Aggregation Rates in Montmorillonite Clay Measured by Light Extinction
Abstract
The aggregation rate and configuration of colloidal clay aggregates are important factors in soil clotting and marine-estuary sedimentation. Colloidal aggregation occurs in either of two regimes: 1) reaction limited colloidal aggregation (RLCA) or slow aggregation, in which particles must overcome a potential barrier to adhere, and aggregation rates are determined by the height of the barrier which determines the average number of collisions before adhesion; and 2) diffusion limited colloidal aggregation (DLCA) or fast aggregation, in which there is no barrier to adhesion, and aggregation is limited by the time between collisions. The universality principle dictates that the fractal dimension in the DLCA and RLCA are 1.7-1.9 and 2.1-2.3, respectively. We employ turbidity measurements to determine the aggregation rates and fractal dimension of homoionic smectite suspended in electrolyte solutions over a range of cation concentrations. Early stage aggregation rates (proportional to the time rate of change of the turbidity) and the stability ratio were determined as a function of cation concentration (Ca2+, Mg2+ and K+) from turbidity measurements. At low cation concentrations, the aggregation rates increased with cation concentration until a saturation concentration was reached. The "saturation" concentration, or critical coagulation concentration (CCC), is 3 mM for CaCl2, 4 mM for MgCl2, and 70 mM for KCl. The stability ratio exhibits a z-1 dependence (z is cation valence) rather than the widely accepted z-2 relationship predicted by Reerink and Overbeek. The analysis of turbidity results shows that RLCA occurs at very low concentrations while DLCA extends for a range of concentrations below the CCC. The fractal dimension of the Ca2+ and Mg2+ induced aggregates are 1.65 and 1.75, respectively for concentrations near and above the CCC. The fractal dimension of the K+ induced aggregates varies from 1.35-1.95 as the concentration is elevated, indicating that more dilute K+ aggregation is in the lower density E-F mode while higher K+ level aggregates form F-F associations. We hypothesize that our observed differences from the DLVO (Derjaguin and Landau, Verwey and Overbeek) model are related to the unique plate-like configuration and differential charge balance in the clay structure. Stability Ratios
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B53D0703K
- Keywords:
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- 3934 MINERAL PHYSICS / Optical;
- infrared;
- and Raman spectroscopy;
- 8032 STRUCTURAL GEOLOGY / Rheology: general