Fate and Transport of Nanomaterials in Aquatic Environments - Nanotechnologies for Water Environment Applications - page 489

water systems, such as the deepest areas and reservoirs, the shallow back-water areas of
streams, and behind dams in reservoir or obstacles in streams.

Transport of sediment may occur as a suspended load (for fine-grained sediment
or NMs themselves) or as bed load (for coarser particles). NMs themselves or NMs
associated with fine-grained suspended sediment may be redeposited and resuspended in
and out of bed sediment until a long-term depositional area is reached. Even these
depositional areas may be scoured in rivers that are subject to high seasonal flows or
flooding, causing the bed sediment to be resuspended and carried downstream.

As fresh sediment continues to accumulate, a sorbed NM associated with
previous accumulation will be slowly buried. Once a NM has been buried to a depth
beyond the mixing zone (at least in lakes), it probably will not reenter the water column.
However, NMs that remain in the upper few centimeters of the bed sediment may
comprise a long-term source of contamination to the water column through a variety of
processes. NMs may be reintroduced into the water column either in its dissolved state
(by desorption, followed by chemical diffusion) or in association with sediment
particles. Resuspension of bottom materials, driven by energy inputs into the system
(e.g., strong wind-induced current, lake turnover, large releases from reservoirs, and
storms resulting in large water discharges), can erode the long-term sedimentation areas
and move sediment associated NMs into the water column. The movement and feeding
activities of benthic organisms in the upper layers of sediment (~20 cm) mixes newly
deposited sediment with older materials. This process, referred to as bioturbation,
moves buried particles and their associated NMs back toward the sediment-water
interface where they can more easily be resuspended. As usual, the depth of sediment
affected by bioturbation tends to be shallower than that by physical erosion.

When NMs or NPs are added (or accidently released or settled down) to a
sediment, continuing sedimentation of the NMs or NMs attached onto the sediment
particles will cause the consolidation of the sediment, which makes invalid classical
solutions for NPs' transport through porous media (see Section 15.2.2.3) that are derived
strictly for rigid media (Ogata, 1970; Bear, 1972). The one-dimensional consolidation
and the transport of NPs in such a system can be described with two imaginary columns
of elements that follow the motion of the fluid and solid phases separately. Each element
has a unit cross sectional area, initial height, initial void volume (or fluid volume), initial
concentration and mass of NPs. The initial concentration of the sorbed NPs on the
solids of each element is assumed to be in equilibrium with the corresponding fluid NP
concentration. At a certain time of consolidation, the mass balance of NPs in both the
liquid and solid phase is

M(t) = M s (t) + M w (t)

(Eq. 15.35)