Targeting sorbed PAHs in historically contaminated soil - Can laccase mediator systems or Fenton's reagent remove inaccessible PAHs?

This laboratory study investigates the potential of two innovative laccase-mediator systems for removing PAHs from historically contaminated field soil and focuses on the treatment effect on the accessible and desorption resistant PAH fraction. Laccase degraded accessible PAHs when applied in combination with the mediator TEMPO (up to 24 % within 48 h). The mediator HBT did not induce degradation but mobilized desorption resistant PAHs from high affinity sorption sites via a competitive sorption mechanism. Enzymatic degradation of inaccessible PAHs was not observed with neither of the two enzyme-mediator systems. To verify a potential radical susceptibility of contaminants inaccessible to microorganisms, PAH contaminated biochar was treated with hydroxyl radicals generated by Fenton's reaction. These radical species reduced the desorption resistant fraction of phenanthrene (13 +/- 10 %), fluoranthene (33 +/- 8 %) and benzo(a)pyrene (69 +/- 5 %). In conclusion, laccase-mediator systems can interact with accessible and inaccessible PAHs, whereas direct degradation of desorption resistant contaminants required highly active hydroxyl radicals. Further studies should develop enzyme-mediator systems establishing a sufficient oxidation potential to attack the desorption resistant contaminant fraction.

Automated summary

This laboratory study examines the potential of two innovative laccase-mediator systems in removing PAHs from contaminated field soil. The results show that laccase in combination with TEMPO can degrade accessible PAHs, while HBT can mobilize desorption resistant PAHs. However, neither of the enzyme-mediator systems can effectively degrade inaccessible PAHs. Additionally, hydroxyl radicals generated by Fenton's reaction can reduce the desorption resistant fraction of certain PAHs.