The many chemicals (Raghava Rao et al., 2003). Dyeing

The most
important challenges for leather industry are to develop products meeting the
performance property demands of the consumers, and minimizing environmental
pollution from its processes (Gutterres and Santos, 2009). Leather industry is
notable for its wastes, and is also interrogated regarding the requirements for
low environmental impacts because of global quality life claims. The procedures
involved in turning hides into the leather cause liquid and solid pollution
loads at different processing stages such as liming, pickling, tanning,
fatliquoring, dyeing with many chemicals (Raghava Rao et al., 2003).

Dyeing is one of
the more important step in leather making as it is usually the first property
of leather to be assessed by the consumer. The appearance of leather can be
improved through dyeing keeping pace with popular style, satisfying fashion
requirements of people, adapting to the needs of variable uses (Covington,
2009). The coloration of leather is typically made with azo dyes. Azo colorants
are substances that have a coloring effect one or more azo groups (-N=N- double
bonds) in their chemical structure (Christie, 2001). These dyes have rather
complex structures and they always have one part analogous to substrate,
allowing molecule’s coupling, and one part which provides the colour, that is
to say the visually perceived part (Przysta? et al., 2012). They are the
largest group of synthetic colorants and can be used in leather fibers,
synthetic and natural textile fibers, plastics, paper, mineral oils,
foodstuffs, cosmetics etc. Azo group of dyes represents about 70% of the dyes
produced annually in the world and colored wastewater is particularly
associated with their presence (Dos Santos et al., 2007; Bruschweiler et al.,
2014).

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The
environmental awareness and its sanctions that have appeared over recent years
put the usage of many chemicals in leather production into question. Reductive
cleavage of azo dyes is an oxidation-reduction reaction and electrons releasing
from oxidation of organic compounds in the compound goes through the azo dye
and cleaves the azo bond and generate the aromatic amines (Figure 1). These
aromatic amines can be harmful, and some of them have been classified as
particularly carcinogenic, allergenic and genotoxic by International Agency for
Research on Cancer (WHO, 2010). Aromatic amines are also declared as
bioaccumulative and toxic to aquatic life (Pinheiro et al., 2004). Owing to the
growing concern about the potential risk of aromatic amines to consumers, European
Parliament issued European Directive 2002/61/CE (European Directive
2002/61/CE). This directive restricts the commercial use of azo dyes that,
after degradation, release any of the 22 aromatic amines listed. Azo dyes which
may release one of the 22 aromatic amines yet regulated carcinogenic aromatic
amines are banned from clothing textiles in European Union (Annex XVII of the
REACH regulation; No, 1907/2006) (EC, 2009). The amines listed are those
considered to be harmful in concentrations above 30 mg/kg (0.003% by weight) in
textile or leather products that are directly and prolonged contact with the
human skin as clothing, footwear, gloves, hats, etc (Ahlstrom et al., 2005).
Besides, it is known that large amounts of azo dyes entering activated sludge
treatment plants, will pass through unchanged and will be discharged into the
environment together with their potentially carcinogenic degradation products
(Dos Santos et al., 2007).

 

Although the use
of azo dyes containing banned aryl amine compounds is restricted, it can still
be used today by leather producers intentionally or unintentionally. In case this
restricted aryl amine content is determined during the controls of the export
of finished leathers, tons of leather can be destroyed for the companies without
any economic gain.

Numerous studies
presently exist on degradation of azo dyes. However, only few studies are
available in literature related with the application of bleaches on removal of
azo dye in leather, whereas many works have been reported about the dyeing
effluents of azo colorants.

Chemical
oxidation methods enable the destruction or decomposition of dye molecules. Oxidation
of dyes and its wastewater has recently received great attention because of
their high efficiency in the oxidation of dyestuff, ease of operation and low
cost (Bigda, 1995). Modification in the chemical composition of azo compound
takes place in the presence of oxidizing agents, and thus the dye molecules
become susceptible to degradation (Metcalf, 2003; Saratale et al., 2011). Oxidative
degradation by chlorine and ozone are the important alternatives for dealing
with the removal of azo dyes. Although chlorine is a powerful oxidant,
chlorination has a disadvantage of generating carcinogenic by-products and
started to be phased out (Sarasa et al., 1998). On the other hand, ozone and
its combination are still valid as effective treatment methods, providing
efficient color removal by breaking the conjugated double (–N=N–) bonds
associated with the dye (Srinivasan et al., 2009; Onem et al., 2017). In
addition to ozonation, H2O2-released oxidations have been
successfully employed to degrade dyestuffs and intermediate in the
manufacturing effluent (Kang et al., 1999). Bleaching agents can whiten or
decolorize a substance by reacting with the chromophores that are responsible
for the color of the substance. Depending on the nature of the chromophores,
the bleaching agent will either be an oxidizing or reducing agent (Holst,
1954). Oxidizing bleaches works by altering the chemical bonds of a chromophore
so that it has no color. Oxidative bleaching agents, such as hydrogen peroxide,
sodium perborate and sodium percarbonate have been widely used as bleaching
components of fabric bleach compositions (Fujiwara et al., 1995).

There are other
alternative methods have been tried by different scientists for degradation of
azo dyes. Electrochemical processes have been proposed as one of the
alternative methods for treating wastewaters containing azo dyes with providing
good decolourisation yields. (Lopez-Grimau et al., 2013). Biotechnological
approaches have attracted interest with regard to tackling azo dye pollution in
an ecoefficient manner, mainly with the use of bacteria, fungi and yeast (Pandey
et al., 2007; Dawkar et al., 2009; Ghodake et al., 2009; Saratale et al.,
2011). Supercritical water oxidation have been effectively treated a wide
variety of industrial wastes including azo dyes in a significantly short
residence times. But, it is an expensive technology due to its operation at
high temperature and pressure (Sogut and Akgun, 2007).

In this study, regain
of the 300 half back lining leathers which consist highly rate of harmful azo
colorants was investigated and aimed to determine best bleaching effect using
different bleaches as decolorant for safely removing of azo dyed leather
surfaces. This practically applicable way is related to direct removal of azo
dyes in leather in the presence of oxidative bleaches with different
concentrations.