Preparation of Chitosan—Graphene Oxide Composite Aerogel by Hydrothermal Method and Its Adsorption Property of Methyl Orange

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Article Preparation of Chitosan—Graphene Oxide Composite Aerogel by Hydrothermal Method and Its Adsorption Property of Methyl Orange

Wei Zhu 1,2, Xueliang Jiang 1,3,4,*, Fangjun Liu 1,3, Feng You 1,3 and Chu Yao 1,3 1 School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, China; [email protected] (W.Z.); [email protected] (F.L.); [email protected] (F.Y.); [email protected] (C.Y.) 2 The College of Post and Telecommunication of WIT, Wuhan 430073, China 3 Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, Wuhan 430205, China 4 Key Laboratory of Green Preparation and Application for Functional Materials, Ministry of Education, Hubei University, Wuhan 430062, China * Correspondence: [email protected]; Tel.: +86-27-8719-5661

Received: 23 July 2020; Accepted: 9 September 2020; Published: 22 September 2020

Abstract: Graphene based aerogel has become one of the most likely functional adsorption materials that is applicable to purify various contaminated water sources, such as dye wastewater, because of its high porosity, structural stability, large specific surface area, and high adsorption capacity. In this study, chitosan and graphene oxide were first selected as the matrix to prepare the composite hydrogel through the hydrothermal method, which was further frozen and dried to obtain the target aerogel. The microscopic structures and adsorption capacity of the composite aerogel were then characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and N2 (nitrogen) physical adsorption and desorption tests. The results show that the specific surface area of the composite aerogel was reached at 297.431 m2/g, which is higher than that of graphene oxide aerogel and chitosan aerogel. The aperture was reduced to about 3 nm. The adsorption rate of the composite aerogel for the methyl orange solution was as high as 97.2% at pH = 1, and the adsorption capacity was 48.6 mg/g. The adsorption process of the composite aerogel satisfies the Langmuir equation and can be described by the second-order adsorption kinetics. In addition, it is worth noting that this composite aerogel can provide a striking adsorption characteristic on methyl orange due to the combining effects from massive amino groups on chitosan and the structural conjugation of graphene oxide.

Keywords: graphene oxide; chitosan; aerogel; hydrothermal method; adsorption

1. Introduction With the improvement to the aesthetic requirements of products, dyes have been widely used in many fields. Tremendous dye wastewater will be continuously produced and released to the surrounding areas during the period of industrial production, which results in hazardous environmental pollution [1–3]. Therefore, it is very important to effectively remove dyes from polluted water sources. Adsorption is one of the most promising and reliable techniques for removing pollutants from wastewater [4]. Graphene oxide aerogel is a three-dimensional network structure material with graphene as the cross-linked body and gas as the dispersion medium. Graphene oxide aerogel not only retains the nano-characteristics of graphene, but also solves the dispersion difficulties of graphene. It is also a new material that can be used as an adsorbent. Chitosan and chitosan-based materials are considered to be good adsorbents due to their unique high reactivity, chemical stability and high

Polymers 2020, 12, 2169; doi:10.3390/polym12092169 www.mdpi.com/journal/polymers Polymers 2020, 12, 2169 2 of 16 affinity [5,6]. By combining graphene with chitosan, the advantages of specific surface area and porosity of graphene materials can be fully utilized while retaining the adsorption capacity of chitosan itself. The interlayer of chitosan can reduce the overlaps between graphene sheets, inhibit agglomeration, increase the specific surface area of graphene and improve adsorption capacity [7]. In recent years, the relevant research into chitosan/graphene composites has been reported successively. For example, Liao et al. [8] prepared 3D porous graphene oxide/polydopamine/chitosan (GO/PDA/CS) aerogels for crosslinking PDA with CS and assembled it. The active site was conducive to the removal of U (VI), and the maximum adsorption capacity was up to 415.9 mg/g at 298 K. Omid Ghasemi et al. [9] used graphene oxide (GO), chitosan and glutaraldehyde as raw materials to prepare the composite aerogel with a maximum adsorption capacity of 48.22 g/g for oil products in seawater. Liu et al. [10] prepared the chitosan/graphene oxide(CS/GO) composite material by adding a small amount of GO as a matrix and crosslinking it with glutaraldehyde. The maximum adsorption capacities were 1076.649 mg/g for Au(III) and 216.920 mg/g for Pd(II), respectively. Sabzevari [11] cross-linked graphene oxide (GO) with chitosan and prepared a composite material (GO-LCT) for the adsorption of methylene blue (402.6 mg/g). Wang et al. [12] prepared porous chitosan aerogels doped with graphene oxide (CSGO aerogels) by crosslinking and freeze drying and used them as highly effective adsorbents for two azo dyes, methyl orange (MO) and amido black 10B (AB10B). However, the preparation methods in the most reported literatures require the addition of cross-linking agents, such as epicloropropane, formaldehyde or glutaraldehyde, which are highly toxic and the experimental methods are relatively complex. Therefore, it is also of great importance to develop a simple way to synthesize the target composite aerogels and to reveal and analyze their absorption mechanisms. The preparation method adopted in this paper is the hydrothermal method, which is simple and does not require the addition of a cross-linking agent. The production process is green, environmentally friendly, and has a low production cost. The structure and adsorption properties of chitosan and graphene oxide (CS/GO) composite aerogels prepared by three methods (hydrothermal, chemical crosslinking and sol-gel) were compared. The microstructure, specific surface area and porosity of the CS/GO aerogels were systematically investigated. To this end, the adsorption mechanism, the adsorption model, and the desorption and recycling of the CS/GO aerogel will analyzed and developed, respectively.

2. Experimental

2.1. Raw Materials Natural graphite powder (97%), potassium permanganate (99.9%), concentrated sulfuric acid (98%), hydrogen peroxide solution (5%), deacetylation chitosan, sodium ascorbate, glutaraldehyde, methanol, phosphoric acid, hydrohdloric acid, acetic acid, sodium hydroxide, and methyl orange (MO) were purchased from Sinopharm Chemical Regent Co. Ltd., Shanghai, China.

2.2. Preparation of Chitosan (CS) Aerogel by Sol-Gel Method At ﬁrst, the homogeneous chitosan solution was obtained by adding 3 g chitosan into 100 mL dilute hydrochloric acid (2.5 wt%). Subsequently, a certain amount of chitosan solution was slowly dropped into 1 mol/L NaOH solution (at a controlled rate of 60 drops/min) and then reacted for 120 min. The reaction product was ﬁltered, washed with distilled water to pH = 7, and freeze-dried at 50 C − ◦ (48 h) to obtain the target CS aerogel.

2.3. Preparation of Graphene Oxide (GO) Aerogel by Hydrothermal Method GO was prepared by the improved Hummers method and dispersed through an ultrasound in deionized water. 50 mL GO dispersion (4 mg/mL) was mixed with sodium ascorbate (the mass ratio is 1:1) and the mixture was then reacted in a hydrothermal kettle for 8 h at 120 ◦C. After this, the resulting Polymers 2020, 12, 2169 3 of 16 Polymers 2020, 12, x FOR PEER REVIEW 3 of 16 productsresulting were products freeze-dried were freeze-dried at 50 ◦C (48 at h) − to50 obtain°C (48 the h) GOto obtain aerogel. the The GO schematic aerogel. mechanism The schematic for − themechanism synthesis for of thethe GOsynthesis aerogel of is the shown GO aerogel in Figure is 1showna. in Figure 1a.

(a)

(b)

FigureFigure 1. 1.Schematic Schematic illustration illustration of of the the synthesis synthesis mechanism: mechanism: ( a(a)) GO GO aerogel, aerogel, ( b(b)) CS CS/GO/GO aerogel. aerogel.

2.4.2.4. Preparation Preparation of of Chitosan Chitosan/Graphene/Graphene Oxide Oxide (CS (CS//GO)GO) Composite Composite Aerogel Aerogel by by Hydrothermal Hydrothermal Method Method AA certain certain amount amount ofof chitosan chitosan waswas addedadded toto 100100 mL mL acetic acetic acidacid (2.5 (2.5 wt%) wt%) and and was was put put in in an an ultrasoundultrasound for for 2 2 h h to to completely completely dissolve dissolve the the chitosan, chitosan, forming forming a a uniform uniform light light yellow yellow transparent transparent viscousviscous liquid. liquid. ChitosanChitosan solutions of different diﬀerent qualit qualitiesies were were added added to to the the GO GO dispersion dispersion solution solution (4 (4mg/mL) mg/mL) so so that that the the mass mass ratio ratio of ofGO: GO: CS CS was was 3:1, 3:1, 5:1, 5:1, 10:1, 10:1, 15:1, 15:1, and and 20:1. 20:1. The The mixture mixture was was added added into intothe thehydrothermal hydrothermal reaction reaction kettle kettle and andreacted reacted at 120 at °C 120 for◦C 12 for h 12to obtain h to obtain the CS/GO the CS hydrogel./GO hydrogel. After After the CS/GO hydrogel was freeze-dried at 50 C (48 h), the CS/GO aerogel was obtained. Figure1b the CS/GO hydrogel was freeze-dried at −50− °C◦ (48 h), the CS/GO aerogel was obtained. Figure 1b illustratesillustrates the the synthesis synthesis mechanism mechanism of of the the CS CS/GO/GO aerogel. aerogel. 2.5. Preparation of CS/GO Aerogel by Chemical Crosslinking 2.5. Preparation of CS/GO Aerogel by Chemical Crosslinking Firstly, the pH value of GO dispersion was adjusted to 10 with the NaOH solution (1 mol/L), Firstly, the pH value of GO dispersion was adjusted to 10 with the NaOH solution (1mol/L), and and then the GO dispersion was mixed with the chitosan solution (2.5 wt%). After 30 min of ultrasound, then the GO dispersion was mixed with the chitosan solution (2.5 wt%). After 30 min of ultrasound, an appropriate amount of acetic acid was added to the mixture to adjust the pH to about seven. CS/GO an appropriate amount of acetic acid was added to the mixture to adjust the pH to about seven. hydrogel was obtained by adding glutaraldehyde solution and heating the mixture in a water bath at CS/GO hydrogel was obtained by adding glutaraldehyde solution and heating the mixture in a water 95 C for 24 h. Finally, the CS/GO aerogel was obtained by freeze-drying at 50 C for 48 h. bath◦ at 95 °C for 24 h. Finally, the CS/GO aerogel was obtained by freeze-drying− ◦ at −50 °C for 48 h. 2.6. Preparation of CS/GO Aerogel by Sol-Gel Method 2.6. Preparation of CS/GO Aerogel by Sol-Gel Method 0.0255 g GO was added into the 20 mL acetic acid solution (1%, v/v) and ultrasonic was used at room temperature0.0255 g GO forwas 30 added min tointo form the a 20 uniform mL acetic suspension. acid solution Then, (1%, 0.5 gv/ chitosanv) and ultrasonic powder waswas added,used at androom the temperature chitosan was for completely 30 min to form dissolved a uniform by ultrasound suspension. for Then, 1 h. After0.5 g chitosan standing powder for 12 h, was a syringe added, (pinholeand the forchitosan 0.7 mm) was was completely used to dropdissolved 200 mL by ofultrasound the NaOH for solution 1 h. After (3% standingw/v) into for the 12 suspensionh, a syringe (pinhole for 0.7 mm) was used to drop 200 mL of the NaOH solution (3% w/v) into the suspension drop-by-drop (control speed 60 drops/min) to form a beadlike liquid. After standing for 24 h, the

Polymers 2020, 12, 2169 4 of 16 drop-by-drop (control speed 60 drops/min) to form a beadlike liquid. After standing for 24 h, the beads gradually solidified, then washed to neutral with deionized water, filtered and poured into a 250 mL flask. Then, 30 mL of methanol and 1.5 mL glutaraldehyde solution (50%, v/v) was added and stirred at room temperature for 5 h. After filtering the formed beads, they were washed with ethanol and deionized water three times, respectively, and freeze-dried at 50 C for 48 h to obtain the grayish − ◦ brown CS/GO composite aerogel.

2.7. Material Characterizations Fourier transform infrared (FTIR) spectra were recorded on the Thermo Electron Nicolet 6700 spectrometer (Thermo Electron Corporation, Waltham, MA, USA) with the potassium bromide (KBr) pellet technique. The morphology and structure of the products were studied by a scanning electron microscope (SEM, JSM-5510LV, JEOL, Tokyo, Japan). The phase composition of the samples was determined using powder X-ray diffraction (XRD, Bruker D8-Advance, Cu Ka radiation, λ = 0.15418 nm, Bruker, Karlsruhe, Germany) at 40 kV and 20 mA. The ultraviolet-visible spectrometer (PerkinElmer, lambda35 Perkin Elmer, Waltham, MA, USA) was used to record the ultraviolet visible-light diffused reflection spectrums (UV-DRS) of the samples, and BaSO4 was used as the referential sample. A full automatic specific surface area and aperture distribution analyzer (Autosorb IQ, Quantachrome, Boynton Beach, FL, USA) obtained the adsorption and desorption isotherm curve and the corresponding pore size distribution of the sample at the nitrogen (N2) temperature, calculated the specific surface area by baxrett-emmett-teller (BET) method, and calculated the pore size distribution by the berret-joyner-halenda (BJH) model.

2.8. Adsorption Characterization The adsorption activity of the samples was evaluated by measuring the adsorption rate of methyl orange (MO) solution. A certain concentration of methyl orange solution was prepared. Then, 50 mL of the solution, with initial concentrations, were shaken for 5 h at room temperature with 20 mg samples. Further, the pH was adjusted in the range of one to nine using HCl and NaOH solutions. After the speciﬁed time, the solid and liquid were separated immediately. The concentration (Ct) of the centrifuged solution and the initial concentration (C0) of the above solution were monitored immediately using an ultraviolet-visible spectrometer. The absorption rate was calculated as Ct/C0. The adsorption capacity (qt) was calculated by Equation (1).

C Ct q = 0 − V (1) t m

3. Result and Discussion

3.1. The Structure and Adsorption Properties of CS/GO Aerogels Prepared by Three Experimental Methods In this paper, CS/GO aerogels were prepared by hydrothermal, chemical cross-linking and sol-gel methods (a brief description is provided in the following sections), and were denoted as CS/GO, CS/GOc, and CS/GOS, respectively. Physical adsorption and desorption tests of N2, and SEM were used to characterize the structure of the three aerogels. Figure2A shows the scanning electron microscopy of the three aerogels. The pore characteristics of the three aerogels are shown in Table1. Obviously, it can be seen from the morphology that the CS/GO aerogel prepared by the hydrothermal method had more surface folds and more dense pores, which made the specific surface area of the CS/GO aerogel higher than CS/GOc and CS/GOS. Figure2B shows the results of the MO-adsorption performances of CS /GO aerogels prepared by chemical crosslinking, hydrothermal, and sol-gel methods, respectively. The data demonstrate that the MO-adsorption efficiency of the CS/GO aerogel, prepared by the hydrothermal method, reached as far as 96.6%, and was significantly higher than that of the aerogels obtained by the chemical crosslinking method (89.2%) and sol-gel method (59.45%). This was because the high-pressure chemical environment of hydrothermal method [13] causes a series of benefits, including Polymers 2020, 12, 2169 5 of 16

Polymers 2020, 12, x FOR PEER REVIEW 5 of 16 a three-dimensional structure, many holes, a large speciﬁc surface area, as well as superior adsorption for the synthesizedcauses a series CS/ GOof benefits, aerogel. including a three-dimensional structure, many holes, a large specific surface area, as well as superior adsorption for the synthesized CS/GO aerogel.

Figure 2. (A) SEM photographs of aerogels: (a1,a2) CS/GOc, (b1,b2) CS/GOS, and (c1,c2) CS/GO, (B) Figure 2. (A) SEM photographs of aerogels: (a1,a2) CS/GOc, (b1,b2) CS/GOS, and (c1,c2) CS/GO, adsorption performance of CS/GO aerogels prepared by three experimental methods on methyl (B) adsorption performance of CS/GO aerogels prepared by three experimental methods on methyl orange solution. orange solution. Table 1. Hole structure analysis of the aerogels. Table 1. Hole structure analysis of the aerogels. Sample BET (m2/g) Pore Volume (cm3/g) Pore Diameter (nm) Sample CS/GOBETS (m56.9162/g) Pore Volume 0.267 (cm3/g) Pore 15.275 Diameter (nm) CS/GOc 147.49 0.680 13.609 CS/GOS CS/GO56.916 297.431 0.267 0.238 3.057 15.275 CS/GOc 147.49 0.680 13.609 The studyCS/GO of CS/GO 297.431 aerogels prepared by 0.238three experimental methods 3.057 shows that the CS/GO aerogel prepared by the hydrothermal method was superior to the other two in terms of structure and adsorption performance. The following is a study of the properties of the CS/GO aerogel The studyprepared of CSby hydrothermal/GO aerogels method prepared and the bycontrol three factors experimental of the adsorption methods process. shows that the CS/GO aerogel prepared by the hydrothermal method was superior to the other two in terms of structure and 3.2. FTIR Analyses of Various Aerogels adsorption performance. The following is a study of the properties of the CS/GO aerogel prepared by hydrothermal methodTo better understand and the control the molecular factors structures of the adsorptionof prepared aerogels process. of CS, GO, and CS/GO, the FTIR spectra was investigated and is displayed in Figure 3. For the CS aerogel, the peaks at around 1378 cm−1 and 897 cm−1 belong to the bending vibration of the C–H from –CH3 and the bean-ring 3.2. FTIR Analyses of Various Aerogels structure of chitosan, respectively. Similarly, for the GO aerogel, except for the stretching vibration −1 −1 −1 To betterpeak understandat nearly 3452 cm the, the molecular other peaks structures at 1621 cm and of prepared1075 cm were, aerogels respectively, of CS, sourced GO, from and CS/GO, the factors of C=C and C–O in benzene. With regard to the CS/GO aerogel, it included not only the the FTIR spectracommon was characteristics investigated from and the is bean-shaped displayed appearance in Figure3 .of For chitosan the CS at aerogel,892 cm−1 theand peaksC=C on at around 1 1 1378 cm− grapheneand 897 oxide cm− at belong1615 cm−1 to, but the also bending the remaining vibration strong ofbending the C–H vibration from peak –CH from3 and –NH the2 or – bean-ring structure ofNH chitosan,3+ at 1549 cm respectively.−1. This result indicates Similarly, that forthe CS/GO the GO aerogel aerogel, can be except fabricated for through the stretching a chemical vibration interaction between1 chitosan and graphene oxide. 1 1 peak at nearly 3452 cm− , the other peaks at 1621 cm− and 1075 cm− were, respectively, sourced from the factors of C=C and C–O in benzene. With regard to the CS/GO aerogel, it included not only 1 the common characteristics from the bean-shaped appearance of chitosan at 892 cm− and C=C on 1 graphene oxide at 1615 cm− , but also the remaining strong bending vibration peak from –NH2 or 3+ 1 –NH at 1549 cm− . This result indicates that the CS/GO aerogel can be fabricated through a chemical

interactionPolymers between 2020, 12, chitosanx FOR PEER REVIEW and graphene oxide. 6 of 16

FigureFigure 3. FT-IR 3. FT-IR spectra spectra ofof aerogels: (a) (CS,a) CS,(b) GO, (b) and GO, (c) and CS/GO. (c) CS/GO.

3.3. XRD Analysis of Various Aerogels The crystal structure of the CS, GO, and CS/GO aerogels was comparatively illustrated in corresponding XRD patterns, as shown in Figure 4. From the pattern of the CS aerogel, it is obvious that there is a diffraction peak emerging at 20°, which is attributed to the anhydrous crystal structure [14]. As for the GO aerogel, the diffraction peak appeared at 25°, and was associated with the interlayer spacing of 0.36 nm, calculated by the Bragg equation, whereas the peak intensity was not that strong. This results state that most of the oxygen-containing functional groups can be removed after hydrothermal reduction, and the resulting GO aerogels were randomly accumulated and distributed between graphene sheet layers. In addition, the diffraction peak at 20°, belonging to the structural characteristic of CS, disappeared and a new peak appeared at 24° in the pattern of the CS/GO aerogel, indicating that the addition of GO presents a great negative effect on the anhydrous crystal structure of chitosan. According to the Bragg equation, the interlayer spacing of the CS/GO aerogel was calculated as 0.37 nm, which shows less of an increase in contrast to the GO aerogel. This result indicates that the slight interlayer expansions of GO sheets happen due to the insertion of CS, leading to an effective preparation of CS/GO aerogel by way of a grafting modification.

Figure 4. XRD patterns of aerogels: (a) GO, (b) CS/GO, and (c) CS.

3.4. SEM Analyses of Various Aerogels Figure 5 exhibits the SEM images of the CS aerogel, GO aerogel and CS/GO aerogel. As observed, the morphological characteristic of the GO aerogel is a three-dimensional structure, which reflects

Polymers 2020, 12, x FOR PEER REVIEW 6 of 16

Polymers 2020, 12, 2169 6 of 16 Figure 3. FT-IR spectra of aerogels: (a) CS, (b) GO, and (c) CS/GO.

3.3.3.3. XRD Analysis of Various AerogelsAerogels TheThe crystalcrystal structurestructure ofof thethe CS,CS, GO,GO, andand CSCS/GO/GO aerogelsaerogels waswas comparativelycomparatively illustratedillustrated inin correspondingcorresponding XRD XRD patterns, patterns, as as shown shown in in Figure Figure4. From4. From the the pattern pattern of the of CSthe aerogel, CS aerogel, it is obviousit is obvious that therethat there is a di isff araction diffraction peak peak emerging emerging at 20 ◦at, which20°, wh isich attributed is attributed to the to anhydrous the anhydrous crystal crystal structure structure [14]. As[14]. for As the for GO the aerogel, GO aerogel, the diff ractionthe diffraction peak appeared peak appeared at 25◦, and at was25°, associatedand was associated with the interlayer with the spacinginterlayer of spacing 0.36 nm, of calculated0.36 nm, calculated by the Bragg by the equation, Bragg equation, whereas whereas the peak the intensitypeak intensity was notwas that not strong.that strong. This This results results state state that mostthat most of the of oxygen-containing the oxygen-containing functional functional groups groups can be can removed be removed after hydrothermalafter hydrothermal reduction, reduction, and the and resulting the resulting GO aerogels GO wereaerogels randomly were accumulatedrandomly accumulated and distributed and betweendistributed graphene between sheet graphene layers. sheet In addition, layers. In the addition, diffraction the peakdiffraction at 20◦ peak, belonging at 20°, tobelonging the structural to the characteristicstructural characteristic of CS, disappeared of CS, disappeared and a new peak and appeareda new peak at 24 appeared◦ in the pattern at 24° ofin thethe CS pattern/GO aerogel, of the indicatingCS/GO aerogel, that the indicating addition that of GO the presents addition a greatof GO negative presents eff aect great on thenegative anhydrous effect crystal on the structureanhydrous of chitosan.crystal structure According of chitosan. to the Bragg According equation, to the the interlayer Bragg equation, spacing the of the interlayer CS/GO aerogelspacing was of the calculated CS/GO asaerogel 0.37 nm, was which calculated shows as less 0.37 of nm, an increasewhich shows in contrast less of to an the increase GO aerogel. in contrast This resultto the indicatesGO aerogel. that This the slightresult interlayerindicates that expansions the slight of interlayer GO sheets expansio happenns due of toGO the sheets insertion happen of CS,due leading to the insertion to an eff ectiveof CS, preparationleading to an of effective CS/GOaerogel preparation by way of CS/GO of a grafting aerogel modification. by way of a grafting modification.

FigureFigure 4.4. XRDXRD patternspatterns ofof aerogels:aerogels: (a)) GO,GO, ((b)) CSCS/GO,/GO, andand ((cc)) CS.CS.

3.4.3.4. SEM Analyses of Various AerogelsAerogels FigureFigure5 5 exhibits exhibits the the SEM SEM images images of of the the CSCS aerogel,aerogel, GOGO aerogelaerogel andand CSCS/GO/GO aerogel. As observed, thethe morphologicalmorphological characteristiccharacteristic ofof thethe GOGO aerogelaerogel isis aa three-dimensionalthree-dimensional structure,structure, whichwhich reflectsreflects the typical structural nature of graphene’s lamellar folds where the space holes and smooth and compact surfaces simultaneously exist depending on the special structural effects of numerous folds. With respect to the surface morphology of the CS aerogel, it presents a relatively rough surface relating to amorphous dense accumulation, uneven voids, a loose porous structure, and fuzzy edges. For the CS/GO aerogel, the morphological appearance indicates that smaller aperture, narrower distribution, larger specific surface area, and denser holes were formed, which made the adsorption rate of the CS/GO aerogel higher than that of the CS aerogel and GO aerogel. In other words, the CS/GO aerogel prepared by this method had a uniform three-dimensional structure and was reliable for applications in the absorption of MO. Polymers 2020, 12, x FOR PEER REVIEW 7 of 16 the typical structural nature of graphene’s lamellar folds where the space holes and smooth and compact surfaces simultaneously exist depending on the special structural effects of numerous folds. With respect to the surface morphology of the CS aerogel, it presents a relatively rough surface relating to amorphous dense accumulation, uneven voids, a loose porous structure, and fuzzy edges. For the CS/GO aerogel, the morphological appearance indicates that smaller aperture, narrower distribution, larger specific surface area, and denser holes were formed, which made the adsorption rate of the CS/GO aerogel higher than that of the CS aerogel and GO aerogel. In other words, the PolymersCS/GO 2020aerogel, 12, 2169 prepared by this method had a uniform three-dimensional structure and was reliable7 of 16 for applications in the absorption of MO.

Figure 5. SEM photographs of GO aerogel, CS aerogel and CSCS/GO/GO aerogel (5000(5000×).). × 3.5. Physical Adsorption and Desorption Test of N 3.5. Physical Adsorption and Desorption Test of N22 Figure6 6 illustrates illustrates the the BJH BJH pore pore size size distribution distributi curveson curves of the of CS the aerogel, CS aerogel, GO aerogel GO aerogel and CS /andGO CS/GOaerogel, aerogel, and the and corresponding the corresponding porous characteristicsporous characteri arestics presented are presented in Table 2in. ItTable is clear 2. It that is clear the BETthat 2 valuethe BET of value the CS of/GO the aerogelCS/GO reachedaerogel reached 297.431 297.431 m /g, which m2/g, waswhich significantly was significantly higher higher than that than of that the ofGO the aerogel GO aerogel and CS and aerogel. CS aerogel. Meanwhile, Meanwhile, the pore the size pore of size the CSof /theGO CS/GO aerogel aerogel was the was smallest the smallest among amongthe studied the studied aerogels. aerogels. The reaction The re ofaction CS and of GOCS couldand GO not could destroy not the destroy graphene the graphene skeleton structure.skeleton structure.Chitosan wasChitosan grafted was on grafted the surface on the of thesurface graphene, of the reducedgraphene, the reduced reunion the of reunion graphene of layers graphene and layersformed and a more formed dense a more hole, dense which hole, was which conducive was cond to theucive free to di theffusion free diffusion of dye molecules of dye molecules to increase to increaseopportunities opportunities for interaction for interaction with adsorption with adsorp sites ontion the sites adsorbent on the [adsorbent15]. Overall, [15]. the Overall, interaction the interactionbetween CS between and GO contributedCS and GO tocontributed the improvement to the improvement of the specific of surface the specific area of surface the CS/ GOarea aerogel of the CS/GOin order aerogel to enhance in order its adsorption to enhance capacity. its adsorption capacity.

Table 2. Hole structure analysis of the samples.

Sample BET (m2/g) Pore Volume (cm3/g) Pore Diameter (nm) CS 88.993 0.1254 5.64 GO 31.88 0.137 8.287 CS/GO 297.431 0.238 3.057 Polymers 2020, 12, x FOR PEER REVIEW 8 of 16

Polymers 2020, 12, 2169 8 of 16 Polymers 2020, 12, x FOR PEER REVIEW 8 of 16

Figure 6. BJH pore size distribution curves of aerogels: (a) CS, (b) GO, and (c) CS/GO.

Table 2. Hole structure analysis of the samples.

Sample BET (m2/g) Pore Volume (cm3/g) Pore Diameter (nm) CS 88.993 0.1254 5.64 GO 31.88 0.137 8.287 CS/GO 297.431 0.238 3.057 FigureFigure 6. BJH6. BJH pore pore size size distribution distribution curves curves of aerogels:of aerogels: (a) ( CS,a) CS, (b )(b GO,) GO, and and (c) ( CSc) CS/GO./GO. 3.6. Effect of Adsorbent Types 3.6. Effect of Adsorbent Types 50 mL solutions with initialTable concentrations 2. Hole structure (20 mg/L)analysis were of the shaken samples. for 5 h at room temperature with 5020 mLmg solutionsof GO aerogel, withSample initial CS BET aerogel concentrations (m2/g) CS/GO Pore aerogel,Volume (20 mg/ (cmL)respectively. were3/g) shakenPore DiameteFigure for 5 h7Ar (nm) at shows room temperaturethat CS/GO withaerogel 20 mghad of the GO best aerogel, adsorptionCS CS aerogel performance,88.993 CS /GO aerogel, with 0.1254 respectively.an adsorption Figure rate7 ofA 5.64 shows95.3% thatand CSan/ GOadsorption aerogel hadcapacity the best of 47.65 adsorption mg/g GO performance,at 180 min.31.88 The with oxygen-co an adsorption 0.137ntaining rate groups of 95.3% on and 8.287the an GO adsorption aerogel surface capacity can of 47.65absorb mg cationic/g at 180 dyes min. inCS/GO Thewater oxygen-containing due297.431 to electrostatic groups action, 0.238 on so the the GO adso aerogelrption 3.057 surface rate of can such absorb anionic cationic dyes dyesas MO in is water not high—at due to electrostatic only 45.3%. action, CS aerogel, so the adsorptiondue to its small rate of specific such anionic surface dyes area as and MO limited is not high—atadsorption3.6. Effect only sitesof 45.3%.Adsorbent on the CS aerogel,Types aerogel, has due a to low its smalladsorption specific rate surface (56.1%). area Compared and limited to adsorption pure GO aerogel, sites on the aerogel, has a low adsorption rate (56.1%). Compared to pure GO aerogel, CS/GO aerogel prepared CS/GO 50aerogel mL solutions prepared with by initial the hydrothermal concentrations method (20 mg/L) improved were shaken the porosity. for 5 h at Due room to temperaturethe large by the hydrothermal method improved the porosity. Due to the large amount of amino groups on the amountwith 20 of mgamino of GO groups aerogel, on theCS surfaceaerogel ofCS/GO the GS aerogel,/GO aerogel, respectively. the adsorption Figure 7A point shows and that specific CS/GO surface of the GS/GO aerogel, the adsorption point and specific surface area were increased, and the surfaceaerogel area had were the increased,best adsorption and the performance, diffusion mechanism with an adsorption between therate pollutants of 95.3% and and an adsorbents adsorption diffusion mechanism between the pollutants and adsorbents was improved, so the adsorption of the wascapacity improved, of 47.65 so the mg/g adsorption at 180 min. of the The CS/GO oxygen-co aerogelntaining was better groups than on thethe otherGO aerogel two unmodified surface can CS/GO aerogel was better than the other two unmodified adsorbents. adsorbents.absorb cationic dyes in water due to electrostatic action, so the adsorption rate of such anionic dyes as MO is not high—at only 45.3%. CS aerogel, due to its small specific surface area and limited adsorption sites on the aerogel, has a low adsorption rate (56.1%). Compared to pure GO aerogel, CS/GO aerogelA prepared by the hydrothermal method improvedB the porosity. Due to the large amount of amino groups on the surface of the GS/GO aerogel, the adsorption point and specific surface area were increased, and the diffusion mechanism between the pollutants and adsorbents was improved, so the adsorption of the CS/GO aerogel was better than the other two unmodified adsorbents.

A B

Figure 7. ((AA)Effect)Eﬀect of of adsorbent adsorbent types types on on adsorpti adsorptionon properties properties of ofmethyl methyl orange: orange: (a) CS (a) aerogel, CS aerogel, (b) (GOb) GO aerogel, aerogel, (c) (CS/GOc) CS/GO aerogel, aerogel, and and (B ()B effect) eﬀect of of the the raw raw material material ratio ratio on on the the adsorption adsorption performance performance of the CSCS/GO/GO aerogel.

3.7. Effect of Raw Material Ratio on the Adsorption of CS/GO Aerogel Figure7B plots the MO-adsorption e fficiency of the CS/GO aerogels with different mix proportions of GO and CS. In this study, various CS/GO aerogels were prepared with variables of mass ratio of GO to CS at 3:1, 5:1, 10:1, 15:1, and 20:1. For a brief description, as shown in the following sections, they are Figure 7. (A)Effect of adsorbent types on adsorption properties of methyl orange: (a) CS aerogel, (b) denoted as CS/GO , CS/GO , CS/GO , CS/GO , and CS/GO , respectively. As the proportion of GO aerogel, (3c) CS/GO aerogel,5 and10 (B) effect 15of the raw material20 ratio on the adsorption performance of the CS/GO aerogel.

Polymers 2020, 12, x FOR PEER REVIEW 9 of 16

Polymers 2020, 12, 2169 9 of 16 3.7. Effect of Raw Material Ratio on the Adsorption of CS/GO Aerogel Figure 7B plots the MO-adsorption efficiency of the CS/GO aerogels with different mix grapheneproportions oxide inof GO the systemand CS. increased,In this study, the various adsorption CS/GO rate aerogels increased. were prepared It is clear with that variables the adsorption of of CSmass/GO ratio aerogel of GO first to CS increased at 3:1, 5:1, and10:1, then15:1, and decreased 20:1. For as a brief the description, mass ratio as of shown GO/CS in rose.the following The results indicatesections, that they the are adsorption denoted as effi CS/GOciency3, CS/GO of the5, CSCS/GO/GO10 aerogel, CS/GO15 was, and largelyCS/GO20 dependent, respectively. on As the the mass ratioproportion of GO/CS, of which graphene could oxide reach in the a peaksystem value increased, of nearly the adsorption 96.6% when rate theincreased. mass ratioIt is clear was that fixed at the adsorption of CS/GO aerogel first increased and then decreased as the mass ratio of GO/CS rose. around 10:1. This result reflects that the optimal porous microstructure and specific surface area of The results indicate that the adsorption efficiency of the CS/GO aerogel was largely dependent on the the synthesized CS/GO aerogel with the GO/CS mass ratio of 10:1 can be obtained to hold more MO mass ratio of GO/CS, which could reach a peak value of nearly 96.6% when the mass ratio was fixed molecules,at around in comparison 10:1. This result to other reflects mass that ratios.the optimal If the porous mass ratiomicrostructure continued and to bespecific increased surface by area more of than 10:1,the synthesized adsorption CS/GO response aerogel would with present the GO/CS a slight mass decrease ratio of 10:1 that can was be not obtained expected to hold for more dye removals.MO The reasonmolecules, for thisin comparison result may to be other that mass the excessive ratios. If the amounts mass ratio of GO continued will cause to be the increased reduction by ofmore relative concentrationthan 10:1, ofthe functional adsorption groups response on thewould molecular present structurea slight decrease of chitosan that andwas thusnot expected negatively for influencedye the absorptionremovals. The of thereason CS /forGO this aerogel. result may be that the excessive amounts of GO will cause the reduction of relative concentration of functional groups on the molecular structure of chitosan and thus 3.8. Enegativelyffect of pH influence the absorption of the CS/GO aerogel.

The3.8. Effect pH valueof pH was also an important determinant affecting the adsorption of aerogels, which was designed to range from one to nine for the adsorption checking in this study [16]. The 50 mL The pH value was also an important determinant affecting the adsorption of aerogels, which methyl orange solution (20 mg/L) was used as the adsorption object, and the CS/GO aerogel (20 mg) was designed to range from one to nine for the adsorption checking in this study [16].10 The 50 mL was used as the adsorbent at room temperature. Figure8A describes the MO-adsorption e fficiency methyl orange solution (20 mg/L) was used as the adsorption object, and the CS/GO10 aerogel (20 mg) of thewas CS used/GO as aerogel the adsorbent with a at changing room temperature. pH value. Figure It was 8A describes found that the theMO-adsorption whole absorption efficiency of the CS/GOof the aerogel CS/GO consistently aerogel with a decreased changing pH as value. the pH It was value found increased. that the whole However, absorption it is of worthy the CS/GO to note that theaerogel adsorption consistently effi ciencydecreased remained as the pH relatively value increa stablesed. within However, the pHit is rangeworthy of to 1–5 note and that decreased the rapidlyadsorption when the efficiency pH value remained exceeded relatively five. This stable depended within the upon pH range the methyl of 1–5 orangeand decreased having rapidly a kun-type structurewhen in the the pH acidic value condition, exceeded andfive. the This sulfate depended end ofupon its molecule the methyl being orange negatively having a charged, kun-type which couldstructure generate in electrostaticthe acidic condition, adsorption and the with sulfate –NH end3+ ofon its chitosan molecule to being achieve negatively the purpose charged, of which removing could generate electrostatic adsorption with –NH3+ on chitosan to achieve the purpose of removing MO. Under alkaline conditions, methyl orange was negatively charged and was repulsive to the MO. Under alkaline conditions, methyl orange was negatively charged and was repulsive to the oxygen-containing functional groups of chitosan and graphene, causing the adsorption effect to behave oxygen-containing functional groups of chitosan and graphene, causing the adsorption effect to less wellbehave than less under well acidicthan under environments. acidic environments. In a word, In the a word, acidic the chemical acidic chemical environments environments are beneficial are to providebeneficial a high to possibility provide a high for thepossibility prepared for CSthe/ GOprepared aerogel CS/GO to dispose aerogel andto dispose remove and the remove dye inthe water, whiledye its adsorptionin water, while capacity its adsorption can reach thecapacity maximum can reach level the with maximum the adsorption level with effi ciencythe adsorption of 97.2% and the adsorptionefficiency of capacity 97.2% and of the 48.6 adso mgrption/g when capacity the pH of 48.6 value mg/g is one.when the pH value is one.

A B

FigureFigure 8. (A 8.)E (ﬀAect) Effect of pH of value pH onvalue adsorption on adsorption performance performance of CS /ofGO CS/GO aerogel. aerogel. (B)E ﬀ(ectB) ofEffect adsorption of timeadsorption on adsorption time performance on adsorption of theperformance CS/GO aerogel. of the CS/GO aerogel.

3.9. Inﬂuence of Adsorption Time

Figure8B presents the adsorption e fficiency of the CS/GO aerogel with the elapsing time. It is significant that the adsorption rate of the CS/GO aerogel was fast in the early stage, within and around 120 min, and then increased slowly until reaching the adsorption equilibrium. The explanation for this is that the unique porous structure and advantageous that were specific to the surface area of Polymers 2020, 12, x FOR PEER REVIEW 10 of 16

3.9. Influence of Adsorption Time Figure 8B presents the adsorption efficiency of the CS/GO aerogel with the elapsing time. It is significant that the adsorption rate of the CS/GO aerogel was fast in the early stage, within and

Polymersaround2020 120, 12 ,min, 2169 and then increased slowly until reaching the adsorption equilibrium.10 ofThe 16 explanation for this is that the unique porous structure and advantageous that were specific to the surface area of the CS/GO aerogel provided a tremendous and special potential in the absorption of theMO CS molecules,/GO aerogel and provided the absorption a tremendous will finish and when special the potentialinternal space in the of absorption the CS/GO of aerogel MO molecules, is totally andfilled. the absorption will ﬁnish when the internal space of the CS/GO aerogel is totally ﬁlled.

3.10.3.10. DesorptionDesorption AtAt present,present, the the commonly commonly used used regeneration regeneration methods methods of adsorbentof adsorbent include include the solventthe solvent method method [17], heating[17], heating method method [18], electrochemical [18], electrochemical method [method19], oxidation [19], oxidation method [20 method], and more. [20], Inand this more. experiment, In this theexperiment, solvent method the solvent was adoptedmethod was and adopted 1.5 mol/ Land NaOH 1.5 mol/L solution NaOH was solution used for was desorption. used for desorption. AfterAfter the adsorption adsorption experiment, experiment, the the remaining remaining methyl methyl orange orange solution solution was was poured poured out. out.The Theadsorbent adsorbent with withdistilled distilled water water was washed was washed three ti threemes to times remove to remove the methyl the methylorange that orange had that not hadadsorbed not adsorbed firmly on firmly the surface on the of surface the adsorbent. of the adsorbent. Then, the Then, adsorbent the adsorbent was added was to added30 mL toof 30NaOH mL ofsolution NaOH (1.5 solution mol/L) (1.5 and mol placed/L) and in placedan oscillating in an oscillating water bath water for 60 bath min. for During 60 min. this During period, this the period, color theof the color solution of the solution deepened, deepened, indicating indicating that the that dy thee dyemolecules molecules were were gradually gradually desorbing desorbing from from thethe adsorbent.adsorbent. AfterAfter fullfull shock,shock, thethe adsorbentadsorbent waswas removed,removed, washedwashed withwith distilleddistilled waterwater once,once, andand thenthen desorbeddesorbed withwith NaOH NaOH solution solution one–two one–two times times until until the the solution solution became became transparent, transparent, indicating indicating that that the dyethe dye on the on adsorbentthe adsorbent was was basically basically removed. removed. The The remaining remaining NaOH NaOH solution solution was was removed removed from from the desorbedthe desorbed adsorbent adsorbent with distilledwith distilled water (two–threewater (two–t times)hree untiltimes) the until solution the wassolution neutral. was The neutral. desorbed The adsorbentdesorbed adsorbent was dried was to a constantdried to a weight constant at 60weight◦C for at the 60 next°C for adsorption the next adsorption experiment. experiment. TheThe structurestructure ofof thethe CSCS/GO/GO aerogelaerogel afterafter desorptiondesorption (Figure(Figure9 b)9b) had had no no significant significant change change comparedcompared toto thatthat beforebefore adsorptionadsorption (Figure(Figure9 9a).a). FigureFigure9 c9c shows shows the the adsorption adsorption e effectffect ofof thethe secondsecond adsorptionadsorption experimentexperiment afterafter thethe firstfirst desorption.desorption. AfterAfter desorption,desorption, thethe adsorptionadsorption eefficiencyfficiency ofof thethe secondsecond adsorptionadsorption waswas 93.24%,93.24%, asas comparedcompared toto thethe firstfirst adsorptionadsorption eefficiencyfficiency ofof 96.6%,96.6%, andand thethe adsorptionadsorption eeffectffect waswas basicallybasically stable.stable. ThisThis showsshows thatthat thethe CSCS/GO/GO aerogelaerogel cancan beberecycled. recycled.

a b c

FigureFigure 9.9. ((aa)) SEMSEM imageimage ofof CSCS/GO/GO aerogelaerogel beforebefore adsorption.adsorption. ((bb)) SEMSEM imageimage ofof CSCS/GO/GO aerogelaerogel afterafter desorption.desorption (.c ()c The) The e ffeffectect of of readsorption readsorption after after desorption. desorption. 4. Adsorbing Mechanism Analysis of CS/GO Aerogel 4. Adsorbing Mechanism Analysis of CS/GO Aerogel 4.1. Adsorption Kinetics Study 4.1. Adsorption Kinetics Study The adsorption data were fitted based on the quasi-first-order adsorption kinetics model (EquationThe adsorption (2)) and the data quasi-second-order were fitted based adsorption on the kinetics quasi-first-order model (Equation adsorption (3)) [21 kinetics], as shown model in Figure(Equation 10, and(2)) theand relevant the quasi-second- parametersorder are presentedadsorption in kinetics Table3 .model (Equation (3)) [21], as shown in Figure 10, and the relevant parameters are presented in Table 3. Quasi-first-orderTable adsorption 3. Results kinetic of CS/GO model: aerogel adsorption kinetics model fitting.

1 1 1 2 Model k/(mg g min ) qe/(mg g ) R · − · − · − quasi-ﬁrst-order

adsorption 0.01219 30.79 0.96239 kinetics model quasi-secondary adsorption 0.000641 52.6 0.99527 kinetics model Polymers 2020, 12, x FOR PEER REVIEW 11 of 16

−= − ln(qqeet ) ln qkt1 (2) Quasi-secondary adsorption kinetic model: tt=+1 2 (3) qqt kq2 ee

In formula, qe and qt are adsorption equilibrium and adsorption capacity (mg/g) at time t,

Polymersrespectively;2020, 12 t, 2169is the adsorption time (min); and k1 and k2 represent the first-order adsorption kinetics11 of 16 constants (min−1) and the second-order adsorption kinetics constants (g·mg−1·min−1), respectively.

Figure 10. Fitting diagram of quasi-ﬁrst-order adsorption kinetics model (left) and quasi-secondary Figure 10. Fitting diagram of quasi-first-order adsorption kinetics model (left) and quasi-secondary adsorption kinetics model (right). adsorption kinetics model (right). Quasi-ﬁrst-order adsorption kinetic model: Table 3. Results of CS/GO aerogel adsorption kinetics model fitting.

ln(qe qt) = ln qe k −1t −1 −1 2 (2) Model − k/(mg·g− 1 ·min ) qe/(mg·g ) R quasi-first-order adsorption kinetics model 0.01219 30.79 0.96239 Quasi-secondaryquasi-secondary adsorption adsorption kinetic kinetics model: model 0.000641 52.6 0.99527 t 1 t Table 3 shows the comparison of the regression= + coefficients, R2 (quasi-secondary) > R2 (quasi-(3) q k q 2 q first-order), and the maximum adsorption capacityt 2 e calculatede by the quasi-second-order adsorption kineticIn model, formula, whichqe and wasq t52.6are mg/g—close adsorption equilibriumto the maximum and adsorptionadsorption capacitycapacity (mgmeasured/g) at timeby thet, respectively;experiment oft is48.68 the adsorptionmg/g. The result time (min); shows and thatk 1thande quasi-second-orderk2 represent the first-order adsorption adsorption kinetic model kinetics is constantsmore suitable (min to1 describe) and the the second-order adsorption adsorptionprocess that kinetics is controlled constants by the (g activemg 1 minsites 1of), chemisorption respectively. − · − · − [22]. TableMeanwhile,3 shows the the quasi-second-order comparison of kinetic the regression model appears coe ffi tocients, be moreR2 reliable(quasi-secondary) and real to reflect> R2 (quasi-first-order),the adsorption capacity and of the the maximum CS/GO aerogel adsorption on the capacityMO molecules. calculated by the quasi-second-order adsorption kinetic model, which was 52.6 mg/g—close to the maximum adsorption capacity measured by4.2. the Isothermal experiment Adsorption of 48.68 Model mg /g. The result shows that the quasi-second-order adsorption kinetic modelLangmuir is more suitable[23] and toFreundlich describe the[24] adsorptionadsorption processisotherm that equations is controlled were byused the to active evaluate sites the of chemisorptionadsorption system [22]. and Meanwhile, adsorption the behavior. quasi-second-order kinetic model appears to be more reliable and real toThe reflect linear the expression adsorption of capacityLangmu ofir equation the CS/GO is aerogelas follows: on the MO molecules.

4.2. Isothermal Adsorption Model 𝐶 𝐶 1 = + (4) Langmuir [23] and Freundlich [24𝑞] adsorption𝑞 isotherm𝑘𝑞 equations were used to evaluate the adsorptionwhere Ce—equilibrium system and adsorptionmass concentration behavior. of the dye solution/(mg/L) qe—equilibriumThe linear expression adsorption of capacity/(mg/g) Langmuir equation is as follows: qmax—monolayer saturated adsorption capacity/(mg/g) Ce Ce 1 kL—Langmuir constant /L/mg related to adsorption= +capacity and adsorption rate (4) q q k q The basic characteristics of Langmuire adsorptionmax isothermsL max can be represented by dimensionless equilibrium parameters (RL): where Ce—equilibrium mass concentration of the dye solution/(mg/L)

qe—equilibrium adsorption capacity/(mg/g) qmax—monolayer saturated adsorption capacity/(mg/g) kL—Langmuir constant /L/mg related to adsorption capacity and adsorption rate The basic characteristics of Langmuir adsorption isotherms can be represented by dimensionless equilibrium parameters (RL): 1 RL = (5) kLC0 + 1 Polymers 2020, 12, x FOR PEER REVIEW 12 of 16

1 Polymers 2020, 12, 2169 𝑅 = 12 of(5) 16 𝑘𝐶 +1 The Freundlich equation describes the adsorption process on an uneven surface. The equation is as Thefollows: Freundlich equation describes the adsorption process on an uneven surface. The equation is as follows: 11 lnlnq 𝑞 == lnln𝑘k ++ ln 𝐶C (6)(6) e F n𝑛 e wherewhere kkFF and nn areare FreundlichFreundlich constantsconstants relatedrelated toto adsorptionadsorption capacitycapacity andand adsorptionadsorption strength, respectively.respectively. The The larger larger kkFF andand nn valuesvalues represent represent the the adsorbent’s adsorbent’s better better adsorption adsorption performance. performance. If If(1/ (1n/)n <) <1, 1,the the adsorption adsorption conforms conforms to to the the normal normal Langmuir Langmuir isotherm isotherm model. If (1(1//n)) >> 1, collaborative adsorptionadsorption occursoccurs [[25].25]. LangmuirLangmuir andand FreundlichFreundlich adsorptionadsorption isothermisotherm equationsequations werewere usedused toto analyzeanalyze thethe adsorptionadsorption behaviorbehavior ofof CSCS/GO/GO aerogelaerogel toto MO,MO, asas shownshown inin FigureFigure 11 11::

FigureFigure 11.11. Langmuir adsorption isotherm ((leftleft)) andand FreundlichFreundlich adsorptionadsorption isothermisotherm ((rightright).).

2 TheThe datadata inin TableTable4 4show show that that the the correlation correlation coe coefficientﬃcient of of the the Langmuir Langmuir equation equation (R (R=20.9932) = 0.9932) is 2 higheris higher than than that that of of the the Freundlich Freundlich equation equation (R (R=2 =0.81199), 0.81199), so so the the former formermodel model isis moremore suitablesuitable toto reﬂectreflect thethe adsorption.adsorption. InIn addition,addition, TableTable5 5 presents presents the theR RLL valuevalue ofof equilibriumequilibrium parameterparameter atat variousvarious concentrations,concentrations, whichwhich can can be be used used to to judge judge the th naturee nature of of the the adsorption adsorption process process [26 [26].]. The TheRL RvaluesL values at variousat various concentrations concentrations are allare less all less than than one, one, indicating indicating that thethat adsorption the adsorption of the of CS the/GO CS/GO aerogels aerogels under theunder designed the designed concentrations concentrations can easily can happen. easily Ithappen. is also worth It is also noting worth that thenoting RL valuethat the decreased RL value as thedecreased concentration as the concentration rose, thus demonstrating rose, thus demonstrat that the chemicaling that the adsorption chemical wasadsorption more likely was more to occur likely at higherto occur concentrations at higher concentrations of the MOsolution. of the MO Since soluti theon. adsorption Since the ofadsorption MO is the of electrostatic MO is the electrostatic interaction betweeninteraction positively between charged positively amino charged groups amino on the grou CS chain,ps on the the adsorption CS chain, of the dye adsorption molecules forof dye the aerogelmolecules is the for monolayer the aerogel and is the the active monolayer sites are homogeneous.and the active Moreover, sites are accordinghomogeneous. to the FreundlichMoreover, equation,according its to constantthe Freundlich of 1/n = equation,0.09478 was its constant less than of one, 1/n which = 0.09478 indicates was less that than the adsorptionone, which conformsindicates tothat the the normal adsorption Langmuir conforms isotherm to the model. normal Langmuir isotherm model.

TableTable 4.4. IsothermalIsothermal adsorptionadsorption parametersparameters ofof LangmuirLangmuir andand Freundlich.Freundlich.

ModelModel LangmuirLangmuir Freundlich Freundlich q qmax k kL t/ Ct/℃ max L RR22 kkF (mg/g(mg/g (L/mg) (L/mg)1/n1)/n ) n n R2R 2 ◦ (mg(mg·gg 1) −1) (L(L/mg)/mg) F · − 25 38.67 5.002 0.9932 28.84 10.55 0.81199 25 38.67 5.002 0.9932 28.84 10.55 0.81199

Table 5. RL value of methyl orange adsorbed by CS/GO aerogel. Table 5. RL value of methyl orange adsorbed by CS/GO aerogel. Concentration (mg/L) 10 20 40 50 Concentration (mg/L) 10 20 40 50 RL 0.0196 0.0099 0.00498 0.00398 R L 0.0196 0.0099 0.00498 0.00398

Polymers 2020, 12, 2169 13 of 16

4.3. Adsorption Mechanism The adsorption mechanism of the graphene-like gels has been extensively studied. Zhang et al. [27] prepared GO/CS composites by chemical cross-linking, believing that materials with high porosity can provide enough space to absorb substances, facilitating material exchange. Singhi et al. [28] prepared chitosan-reduced graphene oxide(RGO) composite hydrogel for adsorption of organic dyes, suggesting that the initial dye concentration provides the necessary driving force to overcome the mass transfer resistance of the dyes at the active site of the adsorbent. Ruomeng, Yu et al. [29] prepared multifunctional GO/CS aerogel microspheres, which have good adsorption properties for heavy metal ions and dyes. It is believed that the -NH3+ group on the composite gel becomes the active site and is easily bound to the adsorption object through electrostatic interaction [30]. Yang et al. [31] constructed a novel 3D RGO aerogel. It is considered that the π–π and electrostatic interactions between aerogel and dye molecules and the synergistic effect of adsorbent polyhedron interface are the main driving factors of the ultra-high adsorption performance. Both the GO aerogel and CS aerogel have the potential to be good adsorbents, but it is difficult to obtain satisfactory adsorption effects for the GO aerogel obtained by high temperature hydrothermal reaction, due to the mutual attraction and aggregation of the lamellar from the π–π action, and chitosan is easy to dissolve under acidic conditions, so it is not a practical adsorbent. Aromatic organic dyes, such as MO, tend to react with π–π action, so the structures, such as non-oxidized zone and aromatic, clustering on the GO, have a high affinity for them. Through the modification of GO surface graft CS, the three-dimensional network structure of graphene remains relatively intact, the increase of layer spacing between the GO lamellar layers, due to the insertion of CS, except that the lamellar spacing increases and the volume of holes in the middle increases, so that it can accommodate more pollutants. 3 However, MO discharges negatively charged D-SO − in the aqueous phase. Due to the presence of abundant cationic groups on the surface of chitosan, it is easy to attract MO through electrostatic action, thus enhancing its adsorption performance. The CS/GO aerogel is a three-dimensional porous structure that can be prepared through a simple and controllable hydrothermal method. Compared to other literatures (Table6), it shows a larger specific surface area and a better adsorption effect.

Table 6. Comparison of adsorption of diﬀerent adsorbents for MO [32–38].

Pore Diameter Experimental Adsorbents q (mg/g) BET (m2/g) References e (nm) Methods Chemical GP55 30 453.4 3–4 [32] crosslinking RGO-MMT 32.5 43.17 123.28 Sol-gel method [33] Chemical M-CS/γ-Fe O /MWCNTs 31.44 — — [34] 2 3 crosslinking CSGO 32.73 — — Sol-gel method [35] Template L-Cys-GA 75 154 3.7 [36] method Chemical GCAs 20 68.11 — [37] crosslinking γ-Fe O /chitosan solution casting 2 3 29.41 — — [38] composite ﬁlms method Hydrothermal CS/GO 48.6 297.431 3 this study method

As mentioned above, the prepared CS/GO aerogel showed a good adsorption performance on MO. In Figure 12, compound gel on the GO has not been oxidized in the conjugate area shown in the π–π conjugate active adsorption sites, CS chain containing amino groups, with the H+ form –NH3+ in the water and positively charged, as positive active adsorption sites. MO are anionic dyed and the surface is negatively charged, based on the CS/GO composite aerogel on the adsorption of CS molecular chains Polymers 2020, 12, x FOR PEER REVIEW 14 of 16 Polymers 2020, 12, 2169 14 of 16 the water and positively charged, as positive active adsorption sites. MO are anionic dyed and the surface is negatively charged, based on the CS/GO composite aerogel on the adsorption of CS ofmolecular amino and chains dye of molecules amino and that dye electrostatically molecules that interact. electrostatically Further, the interact. conjugate Further, eﬀect the between conjugate the unoxidizedeffect between conjugate the unoxidized region of conjugate the GO and region the of dye the molecules GO and the may dye also molecules contribute may to also the increasecontribute in adsorptionto the increase capacity. in adsorption Therefore, capacity. the combination Therefor ofe, electrostaticthe combination action of and electrostaticπ–π conjugate action action and is π the–π adsorptionconjugate action mechanism is the adsorption of the CS/GO mech compositeanism of aerogel. the CS/GO composite aerogel.

Figure 12. Schematic diagram of the adsorption of methyl orange by the CSCS/GO/GO aerogel.

5. Conclusions This studystudy mainly mainly aimed aimed to prepareto prepare a high-absorption a high-absorption CS/GO CS/GO aerogel aerogel for application for application in removing in methylremoving orange. methyl Based orange. on Based this, CS, on GO,this, andCS, CSGO,/GO and aerogels CS/GO were,aerogels respectively, were, respectively, prepared prepared through variousthrough methods various inmethods order to in obtain order comparativeto obtain compar investigationsative investigations of their microstructures of their microstructures and adsorption and properties.adsorption Finally,properties. some Finally, valuable some and valuable interesting and findingsinteresting were findings reached, were as below:reached, as below: (1)(1) TheThe hydrothermal hydrothermal method is an acceptable and applicable method to prepare the CS/GO CS/GO aerogel with a high adsorptionadsorption raterate (96.6%),(96.6%), which which proved proved to to be be better better than than the the other other approaches, approaches, such such as asthe the sol-gel sol-gel method method (59.45%) (59.45%) and and chemical chemical crosslinking crosslinking method method (89.2%); (89.2%); (2)(2) TheThe microstructural microstructural characterizations indicate th thatat the CS/GO CS/GO aerogel has a combined structure provided by CS and GO aerogels, which has a larger specificspecific surfacesurface areaarea (297.431(297.431 mm22//g),g), more micro-holes,micro-holes, and higher absorption volume; (3)(3) TheThe experimental data ofof thethe CSCS/GO/GO aerogel aerogel were were well well fitted fitted by by the the Langmuir Langmuir isotherm isotherm model model in inthe the case case of of MO. MO. The The quasi-secondary quasi-secondary adsorption adsorption kinetic kinetic model model provided provided a better a better correlation correlation of ofthe the adsorption adsorption data data than thethan quasi-first the quasi-first adsorption adsorption kinetic model,kinetic implying model, thatimplying the adsorption that the adsorptionprocess is controlled process is bycontrolled the active by site the of active chemisorption; site of chemisorption; (4)(4) TheThe CS/GO CS/GO aerogel can enhance the adsorption capa capacitycity of MO and it can be well used for the adsorptionadsorption of MO in a larger pH range (pH = 1~9);1~9); and and (5)(5) CSGOCSGO is a a promising adsorbent for the adsorption of MO in practice. Despite all this, this study mainlymainly focused focused on on the adsorption properties of of CSGO for MO, and not for other sources of contaminants.contaminants. Therefore, Therefore, future future research research should should be be more more concerned with cationic dyes andand heavyheavy metals. metals.

Author Contributions: Conceptualization, W.Z. and X.J.; software, W.Z.; formal analysis, W.Z., X.J. and F.Y.; Author Contributions: Conceptualization, W.Z. and X.J.; software, W.Z.; formal analysis, W.Z., X.J. and F.Y.; investigation, W.Z. and X.J.; data curation, W.Z. and F.L.; writing—original draft preparation, W.Z.; writing— investigation, W.Z. and X.J.; data curation, W.Z. and F.L.; writing—original draft preparation, W.Z.; writing—review andreview editing, and X.J.,editing, F.Y. andX.J., C.Y.F.Y. All and authors C.Y. All have authors read and ha agreedve read to and the publishedagreed to versionthe published of the manuscript. version of the manuscript. Funding: The authors thank the supports of the National Natural Science Foundation of China (51273154) and the NaturalFunding: Science The authors Foundation thank of the Hubei supports Province of the (2017CFB289). National Natural Science Foundation of China (51273154) and Conﬂictsthe Natural of Science Interest: FoundationThe authors ofdeclare Hubei Province no conﬂict (2017CFB289). of interest. Conflicts of Interest: The authors declare no conflict of interest.

Polymers 2020, 12, 2169 15 of 16

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