Absorption describes the transfer of a chemical across the biological membrane into the blood circulation.a

Paustenbach et al., 1997

Biostabilization refers to the biodegradation of the more labile HOC (hydrophobic organic compound) fraction leaving a residual that is much less available and mobile.

Luthy et al., 1997

aIn this report, “absorption” is used generically for non-mammalian organisms to be synonymous with “uptake.”

absorbed and able to reach systemic circulation in an organism. Another view of bioavailability is represented by a chemical crossing a cell membrane, entering a cell, and becoming available at a site of biological activity. Others might think of bioavailability more specifically in terms of contaminant binding to or release from a solid phase. These different viewpoints of bioavailability create a semantic stumbling block that can confound use of the term across multiple disciplines—hence the reason that “bioavailability processes” is used in this report.

Figure 1-1 is a depiction of bioavailability processes in soil or sediment; it incorporates exposure by release of solid-bound contaminant and subsequent transport, direct contact of a bound contaminant, uptake by passage through a membrane, and incorporation into an organism. “A”—contaminant binding and release—refers to the physical and [bio]chemical phenomena that bind/unbind, expose, or solubilize a contaminant associated with soil or sediment. This may include geological processes like weathering and scouring, chemical processes like redox reactions or complexation, and biochemical processes through the action of biosurfactants or hydrolytic enzymes. Binding may occur by adsorption on solid surfaces, by absorption within a phase like natural organic matter, or by a change in form as in covalent bonding. “B” in Figure 1-1 involves the movement of a released contaminant to the membrane of an organism. Transport may result from diffusion and advection to target receptors such as microbes, plants, and humans. Thus, bioavailability processes A and B comprise exposure via various chemical and biochemical phenomena that affect release and subsequent transport of dissolved contaminants. “C” involves the movement of contaminants still bound to the solid phase, which can play a role in dermal contact of soils, oral ingestion of soil or sediment, or exposure to burrowing organisms in soil or sediment. It should be noted that processes A, B, and C can occur internal to an organism such as in the gut lumen, although they are depicted in Figure 1-1 as occurring in the external environment.

The bioavailability process depicted as D in Figure 1-1 entails movement across membranes. Here the contaminant passes from the external environment through a physiological barrier and into a living system. An example is transport

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