Decolorisation of Reactive Orange using Coupled Oxidation Process.

 

K. Gowthami Balashri* and S. Kanmani

Centre for Environmental Studies, Anna University, Chennai, India.

 

ABSTRACT:

Increasing urbanization and industrialization have resulted in a dramatic increase in the volume of wastewater. Global water pollution scenario suggests that nearly 1.5 billion people lack safe drinking water and at least 5 million deaths are attributed to waterborne diseases such as cholera, hepatitis every year. The textile industry plays an important role in Indian economy. The textile dyeing wastewaters are highly coloured due to the release of unfixed dye molecules in the dyeing process. The traditional treatment techniques applied in textile wastewaters, such as coagulation/flocculation, membrane separation (ultrafiltratation, reverse osmosis) or elimination by activated carbon adsorption, only do a phase transfer of the pollutant, and biological treatment is also not a complete solution to the problem. So Advanced Oxidation Processes (AOP´s), like ozonation and ozonation combined with hydrogen peroxide and UV source, could be a good option to treat and eliminate textile dyes. Advanced Oxidation Processes are the one that offers a highly reactive, non-specific oxidant namely hydroxyl radicals (HO•), capable of destroying wide range of organic pollutants in water and wastewater. In the present study, colour removal on synthetic textile dyeing wastewater (SWW) Reactive orange was performed using ozone and ozone based AOPs such as O₃/UV, O₃/H₂O₂ and O₃/UV/H₂O₂. The decolorization of dye has been investigated in terms of change in color by measuring absorbance.

 

INTRODUCTION:

Textile dyeing industry is one of the major industries consuming large amount of water for its various operations and also discharging the wastewater in large quantity. The textile units use a number of dyes, chemicals and other materials to impart desired quality to the fabrics. These units generate a substantial quantity of effluents, the quality of which in most of the cases are unsuitable for further use and can cause environmental problems, if disposed of without proper treatment. During the past two decades, several decolorization techniques have been reported, few of which have been accepted by industries. There is a need to find alternative treatments that are effective in removing dyes and colorants from large volume of effluents. The color of effluent is as indication of pollution if it discharges into water bodies. The color of water, polluted with organic colorants, reduces when the cleavage of the –C=C-bond the –N=N- bond and heterocyclic and aromatic ring occurs.

 

There are various methods which are formed by the physical, chemical and biological methods for treatment of textile industry wastewater Chemical methods like coagulation and flocculation are generally used in order to eliminate organic materials. Coagulant materials are usually effective on decomposed dyeing substance. Coagulation is effective for treatment of insoluble dyestuff wastewater but not so effective for soluble dyestuff wastewater (1).

 

Chemical oxidation treatment technologies are of industrial interest for the depuration of bio-refractory wastewaters. Among them, ozonation and Fenton’s process, operating at room conditions of pressure and temperature, are economically preferable avoiding operational costs associated with the reactor heating up (2) the chemical oxidation can destroy the colour of dyes and other contaminated materials and this method is very effective to decolorize the soluble dyes. In the last decade, a lot of research has been addressed to a special class of oxidation technique known as Advanced Oxidation Processes (AOPs), pointing out its potential prominent role in the wastewater treatment (3). AOPs were defined by Glaze et al (1987) as near ambient temperature and pressure water treatment processes which involve the generation of highly reactive hydroxyl radical (OH°) in sufficient quantity to effect water purification. The hydroxyl radical is a powerful, nonselective chemical oxidant which reacts very rapidly with most organic compounds.

 

In the present study, the potential of the oxidizing agents such as UV-C, H₂O₂ and O₃ in decolorizing reactive orange107 (RO107) was investigated using batch scale reactor. The optimal dose and levels of pH, contact time for the enhanced decolorization was tested on the basis of experiments possessing higher average decolorization rate. The influence of all the oxidants on concomitant decolorization with mineralization was studied. To confirm whether decolorization is due to degradation, the UV-vis spectrophotometric analysis was carried out.

 

MATERIALS AND METHODS:

Dyes, chemicals and instruments:

RO235 (λ_max=417 nm) was obtained from Sigma Chemical Company (St Louis, MO, USA). Double distilled water was used for preparing dye solutions throughout the study. The stock solutions of the dyes were stored in the dark at room temperature. All the chemicals used were of the highest purity and of analytical grade. The concentrations of the dye solutions were estimated using the absorbance recorded on Shimadzu UV-1800 spectrophotometer model number (Tokyo, Japan). Ozone was produced from air using Labozone ozone generator (Ozone technology solutions, India), which is capable of producing up to (10.4 mg/min) of ozone using oxygen (>99.9% purity) as the feed gas.

 

Reactor set up:

The reactor consists of plexiglass box (length 16 cm, breadth 16 cm and height 24 cm).  In all experiments, the system was operated in a recirculation mode. A peristaltic pump provided the flow of wastewaters into the reactor. The reactor was provided with  an inlet and outlet for feeding and collecting the water samples.  O₃ gas was purged into the reactor from the bottom. 

 

The excess or unreacted O₃ from the reactor after the reaction was collected from the top of the reactor and allowed into buffered KI traps to measure the amount of unreacted ozone and then into fume hood.  The whole set up was placed inside a plywood box which housed three UV lamps (6 W, Philips, 254 nm) at the bottom side of the top cover. The illumination unit was aligned in parallel to reactor surface at a distance of 5 cm. The plywood box was also provided with two exhaust fans to maintain the temperature inside the reactor setup. The Schematic representation of the reactor is shown in the Fig. 1.

 

The samples were collected from the selected industry and cooled as its temperature was higher than the atmospheric temperature. The ozone from the Ozonator was passed into the reactor using fine bubble diffuser for diffusion of ozone. The H₂O₂ was added as required into the reactor

 

The lab scale reactor study was initiated to evaluate the performance of the reactor. In the performance evaluation study, effect of H₂O₂ dosage, contact time, ozone dosage, effect of pH, initial concentration was conducted. The laboratory – scale feasibilty studies had been carried out in order to study the effects of operating parameters such as ozone dosage (62 – 210 mg/L), H₂O₂ dosage (100 – 400 mg/L), dye concentration (20 – 80 mg/L), volume of wastewater ( 3& 4 litre), lamp power (6W – 18 W) and contact time of 60 min for decolorisation of textile dyeing wastewaters.

 

Fig.1. Schematic representation of laboratory scale reactor

RESULTS AND DISCUSSION:

Decolorization studies:

Experimental investigations were conducted on the decolorization  studies on ozonation and ozonation assisted with UV and H₂O₂ in reactive orange by laboratory – scale reactor  (Rectangular). The laboratory - scale treatability studies were carried out in order to study the effects of operating parameters such as ozone dosage (62–210 mg/L), H₂O₂ dosage ( 100 – 400 mg/L), dye concentration   (20–80mg/L), pH (3–12) and lamp source for decolorisation of the dye reactive orange.

 

Effect of ozone dosage in decolorization:

The experiments were performed with initial concentration of 40mg/L with various ozone dosages of 62mg/L, 82mg/L, 157mg/L & 210mg/L and were analyzed to assess the contribution of ozone in decolorization of the dye. The results of the effect of ozone dosage are depicted in Figure 3.1.

 

Fig.2. Effect of ozone dosage on reactive orange

 

It was observed that the percentage colour removal of reactive orange for various ozone dosages can be interpreted that on increasing the ozone dosage, the colour removal efficiency increased. It is also observed that the reaction is very fast as complete decolorization of SWWs was achieved in 60min. of contact time. This may be due to the contribution of ozone in producing the hydroxyl radicals which are even more powerful oxidants than ozone

The mechanism of ozonation is as follows                                                                                  

 

The decomposition of ozone is initiated by means of one or more reactions.

 

The ozonide ion (O₃^•-) ultimately decomposes to form OH^• radicals by means of the above given reactions (4)

 

The mechanism describes the formation of hydroxyl radicals and thereby its contribution in colour removal. Though the dye was treated with various ozone dosages, significant results were achieved at 157mg/L of ozone dosage where the colour removal was about 99.6%,  for the ractive orange. On further increase to 210mg/L, the decolorization  remained constant which may be due to the escape of ozone without reacting to the pollutant completely and rapidly reaches the surface of the reactor easily. The decolourisation of RR 194 and RY 145 dye solutions were almost complete at the end of 30 min. ozone treatment.(5)

 

Effect of H₂O₂ dosage on colour removal:

Hydrogen peroxide (H₂O₂) acts as a catalyst and accelerates the decomposition of ozone to hydroxyl radicals. In order to study the influence of H₂O₂ in decolorization, its concentration was varied from 100 mg/L to 400 mg/L with 40 mg/L initial concentration of the dye  and ozone dosage of 157 mg/L. The results are depicted in Fig. 3.

 

Fig. 3. Effect of H₂O₂ dosage

 

From the results, it was observed that the colour removal decreased on increasing the H₂O₂ concentration. This can be attributed to the scavenging effect of hydroxyl radicals where the activity of OH^. gets reduced on increasing the H₂O₂ concentration. Maximum colour removal of 93.2%, was achieved at 100mg/L of H₂O₂ dosage in 60 min.. The higher decolorization efficiency with low dosage of H₂O₂ may be explained with more efficient generation of OH radicals according to the follows equations.

 

The mechanism of H₂O₂ is as follows,

Excess of H₂O₂ could lead to the consumption of the active oxidizing hydroxyl radicals other than the dye degradation reaction and consequently reduce the rate of the reaction. This phenomenon is referred to as scavenging effect (6).

 

Effect of initial dye concentration:

Pollutant concentration is an important parameter in wastewater treatment. Hence, in order to study the feasibility studies of ozonation process to different concentration of dyes in wastewater, experiments were conducted at pH 6.5, O₃ dosage of 157mg/L  by varying the initial concentration of dye from 20 - 80 mg/L. The results of the experimental studies were presented in Figure 3.3.

 

Fig.4. Effect of initial concentration of reactive orange

 

Decolorization efficiency of 99.6% for the reactive orange dye was observed for 40mg/L of dye concentration for a contact time of 60min. in lab scale reactor. From the Figure 4.3, it was observed that the colour removal pattern was similar with a rapid decolorization in the initial stages, followed by a relatively slower decolorization at later stages and also the extent of decolorization decreased with increase in initial concentration of dye. Increasing the dye concentration from 50 to 250 mg/L the decolourisation efficiency decreased. (7)

 

Influence of light source:

Generally, the combined use of ozone and UV is often reported to have increased the efficiency of the process (8). To study the influence of UV source on the ozonation processes, the experiment was performed with various lamp power of UV source at the optimised conditions of O₃ dosage and H₂O₂ concentrations. Hence, the experiments were carried out at pH 6.5 in the presence of UV lamp at 254 nm with varying lamp power from 6-18 W. The results of the experimental studies were presented in  Table 1 .

 

Table 1 Influence of light source on colour removal (Process – UV/O₃  & UV/O₃/H₂O₂)

 

Photolysis of ozone in water with UV radiation in the range of 200-280 nm can lead to yield of hydrogen peroxide. Hydroxyl radicals can be generated by the produced hydrogen peroxide under UV radiation and / or ozone and their mechanism are presented in Eqn. 6 and 7.

The mechanism of O₃/UV and O₃/UV/H₂O₂ is as follows,

From the table 4.1, it was observed that for UV/O₃/H₂O₂, the colour removal of 99.8%  was observed. In UV/O₃, colour removal of 99.2% was observed using 18W of UV light irradiation. The O₃/UV process makes use of UV photons to activate ozone molecules, thereby facilitating the formation of hydroxyl radicals (9). UV radiation alone is not capable to destroy reactive dyes which are resistant to direct UV light exposure (Rierra et al, 2010). Therefore addition of H₂O₂ to the O₃/UV process accelerates the decomposition of ozone, which resulted in an increased rate of OH radicals generation (10).

 

CONCLUSIONS:

After optimizing the ozone dosage, H₂O₂ dosage, UV source and initial  concentration, the SWWs was tested for the efficacy of the individual and coupled AOP processes (which can also be called as oxidation systems) on colour removal at neutral pH in lab scale reactor . Under optimized conditions of initial concentration for 40mg/L, volume of 4 litres, ozone dosage of 157mg/L, H₂O₂dosage of 100mg/L and UV light irradiation of 18W,  the decolouristion studies were carried out for  reactive orange respectively. The best results were obtained from the coupled oxidation system (O₃/UV/H₂O₂) where the colour removal was found to be 99.2 orange dye.

 

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Received on 27.08.2013                             Accepted on 01.09.2013        

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