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Cohesive effects between fine-grained sediment particles greatly influencetheir effective settling rate and erodibility. Many studies have observed aqualitative difference in settling dynamics between clays in freshwater, whereparticles remain dispersed, and in saltwater, where aggregates form and settlerapidly. The critical coagulation concentration (CCC) of salt that separatesthe two regimes however remains under-investigated, even though knowledge ofthe CCC is crucial to understanding aggregation in settings such as estuaries,where large salt concentration gradients occur. Furthermore, no simplecriterion exists to predict the CCC for clay suspensions. In this study,systematic experiments are performed to determine the CCC, by measuringtransmitted light intensity through clay suspensions. To investigate the effectof ion valence, sodium chloride (NaCl) and calcium chloride (CaCl$_2$) areused. For kaolin clay, the results show a CCC of 0.6mM NaCl ($\approx 0.04$pptNaCl $=$ 0.04 PSU), and of 0.04mM CaCl$_2$ ($\approx 0.004$ppt CaCl$_2$).Because these salinities are lower than those commonly observed in nature,these findings indicate that kaolin clay should flocculate in nearly allnatural aquatic environments. Furthermore, due to the fact that tap water oftenhas salinities higher than this threshold, these results imply that great careis needed in experiments, especially in large facilities where using distilledwater is not feasible. In addition, a simple criterion to estimate the CCC fora kaolin clay suspension is derived. This criterion predicts that flocculationoccurs at extremely low salt concentrations and is approximately independent ofclay concentration, in agreement with the experimental observations andconsistent with experimental evidence from the literature.

A simple criterion and experiments for onset of flocculation in kaolin clay suspensions