Safer bioremediation for contaminated soil
Scientists know plenty about how microbes can break down major soil pollutants Polycyclic Aromatic Hydrocarbons or PAHs, but what happens with other contaminants, including oxygenated PAHs (oxy-PAHs) or Nitrogen-containing PAHs (N-PAHs), is often overlooked. Another uncertainty is whether the action of the microbes may end up increasing the overall toxicity of the soil by creating new transformation products. These are reasons why current biorestoration technologies are still not effective enough at cleaning up contaminated soils. The Spanish-run project NETPAC aims to identify the microbial communities relevant for these neglected polycyclic aromatic compounds or PACs and find new, more sustainable ways of using them to restore PAH-impacted soils. Bioremediation — a technique which involves using organisms to remove or neutralise pollutants — began to be used in the 1960s. There has been growing interest in the method in recent years as European countries look for more environmentally-friendly ways of cleaning up contaminated sites. But regulation of this technique is, in most countries, still based on a list of 16 compounds drawn up by the US Environmental Protection Agency back in the 1970s. Contaminants overlooked “When a company is doing bioremediation, they will have to check if the compounds on the list have been reduced in concentration in the soil, but they don’t consider others,” says Joaquim Vila, a lecturer in microbiology at the University of Barcelona, Spain, and main researcher on the project. “There is a large variety of compounds where we do not know if bioremediation is removing them or not.” Many of these are members of the PAC family. The NETPAC team used high resolution mass spectroscopy combined with Kendrick mass-defect filtering to analyse four PAH-contaminated samples from different sites. They managed to identify more than 230 isomers of nitrogen-containing PAHs. “Now we are trying to see if there are bacteria in the soil which can degrade these N-PAHs,” says Dr Vila. “We have evidence that they can be degraded and we are starting to look at which bacteria can do this.” Identifying some of these overlooked contaminants was one major result of NETPAC. Another was identifying a bacterial metabolite of pyrene which is implicated in the increase of soil toxicity after bioremediation. The project also considered the agents responsible for its formation. Proof of microbial action The team used an approach combining effect-directed analysis with metabolite profiling to compare PAH-contaminated soil samples from a former gas works plant before and after treatment. The treated soil was found to contain a new toxic compound which was shown to be of biological origin. “This is the first evidence of the direct implication of a specific bacterial metabolite in increased soil genotoxicity after bioremediation,” says Dr Vila. These findings point to the need for a stronger, more comprehensive regime for managing the risks of using bioremediation to clean up PAH-contaminated sites. “We have demonstrated there are these toxicology-relevant compounds in the soil we need to deal with,” says Dr Vila, “so from this point there are other groups and regulators who can make decisions to lengthen the list in terms of bio-regulation.”
Keywords
NETPAC, bioremediation, soil contaminants, PAHs, PACs, risk management, regulation
