Industrial dye degradation and detoxification by basidiomycetes belonging to different eco-physiological groups
Introduction
Textile effluents are one of the most difficult-to-treat wastewaters on account of the massive presence of weakly biodegradable and often toxic substances such as additives, detergents, surfactants and dyes [1]. Traditional technologies have proven to be markedly ineffective for treating textile effluents because most of the synthetic dyes are highly resistant to biological, physical or chemical treatments and therefore dyes and their derivates accumulate in the environment [2]. Actually, the release of coloured wastewater in the ecosystem is a remarkable source of eutrophication and perturbations in aquatic life. The presence of dyes or their degradation products in water even at very low concentrations can be toxic, and sometimes carcinogenic, mutagenic or teratogenic to various organisms, man included [3], [4], [5].
The implementation of strict legislation in many countries, combined with the awareness of the negative environmental impact of dyestuffs, has resulted in recent years in an increasing number of environmental researches for the development of processes that can effectively treat textile effluents. White-rot fungi (WRF) have proved to be the microorganisms most efficient in degrading synthetic dyes [6]. Their extracellular enzymatic system, which is involved in lignin degradation, consists mainly of oxidative enzymes like laccases (Lac), lignin peroxidases (LiP) and manganese peroxidases (MnP). Recently, another type of peroxidase named versatile peroxidase (VP), sharing the catalytic properties of LiP and MnP, was described in several species from the genera Pleurotus and Bjerkandera [7] and was demonstrated effective against a wide range of industrial dyes [8]. In recent years, some authors have highlighted the degradation potentialities of other eco-physiological groups of basidiomycetes (i.e. brown-rot fungi—BRF, compost fungi), underlining the importance to investigate other fungal species till now neglected [9], [10].
However, most of the screenings have been performed so far on single model dyes at low concentrations, but these conditions are poorly predictive of the actual decolourisation efficiency of real effluents in which dyes are usually present as a mix and often in quite high concentrations. Therefore, the selection of representative compounds to carry out a reliable and significant screening is a key point since model dyes biotrasformations cannot always be extrapolated to industrial dyes with apparent similar structure [11].
Another critical point is the evaluation of the detoxification after fungal treatment, a parameter that is often not taken into account, even if required by recent Italian legislation (Legislative Decree 152/2006), and that is the prerequisite to hypothesize a real application in the treatment of coloured wastewaters. Actually, decolourisation does not imply that the resulting molecules are less toxic than the parent ones. On the contrary, it has been shown that anaerobic degradation leads to reduction and cleavage of the azo-bonds of dyes derived from benzidine and the formation of potentially carcinogenic aromatic amines [5].
In the present work the decolourisation efficiency of 25 basidiomycetes ascribable to different species and eco-physiological groups against 9 commercially important industrial dyes was investigated in solid conditions. Several culture conditions were tested, in order to select fungi endowed with high degradation capabilities over a wide range of conditions and, hence, more suitable to be exploited in the treatment of different wastewaters. The 9 best strains were studied in greater detail, in liquid conditions, investigating the decolourisation activity towards simulated effluents containing single or mixed dyes and the enzymatic mechanisms involved. A very promising strain of Bjerkandera adusta was then cultured under specific conditions in order to optimize its degradation yield and the Lemna minor ecotoxicity test was performed in order to estimate the evolution of toxicity due to the fungal treatment.
Section snippets
Dyes
Nine industrial dyes kindly provided by Clariant Italia S.p.A. were selected for decolourisation assays because of their recalcitrance to biodegradation by conventional wastewater treatment plants. They are all commercially important dyes, with a wide range of applications across the textile industries and are representative of the most used structural dye types, comprising a variety of anthraquinonic, mono-, di-, polyazoic and phtalocyanin chromophores. Their acronyms, chemical groups, maximum
Dye decolourisation on agar plate
The strains of B. adusta MUT 2295, MUT 2843 and MUT 3060 were able to decolourise all dyes on all media, showing a significant physiological versatility (Table 2). These results are in agreement with Nordstrom et al. [17] who found in plate experiments that a Bjerkandera sp. strain had the potential to decolourise several dyes regardless of a different content of nitrogen. The finding of strains efficient against a wide range of structurally different dyes and not strictly dependent on culture
Conclusions
The results obtained from this wide screening on the degrading potentialities of basidiomycetes belonging to different eco-physiological groups against industrial dyes, allow to draw several conclusions:
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Most of the tested isolates showed the capability to decolourise a broad spectrum of structurally different molecules and some strains were found effective in the decolourisation of all dyes on all media, displaying a physiological versatility which is a very crucial aspect with a view to
Acknowledgements
This work was supported by MIUR—Ministero Italiano dell’Università e della Ricerca (University of Turin, ex 60%), by the project “Sviluppo di procedure di biorisanamento di reflui industriali (BIOFORM, Compagnia di San Paolo, Turin—Italy), and by Marcopolo Engineering S.p.A. (Borgo S. Dalmazzo, Italy).
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