Adsorption-enhanced photocatalytic property of Ag-doped biochar/g-C3N4/TiO2 composite by incorporating cotton-based biochar

Abstract In this study, the biochar obtained from waste cotton fibers was introduced into the Ag-doped g-C 3 N 4 /TiO 2 hybrid 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-C 3 N 4 /TiO 2 and Ag-doped biochar/g-C 3 N 4 /TiO 2 were 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-C 3 N 4 /TiO 2 was significantly enhanced through the synergic effects compared with the Ag-doped g-C 3 N 4 /TiO 2 . This was due to the uniform doping of multi-scale porous biochar with g-C 3 N 4 nanosheet, Ag and TiO 2 nanoparticles. 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-C 3 N 4 /TiO 2 , which was probably due to the coexistence of multi-valence Ti +4/+3 and Ag 0/+1 species and oxygen-containing groups of biochar. The major reactive species of the Ag-doped biochar/g-C 3 N 4 /TiO 2 were 1 O 2 and h + . The MO dye adsorption onto the Ag-doped biochar/g-C 3 N 4 /TiO 2 followed 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-C 3 N 4 /TiO 2 exhibited a promising application for the treatment of wastewater containing multi-pollutants especially organic dyes and heavy metal ions.