A novel graphene aerogel synthesized from cellulose with high performance for removing MB in water

1. Introduction

With modern industries developing, environmental pollution becomes more and more aggravated such as the water pollution, resulting from discharge, leakages or spills of hazardous chemicals from some fabrication process [1,2]. In recent decades, water polluted by dye contamination has caused serious risks to the environmental safety, and threats to the long-term sustainable evolution of human society [3]. It is a global concern that a great number of dyes have been discharged into ground water every year, from industries such as textile, leather, paper, plastics, etc. The dyes are very stable and not easy to be naturally degraded in the surrounding, because of their molecular structure based on polycyclic aromatic group [4]. The contaminated dye of polycyclic aromatic compound in the ground water like methylene blue will impart toxicity to aquatic life and be assimilated by human through food chain, which can cause severe disease to human beings, such as dysfunction of kidney, brain, liver and central nervous systems, etc [5]. When the contaminated dyes in water degraded by means of physical, chemical or biological treatments, some toxic and carcinogenic by-product will be brought up simultaneously. Regarding the current environmental situation, water pollution caused by dyes has become a serious environmental problem in worldwide.

It is really an urgent issue to find some effective technologies for safely and easily removing dye contamination in water. Many researchers all over the world keep working on this issue, and various methods for water treatment have been reported such as photocatalytic degradation, ultrafiltration, cation exchang, distillation, extraction and adsorption/precipitation processes etc [[6], [7], [8]]. Among these reported technologies, adsorption may be a promising treatment for removing dyes in water, owning to its particular advantages such as low cost, high efficiency, easy operation, eco-friendliness and no formation of harmful substances during treatment [[9], [10], [11]]. While, design of effective and low-cost adsorbent is the critical key for adsorption technologies applied in effective removal of dye in water.

Traditional adsorbents like zeolite, perlite, clay and activated carbon etc., cannot satisfy to treat dye pollutants in water because of the drawbacks of relative low efficiency, poor adsorption ability, high cost and second pollution [[12], [13], [14]]. It is still a challenge to develop an absorbent with the excellent adsorption properties for removing dye in water, such as high efficiency and low cost. So, many efforts have been devoted to prepare novel advanced materials for removing organic pollutants effectively from water [[15], [16], [17]]. Recently, carbon-based porous materials have been regarded as an idea adsorbent material in water treatment, due to their properties of chemical inertness, good mechanical stability, homogenous components, and low contaminant content [[18], [19], [20], [21]]. Besides, the used carbon materials can be discharged in soil without harm to environment, since carbon substance is benign to improvement of soil texture and its ecosystem for soil microorganism growth.

Nowadays, hierarchical carbon porous materials has attracted growing attention for it applying as adsorbent, since the hierarchical pore structure in the adsorbent can endow the transport and absorption of adsorbate into the adsorbent at a high rate, which makes this material be a promising candidate for the application in advanced absorption field. Graphene aerogel (GAs) is a relatively novel emerging carbon-based material with well-developed hierarchical porous structure [[22], [23], [24]], characteristic of the exhibits excellent properties of high porosity, large surface area, and good chemical stability, and have been widely explored as adsorbent materials for treating wastewater contaminated with oil or organic pollutant in the recent years [[25], [26], [27]].

Although GAs have been demonstrated with potential application for overcoming the increasing environment problems like water pollution [28,29], its massive production and application are dramatically restricted by the high cost, such as the harmful and expensive precursors, complicated processes and complex equipment involved in fabrication. Up to now, the methods adopted for graphene aerogel fabrication include chemical vapor deposition (CVD), hydrothermal reduction, and chemical reduction process [30,31]. While, CVD method needs expensive precursors and complex equipment; the other two strategies go with the high energy wastes and expend a large amounts of chemical reagents. These expensive cost and redundant preparation process seriously prevent GAs applying in the wide fields [32].

With green industries developing, biomass-derived carbon materials have drawn significant interest in the past decades [[33], [34], [35]], since biomass are not only renewable but also inexpensive. There are so such abundant natural biomass sources of carbon precursor that can be easily converted into carbon product, such as cellulose [36], saccharides [37,38], coconut [39], starch and dead leaves et al. [40,41]. So, the biomass may be a green alternative precursor of carbon materials produced in the industrial scale. Therefore, exploring the novel routes taken biomass as precursor to prepare carbon-based aerogels like graphene aerogel may be of an attractive interesting to both the industry and academic word.

In this study, we provided a novel approach to design and synthesize graphene-aseembled aerogel from cellulose aerogel via an effective and low-cost way. The adsorption properties of this kind of graphene aerogel applied as the adsorbent have been investigated in removing cationic dye like methylene blue (MB) in water, and the proposed graphene aerogels of CGA-2, as adsorbent for MB removing, exhibits a high efficiency. It can be believed that this novel kind of graphene aerogel have great potential application as an effective adsorbent for removing cationic dye in water.

2. Materials and methods

2.1. Materials

Microcrystalline cellulose (MCC), Sodium hydroxide (NaOH, 96 %), and Urea (CO(NH₂)₂, 99 %) were purchased from Sinophrm Chemical Reagent Co., Ltd. Methylene blue (MB) was procured from Shanghai Aladdin Bio-Chem Technology Co., Ltd, Phenol formaldehyde resin (PF) was supplied by Wuhan create Technology Co., Ltd. All reagents used in the experiment were analytical reagent grade and used as received. Deionized water was used for all experiments.

2.2. Synthesis of graphene aerogel from cellulose

In a typical synthesis, 7 g of NaOH, 5 g of microcrystallinecellulose (MCC) and 12 g of urea were dissolved into 200 mL of H₂O in sequence under -12 °C, obtaining a mixture solution. Then, a certain phenolic resin (PF) like 0.5.% (w/v) was added into the mixture solution stirring to be homogenous; After that, the mixture solution was transferred to a beaker for solidification about 24 h at 50 °C; the solidified jell was dried at 110 °C for 12 h to obtain a cellulose aerogel composited with PF, which was denoted as C_pAs where s indicates the different amount of PF composited in the cellulose aerogel. Finally, the prepared C_pAs was carbonized in nitrogen air under 600 °C to obtain a carbon-based aerogel, which was denoted as CGAs that was demonstrated as graphene aerogel. The synthetic route to the prepared sample is illustrated in Fig. 1, in which cellulose aerogel (CA) and carbon aerogel (CCA) based on CA were prepared in the same way without PF addition.

2.3. Character of the samples

Thermal stability and carbon yield of the carbon precursors were determined by thermo-gravimetric (TG) analyzers (GA; STA449C; Netzsch, Germany). The specific surface area and pore structure parameters of the prepared samples were determined from the adsorption isotherm of nitrogen at 77 K by Tri-star 3020 (Micromeritic, USA). The morphologies of samples were observed by scanning electron microscopy (SEM, Hitachi S4800, Japan) and a transmission electron microscopy (TEM; JEM-3010, Japan). The phase composition of the graphene in the prepared samples was identified by X-ray diffraction (XRD; D8 Focus, Germany) and Raman spectroscopy (Raman; invia-reflex, England) performed using a 532 nm laser as the excitation source. The evidence for the surface composition of the prepared sample was inferred from X-ray photoelectron spectroscopy (XPS) by ESCALAB250Xi (Thermo Scientific, USA). Zeta potential value was determined by zeta potential analyzer (Brookhaven Instrument, USA).

The amount Q_t (mg/g) of the absorbed MB on the tested adsorbent at time t (min) was calculated as following:

Q_t=$\frac{V(C_0-C_t)}{m}$ (1)

where C₀ (mg/l), C_t (mg/l) are the concentrations of MB in a solution at the initial and final adsorption. V (ml) is the volume of the solution, and m (g) is the weight of the tested adsorbent. And all the batch experiments are conducted in duplicate, only the mean values have been reported.

3. Results and discussion

3.1. Synthesis and characterization of CGAs

Thermo-gravimetric analysis of pyrolysis of three carbon precursors is shown in Fig. 2. It can be seen that CA has a pyrolytic temperature nearby 300 °C, giving a pyrolytic carbon yield about 10 % at 600 °C; while Phenolic resin (PF) has a pyrolytic temperature nearby 400 °C, giving a pyrolytic carbon yield about 55 % at 600 °C. So, C_pA composited of CA and PF has a wide pyrolyzed temperature range from 120 °C to 250 °C, giving a pyrolytic carbon yield about 48 % at 600 °C. These results show the carbon yield of carbon precursor of cellulose aerogel can be greatly increased by addition of PF. This may because that PF has served as binder agent in sol-gel reaction, which benefits cellulose molecule in colloid solution polymerized to aerogel form in a high degree of polymerization. The cellulose aerogel with a higher polymerization degree can be derived to carbon aerogel with a higher carbon yield after carbonization.

Besides serving as binder in the sol-gel reaction of cellulose colloid, PF may also play a role of structure inducer agent for developing a stable network in cellulose aerogel. As a hydrophobic molecule, PF can induce cellulose colloid forming micellar mesophase in sol-gel reaction; and the formed micellar structure will be settled in the obtained cellulose aerogel as a fine porous network. It can be seen in Fig. 3, that the porous structure of the cellulose aerogel with PF like C_pA shown in Fig. 3a(2) is much better than those of cellulose aerogel without PF like CA shown in Fig. 3a(1). This indicates PF added into cellulose colloid can greatly promote porous network formed in the cellulose aerogel obtained from sol-gel reaction.

Because the pyrolytic temperature of PF is about 400 °C, it is much higher than the pyrolytic temperature of cellulose aerogel about 300 °C as shown in Fig. 2, which lets the composited PF in cellulose aerogel keep its resin properites when the pyrolysis of cellulose aerogel occurring. So, the composited PF can stand up the pyrolyzed carbon of cellulose aerogel assembling to a cross-linked network, avoiding the structure collapse during the carbonization process of cellulose aerogel. By which, PF will work like a template for reinforcing the textural structures of the pyrolytic carbon of cellulose aerogel. Benefitting from this work of PF during carbonization, the final product of carbon-based aerogel obtained from cellulose aerogel can remain a nearly intact porous structure from its precursor of cellulose aerogel. It can be seen in Fig. 3b(2), that the porous structure of carbon-based aerogel like CGA in Fig. 3b(2) has been perfectly remained from its precursor of cellulose aerogel like C_pA in Fig. 3a(2) after carbonization; while some fine porous structures of cellulose aerogel without PF working like CA in Fig. 3a(1) has been partly lost in its product of carbon-based aerogel like CCA in Fig. 3b(1) after carbonization.

X-ray diffraction (XRD) is a useful analysis technique for identification of phase and lattice structure, so it is often used for recognition of graphene. The XRD patterns of the prepared samples like CGAs that listed in Table 1, obtained from carbonization of cellulose aerogel, exhibit a characteristic of the typical amorphous carbon as Fig. 4(a) shown; there are two broad diffraction peaks are located at 24.5° in (002) plane and 43.2° in (100) plane, which can indicate the samples presenting structure of graphene material [42,43].

Raman spectroscopy is possibly a powerful method to characterize carbonaceous materials, particularly for distinguishing graphene materials. Two main peaks in the Raman spectrum assigned to the characteristic peaks of graphene-based materials can be observed at ∼1580 cm^-1 as G-band peak and ∼1343 cm^-1as D-band peak, due to the sp² hybridization of carbon atoms and the structure defect of sp³ carbon atoms. The Raman spectrum of CGAs prepared in this study display a strong G-band peak and D-band peak as shown in Fig. 4(b), which may indicate the present samples characteristic of graphene materials. The I_D/I_G ratios calculated from the Raman spectra can be used as an indicator of the structure quality of graphene materials, since the degree of crystallinity is directly proportional to the intensity ratio of D & G-peaks [44]. It can be found in Fig. 4(b) that I_D/I_G ratio of the prepared sample of CGAs listed in Table 1 are 0.91 for CGA-1, 0.88 for CGA-2 and 0.89 for CGA-3, which may demonstrate CGA-2 has the best structure of graphene materials among these three samples.

The TEM measurement of the CGAs listed in Table 1 are shown in Fig. 5, it can be seen in Fig. 5(a, b, and c), the morphology of all three samples exhibits a characteristic feature of graphene flake. A hierarchical porous network can be clearly observed in the sample of CGA-2, while, many parallel arrays of carbon atom in CGA-2 can be found in Fig. 5(d), confirming with the results obtained in XRD and Ramon spectrum analysis [[42], [43], [44]]. By these results, the multilayer structure of graphene sheet in GCA-2 may be identified.

Adsorption-desorption isotherms of nitrogen and pore size distribution curves of the prepared samples at 77 K are shown in Fig. 6(a) and (b). The isotherm plot of the CCA shown in Fig. 6(a) is type I with a sharp inflection points are present at a low pressure, showing the transition from micropore filling to multilayer adsorption; while the isotherm plot of CGAs are type IV with a hysteresis loop exhibited at relative high pressure, indicating the dominance of mesopores and macropores in these samples.

The pore size distributions are shown in Fig. 6(b), and it can be seen there are a hierarchical mixture of micropore, mesopore and macropore presented in CGA. The specific porous structure of the adsorbent is very important to its adsorption activity, since a hierarchical pore structure in the adsorbent will be beneficial to the adsorption process. Mesopore and macropore have a transitional function as channel and container, making the adsorbate easily transported to the micropore inside the adsorbent for adsorption. The pore structure parameters of the different samples are shown in Table 1. It can be seen that 0.5.% (w/v) of PF adding in cellulose colloid for sol-gel reaction gives the final product of CGA-2 the largest S_BET of 1426.11 m²/g and total volume of 1.02 cm³/g, which is the favorable porous structure for MB adsorption.

3.2. The performance of the prepared samples for MB removal

The adsorption capacities of the prepared samples are tested under 25 °C in the neutral solution with different MB concentration, and it can been seen the adsorption capacities of the different samples present a trend of increase with the initial MB concentration of the test solution, and their MB removal ratios decrease with the increase of the initial MB concentration as shown in Fig. 7(a). CGA-2 has a maximum adsorption capacity about 610.85 mg/g when the initial concentration of MB is more than 600 mg/L. While, the MB removal ratio will decrease from 99 % to 45 % in the solution with the initial concentration of MB increasing from 100 mg/L to 600 mg/L, as shown in Fig. 7(b).

Fig. 8 shows the effect of contact time on MB adsorption of CGA-2, in which the tests are carried in a neutral water solution with initial MB concentration of 100 mg/L under 25 °C. It can be found that MB adsorption equilibrium has been established on CGA-2 in 10 min with a removal ratio of 99 %. The inset picture in Fig. 8 shows the photographs of MB solution before (left) and after (middle, right) adsorption by CGA-2, from which it can be seen the test solution becomes very clear after 10 min adsorption by CGA-2.

The pH value of MB solution can greatly effect on the adsorption process, which has been studied in an aqueous solution with the initial MB concentration of 300 mg/L under 25 °C. It can be seen that the MB adsorption ability of CGA-2 increases with the initial pH value of the MB solution and the removal ratio of MB on CGA-2 can increase from 34 % to 95 %, when the initial pH value of the MB solution changed from 2 to 11 as shown in Fig. S1. In a solution with pH of 11 and MB initial concentration of 1.0 g/L, the adsorption capacity of CGA-2 can reach to 1.2 g/g. The XPS spectra in Fig. S2 show some oxygen and nitrogen incorporated with carbon in CGA-2, which can induce charge delocalization on the surface of CGA-2 to provide chemisorptions site for adsorption of cationic dye like MB. It has been also found that the zeta potential value of CGA-2 is about -23.08 mV in the neutral water solution, so that the basic condition is favorable for MB adsorption on CGA-2.

Therefore, 0.1 M hydrochloric acid can be used as striping solution for regeneration of CGA-2, of which the adsorption-desorption cycle is shown in Fig. 9. It can be seen in Fig. 9 the adsorption capacity of CGA-2 still keeps about 90 % over regeneration for four times, implying that CGA-2 may be a cost-effective adsorbent with recyclability for removing cationic dyes like MB.

3.3. The isotherms and kinetics of MB adsorption on CGA-2

Isotherm of adsorption on an adsorbent is very important to understand the behavior between adsorbent and adsorbates. In the present study, the experimental data of MB adsorption on CGA-2 were analyzed according to Langmuir, Freundlich and Temkin isotherm models [48,49].

The results are shown in Fig. 10, in which it can be found that Langmuir model fits the experimental data very well with a correlation coefficient R² of 0.99 that can be seen in Fig. 10(a). Langmuir model is much better than both Freundlich and Temkin models with R² about 0.75 and 0.76 that can be seen in Fig. 10(b), indicating MB adsorbed on CGA-2 as a monolayer.

The kinetic analysis can give the insight into the adsorption mechanism which is primarily used to design an effective adsorption route for application. To understand the adsorption kinetics of MB on CGA-2, the experimental data were fitted to well-established kinetic models such as pseudo-first-order and pseudo-second-order. The mathematical equations of pseudo-first-order kinetic model can be expressed as Eqs. (2) and (3) [45,46], respectively

ln(Q_e-Q)=lnQ_e-k₁t (2)

$\frac{t}{Q}=\frac{1}{k_2Q_e^2}+\frac{t}{Q_e}$ (3)

where Q (mg g^-1) is the adsorption capacity at time t (min), Q_e(mg g^-1) is the adsorption capacity at adsorption equilibrium, k₁ (min^-1) is the rate constant of the pseudo-first-order kinetics and k₂ (g mg^-1 min^-1) is the pseudo-second-order rate constant, respectively. The values of k₁ and Q_e can be obtained from the slope and intercept of ln(Q_e- Q) versus t plots and the values of Q_e and k₂ can be obtained from the fitting line of t/Q versus t as shown in Fig. 11. In this work, the regression coefficient (R²) is used to evaluate the fitting results.

It can be seen in Fig. 11, the experimental data fit well with the pseudo-second-order kinetic model, which is reflected by the regression coefficient R²>0.9996. Parameters of the two kinds of model are shown in Table 2, from which it can be seen the experimental values of Q_e.exp are in close agreement with values of Q_e.cal calculated from the pseudo-second order model. While experimental values of Q_e.exp are varied greatly to the values of Q_e.cal calculated from the pseudo-first-order model. The results suggest that the pseudo-second-order kinetic model is preferable to describe the adsorption behavior of MB onto CGA-2.

By the data from the solution with MB concentration of 300 mg/L under temperature of 298, 313 and 333 K as shown in Table 2, the activation energy (Ea) for MB adsorption on the CGA-2 can be obtained about 1.47 kJ/moL which is calculated according to the method in reference [21]. It may indicate a low potential barrier existing in this adsorption process, and there is a strong interaction should be existed between CGA-2 and MB molecule in this adsorption process. The Adsorption thermodynamic parameters of MB adsorption on CGA-2 have be also studied and the results are shown in Table S1

The adsorption process in a solution may include chemical adsorption physical adsorption and hydrogen bond adsorption. The pseudo-second order model conforms to chemical adsorption process, in which the adsorbate interacts with the surface of adsorbent by a strong interaction such as the electrostatic attraction or covalent chemical bond. The adsorption kinetic data may indicate CGA-2 providing a strong surface potential for adsorption of cationic dye like MB, due to its graphene flake structure inside, which conforms CGA-2 has a high performance in adsorption of MB process such as the high adsorption capacity and high adsorption rate, etc.

Comparing many other adsorbents that recently reported for MB removing, the prepared adsorbent in this work like GCA-2 has more obvious advantages than the current reported, such as high adsorption capacity, high efficiency, environmental friend and low-cost, which can be seen in Table 3.

4. Conclusion

We present here an easy and low-cost method to fabricate graphene aerogel from biomass like cellulose, in which PF has played important roles as structure inducer, binder and reinforcement during sol-gel reaction and carbonization. By this way, the carbon yield and structure of the final product has been greatly improved, resulting in the prepared sample of CGA-2 with a remarkable adsorption performance for removing MB in water, due to its special pore structure assembled by graphene flake. The high adsorption performance of CGA-2 implies this novel material may have potential application as advanced adsorbent in treating dye pollution in water. The strategies in this study also provide a novel pathway for synthesis of graphene aerogel with specific function.

Declaration of Competing Interest

None.

Acknowledgments

This work was supported by open fund from Hunan province key laboratory for advanced carbon materials and applied technology, Hunan University.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.09.019.

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