Light-activated polydopamine coatings for efficient metal recovery from electronic waste

Abstract

Metal ion adsorption from industrial wastewater has received increasing attention for the elimination of heavy metals and selective recovery of precious metals. Among the precious metals found in E-waste, gold has attracted considerable attention because of its wide range of practical applications and high economic value. However, the adsorbents used at present for the recovery of precious metals have very low adsorption capacity and poor metal selectivity for commercial uses. Herein, we introduce a new photochemical route for the selective and efficient adsorption of gold ions using bio-inspired metal-philic coatings. Internal surfaces of the mesoporous polymer microspheres were coated with polydopamine via the oxidative polymerization of catecholamines. The polydopamine layer served as a selective photo-active reductant for gold ions. Under 1-sun simulated illumination, the polydopamine layer selectively reduced gold ions to generate metallic gold nuclei. Moreover, once the metallic gold nanostructures were formed, localized surface plasmon further enhanced the reduction of gold ions. The combined effects increased the maximum amount of gold ions adsorbed per unit mass of the adsorbent up to 26 times compared to that in the dark. The adsorbed metallic gold was re-dissolvable in a thiourea solution for the complete recovery of gold ions. The selectivity toward gold ions among various metal ions was demonstrated using a solution mixture containing eight different metal ions that are commonly found in industrial wastewater. Density functional theory calculations revealed that reduction of gold on the polydopamine layer was energetically favorable, while the reduction of other metal ions was not. The dramatic increase in the maximum adsorption capacity and selectivity owing to the combined effect of the photochemical activation and polyphenol chemistry renders this process a promising approach toward urban mining of novel metal ions from electronic wastes.