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. Author manuscript; available in PMC: 2021 Apr 1.
Published in final edited form as: J Environ Manage. 2020 Jan 9;259:110044. doi: 10.1016/j.jenvman.2019.110044

Pillared graphene oxide composite as an adsorbent of soluble hydrocarbons in water: pH and organic matter effects

C E Flores-Chaparro *,#, CJ Castilho **,#, I Külaots **, Robert H Hurt **, J R Rangel-Mendez *
PMCID: PMC7517627  NIHMSID: NIHMS1548804  PMID: 31929029

Abstract

Graphene oxide (GO) is a single-atom-thick sheet of carbon with oxygen-containing functional groups decorating its basal plane and edge sites. Most of its high surface area can be lost due to restacking of individual layers during the synthesis and drying of GO-based bulk sorbents. There is great interest to increase the specific surface area of graphene-based sorbents by introducing organic molecules as “pillaring agents” between GO sheets to hinder the stacking process and create sorbents with elevated surface area. This work synthesizes pillared GO by introducing chitosan (CS), a linear polysaccharide with various molecular weights. A composite of low molecular weight CS at a CS/GO ratio of 0.1 is shown to have the highest specific surface area (up to 70.5 m2/g) in comparison to the medium and high CS molecular weight, pristine GO, and the CS/GO composite materials. The affinity of the optimized GO/CS composites towards benzene, toluene, and naphthalene was evaluated at 19.3 mg/L of organic matter content while altering pH. Sips and Langmuir adsorption isotherm models well described the adsorption behavior, and benzene adsorption performance was reduced at low pH. Related to the presence of dissolved organic matter (DOM) in solution, lower diffusivity constants (k1) in hydrocarbon systems were recorded. Our results demonstrate the feasibility of CS as a potential pillaring agent in CS/GO composites to increase specific surface area and enhance the capture of soluble hydrocarbons from aqueous solutions.

Keywords: Graphene oxide, chitosan, pillared composite, soluble-hydrocarbons

Graphical Abstract

graphic file with name nihms-1548804-f0007.jpg

1. Introduction

The mono-aromatic hydrocarbons benzene, toluene, and the poly-aromatic naphthalene are common pollutants that are constituents of gasoline and are widely used in numerous applications (Yakout, 2014). These pollutants are a matter of significant concern and have a relatively high water solubility compared to many others hydrocarbons, and thus a high bioavailability to aquatic organisms (Neff, 2002). Recent research on the remediation of water contaminated with mono-aromatic hydrocarbons uses a variety of physicochemical and biological approaches such as advanced oxidation, membrane filtration, and thermal processes (Bustillo-Lecompte et al., 2018; Jiménez et al., 2018; Ma et al., 2018; Xue et al., 2018). Adsorption-based technologies are the most widely applied treatment options due to their flexibility, high efficiency, and cost-effectiveness. Different types of organic (Tran et al., 2015) and inorganic (Wang and Peng, 2010) media have been evaluated as adsorbent materials for capturing aromatic compounds. Among them, activated carbons (AC) are particularly attractive due to comparatively high surface area, availability of adsorption sites, chemical stability, and low cost. New carbon-based nanomaterials have been considered as potential alternatives to conventional AC, and these include carbon nanofibers, carbon nanotubes, and fullerenes (Wang et al., 2014).

Recently graphene-based materials have been evaluated for removal of a wide range of pollutants from water (Wang and Zhao, 2016). Monolayer graphene and graphene oxide have ultra-high surface area, but upon drying often spontaneously restack to form ordered aggregates with greatly reduced area (Chen, 2015). Self-agglomeration of dried GO particles has restricted its application in large-scale adsorption processes (Kong, 2015). Hence, modification and functionalization of graphene sheets to prevent restakcing is a critical challenge in the synthesis of bulk sorbents from 2D nanosheet precursors.

One approach is to introduce organic molecules or polymers that spontaneously associate with the faces of graphene nanosheet and stearically hinder restacking. A cationic flocculating agent such as chitosan (CS) may be effective at the spontaneous association with negatively charged GO nanosheets through its abundant amine and hydroxyl groups, and also has its own high affinity for water-based pollutants such as metals and dyes that may contribute to the adsorption power of the composite (Crini and Badot, 2008; Wan Ngah et al., 2011). We hypothesized that such pillared graphene-based composites would have the potential for environmental capture of hydrocarbons with significant water solubility, such as benzene, toluene, and naphthalene.

Previous adsorption studies with the CS/GO composites employed pre-established ratios of both precursors for the synthesis of the final adsorbent. Here we show that a specific amount of soluble CS polymer promotes an optimal pillaring effect for the GO sheets, resulting a noticeable increase in the final CS/GO composite product specific surface area and adsorption capacity for water-based aromatic pollutants. This study also examines the effect of chitosan molecular weight in the optimization of composite sorbents. The composites are applied to three key water pollutants such as benzene, toluene and naphthalene, and we study the effects of pH and the fundamental adsorption mechanisms.

2. Material and methods

2.1. Materials

Three chitosan (CS) samples of low (50 – 90 kDa), medium (190 – 310 kDa) and high molecular weight (> 310 kDa) with a given degree of deacetylation of ≥ 75 % were purchased from Sigma Aldrich Co., Ltd. The source materials, solvents and adsorbates were of analytical reagent grade.

2.1.1. Synthesis of GO

Graphene oxide was prepared by a modified Hummers method by a protocol provided elsewhere (Hummers and Offeman, 1958). The GO sample was treated by a two-step acid–acetone wash to remove the salt by-products (Guo et al., 2014). The GO sample was dispersed in water to a final concentration of 2.3 mg/L. Benzene, toluene, and naphthalene were obtained from Sigma‒Aldrich with 99% purity.

2.1.2. Synthesis of CS/GO composites

Chitosan-graphene oxide (CS/GO) composites were synthesized following the method of Jiang et al. (2016) (Jiang et al., 2016). The CS/GO mass fraction was systematically varied from 0 to 1. Briefly, a specific volume of GO solution at 2.3 mg/L concentration was dispersed into 10 mL of deionized water and sonicated for 10 minutes. An acidic CS solution was prepared at the same concentration level as GO suspension by mixing a specific mass of CS into 2.5 % (vol./vol.) acetic acid. In a typical experiment, different CS ratios were slowly added to the GO suspension and then sonicated for 5 minutes. The final mixture was stirred for at least 12 h to obtain a homogeneous dispersion. Three types of chitosan were evaluated in the composites: low, medium and high molecular weight samples provided by Sigma-Aldrich Co. Ltd.

2.2. CS/GO characterization

N2 adsorption isotherms were recorded by the Quantachrome Autosorb-1 instrument, from which specific surface area (m2/g) was calculated by applying the BET model. The analysis of pore-size distribution (PSD) was conducted in the SAIEUS software using the 2D-NLDFT model. The surface morphologies of GO, CS, and CS/GO were evaluated using a field emission Scanning Electron Microscope LEO-SEM 1530 VP operating at 5.0 kV for all ranges of resolution. Before imaging, the samples were coated with a layer of AuPd (~2 nm) to reduce charging. X-ray diffraction spectrometry was performed with a Bruker AXS D8 with Cu KR radiation, λ = 1.5418 Å. Raman spectroscopy was performed with a Witec Alpha 300 Confocal Raman Microscope. FTIR spectra of the GO, CS and GO/CS composites were collected with a Jasco Instruments FT/IR-4100 in the ATR attenuated total reflectance mode. The zeta potential of the suspensions and the average particle size of CS was determined by dynamic light scattering on a Malvern Zetasizer Nano – ZS. The DLS measurements were repeated at least three times with a wavelength of 235 nm at 22 °C with an angle detection of 90°.

2.3. Adsorption experiments

The hydrocarbon (HC) uptake by pillared CS/GO composites was determined for a range of starting HC concentrations from the dilute state to saturation (Table S1) (Zhang et al., 2016). In brief, 14 mL of HC solution and 5 mg of the CS/GO composite were added into amber glass flasks. The flasks were sealed and placed in a shaker operating at 25 °C and 110–120 rev/min for at least 4 h. Adsorption uptakes were studied by adding exactly 5 mg of adsorbent to a series of HC solutions that had the same initial HC concentration. The bottles were incubated (25 °C and 110 rev/min), and the remaining concentration of HC was determined at different times up to 1 h. The initial and equilibrium concentrations of adsorbates were determined by UV-VIS spectrophotometry (Thermo Aquamate) at λ= 254.5, 261 and 284 nm for benzene, toluene, and naphthalene, respectively.

2.3.1. Effects of pH and dissolved organic matter

The pH is an important parameter that governs the net charge of both GO and chitosan, thus a wide range of 3–11 was selected for the test matrix (Table S1). The presence of dissolved organic matter (DOM) was tested with a natural water sample collected from a reservoir located in Mexico (22° 9’ 00” N 101° 3’ 15” W) and prefiltered through Millipore 0.22 μm nylon filters. The total organic carbon content of the natural water sample was measured using a Shimadzu TOC analyzer. A Varian Model 730-ES, Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, Varian 730-ES) was employed to determine soluble metal concentrations and capillary electrophoresis was used for anion quantification (Agilent 1600).

3. Results and discussion

3.1. Characterization of the composites

Different mass fractions and molecular weights of CS were evaluated to optimize the pillaring effect for surface area enhancement. After many tests with various CS/GO mass fractions, the composite with 10% mass fraction of unmodified low-molecular-weight (LMW) CS provided the highest specific surface area (~70 m2/g) (see Fig. 1a). In comparison, the specific surface areas of GO and CS precursors were below 2.6 m2/g, close to the detection limit of the vapor adsorption method. The 10% CS composite represents a significant area enhancement, while still offering adequate adsorbent characteristics due to the mesopores (pore from 2 nm up to 10 nm) generated in the composite (see Fig. 1b). For the remainder of the paper, the selected CS/GO composite with CS mass fraction of 0.1 will be referred to as ‘CS/GO_0.1’, and subjected to full characterization and HC adsorption testing. The CS/GO composites synthesized with medium and high molecular weight CS were not effective in generating high-area pillared GO composites, with areas ranging from 1.5–4 m2/g at CS mass fractions around 0.3–0.4.

Fig. 1.

Fig. 1.

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(a) Composite surface area (m2/g) of different CS/GO mass fractions by using CS of low, medium and high molecular weight (MW). The CS/GO fractions of zero means 100% GO, and a CS/GO fraction of 1 means 100 % CS, (b) Pore size distribution for selected (CS/GO = 0.1) composite calculated using SAIEUS (2D-NLDFT model).

The surface morphology of the precursor GO and CS/GO_0.1 composite structures is presented in Figure 2a and Figure 2b-e. Due to face-to-face intermolecular forces, all the vast monolayer surface area is lost on GO sheets and inter-sheet gallery becomes unavailable for the potential adsorbates to be removed from water solution (Fig. 2a) (Raad et al., 2016). The introduction of CS molecules certainly enhances the available specific surface area of the composite, but a great portion of the interlayer surface of GO sheets is unavaliable (Fig. 2b-e). Moreover, the opposite charges within the work solution (pH ~ 4), promotes the bonding force and the charge stability through the opposite charge with precursors. The SEM images showed the agglomerated CS particles inside the GO layers with the average individual particle sizes between 300 ± 24 nm determined by DLS measurements. The size distribution is similar to the CS micelles observed in Fig. 2b, c and d.

Fig 2.

Fig 2.

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(a) SEM picture of GO; (b, c, d, e) SEM images of the CS/GO_0.1 composite. The arrows indicate the presence of Cs particle between the GO layers; (f) Zeta potential profile of GO, CS and CS/GO composites; (g) X-ray diffraction pattern of GO, CS and CS/GO composites, and (h) Raman spectra of GO and CS/GO composite at its optimum ratio for the surface area.

The Zeta potential values of the CS/GO solution with the composite versus pH profiles in this study is presented in Fig. 2f. The Zeta potential characterization is a powerful tool for determining the electrical properties and colloidal stability of the sample tested. The Zeta potential of the CS was positive at pH 3, and slightly negative at pH 10, which could be associated with the presence of cationic amino groups along the polymer backbone (Espinal-Ruiz et al., 2014). GO, as expected, has negative value of the Zeta potential, which is consistent with previous studies (Konkena and Vasudevan, 2012) and responsible for the colloidal stability of GO (Hunter and Sanders, 1990). The opposite electrical charges in CS and GO over the relevant pH range for our processing likely causes extensive CS/GO association that enables pillaring. The CS/GO association may be additionally promoted by hydrogen bond formation involving the ether and hydroxyl content of chitosan (Abolhassani et al., 2017).

Zeta potential curves as a function of pH for two example composites are shown in Fig. 2f. The Zeta potential of the CS/GO = 0.25 composite changes from large positive value at pH 3 to the highly negative Zeta potential values at pH 10. The Zeta potential of the CS/GO = 0.1 is negative which is consistent with the Zeta potential of GO (Konkena and Vasudevan, 2012), the main component.

The GO precursor XRD spectra has a peak at 2θ = 10.54 (Figure 2g), from which the corresponding GO interlayer spacing (0.843 nm) is calculated by using Bragg’s Law (Bragg and Bragg, 1913). Our calculated GO interspacing value is quite similar to the previously reported values available in the literature (Yang et al., 2010). However, CS XRD spectra has two peaks. One at 2θ = 20.5 and the other one, a broad peak at 2θ = 10.7. These peaks can be attributed to hydrated and anhydrous crystals (Forms I and II), respectively as shown elsewhere (Chen et al., 2013). The CS/GO_0.1 composite XRD spectra does not possess any characteristic peak related to CS, which suggests that CS is well distributed inside the GO sheets. The peak at 2θ = 9.7 in the XRD spectra of the CS/GO_0.1 composite corresponds to a GO interlayer Bragg spacing value of 0.915 nm, with the enlarged spacing a direct consequence of the molecular pillaring. The small displacement of the first CS/GO_0.1 composite peak at 2θ = 9.7, and the two additional peaks located at 2θ = 19.03 and 22.36, are similar to previously reported composites synthesized at a higher CS/GO ratios (Kumar and Jiang, 2016; Zhao et al., 2015). In comparison with GO, a decrease in crystallinity of CS/GO_0.1 composite is explained due to electrostatic and intermolecular bonding with the biopolymer and the surface active sites of GO.

Raman spectroscopy is a valuable tool to characterize carbon-based composite materials. Raman spectra obtained from the precursor GO and the CS/GO_0.1 composite are presented in Fig. 2h. For the precursor GO the three bands shown are centered at wavelengths 1592, 1352, and 2682 cm−1, which can be assigned as the G, D, and 2D bands, respectively. The G band is a result of a C-C bond stretch, being common for all sp2 carbon forms (Krishnamoorthy et al., 2012) and is formed from first-order Raman scattering. The D peak is related to vacancy defects and oxidized domains in GO. The D/G intensity ratio can provide information on the size of the graphenic (sp2 bonding) domains in carbons (Dresselhaus et al., 2010). In this research, the results show that the D/G ratio value increases from 0.98 to 1.03 after the association of CS on graphene oxide layers, as seen in Figure 2h, this is related to additional junctions between GO and CS functional groups, indicating the attachment of both molecules in the carbon based composite. The increase of the D/G ratio was previously reported on a graphene oxide-chitosan composite hydrogel (synthesized at a CS/GO ratio of 0.6) with an increase toward 0.97 to 1.02 after self-assembly process (Zhao et al., 2015).

The interaction between the amino polysaccharide CS and GO sheets could be attributed to the following: (1) the electrostatic interactions of the negative surfaces of GO in solution with the cationic CS, (2) hydrogen-bonding interactions between CS-NH2 groups and oxidized functional groups (carboxylic and phenolic) on GO surfaces, and (3) the amide bond synthesis between the amine groups of CS and the carboxylic groups of GO (Zhang et al., 2016). The presence of certain functional groups on the surface of the precursor GO and CS/GO_0.1 composite were evaluated with the FT-IR spectroscopy and potentiometric titrations. Changes in the absorption peaks at 1720 and 1056 cm−1 are attributed to carboxylic and amide groups, respectively (Fig. 3a). Furthermore, the conducted potentiometric titrations detected a decrease in the peaks of carboxylic (0.152 mmol/g) and phenolic groups (1.448 mmol/g), which suggests the favored interaction between GO and CS (Fig. 3b).

Fig. 3.

Fig. 3.

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(a) FTIR spectra of the CS/GO_0.1 composite and the precursors, (b) pKa distribution for GO and CS/GO_0.1 composite.

The detailed characterization above suggests a mechanism for the assembly of the CS/GO composites. Chitosan polymer and GO nanosheets appear to spontaneously associate in suspension, driven primarily by electrostatic attraction, but with the possibility of secondary hydrogen and covalent bonding. This association partially inhibits the restacking process that would normally occur upon drying, leading to factor of at least 20 in surface area enhancement. Low molecular weight makes chitosan more effective as a restacking inhibitor. As an explanation, we suggest that effective pillaring requires formation of atomically thin adsorbate films on the GO surfaces, which eventually become interstitial organic matter in the final composites. The high molecular weight CS (which has a hydrodynamic radius reported as high as 46.4 nm (Cölfen and Dautzenberg, 2001; Weinhold et al., 2009), would require the most extensive polymer conformational changes to form such films and uniformly bind to all available GO surfaces, and is thus less effective as a pillaring agent at equal CS/GO mass ratio.

4. Adsorption experiments

4.1. Adsorption experiments

Fig. 4 shows the benzene, toluene, and naphthalene adsorption isotherms of the modified composite at the optimal surface area enhancement. The adsorption isotherms of GO were also recorded. The CS/GO_0.1 composite enhbited a noticeable affinity for hydrocarbons, being their adsorption capacity of 147, 60, and 6 mg/g for benzene, toluene and naphthalene, respectively. Three adsorption models known as Langmuir, Freundlich and Sips were proposed to describe the relation between the amount of substance adsorbed per unit mass. These models are well-described in the previous report of Syafiuddin et al., 2018. The Sips model best followed the CS/GO_0.1 composite adsorption data, while the Langmuir model fitted the best with the precursor GO (Table 1). The main advantage of Sips model is that it includes Freundlich and Langmuir characteristics in the determination of the equilibrium approach at low and saturation concentrations, respectively. The heterogeneity constant (n) for all the samples recorded small differences from unity and denotes a homogeneous adsorption on the surface of the adsorbent. On the other hand, according to the Langmuir model, the GO adsorption data described homogeneous adsorption with a finite number of identical sites (Nethaji et al., 2013). Similar results were obtained for the removal of benzene and chlorobenzene by carbon nanotubes and graphite (Chen et al., 2007).

Fig. 4.

Fig. 4.

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Adsorption isotherm of (a) benzene, (b) toluene and (c) naphthalene by (●) CS/GO, (♦) GO at initial pH 6 – 7, and 25°C. The lines represent the Sips adsorption isotherm equation for CS/GO systems and Langmuir for GO systems.

Table 1.

Parameters of the isotherm equations, correlation coefficients (R2) and error function values of the models for organic compounds adsorption.

CS-GO composite GO
B T N B T N
Langmuir
qmax (mg/g) 278.7 75.77 9.705 34.36 19.02 6.560
b (L/mg) 7×10−4 0.018 0.149 0.003 0.008 0.026
RL 0.442 0.097 0.176 0.156 0.195 0.550
SSE 204.6 29.41 0.359 4.320 2.560 0.044
R2 0.993 0.992 0.994 0.993 0.987 0.994
Freundlich
N 0.664 0.274 0.432 0.329 0.346 0.719
KF 1.169 1.169 1.960 2.573 1.808 0.257
SSE 529.7 529.7 1.230 19.50 8.099 0.101
R2 0.982 0.982 0.981 0.970 0.961 0.988
Sips
B 3×10−4 0.020 0.149 0.013 0.010 0.041
qmax (mg/g) 260.2 76.83 9.636 41.44 19.23 4.764
n (L/mg) 1.124 0.961 1.011 0.703 0.957 1.032
RL 0.649 0.088 0.176 0.041 0.163 0.436
SSE 100.1 29.09 0.360 9.840 2.826 0.097
R2 0.997 0.993 0.995 0.985 0.986 0.988
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Comparative adsorption values between the CS/GO_0.1 composite and precursor GO (Fig. 4a, b, c) revealed that CS introduction increased adsorption capacity by 2 to 4-fold. Reports related to the adsorption of organic pollutants by graphene oxide established the existence of the hydrophobic effect and π-π interactions (Raad et al., 2016). The presence of CS between the interlayers of CS/GO_0.1 composite promoted a greater accessibility of available sites for the adsorbates. In addition, hydroxyl groups of chitosan could produce van der Waals interactions acting between the C‒3 of chitosan chains and aromatic molecules (Rangel-Mendez et al., 2009). These main types of binding forces involved in the adsorption process are illustrated in Fig. S1.

4.2. Adsorption rate kinetic studies

Figure 5 shows equilibration times less than 15 min. The kinetic results were fitted by nonlinear convergence to pseudo-first, pseudo-second and Weber-Morris equations to obtain the kinetic parameters (Syafiuddin et al., 2018). According to correlation coefficients (R2 and SSE values), the pseudo-first-order rate model best described the kinetic data for all the systems tested. This evidence suggests that the turnover over rate of the hydrocarbons is proportional to its current concentration. The quick adsorption at the initial stage suggests a large number of binding sites with a high affinity for the aromatic pollutants.

Fig. 5.

Fig. 5.

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Adsorption rate of benzene (Co = 1800 mg L−1), toluene (Co = 515 mg L−1), and naphthalene (Co = 515 mg L−1) by CS/GO_0.1 composite in deionized water and the presence of organic matter (DOM). Benzene biosorption rate onto (●) benzene, (○) benzene + DOM; (▲) toluene, (Δ) toluene + DOM; (■) naphthalene, (□) naphthalene + DOM.

Figure 5 shows that dissolved organic matter (DOM) had only a small effect on the adsorption of the aromatic compounds. DOM slightly decreased the early-stage adsorption rate, but not no effect on the equilibrium adsorption capacity. This observation was reflected in the comparative lower pseudo-first kinetic constants (k1) (Table 2). Note that DOM here is defined as the fraction of organic substances in the natural water sample that passes a 0.22 μm filter, and is composed of three primary sources: a) terrestrial matter from soils, b) phytoplankton occurring in surface water, and c) synthetic substances of anthropogenic origin. The organic and inorganic fraction was fully characterized and is presented in Table S2. The aromaticity in the DOM samples was verified by the light-absorbing fraction (SUVA254 = 0.976 L/mg·m), which also corroborated the presence of large organic components like humic acids (E2: E3 ratio = 5.03). Humic compounds have hydrophobic as well as hydrophilic character, and have a variety of chemical groups and charge density (Chon et al., 2017).

Table 2.

Pseudo-first order, pseudo-second order and Weber-Morris rate equation parameters for removal of benzene, toluene and naphthalene.

CS-GO Composite
Bα B + DOMβ Tγ T + DOM Nδ N +DOM
Pseudo-first order
k1 (h−1) 21.77 16.24 15.94 16.36 10.12 4.483
qe (mg g−1) 148.6 175.4 73.01 98.89 6.577 11.01
SSEβ 11.80 155.9 187.2 82.06 4.177 8.330
R2 0.999 0.994 0.966 0.991 0.910 0.936
Pseudo-second order
k2 (mg g−1 h−1) 0.236 0.129 0.259 0.229 1.819 0.295
qe (mg·g−1) 157.7 190.8 80.79 108.4 7.390 14.08
SSE 206.1 815.3 374.3 42.78 5.182 12.16
R2 0.988 0.970 0.931 0.995 0.888 0.905
Weber-Morris model
k3 (mg g−1 h1/2) 119.9 150.4 69.18 98.47 6.574 12.41
C 57.01 59.03 21.60 27.20 1.436 0.126
SSE 7217 1088 1920 2614 13.31 25.03
R2 0.779 0.779 0.648 0.850 0.714 0.807
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α

= Benzene

β

= Dissolved Organic Matter

γ

= Toluene

δ

= Naphthalene

4.3. The effect of pH

Fig. 6a shows that the initial pH (3–11) has a noticeable effect on the removal of hydrocarbons. In general, the removal capacity of the adsorbates decreases with increasing [H+]. At pH values below the pKa for CS (6.3), protonation of amino groups lowers the hydrogen bonding among CS molecules (Zhang and Xia, 2014). The hydrolysis of the chains may promote the release of CS from the CS/GO_0.1 composite to the solution. The presence of positive charge CS units in the aqueous media probably promoted interference effects of competition with the adsorbates and hindered the accessibility for CS/GO_0.1 actives sites. A minor effect was observed for toluene and naphthalene molecules probably by the higher hydrophobic character (Kow values) of the aromatic molecules (Fayemiwo et al., 2017). Finally, the adsorption amounts were found to increase at about neutral pH but remained similar as pH increases further to 11. These results are consistent with the predominance of van der Waals and π-π forces as the main adsorption mechanism involved.

Fig. 6.

Fig. 6.

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(a) Effect of initial solution pH effect on benzene (Co = 1800 mg/L), toluene (Co = 515 mg/L) and naphthalene (Co = 31.5 mg/L) uptake by CS/GO_0.1 composite at 25°C, 120–130 rev/min.

5. CS/GO composite as an adsorbent of aromatic hydrocarbons

Different pillaring agents for graphene and graphene oxide have been reported in the literature, which mainly included carbon-based materials like carbon nanotubes, nanocarbon fibers, carbon black and fullerenes (Guo et al., 2014). Another type of agents included polymers, metallic cations and in minor proportion organic polymers. For that reason, it is difficult to establish a clear comparison of the pillaring capability of CS. However, similar CS/GO composites reported in the literature, and the organic target pollutants are presented in Table 3. In the present study, the experimental data confirmed the capability of CS, as a soluble polymer to join the interspaces of GO. This approach significantly improved the original surface area and removal capability of GO.

Table 3.

Compilation of the main studies regarding the synthesis of CS-GO for the removal of organics in water.

Composite precursors Target pollutants Removal capacity (mg g−1) Reference
CS; GO Methylene blue
Eosin Y		 320
240		 (Chen, 2013)
CS; GO;
Fe3O4 		Methylene blue 180.83 (Fan, 2012)
CS; GO; FeCl3·6H2O;
FeCl2·4H2O;
gluteraldehyde		 Reactive black 5 221 (Travlou, 2013)
CS; GO; FeCl3·6H2O;
FeCl2·4H2O; gluteraldehyde;
EPC; β-ciclodextrin		 Methylene blue 50.12; 84.32 (Fan, 2013)
Cellulose; GO Methylene blue 70.63 (Shi, 2013)
CS; GO; FeCl3·6H2O AO7 42.7 (Shesmani, 2014)
CS; GO; FeCl3·6H2O;
FeCl2·4H2O;		 Fuchsine 75.31 (Leilei, 2014)
CS; GO; Fly ash; Acid red and cationic red 40
60		 (Sheng, 2016)
CS/GO ratio = 0.1 Benzene
Toluene
Naphthalene		 147
60
8		 This study
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6. Conclusions

The present study showed out that a chitosan/graphene oxide composite prepared with a low-molecular-weight chitosan achieved an optimized specific surface area of 70 m2/g by pillaring the interlayers of graphene oxide (GO) nanosheets. Medium and high molecular weight chitosan molecules produce a little or no pillaring effect on GO registered at a CS/GO ratio between 0.2 – 0.4. For the optimized composite, CS/GO_0.1, a suite of characterization techniques verified the presence of chitosan molecules between the GO sheets, producing a more hydrophobic material with a significant increase in adsorption capacity compared to un-modified GO. In addition, it was found that the soluble hydrocarbons benzene, toluene and naphthalene, with an initial concentration of 1800, 515, and 31.5 mg/L, respectively, had an equilibrium adsorption capacity of 155, 67 and 24 mg/g. Dispersive π-π interactions and van der Waals forces were involved in the adsorption mechanisms, which was described by the Langmuir and Sips isotherm equations. The reported composite, CS/GO_0.1, is easy to synthesize and possess a competitive hydrocarbon removal capacity.

Supplementary Material

1

Highlights.

  • Low-molecular-weight chitosan optimized a pillared effect on graphene oxide

  • An optimized chitosan/graphene oxide ratio of 0.1 achieved a surface area of 70 m2/g

  • Medium and high molecular weight chitosan produced a little or no pillaring effect

  • Composite had 2 to 4-fold the adsorption capacity of graphene oxide

  • The removal decreased with increasing [H+] due to the amino groups protonation

7. Acknowledgements

This work was supported by grant PDCPN-01-247032 (Mexico). Carlos E. Flores-Chaparro acknowledges a doctoral fellowship and a stay of research fellowship from CONACYT no. 424187. This research was also supported by the Superfund Research Program of the National Institute for Environmental Health Sciences under grant P42 ES013660. The authors express their gratitude to Z. Saleeba, R. Spitz, E. Isaacs, D.I. Partida, G. Vidriales, J.P. Rodas, and M.C. Rocha for their invaluable assistance throughout the investigation.

Footnotes

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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