Environmental Engineering Reference
In-Depth Information
The Biodegradation of Azo Dyes
by Actinobacteria
Azeem Khalid and Shahid Mahmood
1 Introduction
Synthetic dyes have wide application in the textile, leather, pharmaceutical, cos-
metic, paper and food industry. According to an estimate, global production of
synthetic dyes is more than 700,000 tonnes and textile sector consumes about 60 %
of the total production of dyes (Robinson et al. 2001 ; Shinde and Thorat 2013 ).
Since dyeing process is not very ef
cient, production of highly colored wastewater
is enormous. The amount lost in wastewater is a function of the class of dyes and in
general, their loss through discharge in the wastewater can be 2 % of the initial
concentration of basic dyes to as high as 50 % of a reactive dye (Tan et al. 2000 ;
Boer et al. 2004 ).
Azo dyes are aromatic compounds containing one or more azo (
N=N
)
chromophores. Such dyes are considered to be electron-de
cient compounds, as
) and sulfonic acid (SO 3 ) electron withdrawing
they possess the azo (
N=N
groups, resulting in a de
cit of electrons in the molecule which renders the com-
pound more sensitive to oxidative catabolism by bacteria. Hence, azo dyes tend to
persist under aerobic environmental conditions (Rieger et al. 2002 ). Because of the
persistence nature, azo dyes have negative impact on the environment in terms of
total organic carbon (TOC), chemical oxygen demand (COD) and biological
oxygen demand (BOD) (Saratale et al. 2009 ). Many synthetic dyes and their
metabolic intermediate products are found to be toxic, mutagenic and carcinogenic
(Dafale et al. 2010 ; Poljsak et al. 2010 ; Sellamuthu et al. 2011 ; Yang et al. 2013 ).
The treatment of dye-contaminated wastewater in an environmentally safe manner
is essentially required prior to its disposal. Various physical, chemical and
biological strategies can be used for the treatment of azo dyes (Qin et al. 2007 ;
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