Bioattenuation/humification strategies are currently being seriously considered as alternatives to mineralization (Hawari et al. 2000a,b; Isbister et al. 1984; Weber et al. 2002). In many experiments, mass balance equations prove that mineralization does not occur, but TNT breakdown products "disappear." The process appears to be due to irreversible binding (sorption) of TNT breakdown products with the soil humic and clay fraction (Achtnich et al. 1999; Elovitz and Weber 1999; Weber et al. 2002). Sorption mechanisms include electrostatic interactions and co-valent bonding (Head 1998) (Figure 3). Sorption interactions will also depend on the chemistry of the pollutant, and the amount of clay and organic content of the soil. Binding can also be mediated via oxido-reductive enzymes including laccase (Bollag et al. 1992). The driving force behind accepting that "disappearence" is a satisfactory end point in bioremediation is the poor results so far obtained from in situ remediation of soil using fungal inoculants. Such an approach needs careful evaluation as Palmer et al. (1997) found that apparently irreversibly bound products could release sufficient residues under certain conditions to be detectable in mammalian bioassays. The presence of these pollutants was not detectable by conventional analytical chemical techniques. Sorption is influenced by pH and high will release sorbed molecules (Head 1998). Natural surfactants produced by indigenous soil microbes can also release sorbed products. There is an increasing awareness that irreversible binding may not
actually occur, and there are many on-going studies on the use of toxicity assays to monitor composts produced by bioremediation (Gundersen et al. 1997; Rocheleau et al. 1999).
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