Adsorption-enhanced photocatalytic property of Ag-doped biochar/g-C3N4/TiO2composite by incorporating cotton-based biochar

Abstract

In this study, the biochar obtained from waste cotton fibers was introduced into the Ag-doped g-C₃N₄/TiO₂hybrid composite through a facile one-step hydrothermal process. The morphology, elemental composition, crystal structure, microstructure, specific surface area, chemical bonding state, energy band structure, and separation efficiency of photoinduced charge carriers of the resultant composite were examined using scanning electron microscope, energy dispersive x-ray spectrometer, x-ray diffractometer, transmission electron microscope, surface area analyzer, x-ray photoelectron spectroscope, ultraviolet-visible spectrophotometer, ultraviolet photoelectron spectroscope, and photoluminescence spectroscope. The adsorption isotherms, kinetics and thermodynamics of the biochar, Ag-doped g-C₃N₄/TiO₂and Ag-doped biochar/g-C₃N₄/TiO₂were evaluated using the model methyl orange dye. The photoacatalytic degradation of the model pollutants including methyl orange, methylene blue, congo red, and tetracycline hydrochloride and the photocatalytic reduction of Cr(VI) ions were also assessed under visible light. Experimental results indicated that the photocatalytic property of the Ag-doped biochar/g-C₃N₄/TiO₂was significantly enhanced through the synergic effects compared with the Ag-doped g-C₃N₄/TiO₂. This was due to the uniform doping of multi-scale porous biochar with g-C₃N₄nanosheet, Ag and TiO₂nanoparticles. The adsorptive enhancement induced by the biochar resulted in the narrowed band gap, suitable electronic energy band structure, and fast separation of photoinduced charge carriers of the Ag-doped biochar/g-C₃N₄/TiO₂, which was probably due to the coexistence of multi-valence Ti^+4/+3and Ag^0/+1species and oxygen-containing groups of biochar. The major reactive species of the Ag-doped biochar/g-C₃N₄/TiO₂were¹O₂and h⁺. The MO dye adsorption onto the Ag-doped biochar/g-C₃N₄/TiO₂followed the Langmuir isotherm model, pseudo-first-order and pseudo-second-order kinetic models, and the adsorption process was an endothermic reaction with entropy reduction effects. As such, the Ag-doped biochar/g-C₃N₄/TiO₂exhibited a promising application for the treatment of wastewater containing multi-pollutants especially organic dyes and heavy metal ions.

Keywords: Ag-doped biochar/g-C3N4/TiO2; adsorption; cotton-based biochar; photocatalytic.