Purification of aqueous orange II solution through adsorption and visible-light-induced photodegradation using ZnO-modified g-C3N4 composites

Abstract

Semiconductor-based remediation enables environmentally friendly methods of removing aqueous pollutants. Simply fabricated ZnO modified g-C₃N₄ composites were utilized as bifunctional adsorptive photocatalysts for orange II removal from aqueous solution through adsorption and photocatalysis processes. The adsorption isotherm data of the g-C₃N₄ (g-CN) and ZnO modified g-C₃N₄ (ZCN) composites on orange II solution were better fitted with the Langmuir isotherm compared to the Freundlich isotherm. The maximum adsorption capacity for ZCN-2.5 was slightly higher than that of bare g-CN. According to the adsorption thermodynamics investigation of ZCN-2.5 in orange II solution, the positive values of Gibb's free energy change (ΔG⁰) suggested a non-spontaneous adsorption process. Furthermore, the negative values of entropy change (ΔS) and enthalpy change (ΔH) indicated the decrement of randomness and exothermic nature during the adsorption process, respectively. The photocatalytic degradation kinetics of g-CN and ZCN composites indicated that the degradation process follows the pseudo-first-order reaction kinetic. The degradation rate of orange II with the ZCN-2.5 composite was 6.67 times higher than that obtained with bare g-CN. Possible adsorption and photocatalytic mechanisms have been proposed.

Fig. 1. (a) XRD patterns of ZnO, g-CN and different ZCN composites and (b) FTIR spectra of g-CN and all fabricated ZCN composites.

Fig. 2. (a) Survey XPS spectra of g-CN and ZCN-2.5; overlap high resolution XPS (b) C 1s, (c) N 1s and (d) O 1s spectra of g-CN and ZCN-2.5 and (e) High resolution XPS Zn 2p spectra of ZCN-2.5.

Fig. 3. (a) TEM image and (b–f) EDS elemental mapping with corresponding SEM image of ZCN-2.5.

Fig. 4. (a) Kubelka–Munk function of UV-vis DRS and (c) PL spectra (upon the excitation at 340 nm wavelength) of ZnO, g-CN, and all fabricated ZCN composites. (b) Tauc plot and (d) EIS resultant Nyquist plot of g-CN and different ZCN composites.

Fig. 5. (a) Effect of contact time on the adsorption of orange II by prepared pure g-CN and different ZCN composites; Orange II solution, 10 mg L⁻¹ (30 mL); adsorbents, 30 mg. (b) Effect of equilibrium concentration on the adsorption of orange II prepared pure g-CN and different ZCN composites; Orange II solution, 30 mL (3–100 mg L⁻¹); adsorbents, 30 mg.

Fig. 6. (a) Langmuir isotherm and (b) Freundlich isotherm for adsorption of Orange II dye by pure g-CN and different ZCN composites.

Fig. 7. (a) Photocatalytic degradation of orange II dye solution using different photocatalysts with the irradiation of visible light and (b) the plot of −ln(C/C₀) versus irradiation time; orange II: 10 mg L⁻¹ (30 mL), photocatalyst: 30 mg.

Fig. 8. Proposed mechanism of (a) adsorption and (b) photocatalytic degradation of orange II dye solution using ZCN composites.