Differential Settling Tendencies of Clay Minerals in Saline Waters
Authors: Whitehouse U. Grant , Jeffrey Lela M. , Debbrecht James D. 
Published: August 2006

Differential settling velocities of individual clay mineral types and clay mineral mixtures in quiet saline water are reported for ocean water chlorinity range 0–18‰, brackish water ionic strength range 0.0–0.686 moles-(unit charge)2/kg, temperature range 6–26°C, clay mineral concentration range 0.01–3.6 g/1., and pH range 6.5–9.8. The materials employed included natural deposit clay minerals and clay minerals extracted from marine sedimentary matter and from terrestrial soils.

Settling velocities at 26°C for illitic and kaolinitic materials reached values of 15.8 and 11.8 m/day, respectively, at an ocean water chlorinity of 18‰ and exhibited little dependence upon chlorinity above a chlorinity of 2‰. Settling velocities for montmorillonites were found to be functions of chlorinity over the entire chlorinity range 0–18‰ and to increase exponentially to a limit of 1.3 m/day at 26°C. The settling velocities were determined by pipette analysis, Oden balance techniques, Kelley-Wiegner manometer methods, and spectrophotometric methods, using artificial sea-water and filtered Gulf of Mexico water.

In quiet brackish water, variations in ionic ratio composition alter the settling rates of illites and kaolinites less than 15 percent from such rates in ocean water, at constant, brackish water, ionic strength of 14 or greater. In contrast, montmorillonitic settling rates in such water varied by 40 percent or more from ocean water rates, at constant ionic strength unless the magnesium—potassium or magnesiun-strontium ionic ratios of the brackish water were kept constant. These induced variations were not sufficient in magnitude, however, to change the general relative order of settling rates for the clay minerals.

Decreasing temperatures over the range 26°-6°C decreased settling rates (of all clay types) progressively up to about 40 percent in accordance with temperature-induced changes in the viscosity and density of the saline water medium.

The influences of fifty-seven different organic compounds or materials (carbohydrates and proteins dissolved or dispersed in the water) upon the settling velocities are cited. In general, carbohydrates increased the settling rates of montmorillonitic materials as much as 25 percent, and proteins decreased such rates a maximum of 1–5 percent. Kaolinitic materials suffered a 30–40 percent decrease in settling velocity under the influence of some proteins. So-called “humic acids,” derived from quinone and soil fractions, decreased kaolinitic and montmorillonitic settling rates to lesser extent. No significant alterations of illitic settling rates by organic materials were noted.

Chlorite-montmorillonites were found to settle slightly faster than sodium and calcium montmorillonites. Potassium-saturated montmorillonites settled from two to three times as rapidly as the reference montmorillonites. Chlorite settling rates, of magnitude comparable to rates found for kaolinites, and vermiculite settling rates, comparable at higher chlorinities to illite settling rates, are also reported.

The apparent interaction of illite and montmorillonite to form illitic-montmorillonitic settling entities in some clay mineral mixtures was noted. Other mixtures, exposed to artificial sea-water for 3–6 years, exhibited a tendency to transport 5–20 percent kaolinite within a developed illitic-chloritic mix, when reagitated.

Evidence is also presented to support the argument that clay minerals do not settle in single solid particulate units in saline waters. The effective settling unit, after flocculation, is described as a coacervate, i.e. as a thermodynamically reversible assembly of solid clay particles or strands within a settling solid-rich liquid unit phase. Settling rate increases are thereby not a consequence of any irreversible formation of larger solid particles or solid aggregates by coalescence of fresh water particles at or beyond the fresh-water-saline-water interface.

Differential transport of clay minerals by the turbulent flow of saline water in a pipe is quantitatively described. Flow rates of about 6 miles/hr were required to eliminate differential transport of the clay minerals. Clay mineral concentrations over the range 0.01–15.0 g/l. were considered.

Chemical data, electron and x-ray diffraction data, base exchange data, and electron micrographs support the settling velocity information.