Influence of the pH on electrokinetic desalination
of porous materials

Electrokinetic phenomena involve the movement of fluid (electro-osmosis) and charged particles (electro-migration) under the effect of an applied electric field. 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 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, and damage the mortar in masonry structures. Here 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. The setup as used in the measurements is given in figure 1.

Fig. 1. (a) Schematic representation of the desalination cell, where the specimen is sandwiched between platinum electrodes and sponges,
 which provide a pathway
for the salt ions to enter the water reservoir. (b) The photo shows the platinum wires inserted in the specimen to measure the potential distribution across the specimen. The reference sample is shown on the right side.


    Experiment showing the effect of pH on the salt transport
The Na+ concentration profiles as measured by NMR under the influence of an applied potential difference of 9 V between the electrodes are shown in Fig. 2a. In Fig. 3a the first profile, at t = 0, represents the initial Na+ concentration in the specimen that was measured by NMR before exposing the specimen to the sponges and
the electrical potential.

Fig. 2. Experimental data (a–c) and model results (d–f) for; the Na+ concentration (a and d); the acidic and alkaline fronts (b and e); and the electrical potential distribution (c and f) as function of distance and time. The experimentally applied electrical potential was 9 V, while in the model it was set to 6 V. The solid lines in (b) represent the results from (e) by taking the front positions at pH = 1 and at pH = 13. In (c) the lines are given as a guide to the eye.

After applying the potential difference, the sodium starts to deplete at both edges of the specimen. This depletion front progresses inward with a velocity of 8.2 × 10−7 m/s until it stops after moving approximately one third of the specimen length, after which the desalination proceeds by the removal of Na+ from the negatively biased cathode (x = 9 cm) only by diffusion.  In Fig. 2b the progression of both the acidic and alkaline fronts with time under the effect of an applied potential difference that was determined by pH-indicator paper is shown. The results show that both fronts collide at approximately the same position where the Na+ depletion front stagnates. The evolution in electrical potential distribution across the specimen under the effect of an applied electrical potential difference of 9 V is shown in Fig. 2c. The actual electrical potential drop across the specimen is ∼6 V, which is smaller than the applied voltage due to polarization of the electrodes. It can be seen that the potential as a function of distance is approximately linear immediately after the application of the electrode potential and this linearity remains up to ∼10 h of desalination. However, after ∼10 h the potential gradient in the vicinity of both electrodes decreases rapidly and a sharp variation in electrical potential at a distance of approximately 25–30 mm from the anode in a narrow region of 5 mm is observed. This is approximately the same position where the Na+ depletion front stagnates (Fig. 2a) and both the acidic and alkaline fronts collide (Fig. 2b), hence showing the influence of the pH fronts entering the material.
      Conclusion and discussion
It is shown that acidic and alkaline regions severely affect the transport of salt ions and that the Na+ depletion front stagnates at the position where both these regions collide. A model based on the Poisson–Nernst–Planck equations showed a good agreement with the experimental observations. The simulation results show
that a large deficit in charge carriers develops where the acidic and alkaline regions collide.

Kashif Kamran, Leo Pel, Alison Sawdy, Henk Huinink, Klaas Kopinga, Desalination of porous building materials by electrokinetics: an NMR study, Materials and Structures 45:297–308 (2012)

K. Kamran, M. van Soestbergen, H.P. Huinink, L. Pel,  Inhibition of electrokinetic ion transport in porous materials due to potential drops induced by electrolysis, Electrochimica Acta 78 229– 235 (2012)

K. Kamran, M. van Soestbergen, L. Pel, Electrokinetic Salt Removal from Porous Building Materials Using Ion Exchange Membranes, Transport in Porous Media, 2012 (DOI 10.1007/s11242-012-0083-0)

K. Kamran, Electrokinetic desalination of porous building materials , Eindhoven University of Technology (2012). (Download 4.1 Mb)