Development of Photocatalytic Reduction Method of Cr(VI) with Modified g-C₃N₄  ^†

Abstract

Hexavalent chromium (Cr(VI)), a common contaminant in industrial wastewater, poses severe health risks due to its carcinogenic and mutagenic properties. Consequently, the development of efficient and environmentally friendly methods to reduce Cr(VI) to the less toxic trivalent chromium (Cr(III)) is of great importance. In this study, we present a cost-effective photocatalytic approach using graphitic carbon nitride (g-C₃N₄) modified with 1,3,5-trihydroxybenzene via one-step thermal condensation. The modified photo-catalyst exhibited improved surface area, porosity, visible-light absorption, and a narrowed band gap, all of which contributed to enhanced charge separation. As a result, nearly complete reduction in Cr(VI) was achieved within 90 min under visible-light irradiation. Further optimization of catalyst dosage and EDTA concentration gave even higher reduction efficiency. This work offers a promising strategy for the design of high-performance photocatalysts for environmental remediation.

1. Introduction

The water pollution caused by heavy metal is a serious environmental issue that humanity is facing. Hexavalent chromium (Cr(VI)) is a common toxic heavy-metal contaminant, mainly originating from the electroplating, tanning, chromium salt production, and metal processing industries [1]. Cr(VI) can be released into the aqueous environment by means of leakage or improper treatment methods. The toxicity of Cr(VI) is much stronger than that of Cr(III) [2]. The presence of Cr(VI) in water can cause skin allergy, tissue cell damage, and cancer. Cr(III) is less toxic than Cr(IV) and can be easily removed because it exists as Cr₂O₃ or Cr(OH)₃. To date, several treatment techniques including membrane filtration, adsorption, reverse osmosis, photocatalytic reduction, chemical precipitation, and ion exchange have been performed for the treatment of Cr(VI). However, all these methods suffer from issues like high cost, cumbersome operation, secondary pollution, and low removal efficiency [3]. In recent years, photocatalytic technology has become a research hotspot in environmental pollution treatment due to its advantages of environmental friendliness and high efficiency. Photocatalytic technology can effectively treat organic matter, drugs, and heavy metal ions in wastewater [4]. An inorganic semiconductor, graphitic carbon nitride (g-C₃N₄), has been intensively applied in pollutant treatment, hydrogen production, sensing, and solar cells, owing to its high thermal and chemical stability as well as its ability to be activated by visible light. However, the photocatalytic reduction of Cr(VI) by g-C₃N₄ is significantly hindered by its inherent limitations, including a narrow visible-light response range, low specific surface area, and poor electron–hole separation efficiency [5]. To improve the photocatalytic performance of g-C₃N₄, various strategies have been employed, such as morphological engineering, elemental doping, construction of heterojunction structures, and surface modification [6]. Nevertheless, the precise control of elemental doping remains challenging, and the introduced heteroatoms may act as defect sites, facilitating the recombination of photogenerated charge carriers and thereby significantly reducing the photocatalytic activity [7]. To address these issues, a novel modification strategy involving the incorporation of aromatic rings into the g-C₃N₄ framework has recently been developed. For instance, Choi et al. reported that the incorporation of benzene linkages into the g-C₃N₄ structure enhances charge separation and enhances visible-light photocatalytic activities [8]. In this study, modified g-C₃N₄ was synthesized via one-step thermal condensation of 1,3,5-trihydroxybenzene (THB) and urea. Various THB loadings were investigated for the optimal photocatalytic reduction of Cr(VI), and the sample T7.5-g-C₃N₄ exhibited maximum reduction activity. The rate constant for Cr(VI) reduction reached 0.0294 min⁻¹, which is approximately 150 times higher than that of pristine g-C₃N₄. This enhanced photocatalytic performance can be attributed to the increased number of active sites and improved light absorption, which together promote the efficient separation and migration of charge carriers.

2. Material and Methods

2.1. Chemicals

All reagents used in this study were of reagent-grade quality and were employed without any further purification. Urea (CH₄N₂O, >99.0%), 1,5-Diphenyl carbonohydrazide (C₁₃H₁₄N₄O), and sulfuric acid (H₂SO₄, >95.0%) were obtained from Fujifilm Wako Pure Chemicals Co., Inc. (Osaka, Japan). 1,3,5-Trihydroxybenzene (Phloroglucinol, >99.0%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Ethylenediaminetetraacetic acid disodium salt, 2-hydrate (EDTA-2Na, >99.5%) was purchased from Kishida Chemical Co., Inc. (Osaka, Japan). Acetone (CH₃COOCH₃, >99.5%) was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Pure water was obtained from an ultrapure water system from Advantec MFS Inc. (Dublin, OH, USA). Potassium persulfate (KSP, 95.0%) was purchased from Nacarai Tesque Co., Inc. (Kyoto, Japan)

2.2. Synthesis of g-CN Photocatalysts

Modified g-C₃N₄ was synthesized via a one-pot thermal polycondensation method. Urea (20 g) and varying amounts of 1,3,5-trihydroxybenzene (0, 2.5, 5.0, 7.5, or 10.0 mg) were dissolved in 10 mL of water by ultrasonic treatment. The resulting solution was then heated at a rate of 2 °C/min up to 550 °C and calcined at this temperature for 2 h. The obtained product was collected, ground into powder, and labeled as T0.0-, T2.5-, T5.0-, T7.5-, or T10.0-g-C₃N₄, corresponding to the amount of 1,3,5-trihydroxybenzene, respectively.

2.3. Characterization

Fourier-transform infrared (FT-IR) spectra were recorded using a Spectrum 100 spectrometer (PerkinElmer, Shelton, CT, USA) equipped with an attenuated total reflectance (ATR) accessory. The microstructure and surface morphology of the catalysts were examined by scanning electron microscopy (SEM, JSM-IT700HR, JEOL, Shelton, CT, USA). Specific surface areas were determined from N₂ adsorption–desorption isotherms measured using a BELSORP-mini II analyzer (BEL, Osaka, Japan). UV-visible diffuse reflectance spectra (DRS) were collected using a UV-vis spectrophotometer (V-750 ISV-922, Jasco, Maryland, United States). The chemical states of the samples were investigated using X-ray photoelectron spectroscopy (XPS) with an Al-Kα radiation source, specifically utilizing the PHI Quantera SXM (Ulvac-Phi, Kanagawa, Japan). Photoluminescence (PL) spectra of the samples were recorded using a Quantaurus-Tau (C11367-21, HAMAMATSU Photonics, Shizuoka, Japan).

2.4. Photocatalytic Activity Testing

The photocatalytic activity of the prepared samples was evaluated based on the reduction of hexavalent chromium (Cr(VI)) under visible light irradiation using a blue LED (λ = 450 nm). In this experiment, 0.3 g L⁻¹ of catalyst was added to 50 mL of an aqueous K₂Cr₂O₇ solution (30 ppm), together with EDTA-2Na (300 ppm) as a hole scavenger. The suspension was stirred in the dark for 30 min to reach adsorption–desorption equilibrium before irradiation. During the reaction, 1.4 mL of the solution was sampled at regular intervals, and the catalyst was removed by centrifugation. The concentration of Cr(VI) was determined by the diphenylcarbazide (DPC) spectrophotometric method. Five hundred mg of DPC was dissolved in 50 mL of acetone, thoroughly mixed, and stored in a bottle wrapped in aluminum foil and kept in the dark. After the photocatalytic reaction, 600 μL of the reaction solution, 500 μL of sulfuric acid (2 M), and 200 μL of DPC solution (10 g L⁻¹) were mixed and diluted with deionized water to 25 mL. The mixture was left to stand for 10 min, and the absorbance at 540 nm was measured using a UV-vis spectrophotometer (V-750 ISV-922, Jasco, MD, USA).

3. Results and Discussion

3.1. Structural Characterizations

The crystal structures of pristine and modified g-C₃N₄ catalysts were investigated using X-ray diffraction (XRD) analysis. As shown in Figure 1a, all photocatalysts exhibited two characteristic diffraction peaks at 12.8° (the in-plane structural repetition of tri-s-triazine units) and 27.7° (the interlayer stacking of aromatic conjugated systems), corresponding to the (100) and (002) planes of g-C₃N₄, respectively (JCPDS No. 87–1526) [9]. The diffraction peak at 12.8° progressively weakened with increasing THB doping content. This decline suggests a disruption in the in-plane ordering of triazine units, indicating that the planar structure has become more disordered due to the modification.

Fourier-transform infrared (FTIR) was employed to investigate the molecular framework of both pristine and modified g-C₃N₄ catalysts, as shown in Figure 1b. The FTIR spectra of all samples exhibited similar vibrational modes of the molecular skeleton at 810 cm⁻¹ (the breathing mode of tri-s-triazine ring), 1200–1700 cm⁻¹ (the stretching vibrational properties of the C–N heterocycle), and 3000–3300 cm⁻¹ (the N–H stretching vibrational mode) [10]. There was no significant change in peak intensity as the amount of THB changed. It may be owing to this that the amount of THB added was so small as to be insufficient to produce notable differences.

The elemental composition was analyzed by X-ray photoelectron spectroscopy (XPS). Figure S1a showed both displaying peaks of C1s and N1s, which indicated the presence of elements C and N in each sample. In Figure S1b, the C1s spectra of T0.0-g-C₃N₄ could be deconvolved into three peaks centered at 284.8 (C–C), 286.63 (C–N–C), and 288.23 eV (N–C–N). As shown in Figure S1c, the N1s spectrum could be fitted with four peaks located at 398.73 (C-N=C), 399.83 (N-C₃), 400.83 (C-NH), and 404.43 eV (π-π* transition). According to the C 1s and N 1s spectra, the modified catalyst exhibits a slight shift toward lower binding energies, indicating an increase in electron density induced by the modification. In addition, the peak area around 400.83 eV in the N 1s spectrum slightly decreases, which suggests that the modifier has partially substituted the terminal amino groups in the carbon nitride framework.

Scanning electron microscopy (SEM) was employed to investigate the morphology of the photocatalysts. As shown in Figure 2a, the unmodified g-C₃N₄ exhibited an aggregated nanosheet-like structure. A similar stacked nanosheet morphology was observed for T-g-C₃N₄ (Figure 2b). However, in the THB-modified catalysts, nanoscale pores were clearly observed at various locations across the sheets. This porous structure likely resulted from the structural distortion and disruption of the planar layers caused by THB incorporation during the thermal polymerization process. The specific surface area of the photocatalysts was analyzed using N₂ adsorption–desorption isotherms. The unmodified sample exhibited a surface area of 58.2 m² g⁻¹, whereas T7.5-g-C₃N₄ showed a significantly higher value of 116.8 m² g⁻¹, indicating that the modification nearly doubled the surface area. This substantial increase suggests a higher number of accessible active sites and improved charge carrier separation, both of which contribute to the enhanced photocatalytic reduction performance.

UV-visible DRS was employed to investigate the optical absorption properties and band structure of synthesized catalysts. As shown in Figure 3a, unmodified catalyst has strong adsorption in the ultraviolet region and adsorption edge at 420 nm of wavelength. With the increase in THB doping content, the adsorption edge of modified g-C₃N₄ was red shifted. Notably, the absorption edge of T7.5-g-C₃N₄ extended to approximately 460 nm. In addition, the modified catalysts exhibited a distinct two-step visible light absorption behavior, indicating a successful extension of the light response range because of THB-induced structural modification. The acquired diffuse reflectance spectra were converted using the Kubelka–Munk transformation to calculate the band gap via a Taucplot analysis (Figure S2). The π–π*-derived bandgaps were as follows: T0.0-g-C₃N₄; 2.94 eV, T2.5-g-C₃N₄; 2.81 eV, T5.0-g-C₃N₄; 2.77 eV, T7.5-g-C₃N₄; 2.78 eV and T10.0-g-C₃N₄; 2.70 eV. In contrast, the modified g-C₃N₄ exhibited an intermediate band derived from n–π^⁎ transitions, and the corresponding band gaps were as follows [11]: T5.0-g-C₃N₄; 2.26 eV, T7.5-g-C₃N₄; 2.23 eV and T10.0-g-C₃N₄; 2.27 eV. In addition, the valence band positions of T0.0-g-C₃N₄ and T7.5-g-C₃N₄ were determined by valence band X-ray photoelectron spectroscopy (VBXPS) to be 1.87 eV and 1.56 eV vs. NHE, respectively. Using these values and the corresponding band gaps, the conduction band positions were calculated to be −1.07 eV for T0.0-g-C₃N₄ and −1.22 eV for T7.5-g-C₃N₄.

Photoluminescent emission (PL) was employed to investigate the rate of recombination for the photo-excited carriers. Strong fluorescent emission was observed at 447 nm for T0.0-g-C₃N₄, implying rapid electron/hole recombination. However, the PL intensity gradually decreased with increasing modification amount, as a result of the formation of additional sublevels that received the photoexcited charges. This indicates a significant reduction in charge carrier recombination, which contributes to the enhanced Cr(VI) photoreduction activity.

3.2. Photoreduction in Cr(VI)

Figure 4 shows the photocatalytic reduction ability of various amounts of modified catalysts towards Cr(VI) under visible light. In contrast to the unmodified catalyst, which exhibited negligible reduction activity, the Cr(VI) reduction rate increased progressively with the increasing amount of THB used for modification. T7.5-g-C₃N₄, having the best reaction rate, achieved a Cr(VI) reduction efficiency of 96.3% within 90 min. However, in the case of T10.0-g-C₃N₄, the reduction efficiency decreased, which may be attributed to structural damage of the catalyst caused by excessive THB modification.

Figure 5a shows the effect of EDTA solution concentration on the photocatalytic reduction of Cr(VI) by T7.5-g-C₃N₄. The reduction rate reached a maximum at an EDTA concentration of 300 ppm, while the reduction efficiency decreased at 500 ppm and 700 ppm. Since the theoretical consumption of EDTA is estimated to be less than 1% of the added amount, this decrease is unlikely to be due to EDTA depletion. Possible explanations include the excessive EDTA covering the catalyst surface, thereby inhibiting the adsorption of Cr(VI) onto active sites, or the scavenging of reactive species such as superoxide radicals. Although further experiments are necessary to clarify the detailed mechanism, the optimal EDTA concentration was determined to be 300 ppm in this study.

To find the optimal catalyst dosage, the catalyst dosage was varied (0.1, 0.2, 0.3, 0.4 g L⁻¹, as shown in Figure 5); by increasing the catalyst dosage from 0.1 g L⁻¹ to 0.3 g L⁻¹, the Cr(VI) photoreduction efficiency was enhanced due to the increased number of active sites on the photocatalyst surface. The highest reduction rate was achieved at a catalyst dosage of 0.3 g L⁻¹, with 97.6% of Cr(VI) reduced within 90 min. However, when the dosage was further increased to 0.5 g L⁻¹, the reduction efficiency declined to 86.5% at 90 min. This decrease is likely due to excessive catalyst dosage preventing visible light penetration and causing particle agglomeration, which reduces the accessibility of Cr(VI) and radiation to the catalyst’s active sites [12].

3.3. Proposed Mechanism for Cr(VI) Photoreduction

The possible mechanisms for the photocatalytic reduction of Cr(VI) by the catalysts before and after modification in this study are illustrated in Figure 6. For the unmodified catalyst, the photocatalytic reduction mechanism follows the typical process. Upon visible light irradiation, electrons in the valence band (VB) are excited to the conduction band (CB). The electrons accumulated in the CB can readily reduce Cr(VI) to Cr(III), since the reduction edge potential of Cr⁶⁺/Cr³⁺ [E⁰ (Cr⁶⁺/Cr³⁺) = +1.33 V versus NHE] is appropriately positioned [13]. The fact that electrons are the active species in this reaction is supported by the radical scavenger experiment shown in Figure S3, where the addition of K₂S₂O₈, an electron scavenger, led to a decrease in the Cr(VI) reduction rate [14]. For the modified catalyst, in addition to the above electron excitation, the electron transition within the intermediate band arising from the n–π* transition activated by the modification further promotes electron accumulation in the CB. Moreover, the increased porosity enhances carrier separation, further improving the reduction efficiency. The addition of EDTA traps holes in VB and promotes reduction on CB.

4. Conclusions

In summary, g-C₃N₄ was successfully modified by incorporating 1,3,5-trihydroxybenzene (THB) during thermal polymerization. BET analysis showed that the specific surface area of the modified catalyst nearly doubled compared to pristine g-C₃N₄, and DRS measurements confirmed enhanced light absorption due to newly introduced n→π^⁎ transitions and a narrowed band gap. Cr(VI) reduction experiments showed that T7.5-g-C₃N₄ achieved a maximum reduction efficiency of 96.3% within 90 min, with a rate constant of 0.0294 min⁻¹—approximately 150 times greater than that of the unmodified sample. These findings demonstrate a promising and cost-effective strategy for the development of highly efficient g-C₃N₄-based photocatalysts for hexavalent chromium removal and broader environmental remediation applications.