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

Abstract

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 (O₂^·−) 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.

Keywords

Nano-biochar / Silver ion reduction / pH regulation / Superoxide radical

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Funding

Key Research Project of Natural Science in Colleges and Universities of Anhui Province(2025AHGXZK30307)

Funding Project for Young Backbone Teachers to Visit and Study in China(JNFX2023061)

Chuzhou University Research Initiation Fund Project(2023qd49)

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