Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2024 Jun 20;9(26):28626–28636. doi: 10.1021/acsomega.4c02705

Efficient Adsorption of Azo Dye Acid Brilliant Red on Graphite Carbon Nitride in Aqueous Solution

Huiwen Sun 1, Yongjun Liu 1,*
PMCID: PMC11223255  PMID: 38973842

Abstract

graphic file with name ao4c02705_0017.jpg

In this study, two types of graphite carbon nitrides were prepared by directly calcinating urea (U-g-C3N4) and melamine (M-g-C3N4) in a muffle furnace. Their adsorption performances on acid brilliant red (ABR) from aqueous solution were examined and compared. Results showed that, at the optimum calcination temperature of 580 °C, both the adsorption capacity of U-g-C3N4 and that of M-g-C3N4 increased strongly with decreasing solution pH. U-g-C3N4 exhibits higher adsorption capacity than M-g-C3N4 at an initial pH > 2.0. However, at an initial pH of 1.0, M-g-C3N4 displayed a much higher adsorption capacity than U-g-C3N4, where the maximum adsorption capacity of M-g-C3N4 can reach 25 635.64 mg g–1, being the highest reported to date. Adsorptions of both adsorbents followed pseudo-second-order kinetic models and the Langmuir adsorption isothermal models. The adsorption is spontaneous and exothermic and occurs mainly through electrostatic attraction between the protonated g-C3N4 and the negatively charged ABR. In addition, the used U-g-C3N4 can be easily regenerated with ethanol and the renewed U-g-C3N4 possesses comparable adsorption capability of its original form, showing its superior recyclability and broad industrial application prospects.

1. Introduction

Dyeing and printing processes in the textile industry have seriously contributed to the pollution of water bodies. Acid dyes, often containing azo groups, constitute the largest portion of the artificial dyes. These dyes are discharged into the environment in the form of wastewater, resulting in considerable deterioration of the receiving water quality.1 Wastewater contaminated with acid dyes is characterized by high chromaticity, poor biodegradability, and high concentration.2,3 Removal of acid dyes from wastewater is becoming a challenge due to the antioxidation design and increasingly stringent discharge standards.

Acid dyes in wastewater can be removed via photocatalytic decomposition, chemical reduction, biological oxidation, etc.4 Photocatalytic decomposition is usually energy-consuming and process complexing if an artificial light source is used.5,6 Chemical reduction uses chemical reductants to destroy the azo bond and results in a lot of toxic byproducts.7 Biological oxidation is cost-effective; however, the long operation time and the requirement of strict operating conditions limit its application in real dye wastewater treatment.8

Adsorption9−11 is an effective and environmentally friendly method that has been applied in many dye wastewater treatments. Activated carbon (AC) is the most utilized adsorbent for the removal of dyes from water.9 However, the adsorption capacity of AC is low, and the recyclability is poor. Development of highly efficient and reusable adsorbents is urgently desirable.12−14

At present, graphite carbon nitride (g-C3N4) is being extensively studied in the photocatalytic decomposition of aqueous organic pollutants15,16 since Wang et al. reported that it can split water into H2 under visible light irradiation.17g-C3N4 is a stable and nonmetallic semiconductor with a band gap of 2.7 eV, which means that it can utilize solar energy to drive the photocatalytic reactions in aqueous solution. Tremendous work has been reported in terms of organic pollutant degradation.15 It shows that g-C3N4 also adsorbs the target pollutants during the degradation.18 Zhu et al.19 studied the isoelectric point and found that methylene blue was moderately adsorbed and methyl orange was hardly adsorbed by g-C3N4. Stefa et al.20 revealed that g-C3N4 nanosheets exhibit no adsorption on anionic ones. As g-C3N4 is rich in nitrogen-containing groups, it can offer lone pair electrons to form complexes with metallic cations or salts with acids. g-C3N4 has been shown to adsorb heavy metal ions in an aqueous solution. g-C3N4 possesses a highly ordered tris-s-triazine (C6N7) unit, which should adsorb organic dyes via hydrophobic effects and π–π interactions.21 On the other hand, hydrogen bonding also plays an important role in the adsorption.22 Selective adsorption of humic acids from landfill leachate can also be achieved.23 In contrast to photocatalysis, using g-C3N4 for removing acid dyes occurs quite little.24−26

In this study, g-C3N4 was prepared using urea (U-g-C3N4) and melamine (M-g-C3N4) as precursors. In addition, their respective adsorption properties on acid brilliant red (ABR) were evaluated and compared in detail. ABR is a typical acid dye, widely applied in colorizing silk, wool, polyamides, and acrylic fibers.27 Chitosan,12 modified nanomaterials,13 and AC14 have been employed for removal of ABR from water. Effects of some important parameters on adsorption were also examined. Adsorption kinetic and thermodynamic analyses were also performed to elucidate possible adsorption mechanisms. In addition, reusability experiments were conducted.

2. Materials and Methods

2.1. Materials

Melamine [C3N3(NH2)3] and urea [(NH2)2CO] were purchased from Kermel Chemical Reagent Company (Tianjin, China). ABR was obtained from Wu Jiang Tong Luo Dyestuff Chemical Company (Jiangsu, China). Reagents mentioned above were of analytical reagent grade, without further purification. Hydrochloric acid [HCl] and sodium hydroxide [NaOH] were purchased from HoWei Pharmaceutical Company (Guangzhou, China). Distilled water was used throughout all of the experiments.

2.2. Synthesis of U-g-C3N4 and M-g-C3N4

U-g-C3N4 and M-g-C3N4 were prepared by directly heating urea and melamine in a muffle furnace, respectively.28 Namely, 18.0 g of urea was heated at 400–650 °C for 2 h, and the heating rate was set to 5 °C/min. The obtained powder was termed U-g-C3N4. M-g-C3N4 is prepared by calcining melamine: 6.0 g of melamine was heated at 520–600 °C with a heating rate of 5 °C/min for 2 h. The yielded products were grounded into powder; And the powder was then heated at 520–600 °C with a heating rate of 5 °C/min for 1 h.

2.3. ABR Adsorption Experiments

A 100 mg portion of g-C3N4 was dispersed in 200 mL of ABR solution stirred at different temperatures (298.15–318.15 K) and different initial pH values (1.0–12.0) with a certain initial ABR concentration of 75–700 mg L–1. During the adsorption, 2.0 mL of the suspension was withdrawn and centrifuged for subsequent analysis. The concentration of ABR was determined using a UV–vis spectrometer (UV-2600, Shimadzu) by measuring the absorbance of the centrifuged solutions at 492 nm. The adsorption amount (Qt (mg g–1)) of ABR was determined using eq 1:

2.3. 1

where V is the volume of ABR solution (L), C0 and Ct are the initial concentration of ABR and the concentration of ABR at adsorption time t (mg L–1), and m is the mass of g-C3N4 (g). It should be noted that the adsorption capacity (Qe) is obtained from eq 1 when the adsorption gets to equilibrium. The solid separated from the centrifugation was washed with deionized water and dried for the structural analyses.

2.4. Characterizations

Crystallographic structures of the calcined samples were analyzed using X-ray diffraction (XRD, D\/max-Ultima+, Rigaku; Bruker D2 Phaser, Cu K, = 1.5406 Å, 30 kV and 10 mA). Morphologies of the samples were observed by scanning electron microscopy (SEM, SUPRA55). N2 adsorption–desorption isotherms were conducted on an ASAP2420 surface area analyzer at 77 K. Before the measurements, the samples were degassed at 200 °C for 6 h. Brunauer–Emmett–Teller (BET, TriStar II3030) specific surface areas were determined using the desorption data. Pore size distribution was determined using the Barrett–Joyner–Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific K-AlphaTM+ spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 100 W. Samples were analyzed under a vacuum (P < 10–8 mbar) with a pass energy of 150 eV (survey scans) or 50 eV (high-resolution scans). All peaks were calibrated with a C 1s peak binding energy at 284.8 eV for adventitious carbon. Fourier transformed infrared (FT-IR) spectra were recorded on an FT-IR spectrometer (Nicolet is5 Thermo) using a standard KBr pellet technique.

3. Results and Discussion

3.1. Effect of Calcination Temperature on the Yield and Adsorption Performances

The effect of calcination temperature on the yield and the adsorption capacity (Qe) of U-g-C3N4 or M-g-C3N4 are shown in Figure 1.

Figure 1.

Figure 1

Open in a new tab

Dependences of yield (a) and adsorption capacity (b) of U-g-C3N4 and M-g-C3N4 on calcination temperature (adsorbent dose, 100 mg; initial ABR concentration, 100 mg L–1; initial pH of ABR solution, 3.0; solution volume, 200 mL; adsorption temperature, 298.15 K).

It can be illustrated from Figure 1a that both the yield of U-g-C3N4 and that of M-g-C3N4 decrease with an increase in calcination temperature. In addition, the yield of M-g-C3N4 is higher than that of U-g-C3N4 at an identical calcination temperature. At 500 °C, the yield of M-g-C3N4 is ca. 50% while that of U-g-C3N4 is only 5%, meaning that urea releases more gases (e.g., CO2 and NH3)28 than melamine during the calcination, which may endow U-g-C3N4 with many more pores than M-g-C3N4, as evidenced by the following BET analysis.

As shown in Figure 1b, the adsorption of ABR by U-g-C3N4 and M-g-C3N4 exhibit a similar trend: the adsorption capacity peaks at 580 °C, where it is 172.8 mg g–1 for U-g-C3N4 and 69.6 mg g–1 for M-g-C3N4. Therefore, the g-C3N4’s prepared at 580 °C (M-g-C3N4-580 and U-g-C3N4-580) were employed as adsorbents in the subsequent investigations.

3.2. Structural Characterizations

SEM images of U-g-C3N4-580 and M-g-C3N4-580 are presented in Figure 2. It can be shown that U-g-C3N4-580 has an observable lamellar folded structure consisting of small flakes with thicknesses of approximately 10 nm, which may provide numerous adsorption sites for the adsorption of dyes.26 M-g-C3N4-580 exhibited a similar structure to that of U-g-C3N4-580. It is a blocky structure formed by dense lamellar overlapping, with a smooth surface29 that indicates that the planar surface possesses abundant nitrogen sites.30

Figure 2.

Figure 2

Open in a new tab

SEM images of U-g-C3N4-580 (a,b) and M-g-C3N4-580 (c,d).

Figure 3a and b show the XRD patterns and FT-IR spectra of U-g-C3N4-580 and M-g-C3N4-580, respectively. As shown in Figure 3a, two obvious diffraction peaks can be observed in the two samples. The weak peak at 12.9° is related to the repeated units (tri-s-triazine) of the in-plane structure and corresponds to the (100) lattice plane. Another strong diffraction peak, (002), centered at 27.6°, corresponds to the conjugated aromatic system stacking plane.31 The diffraction peaks of M-g-C3N4-580 were much stronger than those of U-g-C3N4-580, suggesting that the crystallite size of M-g-C3N4-580 is larger than that of U-g-C3N4-580.

Figure 3.

Figure 3

Open in a new tab

XRD pattern (a) and FT-IR spectra (b) of U-C3N4-580 and M-C3N4-580.

As shown in Figure 3b, broad peaks at 3000–3289 cm–1 correspond to the stretching vibrations of the N–H bond in the primary (−NH2) and secondary (=NH) amine groups, suggesting that the precursor was not fully condensed during the calcination. Peaks at 1639 cm–1, 1573 cm–1, 1464 cm–1, and 1410 cm–1 were assigned to the stretching vibrations of tri-s-triazine repeating units while peaks at 1323 cm–1 and 1245 cm–1 were attributed to the out-of-plane bending vibrations of the tri-s-triazine.32 The sharp peak at 809 cm–1 represents the vibration of the triazine units. The FT-IR spectra results clearly indicated that M-g-C3N4-580 possesses more tri-s-triazine units than U-g-C3N4-580 and that U-g-C3N4-580 contains more triazine rings than M-g-C3N4-580. Such a difference may result in different interactions with ARB.32

Wide scan and C 1s and N 1s high resolution XPS spectra of U-g-C3N4-580 and M-g-C3N4-580 were presented in Figure 4. Figure 4a shows that M-g-C3N4-580 is mainly composed of carbon and nitrogen as well as a small amount of oxygen, while U-C3N4-580 comprises only carbon and nitrogen. The weak O 1s peak may be originated from oxygen species, such as H2O and OH, absorbed on the surface.33 The high-resolution C 1s spectra consist of three peaks at binding energies of 284.8, 286.8, and 288.3 eV, assigned to the surface adventitious carbon or defect-containing sp2 hybridized carbon atoms,16 C–N=C bonding in the aromatic ring of g-C3N4 units,34 and the sp3 hybridized C–N bonding, respectively.24 The high-resolution N 1s spectra can be deconvoluted into three peaks at binding energies of 398.8, 400.6, and 401.5 eV, corresponding to the pyridinic-like N, pyrrolic-like N, and graphitic-like N, respectively.35−37

Figure 4.

Figure 4

Open in a new tab

Wide scan XPS spectra (a) and C 1s (b) and N 1s (c) high resolution XPS spectra of U-g-C3N4-580 and M-g-C3N4-580.

Figure 5a and b shows the N2 adsorption–desorption isotherms of U-g-C3N4-580 and M-g-C3N4-580, respectively. The insets give the corresponding pore size distribution curves obtained from adsorption branch of the isotherms with the Barrett–Joyner–Halenda (BJH) method. As shown in Figure 5a and b, all samples exhibit type IV isotherms with H3-type hysteresis, at a relative pressure range of 0.6–1.0, implying the possible existence of flake-like pores. The pore size distribution curves reveal that the samples mainly contain flake-like pores centered around 2–5 nm, which means that the samples are mesoporous materials.38

Figure 5.

Figure 5

Open in a new tab

N2 adsorption–desorption isotherm and pore size distribution curves (inset) of (a) U-g-C3N4-580 and (b) M-g-C3N4-580.

The specific surface area (SSA), pore volume (Vpore), and pore size (Dpore) of U-g-C3N4-580 and M-g-C3N4-580 are listed in Table 1. The calculated Brunauer–Emmett–Taylor (BET) specific surface areas of U-C3N4-580 and M-C3N4-580 were 72.07 and 26.31 m2 g–1, respectively. The results indicated that the use of urea as the precursor could increase the specific surface area of g-C3N4 by 2.74 times.

Table 1. Specific Surface Area, Pore Volume, and Pore Size of U-g-C3N4-580 and M-g-C3N4-580.

sample U-g-C3N4-580 M-g-C3N4-580
SSA (m2/g) 72.0686 26.3141
Vpore (cm3/g) 0.329988 0.175860
Dpore (nm) 12.3262 16.8137
Open in a new tab

3.3. Adsorption on ABR

Variations in the value of Qt with adsorption time were demonstrated in Figure 6. It can be manifested that the value of Qt increases steeply with increasing adsorption time in the first hour, and then a plateau was reached, suggesting that ABR was adsorbed onto g-C3N4-580 as a monolayer, rather than as multiple layers. In addition, Qt increased little in an adsorption time longer than 2 h. Therefore, 2 h can be regarded as the equilibrium time in practice. U-g-C3N4-580 has a higher adsorption capability than M-g-C3N4-580 under the same experimental conditions. Therefore, most of the next experiments used U-g-C3N4-580 as the research target.

Figure 6.

Figure 6

Open in a new tab

Adsorption of ARB on U-g-C3N4-580 and M-g-C3N4-580 with adsorption time (adsorbent dose, 100 mg; initial ABR concentration, 150 mg L–1; initial pH of ABR solution, 2.0; solution volume, 200 mL; and adsorption temperature, 298.15 K).

In order to better describe the adsorption, two kinetic models were attempted to fit the experimental data to describe the adsorption kinetics. The pseudo-first-order model33 was

3.3. 2

and the pseudo-second-order kinetic model39 was

3.3. 3

where the amounts of ARB adsorbed at equilibrium and at time t (min) are represented by Qe and Qt (mg g–1), respectively; k1(min–1) and k2 (g mg–1 min–1) stand for the rate constants of pseudo-first order and pseudo-second-order adsorption, respectively. The linear plots of ln(Qe – Qt) versus t and (t/Qt) versus t were drawn for the pseudo-first-order and the pseudo-second-order models, respectively, where the rate constants k1 and k2 can be derived. The fitting results are given in Figure 7, and the corresponding kinetic parameters and correlation coefficients are listed in Table 2.

Figure 7.

Figure 7

Open in a new tab

Plots of the pseudo-first-order (a) and pseudo-second-order kinetic models (b) for ABR adsorption on U-g-C3N4-580 and M-g-C3N4-580 (data from Figure 6).

Table 2. Adsorption Kinetic Parameters of ABR over U-g-C3N4-580 and M-g-C3N4-580.

sample pseudo-first-order model
 pseudo-second-order model
parameters k1 (min–1) Qe (mg g–1) R2 k2 (g mg–1 min–1) Qe (mg g–1) R2
U-g-C3N4-580 0.04 140.66 0.98 5.49 × 10–5 198.02 0.99
M-g-C3N4-580 0.03 139.99 0.98 4.31 × 10–5 181.16 0.99
Open in a new tab

Table 2 demonstrates that the correlation coefficient of the pseudo-second-order kinetic model (0.99) is higher than that of the pseudo-first-order kinetic model. Therefore, the adsorption of g-C3N4-580 on ABR is more consistent with the pseudo-second-order kinetics. In addition, the values of Qe calculated from the pseudo-second-order kinetic model for U-g-C3N4-580 and M-g-C3N4-580 were 198.02 and 181.16 mg g–1, closer to the actual adsorption capacity. The adsorption kinetic constant for U-g-C3N4-580 is higher than that for M-g-C3N4-580, which shows a more favorable interaction between ABR and U-g-C3N4-580.39,40

3.4. Effect of Initial pH Value on the Adsorption

pH usually plays an important role in the adsorption. In this study, the adsorption was carried out on ABR solutions at initial pH from 1.0 to 12.0, and the results are illustrated in Figure 8.

Figure 8.

Figure 8

Open in a new tab

Effect of initial pH of ABR solution on its absorption on U-g-C3N4-580 and M-g-C3N4-580 (adsorbent dose, 100 mg; initial ABR concentration, 150 mg L–1; solution volume, 200 mL; adsorption temperature, 298.15 K).

As indicated in Figure 8, the adsorption capacity (Qe) of both U-g-C3N4-580 and M-g-C3N4-580 increased markedly with a decreasing initial pH value. U-g-C3N4-580 showed superior adsorption performance to that of M-g-C3N4-580 when the initial pH value was larger than 2.0. However, the Qe of M-g-C3N4-580 is almost equal to that of U-g-C3N4-580 at initial pH 1.0. It should be noted that both the adsorption ratios reached over 99% in conducting the experiments at an initial pH of 1.0, and this may lead to errors in determinations. Therefore, adsorption experiments of both materials at pH 1.0 with lower adsorbent dosages were further investigated in Figures 9–11.

Figure 9.

Figure 9

Open in a new tab

Adsorption isotherms of ABR on U-g-C3N4-580 at different initial pH values (solution volume, 200 mL; adsorption temperature, 298.15 K; adsorbent dose, 50 mg for initial ABR solution pH 1.0 and 100 mg for initial pH of ABR solution 1.5, 2.0, and 3.0).

Figure 11.

Figure 11

Open in a new tab

Adsorption isotherm of ABR on M-g-C3N4-580 fitted with Langmuir at initial pH 1.0 (a; adsorbent dose, 10 mg; solution volume, 200 mL; temperature, 298.15K) and pH 2.0 (b; adsorbent dose, 100 mg; solution volume, 200 mL; adsorption temperature, 298.15 K).

In acidic solution, ABR was in the anionic form R-SO3– and g-C3N4 was positively charged.29 As a result, an acidic medium is favorable for the adsorption because of the electrostatic attractions between R-SO3– and protonated g-C3N4. However, as observed in Figure 7, under alkaline conditions even at pH 12.0, where almost no protonated g-C3N4 exists, the adsorption capacity is still above 100 mg g–1, indicating that interactions other than the electrostatic one between ABR and g-C3N4 must be present, such as hydrogen bonding and π–π interactions.41,42

Information on the surface properties, adsorbate affinity, and adsorption capacity of an adsorbent can be obtained from an adsorption isotherm. The Langmuir and Freundlich isotherm models are two extensively used mathematical models. The Langmuir model assumes a monolayer coverage and that all of the adsorbent sorption sites are the same. The Freundlich isotherm model assumes that the coverage is multilayer and that all of the adsorption sites are heterogeneous43 as follows:

3.4. 4
3.4. 5

where Qe and Qm (mg g–1) are the adsorption capacity and the maximum adsorption capacity (corresponding to complete monolayer coverage), respectively; Ce (mg L–1) represents the adsorbate concentration at adsorption equilibrium; n is the Freundlich constant; KL and KF are the constants corresponding to the energy of sorption (L mg–1) and adsorption performance (mg g–1 (L mg–1)1/n), respectively. Qm and KL can be calculated according to the slope and intercept of the linear plot of Ce/Qe against Ce. n and KF can be calculated from the slope and intercept of the linear plot of ln Qe versus ln Ce.

Adsorption isotherms of U-g-C3N4-580 at initial pH’s of 1.0, 1.5, 2.0, and 3.0 were performed, and the results are shown in Figure 9. It can be observed from Figure 9 that the adsorption capacity of U-g-C3N4-580 increases with increasing Ce at each initial pH value. These curves are typical type L1 isotherms, implying high affinity between adsorption sites of U-g-C3N4-580 and ABR.44 U-g-C3N4-580 has the highest adsorption capacity at the initial pH of 1.0. Fittings of adsorption isothermal curves of Figure 9 are displayed in Figure 10.

Figure 10.

Figure 10

Open in a new tab

Adsorption isotherms of ABR on U-g-C3N4-580 fitted with Langmuir (a) and Freundlich (b) models at the initial pH values of 1.0 (inset), 1.5, 2.0, and 3.0 (adsorbent dose, 100 mg; solution volume, 200 mL; adsorption temperature, 298.15 K).

As shown in Figure 10a, the Langmuir fitted adsorption isothermal curves showed a gradual increase in the slope and a gradual decrease in the fitted Qm as the initial pH increased. Parameters calculated from the linear forms of the two isotherms are summarized in Table 3.

Table 3. Langmuir and Freundlich Constants for the Adsorption of ABR over U-g-C3N4-580 at Different Initial pH Values.

  Langmuir adsorption isotherm
 Freundlich adsorption isotherm
initial pH Qm (mg g–1) KL (L mg–1) R2 n KF (L g–1) R2
1.0 2063.04 0.03 0.99 3.27 365.56 0.99
1.5 490.20 0.12 0.99 5.57 68.88 0.95
2.0 252.53 0.10 0.99 3.43 67.87 0.98
3.0 164.20 0.12 0.99 1.29 0.01 0.89
Open in a new tab

The Qm values fitted at initial pH’s of 1.0, 1.5, 2.0, and 3.0 were 2063.04 mg g–1, 490.20 mg g–1, 252.53 mg g–1, and 164.20 mg g–1, respectively. The pH of the solution also affects the degree of the protonation U-g-C3N4-580, usually increased with decreasing pH, thus attracting more ABR molecules.45 The Langmuir adsorption model has a stronger correlation and a better fit compared to the fitting of Freundlich adsorption isothermal curves, as shown by the values of R2 in Table 3.

Figure 11 shows Langmuir isothermal models of M-g-C3N4-580 at initial pH’s of 1.0 and 2.0, where the Langmuir constants for the adsorptions are summarized in Table 4.

Table 4. Langmuir Constants for Adsorption of ABR by U-g-C3N4-580 and M-g-C3N4-580 at Initial pH’s of 1.0 and 2.0 (Conditions from Figures 10 and 11).

initial pH sample Qm (mg g–1) KL (L mg–1) R2
1.0 U-g-C3N4-580 2063.04 0.03 0.99
M-g-C3N4-580 25635.64 0.01 0.99
2.0 U-g-C3N4-580 252.53 0.10 0.99
M-g-C3N4-580 176.68 0.18 0.96
Open in a new tab

At an initial pH of 2.0, the Qm of M-g-C3N4-580 was 176.68 mg g–1, less than that of U-g-C3N4-580, 252.53 mg g–1, which has the same trend of the results shown in Figure 6. But at the initial pH of 1.0, the Qm value of M-g-C3N4-580 reached 25635.64 mg g–1, being 12.4-fold greater than that of U-g-C3N4-580 2063.04 mg g–1. Such unexpected results can be explained in that the M-g-C3N4-580 is rich in tri-s-triazine units (Figure 3b), which can be further protonated at a low pH to attract ABR, while triazine units in U-g-C3N4-580 cannot.46 Previous studies showed that anionic dye was hardly19 adsorbed on g-C3N4; however, the present study gives the opposite result.

3.5. Effect of Solution Temperature on the Adsorption

Figure 12a shows adsorption isotherms of ABR on U-g-C3N4-580 at different adsorption temperatures. It can be seen from Figure 11a that the adsorption capacity of U-g-C3N4-580 decreases with increasing adsorption temperature. Adsorption is nearly always a diffusion process with exothermic nature. High temperature can result in an increase in the mobility of the acidic compounds but can decrease the interaction of ABR with active sites of U-g-C3N4-580, which causes a decrease in the adsorption capacity.46

Figure 12.

Figure 12

Open in a new tab

Adsorption isotherms (a) and Van’t Hoff plots (b) of U-g-C3N4-580 on ARB (adsorbent dose, 100 mg; solution volume, 200 mL; initial pH of ABR solution, 1.5).

Thermodynamic parameters such as ΔG (standard Gibbs free energy change), ΔH (standard enthalpy change), and ΔS (standard entropy change) were calculated from the following equations:47

3.5. 6
3.5. 7
3.5. 8

where T is the adsorption temperature (K), CAe and Ce are the equilibrium concentrations of ABR in g-C3N4 and solution (mg L–1), and R is the universal gas constant (8.314 J mol–1 K–1). The van’t Hoff plots for the adsorption of U-g-C3N4-580 are shown in Figure 12b, and the calculated thermodynamic parameters are given in Table 5.

Table 5. Thermodynamic Parameters of U-g-C3N4-580 for ABR Adsorption.

C0 (mg L–1) adsorption temperature (K) ΔG (kJ mol–1) ΔS (J mol–1 K–1) ΔH (kJ mol–1)
150 298.15 –6411.94 –170.94 –56.94
308.15 –3394.42
318.15 –3048.95
200 298.15 –5466.51 –162.70 –53.57
308.15 –2679.73
318.15 –2241.20
250 298.15 –3758.76 –107.70 –35.60
308.15 –1931.13
318.15 –1637.42
300 298.15 –2723.26 –100.34 –32.41
308.15 –1038.80
318.15 –745.52
Open in a new tab

The negative value of ΔG in Table 5 indicates that the adsorption is spontaneous. The increase in ΔG value with increasing adsorption temperature indicates that high temperature unfavors the adsorption. Negative ΔH values confirm that chemical bonds between ABR and U-g-C3N4 formed that make the adsorption exothermic; the negative ΔS values reveal decreasing chaos of the solid-solution interface during the adsorption.

3.6. Reusability and Comparison with Other Adsorbents

Reusability is an important parameter in evaluating the performance of the adsorbents. In this study, U-g-C3N4-580 adsorbed with ABR was separated from the solution by centrifugation and was added into 50 mL of anhydrous ethanol sonicated for 2 h, then centrifuged, washed with deionized water, and dried. Then, the regenerated U-g-C3N4-580 was added to the solution to adsorb ABR again, and the results are given in Figure 13, where the adsorption conditions were the same as those for pristine U-g-C3N4-580.

Figure 13.

Figure 13

Open in a new tab

Readsorption properties of U-g-C3N4-580 after desorption with 50 mL of anhydrous ethanol (adsorbent dose, 100 mg; solution volume, 200 mL; initial pH, 1.5; initial ABR concentration for each run, 100 mg L–1).

As shown in Figure 13, the adsorption ratio for ABR could still reach 80% at the fourth repeated adsorption, indicating the wonderful recyclability of U-g-C3N4-580.

The XRD pattern and FTIR spectra of U-g-C3N4-580 after desorption are presented in Figure 14.

Figure 14.

Figure 14

Open in a new tab

XRD pattern (left) and FTIR spectra (right) of U-g-C3N4-580 after desorption with 50 mL of anhydrous ethanol (adsorbent dose, 100 mg; solution volume, 200 mL; initial pH of ABR solution, 1.5; adsorption temperature, 298.15 K; initial ABR concentration, 200 mg L–1).

As shown in Figure 14, no peaks appeared or disappeared in both the XRD pattern and the IR spectra, indicating the excellent regeneration of U-g-C3N4-580. However, the XRD peak intensities of the desorbed U-g-C3N4-580 were much stronger than the pristine one, implying that the crystal size of U-g-C3N4-580 increased during the adsorption and desorption processes. Peak intensities in the range 1246–1639 cm–1 enhanced after desorption demonstrate the increased relative number of tri-s-triazine units in U-g-C3N4-580 during the adsorption.

In order to better understand the adsorption process, pH changes in the adsorption processes are given in Figure 15.

Figure 15.

Figure 15

Open in a new tab

Change in ABR solution pH during the adsorption by U-g-C3N4-580 (adsorbent dose, 15 mg; initial ABR concentration, 800 mg L–1; solution volume, 200 mL; adsorption temperature, 25 °C).

It can be observed from Figure 15 that, when the ABR solution pH is lower than 7.8, the solution pH increased after adsorption, indicating that adsorption proceeds via protonation of the adsorbents. However, the pH decreased in the case of higher solution pH, indicating that the U-g-C3N4-580 also reacts with OH– in the solution.19

According to the experimental results and the related discussions, the protonation mechanism of g-C3N4 for ABR is proposed in Figure 16. It can be observed from Figure 16 that g-C3N4 is protonated in acidic solution where the ABR is in the form R-SO3–, and the ABR is adsorbed by the protonated g-C3N4 due to electrostatic attraction. Usually, the bonding amino groups are the most easily protonated followed by the nitrogen in the tri-s-triazine units.29 Triazine units are hardly protonated in the solution.46,48 The adsorption capacity of U-g-C3N4-580 is higher than that of M-g-C3N4-580 at a pH higher than 2.0 because it possesses a larger surface area. However, as M-g-C3N4-580 contains more tri-s-triazine units than U-g-C3N4-580, M-g-C3N4-580 has much larger adsorption capacity at an initial pH of 1.0.48 Both ABR and g-C3N4 contained aromatic moieties in the structure, and π–π stacking can contribute to the adsorption. Moreover, g-C3N4 contains sec-((R)2-NH) groups; hydrogen bonds can be formed between the hydrogen atoms therein and the oxygen atoms in ABR.42

Figure 16.

Figure 16

Open in a new tab

Possible protonation mechanism for ABR on g-C3N4.

The maximum adsorption capacity of g-C3N4 prepared in this work and those of other adsorbents for ARB are shown in Table 6.

Table 6. Comparison of the Maximum Adsorption Capacity of Various Adsorbents for ARB.

adsorbents T (°C) Qm (mg/g) reference
AC from corn straw prepared by microwave 45 178.57 (14)
U-g-C3N4-580 25 2063.04 this work
M-g-C3N4-580 25 25635.64 this work
NiO(111) nanosheets 25 30.4 (13)
Open in a new tab

As Table 6 shows, the maximum adsorption capacity of M-g-C3N4-580 can reach 25635.64 mg/g at ambient temperature, which is 100 times higher than those of other adsorbents, indicating its superiority in the adsorption field. As the price of the precursor (melamine, ca. $1.00 per kg in the present Chinese market) for preparing M-g-C3N4-580 is very low, the application of M-g-C3N4-580 in acid dye wastewater treatment would be greatly expected.

4. Conclusions

In this study, we report that g-C3N4 synthesized by direct calcination of urea and melamine was highly efficient for the adsorption of the ABR dye from aqueous solution. The optimum calcination temperature for preparing both materials was 580 °C, where the specific surface area of g-C3N4 obtained from urea (U-g-C3N4) was 2.74 times that from melamine (M-g-C3N4). The adsorption capacity strongly increases with decreasing initial solution pH and adsorption temperature. U-g-C3N4 had a superior adsorption effect to that of M-g-C3N4 at pH > 2.0. However, M-g-C3N4 has a much higher adsorption performance at an initial pH of 1.0, where the maximum adsorption capacity can reach 25 635.64 mg g–1, which was 100 times higher than that of other competitive adsorbents reported to date. The pseudo-second-order kinetic model and Langmuir isotherm model can best fit the adsorption data. The adsorption is spontaneous and exothermic. Electrostatic interaction between the protonated g-C3N4 and ABR predominates the adsorption. g-C3N4 can be easily regenerated by ethanol, and the regenerated g-C3N4 can be successfully used for the new adsorption cycle, showing its strong applicability in dye wastewater treatment.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (11005014, 11675031).

The authors declare no competing financial interest.

References

  1. Maheshwari K.; Agrawal M.; Gupta A. B. Dye pollution in water and wastewater. Novel Materials for Dye-containing Wastewater Treatment; Springer: Singapore 2021, 1–25. 10.1007/978-981-16-2892-4_1. [DOI] [Google Scholar]
  2. Yaseen D. A.; Scholz M. Textile dye wastewater characteristics and constituents of synthetic effluents: a critical review. International Journal of Environmental Science and Technology 2019, 16, 1193–1226. 10.1007/s13762-018-2130-z. [DOI] [Google Scholar]
  3. Sun D.; Zhang X.; Wu Y.; et al. Adsorption of anionic dyes from aqueous solution on fly ash. Journal of Hazardous Materials 2010, 181 (1), 335–342. 10.1016/j.jhazmat.2010.05.015. [DOI] [PubMed] [Google Scholar]
  4. Solayman H. M.; Hossen M. A.; Abd Aziz A.; et al. Performance evaluation of dye wastewater treatment technologies: A review. Journal of Environmental Chemical Engineering 2023, 11, 109610. 10.1016/j.jece.2023.109610. [DOI] [Google Scholar]
  5. Luo Y.; Wei X.; Gao B.; et al. Synergistic adsorption-photocatalysis processes of graphitic carbon nitrate (g-C3N4) for contaminant removal: kinetics. models, and mechanisms, Chemical Engineering Journal 2019, 375, 122019. 10.1016/j.cej.2019.122019. [DOI] [Google Scholar]
  6. Cui Z.; Yang H.; Zhao X. Enhanced photocatalytic performance of g-C3N4/Bi4Ti3O12 heterojunction nanocomposites. Materials Science and Engineering: B 2018, 229, 160–172. 10.1016/j.mseb.2017.12.037. [DOI] [Google Scholar]
  7. Wang X.; Deng B.; Yu L.; et al. Degradation of azo dyes Congo red by MnBi alloy powders: performance, kinetics and mechanism. Mater. Chem. Phys. 2020, 251, 123096. 10.1016/j.matchemphys.2020.123096. [DOI] [Google Scholar]
  8. Kishor R.; Purchase D.; Saratale G. D.; et al. Ecotoxicological and health concerns of persistent coloring pollutants of textile industry wastewater and treatment approaches for environmental safety, J. Environ. Chem. Eng. 2021, 9, 105012. 10.1016/j.jece.2020.105012. [DOI] [Google Scholar]
  9. Gómez V.; Larrechi M. S.; Callao M. P. Kinetic and adsorption study of acid dye removal using activated carbon. Chemosphere 2007, 69 (7), 1151–1158. 10.1016/j.chemosphere.2007.03.076. [DOI] [PubMed] [Google Scholar]
  10. Ayati A.; Shahrak M. N.; Tanhaei B.; et al. Emerging adsorptive removal of azo dye by metal–organic frameworks. Chemosphere 2016, 160, 30–44. 10.1016/j.chemosphere.2016.06.065. [DOI] [PubMed] [Google Scholar]
  11. Bingöl D.; Veli S.; Zor S.; et al. Analysis of adsorption of reactive azo dye onto CuCl2 doped polyaniline using Box–Behnken design approach. Synth. Met. 2012, 162 (17–18), 1566–1571. 10.1016/j.synthmet.2012.07.011. [DOI] [Google Scholar]
  12. Muedas-Taipe G.; Maza Mejía I. M.; Santillan F. A.; et al. Removal of azo dyes in aqueous solutions using magnetized and chemically modified chitosan beads. Mater. Chem. Phys. 2020, 256, 123595. 10.1016/j.matchemphys.2020.123595. [DOI] [Google Scholar]
  13. Song Z.; Chen L.; Hu J.; et al. NiO (111) nanosheets as efficient and recyclable adsorbents for dye pollutant removal from wastewater. Nanotechnology 2009, 20 (27), 275707. 10.1088/0957-4484/20/27/275707. [DOI] [PubMed] [Google Scholar]
  14. Ren X.; Wang S.; Jin Y.; et al. Adsorption properties of reactive dyes on the activated carbon from corn straw prepared by microwave pyrolysis. Desalination and Water Treatment 2020, 200, 296–303. 10.5004/dwt.2020.26131. [DOI] [Google Scholar]
  15. Luo Y.; Zhu Y.; Han Y.; et al. g-C3N4-based photocatalysts for organic pollutant removal: a critical review. Carbon Research. 2023, 2 (1), 14. 10.1007/s44246-023-00045-5. [DOI] [Google Scholar]
  16. Pawar R. C.; Son Y.; Kim J.; et al. Integration of ZnO with g-C3N4 structures in core-shell approach via sintering process for rapid detoxification of water under visible irradiation. Curr. Appl. Phys. 2016, 16 (1), 101. 10.1016/j.cap.2015.11.002. [DOI] [Google Scholar]
  17. Wang X.; Maeda K.; Thomas A.; et al. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8 (1), 76–80. 10.1038/nmat2317. [DOI] [PubMed] [Google Scholar]
  18. Xu H. Y.; Wu L. C.; Zhao H.; et al. Synergic effect between adsorption and photocatalysis of metal-free g-C3N4 derived from different precursors. PloS one 2015, 10 (11), e0142616 10.1371/journal.pone.0142616. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Zhu B.; Xia P.; Ho W.; et al. Isoelectric point and adsorption activity of porous g-C3N4. Appl. Surf. Sci. 2015, 344, 188–195. 10.1016/j.apsusc.2015.03.086. [DOI] [Google Scholar]
  20. Stefa S.; Griniezaki M.; Dimitropoulos M.; et al. Highly porous thin-layer g-C3N4 nanosheets with enhanced adsorption capacity. ACS Applied Nano Materials 2023, 6 (3), 1732–1743. 10.1021/acsanm.2c04632. [DOI] [Google Scholar]
  21. Zhang L.; Li L.; Sun X.; et al. ZnO-layered double hydroxide@graphitic carbon nitride composite for consecutive adsorption and photodegradation of dyes under UV and visible lights. Materials 2016, 9 (11), 927. 10.3390/ma9110927. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Zhao L.; Lv W.; Hou J.; et al. Synthesis of magnetically recyclable g-C3N4/Fe3O4/ZIF-8 nanocomposites for excellent adsorption of malachite green. Microchemical Journal 2020, 152, 104425. 10.1016/j.microc.2019.104425. [DOI] [Google Scholar]
  23. Wang J.; Wang C.; Shi A. An innovative approach for landfill leachate treatment based on selective adsorption of humic acids with carbon nitride. Chemical Engineering Journal 2023, 461, 142090. 10.1016/j.cej.2023.142090. [DOI] [Google Scholar]
  24. Pham T.-T.; Shin E. W. Influence of g-C3N4 precursors in g-C3N4/NiTiO3 composites on photocatalytic behavior and the interconnection between g-C3N4 and NiTiO3. Langmuir 2018, 34 (44), 13144–13154. 10.1021/acs.langmuir.8b02596. [DOI] [PubMed] [Google Scholar]
  25. El Messaoudi N.; Ciğeroğlu Z.; Şenol Z. M.; et al. A comparative review of the adsorption and photocatalytic degradation of tetracycline in aquatic environment by g-C3N4-based materials. Journal of Water Process Engineering 2023, 55, 104150. 10.1016/j.jwpe.2023.104150. [DOI] [Google Scholar]
  26. Fronczak M. Adsorption performance of graphitic carbon nitride-based materials: current state of the art. Journal of Environmental Chemical Engineering 2020, 8 (5), 104411. 10.1016/j.jece.2020.104411. [DOI] [Google Scholar]
  27. GB/T 21888-2023; C.I. Acid red131 (Acid brilliant red P-9B 150%); Chinese standard.
  28. Yang W.; Jia L.; Wu P.; et al. Effect of thermal program on structure–activity relationship of g-C3N4 prepared by urea pyrolysis and its application for controllable production of g-C3N4. J. Solid State Chem. 2021, 304, 122545. 10.1016/j.jssc.2021.122545. [DOI] [Google Scholar]
  29. Wang L.; Chen D.; Miao S.; et al. Nitric acid-assisted growth of InVO4 nanobelts on protonated ultrathin C3N4 nanosheets as an S-scheme photocatalyst with tunable oxygen vacancies for boosting CO2 conversion. Chemical Engineering Journal 2022, 434, 133867. 10.1016/j.cej.2021.133867. [DOI] [Google Scholar]
  30. Raaja Rajeshwari M; Kokilavani S.; Sudheer Khan S. Recent developments in architecturing the g-C3N4 based nanostructured photocatalysts: synthesis, modifications and applications in water treatment. Chemosphere 2022, 291, 132735. 10.1016/j.chemosphere.2021.132735. [DOI] [PubMed] [Google Scholar]
  31. Yang H.-C.; Chao M.-W.; Chou C.-J.; et al. Mushroom waste-derived g-C3N4 for methyl blue adsorption and cytotoxic test for Chinese hamster ovary cells. Mater. Chem. Phys. 2020, 244, 122715. 10.1016/j.matchemphys.2020.122715. [DOI] [Google Scholar]
  32. Li J.; Xiong Y.; Wan H.; et al. In-situ investigation of dye pollutant adsorption performance on graphitic carbon nitride surface: atr spectroscopy experiment and md simulation insight. Journal of Hazardous Materials 2021, 418, 126297. 10.1016/j.jhazmat.2021.126297. [DOI] [PubMed] [Google Scholar]
  33. Ren B.; Xu Y.; Zhang L.; et al. Carbon-doped graphitic carbon nitride as environment-benign adsorbent for methylene blue adsorption: kinetics, isotherm and thermodynamics study. Journal of the Taiwan Institute of Chemical Engineers 2018, 88, 114–120. 10.1016/j.jtice.2018.03.041. [DOI] [Google Scholar]
  34. Wang Z.; Chen L.; Du X.; et al. A “pillared” process to construct graphitic carbon nitride based functionalized mesoporous materials. RSC Adv. 2016, 6 (19), 15605–15609. 10.1039/C5RA26192A. [DOI] [Google Scholar]
  35. Yan T.; Chen H.; Wang X.; et al. Adsorption of perfluorooctane sulfonate (pfos) on mesoporous carbon nitride. RSC Adv. 2013, 3 (44), 22480. 10.1039/c3ra43312a. [DOI] [Google Scholar]
  36. Ciğeroğlu Z.; Kazan-Kaya E. S.; El Messaoudi N.; et al. Remediation of tetracycline from aqueous solution through adsorption on g-C3N4-ZnO-BaTiO3 nanocomposite: optimization, modeling, and theoretical calculation. J. Mol. Liq. 2023, 369, 120866. 10.1016/j.molliq.2022.120866. [DOI] [Google Scholar]
  37. Xiong S.; Liu X.; Zhu X.; et al. One-step preparation of well-dispersed spindle-like Fe2O3 nanoparticles on g-C3N4 as highly efficient photocatalysts. Ecotoxicology and Environmental Safety 2021, 208, 111519. 10.1016/j.ecoenv.2020.111519. [DOI] [PubMed] [Google Scholar]
  38. Wang F.; Tang T.; Zhang R.; et al. Magnetically recyclable cos-modified graphitic carbon nitride-based materials for efficient immobilization of gaseous elemental mercury. Fuel 2022, 326, 125117. 10.1016/j.fuel.2022.125117. [DOI] [Google Scholar]
  39. Ren B.; Xu Y.; Zhang L.; et al. Carbon-doped graphitic carbon nitride as environment-benign adsorbent for methylene blue adsorption: kinetics, isotherm and thermodynamics study. Journal of the Taiwan Institute of Chemical Engineers 2018, 88, 114–120. 10.1016/j.jtice.2018.03.041. [DOI] [Google Scholar]
  40. Haque E.; Jun J. W.; Talapaneni S. N.; et al. Superior adsorption capacity of mesoporous carbon nitride with basic CN framework for phenol. J. Mater. Chem. 2010, 20 (48), 10801. 10.1039/c0jm02974b. [DOI] [Google Scholar]
  41. Lafi R.; Montasser I.; Hafiane A. Adsorption of congo red dye from aqueous solutions by prepared activated carbon with oxygen-containing functional groups and its regeneration. Adsorption Science & Technology 2019, 37 (1–2), 160–181. 10.1177/0263617418819227. [DOI] [Google Scholar]
  42. Konicki W.; Aleksandrzak M.; Moszyński D.; et al. Adsorption of anionic azo-dyes from aqueous solutions onto graphene oxide: equilibrium, kinetic and thermodynamic studies. J. Colloid Interface Sci. 2017, 496, 188–200. 10.1016/j.jcis.2017.02.031. [DOI] [PubMed] [Google Scholar]
  43. Maneerung T.; Liew J.; Dai Y.; et al. Activated carbon derived from carbon residue from biomass gasification and its application for dye adsorption: kinetics, isotherms and thermodynamic studies. Bioresour. Technol. 2016, 200, 350–359. 10.1016/j.biortech.2015.10.047. [DOI] [PubMed] [Google Scholar]
  44. Lütke S. F.; Igansi A. V.; Pegoraro L.; et al. Preparation of activated carbon from black wattle bark waste and its application for phenol adsorption. Journal of Environmental Chemical Engineering 2019, 7 (5), 103396. 10.1016/j.jece.2019.103396. [DOI] [Google Scholar]
  45. Anbia M.; Haqshenas M. Adsorption studies of Pb(II) and Cu(II) ions on mesoporous carbon nitride functionalized with melamine-based dendrimer amine. International Journal of Environmental Science and Technology 2015, 12 (8), 2649–2664. 10.1007/s13762-015-0776-3. [DOI] [Google Scholar]
  46. Rehnelt K. Über Salze des 2, 4, 6-Triamino-triazins-(1, 3, 5) [Melamins]. Monatshefte für Chemie und verwandte Teile anderer Wissenschaften 1953, 84, 257–262. 10.1007/BF00899188. [DOI] [Google Scholar]
  47. Qu W.; Yuan T.; Yin G.; et al. Effect of properties of activated carbon on malachite green adsorption. Fuel 2019, 249, 45–53. 10.1016/j.fuel.2019.03.058. [DOI] [Google Scholar]
  48. Bann B.; Miller S. A. Melamine and derivatives of melamine. Chem. Rev. 1958, 58 (1), 131–72. 10.1021/cr50019a004. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES