Enhanced biodegradation and detoxification of malachite green by Trichoderma asperellum laccase: Degradation pathway and product analysis

The central composite design (CCD)-based response surface methodology (RSM) was engaged in optimizing the biodegradation process of malachite green (MG), a potentially toxic dye, by a laccase identified from Trichoderma asperellum with various parameters viz., enzyme concentration, dye concentration, reacting pH and incubation time. The maximal degradation of MG was obtained to be 97.18% under the optimum conditions with the enzyme concentration of 1.50 U/mL, the dye concentration of 122.66 mg/L, pH of 6.75and the incubation time of 98.58 min. The kinetics investigation of MG degradation was studied at various concentrations of MG, and the values of Km and Vmax were determined as 87.50 mg/L and 1.28 mg/L/min, respectively. Through the UV-Visible spectroscopy, FTIR, HPLC and GC-MS/MS analysis, the metabolites during the biodegradation of MG were identified to include Michler's ketone,4-(dimethylamino)benzophenone, 4-aminobenzophenone and 4-(dimethylamino)benzaldehyde, indicating that the hydroxylation, oxidation, deamination and demethylation processes were all involved in this laccase-mediated MG degradation without the formation of leuco-malachite green derivatives. The genotoxicity studies using the Allium cepa assay have further confirmed that the metabolites are less minimally toxic than the untreated MG. Therefore, this T. asperellum laccase-mediated dye degradation possesses the potentially promising biotreatment process.