Electromigration of K⁺ and NO₃-natural stone and brick under application of a constant voltage

Transport of ions in an applied electric field (electromigration) can be used in conservation actions both for removal (electro-desalination) and supply (electro-precipitation) of ions out from or into the substrate. For the further development of these methods, increased understanding of the influence from side effects on the transport of the target ions is necessary. Electromigration has most often been investigated under application of a constant current. In this work, a constant voltage is applied, and it reports a direct comparison of electromigration of K⁺ and NO₃- through substrates with different porosities. Prisms, one brick and four natural stones, were prepared and contaminated with NaCl in exactly the same way. Electromigration experiments were made under the same applied constant voltage. During the first hours, the current was very different between the experiments (from 2.3 to 72 mA), showing major difference in the electrical conductivity of the substrates - the higher the porosity, the higher the conductivity (and current). The current developed differently between the experiments. By the end of the 3 days experiments, the current was between 3.4 and 9.2 mA, and independent of the substrate porosity. During the experiments, the electrode processes and the connected side effects influenced the electromigration of K⁺ and NO₃- to different extents in the different substrates, and the most in the substrates with high porosity. The experiments underline that the electromigration of target ions depends strongly on the substrate when applying a constant voltage, because the substrate conductivity determines the current and thus the side effects. Applying a constant current instead, as in most of the previous works, enables better managing of the side effects.