Biogenic bimetallic Ag-Au/chitosan aerogel for ultrafast reduction of nitroaromatic pollutants

Developing sustainable, recyclable catalytic materials capable of simultaneously achieving rapid reduction of nitroaromatic contaminants from aqueous media remains a significant challenge in green separation and purification processes. Herein, we report the synthesis of a biogenic Ag-Au/chitosan aerogel (Ag-Au/CSA) using orange-peel extract as a natural reducing and stabilizing agent, which integrates bimetallic electronic synergy with porous biopolymer-templated nanoconfinement. The resulting catalyst exhibits an impressive apparent rate constant (k_app = 0.857 min^-1) for the NaBH₄-assisted reduction of p-nitroaniline (p-NA), fivefold higher than that of the monometallic Ag/CSA analogue. The formation of intimate Ag-Au bimetallic interfaces may induce subtle interfacial electronic effects that are favorable for hydride activation and electron transfer in nitro-group reduction reactions, thus lowering the apparent kinetic barrier without implying a directly measurable charge transfer. Meanwhile, the three-dimensional chitosan aerogel framework offers abundant coordination sites and a hierarchically porous architecture that promotes uniform nanoparticle dispersion, efficient reactant diffusion, and structural robustness. The architecture also enables instantaneous catalyst recovery via vacuum filtration within seconds, ensuring full recyclability with no considerable material loss. Strikingly, the optimized Ag-Au/CSA catalyst retains over 95% of its initial activity after nine consecutive cycles and displays broad-spectrum reactivity toward diverse nitroaromatic substrates. Distinct from previously reported monometallic Ag/CSA systems, this work systematically demonstrates how bimetallic Ag-Au electronic coupling, when integrated into a three-dimensional chitosan aerogel via a biogenic fruit-extract-mediated route, can markedly enhance catalytic kinetics, stability, and recyclability. Collectively, these findings establish intermetallic electronic synergy combined with green biopolymer templating as a robust and scalable strategy for designing recyclable, high-performance catalysts for aqueous-phase environmental remediation and sustainable chemical transformations.