Distinguishable transport behavior of zinc oxide nanoparticles in silica sand and soil columns

As part of ongoing risk assessments of ZnO nanoparticles (nZnO) in the natural environment, transport behaviors of nZnO in soil need investigation. This work comparatively studied the transport and retention behavior of nZnO in silica sand versus soil, where the effect of input concentration (C₀=34~430 mgL(-1)) and ionic strength (IS=1~50 mM) were investigated. In silica sand, nZnO were highly mobile, especially at low C₀ and the efflux of nZnO generally decreased with increasing C₀ at all tested IS. Conversely, at low C₀, n ZnO were almost entirely immobile in soil and the efflux of nZnO increased with C₀ 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 C0 in silica sand, but decreased with C0 in soil. Such differences were mainly responsible for the various transport behaviors of nZnO in silica sand and soil.