For a bioremediation process to be successful the microorganisms,
or their enzymes, need to be in physical contact with
the organic contaminant. Both properties of the soil and the type of
the contaminant determine bioavailability and bioaccessibility
(Harms, 2011). Bioavailability represents the fraction that is uptaken
by the cells, and can cause toxic effects, or can be biodegraded
by intracellular mechanisms. The term bioaccessibility,
often also called environmental availability, considers the fraction
that is potentially available for biota in soils. From the risk assessment
point of view this phenomenon is more important than the
total concentration, because toxic effects can be attributed to a
contaminant only when it is accessible (
Cvancarová et al., 2013).
This is also true for biodegradation phenomena that include the
action of extracellular degradative enzymes, which do not require
penetration into the cells and are often overlooked (Leonardi et al.,
2007; Covino et al., 2010). Particularly, high molecular weight PAHs
have low water solubility and high solid/water distribution ratio. In
aged contaminated soil, they might not be bioaccessible because of
sorption (adsorption to solid surfaces, absorption in organic matter),
physical entrapment (immobilization in micropores) and covalent
bonding (oxidative coupling to phenolic compounds)