Impact of Natural Soil Nanoparticles On the Mobility of Polyaromatic Hydrocarbons In Soils.

Nanoparticles in soils participate in essential ecological services, such as element cycling, sorption and transport of nutrients, organics and pollutants. Their impact is more significant than previously thought due to the special characteristics arising from their nanoscale size and large surface area, and this recognition gap may lead to a lack of understanding of their role in many important biogeochemical processes. In this paper, we focused on the aggregation of extracted soil nanoparticles (SNPs), as well as their transport and deposition behavior in porous media.  In particular, we investigate how they influence the mobility of polyaromatic hydrocarbons (PAHs) in soil.  Nanoparticles were extracted from soils under different conditions. A dynamic light scattering technique was used to monitor the time-dependent average hydrodynamic diameter of SNPs dispersed in aqueous solutions. Also, transmission electron microscopy was applied to visualize the aggregation of SNPs. Column experiments were performed to identify the mobility of SNPs in water-saturated quartz sand (300-355 μm) under physicochemical and hydrodynamic conditions simulating the actual soil environment. The results were qualitatively explained by Derjaguin-Landau-Verwey-Overbeek theory to explain the interaction between SNPs and grain media surfaces, as well as that among the population of nanoparticles. At soil conditions of high ionic strength and normal soil pH range, the life cycle of nanoparticles was generally short because they easily aggregated themselves or deposited on the surfaces of grain media. The significance of the transport of SNPs is that they serve as vectors for the transport of contaminants through soil, such as PAHs, a group of the most common soil pollutants. Adsorption of PAHs onto SNPs, and removal of PAHs from water-saturated quartz sand by eluting SNPs suspensions were both carried out to determine the impact of SNPs on PAH mobility. The study demonstrated that the apparent aqueous solubility of PAHs was enhanced by its sorbtion to the SNPs. However, this increasing trend declined as the nanoparticle concentrations increased, probably due to increasing nanoparticle aggregation/followed by precipitation, inducing the removal of PAHs adsorbed on them from the aqueous phase. Also, the mobility of PAHs in soils was not obviously increased during the eluting processes, probably due to the transient existence of discrete SNPs before they aggregated or precipitated.