Stability of SiO 2 nanoparticles with complex environmental conditions with the presence of electrolyte and NOM

SiO 2 -engineered nanoparticles (SiO 2 -ENPs) are one of the most commonly employed nanomaterials. There was a lack of research on the comprehensive effects of environmental factors on the stability of SiO 2 -ENPs. In the present work, the aggregation size, surface charge, and percentage of nanoparticles have been studied under the presence of monovalent/divalent cations combined with natural organic matter (NOM), in an effort to provide a comprehensive investigation of the stability of SiO 2 -ENPs in complex environmental conditions. Results show that NOM adsorbed to the particles and made their zeta potential more negative, hence stabilizing them by electrostatic repulsion, and the stability of nano-SiO 2 in NOM solutions follows the order of pH 9 > pH 7 > pH 5. In addition, SiO 2 -ENPs size in solutions with humic acid (HA) was larger than that with fulvic acid (FA). The aggregation of SiO 2 -ENPs enhanced with the increase of cation concentrations, and divalent cations (Ca 2+ ) show stronger influence on SiO 2 -ENPs aggregate than monovalent cations. Divalent cations partially neutralized the adsorbed organic matter, and aggregation was enhanced with Ca 2+ , suggesting the presence of specific Ca 2+ -NOM bridges. The aggregation of SiO 2 -ENPs in electrolyte (CaCl 2 or NaCl) and CaCl 2 + NOM solutions follows in the order of pH 5~pH 9 > pH 7, showing that weakly acidic or alkaline conditions are not favorable for the stability of SiO 2 -ENPs in complex environmental systems.