Azo dyes are synthetic colorants widely used in industries like textiles. They contain an azo group that imparts vibrant colors but some may release carcinogenic aromatic amines under certain conditions. Photo-catalysis using titanium dioxide or other catalysts can effectively degrade azo dyes in wastewater by breaking the azo bonds. This treatment reduces toxicity and color, though complete mineralization may not occur. Research continues on developing safer photocatalytic methods for sustainable azo dye wastewater remediation.

1. introduction
• Azo dyes are a class of synthetic colorants that are widely used in various industries, including
textiles, printing, and cosmetics. They are characterized by a chromophore called the azo group (-
N=N-), which imparts intense and vibrant colors to the dyes. Azo dyes are known for their
versatility, affordability, and ease of application, making them popular choices for coloring a wide
range of materials.
• The chemical structure of azo dyes consists of one or more azo groups (-N=N-) attached to
aromatic rings or other molecular structures. These dyes can be synthesized by coupling
reactions between a primary aromatic amine and a diazonium salt, resulting in the formation of an
azo bond. The number and arrangement of azo groups, as well as the substituents on the
aromatic rings, contribute to the diverse range of colors that can be achieved with azo dyes.

2. • One of the key advantages of azo dyes is their high color strength and lightfastness. They can produce bright
and intense colors even with small amounts of dye, making them economical for large-scale applications.
Additionally, azo dyes often exhibit good resistance to fading when exposed to light, making them suitable for
use in outdoor applications and textiles.
• However, it is important to note that some azo dyes have been found to pose potential health and
environmental risks. Certain aromatic amines, which can be released from azo dyes under certain conditions,
have been identified as potential carcinogens or allergens. As a result, regulations and standards have been
established in many countries to restrict or ban the use of certain azo dyes in consumer products.
• To address these concerns, there has been an increased focus on developing safer and more sustainable
alternatives to traditional azo dyes. Research efforts are directed towards the development of eco-friendly and
non-toxic dyes, such as natural dyes derived from plant sources or innovative dyeing techniques that reduce or
eliminate the need for synthetic dyes altogether.
• azo dyes are synthetic colorants that have been widely used in various industries due to their intense colors,
affordability, and ease of application. While they offer many advantages, their potential health and
environmental impacts have led to increased scrutiny and the development of safer alternatives.

3. the range of impurities commonly found in textile
industries
Impurity Description
Azo Dyes
Synthetic colorants with an azo group (-N=N-) as a chromophore,
widely used in textiles. Some azo dyes have been found to release
aromatic amines, which can be potential health hazards.
Formaldehyde
A colorless gas used in textile finishing processes. It can cause skin
irritation, respiratory issues, and is classified as a probable human
carcinogen.
Heavy Metals
Metallic elements such as lead, cadmium, mercury, and chromium,
which can be present in dyes or dyeing processes. They are toxic and
can cause serious health and environmental issues.
Aromatic Amines
Chemical compounds that can be released from azo dyes. Certain
aromatic amines have been identified as potential carcinogens or
allergens.
Chlorinated Solvents
Organic solvents containing chlorine atoms, commonly used in dyeing
and printing processes. They are harmful to human health and the
environment and can cause groundwater contamination.
Phthalates
Chemical compounds used as plasticizers in textile printing inks. They
are potential endocrine disruptors and can have adverse effects on
human health, particularly reproductive health.
Volatile Organic Compounds (VOCs)
Organic compounds that evaporate at room temperature. They are
emitted by various textile processing chemicals and can contribute to
indoor air pollution and respiratory issues.

5. Photo catalyst in azo dye
Photo-catalysis is a very promising technology on azo dye waste- water treatment, which
relies on performance of the photo-catalysts to generate reactive oxygen species (Kalpana and
Selvaraj, 2015, 2016). Bilal et al. (2018) used TiO2 assisted photocatalysis to remove RB5.
The decolorization efficiency of RB5 was 96% with 74-82.3% COD removal. The
chromophore of RB5 was attacked and broken down, and aromatic amines were generated.
Bio-toxicity assessment using brine shrimp nauplii showed that the toxicity of RB5 was
reduced significantly after photocatalytic treatment. The mortality of brine shrimp nauplii was
31.43-73.4% before the treatment, and reduced to 17.23-47.21% after treatment. Maroudas et
al. (2021) investigated the degradation behav- iors of three azo dyes (Dermacid Red,
Dermacid Black RVE, and Der- macid Brown) by the synergetic decolorization treatment
using ultrasounds, photocatalysis and photo-Fenton reaction. The decolor- ization was due to
the destruction of azo bonds, and aromatic amines were produced. They also suggested that
because of their diverse structures, each azo dye showed different degradation levels using
different combinations of the above-mentioned techniques, and these dyes were not
completely mineralized after the treatment. Zhang et al. (2019) used a Fe-based catalyst (MIL-
101) to activate Oxone for amaranth degradation. They revealed that MIL-101 could be
function- alized with amino groups, and exhibited visible-light-responsive pho- to-catalytic
activities to enhance Oxone activation and amaranth degradation. This gatalytic
transformation also could lead to the for- mation of hydroxyl radicals to improve dye
degradation efficiency. A potential amaranth degradation pathway was proposed