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Colloid Stability in Vadose Zone Hanford Sediments
journal contribution
posted on 2005-03-15, 00:00 authored by Szabolcs Czigány, Markus Flury, James B. HarshWe experimentally determined colloid stability of natural
colloids extracted from vadose zone sediments from the U.S.
Department of Energy's Hanford Reservation. We also
used reference minerals, kaolinite, montmorillonite, and
silica, for comparative purposes. Colloid stability was assessed
with two different methods: the batch turbidity method
and dynamic light scattering. Critical coagulation concentra
tions (CCCs) were determined for pure Na and pure Ca
electrolyte solutions, as well for mimicked Hanford vadose
zone pore waters with varying sodium adsorption ratios
(SARs). Critical coagulation concentrations obtained from
the batch turbidity method were sensitive to initial
colloid mass concentrations, settling time, and CCC criteria.
The lower the initial colloid concentration and the
shorter the settling times were, the larger was the CCC.
The CCCs determined from the dynamic light scattering,
where diluted colloidal suspensions are used, were not
dependent on settling time and arbitrary CCC criteria, so
dynamic light scattering is therefore the preferred method
to determine colloid stability. The CCC values determined
from dynamic light scattering ranged from 90 to 200 mmol/L
for Na systems and 1.7 to 3.8 mmol/L for Ca systems.
The stability of natural colloids was intermediate between
that of pure kaolinite and montmorillonite. The results
indicate that colloids in the Hanford vadose zone form stable
suspensions, i.e., are in the slow aggregation regime.
Nonetheless, due to the long travel times in the vadose
zone, nearly all colloids will aggregate and be removed from
the water column before reaching groundwater levels.