Eco-friendly recovery and upcycling of silver and palladium through in situ adsorption–reduction using wool fibers

Abstract

Recovery of precious metals (PMs) from industrial waste is essential for sustainable resource management. Among PM recovery techniques, adsorption-based technologies are gaining attention owing to their efficiency across many waste sources. However, the fabrication of synthetic adsorbents requires toxic chemicals and high energy consumption, leading to secondary environmental pollution and costly recovery. This study demonstrates the potential of wool fibers as eco-friendly adsorbents for selective PM recovery/recycling and catalytic upcycling. The wool fiber surface possesses a uniformly dense distribution of nitrogen- and sulfur-containing functional groups, and the surface charge can be adjusted by varying the degree of protonation depending on the solution pH. This enables PM ion adsorption under both acidic and neutral conditions. The adsorbed PM ions were reduced to their metallic forms and remained stably immobilized on the wool fiber surface because of their irregular cuticle structures. The dual-charge properties of the wool fibers enable effective silver (Ag) recovery at pH 8 and palladium (Pd) recovery at pH 2, with maximum adsorption capacities of 88.96 mg g−1 and 76.31 mg g−1, respectively. Wool fibers selectively recovered 97 % of Ag from the leachate of wasted photovoltaic solar cells and 99 % of Pd from that of spent catalysts. The calcination of PM-adsorbed fibers enabled the recovery of high-purity PMs (≥99.7 % for Ag and ≥ 99.9 % for Pd), which were directly upcycled as catalysts for 4-nitrophenol reduction, highlighting their strong practical applicability. Therefore, wool fibers are sustainable PM adsorbents which can enhance resource recovery and support eco-friendly industrial practices.