INTRODUCTION:

During last few decades, water pollution became one of most important world problem that threating our environment. Most of pollutants are generally textile and chemical industries using synthetic organic dyes. So that treatment methods are intensively required to remove or at least minimize the effect of pollutants that are affected on the quality of water (drinking or river water), increasing the maintenance cost that is required for drinking water network by making pipes or drains leakage. In some cases this polluted wastewater when reaching into the ground can lead to growth of bacteria and virus¹.

Dye-colored wastewaters effluents enter into the aquatic ecosystem represent both environmental and public health risks because of toxicological effects and bioaccumulation in wildlife. Many dyes can decompose into aromatic amines which are classified as carcinogenic compounds under aerobic conditions which can cause serious damage to humans and animals². Dyes can cause allergy, dermatitis, skin irritation and cancer in humans³. Besides that, colored water especially in the surface prevents light penetration and it may affect photosynthesis and threat aquatic life⁴. There are many treatments technologies that can be used for treatment of dye pollution such as biological methods, coagulation, Nano filtration electrochemical process, activated carbon adsorption and catalytic oxidation processes. However, these methods were found to produce non-harmful small molecules like CO₂, H₂Oand some other inorganic species^5-8.

The photocatalytic degradation techniques are widely used to remove dyes using photocatalyst, in this context using photocatalyst semiconductors is well known and has shown great utility in the full mineralization of organic pollutants. So that, these materials can be considered as promising techniques for treating many current environmental issues^9,10. Heterogeneous photocatalysis reaction is carried out in the presence of a catalyst and light with a proper energy. In this case photocatalytic reaction is initiated by absorbing energy that equal or more than its bandgab. This energy can promote electrons from covalent band to conductive band degrade and trapped by oxygen. The latter is active species (O₂^.-) has ability to destroy dyes, organic acids and crude oil stain in the sea, so inorganic molecules like nitric oxide and removing metals such as chromium¹¹. Photocatalysis technique using catalyst/light such as TiO₂ and ZnO are extensively used for treatment of pollution, self-cleaning of surfaces, solar cells, photocatalytic oxidation, clean fuel production and water purification^12-14. The catalytic action is promoted by absorbing light and many species can be formed which makes action on the adsorbed dye at surface of catalyst and finally the product leaves the surface¹⁵. Photocatalysis processes are mainly depended on species that are formed like OH radicals that are participated in redox reactions with the species that are adsorbed at the surface of the used catalyst. These radicals can be produced by reaction of the positive holes (h⁺) with adsorbed water molecules¹⁶. Generally, improvement surface properties can be performed achieved by applying some methods. One of these methods id the doping of the photocatalyst using some metals dopants. This can lead increase charge separation lifetime and hence increase the efficiency of the photocatalytic process. However, photocatalysts can be modified by loading with metal ions such as Fe³⁺, Cr³⁺ and Ag⁺ and here different methods can be used such as impregnation and photodeposition method. Doping of some metal dopants within the photocatalyst can generate a lower new energy level of conduction band. This newly formed secondary energy levels would ease electron excitation by lowering bandgap energy than that is required by the pure^17-19.

Generally, surface properties of the photocatalyst can be modified via utilization of different methods such as doping transition noble metals^20-22 composite semiconductors^23,24 and substituting oxygen with anions²⁵. The doped ions also act as trapping sites for electrons and hole, hence altering the recombination rate by maximize the life time^26-28. In this context, modification of the catalyst can impart s enhanced surface properties and these involve enhancement of light absorption, improvement of adsorption ability, reduction of recombination rate and to some extent increasing photo-stability²⁹. Znic oxide can be exposed to photo corrosion and this feature inhibits its activity as photocatalyst in acidic or alkaline media, so doping of some noble metals such as Ag, Pd, Pt, Au and Fe can increase its photo stability³⁰.

In current study, zinc oxide nanoparticles would be synthesized by sol gel method. Then the prepared ZnONPs would be modified by doping silver particle. Catalysts activity was investigated via following photocatalytic removal of CRD from aqueous solution. Different reaction parameters and conditions would be also investigated in this work.

EXPERIMENTAL:

Zinc oxide nanoparticles (ZnONPs) were synthesized according to Sol-Gel method. In this method, 10 gram of zinc chloride (ZnCl₂. 5H₂O, 99.95%, Fluka Co.). This was dissolved in 200 mL of distilled water with continuous stirring and the temperature was hold at 45 ºC. Then a solution of 0.1 N of sodium hydroxide (NaOH, 99.9%, B.D.H.) was added on the beaker’s wall of the reaction very slowly. Then the obtained white precipitate kept under stirring with continuous addition of NaOH for four hours until reaching pH=12, and the produced gel of zinc oxide was kept under normal air condition for overnight. The clear solution was removed by decantation and the gel washed with distilled water for carefully to insure removing of sodium ion then it was dried in oven at 100-110 °C for 24 hour. Then the prepared ZnONPs was investigated using XRD, EDX, SEM and FTIR.

Doping of Zinc oxide nanoparticles with silver particles was performed by mixing of 0.01 N of silver nitrate (99%, BDH Company) with 1 gram of zinc oxide using 100 mL of DW and this mixture was put in a beaker of 250 ml. This mixture was completed to a volume of 100 mL of DW. The obtained slurry was stirred continuously for 2 hours in dark. Then it was illuminated with UV light A 70 watt UV light candles (Philips) with continuous stirring until the slurry turned from white to gray color. The produced precipitate was filtered until no chloride was appeared as AgCl, then it was dried at 100-110 ºCfor overnight. The produced ZnONPs/Ag was investigated using FTIR, XRD, SEM and EDX.

Congo red dye that was used in the current study has a molecular formula of (C₃₂H₂₂N₆Na₂O₆S₂); and it has a molecular weight: 696.66 g/mol. This dye was purchased from BDH Company (99.9%). Simulated textile wastewaters of this dye was prepared using DW and the chemical structure of CRD is represented in Figure 1.

Figure1. Chemical structure of the used dye (Congo red dye)

SEM was used to study surface morphology of the prepared catalyst. It was a Scanning Electron Microscope Inspect 550, Netherland. FTIR spectroscopy was used to investigate functional groups of the synthesized materials. FTIR spectra were recorded by using FTIR Perkin Elminer Spectrophotometer. All analyzed samples were dried carefully and then mixed with KBr powder to make them as pellets prior to perform run. Crystalline structure of ZnO and Ag/ZnO was conducted using Simadzu-6000 X-rays diffractmeter. All XRD patterns were recorded in the range of 2Theta= 20- 80 deg.

The results of the photocatalytic activity for removal of Congo red dye over ZnO and Ag/ZnO are represented in Figure 2. For each experiment, same amount of dye was used (100 mL, 50 ppm) over a suspension of the used ZnONPs or Ag/ZnONPs) catalysts (0.10 gram). Reaction mixture was mixed finely using magnetic stirror for 30 minute in the dark for each run in order to approach adsorption equilibrium. After that, irradiation was flushed with UV radiation using candles (Philips) 70 watt UV. Radiation. Illumination of reaction mixture was performed under normal air conditions. Analyzing reaction mixture over reaction time was conducted at each 10 minute by withdrawing of two milliliter of the irradiated mixture. This sample then was subjected to centrifuge at 5000 rpm, and this process was repeated for several times. The efficiency of dye removal over the used catalysts was conducted by measuring the optical density of the clear liquid at a wavelength of 496 nm. with Shimadzu 1180 UV-Vis Spectrophotometer.

Figure 2. Apparatus of homemade photoreactor performing photocatalytic reactions

RESULT AND DISCUSSION:

The obtained results of SEM and EDX are summarized in Figures 3 and 4. These images showed formation an agglomerate powder of both of these materials with an average scale of particle size less than 100 nm. Also these images showed very fine powder of both ZnONPs and Ag/ZnONPs. On the other hand, Energy dispersive X- rays (EDX) was used to conduct the analysis of the prepared materials and the obtained measurements reveal that the catalyst takes the formula Zn_0.49O_0.49Ag_0.02 as shown in Figure 4.

Figure 3. SEM images for ZnONPs and ZnONPs/ Ag prepared catalysts

Figure 4. EDX analysis results for ZnONPs and ZnONPs/Ag catalysts.

The XRD patterns of each of ZnONPs and ZnONPs/Ag are presented in Figure 5. From XRD patterns of ZnO and according to its main peaks it is showed wurtzite type hexagonal structure; (JCPDS 36−1451 card) for Ag-doped ZnO sample^31,32. The peaks at 2θ of 31.7°, 34.5°, 36.2°, 47.5°, 56.5°, 67.94, 69.08, 72.5 and 76.95 are corresponded to the crystal planes (100), (002), (101), (102), (110), (112), (201), (004) and ( 202) of crystalline ZnO. Generally, the percentage of Ag is around 0.02% and this showed a weak diffraction peak at 2θ=38.8 (111) plane). This peak is corresponded well with fcc crystal structure of silver according to JCPDS 01-087- 0720 card^31,32.

Figure 5. XRD patterns of ZnONPs (above) and ZnONPs /Ag (below).

The peak positions or diffraction peaks from any other chemical species, such as silver oxides, was not observed in any of the modified sample. The crystal sizes of all of ZnO and ZnO-Ag catalyst are calculated according to Scherer equation³².

D = mλ/B cosθ 1

According to the above equation, B is the half-height width of the diffraction peak, the symbol m is constant (0.94), θ is the diffraction angle, and λ is wavelength of x-rays that is corresponding to the CuKα radiation. The average particle sizes for ZnO and Ag/ZnO were estimated according to the above relation and these were in the range of 50 - 60 nm. Also, the diffraction peaks after doping silver were shifted to a red shift that due to insertion of silver particles at the surface of ZnO^31,32.

The results of FTIR spectra are presented in Figure 6. FTIR spectra of Ag/ZnO nanocomposite material exhibited some new bands in FTIR spectra for doped ZnO in comparison with FTIR spectrum for neat ZnO. The bands at the range of 900-1500 cm^-1 are attributed to the presence of stretching and bending of oxygen at the surface of the catalyst^33,34. The bands at 465 cm^-1 and 725 cm^-1are attributed to presence of ZnO and Ag ions ³⁴. In case of doping with relatively high level of Ag dopants, for this case band would be observed in the region of 750 cm^-1. This peak is confirmed the presence of doped silver particles within ZnO surface ³⁵.

Figure 6: FTIR spectra fort ZnO (a) and Ag/ZnO (b)

The part of Photocatalytic activity of dye removal over ZnO and Ag/ZnO was carried out via using a series of dye solutions using 100 ml at a concentration of 50 ppm. This mixture was suspended with 0.01 g. of catalyst under applying a pH of mixture of 6.7, and at a reaction temperature of 298 K. The obtained results showed that CRD was removed over ZnONPs with efficiency of around 22% after one hour of irradiation, on the other hand, the efficiency of CRD removal over silver doped zinc oxide was around 62% for the same reaction time and under applying same reaction conditions. The results showed that, the activity of dye removal in case of using ZnO/Ag was increased three times in comparison with that when using neat ZnO. These explanations probably arises to the role of silver particles in formation of active sites at the surface of zinc oxide. This factor can improve the photocatalytic processes and it reduce back electron transfer and retard recombination reaction. On the other hand, doped Ag particles ZnO can act as a sink for capturing of conduction band electrons and preventing their recombination with h⁺_VB. Thus they have a sufficient time to react with pre-adsorbed species and hence increase the efficiency of the photo catalyzed reaction. Generally, these results are in a good agreement with the similar previous studies that were reported in other studies^36,37. These results are presented in Figure 7.

Figure 7: Photocatalytic removal of CR dye over ZnO and ZnO-Ag catalysts

There is another way that azo-dyes may enter photolysis reaction and suffer from self-degradation or inject an electron from excited dye to sensitize the semiconductor and electron will transfer to its conduction band³⁸. From other hand, CR dye may cause a poisoning of ZnO surface which leads to deactivation of ZnO. Modification of the surface of zinc oxide using Ag particles can increase surface activity and stability against attacking by CR dye or its intermediate compounds through the degradation processes when reaction was proceeded more time until fully removal of color of CR dye. For this case the absorbance goes towards near zero and no more dye molecules are being available to be degradable as all dye molecules convert into intermediate molecules or to CO₂ and H₂O. However, this case can be observed after 180 minute of photocatalytic reaction as shown in Figure 8. This time is dependent on the dye kind and the intermediates that are produced through photocatalytic reactions^39,40.

Figure 8. Photocatalytic removal of CR dye over a suspension of ZnO.

To study the effect of the used masses of the catalysts, a series of experiments were conducted under the same reaction temperature with variation in catalyst concentration. The used catalytic materials were suspended in a dye solution (100 mL of 50 ppm). The results were recorded spectroscopic method as mentioned in above part. These results showed that, the efficiency of CRD removal was increased with increase the used mass of catalyst from 0.025 to 0.05 g., and under these circumstances the best results was noted at mass of catalyst around 0.05 g. which showed a removal efficiency around 95% as shown in Figure 9. This observation probably arises from absorbing a high ratio of incident photons in case of the presence of low masses of the of catalyst at this environment. For this case, the formed dense slurry don’t pass light into other part of reaction mixture and so that many catalyst particles don't absorb photons. In case of higher masses of the used catalyst (more than 0.05 g.) catalyst particles make an inner filter which prevents light from passing inside the solution where high concentration of particles work as a shield against the entering light. Thus only the particles outside that against light can absorbs light and the inner particles can’t do that. In these circumstances not all catalyst particles would be photoexcited and only those absorb photon can be excited and generated electron-hole pairs. This process can lead to reduce the activity of the photocatalytic reaction at the surface and hence yield negative results in comparison with lighter masses of the used catalyst^41-43.

Figure 9. Effect of catalyst masses on photocatalytic efficiency of CRD removal.

Removal of CRD molecules in the presence of a heterogeneous photocatalyst can be studied kinetically according to the Langmuir-Hinshelwood kinetics model as follows:

r_o = d_C / dt = kK_C /(1+ K_C) 2

In case of low concentrations of the used dye (Kc) , the denominator is neglected when (K_C<< 1), and integrating with respect to reaction time (t). This reaction is considered to be as the pseudo-first order kinetic model as shown in the following equation:

ln(C_o / C_t) = kKt = K_app t 3

According to the above relation, dC/dt is the degradation rate of dye molecules (mg/L×min), Co and Ct are represented of initial dye concentration and the concentration at a desired time (t) of the dye (mg/L) respectively. The term k, refers to reaction rate constant (min^-1), K is the adsorption coefficient of the dye molecules onto the photocatalyst particle (L/mg) and k_app is the apparent rate constant calculated from the curves (min^-1). From the effect of CR dye concentration on the photocatalytic decolorization^44,45. By increasing of dye concentration gradually starting from diluted concentrations, in this case the amount of dye molecules that were adsorbed on the surface was increased and all active sites were filled which affect the catalytic activity of photocatalyst. So that at high dye concentrations, the rate of degradation was decreased. This finding is probably arises from reduction in OH radicals formation under these circumstances as the active sites at the surface are expected to be fully occupied by the dye molecules^46,47. Moreover the dye itself absorb light at high concentration so the efficiency of light absorbing reduced and light can’t absorb by photocatalyst. The results showed that, this photocatalytic reaction is almost was pseudo-first order kinetics. The rate constant (k) for this reaction is lowered at high dye concentrations and this probably due to these circumstances, this reaction is basally related to light presence. Therefore, light absorbed by dye and work as a filter and no enough light is absorbed by other catalyst particles. The obtained results are presented in Figure 10.

Figure 10. Effect of dye concentration on the efficiency of dye removal over ZnONPs

Figure 11. Langmiur-hanshell wood plot for CR dye removal over catalyst.

Langmiur-hanshell wood mechanism is one of popular description of heterogeneous photocatalysis that can be expressed by arranging equation (2):

1/ K_app= 1/ k K + C_o / k 4

The kinetic of photodegradation of organic dyes is proceeded due to pseudo-first order kinetics model and for this type of reactions dye concentration is the most effecting parameter⁴⁸. According to above equation, kapp is a time independent constant ⁴⁹. By plotting 1/k_app verse C_o of CR a straight line and the slope is 1/k with intercept equal 1/Kk and The results are presented in Figure 11.

According to Arrhenius equation, the rate of photocatalytic reaction is mainly dependent on temperature of the reaction and so that increasing reaction temperature is expected to rise of the rate of photo catalytic dye removal over the used photo catalyst. The obtained results are presented in Figure 12. The apparent activation energy (Ea) can be calculated from the Arrhenius equation as follows:

kapp = A exp [-(Ea/RT)]

Figure 12. The reaction rate for removal of CR dye over the used catalyst at different temperature.

Figure 13. Arrhenius plot of ln k against inverse of reaction temperature in kelvin scale

The linear transform ln (kap) versus (1/T) is shown in Figure 13. This figure gives a straight line with a slope equal to -Ea/R. From the obtained data the apparent activation energy equals to 22.1 kJ/mol⁵⁰.

CONCLUSIONS:

Zinc oxide nanoparticle was synthesized using sol-gel method and photodeposition method was used to dope it with silver particles. The activity of the prepared materials was conducted by following photocatalytic removal of CRD from its simulated textile wastewaters. From the obtained results, Ag/ZnO was more active than ZnO and it was noted that, CRD removal was mainly dependent on some reaction parameters and conditions. These involving the amount of mass of the used catalyst, reaction time, pH of suspension mixture as well as temperature of reaction mixture.

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Received on 15.12.2018 Modified on 22.01.2019

Research J. Pharm. and Tech. 2019; 12(6): 2669-2676.

DOI: 10.5958/0974-360X.2019.00446.3