Dynamic silver speciation as studied with fluorous-phase ion-selective electrodes: Effect of natural organic matter on the toxicity and speciation of silver

The widespread application of silver in consumer products and the resulting contamination of natural environments with silver raise questions about the toxicity of Ag⁺ in the ecosystem. Natural organic matter, NOM, which is abundant in water supplies, soil, and sediments, can form stable complexes with Ag⁺, altering its bioavailability and toxicity. Herein, the extent and kinetics of Ag⁺ binding to NOM, matrix effects on Ag⁺ binding to NOM, and the effect of NOM on Ag⁺ toxicity to Shewanella oneidensis MR-1 (assessed by the BacLight viability assay) were quantitatively studied with fluorous-phase Ag⁺ ion-selective electrodes (ISEs). Our findings show fast kinetics of Ag⁺ and NOM binding, weak Ag⁺ binding for Suwannee River humic acid, fulvic acid, and aquatic NOM, and stronger Ag⁺ binding for Pony Lake fulvic acid and Pahokee Peat humic acid. We quantified the effects of matrix components and pH on Ag⁺ binding to NOM, showing that the extent of binding greatly depends on the environmental conditions. The effect of NOM on the toxicity of Ag⁺ does not correlate with the extent of Ag⁺ binding to NOM, and other forms of silver, such as Ag⁺ reduced by NOM, are critical for understanding the effect of NOM on Ag⁺ toxicity. This work also shows that fluorous-phase Ag⁺ ISEs are effective tools for studying Ag⁺ binding to NOM because they can be used in a time-resolved manner to monitor the activity of Ag⁺ in situ with high selectivity and without the need for extensive sample preparation.