Lignin peroxidase efficiency for methylene blue decolouration: Comparison to reported methods

The oxidative potential and low specificity of peroxidases are distinctive regarding their efficiency for recalcitrant compounds degradation. However, the usefulness of these biocatalysts for environmental biocatalysis needs a stepwise investigation on the reaction conditions that would render these biocatalysts both efficient and cost effective. In a recent work we compared the usefulness of the fungal lignin peroxidase (LiP) to that observed for the plant horseradish peroxidase (HRP) concerning the degradation of methylene blue (MB) and of its demethylated derivatives. We showed that although both enzymes are able to oxidize MB and its derivatives, HRP reactions require higher H2O2 concentrations, present a considerably lower reaction rate, and contrary to LiP, HRP is unable to achieve aromatic ring cleavage. The oxidation potential of LiP is roughly double than that of less effective HRP (∼0.7 V) and this explains relative efficacy. Thus, lignin peroxidase would be more suitable for phenothyazine dyes degradation and colour removal from waste streams. The present work shows that the use of LiP for the decolouration of MB is competitive in comparison to the majority of the reported methods, regarding reaction time, range of substrate concentration and removal efficiency. In reaction mixtures containing 50 mg/L methylene blue and carried out at 30 °C the dye was degraded within 30 min. Reaction conditions were optimized concerning H2O2 addition mode to avoid the inactivation of the enzyme by H2O2 excess, the enzyme concentration to minimize cost, and the reaction temperature. Results indicated that the use of an MB:H2O2 molar ratio of 1:5 resulted in efficient removal of 90% colour in reactions with MB concentrations up to 50 mg/mL. The enzyme stability was not affected by peroxide concentration up to 990 μM and an LiP:H2O2 molar ratio up to 1:900. The stepwise addition of the peroxide extended the possibility of using total peroxide concentrations up to 1980 μM. Lignin peroxidase was stable up to 60 °C.