Fast decolorization of azo dyes in alkaline solutions by a thermostable metal-tolerant bacterial laccase and proposed de
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ORIGINAL PAPER
Fast decolorization of azo dyes in alkaline solutions by a thermostable metal‑tolerant bacterial laccase and proposed degradation pathways Laura E. Navas1,4 · Romina Carballo2,4 · Laura Levin3,4 · Marcelo F. Berretta1,4 Received: 13 April 2020 / Accepted: 26 June 2020 © Springer Japan KK, part of Springer Nature 2020
Abstract Biocatalytic decolorization of azo dyes is hampered by their recalcitrance and the characteristics of textile effluents. Alkaline pH and heavy metals present in colored wastewaters generally limit the activity of enzymes such as laccases of fungal origin; this has led to an increasing interest in bacterial laccases. In this work, the dye decolorization ability of LAC_2.9, a laccase from the thermophilic bacterial strain Thermus sp. 2.9, was investigated. Its resistance towards different pHs and toxic heavy metals frequently present in wastewaters was also characterized. LAC_2.9 was active and highly stable in the pH range of 5.0 to 9.0. Even at 100 mM Cd+2, As+5 and Ni+2 LAC_2.9 retained 99%, 86% and 75% of its activity, respectively. LAC_2.9 was capable of decolorizing 98% of Xylidine, 54% of RBBR, 40% of Gentian Violet, and 33% of Methyl Orange after 24 h incubation at pH 9, at 60 °C, without the addition of redox mediators. At acidic pH, the presence of the mediator 1-hydroxybenzotriazole generally increased the catalytic effectiveness. We analyzed the degradation products of laccasetreated Xylidine and Methyl Orange by capillary electrophoresis and mass spectrometry, and propose a degradation pathway for these dyes. For its ability to decolorize recalcitrant dyes, at pH 9, and its stability under the tested conditions, LAC_2.9 could be effectively used to decolorize azo dyes in alkaline and heavy metal containing effluents. Keywords Thermostable bacterial laccase · Thermus sp. 2.9 · Azo dye · Decolorization
Communicated by I. Cann. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00792-020-01186-w) contains supplementary material, which is available to authorized users.
Abbreviations RBBR Remazol Brilliant Blue R HBT 1-Hydroxybenzotriazole ABTS 2,2′-Azino-di-[3-ethylbenzthiazoline sulfonate] pHBA Para-hydroxybenzoic acid CE Capillary electrophoresis
* Marcelo F. Berretta [email protected]
Introduction
1
Laccases are multicopper oxidases (MCO) that couple the oxidation of a wide range of substrates with the reduction of molecular oxygen to water. Due to their ability to oxidize phenolic compounds, laccases can act on a variety of persistent environmental pollutants present in the waste of several industrial processes such as dyes from textile effluents (Singh et al. 2015; Unuofin et al. 2019). The colored effluents discharged into water bodies can cause an increase in biochemical oxygen demand (BOD) and chemical oxygen demand (COD) and decrease the transmittance of light, reducing the photosynthetic activity in these ecosystems. Additionally, the release of dyes into the environment is toxic to li
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