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

Abstract

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 SAg -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 SAg -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 SAg -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.

Retention Site Contribution Toward Silver Particle Immobilization in Porous Media

“…In water unsaturated conditions, similar trends are observed, albeit with additional retention sites available. The slight change in retained particle distribution in the last 10 mm of the column can be explained by a modest increase in saturated at the lower boundary (Leuther et al., 2020). In both water saturation systems the SW interface contributes significantly, though not exclusively to particle retention.…”

“…Thus, sound evidence is lacking to confidently pair the pore‐scale mechanisms represented in the models with the DP behavior they produce. High resolution X‐ray computed tomography (XCT) has recently gained traction to address aspects of this knowledge gap (Leuther et al., 2020; Li et al., 2006a; Perez et al., 2020).…”

“…Furthermore, the occurrence of chemical heterogeneities and roughness on any of the approaching surfaces can significantly modify their interfacial interactions (Johnson et al., 2010; Torkzaban & Bradford, 2016; Tufenkji & Elimelech, 2005). Relevant xDLVO interactions to this work include: charge screening from changes in ionic strength, which affect the EDL (Mills et al., 1994; Molnar, Johnson, et al., 2015; Polemio et al., 1980); steric interactions (ST) from surface adsorption of naturally occurring dissolved organic matter (Babakhani et al., 2017; Chen & Elimelech, 2006; Leuther et al., 2020; Molnar, Gerhard, et al., 2015; Morales, Sang, et al., 2011; Morales, Zhang, et al., 2011; Patiño et al., 2020; Phenrat et al., 2008); polar or Lewis acid‐base interactions (AB) from the electron‐acceptor and electron‐donor nature of the two approaching materials (Van Oss, 2006); and hydrophobic interactions (HYD) between the particles and air bubbles under unsaturated conditions (Birdi, 2008; Crist et al., 2005; Kohli & Mittal, 2015). While substantial progress has been made to account for the pertinent colloidal interactions in typical groundwater systems, relating these surface energetics to pore‐scale and Darcy‐scale observations is not straight forward.…”

Aquatic Ecosystem Risk Assessment Generated by Accidental Silver Nanoparticle Spills in Groundwater

“…The transport of nanoparticles in porous media has received a significant amount of scientific interest in recent years due to its diverse applications, such as the remediation of groundwater with NPs or the use of porous media to filter water containing NPs. Several investigators have made advances in understanding the factors that influence the fate and transport of AgNPs in porous media [ 26 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ] Most of these studies were performed using sand columns to represent the porous media, with concentrations ranging between 1 and 40 mg/L, and several studies have investigated the transport of AgNPs in natural soils [ 26 , 34 , 37 ]. Also, the mobilities of different types of coated AgNPs—such as NPs coated with citrate [ 33 , 38 , 39 ], PVP [ 26 , 35 ], or other substances [ 32 , 33 ]—were tested in these studies.…”

An overlooked mechanism underlying the attenuated temperature response of soil heterotrophic respiration

“…Increasing water content of a dried soil initially increases O 2 dissolution, but once a threshold is reached, increasing soil water content further reduces O 2 supply because of the reduced area of water–air interface across which O 2 must dissolve, and the increased distance for dissolved O 2 to travel to reactive sites [30]. Dissolution of O 2 and its diffusion control biogeochemical reactions in soil [31], but they are difficult to model due to their complexity [32]. Consequently, most SOC models do not consider O 2 explicitly, probably based on an erroneous perception that O 2 in the topsoil is not a limiting factor [33].…”