Recyclable fully biobased chitosan adsorbents spray-dried in one-pot

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Recyclable fully biobased chitosan adsorbents spray-dried in one-pot to microscopic size and enhanced adsorption capacity Zhaoxuan Feng, Takahiro Danjo, Karin Odelius, Minna Hakkarainen, Tadahisa Iwata, and Ann-Christine Albertsson Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00186 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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Recyclable fully biobased chitosan adsorbents spray-dried in one-pot to microscopic size and enhanced adsorption capacity

Zhaoxuan Feng,1 Takahiro Danjo,1,2 Karin Odelius,1 Minna Hakkarainen,1 Tadahisa Iwata,2 Ann-Christine Albertsson1*

1

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology,

Teknikringen 58, 10044 Stockholm, Sweden

2

Department of Biomaterial Sciences, Graduate School of Agricultural and Life

Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

*Corresponding Author. E-mail: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT

A facile one-pot spray-drying process was developed for fabrication and in-situ crosslinking of chitosan microspheres to improve the adsorption capacity by microscopic design. A fully biobased nature was achieved by utilizing genipin (GP) as a crosslinking agent and chitosan derived nano-graphene oxide (nGO) as a property tuner. The produced chitosan microspheres were further proven as powerful adsorbents for common wastewater contaminants such as anionic dyes and pharmaceutical contaminants, here modelled by methyl orange (MO) and diclofenac sodium (DCF). By regulating the amount of GP and nGO, as well as by controlling the process parameters including the spraydrying inlet temperature and post-heat treatment, the surface morphology, size, zeta potential and adsorption efficiency of the microspheres could be tuned accordingly. The adsorption efficiency for MO and DCF reached 98.9 and 100 %, respectively. The microspheres retained high DCF adsorption efficiency after six adsorption and desorption cycles and the recyclability was improved by incorporated nGO. The fabricated microspheres, thus, have great potential as reusable and eco-friendly adsorbents.



KEYWORDS

In-situ crosslinking, microspheres, chitosan, spray drying, genipin, biobased, recyclable


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INTRODUCTION

Bio-adsorbents are emerging as promising alternatives to conventional approaches for removal of contaminants from water. Chitosan (CS), a natural polycationic linear polysaccharide, has abundant primary amines and hydroxyl groups endowing numerous binding sites to entrap heavy metals and organic pollutants1 through chelating and electrostatic effects,2,3 As an example, CS-based adsorbents have been developed for the elimination of dyes,4 pharmaceutical wastes5 and heavy metals.6 However, a large amount of binding sites cannot be reached by the contaminants as they are confined by the strong inter-and/or intra-molecular interactions existing in the natural CS. Also, the specific surface area of CS flakes is low, which leads to a low adsorption capacity and slow adsorption kinetics.7 Hence, further improvement of the adsorption performance of CS or CS-based adsorbents is still an issue. Another important issue with hydrogels for water purification is the long-term stability, which can be improved by additional physical or chemical crosslinking. Different cross-linking agents such as glutaraldehyde, formaldehyde, and tripolyphosphate have been used;8–10 however, the aforementioned



cross-linking agents are synthetic and impart certain toxicity,11 which may bring potential hazards to the aqueous system. Preferably the introduced crosslinkers should be as environmentally benign as chitosan. An interesting approach to physical crosslinking was achieved by grafting polyester chains consisting of lactic acid and glycolic acid from the amino groups of chitosan.12 The physical crosslinking was formed through aggregation and secondary interactions between the grafted hydrophobic side chains.

Size reduction of adsorbents to the micro-scale is anticipated to facilitate the adsorption process, due to the creation of a larger specific surface area and more functionalities being exposed to the contaminants. Of numerous techniques to prepare CS microspheres, spray drying stands out as a rapid, facile, and cost-effective single-step process which can directly convert chemical solutions into dry microspheres through the atomization with a hot gas medium.13–16 So far, various composite microparticles consisting of CS and other matrixes e.g. poly(3-hydroxybutyrate), poly(D,L-lactic-co-glycolic acid), and lactose have been spray-dried mainly for biomedical applications.17,18 For applications in the environmental domain, spray-dried CS microspheres demonstrated effective adsorption


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affinities towards ciprofloxacin, Cd (II), Zn (II), and Cu (II).19–21 However, a multi-step preparation approach has normally been applied by first spray drying the CS-based microspheres, then suspending the obtained microspheres into a solution containing a cross-linking agent for several hours, and finally washing and drying the products, thus making the approach tedious and energy-intensive.

Processes for one-pot in-situ fabrication of cross-linked CS-based microspheres with a green and environmentally friendly crosslinking agent and modifications that in combination with size reduction would enhance the adsorption capacity and recyclability of the systems would be highly desirable. Previously we synthesized biobased nGOs by a microwave-assisted carbonization process22–24 and utilized it as property enhancer for bioplastics23 and biomedical materials.25–27 The incorporation of biobased nGO into CS hydrogels was proven to enhance the drug adsorption efficiency of the hydrogels as well as to improve the hydrogel network stability.28 Spray drying solutions of CS, genipin (GP) as a natural, low-cytotoxic and biocompatible crosslinking agent,29,30 and CS derived nGO could offer a green one-pot process for production of in-situ crosslinked fully biobased



CS/nGO composite microspheres with increased adsorption capacity due to the size reduction. The reusability of the microspheres could be improved by the stabilizing effect of nGO observed in earlier studies,24,27 which could further provide cogent promise for applying this environmentally-benign adsorbent in the wastewater purification.

EXPERIMENTAL Materials. Chitosan (CS) of low molecular weight (Degree of deacetylation (DDA) ca. 88 %, Mv 360 000 g/mol, and molecular weight distribution 2.9) and chitosan of medium molecular weight (DDA ca. 77 %, Mv 444 500 g/mol, and molecular weight distribution 4.2) were purchased from Fluka Biochemika and Sigma-Aldrich, respectively. Methyl orange (MO, dye content 85 %), diclofenac sodium (DCF, 99 %), trifluoroacetic acid-d (TFA-d, ≥99.5 atom % D), nitric acid (HNO3, 70 %), sulfuric acid (H2SO4, 95-98 %), hydrochloric acid (HCl, 37 %) and sodium hydroxide (NaOH, 95–98 %) were also purchased from Sigma-Aldrich. Genipin (GP ≥ 98 %) was purchased from Linchuan Zhixin


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Bio-Tech. Acetic acid (≥ 99.5 % purity) was obtained from Acros Organics. Methanol (≥ 99 %) and ethanol (≥ 96 %) were provided by VWR Chemicals. All chemicals were used as received.

Synthesis of nGO from Chitosan. Chitosan-based nGO was synthesized according to previously described procedure.30 Briefly, six TFM (Teflon Fluor Modified) vessels were prepared with the mixture of 2 g of chitosan (medium molecular weight) and 20 mL of sulfuric acid solution (0.1 g/mL) in each. The hydrothermal carbonization of chitosan was performed in the Milestone flexiWAVE microwave irradiation chamber with maximum power of 1200 W. The irradiation temperature was increased to 220 °C with a RAMP time of 20 min and then kept isothermally at 220 °C for 2 h. The obtained black solid carbon nanospheres (CN) were filtered and washed with deionized water, and dried in vacuum oven for 3 days. To produce nGO, 2 g of CN was dispersed in 200 mL of nitric acid with sonication for 30 min, and then heated in an oil bath at 90 °C for 1 h. After oxidation, water was added to stop the reaction, followed by evaporation of the solvent under vacuum



distillation and freeze-drying process. The purified nGO was dark brown in color. Detailed characterization is presented in previous study.31

Fabrication of Chitosan-based Composite Microspheres by Spray-drying. The composite microspheres were fabricated by one-pot approach. Chitosan (0.5 g, low molecular weight) and a predetermined amount of nGO (see Table 1) were dissolved in 100 mL of acetic acid aqueous solution (2.5 vol %) and then filtered through a Buchner funnel to remove the insoluble particles. A predetermined amount of genipin powder (see Table 1) was dissolved in 60 vol % of ethanol (1 mL) and mixed homogenously with the aforesaid CS/nGO solution before use. The prepared solution was sprayed through a 0.7 mm fluid nozzle in a Mini Spray Dryer B-290 (Büchi, Labortechnik AG, Switzerland). The inlet temperature was set at 100 or 200 °C, respectively, with a solution feeding rate of 6 mL/min. By means of an air flow rate of 600 Nl/h with an aspiration of 90 %, the sprayed droplets were transported into a cyclone chamber and eventually precipitated into particles in a collecting vial. The spray-dried products were retrieved from the collecting vial and stored in a desiccator at room temperature. Post-heat treatment was performed


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on selected samples by dispersing 100 mg of the sprayed microspheres in 100 ml of methanol solution (50 vol %) with continuous stirring at a temperature of 37 °C for 48 h. Samples were collected via centrifugation and dried in the vacuum oven for two days. All the prepared samples were subjected to the same spray drying process but with variations in the composition, inlet temperature or post-heat treatment. The different samples and their compositions and process variables are listed in Table 1.

Table 1. The chitosan-based composite microspheres fabricated by spray drying. Name

Genipin

nGO

Inlet

Post-heat

(wt %)

(wt %)

temperature (°C)

treatment

GP5/nGO0_100 °C

5

0

100

No

GP5/nGO2_100 °C

5

2

100

No

GP5/nGO10_100 °C

5

10

100

No

GP5/nGO10_200 °C

5

10

200

No

GP10/nGO0_100 °C

10

0

100

No

GP10/nGO10_100 °C

10

10

100

No

P-GP10/nGO0_100 °C

10

0

100

Yes

P-GP10/nGO10_100 °C

10

10

100

Yes

Note: The concentration of chitosan for all the samples was 5 mg/mL and the volume 100 mL. The wt % of genipin and nGO were obtained using chitosan weight (0.5 g) as reference.



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Batch Adsorption Experiments. Diclofenac sodium (DCF), an anti-inflammatory drug, and methyl orange (MO), an anionic dye,

were selected as model compounds for the

adsorption study representing common wastewater contaminants. A preliminary adsorption test was conducted by building up a syringe column packed with 50 mg of GP10/nGO10_100°C as adsorbent. A solution of MO was then pushed through the syringe. A systematic study measuring the adsorption efficiencies (%) at equilibrium for all the samples was also performed. Stock solutions were prepared by dissolving 100 mg of the model compounds in the Milli Q-water, and then by further diluting to the required concentrations before use. Batch adsorption experiments were performed by adding 2 mg of microspheres into 4 ml of drug or dye solution and the mixture was shaken constantly at a speed of 400 rpm in the room temperature for a predetermined period of time. After the preset adsorption time, sample solutions were collected after filtering out the solid microspheres by PTFE syringe filter (0.45 μm).

The concentration of the

remaining DCF or MO in the filtered solutions was determined by the absorbance peaks of the UV-Vis spectra at 276 or 465 nm, respectively. The adsorption efficiency (%),


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adsorption capacity at certain time, qt (mg/g) and at equilibrium, qe (mg/g) were defined according to the Eqs. (1) – (3):

Adsorption efficiency(%)=100 (C0- Ct)/C0

(1)

qt=(C0- Ct)V/m

(2)

qe= (C0-Ce)V/m (3)

where C0, Ct, Ce are the pollutant concentrations (mg/L) initially, after a certain time and at equilibrium, respectively. V is the volume (L) of the solution and m is the weight (g) of the dried microspheres. Moreover, DCF adsorption kinetics and isothermal study were carried out at room temperature.

Desorption and reusability. To evaluate the potential for regeneration and reusability of the chitosan-based composite microspheres as adsorbents for DCF, six consecutive cycles of desorption and adsorption were performed. For each cycle, 100 mL of DCF solution (0.01 mg/mL) was employed for the adsorption experiment. After the adsorption



equilibrium was reached, the adsorbent was separated from the liquid by centrifugation and soaked into 20 mL of NaOH solution (0.1 M) for 10 min for desorption, followed by washing with 20 mL of HCl (0.1 M) and 20 mL of deionized water to neutralize. The eluted samples were then dried in the vacuum oven overnight before subsequent experiment.

Characterization. The surface morphology and size distribution of chitosan-based composite microspheres were observed by using an Ultra-High Resolution FE-SEM (Hitachi S-4800). Prior to the SEM observation, samples were sputter-coated with 4 nmthick gold layers.

To verify the molecular structure of the microspheres after spray-drying and to investigate the possible changes in crosslinking intensity after post-heat treatment, FTIR spectra of the prepared samples were recorded by using a PerkinElmer Spotlight 400 FTIR spectrometer equipped with an attenuated total reflectance (ATR) crystal accessory. Each spectrum was recorded as the average of 16 scans within a range between 4000 cm−1 and 600 cm−1. The resolution was set to be 4 cm−1.


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The zeta potentials of the microspheres were determined by a Nano Zetasizer with a 633 nm He−Ne laser from Malvern Instrument U.K., Inc. Samples were prepared by dispersing 1 mg of microspheres in 10 ml of milli-Q water with mild sonication for 10 min, and the supernatant solutions were used for the measurements. Each sample was scanned three times at room temperature and the average zeta potential value was reported.

UV-Vis spectrophotometer (UV-2410) with a wavelength range from 200 to 700 nm was applied to quantitatively examine the adsorption efficiency of microspheres towards DCF and MO. Moreover, the surface elementary information of the microspheres after MO adsorption was obtained by using Energy Dispersive Spectroscopy (EDS) with the help of an 80 mm2 X-Max Large Area Silicon Drift Detector sensor (Oxford Instrument Nanotechnology).



RESULTS AND DISCUSSION

Chitosan-based composite microspheres were fabricated by an one-pot spray-drying approach, allowing simultaneous incorporation of property enhancing fillers and in-situ crosslinking by genipin, as illustrated in Figure 1. The structure and properties of the microspheres and especially their ability to adsorb model contaminants could be altered by regulating the ratio of aforesaid components and other process variables including the inlet temperature and post-heat treatment.

Figure 1. Schematic illustration showing the fabrication process. 16 ACS Paragon Plus Environment

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Morphology of the Microspheres. A series of chitosan-based composite microspheres were successfully fabricated by spray-drying. In general, the obtained microspheres were orange-brown in color (Figure 1) and displayed good spherical shape with certain heterogeneity in size, scaling in the micron order (Figure 2). However, there are notable differences in the surface smoothness of the different microspheres depending on nGO and GP content. As shown in the SEM image, the GP5/nGO0_100°C microsphere with no nGO exhibited a “brain-like” structure with striped curves on the surface as a result of the rapid shrinking after the solid crust was formed.32 Increasing the amount of crosslinking agent from 5 to 10 wt% led to a remarkable change in the surface smoothness. The reason for this could be related to the higher degree of crosslinking between the primary amine groups on CS and GP, which could lead to a more rigid surface structure.33

Increasing the nGO content from 0 to 2 wt% also generated a smooth surface; whereas further increasing nGO content to 10 wt% resulted in a surface with indentations. The chitosan-derived nGO contains some oxygen functionalities including carboxylic, hydroxyl,



and epoxy groups with the calculated C/O ratio around 3.2;31 Hence, secondary intermolecular interactions such as H-bonding and electrostatic attractions could be formed between nGO and the chitosan matrix, leading to reinforcement of the network strength to resist shrinking during the solvent atomization process. Moreover, we previously revealed that nGO has catalytic effect on the GP-induced crosslinking reaction, which is also expected to improve the network strength.31 The excess amount of nGO could, however, cause collapse of the crust structure.34,35

The influence of inlet temperature on the microspheres was also investigated as the spray-drying conditions are known to strongly regulate the microsphere morphology.36 As shown in Figure 2a, the microspheres fabricated at 200 °C (GP5/nGO10_200°C) had a smoother surface as compared to those fabricated at 100 °C (GP5/nGO10_100°C), which is in correlation with the faster drying rate. The higher drying temperature resulted in faster drying or solvent atomization rate, thus leading to the quick formation of the hard and smooth crust with little time to form wrinkles.37–39


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The microspheres GP10/nGO0_100°C and GP10/nGO10_100°C were further modified by subjecting them to a post-heat treatment at 37 °C for 48 h to obtain PGP10/nGO0_100°C and P-GP10/nGO10_100°C . After the treatment the color of the microsphere suspensions changed from yellowish to green-blue, Figure 2b, which is an intrinsic indication of the intensified crosslinking reaction between amine groups of CS and GP.39 The incorporation of nGO resulted in much deeper the green-blue color for the suspension, which further verified the catalytic effect of nGO on the GP-induced crosslinking observed in our earlier study.30 The SEM images (Figure 2a) also illustrate that the post-heat treatment endowed microspheres with higher surface smoothness, which is presumably related to the higher degree of crosslinking.



Figure 2. FE-SEM images of the obtained chitosan-based spray-dried microspheres (a) and photos showing the color change of the microspheres during the post-heat treatment (b).


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To further monitor the differences at the molecular level before and after post-heat treatment,

the

microspheres

GP10/nGO0_100°C,

GP10/nGO10_100°C,

P-

GP10/nGO0_100°C, and P-GP10/nGO10_100°C were examined by FTIR. Fingerprint peaks at 1640 cm-1 and 1551 cm-1 corresponding to the C=O stretching of amide groups and N-H bending vibration, respectively, are clearly seen in Figure 3. For the samples after post-heat treatment, a clear intensity increase at 1640 cm-1 was observed and attributed to the promoted crosslinking reaction, where the secondary amine was formed through the nucleophilic substitution of the ester group on GP by chitosan’s primary amine group.

Figure 3. FTIR spectra of selected microspheres before and after post-heat treatment.



Size and Zeta Potential of the Microspheres. The size of the fabricated microspheres was determined statiscally from SEM images. For each sample, at least 300 individual microspheres were measured and the mean diameter was calculated and presented in Figure 4a. In general, the spray-dried microspheres exhibited relatively heterogenous size distruibutions (Figure 2a), ranging in between ca. 1 to 9 µm. The mean diameters of the microspheres were centered around 1.4 to 2.4 µm (Figure 4a). Although the size differences between the different microspheres were not that significant, it is possible to observe some interesting tendencies. Increasing GP from 5 to 10 wt % resulted in slight decrease of the mean diameter, which could be explained by the constrained network structure with higher degree of crosslinking. The microspheres spray-dried at 200 °C had somewhat larger diameters compared to the microspheres spray-dried at 100 °C. Increasing the inlet temperature can result in reduced bulk density, which consequently would lead to increased microsphere size.42,43 During the post-heat treatment the crosslinked network is strengthened, which could decrease the size. However, the


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opposite was observed. This is explained by the swelling of the microspheres during the post-heat treatment performed in the presence of water/methanol solution.

All the microspheres had positive zeta potential values, verifying the presence of protonated amine groups on the surface of the microspheres, Figure 4b. As expected, the zeta potential values decreased when the amount of GP increased as crosslinking reduces the number of NH2 groups. Likewise, the incorporation of nGO also resulted in the decline of the surface positive charge, which is ascribed to the oxygen functionalities of nGO and the catalyzing effect of nGO on GP-induced crosslinking reaction. Post-heat treatment resulted in a significant decrease in the zeta potential value due to the increased crosslinking reaction, which was in good agreement with the FTIR results.



Figure 4. Size measurements (a) and zeta potential (b) of the microspheres.

Adsorption of Methyl Orange (MO) by the Microspheres. The orange MO solution was pushed through a syringe filled with GP10/nGO10_100°C microspheres. The solution immediately became colorless, when passing the column with MO molecules immobilized on the microspheres, see Figure 5b and Video S1 in the supporting information. The mechanism behind is assumed to be related to the binding effect of CS with anionic dye through the protonated amine and free hydroxyl groups to form electrostatic attraction and hydrogen bonding.44 To further verify the attachment of MO on the surface of the


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microspheres, EDS spectra of GP10/nGO10_100°C before and after MO adsorption was recorded (Figure 5c). Sulphur originating from the MO molecule was only observed on the surface of the microspheres after adsorption, which provided evidence to firmly support the effective MO adsorption by the microspheres. Furthermore, the quantitative analysis of MO adsorption performance was performed by the UV-Vis spectrophotometer. All the microspheres adsorbed MO, with the adsorption efficiency varying from 53.8 up to the high value of 98.9 %. After post-heat treatment, the microspheres showed lower adsorption efficiency compared with non-post-heated samples, which could be speculated to be due to the larger consumption of the amine groups due to further crosslinking reactions during the post-heat process. This is in agreement with the decreased zeta potential value for the post-heated samples as discussed above.



Figure 5. The MO adsorption study. The molecular structure of MO (a); a photograph of the syringe column packed with the microsphere GP10/nGO10_100°C used for the MO solution (0.08 mg/mL) filtration and the solution before and after filtration (b); EDS spectra of GP10/nGO10_100°C before and after MO adsorption (c); The MO adsorption efficiency (%) at equilibrium (d). All the experiments were conducted at room temperature.


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Adsorption of Diclofenac Sodium (DCF) by the Microspheres. The adsorption efficiencies (%) of the different microspheres for the removal of DCF from the aqueous solution are compared in Figure 6. The results show that all the microspheres could effectively adsorb DCF. The DCF adsorption efficiencies at equilibrium varied from 67.9 to 100 % and the following trends were observed: 1) For the non-post-heated microspheres, increasing GP from 5 to 10 wt% led to a higher DCF adsorption performance, and the incorporation of nGO also favored the adsorption process. This trend is in agreement with our previous study on chitosan hydrogels.28 The increased GP concentration resulted in higher crosslinking density, which in turn might disrupt the secondary interactions between the adjacent polymer units, so that the chitosan crystalline structure could be partially destructed and polar groups such as hydroxyl groups could be more free to interact with DCF.45 The decrease of the microsphere size after GP addition (Figure 4a) could also contribute to the higher DCF adsorption due to the increased specific surface area. Moreover, the oxygen functionalities and the delocalized π-domains of nGO could create secondary intermolecular interactions and π-π stacking with the DCF molecules.46,47 2) The microspheres with post-heat treatment exhibited lower DCF adsorption efficiency 27 ACS Paragon Plus Environment


than the non-post-heated samples. Weaker electrostatic interactions between the protonated NH2-groups of the microspheres and the negatively-charged COO- groups of DCF could explain this and is supported by the decrease of zeta potential on post-heated microspheres (Figure 4b). This trend could also be supported by a denser surface and stronger network generated by the excessive crosslinking reactions during the post-heat process, hindering the diffusion of DCF inside the adsorbent. 3) The incorporation of nGO in the microspheres with post-heat treatment impeded the DCF adsorption efficiency. The blue-green color of the post-heat treated samples deepened when nGO was incorporated in the microspheres indicating higher degree of crosslinking and further confirming the catalytic effect of nGO on the GP-induced crosslinking reaction revealed in our previous work28,31 (Figure 2b). This higher degree of crosslinking is a probable cause for the observed impaired DCF adsorption. 4) Microspheres fabricated at lower inlet temperatures had higher adsorption capacity than those fabricated at higher temperature. As an example GP5/nGO10_100°C had rougher surface and smaller size compared to GP5/nGO10_200°C leading to larger specific surface area for the attachment of DCF molecules. 28 ACS Paragon Plus Environment

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Figure 6. DCF adsorption study comparing the DCF adsorption efficiency (%) of the different microspheres at equilibrium. The orignial DCF concentraton was 0.01 mg/mL, and the experiments were performed at room temperature for 1 h to reach the equilibrium.

DCF Adsorption Kinetics and Isotherms. GP10/nGO0_100°C, GP10/nGO10_100°C, and P-GP10/nGO0_100°C were selected for an adsorption kinetics and isotherms study to reach further insights into the DCF adsorption mechanism. A depiction of the DCF adsorption efficiency as a function of time is given in Figure 7a. All selected microspheres demonstrated a fast and efficient adsorption of DCF. Even within 1 min, the adsorption 29 ACS Paragon Plus Environment


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efficiency reached 86.4, 77.8 and 61.1 % for GP10/nGO10_100°C, GP10/nGO0_100°C and P-GP10/nGO10_100°C, respectively. Both pseudo-first-order (Eq. 4) and pseudosecond-order kinetics (Eq. 5) models were applied to simulate the adsorption process (the kinetics parameters are shown in Figure S1), with equations represented below:

(4)

𝑙𝑛(𝑞𝑒 ― 𝑞𝑡) = 𝑙𝑛𝑞𝑒 ― 𝐾1𝑡

𝑡 𝑞𝑡

1

1

(5)

= 𝐾 𝑞 2 + 𝑞𝑒𝑡 2 𝑒

Where 𝑞𝑒 (mg/g) is the adsorption capacity at equilibrium and 𝑞𝑡 (mg/g) is the adsorption capacity after a specified time t. 𝐾1 (min−1) and 𝐾2 (g mg−1 min−1) are the rate constants of these two models, respectively.


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Figure 7. DCF adsorption efficiency (%) as a function of time ranging from 1 min to 1 h (a), and the corresponding pseudo-second-order kinetics model. The initial DCF concentration was 0.01 mg/mL, and experiments were performed at room temperature.

Comparison of the correlation coefficients (R2) showed that the kinetic curve of DCF fitted better with the pseudo-second order model (Figure 7b), with the calculated values of DCF adsorption capacities being 19.4, 20.0 and 16.7 mg/g which is close to the experimental values of 19.6, 20.2 and 16.9 mg/g on the GP10/nGO0_100°C, GP10/nGO10_100°C, and P-GP10/nGO0_100°C, respectively. As shown in Table 2, the value of 𝑞𝑒 is higher compared to many previously reported bio-based adsorbents, indicating improved



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adsorption performance. For the listed bio-based activated carbons, hydrogen bonding and π-π stacking could be the dominant interactions for the DCF adsorption; whereas for the DCF adsorption by the chitosan-based microspheres the involvement of electrostatic attraction could further favor the adsorption process. In comparison with our previous study on the DCF adsorption properties of chitosan hydrogels with same GP and nGO composition, the spray-dried chitosan microspheres had greatly improved adsorption capacity and shorter equilibrium-time, which is explained by the spherical shape and higher surface area.

Table 2. A comparison of different bio-based adsorbents for the DCF adsorption Adsorbent

𝒒𝒆 (mg/g)

Ref

Chitosan-based microsphere

20.0

Present work

Chitosan-based composite hydrogel

1.9

28

Grape bagasse

11.1

48

Olive waste-based activated carbon

13.8

49

Cellulose-based carbon nanosphere

13.1

50

Cocoa shell-based activated carbon

39.0

51


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The DCF adsorption isotherms were built to describe the relationship between the adsorbate remaining in the solution (Ce) and the adsorbate attached on the surface of the adsorbent (qe) when the adsorption equilibrium is reached. The experimental isotherm data fitted well with the Freundlich isotherm model (Figure 8), with the correlation coefficients close to 1 (Table 3). Based on the assumption of the Freundlich isotherm model, it implies that the chitosan-based microspheres have heterogeneous surface with different adsorption sites.52 In the following equation (Eq.6) proposed by Freundlich:53

1

𝑞𝑒 = 𝐾𝐹 ∙ 𝐶𝑒𝑛(6)

KF represents the Freundlich isotherm constant and n is related with the adsorption intensity. Since 1/n values were above one in this study, a cooperative adsorption could be implied, where DCF containing polar groups is in competition with water for the adsorption sites.54 Moreover, 1/n above one could also indicate S-type isotherms, implying a multilayer adsorption process an adsorbent containing pores with different size.



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Figure 8. DCF adsorption isotherm study with Freundlich model. The DCF initial concentration varied between 0.01 and 0.1 mg/mL. Experiments were conducted at room temperature.

Table 3. Parameters of Freundlich isotherm model Freundlich isothermal model 𝐾𝐹 (mg g−1) (L mg−1)1/n GP10/nGO0_100°C

1/n

33.1

R2 0.995

1.169 GP10/nGO10_100° C

17.7

0.994 1.366


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Reusability of the Microspheres. The recyclability of the adsorbents is an important factor for commercial applications, especially when considering sustainability and costeffectiveness. The microspheres recyclability was, therefore, evaluated through cyclic adsorption and desorption process. GP10/nGO0_100°C and GP10/nGO10_100°C were selected as model microspheres for the recycling study. Figure 9 exhibits the DCF adsorption efficiency for the microspheres as a function of the recycling and reuse cycles. In general, GP10/nGO10_100°C demonstrated higher DCF adsorption performance than GP10/nGO0_100°C during the six-time cyclic process, which indicates that the addition of nGO could facilitate the reusability of the microspheres. Although there was some loss of adsorption efficiency after repeated adsorption and desorption cycles, still the adsorption efficiency was maintained above 70 % after the sixth adsorption cycle for both GP10/nGO0_100°C and GP10/nGO10_100°C, implying promising potential as green and sustainable adsorbent.



Figure 9. DCF adsorption efficiency (%) by GP10/nGO0_100°C and GP10/nGO10_100°C during six cycles of adsorption and regeneration. The initial concentration of DCF solution was 0.01 mg/mL. Experiments were conducted at room temperature.

CONCLUSIONS

The adsorption capacity could be greatly improved and equilibrium time shortened by changing the shape and size of the adsorbent from macroscopic hydrogels to microspheres. A facile and fast one-pot in-situ pathway for the fabrication of crosslinked


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fully bio-based chitosan/nGO composite microspheres was developed by utilizing a spray-drying process. Simple regulation of the feed ratio of chitosan, nGO and genipin crosslinking agent, inlet temperature and post-heat treatment tuned the surface morphology, size and zeta potential of the microspheres. All the prepared microspheres functioned as effective adsorbents towards different types of contaminants modelled by an anionic dye MO (adsorption efficiency ranging from 53.8 to 98.9%) and an antiinflammatory drug DCF (adsorption efficiency ranging from 67.9 to 100%): The good performance was deduced to the probable synergetic contribution of hydrogen bonding and electrostatic attraction between the protonated amine groups on chitosan with negative-charged compounds. The DCF adsorption kinetics study revealed that the pseudo-second-order model fitted the experimental data very well, with the simulated adsorption capacities fitting almost identically with the experimental results. Freundlich isotherm model showed a good fit with the adsorption data, and therefore implied a heterogenous surface of microspheres with different adsorption sites. Finally, the chitosan-based microspheres could be effectively regenerated and reused. The addition of nGO facilitated the reusability of the microspheres leading to 80% of the DCF 37 ACS Paragon Plus Environment


adsorption efficiency still remaining after six adsorption and desorption and cycles. This gives promise for practical applicability of the fabricated microspheres as green and sustainable adsorbents for wastewater contaminants.

ACKNOWLEDGEMENTS

The author gratefully acknowledges the financial support from the China Scholarship Council (CSC). A part of this work by Takahiro Danjo was supported by a Grant-in-Aid for JSPS Research Fellow and JSPS overseas challenge program for young researchers.

SUPPORTING INFORMATION

DCF adsorption kinetics parameters, and the video showing the removal of MO by the microsphere column


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Table of Contents Graphic (TOC)


Recyclable fully biobased chitosan adsorbents spray-dried in one-pot

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