Location: Contaminant Fate and Transport ResearchTitle: Retention and remobilization of stabilized silver nanoparticles in an undisturbed loamy sand soil Author
Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2013
Publication Date: 10/9/2013
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2434.pdf
Citation: Liang, Y., Bradford, S.A., Simunek, J., Heggen, M., Vereecken, H., Klumpp, E. 2013. Retention and remobilization of stabilized silver nanoparticles in an undisturbed loamy sand soil. Journal of Environmental Science and Technology. 47:12229-12237. Interpretive Summary: Stabilized silver nanoparticles (AgNPs) are increasing being used as an antimicrobial agent in commercial products. There is a concern that the release of AgNPs into the environment may pose a risk to ecosystem health. The objective of this study was to investigate the influence of various physical (water velocity and input concentration) and chemical (solution ionic strength and composition) factors on the transport, retention, and release of AgNPs in an undisturbed loamy sand soil. The retention of AgNPs in undisturbed soil was demonstrated to strongly depend on these factors. Release of AgNPs occurred in association with soil colloids when calcium on the soil surface was exchanged by potassium and the ionic strength was lowered. This information will be of interest to scientists and engineers concerned with predicting the fate of nanoparticles, microorganisms, and other colloids in the soil root zone and groundwater environments.
Technical Abstract: Column experiments were conducted with undisturbed loamy sand soil under unsaturated conditions (around 90% saturation degree) to investigate the retention of surfactant stabilized silver nanoparticles (AgNPs) with various input concentration (Co), flow velocity, and ionic strength (IS), and the remobilization of AgNPs by changing the cation type and IS. The mobility of AgNPs in soil was enhanced with decreasing solution IS, increasing flow rate and input concentration. Significant retardation of AgNP breakthrough and hyperexponential retention profiles (RPs) were observed in almost all the transport experiments. The retention of AgNPs was successfully analyzed using a numerical model that accounted for time- and depth-dependent retention. The simulated retention rate coefficient (k1) and maximum retained concentration on the solid phase (Smax) increased with increasing IS and decreasing Co. The high k1 resulted in retarded breakthrough curves (BTCs) until Smax was filled and then high effluent concentrations were obtained. Hyperexponential RPs were likely caused by the hydrodynamics at the column inlet which produced a concentrated AgNP flux to the solid surface. Higher IS and lower Co produced more hyperexponential RPs because of larger values of Smax. Retention of AgNPs was much more pronounced in the presence of Ca2+ than K+ at the same IS, and the amount of AgNP released with a reduction in IS was larger for K+ than Ca2+ systems. These stronger AgNP interactions in the presence of Ca2+ were attributed to cation bridging. Further release of AgNPs and clay from the soil was induced by cation exchange (K+ for Ca2+) that reduced the bridging interaction and IS reduction that expanded the electrical double layer. Transmission electron microscopy, energy-dispersive X-ray spectroscopy, and correlations between released soil colloids and AgNPs indicated that some of the released AgNPs were associated with the released clay fraction.