Page 277 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

262 Principles and Methods

passing through the porous medium) and theoretical values, values
0
of can be calculated for a given particle suspension.
Particle detachment from a collector is dependent on the nature of the
interaction between the two surfaces. In the absence of an energy bar-
rier, the rate of particle detachment from the collector surface will be
controlled by the ability of the particle to diffuse across the diffusion
boundary layer [62]. When an energy barrier is present, however, the
deposited particle must overcome an energy (
) that is equivalent to
T
the depth of the primary minimum (
) plus the height of the energy bar-
1
rier (
) (Figure 7.18) in order to go back into the bulk suspension.
2
Similarly, nanoparticles deposited in a secondary minimum must over-
come an energy that is equivalent to its depth (
min ). As noted in the sec-
tion “Physicochemical Interactions,” the energy barrier height of primary
and secondary minima depths decrease with decreasing particle size.
This being the case, nanoparticle remobilization should be more sensi-
tive to changes in solution chemistry than larger particles.
Particle deposition or attachment may also take place in the attrac-
tive secondary minimum if it is present [6]. The ability of particles to
deposit in a secondary minimum is dependent on their size and on the

dp = 500 nm

dp = 100 nm

φ 2

φ min

φ 1
φ + φ = φ T
2
1

Figure 7.18 Interaction energy profile for two particle sizes
illustrating the total interfacial energy (
T ) that must be
overcome in order for a particle to detach from a collector sur-
face and become resuspended in the bulk suspension.