title: Transport of citrate-coated silver nanoparticles in unsaturated sand.
abstract: Chemical factors and physical constraints lead to coupled effects during particle transport in unsaturated porous media. Studies on unsaturated transport as typical for soils are currently scarce. In unsaturated porous media, particle mobility is determined by the existence of an air-water interface in addition to a solid-water interface. To this end, we measured breakthrough curves and retention profiles of citrate-coated Ag nanoparticles in unsaturated sand at two pH values (5 and 9) and three different flow rates corresponding to different water contents with 1 mM KNO3 as background electrolyte. The classical DLVO theory suggests unfavorable deposition conditions at the air-water and solid-water interfaces. The breakthrough curves indicate modification in curve shapes and retardation of nanoparticles compared to inert solute. Retention profiles show sensitivity to flow rate and pH and this ranged from almost no retention for the highest flow rate at pH=9 to almost complete retention for the lowest flow rate at pH=5. Modeling of the breakthrough curves, thus, required coupling two parallel processes: a kinetically controlled attachment process far from equilibrium, responsible for the shape modification, and an equilibrium sorption, responsible for particle retardation. The non-equilibrium process and equilibrium sorption are suggested to relate to the solid-water and air-water interfaces, respectively. This is supported by the DLVO model extended for hydrophobic interactions which suggests reversible attachment, characterized by a secondary minimum (depth 3-5 kT) and a repulsive barrier at the air-water interface. In contrast, the solid-water interface is characterized by a significant repulsive barrier and the absence of a secondary minimum suggesting kinetically controlled and non-equilibrium interaction. This study provides new insights into particle transport in unsaturated porous media and offers a model concept representing the relevant processes.
authors.affiliation: Department of Soil Physics, Helmholtz Centre for Environmental Research-UFZ, Theodor-Lieser-Strasse 4, 06120 Halle-Saale, Germany. Electronic address: samuel.kumahor@ufz.de.
authors.forename: Samuel K
authors.initials: SK
authors.lastname: Kumahor
authors.affiliation: Interdisciplinary Center for Scientific Computing, University of Heidelberg, Im Neuenheimer Feld 368, Raum 422, 69120 Heidelberg, Germany. Electronic address: pavel.hron@iwr.uni-heidelberg.de.
authors.forename: Pavel
authors.initials: P
authors.lastname: Hron
authors.affiliation: Universität Koblenz-Landau, Institute for Environmental Sciences, Group of Environmental and Soil Chemistry, Fortstr. 7, D-76829 Landau, Germany. Electronic address: metreveli@uni-landau.de.
authors.forename: George
authors.initials: G
authors.lastname: Metreveli
authors.affiliation: Universität Koblenz-Landau, Institute for Environmental Sciences, Group of Environmental and Soil Chemistry, Fortstr. 7, D-76829 Landau, Germany. Electronic address: schaumann@uni-landau.de.
authors.forename: Gabriele E
authors.initials: GE
authors.lastname: Schaumann
authors.affiliation: Department of Soil Physics, Helmholtz Centre for Environmental Research-UFZ, Theodor-Lieser-Strasse 4, 06120 Halle-Saale, Germany; Institute of Soil Science and Plant Nutrition, Martin-Luther-University Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle-Saale, Germany. Electronic address: hans-joerg.vogel@ufz.de.
authors.forename: Hans-Jörg
authors.initials: HJ
authors.lastname: Vogel
mesh_terms: D002951:Citrates; D007700:Kinetics; D008956:Models, Chemical; D053758:Nanoparticles / Q000737:chemistry*; D016062:Porosity; D012822:Silicon Dioxide / Q000737:chemistry; D012834:Silver / Q000737:chemistry*
chemical_list: D002951:Citrates; D012834:Silver; D012822:Silicon Dioxide
keywords: Air–water interface; Engineered nanoparticle; Extended DLVO theory; Pore structure; Solid–water interface; Unsaturated flow