Chemical speciation of silver (Ag) in soils under aerobic and anaerobic conditions: Ag nanoparticles vs. ionic Ag

This study investigated how silver nanoparticles (AgNP) and ionic silver (AgNO₃) undergo phase-transformations in soils under aerobic and anaerobic conditions using extended X-ray absorption fine structure (EXAFS) spectroscopy. After 30 days of aerobic incubation, 88% of AgNP added to the soil remained persistent, whereas AgNO₃ was completely transformed into Ag associated with humus and clay minerals. In the anaerobic soil, 83% of the spiked AgNP was transformed into Ag₂S, accompanied by significant decrease in water- and acid-extractable Ag fractions. About 50% of AgNO₃ spiked to the anaerobic soil underwent transformations into metallic Ag and associations with clay minerals. Oxide (Ag₂O) and carbonate (Ag₂CO₃) forms of Ag were not predominant in aerobic and anaerobic soils. The redox potential of soil had a profound effect on determination of the phase-transformation pathways for AgNP and ionic Ag.