Synthesis and Characterization of Composite WO3 Fibers/g-C3N4 Photocatalysts for the Removal of the Insecticide Clothianidin in Aquatic Media

Abstract

Photocatalysis is a prominent alternative wastewater treatment technique that has the potential to completely degrade pesticides as well as other persistent organic pollutants, leading to detoxification of wastewater and thus paving the way for its efficient reuse. In addition to the more conventional photocatalysts (e.g., TiO₂, ZnO, etc.) that utilize only UV light for activation, the interest of the scientific community has recently focused on the development and application of visible light-activated photocatalysts like g-C₃N₄. However, some disadvantages of g-C₃N₄, such as the high recombination rate of photogenerated charges, limit its utility. In this light, the present study focuses on the synthesis of WO₃ fibers/g-C₃N₄ Z-scheme heterojunctions to improve the efficiency of g-C₃N₄ towards the photocatalytic removal of the widely used insecticide clothianidin. The effect of two different g-C₃N₄ precursors (urea and thiourea) and of WO₃ fiber content on the properties of the synthesized composite materials was also investigated. All aforementioned materials were characterized by a number of techniques (XRD, SEM-EDS, ATR-FTIR, Raman spectroscopy, DRS, etc.). According to the results, mixing 6.5% ^W/_W WO₃ fibers with either urea or thiourea derived g-C₃N₄ significantly increased the photocatalytic activity of the resulting composites compared to the precursor materials. In order to further elucidate the effect of the most efficient composite photocatalyst in the degradation of clothianidin, the generated transformation products were tentatively identified through UHPLC tandem high-resolution mass spectroscopy. Finally, the detoxification effect of the most efficient process was also assessed by combining the results of an in-vitro methodology and the predictions of two in-silico tools.

Keywords: AOPs; Z-scheme; graphitic carbon nitride; photocatalysis; toxicity assessment; transformation products.

Figure 1

XRD patterns of the synthesized (a) WOFs, (b) CNU and CNTU, and (c) composite materials (5%-WCNU, 6.5%-WCNU, 5%-WCNTU, 6.5%-WCNTU) acquired in the 2θ range of 10–90°.

Figure 2

ATR-FTIR spectra of the synthesized (a) WOFs, (b) CNU and CNTU, and (c) composite materials (5%-WCNU, 6.5%-WCNU, 5%-WCNTU, 6.5%-WCNTU) recorded in the wavenumber region of 4000–400 cm⁻¹.

Figure 3

Raman spectra of the synthesized (a) WOFs (532 nm), (b) CNU and CNTU, and (c) composite materials (5%-WCNU, 6.5%-WCNU, 5%-WCNTU, 6.5%-WCNTU) (785 nm).

Figure 4

SEM images of (a,b) WOFs, (c) CNU, (d) CNTU, (e) 5%-WCNU, (f) 5%-WCNTU, (g) 6.5%-WCNU, and (h) 6.5%-WCNTU, acquired using an accelerating voltage of 15 kV under high vacuum (0.1 Pa).

Figure 5

EDS spectra of the synthesized (a) pristine (WOFs, CNU, and CNTU) and (b) composite materials (5%-WCNU, 6.5%-WCNU, 5%-WCNTU, and 6.5%-WCNTU), recorded using an accelerating voltage of 15 kV under high vacuum (0.1 Pa).

Figure 6

Adsorption–desorption isotherms of (a) CNU, (b) CNTU, (c) 6.5%-WCNU, and (d) 6.5%-WCNTU recorded at −196 °C.

Figure 7

[F(R_∞)*hv]^1/2 vs. hv plots for (a) WOFs, (b) CNU, (c) CNTU, (d) 5%-WCNU, (e) 5%-WCNTU, (f) 6.5%-WCNU, and (g) 6.5%-WCNTU, and corresponding determined band gaps.

Figure 8

PL spectra of CNU, CNTU, 5%-WCNU, 5%-WCNTU, 6.5%-WCNU, and 6.5%-WCNTU (excitation wavelength: 320 nm).

Figure 9

(a) Photocatalytic degradation kinetics of CLO (5 mg L⁻¹) using the synthesized pristine (CNU, CNTU) and (b) composite photocatalysts (5%-WCNU, 6.5%-WCNU, 5%-WCNTU, 6.5%-WCNTU) (100 mg L⁻¹) under simulated sunlight (500 W m⁻²). (c) Effect of different pH values on the degradation kinetics of CLO (5 mg L⁻¹) using 6.5%-WCNU (100 mg L⁻¹) under simulated sunlight (500 W m⁻²). (d) Effect of different anions and humic acid (acting as dissolved organic matter) on the degradation kinetics of CLO (5 mg L⁻¹) using 6.5%-WCNU (100 mg L⁻¹) under simulated sunlight (500 W m⁻²).

Figure 10

(a) Photocatalytic evolution kinetics of 2TA-OH using the synthesized pristine (CNU, CNTU) and composite photocatalysts (6.5%-WCNU, 6.5%-WCNTU) (100 mg L⁻¹) under simulated sunlight (500 W m⁻²). (b) Photocatalytic degradation kinetics of CLO (5 mg L⁻¹) using 6.5%-WCNU (100 mg L⁻¹) in AcN under simulated sunlight (500 W m⁻²). (c) Schematic representation of the Z-scheme photocatalytic mechanism in the 6.5%-WCNU.

Figure 11

(a) Microtox bioassay results when 6.5%-WCNU and 6.5%-WCNTU were utilized and (b,c) in-silico predicted mutagenicity and developmental toxicity values for CLO and its TPs formed when 6.5%-WCNU was used.

Figure 12

(a) Evolutionary profiles of CLO’s tentatively identified TPs based on high-resolution MS data and (b) the proposed transformation pathways of CLO.