<p>This study investigated the micro-mechanisms of reverse electro-osmotic flow from the cathode to the anode in kaolin, focusing on the zeta potential, role of carboxyl hydrophilic groups, and the relative movement of hydrated cations and anions. The accumulated volumes of electro-osmotic dewatering, electric current, electric potential, and zeta potential during electro-osmosis in kaolin clay with the addition of acetic acid (0.08, 0.25, and 0.50&#xa0;mol) and citric acid (0.07, 0.14, and 0.50&#xa0;mol) were measured. The results indicated that electro-osmotic flow occurred from the cathode to the anode in kaolin clay with a negative zeta potential, suggesting that the zeta potential is not the only factor that affects the electro-osmotic flow direction. The anions with hydrophilic groups (citrate and acetate), hydrated and migrated toward the anode, contributing to the reverse electro-osmotic flow. The electro-osmotic flow from the anode to the cathode was weakened by the addition of acetic acid (0.08&#xa0;mol) and citric acid (0.07&#xa0;mol) and the reverse electro-osmotic flow was enhanced in kaolin clay with the addition of acetic acid (0.25 and 0.50&#xa0;mol) and citric acid (0.14 and 0.50&#xa0;mol). The results also suggested that the energy gradient theory is applicable to the reverse electro-osmotic flow. The rate constant derived from the electric current was lower than that derived from the accumulated volume of electro-osmotic dewatering. A new model of the micro-mechanism for two electro-osmotic flows in opposite directions was developed based on the relative movement of hydrated cations and anions in kaolin.</p> Graphical Abstract <p></p>