International Journal of Biological Macromolecules 83 (2016) 133–141

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Sodium alginate/graphene oxide aerogel with enhanced strength–toughness and its heavy metal adsorption study Chenlu Jiao a , Jiaqing Xiong a , Jin Tao a , Sijun Xu b , Desuo Zhang a , Hong Lin a , Yuyue Chen a,∗ a b

College of Textile and Clothing Engineering, Soochow University, Suzhou, Jiangsu 215123, PR China Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan

a r t i c l e

i n f o

Article history: Received 17 August 2015 Received in revised form 16 November 2015 Accepted 23 November 2015 Available online 2 December 2015 Keywords: Sodium alginate Graphene oxide Aerogel Strength Toughness Adsorption

a b s t r a c t Ordered porous sodium alginate/graphene oxide (SAGO) aerogel was fabricated by in situ crosslinking and freeze-drying method. GO, as reinforcing filler, can be easily incorporated with SA matrix by self-assembly via hydrogen bonding interaction. Compared with pure SA aerogel, the as-prepared SAGO exhibited excellent mechanical strength and elasticity, and the compression strength of SAGO can reach up to 324 kPa and remain 249 kPa after five compression cycles when 4 wt% GO was added, which were considered significant improvements. SEM result presents that the addition of GO obviously improves the porous structures of aerogel, which is beneficial for the enhancement of strength–toughness and adsorbability. As a consequence, the adsorption process of SAGO is better described by pseudo-second-order kinetic model and Langmuir isotherm, with maximum monolayer adsorption capacities of 98.0 mg/g for Cu2+ and 267.4 mg/g for Pb2+ , which are extremely high adsorption capacities for metal ions and show far more promise for application in sewage treatment. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Generally, for most ionic polysaccharides, the ability to bind divalent cations and form gels is the key to their biological functions and technological applications [1]. As natural polysaccharide derived from brown sea algae, sodium alginate (SA) is a linear polyanionic copolymer composed of (1–4)-linked ␤-d-mannuronic acid (M) and ␣-l-guluronic acid (G) residues [2,3]. The gelling of SA is mainly achieved by the exchange of Na+ from the G residues with the divalent cations (such as Ca2+ ). The divalent cations bind to different chains of G blocks to form a structure like an “egg box” [4,5], resulting in a three-dimensional network between the crosslinking of different chains. Because of its reversible solubility, SA can be fabricated in various forms such as films, fibers, beads and aerogels [6–11]. Recently, there is a growing interest in using biopolymers for aerogel production, which dues to that the resulting aerogels reveal both specific inheritable functions of the initial biopolymer and distinctive features of aerogels (open porous structure with high specific surface and pore volume) [12]. The synergy of properties

∗ Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.ijbiomac.2015.11.061 0141-8130/© 2015 Elsevier B.V. All rights reserved.

has prompted to view biopolymer aerogels as promising candidates for versatile applications. SA aerogel has a great many of favorable properties such as hydrophilicity, biocompatibility, biodegradability, strong ion-exchange and gel-forming abilities, holding great promises for tissue engineering [13–15], drug delivery [16–18], sewage treatment [19,20], thermal insulation [12] and as starting materials for carbon aerogels [21]. Apart from above advantages, pure SA aerogels still display some structural unsatisfactory properties in weak mechanical strength, structural nonuniformity and fragile collapse [22], which will limit their applications in many fields. To handle this problem, an innovative technology that has gained attention is the addition of reinforcing fillers, which has been considered to be an effective method for improving the mechanical performance and toughness of aerogels. Among popular fillers, graphene oxide (GO) exhibits great potential due to its outstanding mechanical properties, high binding potential, high aspect ratio, excellent flexibility and superior processability. GO sheets can be easily produced by thermal oxidation as suggested by Hummers. This procedure introduces abundant oxygen-containing functional groups (hydroxyl, carboxyl, epoxy and ketone groups) [23,24], which facilitate the interfacial interaction between GO sheets and hydrophilic matrix via hydrogen bond, ionic bond and covalent bond. So GO sheets have been increasingly proved to be

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ideal reinforcing fillers for composites [25–27]. However, another challenge still exists, namely that the gelation process of SA is so quick that the formation time of several seconds often results in brittle structure and heterogeneity of pore size [6,22]. Based on the consideration, the SA/GO (SAGO) aerogel possessing homogeneous porous structure with high mechanical strength and favorable resilience was obtained via in situ crosslinking induced by d-glucono-␦-lactone (GDL)-driving-Ca2+ -release process in the presence of GO, and the unidirectional homogeneous porous structure was achieved by freeze-drying treatment. And its surface morphology, mechanical property and adsorption capacity for metal ions were investigated.

Japan). X-ray diffraction (XRD) studies were performed on an X-ray diffractometer (Bruker AXS, D8 Advance, Germany). Fourier Transform Infrared spectroscopy (FT-IR, Nicolet 5700, Thermo Nicolet, USA) and X-ray photoelectron spectroscopy (XPS, Axis Ultra HAS, Shimadzu/Kratos, Japan) were recorded to confirm the chemical and morphological characteristics. Inductively coupled plasma atomic emission spectroscopy (ICP-AES, iCAP 6000 DUO, Thermo Scientific, USA) was used to measure the concentrations of metal ions. 2.5. Porosity of SAGO aerogel

2. Experimental

The porosity of the porous SAGO monoliths was calculated according to the following equation:

2.1. Materials

Porosity (%) =

SA (medium viscosity, M = 250,000 g/mol) was purchased from Sigma–Aldrich Shanghai Trading Co., Ltd (Shanghai, China) and it was used without further purification. Graphite, CaCO3 and dglucono-␦-lactone (GDL) were obtained from Sinopharm Chemical Reagent Co., Ltd (Suzhou, China). Cu(NO3 )2 ·3H2 O and Pb(NO3 )2 served as the sources of Cu2+ and Pb2+ and were acquired from Meryer Chemical Technology Co., Ltd (Shanghai, China).

where Vt (cm3 ) is the total volume of SAGO monoliths, Va (cm3 ) is the actual volume of the material, Wt (g) is the mass of the monoliths, and  (g/cm3 ) represents the density of the material. Each sample was measured in triplicate and the average value was calculated.

2.2. Preparation of GO GO was prepared from purified natural graphite using a modified Hummers’ method [28,29]. 46 mL mixed acid of concentrated H2 SO4 and H3 PO4 (9:1 in volume ratio) was put into a three-necked flask containing 2 g graphite. After magnetic stirring for 0.5 h in an ice bath, 12 g KMnO4 was incorporated and kept for 2 h. Subsequently, the temperature was increased to 35 ◦ C and maintained for 0.5 h, followed by adding excess deionized water and the temperature was increased to 98 ◦ C. Finally, the incorporation of 30% H2 O2 resulted in a bright yellow colored product. The product was filtered and washed for several times with 3% HCl solution and abundant deionized water until the solution became nearly neutral. The resulting filter cake was dried in vacuo and re-dispersed in water with ultrasonic treatment to obtain suspensions with different GO contents. 2.3. Fabrication of SAGO aerogels In a typical synthesis, 2 g SA was added to 100 mL deionized water and stirred vigorously for 3 h to obtain uniform SA solutions, and a suspension with 80 mg GO was added and kept stirring to obtain hybrid sol. Then a certain amount of CaCO3 was allowed to disperse absolutely in the hybrid sol, followed by adding GDL. Here, CaCO3 and GDL were used as a source of calcium ions to initiate gelation [30–32]. Molar ratio of CaCO3 to GDL was set to 0.5 in order to achieve an approximately neutral condition. The mixture was casted into molds, frozen at −50 ◦ C and freeze-dried for 48 h under vacuum (less than 10 Pa), then monolithic SAGO aerogel with GO content of 4 wt% was obtained (abbreviated as SAGO-4). Similarly, pure SA, SAGO-1, SAGO-2, SAGO-3 and SAGO-5 aerogels were prepared according to the same procedure. 2.4. Characterization The morphology analysis of GO was conducted on Atomic Force Microscopy (AFM, Dimension Icon, Bruker, Germany). The compressive strength was conducted via a universal testing machine (Instron-3365, Instron, USA) at a compressing rate of 1 mm/min. The microstructure of SAGO aerogel was studied using field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi,

Vt − Va Vt − Wt / × 100 = × 100 Vt Vt

(1)

2.6. Adsorption capacity study for Cu2+ and Pb2+ Pure SA and SAGO-4 aerogels were soaked in metal ion solutions with initial concentration of 500 mg/L at 30 ◦ C and concentrations of metal ions were measured at various time. The initial pH values were selected as the experimental values, which were 5.0 and 5.5 for Cu2+ and Pb2+ solutions, respectively. Adsorption capac−Ct ity was calculated by the following equation:(2)Qt = Com V where Qt (mg/g) and Ct (mg/L) represent the adsorption capacity and concentration of solution at a given time, Co (mg/L) is the initial concentration of metal ion solution, V (L) denotes the volume of solution, and m (g) is the mass of aerogel. 3. Results and discussion 3.1. Characterization of GO Fig. 1 shows the photos and AFM image of GO suspensions that were used to prepare the monolithic SAGO aerogel. The exfoliated GO can be readily dispersed in deionized water with mild ultrasonic treatment and formed transparent suspensions that can maintain for several months. AFM image demonstrates that the GO sheets consist of one or several layers with each thickness of ca. 1.2 nm. These well dispersed GO suspensions are very useful for fabricating monolithic aerogels with enhanced mechanical strength and structural recoverability. 3.2. Mechanical properties of aerogels Mechanical property is the key to outstanding applicability and recyclability of aerogels. In order to improve the strength of SA-based aerogels, GO was selected as the rigid building blocks. Combined with soft and rigid nature of SA and GO, the asprepared hybrid aerogels exhibited enhanced flexibility. This can be easily proved by the comparative study of SAGO and pure SA aerogels. As shown in the inset of Fig. 2(b), a preliminary comparative study of SAGO and pure SA aerogels was conducted to evaluate the improvement. As predicted, the pure SA aerogel shown permanent deformation and collapse at strain of 60% [33], while the SAGO aerogel rebounded to almost the original height immediately after removing the pressure, which is well consistent with

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Fig. 1. (a) Photos of GO suspensions (0.02% and 0.20%). (b) AFM image of GO sheets with height profiles.

Fig. 2. Compressive strength of (a) SAGO aerogels with various GO concentrations and (b) SAGO-4 aerogel with increasing compressive cycles. (c) Typical compressive stress–strain curves of SAGO aerogels with different GO concentrations.

the stress–strain curves in Fig. 2(c). The result suggests the excellent mechanical strength and elasticity of SAGO aerogel and GO can effectively enhance their strength. Notably, when 4 wt% GO was added, the compressive strength increased gradually from 233 to 324 kPa (Fig. 2(a)) and stop rising when continue increasing the GO content, indicating the dose dependent. Additionally, Fig. 2(b) showed the compressive strength decreased with increasing compressive cycle, but still remained 249 kPa at the fifth compression of SAGO-4, which is slightly higher than the initial strength (233 kPa) of pure SA aerogel.

3.3. SEM characterization of aerogels The excellent mechanical strength and elasticity are closely related to morphological structure. The structural changes of aerogels were studied using scanning electron microscopy as presented in Fig. 3, both of pure SA and SAGO aerogels have local unidirectional porous structures which facilitate the diffusion of adsorbates. The width of pores ranges from 50 to 100 ␮m and becomes uniform and homogeneous with introduction of GO. Fig. 3(a) and (b) present the pure SA aerogel has a tubular unidirectional porous structure

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Fig. 3. SEM photos of (a and b) pure SA aerogel and (c and d) SAGO aerogel with 4 wt% GO. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 4. Porosity of SAGO aerogels with different GO concentrations.

with smooth surface shown in the inset of Fig. 3(b). While Fig. 3(c) and (d) exhibit that the SAGO aerogel possesses a platelike unidirectional porous structure and wrinkly surface (like that in the inset of Fig. 3(d)), which is the typical morphology of GO sheets, indicating GO sheets are uniformly distributed in SA matrix. Additionally, numerous “riband bridges” (red dashed ellipse in Fig. 3(d)) protrude from the pore walls when GO sheets are added, and eventually, these “riband bridges” connect adjacent layers and divide long pores into small parts. Furthermore, the relationship between the porosity and GO concentration is shown in Fig. 4. The results indicate that the porosity is not dependent on GO concentration and is in the range of 90.7–91.8%, which may be the concentration of GO is too low to have a measurable effect [27]. Anyway, all the SAGO aerogels have high porosity and interconnectivity that facilitate adsorption of metal ions.

The assembly procedure and possible mechanism are shown in Scheme 1. The unique architectural feature is believed to be due to the special structure and excellent toughness of GO, and the amphipathic GO sheets are the essential components for the formation of SAGO aerogels. Rather than described as hydrophilic sheets in previous reports, recent researches have proved that GO sheets are actually amphiphilic with hydrophilic edges and largely hydrophobic basal planes, because the oxygen-containing groups exist at the fringe and the polyaromatic rings dominate in the central plane [34,35]. So when the rigid loadbearing units of GO sheets were mixed with flexible loadbearing units of SA chains, an effective hydrophilic assembly may happen driven by the hydrophilic edges of GO sheets and active SA chains via hydrogen bond. As a result, edges of some GO sheets would bridge and interact with adjacent SA chains, and GO sheets were immobilized in SA polymer matrix accompanied with in situ crosslinking process. With the sublimation of water during the freeze-drying process, the left GO sheets formed numerous “riband bridges”. It is believed that the excellent mechanical strength and elasticity of SAGO aerogel can also be partly attributed to this particular “riband bridges” structure. Under the same compressive strain, more pressure can be loaded owing to the “riband bridges” which acts like “spring plates” to resist deformation, and rebound rapidly once the load is removed [36]. Furthermore, the as-formed interconnected porous structure by the combination of rigid GO sheets and flexible SA chains can promote the load transfer and improve the strength–toughness of aerogel.

3.4. XRD, FT-IR and XPS analyses of aerogels The XRD patterns of GO, SA and SAGO aerogel are shown in Fig. 5(a). The diffraction peak of GO appears at 11.0◦ , indicating that the interlayer distance of GO is ca. 0.80 nm which is obviously larger than that of graphite (ca. 0.34 nm). Broad peak of SA at 13.6◦ shows the generally amorphous state of SA [37]. It is noted that no GO peak is observed in the SAGO aerogel, suggesting addition of a small quantity of GO has no obvious impact on the

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Scheme 1. Schematic illustration of fabrication process and formation mechanism of SAGO aerogel.

crystallinity. In addition, it can be obtained that the GO sheets have been well exfoliated and evenly dispersed in SA polymer matrix, and intermolecular interactions can improve the uniformity of components and provide good miscibility to prepare desired aerogels [38]. In order to explore the interactions between components, FT-IR analysis was performed and result is shown in Fig. 5(b). The FT-IR spectra of SA and GO are similar to previous reports [39,40]. The bands of SA at 3427 and 1617 cm−1 are attributed to asymmetric stretching vibrations of OH and COO− . For GO, it presents typical absorption bands at 3395, 1725 and 1621 cm−1 , which can be assigned to stretching vibrations of OH, COOH and C C in the sp2 carbon skeletal network, respectively. For the SAGO aerogel, the peak of C H asymmetric stretching vibration at 2927 cm−1 is obviously weak because the “egg-box” structures formed by SA macromolecules and Ca2+ ions limit the C H stretching [1,26]. However, the signal at 1024 cm−1 that represents the C O C stretching is considerably enhanced in the spectrum of Ca2+ crosslinked SAGO aerogel. Furthermore, the downshifted bands at 3379 and 1614 cm−1 suggest GO sheets interact with SA chains through intermolecular hydrogen bonds [22], which is in agreement with the interaction mechanism. XPS was conducted to obtain more detailed information about chemical structure of SAGO aerogel. The presence of elements C, O, Ca and Na in SAGO aerogel is confirmed by the photoelectron lines of the wide-scan XPS spectrum in Fig. 5(c), and the incorporation of Ca2+ caused the crosslinking with COO− is displayed in Ca2p3/2 (347.6 eV) of Fig. 5(d). The C1s XPS spectrum of SA (Fig. 5(e)) present three peaks at binding energies of 284.6, 286.5 and 287.9 eV, which can be assigned to the carbon atom in the forms of C C/C C, C O and O C O. But the C1s spectrum of SAGO aerogel (Fig. 5(f)) can be deconvoluted into four main bands: C C/C C (284.6 eV), C O (286.7 eV), C O (287.9 eV) and COOH (288.9 eV), which results from the incorporation of GO with abundant oxygen-containing groups and the drift caused by hydrogen bonding between GO and SA. What is more importantly is, when GO sheets were introduced into the SA polymer matrix, strength of SAGO aerogel was enhanced

owing to the hydrogen bonding interactions between the of GO and OH of SA.

COOH

3.5. Adsorption kinetics Kinetic studies provide the adsorption mechanism of metal ions onto SAGO, which are necessary to depict the adsorption rate and control the residual time of the whole adsorption process. The adsorption behaviors of pure SA and SAGO aerogels for Cu2+ and Pb2+ were shown in Fig. 6(a). Obviously, the adsorption rates were high at initial adsorption period, which might be due to the interconnected porous structures and plentiful vacant sites of aerogels. The equilibriums were reached within 180 min for Cu2+ and 240 min for Pb2+ . Compared with pure SA aerogel, 30.9 mg/g and 60 mg/g improvement of adsorption capacities were achieved and reached up to 81.5 and 204.4 mg/g, respectively, for Cu2+ and Pb2+ when 4 wt% GO was incorporated. This improved adsorption capacities can be attributed to the hydroxyl and carboxyl groups located at edges of GO sheets are introduced into aerogel, which makes more attachment sites exposed to accommodate more adsorbates, according to Zhang et al. and Khasbaatar et al. [27,41]. On the other hand, the “riband bridges” structure and increase of specific surface area contribute to the enhancement of adsorption capacity as well [42]. Adsorption kinetics data were analyzed using Lagergren pseudo-first-order model in Fig. 6(b) and pseudo-second-order model in Fig. 6(c), which were given as follows: Pseudo-first-order model

ln(Q1e − Qt ) = lnQ1e − k1 t

Pseudo-second-order model

t t 1 = + 2 Qt Q2e k2 Q2e

(3) (4)

where Q1e (mg/g) and Q2e (mg/g) represent the calculated adsorption capacity of SAGO at equilibrium, Qt is the adsorption amount at t (min), k1 (min−1 ) and k2 [g/(mg min)] are the rate constants of pseudo-first-order and pseudo-second-order kinetics equations, respectively.

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Fig. 5. (a) XRD patterns of SA, GO and SAGO-4 aerogel; (b) FT-IR spectra of SA, GO and SAGO-4 aerogel; (c) XPS spectra of SA and SAGO-4 aerogel; (d) core-level Ca2p of SAGO-4 aerogel; (e) core-level C1s of SA and (f) core-level C1s of SAGO-4 aerogel. Table 1 Kinetic parameters and experimental adsorption capacities for Cu2+ and Pb2+ by SAGO aerogel. Adsorbates

2+

Cu Pb2+

Qexp (mg/g)

81.5 204.4

Pseudo-first-order model

Pseudo-second-order model

Q1e , cal (mg/g)

k1

R2

Q2e , cal (mg/g)

k2 × 104

R2

59.8 129.2

0.0075 0.0062

0.9330 0.9265

94.3 221.7

1.1784 0.7768

0.9921 0.9981

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Fig. 6. (a) Adsorption behaviors of pure SA (dashed lines) and SAGO aerogels (solid lines) for Cu2+ and Pb2+ . (b) Pseudo-first-order kinetic plots of SAGO aerogel. (c) Pseudo-second-order kinetic plots of SAGO aerogel.

The corresponding parameters and experimental adsorption capacities are shown in Table 1. It is observed that the pseudosecond-order kinetic model has higher correlation coefficient values (R2 > 0.99) and the theoretical adsorption capacities (Q2e ) calculated from pseudo-second-order kinetic equation are closer to the experimental adsorption capacities (Qexp ), indicating that the pseudo-second-order adsorption model is predominant.

where Ce (mg/L) is the equilibrium concentration, Qe (mg/g) is the adsorption amount at equilibrium, Qmax (mg/g) denotes the maximum monolayer capacity of adsorbent, and kL (L/mg) is the Langmuir constant that represents the energy of the adsorption process. Qmax and kL were calculated from the slope and intercept of the Langmuir isotherm (Fig. 7(b)), and their values were listed in Table 2. Another essential parameter, RL , called separation factor, is determined by the relation:

3.6. Adsorption isotherm

RL =

The metal ion adsorption is significantly influenced by the initial concentration in aqueous solution. As shown in Fig. 7(a), the adsorption amount at equilibrium for each adsorbate rises dramatically with an increase in initial concentration from 25 to 600 mg/L at first and then tends to level off. Adsorption isotherm is important to determine the adsorption behavior and predict whether an adsorption system is favorable. Here, two important isotherm equations, namely Langmuir and Freundlich isotherms [43], were constructed using the adsorption equilibrium data of Cu2+ and Pb2+ by the SAGO aerogel. The Langmuir isotherm is based on the formation of monolayer coverage of adsorbate on the homogeneous surface. The linearized Langmuir isotherm equation can be expressed as follow:

where RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). The Freundlich isotherm model is an empirical relationship describing the adsorption of solutes from liquid to solid surface, and assumes that multilayer adsorption occurs on a heterogeneous surface. The liner form of the Freundlich is:

Ce 1 Ce = + Qe Qmax kL Qmax

(5)

1 1 + kL Co

lnQe = ln kF +

(6)

1 ln Ce n

(7)

where kF and n are Freundlich constant and heterogeneity factor, which are determined by the intercept and slope of the linear plot (Fig. 7(c)). The estimated model parameters were summarized in Table 2. Owing to high correlation coefficient (R2 > 0.99), suitable Langmuir constant (kL > 0) and feasible separation factor (0 < RL < 1), the Langmuir isotherm appears to be a favorable model to supervise the adsorption process. The Qmax of SAGO is 98.0 mg/g for Cu2+ and

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Fig. 7. (a) Influence of initial concentrations on the absorption capacity of SAGO aerogel. (b) Langmuir adsorption isotherm plots of SAGO aerogel. (c) Freundlich adsorption isotherm plots of SAGO aerogel.

Table 2 Langmuir and Freundlich parameters for adsorption isotherms of SAGO aerogel. Adsorbates

Cu2+ Pb2+

Langmuir

Freundlich

Qmax (mg/g)

KL (L/mg)

R2

RL

KF (L/mg)

n

R2

98.0 267.4

0.0102 0.0066

0.9945 0.9912

0.1404–0.7968 0.2016–0.8584

4.9464 7.2432

2.1240 1.7994

0.8952 0.9343

267.4 mg/g for Pb2+ . It is obvious that SAGO in this work possesses the optimal adsorbability, which is believed to be a promising candidate for application in sewage treatment. 4. Conclusions In conclusion, GO sheets were used as reinforcing fillers for improving the mechanical performance including elasticity of neat SA aerogel. A series of three-dimensional mold-shaped SAGO aerogels were prepared via in situ crosslinking induced by GDL as a gelation promoter. With 4 wt% GO incorporation, the compressive strength can achieve 324 kPa and still remain 249 kPa at the fifth compression cycle. The outstanding mechanical strength and satisfactory elasticity can be attributed to the strong hydrogen bonding interactions between GO sheets and SA chains demonstrated by FTIR and XPS spectra. Obviously, the adsorption capacities of SAGO aerogel for Cu2+ and Pb2+ are significantly increased when GO is added, and the Langmuir maximum adsorption capacities reach up

to 98.0 and 267.4 mg/g, respectively. The results of microstructures, mechanical properties and adsorption capacities of SAGO aerogel confirm that the embedding of GO and in situ crosslinking technology make SAGO aerogel a promising candidate in wastewater treatment. Acknowledgements The authors are grateful for the financial support by the National Natural Science Foundation of China (No. 51403141) and Natural Science Foundation of Jiangsu Province, China (No. BK20140347). References [1] [2] [3] [4]

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graphene oxide aerogel with enhanced strength-toughness and its heavy metal adsorption study.

Ordered porous sodium alginate/graphene oxide (SAGO) aerogel was fabricated by in situ crosslinking and freeze-drying method. GO, as reinforcing fille...
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