Immobilized Laccase Mediator-Catalyzed Oxidation of Aqueous Mixtures of Polycyclic Aromatic Hydrocarbons

Abstract

Although terrestrial and aquatic environments are polluted with mixtures of polycyclic aromatic hydrocarbons (PAHs), much of the research efforts toward bioremediation of these toxic, mutagenic and/or carcinogenic pollutants have focused on individual compounds. The potential of laccase (E.C. 1.10.3.2) from Trametes versicolor immobilized on soil and kaolinite for in vitro oxidation of equimolar concentration of anthracene (AnT), benzo[a]pyrene (BaP), benz[a]anthracene (BaA) and pyrene (PyR) in sole, binary, ternary and quaternary substrate systems, in the presence of 2, 2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1-hydroxybenzotriazole (HBT) as mediator compounds was investigated. Results indicated that HBT was a better mediator compared to ABTS due to favorable redox potential difference (ΔEo) between laccase-HBT and PAH couples. The amount of each PAH oxidized depends on the type and number of PAHs present in the mixture. Due to its low molecular weight and high Eo, AnT inhibited the oxidative transformation of the other PAHs, and appeared to be competing for the same active site of the enzyme with BaP. Except for AnT, the oxidation of all the PAHs decreased with complexity of the mixture, with PyR oxidation inhibited by the presence of BaA. The oxidation of the PAHs was significantly low in the quaternary substrate system, probably due to enzyme inhibition from substrate overload. The results indicated that competitive effect may arise when more than one PAH is present in the reaction solution to interact with the enzyme and offer the possibility of the use of immobilized LMS for remediating complex PAH mixtures in the environment.