Impact of hydrological pattern on silver nanoparticle transport in soils

Joanna Makselon1, Dan Zhou2, Irina Engelhardt2, Diederik Jacques3, Harry Vereecken1, Erwin Klumpp1
1 Institute of Bio- and Geosciences (IBG-3), Forschungszentrum Jülich
2 Department of Hydrogeology, TU Bergakademie Freiberg
3 Institute for Environment, Health and Safety, Belgian Nuclear Research Centre

O 13.5 in Reactive transport modeling

14.04.2016, 15:45-16:00, Audimax A, Geb. 30.95

Silver nanoparticles (AgNP) are one of the most widely used anthropogenic nanoparticles for their excellent physicochemical properties, e.g. high electrical conductivity, catalytic and antimicrobial activity. AgNP can be released into the environment via discharge from industry and waste water treatment plants. In the subsurface, transport of AgNP is driven by their unique physicochemical properties, such as particle size, surface charge, and properties of the porous medium, such as pH value and ionic strength (IS). Currently systematic experimental and numerical investigations addressing the influence of hydrological patterns (e.g. evaporation and irrigation) on the transport of AgNP are still missing. In this study, unsaturated column experiments were conducted in three different hydrological scenarios: continuous irrigation with AgNP followed by i) irrigation with 1 mM KNO3, ii) flow interruption for 3 days followed by irrigation with 1 mM KNO3, iii) flow interruption for 3 days followed by irrigation with 0.2 mM KNO3. Effluent AgNP concentrations were measured by ICP-MS. Laboratory experiments were interpreted with HP1 that couples Hydrus-1D with PhreeqC. The model combines the colloid-filtration theory (CFT) with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Sorption at the soil-water (SWI) and air-water interface (AWI) were reproduced using the CFT and linear kinetic sorption equation, respectively. The DLVO theory was implemented as sub-equations of CFT to calculate the attachment efficiency which involves the impact of IS on molecular repulsive electrostatic forces.

Our experimental results show that the breakthrough curves (BTCs) of AgNP exhibited a sharp decrease after the flow interruption compared to that under continuous flow condition and the decrease was more distinctive by irrigation with 1mM KNO3 than 0.2 mM KNO3 after the interruption. This sharp decrease can be explained by the hydraulic conditions that developed when the flow was interrupted: reduced water saturation, flow velocity and increased IS by evaporation. Both hydraulic and chemical conditions enable a high AgNP retention. The development of SWI and AWI are directly correlated with evaporation which controls water content and drives the attachment of AgNP at both interfaces. However, the simulation results display that AgNP attachment at the SWI was more important for the retardation than that at the AWI. The hydrological induced changes in flow rate and water content altered the AgNP coefficients that are responsible for the attachment-detachment kinetics at the SWI. However, variable IS also affects the attachment-detachment kinetics at the SWI. Therefore when analyzing the transport of AgNP, transient hydraulic and variable saturated conditions must be taken into account since they are sensitively driving the transport of AgNP. Their impact on the development of the SWI and changes in IS are mainly responsible for the attachment-detachment kinetics of AgNP.

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