Influence of redox mediators and metal ions on synthetic acid dye decolourization by crude laccase from Trametes hirsuta

In this paper, the effect of redox mediators on synthetic acid dye decolourization (Sella Solid Red and Luganil Green) by laccase from Trametes hirsuta cultures has been investigated. All the redox mediators tested, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), 1-hydroxybenzotriazole (HBT) and Remazol Brilliant Blue R (RBBR), led to higher activities than those obtained without mediators addition showing the suitability of the laccase/mediator system (LMS) in the decolourization of acid dyes. HBT was by far the most effective mediator, showing a decolourization percentage of 88% in 10 min for Sella Solid Red and of 49% in 20 min for Luganil Green. On the other hand, the stability of laccase against several metal ions, normally found in textile wastewater, was assessed. Laccase was stable at a concentration of 1 mM for 7 d against all the metal ions tested except for Zn +2, CrO 4 - 2 , Cd +2, Cr 2 O 7 - 2 , Fe +2, Cu +2 and especially Hg +2. When the concentration was increased to 10 mM laccase stability decreased against all the metals assayed, in particular against Fe +2. In addition, the effect of metal ions on the decolourization process was also studied. It was found that Hg +2 inhibited the dye decolourization process, being the presence of HBT absolutely required for dye decolourization.