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

Abstract

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.

Effects of Humic and Fulvic Acids on Silver Nanoparticle Stability, Dissolution, and Toxicity

“…The relative impact of the NOM types used here on AgNP dissolution is also consistent with the results of our related study in which ISEs were used to measure Ag + binding to NOM in the absence of AgNPs. 24 Addition of PLFA to a Ag + solution significantly decreased the free Ag + concentration, indicating binding of NOM to Ag + , while SRHA and SRFA induced smaller decreases. In the present study, PVP-capped AgNPs were also observed to be less toxic than citrate-capped AgNPs, consistent with their significantly lower release of Ag + (Figure 3b).…”

“…31 Three types of NOM were used: Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) have similar elemental compositions 32 (SI Table S1) and represent NOM fractions derived primarily from decomposition of vegetation. 33 Pony Lake fulvic acid (PLFA) represents NOM rich in sites with high affinity for metallic silver and Ag + (due to high sulfur and nitrogen content, 34 a subset of which has a high affinity for silver and Ag + ; 24 SI Table S1). It is derived exclusively from microbial matter decomposition.…”

“…The relative NOM ranking observed here is consistent with that observed in a study considering Ag + only. 24 …”

“…Dissolution was monitored over 5 h in an identical buffer using fluorousphase Ag + ISEs, prepared as described elsewhere 24 (see also the SI and Figure S4). Toxicity of AgNPs to Shewanella oneidensis MR-1 was evaluated using the LIVE/DEAD Cell Viability Assay (Invitrogen).…”

Silver(I) Binding Properties of Organic Soil Materials Are Different from Those of Isolated Humic Substances

“…The samples were equilibrated on an end-over-end shaker (Heidolph Reax II) in darkness at 14 °C for 48 hours. According to previous studies made with soil materials 15,17,18 and isolated humic fractions 28 , this would enable equilibration of silver(I) ions in the soil suspensions. Long equilibration times (weeks-months), might result in ageing and transformation reactions 15,16 .…”