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 (MoO) adsorbent with mixed-valence (Mo(V) and Mo(VI)) demonstrates high selectivity (distribution coefficient of 6437.40 mL g) for Ag and an uptake capacity of 2605.91 mg g. 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, MoO 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, MoO 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.