Zero‐Discharge Synthesis of Silver Nanoparticles and Activated Carbon From Agricultural Residues

ABSTRACT Rice husk, straw, and bran are abundant agricultural residues that generate large amounts of solid waste during rice processing. Their valorisation has been explored, with most studies focusing on the production of activated carbons (ACs). However, comparative evaluations of ACs derived from different rice residues remain lacking, and the neutralization effluent produced during AC preparation is typically discarded as liquid waste. Here, we present a zero‐discharge strategy that simultaneously converts rice residues into ACs and silver nanoparticles (AgNPs), while also enabling a direct comparison across three feed stocks. Biochar produced by pyrolysis at 600 °C was chemically activated with KOH to yield ACs with hierarchical porosity and distinct surface features. The neutralization effluents, enriched in phenolic and fatty acids, were directly repurposed as reducing and stabilizing media for AgNP synthesis, thereby eliminating the need for external reagents and establishing a closed‐loop process. The formation of AgNPs was confirmed by UV–vis, FTIR, HRTEM, and particle size analysis, which revealed tunable morphologies (spherical, rod‐like, and core–shell) with an average diameter of ∼5 nm. Mechanistic insights from ^1H NMR and HRMS demonstrated the involvement of effluent‐derived phenolic, thiols, and hydroxyl acids in Ag + reduction and stabilization. Functional assays further revealed potent antioxidant and antibacterial activities. ACs, obtained at ∼64% yield, were systematically characterized using SEM, XRD, BET, TGA, and electrochemical analyses. The results were consistent with previously reported studies on individual residues. This first demonstration of neutralization effluent reuse for AgNP synthesis, combined with a comparative evaluation of ACs from multiple residues, establishes a dual, closed‐loop valorization framework for agricultural waste.