Transport and retention of surfactant- and polymer-stabilized engineered silver nanoparticles in silicate-dominated aquifer material

Authors: ADRIAN, YORCK - Aachen University; SCHNEIDEWIND, UWE - Aachen University; Bradford, Scott; SIMUNEK, JIRKA - University Of California; FERNANDEZ-STEEGER, TOMAS - Technical University Of Berlin; AZZAM, RAFIG - Aachen University

Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2018
Publication Date: 3/7/2018
Citation: Adrian, Y.F., Schneidewind, U., Bradford, S.A., Simunek, J., Fernandez-Steeger, T.M., Azzam, R. 2018. Transport and retention of surfactant- and polymer-stabilized engineered silver nanoparticles in silicate-dominated aquifer material. Environmental Pollution. 236:195-207. https://doi.org/10.1016/j.envpol.2018.01.011.

Interpretive Summary: Engineered silver nanoparticles (Ag-ENPs) are used in many industrial processes, and release of Ag-ENPs into the environment poses a potential risk from their antimicrobial properties. Experiments were conducted to investigate the influence of various environmental factors on the long-term transport of Ag-ENPs. Our results indicate that the transport of Ag-ENPs was enhanced over time as available retention sites filled, and that this filling processes was a strong function of many environmental factors. Previous studies examining the risks of Ag-ENPs do not consider time-dependent filling, and results from this study will be of interest to scientists and engineers concerned with the fate and risks associated with Ag-ENPs.

Technical Abstract: Packed column experiments were conducted to investigate the transport and blocking behavior of surfactant- and polymer-stabilized engineered silver nanoparticles (Ag-ENPs) in saturated natural aquifer material with varying silt and clay content, background solution chemistry, and flow velocity. Breakthrough curves for Ag-ENPs exhibited blocking behavior that frequently produced a delay in arrival time in comparison to a conservative tracer that was dependent on the physicochemical conditions, and then a rapid increase in the effluent concentration of Ag-ENPs. This breakthrough behavior was accurately described using one or two irreversible retention sites that accounted for Langmuirian blocking on one site. Simulated values for the total retention rate coefficient and the maximum solid phase concentration of Ag-ENPs increased with increasing solution ionic strength, cation valence, clay and silt content, decreasing flow velocity, and for polymer- instead of surfactant-stabilized Ag-ENPs. Increased Ag-ENP retention with ionic strength occurred because of compression of the double layer and lower magnitudes in the zeta potential, whereas lower velocities increased the residence time and decreased the hydrodynamics forces. Enhanced Ag-ENP interactions with cation valence and clay were attributed to the creation of cation bridging in the presence of Ca2+. The delay in breakthrough was always more pronounced for polymer- than surfactant-stabilized Ag-ENPs, because the magnitude of their zeta-potential was lower. Our results clearly indicate that the long-term transport behavior of Ag-ENPs in natural, silicate dominated aquifer material will be strongly dependent on blocking behavior that changes with the physicochemical conditions and enhanced Ag-ENP transport may occur when retention sites are filled.