Transport and Retention of Sulfidized Silver Nanoparticles in Porous Media: The Role of Air‐Water Interfaces, Flow Velocity, and Natural Organic Matter

• Published in: Water resources research Vol. 56; no. 9
• Main Authors: Leuther, Frederic, Köhne, John Maximilian, Metreveli, George, Vogel, Hans‐Jörg
• Format: Journal Article
• Language: English
• Published: 01.09.2020
• Subjects: air‐water interface; natural organic matter; phase distribution; silver nanoparticles; transport; X‐ray tomography
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2020WR027074

The sulfidation and aging of silver nanoparticles (Ag‐NPs) with natural organic matter (NOM) are major transformation processes along their pathway in wastewater treatment plants and surface waters. Although soils appear to be a sink for disposed Ag‐NPs, the impact of variable saturation on the transport and retention behavior in porous media is still not fully understood. We studied the behavior of sulfidized silver nanoparticles (S‐Ag‐NPs, 1 mg L−1) in saturated and unsaturated sand columns regarding the effects of (i) the presence of NOM (5 mg L−1) in the aquatic phase on retention, transport, and remobilization of S‐Ag‐NPs and (ii) the distribution and quantity of air‐water and solid‐water interfaces for different flow velocities determined via X‐ray microtomography (X‐ray μCT). Unsaturated transport experiments were conducted under controlled conditions with unit gradients in water potential and constant water content along the flow direction for each applied flux. It was shown that (i) NOM in S‐Ag‐NP dispersion highly increased the NP‐mobility; (ii) differences between saturated and unsaturated transport were increasing with decreasing flux and, consequently, decreasing water contents; (iii) both, solid‐water and air‐water interfaces were involved in retention of S‐Ag‐NPs aged by NOM. Using numerical model simulations and X‐ray μCT of flow experiments, the breakthrough of Ag‐NP could be explained by a disproportional increase in air‐water interfaces and an increasing attachment efficiency with decreasing water content and flow velocity. Key Points The transformation of Cit‐Ag‐NPs to NOM‐S‐Ag‐NPs, as it is expected when released into the environment, decreases their mobility The retardation of particles was mainly dependent on the interfacial areas between the solid‐water and air‐water phase The decreasing amount of particle breakthrough at reduced saturation was dependent on the increasing AWI and the decreasing flow velocity