Graphene Oxide-Chitosan Composite Material for Treatment of a Model Dye Effluent.

Graphene oxide (GO) was cross-linked with chitosan to yield a composite (GO-LCTS) with variable morphology, enhanced surface area, and notably high methylene blue (MB) adsorption capacity. The materials were structurally characterized using thermogravimetric analysis and spectroscopic methods (X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and 13C solid-state NMR) to support that cross-linking occurs between the amine groups of chitosan and the -COOH groups of GO. Equilibrium swelling studies provide support for the enhanced structural stability of GO-cross-linked materials over the synthetic precursors. Scanning electron microscopy studies reveal the enhanced surface area and variable morphology of the cross-linked GO materials, along with equilibrium and kinetic uptake results with MB dye in aqueous media, revealing greater uptake of GO-LCTS composites over pristine GO. The monolayer uptake capacity (Q m; mg g-1) with MB reveals twofold variation for Q m, where GO-LCTS (402.6 mg g-1) > GO (286.9 mg g-1). The kinetic uptake profiles of MB follow a pseudo-second-order trend, where the GO composite shows more rapid uptake over GO. This study reveals that the sorption properties of GO are markedly improved upon formation of a GO-chitosan composite. The facile cross-linking strategy of GO reveals that its physicochemical properties are tunable and versatile for a wider field of application for contaminant removal, especially over multiple adsorption-desorption cycles when compared against pristine GO in its highly dispersed nanoparticle form.

Introduction

The development of advanced materials for the controlled removal of dyes and organic contaminants from effluent originating from printing, food, textile, paper, and pharmaceutical industries[1,2] is an active area of research. Many dye species are known to be carcinogenic and can affect water quality, as shown by undesirable human and ecosystem health effects.[3] Adsorption-based removal is an efficient process for the capture of dyes and contaminants from wastewater systems. Solid phase adsorbents represent an effective remediation strategy because of their technical simplicity, high efficiency, low cost, and potential reuse of the adsorbent.[4,5] Therefore, continued effort is required for the development of an efficient, novel, and cost-effective solid adsorbent material.

Recently, graphene- and graphene oxide (GO)-based materials have been studied for applications in adsorption and water treatment.[6,7] In particular, GO has emerged over the past decade as a next-generation material for wastewater treatment because of its low-cost production, large surface area, and strong interaction with a wide range of anionic, cationic, or neutral dyes in aqueous media.[8,9] Graphene is comprised of sp2-hybridized carbon atoms arranged in a two-dimensional honeycomb lattice. In contrast to graphene, which has a low aqueous solubility, GO is highly dispersible in water and organic solvents because of the presence of polar functional groups and high surface area.[9] The highly negative charge density of GO in aqueous solutions provides effective adsorption sites for cationic dyes,[6] such as methylene blue (MB) and serves as a material for gas capture (e.g., N2 and CO2) applications.[10,11] Although GO is a promising material as an adsorbent, efficient recovery is problematic because of its tendency of forming stable colloids that hinder phase separation. However, the use of polymers to immobilize GO offers an opportunity to produce composite materials with unique properties for solid phase separation and adsorption.

Fabrication of framework structures from individual GO sheets was studied using physical and chemical interactions to yield GO self-assembled films, hydrogels, and aerogels.[12,13] However, produced GO frameworks have limited applications in wastewater treatment because of scalability as a result of repulsive hydration forces between GO layers that result in electrostatic separation.[14] Therefore, the development of stable interconnected GO frameworks is one strategy for the successful application of these materials in wastewater treatment. Previous works reveal that cross-linking and other forms of synthetic modification of GO sheets can improve the sorption properties, as shown by the variation in methylene blue uptake (cf. Table S1, Supporting Information).[15,16] The resulting material would offer higher sorption capacity compared to pure GO because of the increased available adsorption sites as well as improved stability and mechanical structure for producing thin films.[17,18]

GO framework structures can be achieved by cross-linking GO sheets through noncovalent or covalent bonds by using biopolymers as cross-linkers.[15,19] Chitosan (CTS) is among a group of abundant renewable biopolymers[20] that has been used as an adsorbent for removal of organic dyes from aqueous solution because of the presence of active adsorption sites (hydroxyl and amino groups).[21] CTS can undergo an amidation reaction with the carboxyl groups of GO to form a homogenous and well-dispersed GO composite.[22,23] On the basis of the foregoing considerations, a key objective of this study was to develop a facile method for the preparation of a GO composite by cross-linking GO with CTS to obtain a material with improved physicochemical properties for sorption-based applications. To this end, spectroscopic methods [Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and 13C nuclear magnetic resonance (NMR)] were used to characterize the structure and composition of cross-linked GO composites. Also, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were used to analyze the thermal stability and surface morphology of the GO composites. The adsorption properties were evaluated under equilibrium and kinetic conditions using MB in aqueous solution. It will be shown that this study contributes significantly to the field of sustainable GO composites through the development of a suitable adsorbent for contaminant removal with tunable properties that exceed those of GO alone, and also can be reused for multiple cycles of adsorption–desorption. Also, this study advances the field of GO composites by use of a facile cross-linking methodology that is low cost and ecofriendly, where the structure of the composites are supported by several complementary methods.

Experimental Section

Materials

Low molecular weight CTS (LCTS) (Mw = 950 000 Da, 75–85% deacetylation), sodium nitrite (NaNO3), potassium permanganate (KMnO4), sulfuric acid (98%, ACS grade), MB (high purity, biological stain), and hydrogen peroxide (30 v/v %) used in this study were obtained from Sigma-Aldrich Canada Ltd. HPLC-grade methanol was obtained from Fisher Scientific, NJ, USA. Filter paper (Ahlstrom grade 613, 7.5 cm) and graphite flakes, natural, 325 mesh, 99.8% (metals basis) were obtained from Alfa Aesar Thermo Fisher Scientific and further purified by Soxhlet extraction for 24 h using HPLC-grade methanol, followed by drying in a vacuum oven at 60 °C for 12 h to remove impurities.

Synthesis of GO

GO was synthesized from graphite flakes using the modified Hummer’s method.[24] Briefly, 100 mL concentrated H2SO4 was added into a 500 mL flask filled with 4 g of graphite in an ice bath, followed by the addition of 2 g of NaNO3, and stirred for 4 h. Then, 12 g of KMnO4 was gradually added while the mixture was stirred for 2 h. The ice bath was removed, and the system was heated at 35 °C for another 30 min. Subsequently, 240 mL of distilled water was slowly added to the system, which caused a temperature rise to 90 °C, and continued to stir for another 30 min. Then, 160 mL of water and 30 v/v % H2O2 were added to terminate the reaction. The solution was stirred overnight and purified using multiple washings with Millipore water, HCl (30%), and ethanol until it reached neutrality (pH 7). After multiple washings, the solid GO was vacuum-dried at 40 °C to obtain dried GO powder.

Preparation of GO–CTS Cross-Linked Composite

The composite materials (GO-LCTS) were prepared by making the GO solution with a concentration of 3 mg/mL. LCTS solution was produced by dissolving 5 g of LCTS in 500 mL of 1 v/v % glacial acetic acid with stirring. The resulting LCTS solution (1 v/v %) was added drop-wise to the GO solution with continuous stirring for about 4 h. The mixture was neutralized to pH ≈ 7 using 1 M NaOH followed by stirring for 12 h. Herein, several cross-linked GO samples were prepared with variable precursor weight ratios. The results reported herein are focused on GO-LCTS with 1:0.3 (w/w) ratio because this condition affords a minimum amount of cross-linker level to stabilize the GO-LCTS composite. The solution was washed with Millipore water followed by pouring onto a glass surface and dried at ambient conditions for 48 h to obtain the GO-LCTS films with an average thickness ranging from 20 to 60 μm. An outline of the experimental sequence is shown in Figure .

Figure 1

Synthetic procedure for cross-linked GO-LCTS composite materials.

Characterization

Surface Charge (ζ Potential) Measurement

The ζ potential of the GO solutions (0.01 w/v %) were measured at various pH values (4–12) using a Zetasizer Nano ZS (Nano ZS90, Malvern, UK), before and after cross-linking. All samples were diluted to 0.01 w/v % and measured in Milli-Q water at different pH.

Scanning Electron Microscopy

The surface morphology and surface topography of non-cross-linked and cross-linked composite materials were studied using SEM (Hitachi model SU8010). SEM images of samples were collected under an accelerating voltage (5 kV).

FTIR Spectroscopy

A Bio-Rad FTS-40 IR spectrophotometer was used to obtain the IR spectra of the composite materials. The sample powder was mixed with pure spectroscopic-grade KBr (weight ratio: 1:10). The FTIR spectra were obtained in reflectance mode with a resolution of 4 cm–1 over a spectral range of 400–4000 cm–1.

Raman Spectroscopy

The Raman spectra of pure GO and cross-linked GO-LCTS samples were obtained to monitor changes in the degree of order–disorder of the GO structure after cross-linking using a Renishaw Raman spectrophotometer equipped with an inVia reflex optical microscope and a laser excitation wavelength of 514.5 nm over the spectral range 3700–5000 cm–1.

13C Solid-State NMR

The 13C solid NMR spectra of GO and GO-LCTS composite material were obtained using a Bruker AVANCE III HD spectrometer equipped with a 4 mm DOTY cross-polarization with magic angle spinning (CP/MAS). Spectra were acquired using CP/MAS using a solid probe operating at 125.77 MHz (1H spectral frequency at 500.23 MHz). The 13C CP/MAS NMR spectra were acquired using a contact time of 0.75 ms, MAS at 10 kHz, and 1H 90° pulse of 3.5 μs, with a ramp pulse on the 1H channel. All spectral data were recorded using 71 kHz SPINAL-64 decoupling during acquisition with external reference to adamantane at δ = 38.48 ppm.

X-ray Diffraction

The X-ray diffraction (XRD) patterns of GO and cross-linked composite materials were obtained using a PANalytical Empyrean powder X-ray diffractometer equipped with a Co source and X’Celerator detector. The powdered samples were placed in a horizontal mode and mounted for the test. The PXRD patterns were measured in continuous mode over a range of 5–60° 2θ, with a scan speed of 335 ms/step (3°/min).

Thermogravimetric Analysis

Thermal stability and decomposition temperature of the materials were measured using a TA Instruments Q50000IR TGA system, which was operated from 23 to 500 °C with a heating rate of 5 °C min–1 under a nitrogen atmosphere.

Equilibrium Swelling Properties

The swelling properties and water uptake of the GO-LCTS composites and the precursors (graphite, LCTS, and GO) were evaluated by immersing 50 mg of the material in 12 mL Millipore water as the solvent and equilibrated in a horizontal shaker for ∼24 h.[25] The weight of swollen samples (Ws) was determined by weighing hydrated samples after removing excess surface water with filter paper. By drying the hydrated samples in an oven at 60 °C, the dry weight (Wd) was obtained and the water swelling (Sw) was calculated using eq as below

Sorption Studies of GO-LCTS Composite

MB Sorption of GO-LCTS Composite

The equilibrium MB uptake capacity of GO and GO-LCTS composites was evaluated by batch mode using fixed amounts of adsorbent in sealed glass vials. The adsorption isotherms obtained by such batch mode conditions used a dosage of 5 mg adsorbent with 7 mL MB solution (pH = 7) at variable initial MB (100–1000 μM). Samples were mixed on a horizontal shaker (SCILOGEX SK-O330-Pro) for 24 h to ensure equilibrium. A 1 mM stock solution of MB in water was prepared, and other solutions were obtained by appropriate dilution. Absorption measurements were carried out at λ = 664 nm, and a linear calibration curve for MB absorbance at variable concentration was obtained with a slope of 0.0615 = Abs664nm/[MB]. After 24 h shaking to achieve equilibrium, the sample powders were separated from the solution by centrifugation and the residual (MB) was measured using UV–vis spectroscopy (Varian Cary 100 Scan UV–vis spectrophotometer). The equilibrium uptake of MB was calculated using eq .where Qe (mg g–1) is the MB adsorbed per unit mass of adsorbent, C0 (mM) is the initial dye concentration, Ce (mM) is the residual amount of MB, V (L) is the volume of the MB solution, and m (g) is the weight of the adsorbent.

The GO composite was tested for its regeneration properties over several adsorption–desorption cycles with MB, as described in further detail in the Supporting Information.

Kinetic Uptake Studies of MB

Kinetic adsorption studies of GO samples cross-linked with LCTS toward MB as the dye probe in aqueous solution to estimate the sorption capacity of GO before and after cross-linking with LCTS. Kinetic isotherm profiles were obtained by plotting Q versus t using a one-pot method described elsewhere to account for adsorption processes of nanomaterial sorbents.[26] In brief, ca. 100 mg of a powdered sample was placed into a folded filter paper with both ends sealed before adding to the MB solution, where it should be noted that the adsorption of MB by the filter paper was deemed to be negligible overall.[27] The sealed filter paper was immersed in a fixed volume (250 mL) of an aqueous 5 μM MB solution. Aliquots of MB solution were pipetted (3 mL) at variable time intervals and further quantified via UV–vis spectrophotometry (Varian Cary 100 Scan UV–vis spectrophotometer). It is worth mentioning that the filter paper used in our experiment provides reliable results because of the fast diffusion processes and negligible adsorption throughout the experiment.[27] The MB adsorption capacity at variable times for the GO and GO-LCTS materials was calculated using eq where C0 and C are the concentration values of MB initially and at variable time (t), respectively; V is the solution volume; and m is the weight of the adsorbent. The adsorption kinetics can be described by the pseudo-first-order and the pseudo-second-order (PSO) model to evaluate parameters of the isotherm. In this study, the best-fit results were obtained by PSO model as given in eq where Q is the amount of solute adsorbed at time t (mg g–1), Qe is the amount of solute adsorbed at a pseudo-equilibrium (mg g–1) condition, and k2 is the rate constant according to the PSO adsorption model. The kinetic sorption parameters that were deduced from the PSO model in this study provide a comparison of sorption characteristics of GO and its cross-linked form with LCTS.

Results and Discussion

Surface Charge Material Characterization

The point of zero charge of GO at different pH conditions is an important indicator of the nature of adsorptive interactions because the surface charge of the adsorbent phase (GO and GO-LCTS composite) is likely to affect the adsorption capacity of MB. As shown in Figure , the ζ potential of the GO materials was highly negative, and this charge decreases continuously with increasing pH from 4 to 10, and is consistent with previous literature.[28] This negative charge is primarily because of the introduction of various functional groups (such as −OH and COOH) on the GO surface, which form during the oxidation of graphite. Such groups become ionized at higher pH and result in enhanced negative charge.[7] Therefore, it is expected that GO and its modified forms will display favorable affinity with cationic species, especially over the pH range 6–12. GO forms a stable suspension in aqueous solution in the pH range 6–12 because its ζ potential is below −30 mV. Accordingly, particles with ζ potential lower than −30 mV are strong enough to maintain a stable colloidal solution because of repulsive forces between them.[29] Therefore, it would be appropriate to run the cross-linking experiment at any pH above 6. The ζ-potential measurement of GO-LCTS composite further supports the formation of the GO-LCTS composite. The ζ potential of GO was −37.5 mV prior to cross-linking with LCTS, whereas the surface charge of the composite was −2.5 mV.

Figure 2

ζ Potential for GO at variable pH and GO-LCTS composite. The right-hand panel corresponds to solution conditions at ambient pH and 295 K.

SEM Results

Figure shows SEM micrographs for GO and its cross-linked form (GO-LCTS). The SEM images depict the graphite starting material and the synthesized GO, further revealing the layered structure of these materials. By comparison, the cross-linked GO composites have wrinkled edges with irregular shapes of dense interconnected layers. The micrographs reveal that cross-linking of GO alters its regular layered morphology and surface roughness according to the cross-linker type. The SEM results indicate that the composites possess higher surface roughness and porosity when compared with pristine graphite or GO. These variations in morphology and textural properties provide support for unique product morphology that occurs upon formation of a cross-linked composite between GO and LCTS.

Figure 3

SEM micrographs of carbonaceous materials (GO, graphite) and the GO-composite material: (a) graphite, (b) GO, and (c) GO-LCTS.

FTIR and Raman Spectral Characterization

IR spectroscopy was used to monitor the changes in surface functional groups upon the formation of GO from graphite and the structural characterization of the GO-LCTS composite. The FTIR spectra of the cross-linked composite material and its precursors (GO and LCTS) are shown in Figure a.

Figure 4

FTIR (a) and Raman spectra (b) of the precursors and GO-LCTS.

First, the GO preparation was characterized by identifying the characteristic IR bands for −OH stretching at ∼3200–3700 cm–1, carbonyl groups (C=O) at ca. 1680–1730 cm–1, C=C groups at ca. 1550–1650 cm–1, and epoxy groups at ca. 1230–1350 cm–1. For the IR spectrum of LCTS, characteristic bands are centered at 1152 and 895 cm–1, corresponding to signatures assigned to a glucopyranose ring unit, whereas the C=O stretching vibration of amide I (NHCO) and amide II (N–H) bending of NH2 is observed at 1650 and 1590 cm–1, respectively. The presence of the band of both precursors (GO and LCTS) is supported by the similar spectral features of cross-linked composite materials. A noteworthy observation for the cross-linked GO composites includes the absence of some bands or changes of intensity when compared to similar IR bands for the unmodified GO. For instance, the LCTS glucopyranose band at 895 cm–1 was not observed in GO-LCTS. The formation of an amide linkage between GO and LCTS can be demonstrated by the absence of GO peaks at 1730 cm–1, attributed to C=O in the −COOH moiety of GO, where a greater IR intensity of the amide II band at 1595 cm–1 is observed for cross-linked GO composites. This provides support for the formation of a linkage between GO and LCTS as the linker, in agreement with previous reports.[30]

The Raman spectrum in Figure b for GO consists mainly of D- and G-band signatures at 1350 and 1580 cm–1. The G band is characteristic of sp2-hybridized carbon networks of graphene sheets, whereas the D-band results because of structural imperfections created by the attachment of oxygen-based functional groups on the carbon basal plane and its partially disordered structure.[31] In contrast to GO, graphite has an almost insignificant D band because of its highly crystalline structure. However, the intensity of this Raman band increased, and it became broader upon cross-linking. Various studies report that the removal of functional groups from the GO structure results in a greater D/G intensity ratio because of the onset of defects after reduction and also in self-assembly of the GO composites.[32,33] In this study, cross-linking of GO led to changes in the D/G intensity ratio, from 0.98 in GO to 1.26 in the cross-linked GO composite. This change in the D/G signal intensity ratio can be attributed to disruption of the GO layered structure upon cross-linking with CTS.

13C Solid NMR Spectral Characterization

The spectra for pristine GO (Figure ) present 13C NMR lines for epoxide groups (∼60 ppm), −C–OH groups (∼70 ppm), and graphenic group (∼130 ppm), in agreement with a previous report.[34] The spectral differences between the cross-linked GO materials with GO relate to the broadening and 13C spectral shift variation for the C–OH groups (∼70 ppm) of GO, along with new spectral signatures ca. 30 and 100 ppm assigned to the glucosamine and acetylated forms of the co-monomers of LCTS. The upfield signatures (ca. 20–80 ppm) for GO relate to the alkyl and alkenic groups. In addition, the appearance of downfield signatures at ca. 170–180 ppm relate to the presence of carbonyl (acetyl/amide groups of LCTS and −COOH groups of GO). The two 13C carbonyl signatures for the GO-LCTS composite is related to the presence of at least two carbonyl groups, one for the acetyl or amide groups. The NMR results support that cross-linking has occurred, in agreement with FTIR bands at 1655 and 1595 cm–1. The above results are in general agreement with 13C NMR results reported by Mahaninia and Wilson[35] for unmodified and cross-linked CTS with different bifunctional linker systems. The results presented herein provide an example of the first reported solid NMR spectral results for this type of GO composite material (Figure ).

Figure 5

Normalized 13C solid NMR spectra of GO and GO-based cross-linked composites.

XRD Structural Characterization

XRD characterization (Figure a) of GO (after oxidation of graphite) and GO-LCTS (after cross-linking of GO with LCTS) was used to evaluate the structure of each of the samples, as well as monitor changes in the interlayer spacing of GO after cross-linking. The XRD pattern of graphite shows a sharp characteristic peak at 2θ = 31.5°. After the introduction of oxygen functional groups to the structure of graphite, the graphitic peak shifts to 2θ = 12.5°, related to the interlayer distance of 0.72 nm between GO sheets.[36]

Figure 6

XRD patterns (a) and TGA curves (b) of GO, precursors (graphite, LCTS), and GO-LCTS composite.

Also, the XRD pattern of pure LCTS powder displays two broad peaks at 2θ = 9.5° and 22.5°, related to the amorphous hydrated and anhydrous structure of LCTS, respectively.[37] In comparison to the GO and LCTS patterns, cross-linked GO-LCTS shows shifting of the sharp XRD line of GO at 12.5° to a lower 2θ (10.2°) value with an absence of broad LCTS peaks. These changes indicate the exfoliation of LCTS into the GO sheets[38] and an increase in the interplanar distance (0.72–0.87 nm) between GO sheets by the introduction of LCTS. Although the broadening peak in the XRD pattern of GO-LCTS suggests slightly lower crystallinity of the GO-LCTS, shifting of the peak to lower wavenumbers indicates an increase in interlayer distance of GO sheets by 0.15 nm upon cross-linking with LCTS. This indicates that LCTS chains are well introduced and strongly bonded to the oxygen functional groups of GO, while maintaining the stacked structure of GO sheets in the material.[23,39] The higher crystallinity of the graphite and GO sheets agrees with the layered cross-sectional morphology observed in SEM images (Figure a,b), as compared to the slightly separated layered cross-section morphology of the GO-LCTS composite (Figure c). The results for GO and GO-LCTS composites suggest that cross-linking of the GO sheets is a critical technique to tune GO-based material properties, including morphology, degree of crystallinity, and interlayer distance between corresponding GO sheets.

Thermal Stability Properties

The TGA results for the cross-linked GO composite are shown in Figure b, where a sharp thermal decomposition occurs starting near 120 °C for GO, which results in a total weight loss event for the material. The cross-linked GO-LCTS material does not show a comparable thermal event, however; two key thermal events appear at ca. 155 and 250 °C with a weight loss of 15.6 and 35.4%, respectively. These thermal events for the GO composite relate to loss of adsorbed or free water and decomposition of GO oxygen functionalities and the LCTS backbone that yields degradation of the GO-LCTS framework structure. The TGA results confirm that the GO-LCTS structure has greater stability over GO as shown by its stability upon heating to the upper-temperature limit (500 °C).

Solvent Swelling and Dye Sorption Properties

Equilibrium swelling properties of GO-LCTS composite and the precursors are listed in Table . The results indicate that graphite and LCTS have the least swelling, whereas GO and the cross-linked GO composite (GO-LCTS) display greater swelling in water. The GO-LCTS composite has reduced swelling relative to pristine GO that may relate to the fewer ionic sites (−COOH) as a result of amide bond formation because of cross-linking, in agreement with the IR and NMR spectral results above.

Table 1

Water Swelling Properties of the GO-LCTS Composite and Precursors (Graphite and GO)

material	swelling (%)
graphite	510
LCTS	345
GO	9745
GO-LCTS	6500

The greater swelling of GO is attributed to its highly hydrophilic nature imparted by its polar functional groups (e.g., −OH, −COOH, etc.). By contrast, cross-linking of GO with LCTS decreases the overall hydrophilic character of the composite that may alter water infiltration and hydration properties of the system. The denser arrangement of the GO-LCTS 3D framework (according to SEM results) is expected to contribute steric hydration effects that differ from the 2D nature of GO because of the greater abundance and accessibility of polar functional groups in pristine GO. Differences in the hydrophilic character and network structure of GO-LCTS and GO provide an account for the water swelling properties of these materials.

Equilibrium Dye Uptake of MB

The MB sorption behavior of the GO and GO-LCTS adsorbents in aqueous solution was analyzed by fitting the adsorption isotherm results for uptake of MB at variable concentration by various models (Langmuir, Freundlich, and Sips isotherms). The Langmuir model (eq ) describes monolayer sorption for the adsorbate–adsorbent system. By comparison, Freundlich and Sips models describe the relationship for adsorbent–adsorbate systems with variable adsorption sites (eqs and 7). The Sips model accounts for Langmuir and Freundlich behavior, where the maximum adsorption capacity of the adsorbate (Qm) onto the adsorbent surface can be estimated.

In eq , Qe is the equilibrium uptake of dye per unit mass of adsorbent, Ce is the residual equilibrium concentration of dye, and K1 is the Langmuir adsorption affinity constant. In eq , Kf is the Freundlich adsorption capacity constant and nf relates to the intensity of adsorption, where 1/nf < 1.0 represents highly favorable adsorption, whereas 1/nf > 2 denotes unfavorable adsorption process, where 1/nf < 1 obtained for both GO and GO-LCTS materials showed favorable adsorption for these adsorbents. In eq , Ks is the Sips adsorption constant that relates to the adsorption energy, and ns indicates surface heterogeneity of the sorbent, where Ce is defined as in eq .

The adsorption isotherms for MB onto GO and GO-LCTS are shown in Figure , where the best-fit sorption parameters obtained by the Sips model are listed in Table , where favorable correlation coefficients (R2) are obtained (R2 ≈ 0.960–0.972). The MB adsorption results for GO and GO-LCTS material adopt behavior described by monolayer adsorption onto homogeneous adsorption sites. The latter is supported by values of ns near unity (ns = 0.96) for the surface cross-linked GO (GO-LCTS). The maximum adsorption capacity (Qm) of MB, according to the Sips model for GO-LCTS (Qm = 402.6 mg g–1) exceeds that for pristine GO (Qm = 286.9 mg g–1) and LCTS (Qm = 12.0 mg g–1; data not shown here). The adsorption capacity of MB by GO-LCTS reported here is notably higher than a number of reported values for other graphene-based adsorbents (cf. Table S1; Supporting Information).

Figure 7

(A) Isotherm sorption results for MB with GO and GO-LCTS sorbents, and (B) decolorization of MB before and after the sorption process with GO-LCTS.

Table 2

Isotherm Parameters for MB Adsorption with GO and GO-LCTS Sorbent Materials at 295 K

		sorbent material
adsorbate	parameter	GO	GO-LCTS
MB	Qm (mg g–1)	286.9	402.6
	Ks (L mg–1)	0.028	0.111
	ns	0.90	0.96
	R2	0.960	0.972

The Qm value for cross-linked GO increased by 115 mg g–1 compared to unmodified GO, reflecting the enhanced adsorption properties of the composite. The superior Qm values may relate to several factors: (i) alteration of the surface functional groups of GO-LCTS provides additional adsorption sites that favor adsorption, and (ii) cross-linking of GO with LCTS contributes cooperative sorbent–MB dye interactions because of changes in the macromolecular structure, along with alteration of the hydrophile–lipophile balance of the composite over GO. It should be noted that pristine LCTS has negligible uptake of MB, whereas the GO-LCTS material has notably higher uptake of MB. There is a strong binding interaction between MB (cationic dye) and the GO-based materials because of the presence of polar/charged functional groups with Lewis base character on the surface of these materials. MB shows a favorable binding with GO, whereas the GO cross-linked material shows much greater uptake of MB after cross-linking with LCTS. The higher adsorption capacity of GO-LCTS is attributed to its composite structure, which serves to disperse GO and prevent its aggregation in aqueous media. The favorable dispersion of GO through the composite lends to its large surface area and microporous structure that is critical for adsorption applications.[23] The interconnected network structure and higher interlayer distance between GO sheets in the GO-LCTS composite (as shown in XRD result Figure a) allow adsorbate molecules (MB) to diffuse into the active sites of GO-LCTS.[23] Additionally, both GO and LCTS are known to remove dyes via electrostatic interactions.[40,41] Therefore, the combined effect of these precursors contribute excellent sorption properties that enhance the overall sorption capacity of the GO-LCTS composite.

The high removal efficiency of cross-linked GO samples is evidenced by Figure b. At an initial MB dye concentration from 100 to 1000 μM, variable decolorization occurs after the adsorption process relative to the initially turbid colored solutions prior to adsorption.

Kinetic Dye Uptake of MB

The kinetic uptake performance of GO and GO-LCTS materials are shown in Figure , where the MB adsorption capacity increased quickly for both GO and GO-LCTS sorbent materials in the first 50 min and decreased slowly thereafter. The rapid MB uptake is contributed by the negatively charged adsorption sites accessible on the sorbent surface. By contrast, the slower uptake is because of the greater adsorption site occupancy as the surface sites become more saturated with MB at later stages of the isotherm profile. Consequently, preliminary MB adsorption may avoid the diffusion of further species onto the dense GO-LCTS structure, resulting in longer equilibrium times.

Figure 8

Kinetic uptake profile of MB with GO and GO cross-linked composite material, where the solid lines represent the best fit by the PSO model (see eq ).

Table represents the kinetic adsorption parameters for sorbent materials with MB over a 250 min interval, in which the PSO kinetic model provided reasonable fitting results based on favorable correlation coefficients (R2), where R2 ≈ 0.987–0.996. According to the obtained kinetic parameters (k2 and Qe), the GO-LCTS material has greater uptake over pristine GO based on the kinetic profiles. The value of Qe (μmol g–1) and the PSO rate constant values (k2; g/μmol min) for the sorbent materials increased after cross-linking: GO (4.17 μmol g–1) < GO-LTS (5.10 μmol g–1), and GO (0.004 g/μmol min) < GO-LCTS (0.019 g/μmol min), respectively. These trends in uptake are consistent with the results for the TGA and spectral characterization (FTIR, 13C NMR, and Raman) for the GO-LCTS material. Also, a previous report[17] indicates that the intraparticle diffusion and external diffusion play an important role in the adsorption kinetics.[42] It has been previously reported that the structure of monolith aerogels containing CTS is affected by the addition of GO. This leads to considerable changes in the morphological characteristics of both CTS and GO[43] that parallel observations noted in the SEM results for GO-LCTS in Figure . Changes in the macromolecular structure of GO and its composite form are evidenced according to the change in surface area upon swelling, sorption, and storage capacity,[43] as similarly noted in the case of water swelling or strong interactions with gaseous species.[44] Additionally, cross-linking GO with LCTS facilitated the nano-dispersion of components that yield more surface active sites for adsorption that enhance the adsorption capacity of GO-LCTS, as outlined below(cf. Figure ).

Table 3

PSO Kinetic Model Values for GO-Based Materials at 295 K

sorbent material	Qm (μmol/g)	k2 (g/μmol min)	R2
GO-LCTS	5.10	0.019	0.987
GO	4.17	0.004	0.996

The greater kinetic uptake of the composite material provides support that cooperative effects occur because of amide bond formation between GO and LCTS that may yield secondary adsorption sites for MB adsorption. Also, there are more active adsorption sites according to the increasing Qe values as the GO becomes cross-linked with LCTS. The decreasing kinetic trend of all sorbent materials also suggest that diffusion of MB through the pore network of the sorbent materials decreased with increasing contact time between sorbent where MB, along with a decreasing number of available adsorption sites over the kinetic profile as time increases.

Conclusions

GO and its cross-linked composites were synthesized by cross-linking LCTS and GO to yield a framework material. The materials were systematically characterized by several complementary methods to affirm the structure of the GO-LCTS composite, where MB removal efficacy was used to study the adsorption properties of the materials at equilibrium and kinetic conditions. The FTIR and 13C NMR spectra are reported for the first time for GO-LTCS composites that provide support for the formation of amide linkages and a unique framework structure relative to the 2D layered structure of GO. The greater surface roughness of the GO cross-linked composite parallels the variable morphology revealed by SEM that is also supported by the concomitant changes in thermal stability revealed by TGA results. The GO composite displays improved adsorbent properties over GO that are suitable for wastewater decontamination, as shown by its effective MB removal at equilibrium and kinetic conditions. The monolayer adsorption capacity (Qm) of the GO-LCTS material with MB was 402.6 mg g–1, which far exceeds that of pristine GO and CTS. The development GO-based composites derived via biopolymer cross-linking display enhanced adsorption properties with improved stability for reuse in multiple adsorption–desorption processes. This strategy extends the field of application of GO due to the facile nature and versatility of the cross-linking approach for tuning the structure of GO, along with the unique adsorption properties of cross-linked GO–CTS. This work demonstrates the potential utility of GO–CTS composites as versatile candidates in solid phase extraction for potential applications in wastewater decontamination, advanced nanomedicine, and drug delivery.[45−48]