Urban mining, recycling precious metals from waste resources, has attracted increasing attention due to increasing industrial demand, limited reserves, and environmental and economic benefits. Polyphenols have been investigated as promising adsorbents for metal ion recovery because of their well-known metal-ligand charge transfer complexation. However, their adsorption capacity and selectivity for precious metals need to be improved for practical applications. Here we present a method to photochemically boost the adsorption of precious metal ions onto tannin-grafted mesoporous silica particles. Ultraviolet light, absorbed by tannin, can initiate the spontaneous reduction of noble metal ions (e.g., Au3+, Pt4+, and Pd2+) into metallic nuclei, which generate plasmonic hot electrons capable of driving the growth of the nuclei under visible light irradiation. The enhancement effects were dramatic: the adsorption capacity increases 11, 2.6, and 3.0 times for Au, Pt, and Pd despite competitive adsorption. The photochemical boosting works selectively for noble metal ions. No significant adsorption of other metal ion species commonly found in electronic wastewater is observed. The adsorption capacity for Au is 68.5 mmol/g, the highest value ever reported to the best of our knowledge. This work also reveals that the stable covalent linkage and high grafting density of tannin on a heterogeneous substrate is a key success factor for the high adsorption capacity because the photochemical process can facilitate the detachment of metallic particles and metal-tannin complexes as demonstrated for hydrogen-bonded tannin on porous particles. Our findings suggest a promising strategy – a photochemical route for precious metal ion recovery from electronic waste applicable to other adsorbents having redox-active properties if appropriate photosensitizers are employed.

Photo-enhanced urban mining of precious metals on tannin-grafted porous particles

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.