Mixed-valence molybdenum oxide as a recyclable sorbent for silver removal and recovery from wastewater

Silver ions in wastewater streams are a major pollutant and a threat to human health. Given the increasing demand and relative scarcity of silver, these streams could be a lucrative source to extract metallic silver. Wastewater is a complex mixture of many different metal salts, and developing recyclable sorbents with high specificity towards silver ions remains a major challenge. Here we report that molybdenum oxide (MoOx) adsorbent with mixed-valence (Mo(V) and Mo(VI)) demonstrates high selectivity (distribution coefficient of 6437.40 mL g−1) for Ag+ and an uptake capacity of 2605.91 mg g−1. Our experimental results and density functional theory calculations illustrate the mechanism behind Ag+ adsorption and reduction. Our results show that Mo(V) species reduce Ag+ to metallic Ag, which decreases the energy barrier for subsequent Ag+ reductions, accounting for the high uptake of Ag+ from wastewater. Due to its high selectivity, MoOx favorably adsorbs Ag+ even in the presence of interfering ions. High selective recovery of Ag+ from wastewater (recovery efficiency = 97.9%) further supports the practical applications of the sorbent. Finally, MoOx can be recycled following silver recovery while maintaining a recovery efficiency of 97.1% after five cycles. The method is expected to provide a viable strategy to recover silver from wastewater.