Charge characteristics and related dispersion/flocculation behavior of soil colloids as the cause of turbidity

Dispersion/flocculation (D/F) behavior of soil colloids is closely related to numerous environmental issues and regulated by the charge characteristics of the colloids. The charge characteristics of five clays commonly distributed in Japan (montmorillonite, sericite, kaolinite, halloysite and allophane) were determined by the modified ion adsorption method using caesium as an index cation and compared with the D/F behavior under the similar conditions. The flocculation effect of Cs on negatively charged clays enabled accurate charge analysis in the high pH and low electrolyte concentration region where measurement had been difficult by conventional methods due to clay dispersion. The dispersion ratio of montmorillonite was evenly high in all pH regions reflecting its large permanent charge. The pH dependence of sericite charge was rather variable than permanent. The dispersion ratio of sericite was increased abruptly above pH 6.5. The pH dependence of kaolinite negative charge was small, indicating the predominance of permanent charge. Both negative and positive charges were detected from halloysite in large pH regions. The dispersibility of kaolinite and halloysite was increased above pH 7. To explain the observed discrepancy between the charge characteristics and the D/F behavior, the significant role of edge surfaces in flocculation was emphasized for sericite, kaolinite and halloysite. In case of halloysite, the reverse imogolite structure curling with the Al-octahedral layers inside was proposed to account for the constant negative charge detected in all pH regions. Allophane showed the charge characteristics typical of variable charge minerals and dispersed in both acid and alkali regions. The difference between the charge characteristics and the dispersibility of allophane was attributed to the heterogeneity of its charge. From the aspect of environmental conservation, keeping the suspension pH below 6-7 so as to prevent edge surfaces from dissociating is likely to be primarily important to control clay dispersion depending on the clay mineralogy. By combining the information derived from this Cs adsorption method with the proper land management, more effective techniques to reduce soil losses and water pollution will be established according to field conditions.