The electrokinetic remediation of contaminated soils relies in two main transport mechanisms: electromigration and electro-osmosis. The electro-osmotic flow depends on fluid characteristics (dielectric constant and viscosity) and soil surface characteristics represented by Z-potential, as well as voltage gradient. In this work, the influence of voltage gradient in the development and maintenance of electro-osmotic flow is studied. The voltage gradient directly affected the electro-osmotic flow but it also affected the geochemistry of soil that had a critical effect on electro-osmosis. It was demonstrated that 1 V/cm resulted in the development and maintenance of a large electro-osmotic flow. The use of organic acids, such as citric acid, induced large electro-osmotic flow due to the interaction of the organic acids with the soil particles. The use of citric acid was considered to be very promising for large-scale operation. (C) 2015 Elsevier Ltd. All rights reserved.