In recent years there has been a considerable amount of research work done relative to the effect of pretreatment and aging in water on the surface properties of both simple metal oxides and more complex oxides such as the silicate minerals.
The present paper investigates the change of point of zero charge (PZC) of a number of simple oxides and several complex oxides as a function of aging time in water. In some cases acid pretreatment of minerals was performed. However, in most cases the minerals were not acid pretreated. In several systems pH change as a function of aging time in water was monitored.
Pure mineral specimens were prepared for study by first hand-picking best appearing specimens and reducing them in particle size in either an agate mortar and pestle or, in the case of asbestiform minerals, in a tungsten carbide pica mill. Next, the minerals were separated into plus and minus 325 mesh fractions. The latter were retained for experimentation. In addition to naturally occurring minerals, synthetic Al2O3 (Linde A) was also studied. In all cases minerals without further treatment were stored in plastic vials.
Electrophoretic mobility studies were performed with a commercially available microelectrophoresis apparatus, the zeta meter. Procedures for using this equipment have been adequately described elsewhere. Two separate methods were used for preparing minerals for measurement of PZC’s as a function of aging time. Firstly, for all materials, a rather conventional procedure was employed.
The second procedure which was only used for asbestiform minerals and serpentine, involved placing dry minerals in proper amounts of water that had been preadjusted to various pH values (2, 4, 6, 8, 10, 12-for example). The suspensions were then allowed to age in a CO2 free atmosphere. The pH of the suspensions were measured as a function of aging time. Also, at various aging times samples were withdrawn and mixed with water (for proper sample opacity) adjusted to the same pH as the suspension at that particular aging time.
An interesting companion to the PZC studies show effect of aging time on suspension pH for suspensions of chrysotile, crocidolite and serpentine treated in various ways. Figure 7 notes the variation of pH as a function of aging time for acid and non-treated suspensions of the three minerals. For the suspensions noted on this figure the minerals were placed in pure water from which CO2 had been removed. For the untreated minerals the pH quickly rose to values near pH 10 and then slowly dropped as a function of aging time until pH values near 9 were reached after long aging.
Discussion of Results
Variation of PZC of Simple Oxides as a Function of Aging Tittle in Water: Parks has noted that the solubility products, Kso’s of many simple oxides can be influenced by past history of the oxides. He has also demonstrated by a simple mathematical approach that factors causing changes in Kso should be expected to also cause changes in PZC if hydroxo-complexes are among the charge-determining ions or PDI and polynuclear species are present in the system of complexes.
During sample preparation a disturbed amorphous layer of “alumina” should form on the surface of the ground mineral much in the manner of the disturbed surface layer on fractured quartz reported by van Lier, de Bruyn and Overbeek. The layer on quartz appeared to be of the order of 300 A° thick. When placed in water the mineral then should present this amorphous surface to water. Smith has reported the PZC of freshly precipitated amorphous Al2O3 to be about at pH 7.5, not far from the PZC value (7.1) reported here for corundum placed in water for less than one hour. The amorphous layer should be relatively soluble in water and the layer therefore should go into solution in a certain length of time. Some quantitative data on this rate for Linde A (synthetic ∝ Al2O3) is to be found in the work of Paces.
Behavior of complex oxides aged in water was more complicated than for the simple oxides. In some cases PZC dropped with time, in others it remained essentially constant, in others it rose with time and in yet other cases it appeared to first rise and then later to drop.
Considering the ferro-magnesium silicates, interesting results were obtained, particularly concerning the data from the two procedures for measuring PZC. For chrysotile the PZC decreases as a function of time, appearing to roughly level off near 9 after long aging time. Essentially identical curves were observed using both procedures. Presumably, magnesium is leaving the chrysotile surface as a function of time shifting the PZC to more acid values.
The curve for serpentine as measured by method 1 is quite different in nature, showing lower PZC values for the mineral. For each PZC determination acid had to be added to the mineral suspensions in order to obtain the points, necessary to measure the PZC. Thus, it is probable that some magnesium was always leached from the mineral giving a surface that always more acid than the original surface before adding acid. It would seem then that the curve gives too low PZC values.