ABSTRACT

The industrial waste water is always characterized by intense colour, high toxicity, concentrated subtracts and salts, decolourization, difficulty to bio-degrade as well as pollutant to ecosystem. In the present work, the decolourization and degradation on of synthetic effluent (a mixture of two reactive dyes) and different industrial effluents have been studied using nano-scale Zero-valent iron (nZVI) as a catalyst. Different experimental parameters like pH, catalyst dose, hydrogen peroxide dose, light intensity & temperature has been optimized using synthetic effluent. The experiments have been conducted in the solar light as well as artificial light. During the study, it was monitored that solar assistance process was more proficient in degradation of reactive dyes with respect to artificial method owing to the production of large number of OH^- radicals by the UV irradiation and allowed the nearly total decolourization of reactive dyes solution.

KEYWORDS

Nano Scale Zero Valent Iron Catalyst, Decolourization, Synthetic Dyes

TO CITE THIS ARTICLE

Samiuddin , Muhammad Asif Tahir , Sara Batool , Mehwish Naz , Farah Naeem , Bukhtawar Raja , Nabila Nasreen and Javeria Ilyas 2020. Photocatalysis with Nano Scale Zero Valent Iron (nZVI) for Degradation of Textile Waste Water Contaminated with Reactive Dyes. Journal of Applied Sciences and Research, 1: 19-32
URL: https://www.sciworldpub.com/article-abstract?doi=44-jasr-20

1. INTRODUCTION

Dyes can be referred as “substances which on interaction with substrate give color by a process that alters temporarily or permanently, any crystal structure of the colored substances [8]. These substances having appreciable coloring capacity show wide applications in the pharmaceuticals, textile, food, paper plastics, cosmetics and photographic industries. Such dyes are able to adhere at compatible surfaces either by solution formation, by forming complexes with salts or metals or covalent bond, by mechanical retention or physical adsorption. Dyes are distinguished onthebasis of their use and composition. Dyes are composed of chromophores which impart those dyeing colours [2]. Such chromophore containing centers carry diverse functional groups, for example azo methine, anthraquinone, arilmethane, nitro, carbonyl and others. Additionally, electrons donating, or withdrawing substituents intensifying the color of the chromophores are termed as auxochromes. The most commonly employed auxochromes are carboxyl, amine, hydroxyl and sulfonate [4]. Annually 700000 tons of dyes are produced all over the world. Various industries include textile, tanning, leather, paper, and pulp mills are using synthetic dyes. The major source of environmental pollution is textile industry wastewater because it contains non-biodegradable synthetic dyes [16].

The main sources of water pollution are the dyes and the products obtained after the degradation process may cause cancer and toxicity in mammals. Nearly half of total production of synthetic dyes used in textile industry (7×10⁵ tons per year), worldwide is categorized as production of azo compounds. The molecular structures of these azo compounds have the chromophore with one or more azo bonds -N= N- [6,17].

Usually, azo dyes are suspected to be carcinogenic, mutagenic and harmful. Half of the lifetime of most of the azo dyes is in sunlight; that is more than 80 days, due to it shows resistant to chemical as well as biological degradation. Resultantly, lower concentrations of azo dyes are also found hazardous to the environment. The common chelating agent in paper pulp bleaching; Ethylenediaminetetraacetic acid (EDTA) is also used in manufacturing of textiles as well. Large amount of EDTA is unavoidable part of the effluents of textile industry. Some of the common damages caused from EDTA pollution are; the possible mobilization of toxic heavy metals, extended biological availability to aquatic life and many other risk factors for ground water as well as drinking water. It has been demonstrated that common facilities for wastewater treatment do not serve the purpose of degrading most of the industrial chelating agents. These dyes must be removed because their low concentration dyes may damage the fiber quality in nutrition and marine vegetation. Due to the deleterious effects of many organic dyes, it is critical to remove them from waste materials [25]. Reactive dyes belongs to the extremely coloured organic materials and mostly used in tanning industries. They attached to the available substrate through a strong covalent bond between the dye molecules and that of fiber. Reactive dyes are being used commonly by the industries in modern era. Reactive dyes are mostly applied in weakly acidic medium. These dyes are mostly used in fewer amounts as compared to the other dyes. Dyeing industries are using reactive dyes to dye wool as well as cotton and polyamaide fibers due to their specific properties such as vast colour shade variety, high wet fastness profile, their brilliant colour and ease of applications [5].

Reactive dyes are soluble anionic dyes which, in solution, are repelled by the negatively charged surface of the cotton fiber. If water becomes polluted, it loses its value to us economically and aesthetically and can become a threat for our health and other living bodies present in it [15]. These dyes are mostly based upon nitrogen chromophors associated with vinyl sulfone, trichloropyrimidine, aschlorotriazine, difluorochloropyrimidine and many other different reactive groups. The nature and number of functional group combined with the molecular structure of reactive dye has distinct impact on dyeing behavior. Due to their carcinogenic properties [20] and being very reactive chemically, the dyes are harmful and have very adverse effects on the ecosystem components at both macro as well as molecular levels [9]. The fixation procedure which is mostly used at industrial scale is the vinylsulfonyl group. Just like the chlorotriazines, the functional group makes interactions with the hydroxyl groups present in the cellulose. Most common version is remazol. This dye first makes bonding with ethylsulfonyl group.

The main objective of the study is to optimize different physio-chemical parameters like conc. of synthetic effluent, conc. of ZVI, H₂O₂, time of incubation, pH etc using zero valent iron to decolorize synthetic effluent using solar light and artificial light.

2. MATERIALS AND METHOD

The current project was designed to introduce an economical, eco friendly and nontoxic process to degrade the dyes present in the synthetic wastewater. This research work was carried out in the Laboratory of Chemistry Department, Govt College University, Faisalabad, Pakistan. The optimization of reaction parameters for decolonization/ degradation of synthetic solution of reactive dyes by using nano scaled zero valent iron particles (nZVI) was done in the first part of the study. In the 2^nd part, optimized parameters were applied for decolonization / degradation of synthetic solution of reactive dyes by n ZVI using UV-Visible and solar light sources.

All the reagents and required chemicals used in this research work were of analytical grade principally composing on Hydrogen peroxide (H₂O₂) (1 x 10^-2M), Sodium Borohydride (NaBH₄), Ferric Chloride (FeCl₃.6H₂O), ethanol, Distilled water, Deionized water, Sulphuric acid (H₂SO₄), Hydrolyzed starch, Sodium sulphate (Na₂SO₄), Sodium hydroxide (NaOH), Potassium dichromate (K₂Cr₂O₇) and glucose (organic carbon) were purchased form Merck (Pvt) Limited and used without further purification. However reactive dyes were purchased by local market Faisalabad for research purpose.

The nZVI particles was prepared by reduction of ferric using strong reducing agent sodium borohydride. For synthesis of nZVI, 5.425 g of Ferric Chloride (FeCl₃.6H₂O) was added in a 4:1(v/v) ethanol / water mixture (240 mL ethanol and 60 mL deionized water) and stirred well to dissolve completely. On the other hand 3.783 g of NaBH₄ was dissolved in 100 mL of deionized water to make a 0.1 M solution. An extra solution was prepared to confirm complete reduction of ferric into (Fe⁰) particles. Then this solution of NaBH₄ was slowly added to FeCl₃ solution and black precipitates are formed immediately. Both solutions were completely mixed and then filtered to collect the nZVI black particles. After that, these particles were washed with ethanol three times to remove all the water to prevent them from oxidation of nZVI particles. The finely prepared nanoparticles were eventually dried in oven for whole night at 323 K. A thin layer of ethyle alcohol was added to prevent the oxidation of the nano iron particles [18,20,26]. The whole reaction is presented in Eq. 1.

(1)

A characterization of nZVI particles were performed by SEM (Scanning Electron Microscopy) analysis.

Solutions used in this research were prepared as per details given below;

Stock solution of 100 ppm was prepared by dissolving three reactive dyes 0.01 g of each dye in 100 mL of deionized water. Further concentrations of 20, 40, 60 and 80 ppm solutions of given dye were prepared by further dilutions [19].

Different amounts of nZVI particles ranging from 0.01 g to 0.9 g were added in 100 mL of deionized water for solution preparation.

Standard calculations were used to prepare the required solution of hydrogen peroxide. 30% H₂O₂ is available commercially and 0.38 mL of it was taken in measuring flask and diluted up to 500 mL with distilled water.

2.1. Statistical Analysis
All experimental treatments were run in three replicates and analyzed in triplicate. The data were presented as a mean ± S.D (standard deviation) [21].

3. RESULTS AND DISCUSSION

The current work was performed to introduce a non toxic, environmental friendly and economical process to degrade and mineralize of industrial waste water having reactive dyes. Firstly, reaction conditions were optimized using waste water having reactive dyes with thehelp of nZVI. The conditions were applied to decolorize or degrade the reactive dyes present in wastewater in the presence of solar light and UV-visible light sources.

3.1. Determination of wavelength of maximum absorbance for synthetic dyes wastewater under study
Theconcentration of the solution containing three reactive dyes can be calculated by determining the amount to be absorbed. For such type of spectral analysis, first the wavelength is to be measured where the absorbance starts to generate. The spectrophotometer is very responsive instrument to show the changes in absorbance at a specific desired wavelength. Therefore the wavelength having maximum absorbance must be used for the analysis. Wavelength with maximum absorbance can be determined by measuring the absorbance over a range of 400 to 650 nm with the interval of 25 nm. For the wavelength of maximum absorbance determination, the data can be determined or graphed. The λ_max was calculated to be 520 nm Table 1.

3.2. SEM analysis
The SEM image of freshly prepared iron Nanoparticles is shown in the Fig. 1. Nanosphere of iron particles can be seen. These nanosphere are formed by the contact of particles with each other for the formation of long chains having 50 to 100nm diameters. Actually magnetic properties of iron are responsible for such kind of linear orientation.

Fig. 1.	SEM image of iron

Table 1.	Wavelengths with maximum absorbance

Fig. 2.	TEM image of iron

In Fig. 2, the image of nano zero valent iron particles is shown by Transmission Electron Microscopy (TEM). The spheres with 100 nm diameters can be recognized from one another and is in agreement with SEM results. Closer inspection shows that, the metallic iron and iron oxide phases can be differentiate with the help of corresponding color contrast present in the TEM images. The lighter regions are mostly concentrated on the particle surface while the dark regions are mainly present in the middle of the particle [11].

3.3. Optimization of conditions for decolourization of reactive dyes
In first part, the study of reactive dyes wastewater was done for the optimization of different physio-chemical parameters with the help of different treatment methods to attain decolorization (%) of dye under investigation at maximum level. In photocatalytic treatment the main factors which affect the rate of photocatalytic reaction are light intensity, temperature, the molar concentration of the oxidant (H₂O₂), nZVI concentration, synthetic dye concentration and pH.

3.4. Optimization of synthetic dye concentration
For solar assisted process and photocatalytic water treatment, an essential parameter is concentration of synthetic dye.

Fig. 3(a-e).	Decolorization of synthetic effluent by artificial photocatalysis treatment at (a) 5%, (b) 10%, (c) 15%, (d) 20% and (e) 25% of stock solution

Fig. 4.	Accumulative results of decolorization of synthetic effluent by artificial photocatalysis treatment

The result of different dye concentration on the photolytic decolourization was studied from 5% to 25% of stock solution. It can be observed by the results that decolourization varies as the concentration increases and the maximum concentration was observed for 5% solution in 90 min. This behavior is due to the increase in the extent of absorption on the surface of the catalyst which is the main reason for the reduction of catalytic activity (Fig. 3,4).

3.5. Optimization of pH
Oxidation of synthetic dyes with nZVI is strongly affected by the change in pH because the solar assisted and the photocatalytic treatment reactions are reasonably dependent on pH. In acidic media the nZVI is protonated while in alkaline medium, nZVI surface is deprotonated. When the pH is low then the surface of nZVI gets positive charge while the dye molecule gets negative charge. These oppositely charged ions on nZVI and dye molecule attract each other. While on the higher pH, the ferrous ions are attracted towards the nZVI and occupy the surface by dissolving in it. Then these ions are become capable to combine with hydroxyl ions to produce oxides and hydroxides of Fe(II)and Fe(III) in alkaline solution. The precipitates of these compounds fill the nZVI surface and block the active sites which in turn inhibit the reaction.

The pH effect on the ability of degradation of initial stock solution by nZVI is shown in the Fig. 5 with the initial concentration of dyes solution 5%. The nZVI particles were used to observe the effect of pH on degradation of dye. The results depict that with the decrease in pH, the degradation efficiency of nZVI increases and it was maximum at pH 3 to 6. The degradation with maximum value was observerd at pH 4 with 49.6% dyes degradation due to the reduction of nZVI as shown in Eq. 2-5. At maximum pH 7, the cause of decolourization is the sorption of reactive dyes on the surface of ferrioxy hydroxide (FeOOH) which is actually formed onto the surface of nZVI particles. In fact, in aqueous media, iron undergoes chemical reaction with oxyhydroxide.

(2)

(3)

(4)

(5)

The results clearly show that the dye is effectively degraded by nZVI at lower pH but at high pH, it does not give appreciable results. The pH of the solution has very complicated effects on the rates of photolytic oxidation reactions. The lesser degradation of dye in alkaline media can be explained by the fact that some products are formed on the surface of the particles of nZVI which in turn reduce the reduction ability of the particles. This depicts that the colour removal efficiency is maximum in acidic media (at pH 4) Fig. 5,6. My experiment was in accordance with Chatterjee et al. [3].

3.6. Optimization of nZVI dose
Various quantities of nano zero valent iron ranging from 0.01g to 0.3g were used to check the effect of dosage on the degradation of the reactive dyes in the synthetic solution. The reaction was performed at 25^oC, at pH 5 for about 90 min. It was noted that decolourization of the dye was enhanced linearly with the increase in nZVI dosage from 0.01 g/100mL to 0.1 g/100 mL. The reason is that when the number of nZVI particles increases, the surface area also increases which offer more active places for the reactants and dye is degraded more effectively with the increased number of nZVI particles.

Fig. 5(a-e).	Decolorization of synthetic effluent by artificial photocatalysis treatment at (a) pH 3 (b) pH 4, (c) pH 5, (d) pH 6 and (e) pH 7

Fig. 6.	Accumulative results of decolorization of synthetic effluent by artificial photocatalysis treatment at different levels of pH

Fig. 7(a-e).	Decolorization of synthetic effluent by artificial photocatalysis treatment at (a) 0.01 M ZVI, (b) 0.04 M ZVI, (c) 0.07 M ZVI, (d) 0.1 M ZVI and (e) 0.3 M ZVI

Fig. 8.	Accumulative results of decolorization of synthetic effluent by artificial photocatalysis treatment at different ZVI doses

But this linear relationship sustains upto a certain limit because at a very high concentration of nZVI, the nZVI particles aggregate to form larger particles and thus its degradation capacity decreases. The maximum noted decolourization capacity was 0.1 g/100 mL [13] (Fig. 7,8).

3.7. Effect of temperature
Temperature plays an important role for the elimination of dyes from the synthetic solution with the help of nZVI. The degradation of mixture of reactive dyes was studied at the temperature of 30^oC, 40^oC, 50^oC, 60^o and 70^oC. The experimental results clearly depicts that the rate of decolourization reaction is strongly affected by temperature. The results showed that the degradation increases by increasing the temperature first but after very short interval, it started to decrease (Fig. 9,10). The optimum temperature observed during the experiment was found 50^oC after that the degradation process slows down. In other words, nZVI particles show maximum working at low temperature and working efficiency decreases at high temperature [10].

Ahuja et al. discussed the thermal stability of nZVI particles. It was shown that the nZVI particles are highly stable at lower temperature but as temperature increases, slight weight loss was observed and this weight loss was more obvious at high temperature due to the decomposition of materials at high temperature [1]. Mielczarski et al. observed the decomposition of the dyes in aqueous media by nZVI at pH range 4 to 5 with the temperature range 20 to 50 ^oC. They declared that various types of corrosion products are the main cause of the acceleration of the degradation of dye at higher temperature [12].

Fig. 9(a-e).	Decolorization of synthetic effluent by artificial photocatalysis treatment at (a) 30oC, (b) 40oC, (c) 50oC, (d) 60oC, and (e) 70oC

Fig. 10.	Accumulative results of decolorization of synthetic effluent by artificial photocatalysis treatment at various temperatures

Fig. 11(a-e).	Decolorization of synthetic effluent by artificial photocatalysis treatment at (a) 0.01M H2O2, (b) 0.02M H2O2, (c) 0.03M H2O2,(d) 0.04M H2O2 and (e) 0.05M H2O2

Fig. 12.	Accumulative results of decolorization of synthetic effluent by artificial photocatalysis treatment at various concentrations of H2O2

3.8. Optimization of H₂O₂
H₂O₂ behaves as a good oxidizing agent in photocatalytic reactions. H₂O₂ is of low cost and therefore has great importance in decolorizing the synthetic dye solution by artificial photocatalytic reactions as well as by solar assisted photocatalysis reactions. After the series of experiments, the optimal value of the H₂O₂ concentration was found. It is clear from the experimental data that with the increase in H₂O₂ concentration above the optimum value, the degradation of dye solution decreases because of rummage property of OH^- radical. Experiments show that degradation of dye increases for increasing value of H₂O₂ from 0.01 M to 0.04 M and then start decreasing at 0.05 M (Fig. 11). The optimum concentration of H₂O₂ was 0.03 M. At this concentration maximum degradation of dye was observed. H₂O₂ plays an efficient role in the degradation process but it has a prominent drawback that it cannot be used in basic medium because as the pH increases from 7, it decomposes into oxygen and hydrogen and thus its oxidizing capability vanishes (Fig. 12).

The major source of OH^- ions is the chemical reaction of hydrogen peroxide with Fe⁺² ions. Theoretically, greater amount of hydrogen peroxide must produce more amounts of active OH^- ions if ferrous ions do not act as limiting factors during the reaction. However in the present work, no such phenomenon is seen. In this work, the increased concentration of H₂O₂ has very bad and negative effects in Fenton’s process. This was reported elsewhere [22,24]. In Fenton reactions, the excess amount of hydrogen peroxide than the critical value results the aggregation of OH^- ions and thus OH is depleted. This decrease in the OH^- ions is due to the rummage property of these ions.

3.9. Photocatalysis treatment using UV/visible light source
The effect of light intensity from a UV source on the degradation of dye was studied at 0.05% dye solution, pH 4.0, 0.1 g of nZVI. The results clarify that the degradation rate as well as degradation efficiency increases linearly with the light intensity. Actually as the light intensity increases, the high energy photons are generated and these photons provide energy to the electrons present in valence band. These electrons after taking so much energy are excited into the conduction band of the semiconductor photocatalyst. The wavelength of the photon and the energy of photons are related to each other and the overall energy input for the photocatalytic reactions is based on the intensity of light. A number of experiments were performed using artificial light with various intensity levels such as 500 lux, 1000 lux, 1500 lux, 2000 lux and 2500 lux. It is clear from the experimental data that with the increase in light intensity from 500 lux to 2000 lux, the degradation also increases and above 2000 lux, there is no significant increase in the degradation of the dye (Fig. 13,14).So for the further experiments, the 2000 lux light intensity was taken as optimum value for thedegradation of synthetic dye solution. The present study results are in accordance with literature [23].

3.10. Solar-Assisted photocatalytic treatment:
The experiment was performed at optimized conditions like concentration of dye solution was maintained at 5%, nZVI dose at 0.1g, pH at 4.0, H₂O₂ at 0.03M and temperature at 50^oC. The treatment was carried out for 90 minutes and after every 15 min, degradation values were observed. The maximum decolourization was noted in first 75 min (Fig. 15,16). The role of H₂O₂ was as bleaching agent while that of UV radiation or solar was used for photochemical treatment of textile dyes [7].

Fig. 13(a-e).	Decolorization of synthetic effluent by artificial photocatalysis treatment at (a) 500 lux, (b) 1000 lux, (c) 1500 lux, (d) 2000 lux and (e) 2500 lux

Fig. 14.	Accumulative results of decolorization of synthetic effluent by artificial photocatalysis treatment at different light intensities

Fig. 15(a-c).	Decolorization of synthetic effluent by artificial photocatalysis treatment (a) Forenoon, (b) noon and (c) after noon

Fig. 16.	Accumulative results of decolorization of synthetic effluent by artificial photocatalysis treatment at different interval of a day

The usage of UV radiations in degradation of synthetic solution of dyes in the presence of H₂O₂ was proved a very promising and good technique. From the experimental results, it is clear that with the decrease in pH, the oxidation or degradation also increases and the pH 4 was observed as optimum value where maximum degradation was noted. The effect of solar light was investigated at this optimum value of pH. The comparative study for the decolourization of synthetic solution of dyes was done using nZVI as catalyst in the presence of solar light. The degradation was studied using UV light in comparison with solar light. The results depicted that the percentage decolourization of synthetic solution of different dyes was very close to the values obtained by using solar light. So it was cleared that solar light could be used efficiently for the photocatalytic decolourization of the synthetic solution of different reactive dyes. The efficiency of photocatalytic reactions in the presence of H₂O₂ using UV and solar radiation to decolourize synthetic solution of reactive dyes was studied. It was found that the use of nZVI in the presence of solar light for degradation of dyes solution is an economical process rather than the use of artificial light which is hazardous as well as of high cost [14].

4. CONCLUSION

The advanced oxidationprocess like artificial photocatalysis, solar assisted photocatalysis were used in this research work to check decolourization and mineralization of reactive dyes. Different experimental parameters like dye concentration, pH, H₂O₂, temperature, nZVI dose and light intensity were optimized. Efficient reactive dyes degradation in aqueous solution was achieved with nano scale zero valent iron particles. Dye removal was increased in acidic medium (pH4). Optimum parameters were obtained for nZVI dosage at 0.1 g/100 mL, dye concentration at 5% of stock solution, initial concentration of H₂O₂ at 0.03M and pH4 at room temperature. Under the optimum condition, the dye degradation was investigated in solar light and in artificial assistance UV light. During the study, it was monitored that solar assistance process was more proficient in degradation of reactive dyes with respect to artificial method owing to the production of large number of OH radicals by the UV irradiation and allowed the nearly total decolourization of reactive dyes solution.

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