Highly-sensitive detection and efficient recovery of silver ions by water-soluble sulfur quantum dots†

The treatment and resource utilization of solid waste and wastewater are becoming increasingly important. In this study, we propose a method to synthesize sulfur quantum dots using coked sulfur paste as raw material, which produced sulfur quantum dots with good water solubility, good dispersibility and stable luminescence performance. The synthesized sulfur quantum dots were used as a fluorescent probe for sensitively detecting low concentrations of silver ions (Ag⁺) based on the principle of fluorescence quenching. The detection method has a wide detection range (0–160 μM) with the lowest limit of detection of 7.1 μM and no quenching effect in the presence of 11 common ions, thus enabling the highly selective detection of silver ions. Moreover, a new method for the recovery of silver involving the strong bonding between sulfur quantum dots and silver ions was developed, i.e., even a low concentration of silver ions (under 200 μM) in industrial wastewater was converted into silver sulfide (Ag₂S) and silver sulfate (Ag₂SO₄) for recovery. Results show that the average recovery rate reached 99.03% in the aqueous environment in the presence of five types of hetero-ions, namely, Na⁺, K⁺, Ca²⁺, Mg²⁺ and Al³⁺, showing both high efficiency and selectivity. This investigation realized the resourceful utilization of coked sulfur paste and provides a new pathway for the detection and recovery of low concentrations of silver in industrial wastewater.