In situ variations of salinity are associated with variations of the shear rate, sediment concentration, and OMC, and it is therefore difficult to establish the effect of each of these physical quantities independently. Burt ( 1986) carried out a large series of floc size measurements at different salinities within the Thames estuary, but could not find any correlation between salinity and floc size. ( 1991) observed a decrease of the floc size for increasing salinity. While van Leussen ( 1999) measured, in the Ems estuary, an increase of the floc size for increasing salinity, Eisma et al. An increase in salt concentration decreases the particles’ charge and, therefore, would lead to an increase in the floc size however, this is not always observed in situ. Dissolved (counter) ions can either adhere to the charged particles surface or screen its charge. For instance, at pH = 4, the edges of kaolinite and montmorillonite are positively charged, which leads to fast aggregation and to the formation of larger flocs than at pH = 8, when both edges and faces are negatively charged (Tombacz and Szekeres 2004 Tomback and Szekeres 2006).Ī suspension where particles are likely/unlikely to aggregate is referred to as unstable/stable. However, the charge of mud or clay particles is not uniform and it depends on the suspending medium properties. As a rule of thumb, uniformly charged particles with lower ζ potential (in absolute value) are more likely to aggregate than particles with a higher ζ potential. This charge is often expressed in terms of an electric potential defined as the ζ potential (Kruyt 1983), although the relation between the surface charge of particles and their ζ potential is complex (Chassagne et al. The rate of aggregation and the strength of flocs depend on the electrical charge of/on the particles. Therefore, we seek laboratory investigations and process modeling that can be used to establish relations between relevant parameters, easily measurable in situ, and this behavior. It is technically difficult and costly to monitor cohesive sediment flocculation behavior in situ, and we seek tools to predict this behavior. Seasonal variations of the OMC have been observed by Mikkelsen ( 2002). Burton and Liss ( 1976) observed that the organic matter content (OMC) is one order of magnitude higher in coastal seas (≈ 10 mg/l) than in rivers (≈ 1 mg/l). All these quantities are highly variable in estuaries, where fresh river water mixes with salty marine water, and where the tidal forcing induces variations in shear rate. The rate at which flocs grow and the size they attain depend on hydrodynamic conditions, residence time, sediment properties, pH, and salinity. Flocculation can be regarded as a competition between aggregation and breakup. For cohesive sediment, the settling velocity is affected in a complex way by the sediment properties and the environmental conditions through the process of flocculation. One of the parameters used in sediment transport modeling is the settling velocity. This systematic study can therefore be used for further development of flocculation models. This suggests that a general trend can be established for different and complex types of clays and mud. The response of mud suspensions to variations in salinity and pH is similar to that of kaolinite. (4) Mud with no organic matter at pH = 8 and no added salt flocculates very little. (3) The mean floc size increases with increasing organic matter content. (2) For a given ζ potential, the mean floc size at low pH is larger than observed at pH = 8 for any added salt. The main findings of this paper are: (1) In saline suspensions at pH = 8, the mean floc size increases when the salt concentration and the ζ potential increase. The size of flocs at a given shear rate depends on the properties of the suspension, which affect the electrokinetic properties of the sediment these can be described by means of the ζ potential. From this study, it was found that the mean floc size and the Kolmogorov microscale vary in a similar way with the shear rate for suspensions with different pH and salt concentrations. The mud used for all experiments has been collected in October 2007 in the lower Western Schelde, near Antwerp, Belgium. The results obtained with small-scale flocculation experiments (mixing jar) are compared to results of large-scale experiments (settling column). The purpose of this paper is to establish a relation between a few measurable quantities (the so-called ζ potential, organic matter content, and shear rate) and the flocculation behavior of mud.
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