Abstract:A new concept for the mediating action of humic substances (HS) in the contaminated environment is developed. It defines three scenarios of mitigating activity of HS in the system "living cell-ecotoxicant". The first scenario refers to deactivation of ecotoxicants (ET) by HS due to formation of non-toxic and non-bioavailable complexes. It takes place outside of the cell and is defined as "exterior effects". The second scenario refers to deactivation of ET due to HS adsorption onto the cell wall or membrane and is defined as "boundary effects": sorption takes place on the cell surface and implies changes in permeability and structure of the cell membrane. The third scenario refers to amelioration of contaminant toxicity due to activation of systemic resistance to chemical stress. This implies HS participation in immune response activation and is defined as "interior" effects. Viability of this concept was confirmed by the results of detoxification experiments. It was shown that chemical binding ("exterior effects") played a key role in ameliorating toxicity of ecotoxicants (Hg(II) and PAHs) strongly interacting with HS, whereas enhanced immune response ("boundary and interior" effects) was much more operative for a decrease in toxicity of atrazine weakly interacting with HS. The formulated concept provided satisfactory explanations for a vast pool of reported findings of mitigating activity of HS reviewed in the chapter. Few cases of amplified toxicity reported for weakly interacting contaminants in the presence of low molecular weight HS were related to facilitated penetration and follow up dissociation of humiccontaminant complexes in the cell interior. It is concluded that the developed concept can be used as a prospective tool for both predictive modelling of 249I. Twardowska et al. (eds.), and Water Pollution Monitoring, Protection and Remediation, 3-23.

The possibility of chemical modification of peat humic acids in order to increase their bioactivity by reduction with tin(II) chloride and sodium borohydride and also by carboxylation using the Kolbe method was analyzed.