Impact of the Salt Concentration and Biophysical Cohesion on the Settling Behavior of Bentonites

Abstract

The flocculation behavior of clay minerals in aquatic environments is an important process in estuarine and riverine dynamics, where strong gradients in salinity can locally occur. Various contradicting observations have been reported in the literature on the impact of salt concentration on the settling process of cohesive sediments. To address this issue in a systematic manner, we investigate the settling behavior of clay minerals as a function of the salt concentration of the ambient water. Specifically, we focus on montmorillonite as a prototype clay mineral with a high cation exchange capacity (CEC). To this end, we study suspensions of Wyoming bentonite (Volclay SPV) as a very important constituent for many constructional and industrial purposes. We perform an experimental campaign to study the settling behavior of moderately turbid montmorillonite concentrations in monovalent salt solutions with different salinities (sodium chloride) to represent different environments ranging from deionized to ocean water, respectively. The subsequent settling process was monitored by taking pictures by a camera in regular time intervals over a total observation time up to 48 h. In addition, a modified hydrometer analysis is conducted to determine the grain size distribution (in terms of an equivalent diameter) of the flocculated clay suspension in salt water. Despite the rather high cation exchange capacity of the investigated clay (CEC=88.1), our results show that the settling speed drastically increases within a range of 0.6–1.0 PSU and stays approximately constant for higher salinities. This critical salt concentration is defined here as the critical coagulation concentration (CCC) and lies well below the salinity of natural open water bodies. The hydrometer analysis revealed that 60% of the agglomerates exceed the equivalent grain size of 20 μm. Finally, the findings of this study are supplemented with experiments studying the effect of Extracellular Polymeric Substances (EPS) on the flocculation behavior of bentonite in salt water. Our results demonstrate that salinity is the original trigger for flocculation, whereas EPS allows for even larger floc size but it does not play a significant role for the settling processes of bentonite in estuarine environments.

Investigating cohesive sediment dynamics in open waters via grain-resolved simulations

“…The critical concentration for this transition to take place is called the CCC. To understand its importance in environmental fluid mechanics, experiments on the differential settling of clay suspensions with different salt concentrations have been carried out (Krahl et al, 2022;Rommelfanger et al, 2022;Sutherland et al, 2015). These experiments were meant to supplement previous efforts in field studies to understand whether there is a correlation between the floc size and the salt concentration (e.g.…”

“…Owing to the multitude of competing physical mechanisms, it had been challenging in these studies to identify the governing parameter that controls the critical coagulation conditions. Rommelfanger et al (2022) and Krahl et al (2022) show that for kaolinite and montmorillonite clay, respectively, the CCC is below the salinity of many open water bodies. In these studies, differential settling of clay suspensions was investigated for varying salinities, and a sudden transition to more rapid settling was observed as soon as the salt concentration exceeded the CCC.…”

“…The results of Rommelfanger et al (2022) and Krahl et al (2022) have important consequences for the hydrodynamic properties of suspended clays, such as the settling velocities of flocculated particles. As suggested by figure 3(b), the average floc size appears to increase to values in excess of 20 μm in an ambient fluid with some salinity.…”

“…However, the influence of cohesion on the effective settling rate of flocculated particles is less clear (Liu & Hrenya, 2018;te Slaa, van Maren, He, & Winterwerp, 2015;Winterwerp, 2002). This represents an important knowledge gap, as it has recently been shown that in open water bodies fine-grained sediments exist mostly in a flocculated state (Krahl et al, 2022;Lamb et al, 2020;Rommelfanger et al, 2022). When compared with their non-cohesive counterparts, the settling behaviour of cohesive sediments is quite distinct even for low turbidity levels.…”

“…Such a finite-Reynolds-number effect is nicely illustrated by the drafting-kissing-tumbling scenario described in Fortes, Joseph, and Lundgren (1987). Differential settling is therefore a useful scenario for investigating flocculation due to different settling speeds as a function of the water chemistry that affects the van der Waals forces (Krahl, Vowinckel, Ye, Hsu, & Manning, 2022; Rommelfanger et al., 2022; Sutherland, Barrett, & Gingras, 2015). Nevertheless it represents a somewhat idealized setting because fluid shear is present in nearly all open water bodies.…”