Introduction

Water is important for all living creatures. Hence, the treatment of industrial wastewater is essential in the environmental remediation (Gitis and Hankins 2018). The reuse of industrial wastewater after its treatment is attracted the scientists to solve the shortage of the water problem (Liu et al. 2018). There are many challenges and issues of wastewater treatment; however, the modern protocols for wastewater treatment should be eco-friendly, ease to be performed, low-cost and effective (Rajasulochana and Preethy 2016). Recently, the nanotechnology field has shown several features in water treatment such as a low expense and high working efficiency in removing pollutants and reusable ability (Longwane et al. 2019). Due to the unique properties of nanomaterials like large surface area, low concentration needed to be used; hence, they provided a huge potential activity in treatment of polluted water containing metal toxin substance and different organic and inorganic impurities (Longwane et al. 2019; Gómez-Pastora, et al. 2017). The photo-nanocatalysts showed different response as compared to bulk materials due to their distinct quantum effects and surface properties under light irradiation (Mohamed et al. 2021; Sagir and Tahir 2021). It was reported in several studies that photocatalysis process was used in wastewater treatment through decomposition or mineralization of dissolved organics (Umar et al. 2013; Guya et al. 2016). Recently, many efforts have been devoted to the applications of photoactive nanomaterials-based graphitic carbon nitride (g-C3N4) under visible light irradiation due to its excellent optical and electrical properties (Sheng et al. 2019; Dou et al. 2020; Wang et al. 2020). Due to the unique characteristics of these materials as one of the π-conjugated materials, researchers have modified g-C3N4 nanomaterials for a purpose of improvement of their photocatalytic performance through using various methods such as template method, chemical doping and physical composite methods (Liu et al. 2020; Du et al. 2020). It was proved that the chemical doping can change the electronic structure of g-C3N4 (Pan et al. 2019; Zhou and Qiu. 2019; Miao et al. 2019; Vig et al.2019). Many efforts reported the preparation of photocatalytic materials and their application in the degradation process of organic contaminates by improving the separation efficiency of e-h pairs (Pekakis et al. 2006; Yates 2009). These efforts inclusively have used semiconductor mediated photocatalyzed degradation of organic compounds using light (UV/Visible/Solar). Altogether, the photocatalytic treatment of organic pollutants have been investigated in several reports (Huang et al. 2018; Nie et al.; 2018; Yan et al. 2019; Huang et al. 2020). As a result, this work has been studied the development of new approach for obtaining selenium-doped g-C3N4 as low cost, ease to prepare and effective photoactive nanomaterials and exploring their practical application in treatment of true wastewater from industry contaminated with heavy metal ions.

Our aim was to develop organic modifications on bulk graphitic carbon nitride polymer to improve the photo-response of g-C3N4 polymer toward the maximum visible light absorption in the solar spectrum. It was noted that the g-C3N4 exhibits good to excellent adsorption capacities for heavy metal ions Cu2+, Ca2+, Cd2+, Zn2+, Fe2+, Mn2+. It is indicating that Se-doped g-C3N4 is promising photoactive nanomaterial for wastewater treatment.

Experimental

General

The starting reagents of melamine, 2,4,6-trichloro-1,3,5-triazine, potassium selenocyanate, ethanol were purchased from Sigma Aldrich and were used without further purification.

Characterization of the polymeric nanocomposites

Phase and crystallinity were investigated by X-ray diffraction (XRD) analysis utilizing a Philips PW1710 X-ray diffractometer employing Cu-Kα radiation (λ = 1.54 Å) operated at 30 mA and 40 kV. The morphology, purity, and elemental composition of the samples were characterized using a scanning electron microscope (SEM) supported. The concentration of metal ions were calculated during adsorption process using PerkinElmer AAnalyst AAS. Atomic absorption spectrum was measured through dissociation of the chemical compound.

Methods

Synthesis of pure g-C3N4 polymer

The pure g-C3N4 was prepared by directly heating melamine (10.0 g) at a rate 5 °C min−1 to reach a temperature of 550 °C, and then maintain at this temperature for 2 h in ambient atmosphere. Finally the sample was naturally cooled down to room temperature. The final product was denoted as CN-pure.

Synthesis of selenium-doped g-C3N4 polymer

In a typical preparation, 2,4,6-triselenocyanato-1,3,5-triazine (0.5 g, 1.0 g and 2.5 g, respectively) was mixed with melamine (10 g) under thermal condensation method by at 550 °C for 3h. The formed composites were filtered off and dried to obtain (Se/g-C3N4; 1:5), (Se/g-C3N4; 1:10), (Se/g-C3N4; 1:25. The samples were denoted C1 5%Se/g-C3N4, C2 10%Se/g-C3N4, C3 25%Se/g-C3N4, respectively. The formed nanocomposites were characterized using SEM, XRD, and UV-Visible reflectance.

Application of the as-prepared nanocomposites for Siwa Lake sample (true sample)

Wastewater sample was collected from Siwa Lake in Egypt. To a solution of water sample (100 mL), 50 mg of the prepared composites were added. The treatment process were performed at room temperature in dark for 10 min; and then, it was continued under visible light irradiation for 120 min. Analysis of water sample revealed the metal ions content as follows: Cu2+ (0.456 ppm), Ca2+ (670.02 ppm), Cd2+ (0.661 ppm), Zn2+ (1.633 ppm), Fe2+ (0.1616 ppm), Mn2+ (0.225 ppm).

The removal efficiency of metal ions was calculated according to equation:

The removal efficiency was calculated according to the following equation:

$${{R}}\% = \, \left[ {\left( {{{C}}_{0} - {{C}}_{{1}} } \right)/{{C}}_{0} } \right] \times {1}00$$

where: R%: Removal efficiency, C0: initial concentration of metal ions before treatment, C1: the concentration of metal ions after treatment.

Contact time measurements

Contact time was measured by developing the treatment process in 2 hours. At defined intervals of time, 2 mL of each suspension was sampled using a syringe supported by a filter (2.5 μm pore size).

Results and discussion

Synthesis and characterization g-C3N4 and Se/g-C3N4 polymers

Herein, our approach was initiated by the synthesis g-C3N4 from melamine as a low-cost material followed by the addition of various concentrations of silicon precursor under ultrasound irradiation conditions to obtain Se/g-C3N4 polymeric nanocomposites (Scheme 1).

Scheme 1
scheme 1

Synthesis of Se/g-C3N4 polymeric nanocomposite

Full size image

SEM analysis

The electronic and photoelectric properties of the modified carbon nitrides were then investigated to elucidate their enhanced activity for attacking the heavy metal ions from water on their surface that was assisted by visible light irradiation.

The morphology and microstructures of pure g-C3N4 and Selenium/g-C3N4 composites C1, C2 and C3 were studied by scanning electro-microscope (SEM) analysis as represented in Fig. 1a-d. Pure g-C3N4 represented in (Fig. 1a) as 2D nanosheet, the selenium was appeared as shinny spots in C1, C2 and C3 composites that indicated the incorporation of Se atom in the crystal lattice of g-C3N4 (Fig. 1c-d).

Fig. 1
figure 1

SEM analysis g-C3N4 and exfoliated products C1, C2 and C3 nanocomposites

Full size image

XRD analysis

Identification on the crystal structure and phase of the as-prepared samples were determined through their XRD patterns. As depicted in Fig. 3, in the case of all samples (pure g-C3N4 and C1, C2 and C3), a characteristic peaks at are indexed as 27.3°, corresponding to the inter layer structural packing and the characteristic interplanars taking peaks of aromatic systems, respectively (Sun et al., 2017), suggesting the major structure of g-C3N4 was not subjected to any change after the addition of selenium. Whereas, in the case of C1, C2 and C3 composites, small diffraction peaks localized between 10°- 30° in Fig. 2. This may be due to the amorphous nature of selenium or due to the small amount of selenium used in graphitization process.

Fig. 2
figure 2

XRD analysis of g-C3N4, C1, C2, C3.

Full size image

DRS analysis

In conjunction with the XRD analysis, it can be noted that the doping selenium hybridized tightly with g-C3N4. The UV-Visible diffuse reflectance spectra of the all as-synthesized samples was represented in (Fig. 3). It was determined that g-C3N4 had intrinsic band gap absorption (2.79 eV). However, the band gaps of C1, C2, C3 equal to 2.75, 2.70, 2.69 eV, respectively.

Fig. 3
figure 3

UV-Visible spectrometry of g-C3N4, C1, C2 and C3

Full size image

The application of the prepared samples on treatment of real wastewater sample

Different affecting parameters had been demonstrated in this study. These parameters are the following: Effect of doping, effect of reaction conditions (under ambient or visible light conditions), effect of contact time with respect to the co-existed metal ions in true wastewater had been investigated. In this study, the adsorption ability of g-C3N4, C1, C2 and C3 was evaluated through an adsorption experiment of metal ions from industrial wastewater sample at 60 min under ambient without the effect of light and under the visible light conditions.

Removal of the co-existed metal ions in wastewater

Under ambient conditions without the effect of visible light irradiation (in dark condition for 10 min).

The as-prepared nanocomposites showed less adsorptive ability under ambient conditions (Fig. 4). It was observed that the removal efficiency of metal ions was recorded as follows after 60 min: 20.88% for Cu(II), 19.69% for Cd(II), 14.57% for Ca(II), 18.41% for Mn(II), 5.41% for Zn(II), and 14.77% for Fe(II) were removed from the solution with g-C3N4, respectively.

Fig. 4
figure 4

Effect of g-C3N4-based photoactive materials in removal of heavy metals.

Full size image
Under visible light irradiation for 120 min.

After equilibration for 120 min and estimating the catalytic performance of undoped g-C3N4 and Se-doped g-C3N4 was tested under visible light irradiation. By comparing Se-doped g-C3N4 with undoped g-C3N4. It was revealed that the adsorption of heavy metal ions over the surface of C1, C2, and C3 composite had been increased by irradiating the reaction with visible light. It was observed that C3 nanocomposite showed enhancement photocatalytic activity than undoped g-C3N4 and C1 and C2 (Fig. 4). It was shown that after 120 min of treatment of the as-prepared nanocomposites with wastewater sample, the removal efficiency of metal ions were 90.68% for Cu(II), 42.99% for Cd(II), 35.94% for Ca(II), 56.39% for Mn(II), 33.88% for Zn(II), and 14.77% for Fe(II), respectively, were removed from the solution with C3 nanocomposite under visible light irradiation. It was indicating that C3 exhibited super capacities for removal of metal ions.

Proposed mechanism

The effect of the selenium-doped grafted graphitic nitride was studied on the treatment of true wastewater. The metal ions adsorption was investigated in ambient conditions and under visible light irradiation conditions. The functional groups at the nanocomposites surfaces could draw a conception on the mechanism of metal ion uptake the amino-groups are mainly responsible for the metal removal. In addition organoselenium loaded on g-C3N4 is considered to be a good electron-acceptor material to effectively prevent the electron-hole pair recombination due to its π-conjugation structure.

The results can be explained as follows: (a) both the undoped g-C3N4 and its doped Se on the surface of g-C3N4 composites possess π-conjugation structure (c) the π–π stacking exists between Se/g-C3N4 composite and metal ions (Zhang et al. 2010). As a result, the C3 composite showed the improved absorptivity which supported by photocatalytic activity (Scheme 2). From these interesting results, it can be postulated that Se-doped graphitic nitride has a conjugated large π bond structure was proved to be more efficient photo-nanocatalyst than undoped graphitic carbon nitride in removal of heavy metal ions. Moreover the visible light irradiation increases the ability of adsorption of heavy metals. That because the Se-doped g-C3N4 acquires greater surface area and visible light absorption range than undoped g-C3N4. The doping of selenium on the surface of g-C3N4 can be significantly assisted in electron transmission and corresponding photogenerated carrier separation of streams.

Scheme 2
scheme 2

Postulated reaction mechanism of heavy metal ion removal using Se-doped g-C3N4

Full size image

Conclusion

Graphitic carbon nitride is a two-dimensional polymer material which was modified by doping selenium to provide good physical and chemical stabilities, to be used in the field of wastewater treatment photocatalysis. The present study had introduced the preparation, surface modification, characterization and catalytic properties of graphitic carbon nitride and Se-doped graphitic carbon nitride polymer, and their application in wastewater treatment. It was determined that the Se-doped g-C3N4 polymeric nanocomposites showed effective photocatalytic activity toward heavy metal ions removal. It was demonstrated that the C3 nanocomposite can function as a photoactive material BBD-RSM that showed highest performance toward heavy metal ions removal from wastewater.