Anaerobic toxicity of cationic silver nanoparticles

The microbial toxicity of silver nanoparticles (AgNPs) stabilized with different capping agents was compared to that of Ag⁺ under anaerobic conditions. Three AgNPs were investigated: (1) negatively charged citrate-coated AgNPs (citrate-AgNPs), (2) minimally charged polyvinylpyrrolidone coated AgNPs (PVP-AgNPs) and (3) positively charged branched polyethyleneimine coated AgNPs (BPEI-AgNPs). The AgNPs investigated in this experiment were similar in size (10–15 nm), spherical in shape, but varied in surface charge which ranged from highly negative to highly positive. While, at AgNPs concentrations lower than 5 mg L⁻¹, the anaerobic decomposition process was not influenced by the presence of the nanoparticles, there was an observed impact on the diversity of the microbial community. At elevated concentrations (100 mg L⁻¹ as silver), only the cationic BPEI-AgNPs demonstrated toxicity similar in magnitude to that of Ag⁺. Both citrate and PVP-AgNPs did not exhibit toxicity at the 100 mg L⁻¹ as measured by biogas evolution. These findings further indicate the varying modes of action for nanoparticle toxicity and represent one of the few studies that evaluate end-of-life management concerns with regards to the increasing use of nanomaterials in our everyday life. These findings also highlight some of the concerns with a one size fits all approach to the evaluation of environmental health and safety concerns associated with the use of nanoparticles. - Highlights: • At concentrations -1 the anaerobic decomposition process was not impacted. • An impact on the microbial community at concentrations -1 were observed. • At high concentrations (100 mg L⁻¹), the cationic BPEI-AgNPs demonstrated toxicity. • Toxicity was demonstrated without the presence of oxidative dissolution of silver. • A one size fits all approach for the evaluation of NPs may not be accurate.