Reducing the environmental impact of textile dyes containg wastewater by photocatalytic degradation with zincoxide nanocoated fibers.
Coman, Diana ; Vrinceanu, Narcisa ; Popovici, Eveline 等
Abstract: In textile wastewater, strong color is an important witch
component and is very difficult to deal with. The experiments concerning
color removal have been done by many researches using a variety of
treatments like activated carbon, sodium hyperchloride or other
chemicals as agents. Semiconductor photocatalytic oxidation of organic
substances can be an alternative to conventional methods of removal of
organic pollutants from water. In this research work, an attempt has
been made to process to reduce the negative impact caused by pollutants
from sewage factors of textile dyeing. There are presented aspects
related to possible technological solutions discoloration of the water
from the dyeing process with acid dyes (CI Acid Red 127) using zinc
oxide nanocoated fibrous nanocomposites, as well as the wastewater
management for minimizing the environmental impact in case of
potentially polluting processes. The effectiveness of zinc
oxide-mediated solar photocatalytic degradation of the acid dye and
treatment of a textile dye wastewater were examined.
Key words: textiles, dyeing, pollution, wastewaters,
photocatalysis, management
1. INTRODUCTION
Textile finishing sector is characterized by the use of large
quantities of chemicals, textiles that are processed mainly in aqueous
media, part of the industries with the largest quantities of waste water
(Akyol, 2005).
Most of the chemicals used in textile wet finishing processes
(dyeing, printing, final finishing) remain on textiles, but a
significant amount of them are polluting the water sewage reaching or
the aquatic ecological system.
The wastewaters reach the treatment tanks, being then directed to
the purification or recycling plants (Coman, 2010).
The treatment and decolorouring of textile dye waste containing
acid dyes by conventional chemical and biological methods is rather
difficult. Semiconductor photocatalytic oxidation of organic substances
can be an alternative to conventional methods of removal of organic
pollutants from water. Some additional advantages of the photocatalytic
process are noteworthy:
--its mild operating conditions;
--the activation of the semiconductor by sunlight (near UV), thus
reducing significantly the electric power requirement and hence the
operating cost.
Among the semiconductors, notably zinc oxide and titanium dioxide
are known to be photosensitisers or photocatalysts (Joshi, 2006) These
oxides are illuminated by photons having an energy level that exceeds
their band gap energy excites electrons from the valence band to the
conduction band and holes are produced in the valence band. In order to
generate hydroxyl radicals which are strong oxidants, the photogenerated
valence band holes react with either water or hydroxyl ions adsorbed on
the catalyst surface. Titanium dioxide-mediated photocatalytic
degradation of acid dye and decolourisation of textile dye waste
containing acid dyes have been studied (Kanmani, 2003; Reddy, 2005).
Zinc oxide-mediated photocatalytic degradation of acid dye solutions
have also been studied (Akyol, 2005). Taking into account this entire
context, studying the effectiveness of zinc oxide as a photocatalyst for
degradation of a certain acid dye, appeared very appropriate. The
present research was undertaken to examine zinc oxide-nanocoated textile
mediated UV photocatalytic degradation of a acid dye, CI Acid Red 127
and to clarify the most important aspects of reducing environmental
impact using wastewater management.
2. WASTEWATER MANAGEMENT FOR MINIMIZATION OF ENVIRONMENTAL IMPACT
IN TEXTILE INDUSTRY
The impact on textile wet activities on the environment led to
measures of preventing and reducing the pollution quantity and quality,
through sustainable management.
In order to evaluate the environmental impact generated by the
environmental aspects identified in places where waterwaters interfere,
all the characteristics of the resulted wastewaters, as well as the
adequate evaluation criteria of the associated impacts should take into
account.
In order to diminish the environmental impact of wastewater
loading, a hierarchy of the pollutant concentration must be first
established and then solutions for impact management proposed.
For effeciency environmental programs in the textile industry a few
issues, of a great importance:
Wastewater prevention measures:
The possible measures for dimishing of wastewater and pollutant
quantities are:
* The prevention of nocive substances at the selection of dyeing
agents and textile agents in the manufacturing and finishing phase;
* The reusing of some retained and very concentrated wastes;
* The continuously reusage and usage of treated evacuated water;
* Some prevention measures at teams and textile structures;
* Prevention measures at textile wet finishing.
Wastewater treatment measures
* Treatment at the wastewater source; Treatment before the mixing
with the wastewater having another provenience; The final treatment of
wastewater
Environment hazards of polutant factors from textile industry are:
primary chemical agents for textiles (inorganic substances, aliphatic organic acids, reduction and oxidation organic agents, as well as urea);
dyeing agents (dyes and pigments).
Due to the limited degree of fixation, relatively large quantities
of dyes found in textile wastewater and in case of improper purges, the
issue of staining watercourses occurs. In order to evaluate the
environmental impact generated by the environmental aspects identified
in places where wastewater interfere, all the characteristics of the
resulted wastewaters, as well as the correct/appropriate/adequate
evaluation criteria of the associated impacts should take into account.
The environmental impact of wastewaters could be decreased through
a physico-chemical treatment of 80-85% and a biological treatment of
15-20% (Savin, 2009). An alternative to conventional methods of removal
of organic pollutants from water is semiconductor photocatalytic
oxidation of organic substances.
3. EXPERIMENTAL ASPECTS
Textile dye wastewater was obtained from an wellknown romanian
textile company.. A acid dye used in textile dye printing, CI Acid Red
127, was obtained from the abovementioned textile company. The ZnO
textile nano-coated were obtained in a previous research. For the
research a fine-medium weight 100% linen woven fabric was used. In order
to apply the nano-sized ZnO on the cellulosic fibrous support a
'pad-dry-cure' method was used. After padding, to polymerize the acrylic binder, the air-drying and subsequently curing for 3 min at
140 [degrees]C were performed. The fabric thus washed was air-dried. In
order to make a comparison, one set of cellulosic fabrics was coated
with bulk ZnO.
Measuring the absorbance at the wavelength of maximum absorbance
(410 nm) against a standard curve, the concentration of dye or colour of
the dye waste was determined. A filtration of the dye waste was
performed through a 0.45 [micro]m membrane filter before measuring the
absorbance.
In order to perform zinc oxide nano-coated textiles-mediated UV
photocatalytic degradation, some values of dye concentration were
chosen: 20, 50 and 100 mg/1. One litre of the dye solution was taken in
a 11 Pyrex beaker to which zinc oxide was added and kept in suspension
by constant stirring and exposed to UV light. A 20 ml aliquot was
withdrawn at 30 min intervals, filtered through a 0.45 [micro]m membrane
filter and the concentration of the dye in the filtrate was measured.
4. RESULTS AND DISCUSSIONS
Taking into account the previous researches, we assumed that
preadsorption of substrate (organic substance) onto the photocatalyst is
a prerequisite for highly efficient degradation. Previous researchers
have used a dark adsorption period of 15-20 min before illumination for
photocatalytic degradation of dyes to achieve sufficient solid-phase
concentration of the substrate for preventing the recombination reaction. Before illuminating the semiconductor (zinc oxide)-dye
suspension with UV light, in this study, 30 min dark adsorption was
used. In figure 1, degradation of CI Acid Red 127 with varying zinc
oxide dose (0.5-3.0 g/l) is shown. Increasing zinc oxide dose from 2.0,
2.5 and 3.0 g/l, the maximum (98.10%) dye degradation was achieved in 2
h. A zinc oxide dose of 2.0 g/1 was chosen for degradation of CI Acid
Red 127.
[FIGURE 1 OMITTED]
To ascertain whether dark adsorption of the dye onto the
photocatalyst (zinc oxide) was a prerequisite for photocatalytic
degradation, degradation of the dye after 30 min dark adsorption were
studied (figure 2). No significant change in the rate of degradation and
degradation (99.88% and 99.86%), respectively in 2 h was noticed. Hence,
the pre-adsorption step was eliminated in all subsequent experiments.
[FIGURE 2 OMITTED]
5. CONCLUSIONS
Present study is an alternative solution and minimization the
environmental impact of organic pollutants from wastewater with
semiconductor photocatalytic oxidation. Over 99% degradation of 20 and
50 mg/1 CI Acid Red 127 occurred by 2 h illumination under high UV light
intensity and a zinc oxide dose of 2.0 g/1.
The method has demonstrated that photocatalytic oxidation mediated
by zinc oxide-nano-coated textiles is effective in degradation of acid
dyes and certainly to prevent the impact, or rather than reduce it.
Future research will focus on research on the use of other
nanocoated composites fibers in the dyeing wastewaters, for reducing
pollution risk, in fact an important goal for textile wastewater
management.
6. ACKNOWLEDGEMENTS
The results of this research are partially due to the financially
support of POSDRU/89/1.5/S/49944 Project, within European Structural
Funds Framework.
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