Effect of dissolved humic acid on the bioavailability of lead from contaminated soil

Environmental toxicology emphasizes the difference from traditional toxicology in which pure compounds of interest are studied. When the objective is to study the fate and effects of trace elements in the environment, knowledge of the speciation of the elements and their physico-chemical forms is important. Risk-based contaminated site management uses the total amount of contaminant in contaminated environmental media, in particular, for organic contaminants, as a point of exposure; however, with heavy metals that could have different chemical forms in contaminated sites, the portion of heavy metals that actually contributes to toxicity (i.e., bioavailable portion) needs to be considered as a point of exposure in risk characterization. However, current risk assessment for contaminated soil does not consider bioavailability of heavy metals, which highly depends on physicochemical properties of environmental media.

With respect to risk assessment and legislation it becomes more and more clear that failure to consider properly chemical speciation of elements can lead to poor use of our resources. Therefore, an important goal in ecotoxicology is to predict the bioavailability of dissolved metals as a function of their speciation in the environment. Bioavailability of metals in contaminated sites is largely controlled by the physicochemical properties of the environmental media such as organic matter (OM), pH, cation exchange capacity, and oxidation-reduction potential. OM is one of the most influential factors controlling metal mobility, bioavailability, and toxicity. This can be assigned to its large number of functional groups such as carboxylic and phenolic hydroxyl groups that can readily interact with metal cations.

In their new study Korean scientists have now investigated the effect of humic acid (HA), used as a surrogate of organic matter, on Pb toxicity and the subsequent effect on risk characterization in ecological risk assessment. Pb toxicity was assessed via Microtox® bacterial bioassay using Vibrio fischeri in the presence and absence of HA. HA was added to the Pb solution to prepare Pb-HA mixtures that have three chemical forms of Pb - Pb sorbed on particulate HA (pHA) and dissolved Pb, which consists of Pb complexed with dissolved HA (dHA) and free Pb ions. The mixtures were then incubated at room temperature for different periods (0-48 h) to see the effect of increasing contact period on Pb toxicity. Pb sorbed on pHA and dissolved Pb was separated by filtering the mixture through a 0.45 µm filter. Acute Pb toxicity was then determined using both the original mixture and the filtered solution to assess the contribution of the Pb sorbed on pHA and the dissolved Pb on the mixture toxicity. 

The results showed that the dissolved Pb concentrations of the filtrates decreased with increasing contact time and the toxic effects decreased (the EC10 values increased). From this result the authors concluded that the Pb toxicity highly depends on the soluble fraction. Also, reduced Pb toxicity with increasing dHA concentrations, probably due to formation of Pb-dHA complexes, indicated that Pb toxicity largely comes from free Pb ions.

As a final conclusion, the authors emphasize that the incorporation of bioavailable heavy metal concentrations in environmental media as a point of exposure in ecological risk assessment is essential for more realistic risk assessment.

In 2007 we reported about a study investigating the effect of humic acids on the toxicity of lead for marine invertebrates. An international research group was questioning the widely accepted "free ion activity model". According to their results, humic acids increase dissolved lead bioavailability for marine invertebrates. This seems to be an indication that bioavailability also depends on the element of interest, the target organism and the physicochemical characterization of the environmental media and cannot be generally assessed by in vitro experiments and transferred between different environmental compartments.