Towards a Quantitative Structure Activity Relationship (QSAR) of Dissolved Humic Substances as Detoxifying Agents in Freshwaters

Abstract

In freshwater ecosystems, dissolved humic substances (HS) can sorb hydrophobic organic chemicals and thereby reduce their bioconcentration and toxicity. The extent of the sorption depends both on the concentration and on the origin of HS. This paper summarizes work that characterized HS from different origins by various spectroscopic and chemical properties, and related these properties to (1) the sorption of polycyclic aromatic hydrocarbons (PAHs) by HS (measured by a fluorescence quenching method), (2) effects of HS on the bioconcentration of PAHs and (3) effects of HS on the toxicity of PAHs. The most powerful predictor of the association between 26 different HS and the PAHs pyrene, fluoranthene, and anthracene was the content of aromatic carbons of the HS. The influence of HS on the bioconcentration of PAHs was investigated by measuring the effect of seven different humic and fulvic acids on the uptake of pyrene and benzo[a]pyrene into the nematode Caenorhabditis elegans. Again, parameters describing aromatic properties of the HS, such as the content of aromatic carbons, or the specific absorptivity at 254 nm gave the best prediction of HS effects on bioconcentration. Studies on the effect of HS on the toxicity of PAHs (impact on grazing activity of Daphnia magna) gave similar results: The detoxifying ability (= reduction of toxicity) of HS could be correlated to the quotient between aromatic carbons and aliphatic carbons in the HS. Taking these results together, we can describe the relationship between structure and effects of HS on bioavailability and toxicity of PAHs as follows: HS associate with PAHs, making the PAHs unavailable for uptake into organisms, and consequently reducing toxic effects of the PAHs. Aromatic properties of the HS can be used to predict the effect of HS in each of these steps.