Provided they are of limited solubility, surface chemistry will dominate mNP behaviour in most groundwaters. The surface charge on one mNP will tend to repel other mNPs of the same type, thus promoting suspension stability. However, if collisions induced by Brownian motion, flow, or sedimentation are sufficiently energetic, the colliding particles will get close enough for van der Waals forces to overcome the electrostatic repulsion, resulting in particle attachment/aggregation. Figure 16.1a shows that two types of attachment are possible: a primary (energy) minimum attachment, which is effectively irreversible and is attained either by fortuitously energetic collisions or by migration from a secondary minimum attachment; and a secondary minimum attachment, which is reversible upon fall in ionic strength or agitation. As ionic strength increases, the energy barrier between the primary and secondary minimum disappears, thus leading to much easier aggregation (Fig. 16.1b). This is essentially the 'DLVO' model.
Thus for higher ionic strength groundwaters, particle suspensions will tend to aggregate and then either fall out of suspension, or be strained (filtered) out by small pore throats. Though NP concentration does not determine stability, greater concentrations promote quicker aggregation. mNPs will also attach to rock or nNPs, the mechanism being effectively the same.
Though ionic strength is perhaps the single most important factor determining aggregation/attachment, pH, ionic composition, and material type are also important. In addition, many suspensions are stabilized using a variety of compounds, often polymers. The latter form a coating around the particle, and can provide charge and/or steric stabilization. Thus the presence of the stabilizer is also an issue of concern. Finally, particle dissolution needs to be considered in any risk analysis.
All these processes have yet to be reliably quantified in the environmental context, even the well-established DLVO theory is only an approximation even in idealized laboratory settings.
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