Attapulgite Nanofiber-Cellulose Nanocomposite with Core-Shell Structure for Dye Adsorption

Nanocomposite particle used for adsorption has attracted continuous attention because of large specific surface area and adjustable properties from nanocomponent. Herein nanocomposite particle with cellulose core and attapulgite nanofibers shell was prepared. The size of cellulose core was about 2mm and the thickness of nanofibers shell is about 300 μm. Adsorption capacity of nanocomposite particle tomethylene blue can reach up to 11.07mg L and the best adsorption effect occurs at pH= 8; pseudo-firstorder equation and the Langmuir equation best describe the adsorption kinetic and isotherm, respectively; repeated adsorptiondesorption experimental results show that 94.64% of the original adsorption capacity can be retained after being reused three times.


Introduction
Attapulgite nanofiber is a kind of hydrated octahedral layered magnesium aluminum silicate with diameter of 20 nm and length of several hundred nanometers to several micrometers [1,2].Theoretical formula of attapulgite nanofiber is [3].Due to large specific surface area, attapulgite nanofiber has high sorption ability and is widely used in decoloring, drying, removing heavy metal ions or organic contaminants, and other fields [4,5].Attapulgite nanofiber possesses permanent negative charges and exchangeable cations [2].Used as dye adsorbent, attapulgite nanofiber can remove cationic dye such as methylene blue from water.But when added to water, attapulgite nanofibers suspend in water and are difficult to be separated from water after adsorption.The suspended attapulgite will cause secondary pollution to the water and cannot be reused [6].Therefore, finding an excellent carrier for attapulgite is important to avoid secondary pollution and to reuse attapulgite.
Polymer nanocomposites are composites of particlefilled polymers for which at least one dimension of the dispersed particles is in the nanometer range [7][8][9].Polymer nanocomposites containing attapulgite nanofiber have been prepared, in which polymer exists as hydrogel and attapulgite nanofibers are embedded in the hydrogel [10,11].This hydrogel can be used as super adsorbent and adsorbent for heavy metal ions [10,11].
Adsorption usually occurred on the surface of absorbent [12,13].Particle adsorbents have large surface area, which leads to high adsorption ability.Core-shell structure with adsorbent on the shell has low cost and is favorable for particle adsorbent to adsorb dye.Cellulose is one of the most abundant biopolymers in nature [14,15].Nanocomposite particle with cellulose core and attapulgite nanofiber shell has never been reported.But cellulose is difficult to dissolve in common solvent to form cellulose bead.NaOH/urea solution is a "green" and cheap method to dissolve cellulose with no pollution [16].
In this study, we combined attapulgite nanofiber and cellulose to prepare a low-cost adsorbent for water treatment.We prepared cellulose bead by dissolving cellulose in NaOH/urea solution as core to afford necessary strength.Attapulgite nanofibers were coated on cellulose bead as shell to adsorb dye.Coated attapulgite shell can prevent second pollution of attapulgite in water.The structure, adsorption 2 International Journal of Polymer Science behaviors, and reusability of this nanocomposite were investigated.

Preparation of the Nanocomposite Particle.
Cellulose was dissolved in NaOH/urea solution using reported method [17].Solution of NaOH (7 g), urea (12 g), and H 2 O (81 mL) was cooled to −12 ∘ C; then 2 g scoured cotton was immediately added to above solution; then scoured cotton was stirred vigorously for 2 min to obtain a transparent cellulose solution.The resulting solution was dropped into 20 wt% acetic acid solution to form cellulose bead; then the cellulose bead was immersed in water for three days to remove urea and NaOH in bead.
Attapulgite powder was dispersed in 2.5 wt% sodium alginate solutions with agitation and the mass ratios of sodium alginate/attapulgite are 1 : 0, 1 : 1, 0.83 : 1, and 0.5 : 1.Then cellulose bead was coated with above suspension and dipped in CaCl 2 solution to cross-link the sodium alginate.The coated bead was washed with deionized water and dried in an oven at 50 ∘ C for 48 h to get dried composite particle.Composite particles with alginate/attapulgite mass ratios of 1 : 0, 1 : 1, 0.83 : 1, and 0.5 : 1 were labeled as AC0, AC1, AC2, and AC3, respectively.In addition, attapulgite and cellulose solution mixture was dropped into 20 wt% acetic acid solution to form bead coded as AC4.

Morphology and Structure of Composite
Particle.Surface morphology of composite bead was observed by scanning electron microscopy (SEM, SU8010, Hitachi).Microstructure of composite particles was observed by digital microscope (503+ Guangzhou Haote optical instrument company).Fourier-transform infrared (FTIR) spectra are recorded by Smart iTR accessory of FTIR spectrometer (Thermo Fisher Nicolet iS10).Wide-angle X-ray diffraction (XRD) measurement was carried out on an XRD diffractometer (D8-Advance, Bruker).The patterns with the Cu K radiation at 40 kV and 30 mA were recorded in the region of 2 from 5 ∘ to 70 ∘ .TGA (STA 409 PC Luxx NETZSCH) was performed by heating samples to 700 ∘ C at 5 ∘ C/min under a nitrogen flow.

Adsorption Studies
2.4.1.The Adsorption Ability Study.0.1 g adsorbent was immersed into 80 mL methylene blue solution of 12.016 mg L −1 at 25 ∘ C for 36 h with 120 r/min controlled by a full temperature incubator shaker.The adsorption capacity  (mg g −1 ) was calculated using the following equation: where  0 (mg L −1 ) is the initial dye concentrations of the solutions,   (mg L −1 ) is the equilibrium dye concentrations of the solutions,  (L) is the volume of the solution, and  (g) is the dried weight of the adsorbent [18].Methylene blue concentration was measured at 664 nm using UV-Vis spectrophotometer (VARIAN Cary 50).At desired time intervals, 0.5 mL dyes solution was taken out to detect the current methylene blue concentration and 0.5 mL distilled water was added to the bulk solution to keep the volume constant.At time   , the adsorption capacity (  ) (mg g −1 ) was calculated using the following equation [18]: where    (mg L −1 ) is the dye concentration at time   ;   is the volume of solution taken out each time for dye concentration analysis, and  (g) represents the mass of the adsorbent [18].which leads to second pollution to water and difficulty of regeneration.In this study, attapulgite is mixed with sodium alginate solution and is coated on the surface of cellulose bead.Sodium alginate, derived primarily from brown seaweed, is a linear polysaccharide copolymer that consists of two sterically different repeating units, -d-mannuronic acid (M) and -l-guluronic acid (G), in varying proportions [19].Sodium alginate can be cross-linked by Ca 2+ to form hydrogel.Above mixture is cross-linked by Ca 2+ to form a hydrogel layer above the cellulose bead.After drying, the hydrogel becomes an attapulgite shell on the surface of cellulose core, preventing the second pollution of attapulgite powder.The structure of nanocomposite particle is shown in Figure 1.

Results and Discussion
Cellulose is selected to prepare the core for its low cost and being nontoxic and biodegradable.But cellulose is difficult to dissolve in usual solvent.In this study, we use NaOH/urea solution in low temperature as solvent to dissolve cellulose and to prepare cellulose bead as the core.Figures 2(b)-2(f) show the SEM image of cross section and surface of composite particle.Figure 2(b) shows that the shell is adhered closely to the cellulose core and no obvious boundary is found.Figure 2(c) shows the coarse shell with thickness of 300 m (area between two white arrows).Figure 2(d) shows the aggregation area of attapulgite nanofibers (white arrow pointed).Aggregation areas of attapulgite indicate that the dispersion of attapulgite in shell is not well.The aggregation areas are enlarged in Figure 2(e), from which fibrous attapulgite is found.Figure 2(f) shows the pure sodium alginate area existing in the shell.

Morphology and Structure of Composite Particles
FTIR spectra of sodium alginate, attapulgite, and AC2 are shown in Figure 3.In the spectrum of AC2, 1421 cm −1 belongs to the symmetric stretch vibration of C=O of sodium alginate.3614 cm −1 is attributed to the stretching modes of hydroxyls coordinated with the magnesium.3582 cm −1 and 3553 cm −1 belong to the symmetric and antisymmetric stretching mode of molecular water coordinated with the magnesium at the edges of the channels [20].1027 cm −1 belongs to Si-O-Si stretching vibration of attapulgite [21].The results indicate that the shell is composed by sodium alginate and attapulgite.Smart iTR measures the surface structure of sample.So only the shell structure of composite particle can be detected.[22].All belong to the character peaks of attapulgite.The diffraction peaks of 26.54 ∘ are attributed to intergrowth minerals of quartz in attapulgite.In attapulgite shell, no obvious shift of peak position indicates that the crystal structure of attapulgite has no change during mixing.Absence of diffraction peaks of sodium alginate revealed that sodium alginate exists as amorphous.XRD results confirmed that attapulgite is the main component of shell.
TG and DTG curves for the cellulose, attapulgite, and AC1 are presented in Figure 5.The weight loss rate of AC1 is between attapulgite and cellulose.For AC1, below 100 ∘ C, the weight loss was ascribed to the removal of water which included surface water and zeolitic water of attapulgite.Between 200 and 300 ∘ C, the rapid weight loss can be attributed to the decomposition of cellulose and sodium alginate.Above 300 ∘ C, the weight loss corresponds to the degradation of residual decomposition products and loss of coordinated water and structural hydroxyl water in attapulgite [23].The high residue mass of AC1 compared to the cellulose indicates the existence of attapulgite.

Adsorption Capacity.
Figure 6 shows the adsorption effect of AC1 (Figure 6(a), before adsorption; Figure 6(b), after adsorption).The slight color of Figure 6(b) reveals that the nanocomposite particles could effectively remove the methylene blue from water.The clear solution indicates that attapulgite nanocomposite prevents the second pollution of attapulgite nanofiber in water.
Figure 7 compares the adsorption capacities of nanocomposite particles (AC1, AC2, and AC3) with core-shell structure to contrast samples (cellulose bead, AC0, and AC4).Composite particles have better adsorption capacities compared to the adsorption capacities of cellulose bead, AC0, and AC4, which indicate that the shell of attapulgite nanofiber can effectively absorb methylene blue.The adsorption capacities of AC1, AC2, and AC3 are higher than AC0 which only has a shell of sodium alginate, indicating the adsorption mainly resulted from attapulgite nanofiber.AC3 has maximum adsorption capacity of 9.1 mg⋅g −1 .
Attapulgite fiber is a kind of silicate with nanosized rodlike morphology and consists of two double chains of the pyroxene type (SiO 3 ) 2− like amphibole (Si 4 O 11 ) 6− running parallel to the fiber axis [24,25].Attapulgite fiber has high surface area and moderate cation exchange capacity, which is useful for attapulgite as adsorbents to remove dye in wastewater.In addition, attapulgite fiber has negatively charged sorption sites because of isomorphic substitutions in structure.So attapulgite nanofiber can absorb cationic dyes through electrostatic attraction.The higher adsorption capacity of AC3 resulted from the high content of attapulgite nanofiber.From Figure 8, we can see that AC1 has rare dispersion of attapulgite nanofibers on the shell compared to AC2 and AC3, which leads to low adsorption capacity of AC1.
Other adsorption capacities for methylene blue have been reported, such as cross-linked porous starch (9.46 mg g −1 ) [26], sugar extracted spent rice biomass (8.13 mg g −1 ) [27], and attapulgite (51 mg g −1 ) [28].Compared to attapulgite, the relative lower capacity of nanocomposite particle may have resulted in the aggregation of attapulgite on the shell which was observed by SEM photo (Figure 2(d)).The aggregation decreased the specific surface area of attapulgite and then decreased adsorption ability [29].

Effects of pH on the Adsorption.
Figure 9 shows the adsorption capacity of AC2 in the pH range of 2.06∼9.93.As the pH increased from 2.06 to 7.97, adsorption capacity increased from 7.70 to 8.88 mg g −1 ; as the pH increased from 7.97 to 9.93, adsorption capacity decreased from 8.88 to 8.43 mg g −1 .
For attapulgite, some isomorphic substitutions in the tetrahedral layer, such as Al 3+ for Si 4+ , develop negatively charged sorption sites (Si-O − ) on the surface of attapulgite [30].Si-O − can absorb cation dye through electrostatic attraction [30].But, at low pH, these negatively charged  sorption sites are protonated to form Si-OH 2 + by H + , which decrease the negative sites to attract methylene blue.As pH increases, protonated groups become less and negatively charged sites such as Si-O − increase and favor the adsorption of cationic dye.In the alkaline pH range, the decreasing of adsorption capacity could be attributed to competition of Na + ions for the negative adsorption sites [31].
3.5.Adsorption Kinetics.Time dependence of the adsorption capacity of AC1 for methylene blue was tested to study the adsorption kinetics.The initial concentration of methylene blue is 12.016 mg L −1 .The experimental results are shown in Figure 10.About 50% of methylene blue was adsorbed within 7 h.The adsorption equilibrium was achieved after 36 h.These results were similar to cross-linked porous starch whose adsorption equilibrium was reached in about 30 h for methylene blue [26].
To further investigate the adsorption mechanisms, four common kinetic models were used to fit the data, namely, pseudo-first-order model [32], pseudo-second-order model [33], intraparticle diffusion models [34], and the Elovich equation [35].Table 1 shows the equations of these models.In these models,  1 is the rate constant first-order absorption (min −1 ) [32];  eq (mg g −1 ) is the amount of dye adsorbed at equilibrium;   (mg g −1 ) is the amount of dye adsorbed at any time  (min);  2 (g mg −1 min −1 ) is the second-order rate constant [33];   (mg g −1 ⋅min 1/2 ) is the diffusion rate constant [34];  is a constant related to the thickness of boundary layer [34];  (mg g −1 min −1 ) is the initial sorption rate [35];  (g mg −1 ) is the desorption constant related to the extent of surface coverage and activation energy for chemisorption [35].The pseudo-second-order model assumes that the rate limiting step is chemical sorption [36].Elovich model recently has been found to be valid to describe the sorption kinetics of ion exchange systems [18].The fitted parameters of these kinetic models are listed in Table 2.  2 of the linear form for the various models suggests that the pseudo-first-order model is more suitable to describe the adsorption kinetic behavior.Using intraparticle diffusion model, plots of   versus  1/2 do not pass through the origin indicating that the adsorption process is also controlled by film diffusion [36].methylene blue was measured.The equilibrium adsorption data was correlated to four isotherm models: Langmuir [37], Freundlich [38], Sips [39], and Dubinin-Radushkevich [40].Table 3 shows the equations of these models.In these models,  max is the maximum adsorption at monolayer coverage (mg g −1 ) [37];  is equilibrium constant (mL mg −1 ) in Langmuir adsorption [37] or Sips constant related to energy of adsorption in Sips model [39];   is the Freundlich characteristic constants [38]; 1/ is the Freundlich characteristic constants [38];  could be regarded as the Sips parameter characterizing the system heterogeneity;  is a constant related to the mean free energy of adsorption per mol of the adsorbate (mol 2 J −2 ) [40];   is the theoretical saturation capacity (mg g −1 ) [40];  is the Polanyi potential [40], which is equal to ln(1 + 1/  );  (J mol −1 K −1 ) is the gas constant;  (K) is the absolute temperature.The Langmuir isotherm model assumes the adsorption is monolayer adsorption [18].A finite number of identical sites exist on a surface and all sites are energetically equivalent and there is no interaction between adsorbed molecules.Freundlich model is applied to describe that adsorption occurs on a heterogeneous surface.The fitting parameters of the above models are listed in Table 4.Most of the determination coefficients ( 2 ) of the Langmuir model exceed 0.93 compared with those of the other models.This indicates that the Langmuir model is suitable to describe the adsorption behavior of AC2.So it can be concluded that methylene blue molecule is absorbed on the surface with monolayer.From Figure 11, we can see that   is almost near zero at 25 ∘ C, 40 ∘ C, and 50 ∘ C, which indicates that the particle can remove dye in very dilute aqueous solutions.

Regeneration Efficiency.
Reusability is very important for adsorbent in practical applications.In current work, the adsorption and desorption processes were repeated for three cycles to measure the regeneration efficiency.0.1 mol L −1 H 2 SO 4 aqueous solution was used to recover the methylene blue loaded composite particles.Table 5 shows the adsorption capacities and the regeneration efficiency in each cycle.After three cycles, AC2 gets 94.64% recovery, indicating a high regeneration efficiency.This also illuminates that the adsorbent is suitable for practical applications.

Conclusions
The attapulgite nanofibers were coated onto the surface of cellulose bead to form nanocomposite particles with coreshell structure.SEM, FTIR, XRD, and TG all revealed that attapulgite nanofiber exists in the shell of nanocomposite particles.Nanocomposite particles have higher adsorption capacity than cellulose bead.Adsorption capacity is changed with pH of dye solution and largest adsorption capacity occurs at pH = 8.The adsorption equilibrium and kinetics study of composite particles indicate that the adsorption behavior follows Langmuir model and pseudo-first-order equation.Dye-loaded nanocomposite could be regenerated easily and high adsorption ability is reserved.Simple preparation, low cost, being easy to regenerate make the core-shell structured nanocomposite a suitable carrier for attapulgite and an attractive adsorbent for removal of the organic dyes from water.This study also proposes a new approach to use the attapulgite and cellulose as adsorption material.

International Journal of Polymer Science
Study.0.1 g of AC2 was immersed into 80 mL methylene blue solution with concentration of 6.008 mg L −1 , 9.012 mg L −1 , 10.514 mg L −1 , 12.016 mg L −1 , and 15.02 mg L −1 .Above five solutions were shook at 120 r/min at 25 ∘ C for 36 h.Then the adsorption capacity (  ) of each solution was measured based on (1).The same procedure was conducted at 30 ∘ C, 35 ∘ C, 40 ∘ C, and 50 ∘ C. Then the curves of   -  were plotted.2.5.Reusability Property.The methylene blue-loaded AC1 and AC2 were immersed into 0.1 mol L −1 H 2 SO 4 aqueous solutions, washed with distilled water, and then reused in the next cycle of adsorption experiment.The adsorptiondesorption experiments were conducted for four cycles.All experiments were performed at 25 ∘ C.

Figure 1 :
Figure 1: Schematic depiction of core-shell structure of celluloseattapulgite composite particle.
Figure 2(a) shows microscope image of AC2 and particle has a diameter of about 2 mm and has a core-shell structure.

Figure 4
Figure4shows the XRD peaks of attapulgite and attapulgite shell.Strong diffraction peak of 8.48 ∘ is a typical peak of attapulgite corresponding to basal spacing of about 10.55 Å and is attributed to the primary diffraction of the (110) crystal face.Other diffraction peaks are attributed as d(110) (10.55 Å), d(200) (6.43 Å), d(130) (5.43 Å), d(040) (4.48 Å), d(121) (4.14 Å), and d(061) (2.55 Å)[22].All belong to the character peaks of attapulgite.The diffraction peaks of 26.54 ∘ are attributed to intergrowth minerals of quartz in attapulgite.In attapulgite shell, no obvious shift of peak position indicates that the crystal structure of attapulgite has no change during mixing.Absence of diffraction peaks of sodium alginate revealed that sodium alginate exists as amorphous.XRD results confirmed that attapulgite is the main component of shell.TG and DTG curves for the cellulose, attapulgite, and AC1 are presented in Figure5.The weight loss rate of AC1 is between attapulgite and cellulose.For AC1, below 100 ∘ C, the weight loss was ascribed to the removal of water which included surface water and zeolitic water of attapulgite.Between 200 and 300 ∘ C, the rapid weight loss can be attributed to the decomposition of cellulose and sodium alginate.Above 300 ∘ C, the weight loss corresponds to the degradation of residual decomposition products and loss of coordinated water and structural hydroxyl water in attapulgite[23].The high residue mass of AC1 compared to the cellulose indicates the existence of attapulgite.

Figure 2 :Figure 3 :
Figure 2: The morphology of the cellulose-attapulgite composite particles (AC2) ((a) cross section of particle by digital microscope; (b) cross section of particle by SEM; (c) shell structure of particle; (d) aggregation area of attapulgite; (e) amplified aggregation area of attapulgite; (f) area of sodium alginate).

Figure 4 :
Figure 4: The powder X-ray diffraction pattern of the attapulgite and attapulgite shell.

Figure 9 :
Figure 9: The influence of pH on the adsorption capacity of AC2 for methylene blue with concentration of 12.016 mg L −1 at 25 ∘ C.

3. 6 .− 1 )Figure 10 :
Figure 10: The absorbed capacity of AC1 for methylene blue with concentration of 12.016 mg L −1 at 25 ∘ C and pH = 7.0 as a function of time (◼ represents experiment data; the full line represents fit of experimental data with a pseudo-first-order kinetic equation;  represents concentration of methylene blue in solution).
2.1.Materials.NaOH, HCl, CaCl 2 , urea, acetic acid, methylene blue (methylthioninium chloride C 16 H 18 N 3 SCl), and other reagents used in the current work are all A.R. grade reagents.Attapulgite is provided by Jiangsu dianjinshi Au soil Mining Industry Co., Ltd.Distilled water is used in all experiments.Scoured cotton was used in this study as cellulose provided by Xuzhou weicai hygiene material factory Co., Ltd.Sodium alginate is provided by Shanghai Qingxi Chemical Technology Co., Ltd.

Table 1 :
Kinetic models and their equations.

Table 2 :
The kinetic parameters for MB adsorption onto AC1.

Table 3 :
Isotherm models and their equations.
) Figure 11: The curve of   -  of AC2 under different temperatures with duration time of 36 h.

Table 4 :
The isotherms parameters for methylene blue adsorption onto AC2.