Carbothermal synthesis of biochar-supported metallic silver for enhanced photocatalytic removal of methylene blue and antimicrobial efficacy

Highlights: • Biochar-supported Ag° was prepared via thermal reduction between 150−200 °C. • Sorption of methylene blue by Ag^o-based catalysts was not significant. • Hydroxyl, superoxide and singlet oxygen and hole participated in dye degradation. • Thermally-prepared AgH generate more ROS than AgW due to favorable C property. • Supported-Ag^o maintained good antimicrobial efficacy and low Ag⁺ leaching. Pinewood biochar (PBC)-supported metallic silver (Ag^o) was prepared via a one-step carbothermal reduction route (AgH) or a wet-chemistry reduction method (AgW). XRD and SEM confirmed Ag^o was soldered on PBC matrix. Low methylene blue (MB) sorption was observed for unsupported Ag^o nanoparticles (AgNP), AgH and AgW. Under ultraviolet (UV) light irradiation, net MB degradation by AgH (15.88 g kg⁻¹) was higher than that of AgW (12.50 g kg⁻¹) and AgNP (10.27 g kg⁻¹). TOC removal percentages after degradation corresponded largely to reduction of MB concentrations in solution, indicating MB was dominantly mineralized. Electron paramagnetic resonance (EPR) revealed that MB was degraded by reactive oxygen species (ROS) such as hydroxyl radical (OH), superoxide radical (O₂-) and singlet oxygen (¹O₂). The scavenging experiments further suggested that OH scavengers suppressed MB degradation to a greater extent than other quenchers. Compared to AgW, AgH possessed greater abundance of persistent free radicals, which enhance ROS generation. PBC could also improve separation of electron-hole (e^--h⁺) pairs and enhance electron transfer ascribing to favorable carbon structure. Besides, PBC-Ag^o maintained good antimicrobial efficacy over E.coli DH5α. This work presented a facile carbothermal route to prepare Ag^o-based photocatalysts for dye removal and microbial inhibition in industrial wastewater.