Bioavailability of phenanthrene in the presence of birnessite-mediated catechol polymers

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants and contribute to the pollution of aquatic and terrestrial environments. In soil, their fate may be affected by interactions with the soil biological community and soil colloids. This study was conducted to investigate the fate of phenanthrene (Phe), selected as a representative PAH, in simplified model systems, which simulate processes naturally occurring in soil. Phe was interacted with catechol (Cat), an orthodiphenol, and common intermediate in the microbial degradation of PAHs, and birnessite (Bir), an abiotic strong oxidative catalyst abundant in soil. Two experimental conditions were investigated: Cat (5 mM)+Bir (1 mg ml−1)+Phe (0.05 mg ml−1) mixed at the same time and incubated for 24 h at 25°C (Cat–Bir−Phe) and Cat+Bir incubated for 24 h at 25°C before Phe addition and then incubated for a further 24 h (Cat–Bir+Phe). After incubation, the systems were analysed for residual Cat and Phe, supplied with a selected Phe-degrading mixed bacterial culture, and then the microbial degradation of Phe and the growth of cells were monitored. Complex phenomena simultaneously occurred. Cat was completely removed after a 24-h incubation with Bir, and no interference by Phe in the Bir-mediated transformation of Cat was observed. Elemental analysis and UV–Vis and Fourier transfer infrared spectra showed that Cat transformation by Bir produced soluble and insoluble polymeric aggregates involving Phe. The hydrocarbon also interactedwith the surfaces of Bir either previously coated (Cat–Bir+Phe sample) or not by Cat polymers. When a Phe-degrading bacterial culture was added to the systems after Bir-mediated Cat polymerisation, a different behaviour was observed in terms of Phe consumption and bacterial growth, thus suggesting differentiated availability of Phe to the microbial cells. The hydrocarbon was completely transformed in the presence of Bir and/or Bir covered by Cat polymers. By contrast a reduced degradation was measured when the Phe was involved in the polymerisation of Cat and entrapped in the Cat polymers (Cat–Bir−Phe). Although Cat showed a toxic, lethal effect on the bacterial cells, microbial growth was observed in the presence of Cat and Cat polymers, as the only C source. The mechanism leading to the different availability of Phe in the presence of Cat and Bir is still not clear. Further investigations are requested to provide more insight into such a complex phenomenon.