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Article

Controlling Dye Adsorption Kinetics of Graphene Oxide Nano-Sheets via Optimized Oxidation Treatment

by
Abdullah S. Alshammari
Department of Physics, College of Science, University of Hail, Hail P.O. Box 2440, Saudi Arabia
Crystals 2024, 14(1), 49; https://doi.org/10.3390/cryst14010049
Submission received: 29 November 2023 / Revised: 26 December 2023 / Accepted: 26 December 2023 / Published: 29 December 2023
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Abstract

:
Graphene derivatives have demonstrated high potential for various applications, including environmental ones. In this work, graphene oxide nano-sheets were obtained by utilizing a simple chemical method and were tested for water treatment applications. The pollutant adsorption ability of the produced GO was adjusted through a proper oxidation treatment of the graphene nano-sheets. The GO treatment time was systematically varied to control the oxidation level of the graphene nano-sheets and was found to considerably affect the GO’s properties and performance in removing methylene blue. The microscopic studies showed well-exfoliated, few-layer GO nano-sheets. EDS and FTIR techniques were used to probe the presence of oxygen functional groups on the GO surface. The XRD investigations revealed various crystallinity levels of the prepared GO nano-sheets depending on the treatment time. The MB degradation efficiency was maximized by optimizing the GO treatment time. The results showed that the oxidation treatment parameters of GO play a major role in adjusting its properties and can be effectively utilized to boost its performance for water purification applications.

1. Introduction

Graphene derivatives such as graphene oxide and reduced graphene oxide have acquired considerable interest due to their excellent properties and their various applications [1,2]. They have shown great promise to tackle many challenges in various vital fields, including energy, the environment, and water resources [3,4,5,6,7]. Additionally, these nano-carbon structures have been shown to be highly promising for the advancement of various industries, including electronics, batteries, sensors, and catalysts [8,9,10]. For instance, graphene-based transistors with selective, real-time, and high-resolution ion sensing have recently been reported [11]. The addition of functionalized graphene to a hole transport layer has also been shown to lead to considerable enhancements in the efficiency of organic solar cells [12]. These examples and many others justify the considerable interest among researchers in investigating graphene and its derivatives, their preparation techniques, their properties, and their possible applications. Graphene and its derivatives have been prepared using different physical and chemical techniques [4,13]. Additionally, various treatment approaches of graphene-based materials have been reported to control their properties and make them suitable for specific applications [14,15,16]. Among the various derivatives of graphene, graphene oxide exhibits a high water dispersibility and a high functionalization susceptibility for use in various applications [17]. Chemical oxidation of graphene is among the most popular methods used to produce graphene oxide nano-sheets and is usually combined with mechanical exfoliation to disperse the stacked GO layers. One of the most common approaches used to oxidize graphene is that reported by Hummers and Offeman [18]. This approach has been modified by various researchers to produce GO, and new approaches have been introduced as well [19,20,21,22]. Moreover, various studies have focused on investigating the effect of the oxidation level on the properties of the produced GO layers, and they have shown the important role this factor can play in determining the properties of GO layers [23,24,25,26]. On the other hand, the possible applications of GO have been explored in numerous studies, including environmental applications. The utilization of GO on its own or as a composite with other materials for the removal of different pollutants, including methylene blue, methylene orange, methylene violet, and rhodamine B, from water has been reported [27,28,29,30]. The removal of the pollutant molecules occurs mainly through effective adsorption on the GO surface [31]. In this context, massive efforts have been made to maximize the pollutant removal ability of GO by employing different techniques, including GO functionalization [32,33]. Additionally, various parameters have been investigated to improve GO’s pollutant removal efficiency in previous studies, including the adsorbent amount, initial pollutant concentration, temperature, pH, stirring speed, and reaction time [34]. However, although the graphene oxidation method used and the related oxidation conditions can play a key role in modifying GO’s properties and performance in water treatment applications, the correlation between GO processing parameters and its pollutant removal performance is rarely reported in the literature. The oxidation conditions of GO can be used to adjust its oxidation degree, its exfoliation, its crystallinity and related defect density, and, therefore, its performance for water treatment applications. In this study, a simple oxidation method was used to oxidize graphene with an acid mixture only and at a relatively low temperature. Furthermore, the effect of the graphene oxidation time on its properties and its ability to remove methylene blue (MB) from water was investigated. The properties of the produced GO nano-sheets were studied, and their performance in water treatment applications was tested and analyzed in detail.

2. Materials and Methods

Graphene oxide nano-sheets were prepared using a simple chemical method [35,36] and were investigated using different characterization techniques. Acid mixtures of nitride acid (HNO3) and sulfuric acid (H2SO4) at a (3:1) ratio with a volume of 40 mL were prepared in four beakers. Then, about 1.0 g of raw graphite powder was added to each beaker and sonicated for 30 min. The mixtures were then placed on a hotplate at a temperature of 75 °C with continuous stirring at 300 rpm. The mixtures were left for different times: 30, 60, 90, or 120 min. After the treatment time was completed, each sample was left to cool down to room temperature. The samples were washed with DI water several times until a neutral pH value was reached. The treated samples were sonicated in DI water for 5 min to exfoliate the GO nano-sheets and to obtain good dispersions. The samples were then dried in an oven overnight at 75 °C. The prepared GO samples were imaged using a scanning electron microscope (FEI, Quanta 250 equipped with energy-dispersive X-ray spectroscopy, Eindhoven, The Netherland) and using a transmission electron microscope (JEOL, TEM-2100F, Tokyo, Japan). An X-ray diffractometer (Philips-1710) was used to record XRD patterns in a 2θ range from 0 to 90°. A Thermo scientific spectrophotometer (Thermo Scientific, Evolution 300 UV—vis, Waltham, MA, USA) was used to investigate the absorption spectra of the samples and MB. A Thermo scientific spectrometer (evolution 660) was utilized to obtain FTIR spectra.
For the water treatment evaluation experiment, polluted water with 2 × 10−5 M MB was prepared and used. The initial pH value of the prepared MB solution was ~7. The absorption spectrum of MB was firstly recorded and used as a reference for analyzing the samples’ performance. A quantity of 1 mg of GO of each prepared sample per 1 mL of MB solution was used to study the samples’ performance. The spectra of MB-polluted water were collected after 30, 90, and 180 min to investigate MB degradation in the presence of the different prepared GO samples.
The adsorption isotherm investigations were performed at room temperature. A calibration curve for MB was produced and used to calculate the initial MB solution concentrations. Then, 2.5 mg of the treated GO samples was added to 5 mL MB solutions with different MB concentrations in the range 10–40 mg/L. The MB solutions with GO nano-sheets were then left on a shaker at 160 rpm for 24 h, after which the GO nano-sheets were removed from the solutions using a centrifuge at a speed of 1500 rpm for 30 min. The MB amount left un-adsorbed in each solution was estimated using the UV–vis spectrophotometer and the obtained calibration curve. Furthermore, the reusability of the GO samples for MB removal was evaluated over 5 cycles. After each cycle, the GO samples were separated from the solution by centrifugation, washed, dried, and reused again for MB removal evaluation.

3. Results and Discussion

The SEM and TEM images of the prepared GO samples are shown in Figure 1. Figure 1a represents the raw graphite sample before the treatment (left), which shows a stack of many layers of graphene. After the treatment, the figure shows that the staking graphene layers started to detach, and with the help of mechanical sonication, good exfoliation of the GO with few GO nano-sheets was achieved as shown in Figure 1a (right). Figure 1b displays the SEM images of the different GO samples treated for 30, 60, 90, and 120 min. All the samples show good exfoliation of the raw graphite sample and the production of GO with a lesser number of layers. The TEM images of the samples are shown in Figure 1b. As can be seen from the figure and the magnified view of the sample (at the right), GO with few layers was produced.
The produced GO nano-sheet samples were examined by means of energy-dispersive X-ray spectroscopy (EDS) to determine the oxidation level of the prepared GO samples. As shown in Figure 2a, the EDS studies revealed that the untreated graphite sample had about 98% carbon content and about 2–3% oxygen. After graphite acid treatment, both the weight and atomic ratios of carbon decreased, and those of oxygen increased with increasing treatment time from 30 to 120 min, as seen from the figure. The oxygen weight and atomic ratios increased with the treatment time from 3.26% and 2.46% to 48.0% and 40.93% for the untreated graphite sample and the GO sample treated for 120 min, respectively. These results confirm the successful oxidation of the graphene sheets and show the effect of treatment time on the oxidation level of the GO nano-sheets. Figure 2b also shows the EDS spectrum of the raw graphite sample, which is mainly composed of carbon, and that of the treated GO sample, which shows the presence of carbon, oxygen, and a small ratio of sulfur (at about 2.32 KeV) as a result of the acid treatment.
The XRD study results for the prepared GO samples are shown in Figure 3. The main diffraction peaks of the GO nano-sheets are positioned, as seen from the figure, at about 26.02, which corresponds to the diffraction from the crystal planes (002) [37]. These peaks are related to the graphitic structure and generally exhibit high intensity. Moreover, the peak seen from the figure at about 2θ = 10.8° for the sample treated for 30 min is the characteristic peak of GO. The (002) peak position was found to slightly shift towards lower 2θ values with increasing treatment time from 30 to 120 min, which indicates an increase in the layers’ interplanar distance with oxidation treatment [38]. The interplanar distance (d) was calculated using the relation nλ = 2dsinθ and was found to be in the range 0.348–0.353 nm for (002) planes for the samples treated for 30 to 120 min, respectively. The effect of the oxidation treatment on the crystallinity of the GO nano-sheets is also obvious from the XRD spectra. A clear peak full width at half-maximum (FWHM) widening with the oxidation treatment is also observed. The increase in the peak FWHM is strongly correlated to the treatment time, as illustrated in Figure 3b, and is an indication of the structural variations that occur due to the various oxidation levels of the GO nano-sheets. As the treatment time of the GO samples increases from 30 to 120 min, the peak width increases. The observed FWHM widening can be attributed to the reduction in the size of the stacked layers, as well as the increased structural disorder and reduced crystallinity in GO after the oxidation treatment [39]. These observations indicate the conversion of raw graphite material into nano-sheets of GO. The XRD peak broadening and its shift towards lower 2θ values as a result of increased oxidation levels of GO nano-sheets have been reported in the literature [38].
The FTIR spectra of untreated graphite and GO samples are shown in Figure 4a. The characteristic vibrational bands of GO can be seen from the figure after oxidation treatment and exfoliation of graphite. The peak at 1069 cm−1 is attributed to the C–O stretching band, and that at about 1215 cm−1 is attributed to the epoxy C–O–C bending [40]. Moreover, the band appearing at about 1324 cm−1 is associated with the carboxyl C–OH bending vibration, and that at 1648 cm−1 is related to the C=C stretching vibration [41]. The weak bands at about 2875 cm−1 indicate the stretching vibration of the C–H bond [42]. The peak at about 3526 cm−1 is due to O–H stretching vibration of the hydroxyl group [43,44,45]. The presence of oxygen-containing functional groups confirms the oxidation of graphene layers and the formation of GO nano-sheets. The UV–vis absorption spectra of the GO samples were recorded and used to estimate the band gaps of the GO samples by applying Tauc’s relation: (αhv) = A (hv – Eg)n, where α is the absorption coefficient, hv is the energy of the photons, A is a constant, Eg is the band gap, and n is a parameter with a value of ½ for direct transitions and 2 for indirect ones [46,47]. Figure 4b presents the hv vs. (αhv)2 plots used to determine the band gap of GO nano-sheets. It is clear from the figure that variation of the oxidation treatment time leads to GO band gap modulation. The band gap of the GO sample treated for 30 min was about 2.56 eV, increasing to about 2.88 eV for the sample treated for 120 min. Such an increase in the band gap with the oxidation level has been observed in previous studies and is attributed to the effects introduced to the π conjugated system in GO nano-sheets [48,49].
The prepared samples, GO nano-sheets subjected to different oxidation treatment times, were tested for water treatment applications. Polluted water with 2 × 10−5 M MB was used to test the GO nano-sheets and to evaluate their performance for water treatment. Figure 5a,b presents SEM images of the GO nano-sheets and their FTIR spectra after they were used for MB removal from water. At the end of the adsorption experiment (after 180 min), the GO nano-sheets were removed from the polluted water sample, washed, dried, and imaged via SEM. Figure 5a shows clear, efficient adsorption of MB on the GO nano-sheets. The dark-gray regions show the GO surface, and the light ones display the adsorbed MB dye on the GO nano-sheet surface. The adsorption of MB onto the GO nano-sheets’ surface was confirmed further through FTIR investigation, as shown in Figure 5b. A comparison of the two spectra of GO nano-sheets before and after water treatment shows a disappearance and intensity reduction of most oxygen functionality-related vibration bands. Additionally, the appearance of the characteristic MB band at about 1640 cm−1 due to CH=N vibration and the increase in the intensity of the overlapped NH/OH stretching vibration at about 3500 cm−1 in comparison with the GO samples before water treatment indicate the adsorption of MB on the GO nano-sheets’ surface [50,51].
The absorption spectra of MB before and after the addition of GO nano-sheets oxidized for different times are shown in Figure 6a–e. The raw graphite sample showed very weak ability in removing MB, as seen from Figure 6a. However, all the GO samples treated for different oxidation treatment times showed good ability to degrade MB, the pollutant, but at different levels depending on the applied treatment time. This is indicated by the obvious reduction in the MB absorbance values, as seen from Figure 6b–e, after 180 min in the presence of GO samples. Furthermore, it appears that the GO sample treated for 120 min exhibited the worst removal performance, as compared to the other GO nano-sheet samples.
The variation in MB concentration as a function of time is shown in Figure 7a. It is clear from the figure that the concentration of MB decreased after adding the GO samples. All samples showed a reduction in the MB concentration with time; however, the graphite sample exhibited a very low removal performance, as the concentration of MB was reduced only to 93% of its initial concentration 180 min after sample addition. The GO nano-sheets exhibited a far better performance with reductions in MB concentration to about 56%, 61%, 63%, and 74% of its initial value for the samples treated for 30, 60, 90, and 120 min, respectively. These samples had various oxygen contents, according to the EDS results, that increased with increasing treatment time. The sample treated for 30 min had the lowest oxygen content among the treated samples and exhibited the highest MB reduction rate. With increasing treatment time, and as a result of the increasing oxygen content, the GO samples’ MB removal ability was reduced. The reduction in the removal ability with increasing oxygen-containing group density clearly indicates the role of the oxidation level in adjusting the pollutant removal ability of GO nano-sheets.
Figure 7b presents plots of (ln C/Co) vs. time (T), which were used to estimate the reaction constant (K) values. The slope of the straight line in the plots represents the value of K according to the equation ln(C/Co) = ln(A/Ao)= −KT, where C is the concentration at time t, Co is the concentration at time t = 0, and A and Ao are the absorbance and initial absorbance at time t = 0 [52]. The figure shows an obvious difference in the slope of the fitted lines, which reveals a difference in reaction constant (k) for the GO nano-sheets treated for different times. It is also clear from the figure that the reaction constant was low for the raw graphite sample, indicating low MB removal ability. GO samples, however, showed higher reaction constants, with values up to one order of magnitude higher than that of the graphite sample. This indicates the high ability of the GO samples to remove MB. Moreover, GO samples, as seen from the figure, exhibited different slopes of their fitted lines, revealing different reaction constants and different abilities to remove the pollutant depending on their oxidation conditions. GO samples oxidized for 30 min exhibited the highest reaction constant, with a value of about 3 × 10−3 min−1, and higher removal ability as shown in Figure 7c. The reaction constant then decreased with increasing GO sample treatment time and reached about 1.6 × 10−3 min−1 for the GO sample treated for 120 min. These results can be correlated with the XRD and EDS findings. Increasing the treatment time increased the density of oxygen-containing groups on the GO surface, as confirmed by the EDS studies and by the observed crystallinity reduction from the XRD as well. The induced structural disorder affects MB adsorption mechanisms onto the GO surface, namely, MB stacking through π-π interactions, resulting in the observed variation in the reaction constant values. Figure 7d presents the MB removal efficiency of graphite samples and the prepared GO nano-sheets. The graphite sample showed the lowest efficiency among the tested samples, with an efficiency of about 6.7% after 180 min. The figure also shows that the best efficiency was recorded for the GO sample treated for 30 min, at 43.7%. With increasing oxidation treatment time, the GO nano-sheets showed a reduction in removal efficiency to 38.4%, 36.8%, and 26.5% for GO samples oxidized for 60, 90, and 120 min, respectively. Based on the above discussion, it is clear that the introduction of oxygen-containing functional groups to the graphene layers’ surface and their effective exfoliation improved their ability to remove pollutants such as MB. This can be attributed to the increased surface area, which allows for the adsorption of more pollutant molecules on the GO nano-sheets’ surface, as well as possible electrostatic attraction between the functional groups and MB molecules. However, at higher oxidation levels (O/C ratios), the ability of the GO nano-sheets to remove the pollutant is reduced. The removal ability reduction can be explained by considering the mechanism involved in the pollutant removal process. The removal of MB from water mainly occurs through efficient adsorption of MB molecules onto the GO nano-sheets’ surface as a result of different processes, including π-π interaction, electrostatic attraction, and hydrogen bonding [10,53,54]. However, a prolonged oxidation treatment time may lead to considerable structural disorder and crystallinity reduction of the GO nano-sheets, as shown in Figure 8. This, in turn, adversely affects the adsorption, especially through π-π staking, and results in a reduced removal ability. Therefore, by controlling the treatment time, and as a result of the different oxidation levels achieved, the ability of GO nano-sheets to remove MB from water can be adjusted to reach a maximum possible removal efficiency. Yan et al. prepared GO with different oxidation times (0–90 min) and utilizing various doses of KMnO4 and achieved the best MB adsorption results at the longest treatment time. They attributed this result to the change in the preferable parallel π-π staking of MB molecules on graphite into vertical alignment of MB molecules on the GO surface via electrostatic interactions due to destruction of the graphite structure and the increased amount of functional groups induced by the oxidation treatment [55]. However, unlike in their study, the best MB adsorption results in the current work were achieved at the shortest treatment time used (30 min). Additional improvements in the MB removal efficiency could also be possible at a shorter time range than that reported in the current study.
The calibration curve of the MB solution concentration is shown in Figure 9a. The fitting of the experimental data shows a regression coefficient value of R2 = 0.9901, which allows for a precise determination of the MB concentration in the solution from the fitting curve. Langmuir and Freundlich isotherm calculations were performed for the GO sample that was treated for 30 min and showed the best performance results. The equilibrium adsorption of MB onto the GO nano-sheets’ surface was calculated using the relation q e = C o − C e m   V , where qe is the adsorbed MB amount, Co is the initial MB concentration, Ce is the MB equilibrium concentration, V is the solution volume, and m is the GO mass used. The Langmuir isotherm was obtained using the equation C e q e = C e q max + 1 q max K L , where qmax is the maximum MB adsorption amount and KL is the Langmuir constant [32]. Similarly, Freundlich isotherm calculations were performed using the equation ln q e = 1 n ln C e + ln K F , where KF is the Freundlich constant, and the factor 1/n indicates the extent to which the adsorption process is favorable [32]. The maximum adsorption capacity for MB was estimated from Figure 9b and was found to be about 30.65 mg/g. The Langmuir constant and the determination coefficient R2 of the Langmuir model were found to be 0.0383 L/mg and 0.98042, respectively. The obtained R2 value indicates that the experimental data were well fitted by the Langmuir model. Additionally, from the Freundlich isotherm in Figure 9c, the calculated n value was about 2.04, which indicates good adsorption of MB onto the GO nano-sheets’ surface [56]. Moreover, the obtained values of the Freundlich constant and the determination coefficient R2 were 7.6268 and 0.97389, respectively. Figure 9d shows the reusability study results of the GO nano-sheet samples for MB removal over five treatment cycles. All the samples showed a reduction in MB removal efficiency after re-usage. The removal efficiency of the graphite and GO samples was reduced after the fifth cycle by about 11.7% to 47.6% of its initial value. The MB removal efficiency of the GO sample that was treated for 30 min and showed the best performance decreased from 43.67% to 20.82% after it was used in five treatment cycles. Table 1 displays a comparison of the obtained results with those reported previously by other studies. It can be seen from the table that various performance results of GO for MB removal have been reported and were based on the optimization of the adsorbent content, MB concentration, pH value, contact time, and/or stirring speed. The results reported by Yan et al. in the table were achieved by optimizing the oxidation level of GO, and the highest values of isotherm parameters and removal efficiency were achieved for the longest treatment time used [55]. In this work, optimization of the GO performance was performed by adjusting the oxidation level of GO using a simple chemical method, and the best results were achieved for the shortest treatment time used.

4. Conclusions

Graphite was converted into GO nano-sheets using acid treatment for different times. SEM and TEM images showed good exfoliation of the GO nano-sheets. The EDS studies revealed various oxidation levels of the GO nano-sheets, with the oxygen content increasing with increasing treatment time from 30 to 120 min. The presence of oxygen-containing functional groups on the graphene surface was also confirmed through optical investigations. All GO nano-sheet samples were found to be active in terms of pollutant removal ability and were far better than the graphite sample. The removal efficiency was improved from 6.7% for the raw graphite sample to 43.7% for the GO sample with the best removal performance. Moreover, the MB removal ability of the GO nano-sheets was found to considerably depend on the oxidation treatment time. GO nano-sheet samples oxidized for a short time showed a better pollutant removal efficiency than those oxidized for longer times. The decrease in the samples’ MB removal ability is connected to structural disorder and crystallinity reduction, which affects the pollutant molecules’ staking onto the GO nano-sheets’ surface. The GO sample oxidized for a short time also showed an improved MB adsorption capacity of about 31 mg/g in comparison with the other tested samples. The obtained results of this study highlight the role of the GO oxidation treatment time and, as a result, the oxidation levels in maximizing the pollutant removal ability of these materials for efficient water treatment applications. The effect of shorter treatment times and various process temperatures on GO’s pollutant removal ability is to be investigated as it may result in further improvements in GO performance.

Funding

This research was funded by the deanship of scientific research at University of Hail (project No. 0150493).

Data Availability Statement

The data presented in this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. (a) SEM images of graphite, exfoliated GO sheets, and a GO sheet (left to right); (b) SEM images of GO samples treated for 30–120 min; and (c) TEM images of a GO sheet (left) and a magnified image of the sheet’s edge showing the produced GO with few layers (right).
Figure 1. (a) SEM images of graphite, exfoliated GO sheets, and a GO sheet (left to right); (b) SEM images of GO samples treated for 30–120 min; and (c) TEM images of a GO sheet (left) and a magnified image of the sheet’s edge showing the produced GO with few layers (right).
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Figure 2. (a) Variation in the weight and atomic percentages of C and O elements in treated graphene oxide samples with different treatment times, and (b) EDS spectra of graphite (bottom) and graphene oxide (top) samples showing the presence of C and O elements.
Figure 2. (a) Variation in the weight and atomic percentages of C and O elements in treated graphene oxide samples with different treatment times, and (b) EDS spectra of graphite (bottom) and graphene oxide (top) samples showing the presence of C and O elements.
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Figure 3. (a) XRD patterns of exfoliated graphene oxide (GO) with different treatment times, and (b) variation in the XRD peaks’ FWHM with GO treatment time.
Figure 3. (a) XRD patterns of exfoliated graphene oxide (GO) with different treatment times, and (b) variation in the XRD peaks’ FWHM with GO treatment time.
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Figure 4. (a) FTIR spectra of GO and graphite samples, and (b) band gap variation of the prepared GO samples.
Figure 4. (a) FTIR spectra of GO and graphite samples, and (b) band gap variation of the prepared GO samples.
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Figure 5. (a) SEM image of GO nano-sheets after MB adsorption (GO: dark-gray regions and MB: light-gray regions), and (b) FTIR spectra of GO nano-sheets before and after MB adsorption.
Figure 5. (a) SEM image of GO nano-sheets after MB adsorption (GO: dark-gray regions and MB: light-gray regions), and (b) FTIR spectra of GO nano-sheets before and after MB adsorption.
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Figure 6. Absorption spectra of MB at 0, 30, 90, and 180 min with different GO samples treated for (a) 0 (raw graphite sample), (b) 30, (c) 60, (d) 90, and (e) 120 min.
Figure 6. Absorption spectra of MB at 0, 30, 90, and 180 min with different GO samples treated for (a) 0 (raw graphite sample), (b) 30, (c) 60, (d) 90, and (e) 120 min.
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Figure 7. (a) Variation in MB concentration with time, (b) ln C/Co vs. time plots, (c) reaction constant (k), and (d) degradation efficiency of GO nano-sheets treated for different times.
Figure 7. (a) Variation in MB concentration with time, (b) ln C/Co vs. time plots, (c) reaction constant (k), and (d) degradation efficiency of GO nano-sheets treated for different times.
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Figure 8. (a) Adsorption of MB molecules onto GO nano-sheets through π-π staking, electrostatic forces, and hydrogen bonding. (b) Reduced possibility of MB molecule adsorption onto GO nano-sheets through π-π staking due to structural defects induced by prolonged oxidation treatment.
Figure 8. (a) Adsorption of MB molecules onto GO nano-sheets through π-π staking, electrostatic forces, and hydrogen bonding. (b) Reduced possibility of MB molecule adsorption onto GO nano-sheets through π-π staking due to structural defects induced by prolonged oxidation treatment.
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Figure 9. (a) Absorbance–concentration calibration curve of MB solution, (b) Langmuir adsorption isotherm of MB dye onto a GO sample treated for 30 min, (c) Freundlich adsorption isotherm of MB dye onto a GO sample treated for 30 min, and (d) the reusability of the prepared GO nano-sheet samples evaluated for 5 treatment cycles.
Figure 9. (a) Absorbance–concentration calibration curve of MB solution, (b) Langmuir adsorption isotherm of MB dye onto a GO sample treated for 30 min, (c) Freundlich adsorption isotherm of MB dye onto a GO sample treated for 30 min, and (d) the reusability of the prepared GO nano-sheet samples evaluated for 5 treatment cycles.
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Table 1. Removal efficiency and MB adsorption isotherm parameters of GO nano-adsorbents.
Table 1. Removal efficiency and MB adsorption isotherm parameters of GO nano-adsorbents.
SampleLangmuirFreundlichRemoval
Efficiency (%)		Ref.
qmax
mg/g		KL
L/mg		R2nKF
L/mg		R2
GO121.950.0790.9820.6717.9560.94593.00[9]
GO445.90.3890.99002.580.1340.921199.20[53]
GO41.67–598.80.0049–0.46910.8274–0.99771.77–6.690.893–269.10.9751–0.907696.37–97.03[55]
GO17.31.9850.95000.579.3860.920095.00[57]
GO39.810.3350.99843.34412.390.954618.00[58]
GO153.851.440.99725.7190.920.868985.95[59]
GO119.041.580.997−1.331516.860.974295.7[60]
GO30.650.03830.980422.047.62680.9738943.67This Work
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Alshammari, A.S. Controlling Dye Adsorption Kinetics of Graphene Oxide Nano-Sheets via Optimized Oxidation Treatment. Crystals 2024, 14, 49. https://doi.org/10.3390/cryst14010049

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Alshammari AS. Controlling Dye Adsorption Kinetics of Graphene Oxide Nano-Sheets via Optimized Oxidation Treatment. Crystals. 2024; 14(1):49. https://doi.org/10.3390/cryst14010049

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Alshammari, Abdullah S. 2024. "Controlling Dye Adsorption Kinetics of Graphene Oxide Nano-Sheets via Optimized Oxidation Treatment" Crystals 14, no. 1: 49. https://doi.org/10.3390/cryst14010049

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