MEASUREMENT OF CLAY SURFACE AREAS BY POLYVINYLPYRROLIDONE (PVP) SORPTION AND ITS USE FOR QUANTIFYING ILLITE AND SMECTITE ABUNDANCE

Abstract

Blum and Eberl (2004) presented a new technique for determining the surface area of clay minerals in sediment samples by measuring the amount of polyvinylpyrrolidone (PVP) [CAS#9003-39-8] adsorbed by a sample, and explained how the surface-area determination can be used to determine quantitatively the smectite content of the samples. In the previous method for determining surface area by PVP uptake (Blum and Eberl, 2004; also see the erratum relating to that paper published on p. 214 of the present issue), ~50 mg of Nasaturated smectite was dispersed in 5 mL of water, and 1 mL of 10 wt.% PVP-55 (mean MW 55,000) was added. The sample was centrifuged, a portion of the solution decanted, and the PVP concentration in solution determined by mass after drying. The mass of PVP adsorbed on the sample was then computed by difference. This method, while accurate, had several limitations for its practical application which have now been overcome. One of the limitations of the procedure outlined by Blum and Eberl (2004) was that at high clay to solution ratios, samples with >90% Na-saturated high-charge smectites (but not low-charge smectites) tended to aggregate before the surfaces developed a completely adsorbed monolayer of PVP. Observations by X-ray diffraction (XRD) suggested that high-charge smectite particles became separated by only a single PVP layer at high smectite concentrations in solution, reducing the amount of PVP adsorbed (triangles in Figure 1) and, consequently, yielding an underestimate of the sample surface area. As a result, Blum and Eberl (2004) recommended running duplicate PVP-adsorption measurements of uncharacterized samples at different sample to solution ratios to confirm the absence of a sample-size artifact. The authors have now observed that the use of Lisaturated samples (squares, Figure 1) avoids the sampleaggregation problem observed with Na-saturated, highcharge smectites. The use of Li as the interlayer cation prevents smectite aggregation over a very wide range of sample to solution ratios. During the preparation of the Li-saturated samples for the present work, care was taken not to raise the temperature above 25oC. The observed effect is, therefore, unlikely to be the result of Li entering the octahedral sheet and decreasing the fixed charge of the clay. Rather, the larger hydration shell of the Li-saturated clay surface seems to prevent rapid aggregation, allowing enough time for complete PVP adsorption. Blum and Eberl (2004) determined the concentration of PVP remaining in solution by measuring the mass of PVP remaining after evaporation of the solution. This method had several complications. First, PVP is strongly hygroscopic and uptake of atmospheric moisture during weighing can introduce uncertainty into the PVP mass measurement. Second, any residual material in solution will interfere with the measurement of PVP mass. For