Effects of concentration and chemical composition of natural organic matter on the aggregative behavior of silver nanoparticles

Silver nanoparticles (AgNPs) are incorporated in many consumer products and have a high-releasing potential in aquatic environments. In these environments, AgNPs could interact with colloidal natural organic matter (NOM) affecting their behavior and fate. Aggregation kinetics of AgNPs were conducted at 10 NaCl concentrations during 2 h with 2.5 or 25 mgNOM L−1. Four (4) colloidal NOM fractions were extracted from 50 m3 of St.-Lawrence estuarine waters: fulvic acids (SLFA), humic acids (SLHA), hydrophilic (SL4ILE), and hydrophobic (SL4OBE) fractions. The addition of NOM to AgNP dispersion in nanopure water induced a small increase in AgNP size depending on NOM concentration and fraction used. The presence of both salt and NOM significantly altered the size of silver aggregates. With added 2.5 mgNOM L−1, more than 90% of particles have a size smaller than 200 nm, independently of the NOM fraction used. At 25 mgNOM L−1, the particle size distribution was strongly affected by the nature of NOM fractions. Our results show that NOM with highly aromatic carbon content such as SLFA and SLHA promoted the formation of larger aggregates with a hydrodynamic diameter larger than 200 nm. A high content in non-polar aliphatic carbon as observed in SL4ILE and SL4OBE stabilized aggregates at a size below 200 nm. This study highlights the importance of the chemical composition of NOM fractions and their content in non-polar aliphatic and aromatic carbons as they are involved in hydrophobic effects and strongly influence the aggregate size distribution. Sulfur, also present at low concentration NOM fractions, appears to be involved in the aggregation of AgNPs, but its action mechanism was not determined.