Available online at www.sciencedirect.com Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 142–150 Colloid transport through saturated sand columns: Influence of physical and chemical surface properties on deposition Christiane Shani, Noam Weisbrod ∗ , Alexander Yakirevich Department of Environmental Hydrology and Microbiology, The Zuckerberg Institute for Water Research, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sde Boker Campus 84990, Israel Received 5 April 2007; received in revised form 22 August 2007; accepted 30 August 2007 Available online 4 September 2007 Abstract Colloid transport experiments are often conducted in cleaned sand, and the results are then projected onto mechanisms in natural porous media. We conducted tracer experiments to investigate the effect of the sand-cleaning preparations on physical and chemical surface properties and to compare retention mechanisms in natural sand of three levels of cleanliness under saturated conditions. Negatively charged fluorescent microspheres (0.02, 0.1 and 1 m) together with a soluble tracer (LiBr) were applied at the top of vertical sand columns, while effluent samples were taken at the column outlets. At the end of the experiment, columns were dissected and samples were collected at different depths. For each sand, the grain size distribution and surface chemistry were examined. The electrostatic interaction free energies between grain surface and colloid were calculated based on their thermodynamic surface properties according to the traditional and extended DLVO theory. Colloid retention was much higher in natural sand than in its washed counterparts. A comparison of the physical and chemical surface properties of the differently cleaned sands implied that a dust-like particle fraction present in the natural sand had relatively greater impact on colloid retention than changes in grain surface charge resulting from the cleaning procedures. © 2007 Elsevier B.V. All rights reserved. Keywords: Colloid transport; Natural porous media; Saturated flow; Grain size distribution; Electrostatic interaction free energy 1. Introduction The transport of colloidal particles in saturated porous media has been extensively studied in the last two decades [1,2]. Gen- erally, these studies are motivated by environmental concerns, such as colloid-facilitated contaminant transport or transport of biocolloids (viruses and bacteria) in groundwater and soil water [3–5]. Most research is performed in laboratory column experiments using model systems, such as glass beads [6,7] or well-sorted quartz sand [8,9]. Such column experiments have the advantage of allowing control of the experimental condi- tions and isolation of the variables of interest from the rest of the system. The main disadvantage is that the model sys- tem is often not representative of the complexity of natural processes. A compromise between having a controllable sys- tem and a porous medium that still resembles natural soils is the choice of natural sand. This is because of its nearly ∗ Corresponding author. Tel.: +972 8 6596903; fax: +972 8 6596909. E-mail address: weisbrod@bgu.ac.il (N. Weisbrod). round grain shape and relatively uniform mineral composi- tion. However, almost all studies performed in natural sand employ sieving and washing procedures to obtain uniformity and to guarantee repeatability of the experiments [9–12]. As noted by Brown et al. [13], these procedures sometimes have a coun- terproductive outcome. Those authors focused on the sieving procedure and reported that: (a) sieving with stainless-steel sieves may contaminate the sand because of the abrasion of brass and (b) sieving causes inadequate sizing of the sand grains because of sieve clogging, rather than uniformity of the sand batches, and repeatability is therefore at stake. The washing procedures are performed to clean the sand from functional surface groups (washing with acidic or basic solutions) [9,14] or colloidal particles (flushing with pure water or electrolyte solutions of differing ionic strengths) [15,16]. The aim of these treatments is to obtain a well-defined porous medium with known surface properties. However, washing does not just clean the grain surfaces; it also changes the grain size distribution. Fine dust-like particles (<53 m) are washed away and the relative weight of grain sizes with larger diameters is 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.08.047