Coagulation and sedimentation of gold nanoparticles and illite in model natural waters: Influence of initial particle concentration

Abstract

Gold nanoparticles (AuNP) possess unique characteristics that render them adequate for applications and also to be used as a model NP to evaluate the fate and behavior at low NP concentrations due to the ease of detection by modern analytical techniques. Moreover, AuNP may result in some negative effects in the environment and there is a necessity to predict their aquatic environmental concentrations by parameterizing the underlying transport and transformation processes. This study investigated the coagulation and sedimentation of 30nm citrate-coated AuNP under enviro-mimicking conditions, i.e. model natural freshwaters (MNW) covering the range of European water chemistries relevant for colloids and NP (major ions, pH and dissolved organic matter) and artificial seawater (ASW). Firstly, the coagulation rates of AuNP at mg/L concentrations were evaluated using time-resolved dynamic light scattering which showed a decreased rate upon addition of Suwannee river natural organic matter (SRNOM) at low ionic strength (IS) but remained relatively high at high IS. Contrastingly, AuNP approaching environmental realistic concentrations (80μg/L) in quiescent microcosms showed slow decline rates in all MNW and ASW regardless of the presence of SRNOM, as measured by nanoparticle tracking analysis and elemental Au spectrometry. When illite was added as model natural colloid the rates of decline of AuNP remained low as well. This is owing to limited collisions at low particle number concentrations. The results show that besides IS, pH, NOM concentration and type and the intrinsic surface charge of the particles, the particle number concentration and size distribution of both AuNP and natural colloids determine the extent of the large-scale fate of NP in aquatic environments.