Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 255

2 % out of 3,000 commercial dyestuffs tested had LC50 values below 1 ppm. Basic
dyes are found to be most acutely toxic to sh, especially those with a triphenyl-

methane structure. Fish seems to be relatively sensitive to many acid dyes. Mor-
tality tests with rats showed that only 1 % out of 4,461 commercial dyestuffs tested
had LD50 values below 250 mg kg − 1 body weight (Clarke and Anliker 1980 ).
Therefore, the chance of human mortality due to acute dyestuff toxicity is probably
very low. However, acute sensitization reactions by humans to dyestuffs often
occur. Especially, some disperse dyestuffs have been found to cause allergic
reactions, i.e. eczema or contact dermatitis (Specht and Platzek 1995 ).
Chronic effects of dyestuffs, especially of azo dyes, have been studied for several
decades. Researchers had traditionally focused on the effects of food colorants,
usually azo compounds. Furthermore, the effects of occupational exposure of
human workers to dyestuffs in dye manufacturing and dye utilizing industries have
received little attention. Azo dyes in puri

ed form are seldom mutagenic or car-
cinogenic, except for some azo dyes with free amino groups (Brown and Devito
1993 ). However, some of the degradation products of azo dyes are known to be
mutagenic and/or carcinogenic. In mammals, metabolic activation (or reduction) of
azo dyes is mainly due to bacterial activity in the anaerobic parts of the lower
gastrointestinal tract. Various human other organs, especially liver and kidneys, can
reduce azo dyes. After azo dye reduction in the intestinal tract, the released aromatic
amines are absorbed by the intestine and excreted in the urine. The acute toxic
hazard of aromatic amines is carcinogenesis, especially bladder cancer. The car-
cinogenicity mechanism probably includes the formation of acyloxy amines
through N-hydroxylation and N-acetylation of the aromatic amines, followed by O-
acylation. These acyloxy amines can be converted to nitrenium and carbonium ions
that bind to DNA and RNA and induce mutations and tumor formation (Brown and
Devito 1993 ). The mutagenic activity of aromatic amines is strongly related to
molecular structure. Recently, mutagenicity and phytotoxicity tests were performed
on different biological sequencing batch reactor (SBR) systems, which studied for
ef ciency of effectiveness in removing toxic substances from textile wastewaters

and a comparison was made to evaluate the ecotoxicity results observed before and
after wastewater treatment (Giorgetti et al. 2011 ). The biodegradation products of
Congo red and textile industry ef

uent by Pseudomonas sp. SU-EBT were found to
be non-phytotoxic to Sorghum bicolor, Vigna radiata, Lens culinaris and Oryza
sativa plants as compared to Congo red and textile industry ef uent (Telke et al.

2010 ).

7 Conclusions

Presence of color and its causative compounds has always been undesirable in
water used for either industrial or domestic purposes. Amongst complex industrial
wastewater with various types of coloring agents, dye wastes are predominant. The
color in wastewater is highly visible and affects aesthetics, water transparency and