Efficient Removal and Recovery of Ag from Wastewater Using Charged Polystyrene-Polydopamine Nanocoatings and Their Sustainable Catalytic Application in 4-Nitrophenol Reduction

Abstract

This study addresses the long-standing challenges of removing and recovering trace silver (Ag) ions from wastewater while promoting their sustainable catalysis utilization. We innovatively developed a composite material by combining charged sulfonated polystyrene (PS) with a PDA coating. This composite serves a dual purpose: effectively removing and recovering trace Ag⁺ from wastewater and enabling reused Ag for sustainable applications, particularly in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The PS-PDA demonstrated exceptional selectivity to trace Ag⁺ recycling, which is equal to 14 times greater than the commercial ion exchanger. We emphasize the distinct roles of different charged functional groups in Ag⁺ removal and catalytic reduction performance. The negatively charged SO₃H groups exhibited the remarkable ability to rapidly enrich trace Ag ions from wastewater, with a capacity 2-3 times higher than that of positively-N⁺(CH₃)₃Cl and netural-CH₂Cl-modified composites; this resulted in an impressive 96% conversion of 4-NP to 4-AP within just 25 min. The fixed-bed application further confirmed the effective treatment capacity of approximately 4400 L of water per kilogram of adsorbent, while maintaining an extremely low effluent Ag⁺ concentration of less than 0.1 mg/L. XPS investigations provided valuable insights into the conversion of Ag⁺ ions into metallic Ag through the enticement of negatively charged SO₃H groups and the <i>in situ</i> reduction facilitated by PDA. This breakthrough not only facilitates the efficient extraction of Ag from wastewater but also paves the way for its environmentally responsible utilization in catalytic reactions.