bulk dye concentration C at a desired time t is at a time-dependent equilibrium with the aerogel in a system comprising a spherical aerogel with swollen radius R. Upon introducing the aerogel to the solution, the trapped air inside the aerogel pores is nearly instantaneously replaced by the solution, providing a uniform dye concentration inside and outside the adsorbent. The solution is well stirred and the mass transfer resistance from the bulk to the outer aerogel surface (after a concentration gradient formation as a result of dye adsorption) is negligible. The dye diffusion inside the aerogel is considered to be similar to the bulk, because the pore size O(nanocellulose length, ~102 nm) is considerably larger than the dye molecule size (< 1 nm). It


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is also assumed that upon dye adsorption on the aerogel, no surface diffusion takes place. Accordingly, the system can be well described by a time-dependent partial differential equation (PDE) with parameters summarized in Table 3: 1 (1 − )Γ = ! #" $, " " "

(7)

where the left-hand-side term is the time change of bulk concentration and adsorbed dye, balanced by the right-hand-side 1D diffusion in a spherical geometry. Equation 7 can be extended as: 2 (1 − ) ) = !% ' − Γ , " " "

(8)

with *) = + (, − ))(1 − )) − + ), *

where,

)=

Γ − = , Γ Γ

(9)

(10)

At the beginning of the adsorption process (t = 0), the dye concertation C equals the initial bulk concentration C0 (C = C0, initial condition), and at any time (t > 0), the time change in the bulk dye concentration equals to the dye diffusion from aerogel surface to the center (boundary condition 1), and as a result of symmetry at the aerogel center, ∂C/∂r = 0 (boundary condition 2). The governing PDE (Eqn. 8) can be nondimensionalized and converted to a set of ordinary differential equations

50

using central finite differences for spatial derivatives (methods of lines,

MOL). The set of Eqns. 11-13 provide m ordinary differential equations with the initial condition C = 1 at t = 0, describing the time change of dye concentration at m radial positions from the aerogel surface (n = 1) to the center (n = m), which were solved numerically in Matlab.


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*-(,) -(, 1) − 2-(,) -(, − 1) 2 -(, 1) − -(, − 1) = (∆0) *. 1 − (, − 1)∆0 2∆0

(11)

1 (1 − ) − Γ + 23, − )(,)431 − )(,)4 − )(,)5,

! 2≤, ≤−1

with discretized boundary conditions:

and

*-(1) −7 -(1) − -(2) =

1 , *. Δ0 -() = -( − 1),

(12)

(13) 2

where dimensionless C = C/C0, r = r/R, and θ ranges from 0 to 1, A = 4πR , and t = t(D/R2). At the beginning of the process (t = 0), C = 1. Note that in this model, the aerogel tortuosity is considered ~ 1, because the nanoparticles, constructing the adsorbent have at least one large dimension (length L >> dye size). Figure 8 presents the time change in adsorbent coverage, fitting the experimental data by adjusting kads. If the mass transfer is not considered (dashed line), the apparent adsorption rate constant kads ~ 6.25×10-4 s-1, corresponding to an adsorption time constant τads ~ 26.7 min, an implausibly long time for the adsorption of the positively charged MB on the negatively charged functional groups. When the diffusion is taken into account, the best fit (solid line, Figure 8) furnishes kads ~ 0.1319 s-1, corresponding to an adsorption time constant τads ~ 7.6 s, attesting to a diffusion-controlled adsorption process. Note that K = kdes/kads ~ 0.0089 remains constant in all cases, because it is obtained from long-time equilibrium removal data (shown in Figure 5).


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Figure 8.

The fractional surface coverage of the BNCC-CMCT aerogel by MB versus

dimensionless time (t = tD/R2, symbols), and the best theoretical prediction by solving Eqns. 11-13 using kads ~ 0.1319 s-1 (solid line, R2 ~ 0.95). The dashed line presents a similar best fit when diffusion is neglected, which yields an implausibly small kads ~ 6.25×10-4 s-1 (R2 ~ 0.94). Note that the initial MB concentration is 240 mg L-1, corresponding to n0 = 1.53, D ~ 5 m2 s-1, R ~ 5.4 mm, pH ~ 7.5, and T ~ 22 °C.

Table 3. Physicochemical characteristics of the BNCC-CMCT aerogel-assisted dye removal process, used in solving the mathematical model. A = 4πR2

Swollen aerogel outer surface area (m2)

C

Bulk dye concentration (kg m-3)

C = C/C0

Dimensionless bulk dye concentration

C0

Initial bulk dye concentration (0.24 kg m-3)

D

Dye bulk diffusion coefficient (~ 5 m2 s-1)51


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kads

Adsorption rate constant (0.1319 s-1)

kdes

Desorption rate constant (1.17×10-3 s-1)

K = kdes/kads

Langmuir isotherm equilibrium constant (0.0089)

m

Number of discretized points in the radial direction (e.g.,

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400) mads

Adsorbent mass (~ 10-6 kg)

n0 = C0V/(mads Γm)

Initial dye mass per unit mass of adsorbent normalized with maximum removal capacity (kg kg-1)

r

Radial distance (m)

r = r/R

Normalized radial distance

R

Aerogel radius after swelling (0.0054 m)

∆r = R/m

Normalized length of discretized distances in the radial direction

t

Time (s)

t = t(D/R2)

Dimensionless time

V

Solution volume (5×10-6 m3)

ε

Aerogel porosity (0.988 m3 m-3)

θ = Γ/ Γm

Fractional coverage of aerogel (adsorbent) with dye (adsorbate), defined as the ratio of removal at time t to the maximum removal capacity

ρ

Aerogel density (15 kg m-3)

Γ = V(C0-C)/mads

Dye removal at time t (kg kg-1)

Γm

Maximum dye removal capacity of aerogel (0.785 kg kg-1)

Effects of pH Solution acidity influences the charge density of the adsorbent, which in turn affects the adsorption behavior of the adsorbent. Figure 9 shows the results for the adsorption of MB by BNCC-CMCT aerogel at various pHs. The plateau value increases with increasing pH. When the solution pH is increased, carboxyl groups are deprotonated, and the negative charge density of


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the aerogel increases, increasing the number of adsorption sites for MB+. Langmuir plots of MB adsorption at different pHs are shown in Figure 10, and the Langmuir isotherm parameters are listed in Table 4. At pH ~ 3, the maximum adsorption is about 192 mg g-1, which is about 25% of the maximum adsorption at pH ~ 7.5. The adsorption capacity is decreased at low pH as a result of the protonation of carboxyl acid groups. It was noticed that at pH ~ 2, almost no dye was adsorbed to the aerogel, indicating that MB may be desorbed at low pH (< 2) for regeneration and be reused more than once. At pH ≤ 2, besides the protonation of carboxyl groups, it is highly likely that some CMCT amine groups are not crosslinked with aldehyde groups on BNCC, providing positively-charged free amine groups,19 causing repulsion between the cationic dye molecules and the aerogel.

Figure 9. Adsorption isotherm dependency on pH for the BNCC-CMCT aerogel-MB system at T ~ 22 °C.


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Figure 10. Langmuir plots for MB adsorption by the BNCC-CMCT aerogel at various pH and T ~ 22 °C.

Table 4. Langmuir isotherm parameters for MB adsorption by the BNCC-CMCT aerogel at various pH and T ~ 22 °C. pH

Γm (mg g-1)

K

R2

3

192

0.0316

0.990

4

419

0.0147

0.995

7.5

785

0.0089

0.986

As discussed, the rate determining step is the diffusion of MB+ ions to the gel. To saturate the aerogel at high charge densities (high pH), more MB+ ions need to diffuse into the gel than at low pH. Hence, the apparent adsorption rate kads increases by increasing pH from 3 to 7.5. This explains why the slope in Figure 10 increases with decreasing pH, since the slope equals the equilibrium constant K which is inversely proportional to kads. At pH ≤ pKa the adsorption is close to charge stoichiometry. 22 ACS Paragon Plus Environment

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Desorption and regeneration The facile regeneration and decent reusability of an adsorbent can reduce the pollutant removal costs, providing economical remediation processes. The reusability of BNCC-CMCT aerogel was demonstrated in solutions with an initial MB concentration of 50 mg L-1, with each adsorption process taking one hour. The results are shown in Figure 11. The adsorption capacity after the first cycle decreased from 225 mg g-1 to 194 mg g-1, about 86.2 % of the initial capacity, remaining almost constant in the five subsequent cycles. The removed amount is still 188 mg g-1, about 83.5 % of the initial adsorption capacity after six cycles, which shows the aerogel has an excellent reusability. The decrease in performance may be due to part of pre-adsorbed MB being trapped inside the aerogel, reducing the total available negatively-charged carboxyl groups for subsequent adsorption cycles. Decreasing the dye solution pH with HCl protonates the carboxyl groups of the aerogel, forming –COOH and removing MB+ ions. However, MB+ remains adsorbed to the dissociated –COO- groups, which may take a much longer time to desorb from the gel than the time used for the desorption process (10 minutes). Even though it is possible to improve the desorption efficiency by increasing the desorption time, this may not be an economical process, since it is already possible to reach at least 80% of the adsorption capacity even after six cycles.


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Figure 11. Adsorption of MB in BNCC-CMCT aerogel over a few successive adsorptiondesorption cycles. The initial concentration of MB solution is 50 mg L-1 and T ~ 22 °C.

CONCLUSIONS A novel biopolymer-based BNCC-CMCT aerogel was prepared from bifunctional nanocellulose (BNCC) and carboxymethylated chitosan (CMCT) through a Schiff base reaction. BNCC can be obtained from sequential periodate and partially chlorite oxidation of cellulose, followed by a hot water treatment. This highly porous and negatively charged aerogel showed an excellent adsorption performance. At pH ~ 7.5, the maximum methylene blue dye adsorption capacity of the aerogel was 785 mg g-1, obtained by fitting the equilibrium data to the Langmuir isotherm, yielding the highest adsorption capacity for any reported reusable adsorbents prepared from biopolymers. The adsorption capacity of this aerogel can be further improved or tuned by tailoring the functional groups on BNCC / CMCT or by modifying the mass ratio of BNCC /


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CMCT. The aerogel can maintain its adsorption capacity in dye solutions over a wide pH range, be regenerated and successively reused for at least six cycles. A transient adsorption-diffusion model attested that the dye diffusion into the aerogel is the adsorption process rate limiting step. The BNCC-CMCT aerogel is a promising “green” adsorbent, because it is prepared from biodegradable starting materials through an environmental-friendly crosslinking process. This work may open new opportunities for the production of advanced bio-adsorbents.

SUPPORTING INFORMATION The chemical properties of BNCC and CMCT measured by solid-state C-13 NMR. The X-ray diffraction analysis and crystalline properties of BNCC.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors would like to thank the Natural Science and Engineering Research Council of Canada (NSERC), FPinnovations, and Industrial Research Chairs Grants for funding support.

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