Matrix changes and side effects induced by electrokinetic treatment of porous and particulate materials

Transport of ions in an applied electric field holds many applications within both civil and environmental engineering, e.g. for removal of chlorides from concrete to hinder reinforcement corrosion, remediation of heavy metals from soils and other waste materials and recently for desalination of porous stone materials to hinder decay. However, in addition to the removal of target ions in these systems, matrix changes may occur during the electrochemical treatment. For a broader implementation of the electrokinetic methods it is important to understand changes in the matrix composition for different types of materials. The overall aim of this PhD-project is to evaluate matrix changes and side effects induced by electrokinetic treatment of porous and particulate materials.During electro-remediation protons are produced at the anode and hydroxyl ions are produced at the cathode. The consequent pH changes may influence the matrix of the treated materials. The main research objectives were to identify matrix changes during electro-remediation, to investigate the impact of induced matrix changes on the removal of target elements and finally to evaluate the extent of Al dissolution during electrodialytic remediation and the impact on the implementation of soil remediation, e.g. concerning the toxicity of residualsoluble Al in the soil matrix.Electrokinetic treatment was carried out as electrodialytic remediation (EDR) of three particulate matrices (soils and clay) and electrochemical desalination (ED) of three different porous matrices (brick and sandstones). The degree of impact of the induced side effects by electro-remediation on the different types of matrices is related to the subsequent purpose of the treated material. For electrochemical desalination of a porous matrix e.g. sculpture from cultural heritage, the properties of the matrix must remain unaffected by the treatment.Irreversible treatments of such matrices are unacceptable, e.g. in case of increased porosity, the materials is increasingly vulnerable to environmental exposure. The present work has investigated potential pH, chemical and porosity changes of the matrix during ED. In EDR it is not crucial to avoid matrix changes as in ED, but it is important to know which matrix changes are taking place, as these may influence the overall remediation result. In addition, the overall toxicity of the soil must not increase during the treatment e.g. concerning theconcentration of soluble Al.EDR experiments with the three particulate matrices showed that soil weathering occurred during EDR by dissolution of minerals. Pb mobilization was initiated after acidification due to dissolution and removal of calcite from the matrix. In addition to Pb mobilization, acidification caused dissolution of Al from the matrix. Mobilization of Al during EDR was not found to be problematic for the two tested soil matrices, as the mobilization of Al was slower than the electromigration out of the matrix. However, for the clay mineral matrix (illite) the concentration of acid soluble Al increased in the matrix during EDR. The illite matrix was negatively affected by the EDR treatment. Thus the risk of increasing soluble Al in the matrix has to be evaluated in each specific case before implementation of soilremediation.From X-ray powder diffraction it was found that calcite was decomposed in the EDR treated soil. No new mineral phases were developed and no other phases than calcite were fully decomposed. Since dissolution of Al, Fe and Si minerals was observed, it was thus concluded that these elements were dissolved by partial decomposition of soil minerals.The porous matrices studied in this work were Red Brick, Nexø sandstone and Gotland sandstone. The Red Brick and the Nexø sandstone is considered relatively inert matrices while the Gotland sandstone is a limestone and thus adversely affected by acid. To avoid the observed matrix change of dissolution of the bonding material (CaCO₃) resulting in an increased porosity during ED it is important to use a CaCO₃-containing poultice for neutralizing the acid produced at the anode. This generates electromigration of Ca²⁺ from the anode towards the cathode. The ED removal rate of SO₄^2-is observed lower than for Cl^- andNO₃^-. The reason was precipitation of gypsum when SO₄^2- reacts with Ca²⁺ from the poultice. Precipitation of gypsum did not prevent desalination of SO₄^2-, although the ED removal rate was reduced.