ELECTROKINETICS OF MODEL MINERAL SUSPENSIONS

The electrophoretic mobilities (EPMs) of model mineral dispersions in artificial seawater (ASW) and natural seawater (NSW) were measured. The solid samples used in this work can be classified in three groups: (1) clay minerals (kaolinite, bentonite); (2) artificial calcite (SOCAL), natural calcite-raw material (POLCARB); (3) lake and sea sediments. In ASW the EPM values for all natural minerals, kaolinite, bentonite, raw calcite, and lake and sea sediments were between -1.2 x 10(-8) and -3.3 x 10(-8) m2 V-1 s-1 at low salinity (0.13 parts per thousand) and between -0.42 X 10(-8) m2 V-1 s-1 and -1.4 x 10(-8) m2 V-1 s-1 at high salinity (37.2 parts per thousand), higher values being typical for clay minerals and lower values for calcite-rich sediments. In contrast to the behaviour of all natural samples, artificial calcite showed positive EPM values of +1.1 X 10(-8) M2 V-1 s-1 at low salinity and +0.40 x 10(-8) m2 V-1 S-1 in high salinity ASW. The addition of dextran (T-40) to this suspension caused the neutralization of charge of artificial calcite. In NSW of 4 to 38 parts per thousand salinity, the EPM values of all solids including the artificial calcite were negative. Slow 'titration' of simple model solid/10(-3) M NaCl suspensions with NSW of 38 parts per thousand salinity resulted in continuous convergence of measured parameters, pH, specific conductivity and electrophoretic mobility toward the values typical for seawater. Unlike pure ASW, NSW altered positively charged artificial calcite (EPM = +1.4 x 10(-8) m2 V-1 s-1) to a negatively charged surface (EPM = -0.94 x 10(-8) M2 V-1 s-1). The transition ('isoelectric point') was detected at a salinity of about 10 parts per thousand. It was concluded that the electrokinetic (EPM) data under natural conditions, characterized by a high ionic strength, stable pH and a variety of adsorbable species, do not show specific differences between various solids. Therefore, the electrokinetic studies seem to be much more useful when applied to model suspensions, and can be used to reconstruct more complex mechanisms.