Transport And Aggregation Behavior Of Zinc Oxide (nZnO) Nanoparticles In Natural Soil And Water Environment

With industrialization and urbanization soil and water environments are increasingly exposed to nanomaterials and its byproducts, and environmental significance of this new kind of engineered nanomaterials (ENMs) have raised concerns. Zinc Oxide nanoparticles (nZnO) have emerged as one of the most widely used nanomaterials in commercial products and as part of ongoing risk assessments of nZnO in the natural environment, transport behaviors of nZnO in soil need investigation.In the second section of this work comparatively studied the transport and retention behavior of nZnO in silica sand versus soil, where the effect of input concentration and ionic strength were investigated. In silica sand, nZnO were highly mobile, especially at low inlet concentration (Co) and the efflux of nZnO generally decreased with increasing Co at all tested ionic strength. Conversely, at low Co, nZnO were almost entirely immobile in soil and the efflux of nZnO increased with Co at all tested IS. In both media, the retention profiles (RPs) were generally hyper-exponentially shaped suggesting nZnO easily deposited near the column inlet. As indicated by DLVO calculations, previously deposited nZnO on the silica sand surface acted as new deposition sites due to the lower energy barrier (Φmax) between nZnO-nZnO than between nZnO-silica sand. However, previously deposited nZnO in soil could enhance Φmax between the soil surface and nZnO which resulted in unfavorable sites for nZnO deposition. The two-site kinetic attachment model provided good descriptions for the breakthrough curves of nZnO. Detachment was more significant than attachment in silica sand (k1d>k1a), whereas soil surface attachment of nZnO were strong and irreversible (k1a>k1d). The straining interaction parameter (k2Str) increased with increasing Co in silica sand, but decreased with Co in soil. Such differences were mainly responsible for the various transport behaviors of nZnO in silica sand and soil.In the third section, the sedimentation and the aggregation behaviour of nZnO or nTiO2 examined in Grand Canal natural water matrix, when in the presence (unfiltered) and absence (filtered) of different sized natural colloids (NCs). The apparent sedimentation rates (Vs), homoaggregation (khom,crit) and heteroaggregation rates (khet,crit) of nZnO and nTiO2 in natural water were estimated using Stokes equation and a simplified Smoluchowski-based aggregation-settling equation. In the presence of smaller than 1 mm and 25μm NCs in natural water matrix, settlings of nZnO and nTiO2 were regularly similar, which was significantly affected by NCs in natural water. Increasing concentration effect of nZnO was not obvious, but settling of nZnO reduced with co-existence of nTiO2. Calculations of DLVO theory, energy barriers (Φmax) of nZnO-NCs and nTiO2-NCs were significantly smaller than nZnO-nZnO and nTiO2-nTiO2 system, and more negative secondary minimas (Φmin2) occurred in the nZnO-NCs and nTiO2-NCs system, this indicated nZnO and nTiO2 are more easily heteroaggregate with or deposite onto NCs, followed by sedimentation. Settling of nZnO and nTiO2 observed significant differences in filtered natural river water and in deionized water. Due to smaller Φmax and Φmin2, nTiO2 created self-aggregates (homoaggregation) following sedimentation. The highest values of modeling parameter, such as Vs and khet,crit of nZnO and nTiO2 were observed in the presence of NCs in natural water, this means nZnO and nTiO2 easily heteroaggregate with NCs. The mixture of nZnO and nTiO2 in natural water were positively affected the stability of both nZnO and nTiO2, and reduced the apparent Vs and khet,crit.The settlement of nZnO and nTiO2 in fintered natural water differed from unfiltered natural water. In filtered natural water, Vs, khom,crit and khet,crit of nZnO and nTiO2 were significant smaller than in unfiltered natural water. Co-existence of nTiO2 reduced the khom,crit but increased the khet,crit of nZnO. This was related with different effect of dissolved organic matter (DOC) in the natural water.Overall, our results suggest that transport and retention behavior of nZnO through silica sand and soil is remarkably distinguishable, soil medium is cannot be replaced by silica sand medium in the research study. Moreover, the environmental fate and stability of unstable ENMs, such as nZnO, are likely to be influenced by the presence of other stable ENMs, such as nTiO2.