Quantification and prediction of the detoxifying properties of humic substances related to their chemical binding to polycyclic aromatic hydrocarbons

Effects of 27 different humic materials on the toxicity of polycyclic aroma tic hydrocarbons (PAH) were studied for crustacean Daphnia magna. Sources i ncluded isolated humic acids, fulvic acids, and their combination from soil , peat, and freshwater. The PAH used were pyrene, fluoranthene, and anthrac ene. The observed reduction in toxicity of PAH in the presence of humic sub stances (HS) was shown to be a result of the detoxification effect caused b y the chemical binding of PAH to HS and of the direct effect of HS on D. ma gna. An approach was developed to quantify the detoxifying impact of humic materials related to their chemical binding to PAH with a use of the "const ant of detoxification" or "toxicological partition coefficient" K-OC(D). Th e latter was proposed to determine by fitting the experimental relationship s of the detoxification effect versus concentration of HS. The obtained K-O C(D) values were well tracked by the corresponding partition coefficients d etermined by the fluorescence quenching technique (K-OC(fq)): K-OC(D) = b x K-OC(fq), b (mean +/- Cl, n = 26, P = 95%) = 2.6 +/- 0.3, 4.6 +/- 0.6, and 6.0 +/- 1.4 for pyrene, fluoranthene, and anthracene, respectively. The pr edictive relationships between the structure and detoxifying properties of humic materials in relation to PAH were developed. It was shown that the ma gnitude of the K-OC(D) values correlated closely with the aromaticity of hu mic materials characterized with the C-13 NMR descriptors (SigmaC(Ar), Sigm aC(Ar)/SigmaC(Alk)) and atomic H/C ratio. The obtained relationships showed the highest detoxifying potential of the humic materials enriched with aro matics and allowed a conclusion on the chemical binding as the governing me chanism of the mitigating action of HS on the toxicity of PAH.