RAPID DEGRADATION OF AZO DYES USING NANO-SCALE ZERO VALENT IRON

In the present study, nano-scale Zero Valent Iron ( NZVI) was synthesized in ethanol medium by the meth od of ferric iron reduction using sodium borohydride as a reducing agent under atmospheric conditions. The obtained iron nanoparticles are mainly in zero vale nt oxidation state and remain without significant o xidation for hours. A systematic characterization of NZVI wa s performed using XRD, SEM and TEM studies. The obtained iron nanoparticles consist of a zero valen t core surrounding a rest oxide shell. The diameter of i on nanoparticles was predominantly within the range 20 -11 nm. Refractory azo-dye compounds used in the textile industry are commonly detected in many indu strial waste water. In this study the removal effic ien y of three azo dyes, namely, methyl orange, sunset yello w and acid blue a, with laboratory synthesized NZVI particles in relation to the NZVI dosage, dye conce ntration and pH was determined. Increasing the dose of NZVI particles enhanced the decolonization of the d y s. The degradation decreased with increasing solu tion pH and concentration of dyes. These findings demons trated the fast removal of azo dye compounds with N ZVI and the advantage of the synthesized NZVI particles to treat azo dye contaminated wastewater.


INTRODUCTION
Azo dyes comprise of almost half of global production (700, 000 t/y) and during dyeing operation processes about 20 % of them end up in wastewaters (Azbar et al., 2004). Aromatic azo-dyes comprise about half of the total international dye market (Wang and Zhang, 1997;Kulkarni et al., 1985). Textile wastewaters contribute to a significant amount of pollution all over the world, mainly because of the presence of brightly colored dyes and their recalcitrant nature to degradation. This is because of the presence of the chromophore N = N, called the azo bond, which gives it its color (VUS, 2013).
Since the first dye was discovered in 1856 to present time, over thousands of different types of dyes have been invented (Dhuinnshleibhe, 2013;EB, 2013;Susan, 2013). As a result, over hundreds and thousands of tons of dyes were manufactured. As the number of dye production increased so did the amount that was being disposed. The sources of effluent with dyes come from industries mainly, such as, dyeing and printing, textiles, paper and ink manufacturing industries, cosmetics, pharmaceuticals, food (Wallace, 2001;Chequer et al., 2011).
Dye is very harmful to both the environment and the living organisms. Unfortunately, in countries like Bangladesh, these sort of effluents are directly disposed off into water bodies with minimal or no treatment. The resulting effecsts of this are that, a color is imparted on the water body (Chequer et al., 2011). This increases the turbidity and thus reduces the penetration of light into water. The aquatic plants are the biggest sufferers as they are not able to carry out photosynthesis. It also endangers other aquatic lifeforms which eventually leads to a disturbance of the aquatic ecosystem (Chequer et al., 2011).
They can also cause skin cancer thus posing as an occupational hazard for people working in the cosmetic and other dye related industries. In addtion, it can form biotransformation products when interacting with other elements in the environment which in turn pose Science Publications AJES differents threats and risks to humans, animals and the environment (Chequer et al., 2011). Thus, it has become imperative to treat the dyes before their dischargement into the environment.
There are many treatment methods out there but most of them if not all have shortcomings. The azo dyes are highly resistant to micro-organisms making the biological treatment on them ineffective. Also, degradation by physical-chemical treatments such as activated carbon sorption, UV light-degradation, ozo-nation, nanofiltration and chemical oxidation-reduction treatments are equally if not less inefficient (Perey et al., 2002).
Recently, people have been studying and developing the uses of nanotechnology (Shan et al., 2008;Celebi et al., 2007). Nano-scale Zero Valent Iron (NZVI) particles represent a new generation of environmental remediation tech-nologies that could provide cost-effective solutions to some of the most challenging environmental cleanup problems (Shan et al., 2008). Research has shown that nano-scale iron particles are very effective for the transformation and detoxification of a wide variety of common environmental contaminants, such as chlorinated organic solvents, organochlorine pesticides and Polychlorinated Biphenyls (PCBs) (Shan et al., 2008).
NZVI is extremely small with its diameter ranging from 50 to 100 nm, which gives it a large surface area: Volume ratio and makes it highly reactive by its high surface reactivity (Ponder et al., 2000). Thus, it has a greater chance of coming into contact with dye molecules and hence decolorizing them.
Furthermore, their surface areas are up to several times greater than larger-sized powders or granular iron (Ponder et al., 2000;Yuvakkumar et al., 2011). This characteristic makes NZVI particles much more reactive in a reduction-oxidation (redox) process. Because of its extremely small size and high surface area, NZVI is thought to be a more effective technology for dye effluent treatment. An enlarged surface area allows the NZVI particles to react at a much higher rate with azo dyes. This potentially improves degradation performance in high concentration portions of the dyes. Small particle size also allows much more mobility into the dye effluent and NZVI can more easily be injected into shallow and deep dye effluent stores than granular iron (Schmid et al., 2013). Equally important, they provide enormous flexibility for in situ applications (Schmid et al., 2013;EE, 2013). Ability to inject iron can be advantageous when contamination underlies a building.

Preparation of NZVI
For the synthesis of the NZVI particles, 1.0812 g of FeCl 3 . 6H 2 O was dissolved in a 4/1 (V/V) ethanol/water mixture (48 mL ethanol and 12 mL deionized water) and stirred well. On the other hand, 0.7566 g of NaBH 4 was dissolved in 200 mL of deionized water to make a 0.1 M solution. An excess was prepared to ensure complete reduction into NZVI particles. As the NaBH 4 solution was added to FeCl 3 solution, black particles immediately formed. The solutions were mixed completely and then were vacuum filtered to separate the NZVI particles. After that, these were rinsed three times with ethanol to remove all the water. This is a key step as it helps prevent the rapid oxidation of the particles. Then they were stored in a bottle covered with ethanol and placed in a dessicator till further use. Following is the ionic equation for the reduction of ferric iron to NZVI:

Preparation of Dye Solution
100 ppm solutions of each of the three azo dyes were prepared by dissolving 25 mg in 250 mL of distilled water. They were placed inside a vacuum oven for 1 hour at 100°C to absorb any moisture present. Further concentrations of 5, 25, 50 and 75 ppm solutions of each dye were prepared.

Decolonization of Dyes
Pre-weighted amount of NZVI were added to the different dye samples. Absorbance readings of the dye samples were measured using a spectrophotometer (Model-DR/4000U Hach Company, USA) before and after the addition of NZVI.
Different parameters such as pH, concentration of dye and NZVI were varied to study their effects on the degradation process.

Characterization of NZVI Particles
The NZVI particles when preserved with a layer of ethanol stayed without significant oxidation for several weeks. A systematic characterization of the particles were performed using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). The images are given below in Fig. 1 and 2. Figure 2 shows the SEM image of freshly prepared NZVI particles. The SEM image and TEM image are in agreement of the nano particle diameter.

Fig. 2. XRD image of NZVI
They represent nano-spherical iron particles that form chain like aggregations due to its magnetic properties. In addition, the images show that these particles have diameters between 20 to 110 nm, which indicates good quality iron particles. Furthermore, this leads on to prove that NZVI has a large surface area: Volume ratio. Due to this property, it is highly reactive and therefore can participate more in re-actions bringing about the breakdown of the dye particles in the solution.
From the XRD image in Fig. 2, it can be deduced that the wide peak is due to the presence of an amorphous state of iron. Also, the broad peak at 2θ of 45° is characteristic of NZVI and proves the existence of a great amount in the sample (Yuvakkumar et al., 2011).

Effect of NZVI Dosage on Different
Concentrations of Dye Solutions 0.5 g of NZVI was added to 5 mL of 5, 25, 50, 75 and 100 ppm of all the three dye solutions. This was studied to see how the degradation is affected with increasing concentrations of dye molecules while keeping a constant amount of NZVI. A control was set up by keeping equal concentrations (5 mL of 5, 25, 50, 75 and 100 ppm of all the three dye solutions) of the dye samples in the same conditions without any addition of NZVI.
As can be seen from Fig. 3, the absorbance is significantly less after addition of NZVI. The graphs of Fig. 3 show a clear increasing trend of absorbance with the increase in concentration of dye as NZVI was added. All in all, it can be deduced that the absorbance after treatment with NZVI in all the cases is significantly less than that before treatment.

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However, the trend of amount of decolorization for each case may vary a little, but overall, it can be con-cluded, as observed from methyl orange that as the con-centration of the dye increased so did the absorbance after treatment. This was because the amount of NZVI was kept constant, so as the amount of dye increased, there were more particles of the latter that the former could not breakdown due to being outnumbered.
Furthermore, the percentage of dye removed was calculated by the equation given below. In this case, the absorbance taken is that of the mother solutions to which 2 g of NZVI had been added: The results are given in Table 1. It is evident that the percentage removal of the dyes by NZVI was well over 90 % for the first two dyes and almost 80 % for methyl orange. A significant amount as can be seen has been degraded by NZVI.

Effect of Varying NZVI Dosages on Dye Samples
This section attempts to find out the dosage required to achieve a significant reduction of concentration of dye samples. Varying dosages of 0.25, 0.5, 1, 1.5 and 2 g of NZVI were added to 10 mL of all the 100 ppm dye solutions. Further analysis of which will be followed by the graphs given below in Fig. 4. A control was set up by keeping equal concentrations (10 mL of 100 ppm dye solutions) of the dye samples in the same conditions without any addition of NZVI. All the graphs in Fig. 4 prove that with the increasing dosage of NZVI, the decolorization was greater. As there were more particles in the solution to take part in the reaction, with an increased surface area, there were more surface sites available to break down more dye particles and render them colorless.

Effect of pH on Dye Samples Treated with NZVI
This section expresses the amount of degradation with changes in pH level. 0.5 g of NZVI was added to 10 mL of all the 100 ppm dye solutions. The pH of these solutions was adjusted to 3, 7 and 11. A control was set up by keeping equal concentration (10 mL of 100 ppm dye solutions) of the dye samples with varying pH levels (3, 7 and 11) in the same conditions without any addition of NZVI.
All in all, from Fig. 5 it is evident that degradation increases with a decrease in pH. With lower pH, there are a greater number of hydrogen ions present. The iron particles donate two electrons to the H + ions converting them into atoms. These atoms in turn attack the dye particles, breaking them down to amines and rendering them colorless. As the solution gets more acidic, there are a great amount of H + ions present, thus, iron particles can donate electrons to more ions turning them to atoms (Chatterjee et al., 2010). These in turn can attack the azo bonds more at an increasingly acidic condition bringing about disruptions in more dye molecules. This is depicted by the graphs below in Fig. 5 which show a greater degradation with a decreasing pH.

CONCLUSION
From the experiments performed in this study, it can be said that NZVI is an effective degrading agent for dyes. It works best in acidic conditions when the amount of H + ions is greater. Fe 2+ ions are formed after the reduction by NZVI and are quite simple to remove by precipitation.
The removal efficiency is also high with more or less all the dyes being well over 90%.
In countries like Bangladesh, where the disposal of untreated waste water is a serious issue, NZVI could be a promising solution for this problem. It is cheap and readily available. Also, there is hardly any hassle in its preparation. Moreover, the end product is colorless and therefore can be discharged into nearby water bodies without any adverse consequences to turbidity and similar conditions.

ACKNOWLEDGEMENT
A special appreciation and gratitude to Designated Research Institute for Chemical Measurements (DRiCM) Laboratory of Bangladesh Council Scientific and Industrial Research (BCSIR) for permitting to carry out some analytical tests and experiments in their laboratory.