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.

Photolysis, oxidation and subsequent toxicity of a mixture of polycyclic aromatic hydrocarbons in natural waters

“…These compounds share fluorene's basic molecular structure, only differing by one atom in the bridge of the furan ring: C in FLU, O in DBF, and S in DBT (Table 1). Like many PAHs, these compounds are sparingly soluble with low volatility and high K OW (Table 1) and in soils and sediments they strongly associate with dissolved or particulate organic matter [18][19] that may significantly increase apparent solubility [20] yet often reduce bioavailability and toxicity [21]. Interaction with precipitated organic matter may also reduce toxicity of PAHs [22].…”

Modeling the binding of triazine herbicides to humic substances using capillary electrophoresis

“…In particular, the structural characteristics of humic substances such as the charge and size distribution in combination with physicochemical properties derived from their secondary/tertiary structures, cause mobilization (via binding to dissolved organic carbon) and immobilization (via formation of bound residues) of pollutants in the ecosphere, thereby defining transport, bioavailability and toxicity. Many studies have confirmed the relations between active humic binding sites and their specific reactivity toward given structural models (Peuravuori 2001;Steinberg et al 2000). Affinity capillary electrophoresis (ACE) is an accepted method used to study analyte-ligand interactions in pharmacological studies with different types of components.…”

Combined effects of the fungicide propiconazole and agricultural runoff sediments on the aquatic bryophyte Vesicularia dubyana

“…The presence of suspended sediments may interact with pesticides and thereby influence their bioavailability and toxicity. It generally is believed that pesticides associated with suspended sediments are less bioavailable than their dissolved forms (e.g., [3,4]). On the other hand, suspended particles can reduce light penetration in surface water and inhibit photosynthesis of water plants.…”