Colloids and Surfaces A: Physicochem. Eng. Aspects 347 (2009) 230–238 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa Quantification of Au nanoparticles retention on a heterogeneous rock surface U. Alonso a,∗ , T. Missana a , A. Patelli b , D. Ceccato c , N. Albarran a , M. García-Gutiérrez a , T. Lopez-Torrubia a , V. Rigato d a CIEMAT, Environmental Department, Avda. Complutense 22, 28040 Madrid, Spain b CIVEN, Via delle Industrie 9, 30175 Venezia-Marghera, Italy c Universitá di Padova, Dipartimento di Fisica “G. Galilei”, via F. Marzolo, 8, 35131 Padova, Italy d INFN, Laboratori Nazionali di Legnaro, Viale dell’ Università 2, 35020 Legnaro-Padova, Italy article info Article history: Received 30 June 2008 Received in revised form 23 April 2009 Accepted 28 April 2009 Available online 3 May 2009 Keywords: Colloid Nanoparticle Granite Surface retention PIXE Distribution coefficient abstract Colloid-mediated contaminant transport within geological media is still not fully understood, mainly because the mechanisms that lead to colloid retention onto the rock walls are not clear and are difficult to quantify. This study presents an experimental methodology to quantify at a mineral scale colloid surface distri- bution coefficients (K a ) in a heterogeneous rock surface (granite). The retention of negatively charged Au nanoparticles of different size (2, 40 and 100 nm) was analyzed in static (batch) experiments. Two differ- ent pHs were used to account for different rock–colloid electrostatic interactions: the first one when the rock has some positively charged minerals and the nanoparticles are negatively charged (favorable electro- static attraction) and the second case where all granite minerals and the Au nanoparticles are negatively charged (unfavorable electrostatic attraction). The micro-Particle Induced X-Ray Emission (PIXE) technique was used to visualize and quantify the colloid retention on the granite surface. Colloid surface distribution coefficients were measured on the main minerals composing the rock. In the favorable case, higher K a values were observed on the minerals bearing positive charge and a K a dependence on the colloid size was observed. However, non-negligible K a values were measured also on negatively charged minerals, without clear size dependence effects. The main mechanisms responsible for colloid retention in these unfavorable areas were analyzed. They were mostly related to higher porosity of certain minerals or to physical defects of the granite surface (roughness, grain boundaries). The distribution coefficients obtained in this study can be used as input data for theoretical description of colloid transport in fractures. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Colloids are particles of nanometer size (from 1 to 1000 nm) sus- pended in a fluid [1]. The high surface to volume ratio and their electrostatic charge make colloids very reactive to adsorb contam- inants on their surface. If colloids are mobile, they can enhance contaminant transport in the environment [2–4]. The frame of this study is related to the possible colloid- mediated radionuclide (RN) transport in a geological repository for high-level radioactive waste (HLWR) emplaced in a granite massif [5,6]. RN themselves can exist in colloidal form (diameters <1–2 nm). Natural colloids, like iron oxides or clay colloids, can be naturally present in groundwater but also, colloids can be gener- ated from the engineered barriers of the repository, for example ∗ Corresponding author. Tel.: +34 913466183; fax: +34 913466542. E-mail address: ursula.alonso@ciemat.es (U. Alonso). bentonite clay colloids, that have hydrodynamic diameters of about 200 nm, can absorb RN [7]. The real impact of colloids in the RN migration within granite fractures in the repository case is still unclear [5]. In granite media, colloid transport mainly takes place by advec- tion in conductive fractures being their transport significantly different to that of a solute. A fraction of colloids moved unretarded, compared to the water flow but in many cases, and depending on the experimental conditions, a colloid fraction is retained in the medium. Colloid retention was measured even under flow condi- tions and at pH conditions where both the whole rock surface and the colloids are negatively charged, so that repulsive forces must dominate and where higher retention was not expected [8,9]. In those cases, colloid deposition or colloid matrix diffusion alone could not either explain the observed retention [10,11]. The mech- anisms that are retaining colloids in the fracture surface are not yet understood. Theoretical models investigated the relevance of the different retention mechanisms [12,13], even under the unfa- 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.04.046