Advanced Oxidation Process treatment for azo dyes pollutants using ultra-violet irradiation

The research included studying the photocatalytic efficiency of zinc oxide onto photo reaction of Alizarin yellow (A.Y) in the presence of ultraviolet radiation. Several parameters affect the photolysis rate of dye were studied such as: effect of acidic function, different oxidants and supporting of zinc oxide surface. The solution of dye was irradiated using medium pressure mercury lamp 400 W. The results showed that the maximum percentage of dye photolysis was at a natural acidic function using hydrogen peroxide as oxidant and activated charcoal as support. The kinetics of reaction were also studied and the results proved that the reaction from pseudo first order.


Introduction
Pollutants can be defined as materials that change the characteristics of the environment negatively and exposure life of living with the risk [1]. One of the important properties of pollutants is it's the interaction with the other or with other materials which have a more severe impact than would be expected from the effect of each pollutant separately [2]. Therefore, pollution can be defined as any change in the natural conditions of the air, water and soil, which affect human, animal and plant. The environmental pollution can be classified into three types: air pollution, water pollution, soil pollution and radioactive contamination and other [1] - [3].
Water pollution is defined as the presence of any foreign substance, whether it was organic or radiological or biomaterial in the water, which is trying to reduce its quality and quality [4]. Water pollution is the physical or chemical or biological changes occur in the water and lead to change its quality so that it becomes harmful substance [5]. One of the water pollutants is pesticides, industrial dyes, heavy metals and toxic ions that represent a risk to human wellness. The pollution by dyes represents a major source of pollution of the environment that lead to be the case of undesirable not only because of their colors, but because a lot of secretions and outputs toxic incoherent that are carcinogenic [6], [7]. Among these dyes are azo dyes containing chromophoric azo group with double bond (-N = N-) that causes high stability of these dyes which make up more than half of the dyes used in the present time [8]. Alizarin yellow is the type of azo dyes also called Alizarian Yellow Sodium Salt or 5-(m-nitrophenylazo) Salicylic Acid Sodium Salt prepared by Rudolf Nietzki in 1887, producing from dialysis m-nitroaniline and associate with salicylic acid [9] In that respect are three methods employed to treat water pollution are [10]: physical, biological and chemical methods. The advanced oxidation process (AOP) is of the chemical methods and which is referring to a set of processing techniques designed to remove organic and inorganic materials in water and wastewater by oxidation [11]. These methods include the generation of highly effective radicals (especially hydroxyl radicals HO•), and are classified into heterogeneous methods which are include non-photo methods (  Instruments used: the structure of zinc oxide was examined using X-ray Diffraction Shimadzu (LabX XRD-6000), Infrared Spectra were recorded using Shimadzu (FTIR 8000 Series), the electronic spectra was scanned using Ultraviolet Spectrophotometer The conversion % was calculated as following equation: Where Cο = initial concentration of the dye and C = concentration of the dye at any time.
Results and discussion: 1. Study of X-ray diffraction spectrum X-ray diffraction technique was applied to study the crystallinity of the prepared catalyst calcined at 773 K and through one hour. It was noted that the values of the peaks correspond to the hexagonal (wurtzite) of zinc oxide [13], as exhibited in Figure 2. The crystal size of zinc oxide was measured using Debye Scherrer equation [14]:   The infrared spectrum of ZnO was recorded within the range (400-4000) cm -1 using KBr disk. A broad band of IR spectrum was noticed at 3400 cm -1 belongs to stretching vibration of the hydroxyl group. The absorption peak at 1625 cm -1 represents bending vibration of the hydroxyl group and the appearance of absorption bands between 501-401cm -1 represents stretching vibration of Zn-O [15], [16], as shown in Figure 3.

Study the electronic spectrum
The electronic spectrum of ZnO was determined. The appearance of absorption band at 390 nm [17] belong to band gap of ZnO due to transition of electrons from valance band to conduction band (O 2p → Zn 3d) [15].

Dark reactions
The reaction of the dye in the dark was studied in three cases: first case in the presence of oxygen only second case in the presence ZnO only and third case in the presence of oxygen and ZnO.
It has been observed that the highest percentage of dye conversion was using oxygen and zinc oxide equal to 10.296 % as in figure (7) and the highest value of the first order constant was 0.08x10 -2 min -1 .

Effect of pH
The study showed the effect of different values of the acidic function (pH =3, 5,7,9,11) onto A.Y dye using a concentration of 40 ppm and the presence of oxygen and zinc oxide and weight of catalyst 0.3 g. The solution was exposed to ultraviolet radiation for a period 120 min. It can be seen that the conversion % of dye increases with increasing pH values where gave the highest percentage of dye conversion was at the natural pH of the dye (pH = 8). The amphoteric manner of zinc oxide is very significant to hold the rate of reaction that takes space on the airfoil of the particles where the force on the airfoil of the catalyst charge. The zpc of ZnO is 9.0 ± 0.3this mean that the of surface zinc oxide is positive at pH less than 9 and be negatively charged at pH higher due to the adsorption of ions HO -. The presence of large amounts of HOon the surface of zinc oxide particles will form hydroxyl radicals, which the photo stimulation increases with increasing pH due to the dissolution of zinc oxide at low pH [21], [22]. Figure (8) referred that the maximum percentage of photolysis was 92.541% at pH = 8, having rate constant 1.9x10 -2 min -1 . The reason can be explained as a result of electrostatic interactions between the positively charged of the catalyst and anions of dye leading to strong adsorption of the dye on the zinc oxide .In alkaline media, the adsorption of the dye molecules is reduced and the potential causes of this behavior is to be a lot of hydroxyl radicals [21].

Effect of oxidants
The effect of different oxidants (potassium dichromate, persulfate potassium and hydrogen peroxide) was studied onto photoreaction of the dye with concentration equal to 40 mg/L for the oxidizer. the solution of dye was exposed to ultraviolet radiation for a period of 120 min the presence of 0.3 gm zinc oxide, it was noted that the highest percentage of conversion of the dye was by using of hydrogen peroxide which was 93.172% and the rate constant was 2.0x10 -2 min -1 as in figure (9).

Figure (9): conversion % of the dye at different oxidizers
Where the addition of hydrogen peroxide inhibits the recombination of electron-hole by the electron received from photolysis and coming from the conduction band of semiconductor which forms hydroxyl radicals, this electron is regarded as a strong oxidizer [23].

Effect of support onto surface of catalyst
The effect of the nature of the support onto the photo activity of catalyst was studied using different supports (alumina, silica, bentonite, activated charcoal). It has been found that the highest percentage of conversion shown (figure 10) was 99.779% when using activated charcoal and the rate constant was 1.3x10 -2 min -1 .

Figure (10): conversion % of the dye at different supports
This can be excused as the activated charcoal is a good adsorbent material because the high specific surface area. The basic unit of activated charcoal is carbon with the possibility of the existence of charge sites positively charged and the other a negative addition to high porosity [24]. While alumina gave a less percentage of conversion due to existence of positive charges therefor weak functional groups [25].