Behavior of Ag species in presence of aquatic sediment minerals – In context of aquatic environmental safety

Abstract In recent years, there has been a growth in the number of products containing Ag nanoparticles (AgNPs) in many areas. The global use of AgNPs containing products suggests that the water-soil environment may be exposed to the contaminant with different Ag species (isolated nanoparticles, nanoparticle aggregates, or soluble monovalent ions) depending on the environmental conditions and the particles' properties. Both Ag forms (ions and nanoparticles) are toxic to the aquatic and terrestrial microorganisms. In contact with suspension and sediment constituents the Ag can be bound on the active centers of minerals and humic substances and released back into water systems under favorable conditions but their behavior is not well understood. Therefore the sorption of two Ag forms (i.e. Ag(I) ions and nanoparticles – AgNPs) on clay minerals (montmorillonite and kaolinite) and (hydr)oxides of iron (ferrihydrite) as well as the binding strength/mobility of the bonded Ag species were researched. The results show a great sorption potential of clay minerals for both Ag forms and lower sorption capacity of ferrihydrite, in particular for Ag(I) ions. The maximum sorption capacities of montmorillonite, kaolinite and ferrihydrite were 94.39 mg/g, 117.8 mg/g and 26.48 mg/g for AgNPs and 17.92 mg/g, 21.14 mg/g and 3.072 mg/g for Ag(I) ions, respectively. The best-fitted sorption isotherm models for Ag(I) ions uptake onto studied minerals were determined to be in the order: Sips > Langmuir > Dubinin-Radushkevich > Freundlich. The all using equations very good describe the experimental equilibrium data of AgNPs sorption on the minerals. The sequential extraction study indicated different binding mechanisms of the Ag forms onto the clay minerals and ferrihydrite, which depended on the active sites of minerals as well as the Ag species nature in the solution. The contribution of the sorbed Ag(I) ions to the bioavailable fraction of the clay minerals was high but iron (hydr)oxides decreased the Ag(I) mobility. On the other hand, AgNPs bound with the active centers of minerals in a very strong way and were not able to release into water.