Effects of pH and ionic strength on electrokinetic properties of imogolite

Imogolite, an aluminosilicate with thread-like morphology and a nano-size tubular tunnel, is a unique clay mineral not only in morphology but also in colloidal stability. This clay is normally contained in volcanic ash soil and important in characterizing the physical and chemical properties of volcanic ash soil. In the present study, we performed measurements of acid-base potentiometric titration, electrophoresis, sediment volume and electrical conductivity for suspensions of natural imogolite. From these results, we clarified: (i) point of zero charge (PZC) existed near pH 6, where the imogolite has the same amount of positive and negative charges. An exchangeable cation was able to penetrate into the tube, but the counter-ion inside the tube played a small roll in the electrokinetic phenomena. (ii) The sediment volume increased with an increase in pH. The formed flocs were bulky at a high pH in accordance with reduction in the electrical repulsive force. This behavior was more pronounced with an increase in ionic strength. (iii) Surface conductmetry was very useful for detecting the mobile ion in the electrical double layer near the outer surface of the tube. The electrical surface conductance, the sediment volume and the electrophoretic mobility indicated that only the outer mobile ions contributed to the electrokinetic phenomena.