Electrochimica Acta 78 (2012) 229–235 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al hom epa ge: www.elsevier.com/locate/electacta Inhibition of electrokinetic ion transport in porous materials due to potential drops induced by electrolysis K. Kamran a , M. van Soestbergen a,b , H.P. Huinink a , L. Pel a,∗ a Transport in Permeable Media, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands b Materials Innovation Institute, Mekelweg 2, 2628 CD, Delft, The Netherlands a r t i c l e i n f o Article history: Received 27 February 2012 Received in revised form 26 May 2012 Accepted 28 May 2012 Available online 15 June 2012 Keywords: Desalination Porous media Nuclear Magnetic Resonance Poisson–Nernst–Planck theory pH fronts a b s t r a c t In this work we present non-destructive measurements of sodium ion concentration profiles during the electrokinetic removal of sodium chloride from porous materials using Nuclear Magnetic Resonance (NMR). The effect of both protons and hydroxyl ions, generated due to the electrolysis of water, on the transport of the salt ions is studied by tracking the acidic and alkaline fronts using pH-indicator paper. In addition, the electrical potential distribution within the specimen is monitored to assess its influence on the process. To support the observations we compare the experimental results with a theoretical model based on the Poisson–Nernst–Plank equations. In this model we use the chemical equilibrium condition for the self-electrolysis of water in the description of the transport of protons and hydroxyl ions. In addition we use the electro-neutrality condition to compute the transport of salt ions through the material. At the edges of the system the electrical current is distributed over the chemical active species, i.e. protons, hydroxyl and chloride ions, according to the Butler–Volmer description for charge transfer at electrodes. Both the experimental and model results show in the final stage of the electrokinetic remediation process a sharp transition from the acidic to alkaline region at one third of the length of the specimen away from the positively biased electrode, i.e. the anode. From the model results we found that the formation of chlorine gas at the anode does not influence the position of this transition area. In this transition region we also observe a large gradient in electrical potential and a corresponding local deficit of ions. As a result of the large potential gradient in this small transition zone the electrical field in the acidic and alkaline region diminishes. Consequently, electrokinetic ion transport though the material will stagnate. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Electrokinetic phenomena involve the movement of fluid (electro-osmosis) and charged particles (electro-migration) under the effect of an applied electric field. The applications of electroki- netics are wide spread, including desalination of sea (or brackish) water [1], remediation of soils [2–7], desalination of building mate- rials [8], prevention of corrosion in reinforced concrete [9], drug delivery [10], electrochemical treatment of tumors [11], and micro- fluidics [12]. In electrokinetic desalination, which we study in this paper, an electric field is used to enhance the transport of ionic species, and thus their removal rate, while it allows one to control the direction of transport by means of the electrode potential. One of the major application fields of electrokinetic desalination is the removal of salt from building materials, which is necessary to prevent them from salt-induced decay. Electrokinetic desalination ∗ Corresponding author. E-mail address: l.pel@tue.nl (L. Pel). aims to remove salt ions from the zone of deterioration, mainly by electro-migration, with the aid of an externally applied electric field. However, in addition to enhance the transport of salt ions, the applied electric field might also introduce new ionic species due to electrode reactions. Due to electrolysis, e.g., H + and OH - ions can be introduced at the positively and negatively biased electrodes, respectively, resulting in acidic and alkaline fronts. This is a major drawback of electrokinetic desalination since the acidic environment can induce corrosion in reinforced concrete [13], and damage the mortar in masonry structures [14]. In clay soils it was found [3,4] that the electrokinetic transport of ZnCl 2 stopped after a certain amount of time and a sharp front for the Zn 2+ ions was observed. A correlation was found with both a large pH gradient that developed due to the electrolysis of water, and the high potential drop at the point where the Zn 2+ ions formed a front. In this study [4] it was concluded that the inhibition of Zn 2+ ion transport was caused by precipitation of metal hydroxides in the alkaline region. When precipitation in the alkaline region is the only mechanism behind the inhibition of Zn 2+ ion transport, ions that do not precipitate, e.g. Na + and K + , will not be trapped. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.05.123