One-step preparation of a novel SrCO3/g-C3N4 nano-composite and its application in selective adsorption of crystal violet.

A novel kind of nanoparticle SrCO3/g-C3N4 was prepared using strontium carbonate (SrCO3) and melamine (C3H6N6) as raw materials via one-step calcination. The formation of SrCO3/g-C3N4 was confirmed from the X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS) analysis. Its selective adsorption performance was evaluated towards crystal violet (CV), rhodamine B (RhB) and methylene blue (MB). The results showed that the SrCO3/g-C3N4 had selective adsorption ability of CV. Furthermore, adsorption measurements of CV were conducted to investigate the influences of contact time, initial concentration, initial dye solution pH value and adsorbent dosage. The maximum removal rate of CV was 98.56% when the initial concentration was 1600 mg L-1. The kinetic study indicated the adsorption of CV followed the pseudo-second-second model well. The adsorption efficiency of SrCO3/g-C3N4 was greater (97.46%) than that of g-C3N4 (31.30%) and SrCO3 (17.30%). It could be deduced that the synergistic effect of conjugation interaction of g-C3N4 and the electrostatic attraction of SrCO3 might be the main driving force for the superb adsorption of CV. This journal is © The Royal Society of Chemistry.

Introduction

Nowadays, graphitic carbon nitride (g-C3N4) has gained considerable research attention because it possesses excellent advantages such as high chemical and thermal stability under ambient temperature, cost-effective, non-toxic and simple preparation.[1,2] g-C3N4, as a new intriguing class of graphite analogue, consists of conjugated planes containing highly ordered tri-s-triazine (C6N7) units. The layered structure of the tri-s-triazine is connected by weak forces-van der Waal forces. It has been widely used in the photocatalysis field for it has an energy band gap of 2.7 eV which makes it is capable of the visible adsorption.[3-6] As a photocatalyst, it has been widely used in water splitting,[7,8] organic pollutants degradation,[9,10] CO2 reduction,[11] and other fields.[12,13]

Crystal violet (CV, C25H30ClN3, IUPAC name is N-[4-[bis][4-dimethyl-amino]-phenyl]methylene]-2,5-cyclohexadien-1-ylidine]-n-methylmethanaminium chloride), a typical triphenylmethane dye, is widely used in cell biology, paper, leather and textile industry.[14-16] The wastewater containing CV is low biodegradability and high stability (complex aromatic structure) and it could be absorbed through the skin and causing the skin, eye, digestive irritation and even cancer.[17-19] From the aspect of environmental safety and health of life, it is vital to develop an effective way to abate CV in wastewater. Various methods have been adopted for eliminating dye pollution from water, including chemical oxidation,[20,21] photo-catalytic decomposition,[22,23] electro-catalytic degradation[24,25] and non-thermal plasma.[26-28] These methods usually have some defects, such as slow degradation rate, complex, heavy expenses and usually causing secondary pollution. The adsorption technique is gaining more attention for it is high efficiency, simple design, cost-effective and adaptable.[29-35] There have been various kinds of adsorbents developed, such as carbon materials,[36-38] natural clay minerals,[39-42] and bioadsorbents.[43-45] However, certain deficiencies including costly, intricate pre-treatment and low adsorption capability limited the use of some certain adsorbents.[46-53]

Due to the conjugated region, stacking structure and the weak forces of g-C3N4, it has the potential for aggregating functional groups or materials to form nanocomposite with multiform favorable properties.[54-56] This distinctive structure has drawn great interesting in improving the photocatalytic performance.[57,58] But the research of using modified g-C3N4 as adsorbent for clearing up dye pollutant is rarely been reported.[59,60] As far as we know, there has been no report of using g-C3N4-based composite as an adsorbent in dye wastewater treatment. Strontium carbonate (SrCO3), a typical alkaline earth metal carbonate, has been widely used as additives in industrial production.[61-63] Meanwhile, some research has been reported on its adsorption performance attributed to its various architectures.[64,65]

In this work, we firstly induced SrCO3 to incorporate with g-C3N4via one-step calcination method to fabricate a novel adsorbent SrCO3/g-C3N4. The morphology and structure of the composite was characterized by XRD, FT-IR, SEM, TEM, BET and XPS, and its adsorptive capacity and selectivity of CV in aqueous solution were investigated. To the best of our knowledge, this work represents the first example employing g-C3N4-based composite for selective adsorption of CV.

Experimental

Synthesis of SrCO3/g-C3N4

SrCO3, melamine and CV are all AR grade and purchased from Chengdu Kelong Chemical Agents (China), without any further purification. SrCO3/g-C3N4 was synthesized by one-step calcination process in a muffle furnace. SrCO3 and melamine (mass ratio = 1 : 1) were dissolved with deionized water in alumina crucible, and then the mixed solution was dispersion with ultrasonic irradiation for 20 min under ambient temperature. The final solution was transferred into muffle furnace and maintained at 600 °C for 4 hours at heating rate of 5 °C min−1 to obtain the prepared nanocomposite. SrCO3 and melamine were also treated as the forward route for making comparison.

Characterization

X-ray diffraction (XRD, Shimadzu, XRD-6100) patterns were detected with Cu Kα radiation (40 kV, 30 mA, 2θ = 10–80°) to investigate the crystal structures of the samples. Fourier transform infrared spectra (FT-IR, Shimadzu, IR Prestige-21), were recorded in the range of 4500–400 cm−1, using KBr technique to analyze the functional groups on the surface of the composite. Scanning electron microscope (SEM, JEOL, JSM-7800F) and Transmission Electron Microscopy (TEM, JEOL, JEM-2100) were used to observe the morphologies and the microstructures of the samples. The N2 adsorption apparatus (Micromeritics, ASAP 2020) were used to obtain the Brunauer–Emmett–Teller (BET) surface area of the samples. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250xi) was used to determine the binding energy.

Adsorption experiments

The concentration of CV in the solution was determined at the maximum absorbance (λmax = 580 nm) by UV-vis spectrophotometer (UV-vis spectrometer, Tianmei, UV1102). For high concentration, the samples were diluted before measurements. And the initial pH of the dye solution was measured by pH meter (pH meter, Sartorius, PB10).

The adsorption experiments were conducted by a batch method. All experiments were conducted at 7.0 pH value, except those that investigated the effect of initial pH of dye solution. And 0.08 g adsorbent was dispersed in 80 mL CV solution, except that were used to study the effect of the dosage. Similarly, the initial concentration is 1600 mg L−1, except those that investigate the same parameters. The pH of the initial dye solution was adjusted with HCL (0.1 mol L−1) and NaOH (0.1 mol L−1). The kinetic experiments were carried out at the initial concentration of 500, 1000, 1200, 1400, and 1600 mg L−1. It performed on an air bathed shaker. The solution was separated by centrifugation at 5000 rpm for 5 min.

The adsorption capacity at time t q (mg g−1) and removal rate (%) were calculated using the following equation:where C0 and C are the liquid-phase concentrations of the CV (mg L−1) at initial time and time t, respectively. V is the volume of the solution (mL) and m is the mass of the used adsorbent (mg).

Results and discussion

Characterization of the samples

The XRD patterns of the prepared samples are shown in Fig. 1. The g-C3N4 displayed a typical diffraction peaks at 27.46° and 12.96°, corresponding to the (002) and (100) diffraction planes, which represented the interlayer stacking reflection and in-plane structure of aromatic system, respectively.[66,67] This indicated that the g-C3N4 was synthesized by polycondensation approach with pure melamine. Meanwhile, the observed diffraction peaks of calcined SrCO3 are located at 2θ angles = 25.28°, 25.91°, 29.73°, 31.62°, 34.64°, 35.22°, 36.64°, 41.42°, 44.18°, 45.74°, 46.68°, 47.80°, 50.03° which correspond to the planes of (111), (021), (002), (012), (102), (200), (130), (220), (221), (041), (202), (132), (113), respectively. It could be unambiguously indexed to the orthorhombic phase of SrCO3 (JCPDS card no. 05-0418).[68,69] There is no observation of the typical diffraction of g-C3N4 in the synthesized composite, the diffraction peaks were mostly the same as the SrCO3, but the feature peaks positions of SrCO3 shifted slightly toward a lower diffraction angle. And the impure peaks of the composite might be the carbonization of the raw materials or the melamine did not condense completely.

Fig. 1

XRD patterns of the g-C3N4, SrCO3 and SrCO3/g-C3N4.

The FT-IR spectra of the prepared materials are shown in Fig. 2. For the calcined melamine, the typical adsorption peaks in the 1200–1700 cm−1 range and at 808 cm−1 of g-C3N4 could be observed. They are assigned to the typical skeletal stretching vibrations of armomatic C–N heterocycles and the out-of-plane bending vibration of tri-s-triazine rings, respectively.[70,71] The peaks at 698, 858, 1070, 1458 and 1774 cm−1 corresponding to the CO32− of the calcined SrCO3 were observed.[50] In case of the SrCO3/g-C3N4, the typical skeletal stretching vibrations of tri-s-triazine were hardly observed; it might be the adsorption intense of SrCO3 was so strong in order to impede the peaks of g-C3N4. Meanwhile, the peak at 808 cm−1 presented in g-C3N4 shifted to 821 cm−1, the peak at 856 cm−1 presented in SrCO3 shifted to 858 cm−1, the peaks at 1070 cm−1 and 1774 cm−1 were disappeared, these might due to the strong interactions between the carbonate of SrCO3 and tri-s-triazine rings of g-C3N4. Furthermore, the new peak at 2112 cm−1 corresponding to CN groups appeared after calcination indicating that the incorporation of SrCO3 has intense interaction during the condense process of melamine.[72]

Fig. 2

FT-IR spectra of the g-C3N4, SrCO3 and SrCO3/g-C3N4.

To make further investigation of the constitution of the prepared composite, the calcined product was treated with HCl (0.5 mol L−1) and deionized water, because g-C3N4 possessed fascinating acid stability. The treatment was terminated until the pH value was neutral, and then the treated nanocomposite was dried at 60 °C for 24 h. The XRD patterns and FT-IR spectra of the HCl-treated product are characterized in Fig. 3. The feature diffraction peak of g-C3N4, indicating the interlayer stacking, changed from 27.46° to 27.92° is explicitly observed in Fig. 3a. The change of the diffraction peak was corresponded to the stacking distance reduced from 0.325 to 0.319 nm. The above results implied that the interlayer stacking order was improved. And FT-IR spectra of the product were well matched with the pure g-C3N4 except the weak intensity (Fig. 3b). So, it could be indicated that the SrCO3/g-C3N4 was formed with the raw materials SrCO3 and melamine.

Fig. 3

XRD patterns (a) and FT-IR spectra (b) of the HCl-treated SrCO3/g-C3N4.

SEM and TEM images of the calcined samples are displayed in Fig. 4. It revealed the morphology and microstructure of SrCO3/g-C3N4. In Fig. 4a, it could be seen the calcined melamine was predominantly composed of plate-like sheets,[73] and the morphology of calcined SrCO3 was irregular polyhedrons with smooth surface is shown in Fig. 4b. In Fig. 4c, it could be seen clearly that the g-C3N4 covered on the SrCO3 incompletely. Further structural details of SrCO3/g-C3N4 are shown in Fig. 4d. There were some step edges of the layered g-C3N4 and rods-like SrCO3 with some parts overlapping. It might be the amount of g-C3N4 was not enough to disperse on the entire surface of SrCO3; and some areas cannot be wrapped by the g-C3N4. The SEM and TEM image could reveal that there existed some interactions between these two materials. Meanwhile, the specific surface areas of the clained samples were measured. As shown in Table 1, compared with g-C3N4 the composite BET surface area gets smaller with the addition of SrCO3, this might be the SrCO3 interacted with the stacking structure of g-C3N4. These results consistent with SEM results which could further prove that there have been some interactions between SrCO3 and g-C3N4.

Fig. 4

SEM images of the g-C3N4 (a), SrCO3 (b) and SrCO3/g-C3N4 (c); TEM images of the SrCO3/g-C3N4 (d).

Textural properties of g-C3N4, SrCO3 and SrCO3/g-C3N4a

Sample	S BET (m2 g−1)	V total (cm3 g−1)	D (nm)
g-C3N4	27.4	0.126	17.5
SrCO3	1.93	0.003	8.72
SrCO3/g-C3N4	5.86	0.016	9.69

S BET: BET surface area, Vtotal: total pore volume, D: average pore diameter calculated using BJH method.

To further study the chemical state of SrCO3/g-C3N4, the XPS measurements were carried out. Fig. 5a is the general XPS spectra of calcined materials, indicating the presence of C, N, O and Sr in SrCO3/g-C3N4. The XPS spectra of C 1s are shown in Fig. 5b. For pristine g-C3N4 the peaks at 284.2 eV, 287.7 eV were corresponding to sp2-hybridized carbon in C–C group or the adventitious hydrocarbon[74] and sp2-bond carbon in form of C–CN.[75] But after the blended calcinations, the peak at 284.2 eV shifted to 284.6 eV. The N 1s spectra are shown in Fig. 5c. The peak at 398.2 eV was attributed to sp2-hybridized nitrogen in N atom aromatic rings in form of C–NC,[76] and the nearly peaks at 399.3 eV and 401.0 eV were regarded as tertiary nitrogen (N-(C)3) and amino functional groups, respectively.[77] After incorporation, the binding energy peaks slightly shifted. The peaks at 531.1 eV and 533.2 eV of O 1s spectra corresponding to the SrCO3 have no significant change in Fig. 5d. The XPS spectra of Sr 3d at 133.1 eV and 134.6 eV for calcined SrCO3 were observed in Fig. 5e. These peaks was corresponding to the Sr 3d5/2 and Sr 3d3/2, and the peak at 134.6 eV shifted to 134.7 eV after calcination. All these changes after co-calcination suggested that there were strong interactions between the two raw materials. Therefore, according to the analysis of XPS and the above results of XRD, FT-IR, SEM, TEM and BET measurements, it could be concluded that the SrCO3/g-C3N4 was synthesized using SrCO3 and melamine by one-step calcination.

Fig. 5

XPS survey spectra of g-C3N4, SrCO3 and SrCO3/g-C3N4 (a) and high-resolution XPS spectrum: C 1s (b), N 1s (c), O 1s (d) and Sr 3d (e).

The selective adsorption of CV

In our previous study, cationic dyes MB, RhB and CV were chosen as the target dye to conduct the adsorption experiments. We found that SrCO3/g-C3N4 appeared efficient adsorption capacity for CV except for the other dye solutions. To make further investigation of the selectivity of SrCO3/g-C3N4, mixed solutions of CV/MB, CV/RhB and CV/MB/RhB with equal volume ratio were prepared. The concentration of each dye solution was 10 mg L−1, and dosage ratio of SrCO3/g-C3N4 and mixed solutions was 1 g L−1. The adsorption experiments were carried out under ambient temperature and different pH value for 120 min.

The optical pictures and UV-vis measurements of the adsorption process are shown in Fig. 6. It could be apparently seen that CV was decolorized after the test (Fig. 6b), but in the MB and RhB groups the color slightly changed (Fig. 6a–c). In Fig. 6d, the color of CV/MB mixed solution was changed from dark blue to incipient blue (the color corresponding to the MB). For the CV/RhB group, the end color is very close to RhB (Fig. 6e). It suggested that CV in these two mixtures was selectively captured. However, the color in group CV/MB/RhB did not change, because the color of MB and RhB mixed solution is the same as that of CV. To further investigate the selective adsorption of CV, the UV-vis spectra were conducted for the tested groups. In the case of CV/MB, the UV-vis adsorption peaks of CV and MB were at 580 nm and 664 nm before adsorption, respectively. After adsorption, the peak of CV was dropped down drastically while the MB peak was almost unchanged (Fig. 6f). The same situation happened in the CV/RhB group (Fig. 6g). Although the optical of CV/MB/RhB group did not change, but the UV-vis adsorption curve appeared the same situation with the other two mixed groups (Fig. 6h). It could be concluded that the SrCO3/g-C3N4 performed selective adsorption of CV from CV/MB/RhB.

Fig. 6

Optical photographs of single adsorption of MB (a), CV (b) and RhB (c); the selective adsorption of CV from CV/MB (d) and CV/RhB (e) mixture, UV-vis spectra of CV/MB (f),CV/RhB (g) and CV/MB/RhB (h) mixture before and after adsorption.

Furthermore, we also evaluated the effect of the initial pH of the dye solution. Fig. 7 shows the tested results. It could be seen that the initial pH (ranged from 4 to 10) of mixed dye solution did not significantly influence the adsorption process in each tested group.

Fig. 7

Effect of the initial pH of mixed dye solution CV/MB (a), CV/RhB (b) and CV/MB/RhB (c).

Effect of contact time

Fig. 8 shows the effect of different contact time (0–160 min) in the adsorption process. The initial concentration of CV is 1600, 1800 and 2000 mg L−1 and the dosage ratio of SrCO3/g-C3N4 and CV solution was 1 g L−1. For all concentrations, the removal rate of CV became constant after 120 min. Meanwhile, the group of 1600 mg L−1 reached the equilibrium firstly and the removal rate was up to the other concentration. So, the contact time of 120 min and the initial concentration of CV of 1600 mg L−1 were chosen for the following investigation.

Fig. 8

Effect of contact time.

Effect of initial concentration of CV

The effect of initial concentration (50–2000 mg L−1) of CV is shown in Fig. 9. From the obtained results, it was found that the removal efficiency kept steadily before the initial dye concentration up to 1800 mg L−1. This might be attributed to the higher initial concentration offered more numbers of dye molecules that could contact easily with SrCO3/g-C3N4. However, the removal rate was reduced at higher initial concentration indicating the saturation of SrCO3/g-C3N4.

Fig. 9

Effect of initial CV concentration.

Effect of adsorbent dosage

The dosage of the adsorbents is one of the major parameter which influences the adsorption process; an appropriate amount of adsorbents is in favor of the real industrial treatment. Fig. 10 shows the effect of the adsorbent dosage (0.25, 0.50, 0.75, 1.00, 1.25, 1.50 g L−1) of the CV. The removal rate increased with the increased amount of the adsorbent. When the dosage ups to 1.00 g L−1, the removal efficiency did not increase with the increasing dosage. It might due to the binding of almost CV ions onto SrCO3/g-C3N4, which made the equilibrium between solution and adsorbents. So the amount of adsorbent 1.00 g L−1 was the suitable dosage for the adsorption of CV.

Fig. 10

Effect of SrCO3/g-C3N4 dosage.

Effect of pH

The pH of initial dye solution is a key factor in adsorption process, which could influence the interaction between adsorbent and dye. In order to find out the pH effect, the adsorption test was conducted at initial solution pH ranged from 4 to 10. The results in Fig. 11 show that the initial pH of CV solution had little effect on the adsorption capacity. This observation could demonstrate that SrCO3/g-C3N4 is suitable for removing CV in wastewater in a wide pH range, and there is no need to adjust the initial solution pH before treatment.

Fig. 11

Effect of the initial pH of CV.

Adsorption kinetics

In order to study the characteristics of the adsorption process, pseudo-first-order and pseudo-second-order kinetics models were carried out with different concentration (500, 1000, 1200, 1600 mg L−1). The pseudo-first-order and pseudo-second-order are expressed by eqn (3) and (4).[78]where k1 (min−1) and k2 (g (mg−1 min−1)) are the rate constant of pseudo-first-order and pseudo-second-order model, respectively; qe (mg g−1) and q (mg g−1) are the adsorbed amount at equilibrium time and time t.

The rate constant of adsorption could be calculated from the slope of the plot in Fig. 12 and was listed in Table 2. R2 of pseudo-second-order model is much higher than the other, indicating that the pseudo-second order equation is more suitable to describe the CV adsorption with SrCO3/g-C3N4. So the chemisorptions or chemical sorption was the rate-controlling step for the adsorption process.[79]

Fig. 12

Pseudo-first-order (a), and pseudo-second-order (b) kinetic plots for adsorption.

Kinetic parameters of different initial CV concentration

C 0 (mg L−1)	Pseudo-first-order			Pseudo-second-order
	q e (cal) (mg g−1)	k 1 (min−1)	R 2	q e (exp) (mg g−1)	q e (cal) (mg g−1)	k 2 (g (mg−1 min−1)) × 10−5	R 2
1600	16.91	0.014	0.5357	1576.9	1666.7	3.67	0.9425
1800	28.33	0.016	0.7481	1378.6	1428.5	3.53	0.9577
2000	29.53	0.024	0.7822	1143.4	1250.0	8.10	0.9902

Desorption and reuse

In order to evaluate the reusability of SrCO3/g-C3N4, the regeneration process was performed. Adsorption experiment was conducted at the optimal condition as ascribed above in a 250 mL CV solution firstly, after equilibrium the used adsorbent was regenerated by centrifugation and washed several times with methanol and water at room temperature. Finally, the regenerated composite was dried for 24 h at 60 °C for reusing.

It could be seen that the removal rate gradually decreased after regeneration in Fig. 13. It might conclude that the adsorption mechanism of this process is not due to the physical adsorption.

Fig. 13

Reusability of the adsorbent.

Adsorption mechanism of crystal violet

To further study the adsorption performance of CV, the adsorption experiments were conducted with the calcined SrCO3, g-C3N4 and SrCO3/g-C3N4 firstly. 0.08 g tested materials were dispersed in 80 mL CV solution (10 mg L−1), respectively. After 2 h adsorption process, the removal rate of CV for tested materials is exhibited in Fig. 14. It could be seen obviously that the removal ability of g-C3N4 and SrCO3 to CV is much smaller than that of the nanocomposite. The removal rate of g-C3N4, SrCO3 and SrCO3/g-C3N4 were 31.30%, 17.30% and 97.46%, respectively. The adsorption mechanism of g-C3N4 might be attributed to π–π interaction between the g-C3N4 and CV, and that for the SrCO3, the electrostatic attraction between the carbonate generated from the hydrolytic of SrCO3 and the cationic chromogenic groups of CV. And for the SrCO3/g-C3N4, the synergistic effect of π–π interaction and electrostatic attraction highly improved the adsorption properties of the nanocomposite on CV.[50] According to the above results and the crystal structures, we could deduce that superb adsorption capacity of CV might attribute to the synergistic interaction of the conjugation of g-C3N4 and the electrostatic attraction of SrCO3.

Fig. 14

Comparisons of the adsorption effect of the SrCO3, g-C3N4, and SrCO3/g-C3N4.

Furthermore, the FT-IR spectra of SrCO3/g-C3N4 before and after CV adsorption were recorded. As shown in Fig. 15, the peak at 2112 cm−1 assigned to the nitrile groups shifted to the 2107 cm−1 and the peak at 1319 cm−1 assigned to aromatic C–N heterocycles observed clearly. These results indicate that the previously mentioned groups are involved in the adsorption process. Moreover, the intensity of the peak 1458 cm−1 corresponding to C–N heterocycle aromatic became stronger, that was for the CV was adsorbed onto the SrCO3/g-C3N4. All the changes of the FT-IR spectra of the SrCO3/g-C3N4 before and after adsorption could indicate that there have been intense interactions between dye molecules and SrCO3/g-C3N4. Furthermore, the result of desorption test might indicate that there has been intense chemical adsorption between the composite and CV. It could deduced that SrCO3/g-C3N4 performed selective adsorption of CV might due to the chemical structure of CV is more symmetrical and conjugation degree is higher than MB and RhB. On the basis of the above analysis, the conjugation and electrostatic interaction would be the driving force for the selective adsorption of CV.

Fig. 15

FT-IR spectra of SrCO3/g-C3N4 before and after adsorption.

Conclusions

In summary, a new SrCO3/g-C3N4 composite adsorbent was firstly synthesized with SrCO3 and melamine via a simple one-step calcination method. The SrCO3/g-C3N4 performed high and specific selective adsorption for CV. The experiment data was well described by the pseudo-second-order model. According to the crystal structural feature of SrCO3/g-C3N4 and the FT-IR analysis of SrCO3/g-C3N4 before and after adsorption of CV, it could be deduced that the synergies of π conjugation and electrostatic interaction would be the mechanism for the selective adsorption of CV. Our findings indicate that g-C3N4 based composite SrCO3/g-C3N4 could be a promising adsorbent which can potentially be applied for the removal of CV pollutants from aqueous solution.

Conflicts of interest

There are no conflicts to declare.