pH-regulated surface chemistry of nano-biochar for selective silver ion reduction: the pivotal role of superoxide radicals

Silver ion (Ag + ) pollution in aquatic systems poses serious ecological and health threats, demanding effective and controllable remediation technologies. This study investigates the pH-dependent reduction of Ag + by nano-biochar (nano-BC) synthesized via ball milling from wood chip-derived bulk biochar pyrolyzed at 400 °C (nano400) and 700 °C (nano700). Using advanced characterization techniques and free radical detection, we found that the reduction of Ag + to silver nanoparticles (AgNPs) by nano-BC is strongly pH-dependent: no AgNPs are formed under acidic-neutral conditions (pH 6.5–7.5), while weakly alkaline environments (pH 8.5–9.5) promote efficient reduction with the highest yield at pH 9.5. AgNPs formation follows pseudo-first-order kinetics, and nano400 exhibits a reaction rate constant approximately 2.5 times higher than that of nano700, showing significantly faster reduction kinetics. Nano400 outperforms nano700, attributed to its abundant oxygen-containing functional groups (e.g., phenolic hydroxyl groups, 3.01 mmol g −1 ), which enhanced its electron-donating capacity. Mechanistic studies reveal that superoxide radicals (O 2 ·− ) act as the key intermediates, generated via the synergistic effect of deprotonated functional groups and persistent free radicals (PFRs) in nano-BC under alkaline conditions. Excessive nano-BC dosage (> 10 mg L −1 for nano400, > 20 mg L −1 for nano700) inhibits AgNPs formation, likely via reduced interfacial reactive sites and reactive oxygen species (ROS) self-consumption. This study clarifies the core mechanism of pH-regulated Ag + reduction by nano-BC, providing theoretical support for nano-BC remediation of Ag + -contaminated water.