Ultrahigh Water Permeance of a Reduced Graphene Oxide Membrane for Separation of Dyes in Wastewater

Abstract

Membrane separation technology has shown significant potential in the treatment of mixed dye wastewater. In this study, a reduced graphene oxide (AH-rGO) membrane was prepared using an amino-hydrothermal method and applied for the first time in mixed dye separation. These membranes can selectively recover high-value dyes while addressing the technical challenges of balancing permeability and selectivity in traditional membrane materials, which are often at odds with each other in the treatment of mixed dye wastewater. We simulated actual dye wastewater using four dyes: methyl orange (MO), methyl blue (MB), rhodamine B (RB), and Victoria Blue B (VBB). The four combinations of mixed dyes were MO/VBB, RB/VBB, MO/MB, and RB/MB, all of which demonstrated high water permeability and separation efficiency. Notably, the MO/VBB combination achieved a maximum water permeability rate of 118.79 L m⁻² h⁻¹ bar⁻¹ and a separation factor of 24.2. The AH-rGO membrane is currently the highest-permeability membrane available, achieving excellent separation results with typical mixed dye wastewater. Additionally, it demonstrates good stability. The experimental results indicate that the overall performance of the AH-rGO membrane is superior to that ofother graphene oxide (GO) membranes, which reveals the significant application potential of this membrane in the field of mixed dye wastewater treatment.

1. Introduction

The wastewater load from dye emissions produced by the textile, printing, and pharmaceutical industries continues to rise [1,2,3], posing serious threats to ecosystem stability and human health [4,5,6]. Organic dyes such as methylene blue (MB), rhodamine B (RB), methyl orange (MO), and Victoria Blue B (VBB) are the primary pollutants in textile dyeing and finishing processes. These dyes enter the environment through dye leaks and from discharges of process wastewater [7,8,9,10]. MB and RB can induce genetic mutations in mammals, MO is carcinogenic to fish, and VBB causes irreversible damage to human liver cells through bioaccumulation. Given their complex toxicity and their resistance to biodegradation, dye wastewaters must be pre-treated and classified to reduce pollutant concentrations to safe levels, laying a foundation for subsequent treatment or recycling [11,12]. In actual wastewater, dyes are not isolated; such mixed systems, due to their cumulative toxicity and structural stability, pose significant challenges in water body management [13]. Therefore, overcoming the bottlenecks in treating mixed wastewater and developing efficient dye purification and recovery technologies have become urgent needs in the field of industrial wastewater management.

At present, various technologies such as precipitation, adsorption, and membrane separation are used to treat dye wastewater [13]. Membrane separation technology is a promising and widely utilized method in wastewater treatment, owing to its cost-effectiveness, high separation efficiency, minimal energy consumption, and scalability [14,15,16,17]. The efficacy of membrane separation technology relies on the performance of the separation membrane. However, the pursuit of a membrane that exhibits both high permeability and high selectivity remains an arduous undertaking. For example, conventional polymeric membranes are inherently confronted with a “trade-off” dilemma between permeability and selectivity [18,19,20]. When efforts are made to enhance the permeability of membranes, they inevitably lead to a reduction in separation factor, and vice versa [21,22].

Graphene oxide (GO) membranes, constructed through the ordered stacking of two-dimensional nanosheets, have shown promise as novel separation membranes due to their superior filtration performance in water permeance and stability [23,24,25,26]. The GO-PANI membrane, designed by Gui and colleagues, has a water flux of 40.42 L m⁻² h⁻¹ bar⁻¹ and can efficiently retain dyes such as MB, RB, and MO [27]. The AgCO@UiO/GO membrane, developed by Zeng and colleagues, effectively separates MB/MO mixed dyes with a water flux of up to 50 L m⁻² h⁻¹ bar⁻¹ and a separation factor of 2.5 [28]. The Cu (tpa)@GO/PES membrane developed by Makhetha et al. can effectively separate MO/Congo red (CR) dyes with water flux up to 78.67 L m⁻² h⁻¹ bar⁻¹ and separation factor up to 6.1 [29]. The outstanding filtration and separation performance of GO-based membranes is based on their structural characteristics, including tunable interlayer spacing, well-aligned nanopores, and low-resistance water transport channels [30]. Notably, the interlayer spacing of these membranes can be precisely adjusted through chemical cross-linking strategies [31,32,33] or physical structure packaging [34,35]. Generally, reduced graphene oxide (rGO) membranes prepared through controlled reduction processes exhibit both excellent hydraulic permeability and stable structural integrity [24,36,37]. It is worth noting that in the field of membrane separation research, rGO prepared by hydrothermal reduction of amino groups has shown outstanding performance in ion screening applications of wastewater and radioactive solutions, showing excellent selectivity and high water flux [26,31,38], making it a promising candidate material for filtration based on a GOmembrane system.

This study successfully prepared graphene oxide (AH-rGO) films with reduced interlayer spacing by adjusting the hydrothermal reduction time of amino groups, and applied these membranes for the first time in mixed dye separation. Compared to traditional GO membrane materials, the AH-rGO membrane demonstrated significant advantages in both selectivity and water flux, particularly excelling in water flux performance. To systematically investigate its separation capabilities, experiments were conducted using four binary dye systems. The results showed that the MO/VBB dye mixture achieved optimal treatment efficiency—maintaining a high separation factor of 24.2 while delivering a water flux of 118.79 L m⁻² h⁻¹ bar⁻¹. Mechanistic studies confirmed that the membrane primarily achieved efficient separation of different-sized dye molecules through precise size-screening mechanisms. Further experiments systematically evaluated the effects of dye concentration ratios and membrane thickness on AH-rGO performance, verifying its long-term operational stability. Comprehensive experimental results therefore indicate that AH-rGO membranes exhibit outstanding comprehensive performance in mixed dye wastewater separation, demonstrating great application potential in dye treatment fields such as pre-concentration, efficient recovery, and wastewater purification [11,13].

2. Results and Discussion

2.1. Characteristics of the rGO Membranes

In our previous report, the AH-rGO suspension exhibited a zeta potential of −34.1 ± 2.2 mV [31], demonstrating excellent dispersion stability. The morphological characteristics of the AH-rGO (Figure 1a,b) membrane and the GO membrane (Figure S1a) were investigated through scanning electron microscopy (SEM) imaging. As shown in Figure 1a, the membrane fabricated from 10 mL of a 0.17 mg/mL AH-rGO suspension displayed a dry-state thickness of ≈500 nm. The membrane presented a defect-free surface morphology (Figure 1b) and remarkable thickness uniformity, both of which contributed to enhanced water flux and improved ion rejection rates [26,39,40]. The contact angle of rGO was 52.71° in the dry state, decreasing to 34.04° in the actual working state, while the contact angle of GO in the actual working state was 17.93°, demonstrating stronger hydrophilicity (Figure S1c). However, experimental data indicated that the water flux of GO is much lower than that of rGO (Figure S3). This is because the GO surface is rich in oxygen-containing functional groups, which form strong hydrogen bonds with water molecules. Water molecules have to overcome this strong interaction to pass through, significantly increasing transport resistance [25]. Thus, moderate hydrophilicity (such as that of rGO) can ensure a certain degree of wettability while reducing transport resistance, thereby achieving a higher water flux [41].

The interlayer of the AH-rGO membrane and the GO membrane was characterized by X-ray diffraction (XRD) on the mixed cellulose ester (MCE) substrate, as shown in Figure 1c [42]. The XRD analysis indicated that the 7.1 Å peak in the AH-rGO sample originated from partially reduced GO residues, while the broad peak with a 2θ of 25–27° represented the characteristic diffraction of rGO, with its interlayer spacing being reduced to 3.3 Å [43]. The broadening of the peaks suggested that the graphene sheets are disordered after reduction, significantly increasing the complexity and curvature of the interlayer channels. On the one hand, the expanded interaction sites facilitate the rapid directional migration of water molecules through hydrogen bonds and van der Waals forces, effectively enhancing water transport rates [30]. On the other hand, the controlled partial reduction of GO endows the AH-rGO membrane with a unique dual-region structure: a functionalized oxidation zone and an original graphene region. The functionalized oxidation zone, with its hydrophilic oxygen-containing functional groups, acts as a nanoscale spacer, precisely controlling the interlayer spacing and promoting the insertion of water molecules between layers. The original region, with its complete sp²-hybridized conjugated structure, forms ultra-low-resistance water molecule transport channels [30]. This “nanoscale spacer-low-resistance channel” synergy mechanism provides dual guarantees for the efficient permeation of water molecules. The reduced layer spacing establishes a molecular sieving effect, which can effectively realize the size-selective separation of mixed dye molecules.

To further investigate the chemical functional groups and their structural characteristics, we conducted complementary analyses using ultraviolet-visible spectroscopy (UV-Vis), absorption spectroscopy, and Fourier-transform infrared spectroscopy (FT-IR). UV-Vis spectroscopic analysis (Figure S1b) revealed that the characteristic absorption peak of GO at 230–240 nm, attributed to the π→π* transitions of aromatic rings, progressively diminished during the amino-hydrothermal reduction process, giving way to a new peak with enhanced absorbance near 265 nm—a hallmark of restored π-conjugated structures. Concurrently, the shoulder peak at 300 nm, corresponding to the n→π* transitions of carbonyl groups, was markedly attenuated, signifying the effective removal of oxygen-containing functional groups. The FT-IR spectrum of AH-rGO (Figure 1d) further corroborated these structural transformations: the intensities of characteristic absorption bands for the hydroxyl (3400 cm⁻¹), epoxy (1080 cm⁻¹), and carboxyl (1730 cm⁻¹) groups were significantly reduced post-treatment. Specifically, the pronounced weakening of the hydroxyl band was ascribed to nucleophilic substitution by amino groups and concomitant thermal dehydration; the decline of the epoxy band stemmed from ring-opening reactions induced by ammonia molecules; and the near-disappearance of the carboxyl band was attributed to the thermal decarboxylation of ammonium carboxylate intermediates. Additionally, the emergence of a new C–N stretching vibration peak at 1232 cm⁻¹ unequivocally confirmed the successful incorporation of nitrogen in the form of amine groups, thereby achieving the dual objectives of restoring the conjugated network and introducing heteroatomic functionalities. These spectral features show excellent agreement with previously reported data for reduced graphene oxide materials [39,40,44].

2.2. Separation Performance of the AH-rGO Membrane

To systematically evaluate the dye separation performance of the AH-rGO membrane, we measured the rejection rates of methyl orange (MO), methylene blue (MB), rhodamine B (RB), and Victoria Blue B (VBB); these were found to be 5.60%, 96.39%, 36.65%, and >99.80%, respectively. The AH-rGO membrane and MO are negatively charged, while MB, RB, and VBB are positively charged. Although, theoretically, there is an electrostatic attraction between RB and the membrane material, the experimental rejection rate remained low. This suggests that, in the dye separation process, size screening is the primary mechanism, with electrostatic interactions playing a secondary role. This separation method can efficiently separate dyes, having shown great potential in the treatment of mixed-dye-containing wastewater.

Considering that real dye wastewater often contains binary or multivariate dye mixtures, we further constructed four simulated wastewater systems containing binary dye mixtures. Based on the humidity retention strategy reported in Ref. [45], the membrane materials were prepared with non-drying treatment to optimize their hydrophilic properties. Figure 2a illustrates the dynamic filtration process in detail. Membrane preparation and dye separation experiments were carried out under vacuum. Firstly, the AH-rGO membrane was prepared on an MCE substrate via vacuum-assisted filtration (step 1). Subsequently, the mixed dye solution was injected while maintaining the hydrated state of the membrane surface (steps 2–3). Finally, the filtrate was collected by means of vacuum filtration on the osmotic side for subsequent analysis (step 4).

For mixed dye systems with individual dye concentrations of 50 mg/L under 1 bar pressure, the spectral characterization results for four representative separation systems were successfully obtained, as shown in Figure 2b. These included tests of separation of VBB from the RB/VBB mixture, MB from the MO/MB mixture, VBB from the MO/VBB mixture, and MB from the RB/MB mixture. The differences in the absorption spectra between the feeds and filtrates of the mixed dye solutions exhibited distinct spectral separation features, validating the exceptional separation performance of this approach. Subsequent quantitative analyses were made as described in the Materials and Methods section. The results revealed that the water permeations of the RB/VBB, MO/MB, MO/VBB, and RB/MB dye mixtures were 102.71, 81.06, 118.79, and 113.46 L m⁻² h⁻¹ bar⁻¹, with separation factors of 19.5, 10.5, 24.2, and 4.7, respectively (Figure 2c), demonstrating that the AH-rGO membrane provided both high water permeability and sieving efficiency in the treatment of dye waste liquid.

The superior performance of the material system can be attributed to the controlled process of partial reduction of graphene oxide nanosheets. This structural regulation enables the AH-rGO membrane to develop two characteristic regions: functionalized oxidized zones and pristine graphene regions [25]. Specifically, the functionalized oxidized zones act as nanoscale spacers to effectively promote water molecule intercalation between layers, while the pristine regions with intact sp²-hybridized structures provide ultra-low-resistance transport channels for water molecules. The synergistic effect of this unique heterostructure significantly enhances the water permeability of the AH-rGO membrane. In addition, the water permeance of the AH-rGO membrane was 118.79 L m⁻² h⁻¹ bar⁻¹, with a high separation factor of 24.2, which showed that our AH-rGO membrane exhibits superior separation performance with typical dye mixtures, compared with other dye-separation membranes (Figure 2d, Table S2). In fact, the use of GO membranes for dye rejection has already been reported, but this has been more focused on the extraction of single kinds of dye molecules from the dye/metal ion mixture [46,47]. This work is the first time that AH-rGO membranes have been used for the separation of dye mixtures. These results reveal that the highly permeable AH-rGO membrane has potential application in dye separation and effective dye recovery.

2.3. Performance of the AH-rGO Membranes

To further evaluate the performance of the AH-rGO membrane, the MO/VBB mixed dye system was selected as a representative case to analyze the effects of the solution concentration ratio, membrane thickness, and long-term stability on its sieving performance. The reason for this was that the separation factor and water permeation of the AH-rGO membrane were found to be highest for MO/VBB separation (Figure 2c). As shown in Figure 3a, both water permeability and separation efficiency demonstrated excellent stability under different dye concentration ratios (1:1, 1:2, and 2:1). Notably, the separation factor remained above 20.0 as the concentration ratio increased, with the highest water permeation of 24.2 being observed at the 1:2 ratio. This phenomenon can be attributed to the concentration polarization effect during membrane separation. Specifically, as the concentration of retained dye increases, a concentration boundary layer forms on the membrane surface, reducing the effective osmotic pressure gradient and consequently leading to apparent water permeation attenuation.

AH-rGO membranes with controlled thicknesses (250, 500, and 750 nm) were fabricated by adjusting the concentration and volume of the AH-rGO dispersion (Figure 3b). A clear inverse correlation between membrane thickness and water permeability was observed. The 250 nm thick membrane exhibited the highest water permeation (262.91 L m⁻² h⁻¹ bar⁻¹) but demonstrated a separation factor of 19.9 which was relatively lower compared to the thicker membranes. Notably, membranes with 500 and 750 nm thicknesses achieved comparable separation factors while maintaining moderate water permeability. Based on the balance between separation efficiency and permeation performance, the 500 nm thick AH-rGO membrane was identified as the optimal configuration for effective mixed dye separation, combining practical operational efficiency with robust molecular discrimination capabilities.

To further evaluate the long-term stability of the 500 nm thick AH-rGO membrane in terms of water permeability and sieving performance, continuous filtration of the MO/VBB mixture was conducted under vacuum filtration for 60 hours, with water permeation and separation factor measurements recorded every 12 hours (Figure 4). The membrane exhibited high operational stability and water permeation of 100.37 L m⁻² h⁻¹ bar⁻¹ after the 60-hour separation operation, while the separation factor remained consistently above 20.0. The gradual decline in water permeability can be attributed to concentration polarization caused by dye accumulation on the membrane surface during prolonged filtration in our dead-end filtration configuration. Collectively, these results confirm the outstanding stability of the AH-rGO membrane, highlighting its suitability for practical wastewater treatment applications requiring durable separation performance.

2.4. Potential Application in Dye Separation

The AH-rGO membrane shows significant advantages in dye wastewater treatment: it selectively intercepts the target dye (such as high-value VBB) in binary or multivariate dye mixtures, allowing low-value dye or salt to pass through while efficiently enriching the target dye. It not only enables directional recovery to reduce production costs, but also reduces the load of subsequent treatment via pre-concentration [11,13].

Its selective “screening” feature, when integrated with biological treatment and electrochemical oxidation technologies, can form a modular multi-stage treatment system centered on the AH-rGO membrane. First, it removes suspended particles and adjusts pH through pretreatment. Then, it uses a pore-size gradient membrane to selectively retain various dyes of different sizes (such as the molecularly large VBB and molecularly small MO), achieving pre-concentration and recovery of high-value dyes while allowing salts and low-value dyes to pass through. Finally, the permeate is further treated to meet discharge standards or for reuse, forming an efficient “pretreatment–membrane separation–depth treatment” system [12]. This system enables a compliant treatment of dye wastewater which is technically and economically viable and delivers efficient resource utilization.

3. Materials and Methods

3.1. Materials

Phosphorus pentoxide (P₂O₅), sulfuric acid (H₂SO₄), potassium persulfate (K₂S₂O₈), hydrochloric acid (HCl), and 28% ammonia solution (NH₄OH), along with the dyes methyl blue (MB), methyl orange (MO), Victoria Blue (VBB), and rhodamine B (RB) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. Potassium permanganate and hydrogen peroxide were obtained from Shanghai Woke Biotechnology Co., LTD. Deionized water (18.2 MΩ) was used for all experiments. All reagents were used as samples received; no further purification was required.

3.2. Preparation of AH-rGO Suspension

The AH-rGO suspension was synthesized via an amino-hydrothermal method [26,31,38,48]. Graphene oxide (GO) suspension was prepared using an improved Hummers method (specific procedures are detailed in Supplementary Information). For the hydrothermal reduction, 26 mL of 4 mg/mL GO suspension was mixed with 516 mL of DI water and 360 mL of 28% NH₄OH solution. The homogeneous mixture was maintained at 80 °C with vigorous stirring for 6.5 h, followed by additional heating at 90 °C for 1 h, ultimately producing a stable AH-rGO suspension ready for use in subsequent applications.

3.3. Separation Experiments

A 10 mL aliquot of 0.17 mg/mL AH-rGO suspension was vacuum-filtered onto a mixed cellulose ester (MCE; 0.22 μm, JINTENG, Hangzhou, China) substrate. Four binary dye systems (VBB/RB, MO/VBB, MB/RB, MO/MB) were prepared by premixing 25 mL of 100 mg/L primary dye with 25 mL of 100 mg/L secondary dye in the feed chamber of the terminal filtration unit. The filtration process was conducted through the AH-rGO membrane under 1 bar transmembrane pressure. After achieving system stabilization, permeate was collected over a 10-minute steady-state filtration period. The absorbance of the filtrate was subsequently quantified using ultraviolet-visible spectroscopy (UV-Vis).

Finally, the water permeability (J_W, unit: L m⁻² h⁻¹ bar⁻¹), the separation factor of binary hybrid dye (S), and the rejection rate (R, unit: %) were calculated according to Equations (1), (2), and (3) respectively, as follows:

$$J_W={\displaystyle \frac{V}{△t\times A\times P}}$$

(1)

$$S={\displaystyle \frac{\frac{C_{A,p }}{C_{B,p}}}{\frac{C_{A,f}}{C_{B,f}}}}$$

(2)

$$R=1-{\displaystyle \frac{C_p}{C_f}}$$

(3)

where V is the permeate volume, A is the effective membrane area (A = 1.134 × 10⁻³ m²), $△t$ is the permeation time, P is the transmembrane pressure (P = 1 bar), S is the separating factor, C_A,p and C_B,p represent the absorbance of components A and B, respectively, in the permeate, and C_A,f and C_B,f represent the absorbance of components A and B, respectively, in the raw solution. C represents the absorbance at the maximum absorption wavelength for each dye, and subscripts p and f denote the permeate and feed, respectively.

3.4. Characterizations

The membrane morphology was characterized by SEM. The interlayer spacing of the AH-rGO membrane was determined using XRD. FT-IR was employed to analyze the chemical functional groups. The absorbance properties of the AH-rGO suspension were quantitatively evaluated by UV-Vis. The static contact angle of the membrane surface was measured with an optical contact angle goniometer, while the surface charge distribution of the AH-rGO suspension was characterized via Zeta potential analysis. Details of the instrumental analysis can be found in Supplementary Materials Section S1.

4. Conclusions

In summary, the AH-rGO membrane synthesized via the amino-hydrothermal method demonstrated efficient dye separation performance and favorable water permeability across four mixed dye systems (MO/VBB, MO/MB, RB/VBB, and RB/VBB) based on size-selective sieving effects. Notably, the MO/VBB combination exhibited the highest water permeation of 118.79 L m⁻² h⁻¹ bar⁻¹ and a separation factor of 24.2. In summary, in this study we fully demonstrate the potential application of AH-rGO membranes in hybrid dye separation processes. These membranes not only efficiently recover high-value dyes but also combine ultra-high permeability with excellent dye screening performance, which is expected to significantly advance the development of wastewater dye separation and treatment technologies in the future.