SUREAL · Sorption of urea herbicides to black carbon

Final Activity Report Summary - SUREAL (Sorption of urea herbicides to black carbon)

In this work we investigated the binding (sorption) strength of phenyl urea herbicides (PUHs) to Diesel soot. Pyrogenic carbon particles (e.g. soot; collectively termed black carbon; BC) are products of incomplete combustion and are ubiquitously present in the environment. Many previous studies showed that the classical organic pollutants bind several orders of magnitude stronger to BC than to organic matter alone. This implies that the bioavailability of these often toxic pollutants in soils and sediments may be significantly reduced by the presence of soot. PUHs are frequently used herbicides within agriculture as well as biocides in e.g. paints and construction materials. There is therefore a high risk that these chemicals leak into the environment and pose a risk to biota and possibly humans. One of the aims of this study was therefore to study the sorption of PUHs to BC. We further wanted to investigate the influence of molecular structure on sorption strength. Data from our experimental studies showed that PUHs bind 20-170 times stronger to BC than to organic matter alone. These results imply that the risk of many pesticides may be modelled and assessed in the same way (i.e. using a two-phase sorption model) as for the less water soluble and much more studied classical organic pollutants. Computer calculations of the molecular properties of the PUHs in combination with our experimental data further suggested that the molecules that had the most planar three-dimensional structure seemed to bind the strongest to BC. There is limited data on the occurrence of BC in the environment, despite the major significance of BC for the environmental fate of often very toxic organic pollutants. We therefore determined the BC (soot) content in more than 200 sediment samples from rivers and lakes in Switzerland. BC concentrations varied between 0.5-7.2 mgBC/g sediment (dry weight), which is comparable to what was previously measured in other parts of Europe as well as globally. There was a positive correlation between the BC content in lake sediments and the part of traffic and urban land usage in the catchment area (i.e. area surrounding the lake). In a separate study, the toxicity of the PUH diuron to a green algae was investigated in the presence of BC. In line with our findings of strong binding of PUHs to BC, we found that the toxicity of diuron decreased significantly when BC was present in the water. The major part of diuron was bound to the BC particles, and thereby lost its toxic effect to the algae. This research illustrates the effect that BC may have on the toxicity and bioavailability of many very toxic organic pollutants in the environment.

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