Reduction of 2,4-dichlorophenol toxicity to Pseudomonas putida after oxidative incubation with humic substances and a biomimetic catalyst

The effect of a synthetic iron(III)-porphyrin meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate as a biomimetic catalyst in the oxidative treatment of 2,4-dichlorophenol (2,4-DCP) with humic substances and H₂O₂ was evaluated in factorial design experiments conducted at different concentrations of 2,4-DCP (0-25 ppm) and different incubation treatment times (0, 24, 96, or 120 h). In the absence of this treatment, bioassays with the bacterium Pseudomonas putida (ATCC11250) showed decreasing specific growth rates μ (used here to quantify 2,4-DCP toxicity) with increasing concentrations of 2,4-DCP. However, when 2,4-DCP was treated as mentioned above the toxicity of the resulting 2,4-DCP solution was reduced significantly. At low 2,4-DCP concentrations (up to 5 ppm) and long incubation periods (as long as 120 h), the specific growth rate μ was comparable to that of cultures grown in the absence of 2,4-DCP. The reduction in toxicity was directly correlated to a decrease in the concentration of 2,4-DCP in the treated solutions, as measured by high-performance liquid chromatography. The reduced concentrations of 2,4-DCP in the treated solutions could be correctly predicted based on the relationship between the specific growth rates and the 2,4-DCP concentrations in untreated solutions. These results indicate that the oxidative coupling of 2,4-DCP to humic substances catalyzed by the synthetic iron(III)-porphyrin catalyst in the presence of H₂O₂ is responsible for the removal of 2,4-DCP from solutions. This approach appears to be a promising alternative treatment to reduce 2,4-DCP bioavailability and thus toxicity in the environment.