Retention of silica nanoparticles on calcium carbonate sands immersed in electrolyte solutions Yan Vivian Li a,⇑ , Lawrence M. Cathles b a Department of Design and Merchandising, Colorado State University, Fort Collins, CO 80523, United States b Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, United States article info Article history: Received 10 June 2014 Accepted 31 August 2014 Available online 16 September 2014 Keywords: Nanoparticle retention Electrolyte solutions Calcium carbonate (calcite) Interfacial forces Derjaguin–Landau–Verwey–Overbeek (DLVO) theory Surface charge abstract Understanding nanoparticle–surface adhesion is necessary to develop inert tracers for subsurface appli- cations. Here we show that nanoparticles with neutral surface charge may make the best subsurface trac- ers, and that it may be possible to used SiO 2 nanoparticle retention to measure the fraction of solid surface that has positive charge. We show that silica nanoparticles dispersed in NaCl electrolyte solutions are increasingly retained in calcium carbonate (calcite) sand-packed columns as the solution ionic strength increases, but are not retained if they are injected in pure water or Na 2 SO 4 electrolyte solutions. The particles retained in the NaCl experiments are released when the column is flushed with pure water or Na 2 SO 4 solution. AFM measurements on calcite immersed in NaCl solutions show the initial repulsion of a silica colloidal probe as the surface is approached is reduced as the solution ionic strength increases, and that at high ionic strengths it disappears entirely and only attraction remains. These AFM measure- ments and their interpretation with Derjaguin–Landau–Verwey–Overbeek (DLVO) theory shows the cal- cite surface charge is always negative for Na 2 SO 4 solutions, but changes from negative to positive in a patchy fashion as the ionic strength of the NaCl solution increases. Since mixed-charge (patchy) surfaces may be common in the subsurface, nanoparticles with near-zero charge may make the best tracers. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Aggregation and deposition of engineered nanomaterials is largely controlled by physicochemical interactions [1]. In drug delivery and in medical diagnosis it is critical that the nanoparticle remain in the blood long enough to reach target organs to be imaged or tumor cells to be treated [2]. The interactions of nanoparticles with biological units controls the formation of nanoparticle clusters on the membranes, and the formation of the- se clusters improves drug delivery efficiency [3]. The nanofabrica- tion of electronic and optical devices depends on the interaction of nanostructured materials with fabricated devices. These interac- tions are affected by chemical conditions, and manipulation of the- se conditions allows the construction of the dyads, triads, strings, clusters, and other architectures that are the basis of a new gen- eration of nano-devices and smart materials [4,5]. The size, shape, and composition of the particles and hydrodynamic conditions are important, but so also is the surface charge of the particles, and this is controlled by the chemistry of the surrounding solution (e.g. pH, ionic strength, and ionic composition) [1,3,5]. Size-dependence of nanoparticle adsorption has been reported for silica [6], gold [7], silver [8,11] and carbon nanotube particles [9]. The optimal size seems to be that which allows the ordered formation of nanoparticle monolayers [10]. The curvature of the surface of the nanoparticle could influence adsorption and the properties of the adsorbed particles [11]. Nanoparticles with differ- ent shapes interact differently with the same substrates. Pal et al. showed that antibacterial properties of silver nanoparticles under- go a shape-dependent interaction with bacteria [12]. Solution chemistry is a critical external factor controlling nanoparticle behavior [1,13]. The pH, ionic strength, and the ionic composition of the solution controls nanoparticle stability pri- marily by changing surface charge. Maintaining the solution within a range of pH and ionic strength is usually required to maintain a stable nanoparticle suspension. The width of the stability range depends on the nanoparticle chemistry and the ions in solution [14,15]. Increasing the ionic strength of a solution increases the nanoparticle retention in silica sand packed column [16,17], and the mix of aqueous ions affects particle retention [18,19]. Many nanoparticles such as silica, polystyrene latex, and TiO 2 have been injected into sand packed laboratory columns and into the subsur- face, and it has been found that the mineralogy of the nanoparticle and the solid materials it contacts influence transport behavior http://dx.doi.org/10.1016/j.jcis.2014.08.072 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Fax: +1 970 491 4855. E-mail address: yan.li@colostate.edu (Y.V. Li). Journal of Colloid and Interface Science 436 (2014) 1–8 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis