The mostimportant challenges for leather industry are to develop products meeting theperformance property demands of the consumers, and minimizing environmentalpollution from its processes (Gutterres and Santos, 2009). Leather industry isnotable for its wastes, and is also interrogated regarding the requirements forlow environmental impacts because of global quality life claims. The proceduresinvolved in turning hides into the leather cause liquid and solid pollutionloads at different processing stages such as liming, pickling, tanning,fatliquoring, dyeing with many chemicals (Raghava Rao et al., 2003). Dyeing is one ofthe more important step in leather making as it is usually the first propertyof leather to be assessed by the consumer. The appearance of leather can beimproved through dyeing keeping pace with popular style, satisfying fashionrequirements of people, adapting to the needs of variable uses (Covington,2009). The coloration of leather is typically made with azo dyes.
Azo colorantsare substances that have a coloring effect one or more azo groups (-N=N- doublebonds) in their chemical structure (Christie, 2001). These dyes have rathercomplex structures and they always have one part analogous to substrate,allowing molecule’s coupling, and one part which provides the colour, that isto say the visually perceived part (Przysta? et al., 2012). They are thelargest 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 dyesproduced annually in the world and colored wastewater is particularlyassociated with their presence (Dos Santos et al., 2007; Bruschweiler et al.,2014).Theenvironmental awareness and its sanctions that have appeared over recent yearsput the usage of many chemicals in leather production into question.
Reductivecleavage of azo dyes is an oxidation-reduction reaction and electrons releasingfrom oxidation of organic compounds in the compound goes through the azo dyeand cleaves the azo bond and generate the aromatic amines (Figure 1). Thesearomatic amines can be harmful, and some of them have been classified asparticularly carcinogenic, allergenic and genotoxic by International Agency forResearch on Cancer (WHO, 2010). Aromatic amines are also declared asbioaccumulative and toxic to aquatic life (Pinheiro et al., 2004). Owing to thegrowing concern about the potential risk of aromatic amines to consumers, EuropeanParliament issued European Directive 2002/61/CE (European Directive2002/61/CE). This directive restricts the commercial use of azo dyes that,after degradation, release any of the 22 aromatic amines listed. Azo dyes whichmay release one of the 22 aromatic amines yet regulated carcinogenic aromaticamines are banned from clothing textiles in European Union (Annex XVII of theREACH regulation; No, 1907/2006) (EC, 2009).
The amines listed are thoseconsidered to be harmful in concentrations above 30 mg/kg (0.003% by weight) intextile or leather products that are directly and prolonged contact with thehuman skin as clothing, footwear, gloves, hats, etc (Ahlstrom et al., 2005).Besides, it is known that large amounts of azo dyes entering activated sludgetreatment plants, will pass through unchanged and will be discharged into theenvironment together with their potentially carcinogenic degradation products(Dos Santos et al., 2007). Although the useof azo dyes containing banned aryl amine compounds is restricted, it can stillbe used today by leather producers intentionally or unintentionally.
In case thisrestricted aryl amine content is determined during the controls of the exportof finished leathers, tons of leather can be destroyed for the companies withoutany economic gain. Numerous studiespresently exist on degradation of azo dyes. However, only few studies areavailable in literature related with the application of bleaches on removal ofazo dye in leather, whereas many works have been reported about the dyeingeffluents of azo colorants. Chemicaloxidation methods enable the destruction or decomposition of dye molecules.
Oxidationof dyes and its wastewater has recently received great attention because oftheir high efficiency in the oxidation of dyestuff, ease of operation and lowcost (Bigda, 1995). Modification in the chemical composition of azo compoundtakes place in the presence of oxidizing agents, and thus the dye moleculesbecome susceptible to degradation (Metcalf, 2003; Saratale et al., 2011).
Oxidativedegradation by chlorine and ozone are the important alternatives for dealingwith the removal of azo dyes. Although chlorine is a powerful oxidant,chlorination has a disadvantage of generating carcinogenic by-products andstarted to be phased out (Sarasa et al., 1998). On the other hand, ozone andits combination are still valid as effective treatment methods, providingefficient color removal by breaking the conjugated double (–N=N–) bondsassociated with the dye (Srinivasan et al., 2009; Onem et al.
, 2017). Inaddition to ozonation, H2O2-released oxidations have beensuccessfully employed to degrade dyestuffs and intermediate in themanufacturing effluent (Kang et al., 1999). Bleaching agents can whiten ordecolorize a substance by reacting with the chromophores that are responsiblefor 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 chromophoreso that it has no color. Oxidative bleaching agents, such as hydrogen peroxide,sodium perborate and sodium percarbonate have been widely used as bleachingcomponents of fabric bleach compositions (Fujiwara et al.
, 1995). There are otheralternative methods have been tried by different scientists for degradation ofazo dyes. Electrochemical processes have been proposed as one of thealternative methods for treating wastewaters containing azo dyes with providinggood decolourisation yields.
(Lopez-Grimau et al., 2013). Biotechnologicalapproaches have attracted interest with regard to tackling azo dye pollution inan ecoefficient manner, mainly with the use of bacteria, fungi and yeast (Pandeyet al., 2007; Dawkar et al.
, 2009; Ghodake et al., 2009; Saratale et al.,2011).
Supercritical water oxidation have been effectively treated a widevariety of industrial wastes including azo dyes in a significantly shortresidence times. But, it is an expensive technology due to its operation athigh temperature and pressure (Sogut and Akgun, 2007).In this study, regainof the 300 half back lining leathers which consist highly rate of harmful azocolorants was investigated and aimed to determine best bleaching effect usingdifferent bleaches as decolorant for safely removing of azo dyed leathersurfaces. This practically applicable way is related to direct removal of azodyes in leather in the presence of oxidative bleaches with differentconcentrations.