Optimizing photo-mineralization of aqueous methyl orange by nano-ZnO catalyst under simulated natural conditions
© Zyoud et al.; licensee BioMed Central. 2015
Received: 3 December 2014
Accepted: 11 May 2015
Published: 17 May 2015
Photo-degradation of organic contaminants into non-hazardous mineral compounds is emerging as a strategy to purify water and environment. Tremendous research is being done using direct solar light for these purposes. In this paper we report on optimum conditions for complete mineralization of aqueous methyl orange using lab-prepared ZnO nanopowder catalyst under simulated solar light.
Nano-scale ZnO powder was prepared in the lab by standard methods, and then characterized using electronic absorption spectra, photolumenscence emission (PL) spectra, XRD, and SEM. The powder involved a wurtzite structure with ~19 nm particles living in agglomerates. Photo-degradation progressed faster under neutral or slightly acidic conditions which resemble natural waters. Increasing catalyst concentration increased photodegradation rate to a certain limit. Values of catalyst turn over number and degradation percentage increased under higher light intensity, whereas the quantum yield values decreased. The photocatalytic efficiency of nano-ZnO powders in methyl orange photodegradation in water with solar light has been affected by changing the working conditions. More importantly, the process may be used under natural water conditions with pH normally less than 7, with no need to use high concentrations of catalyst or contaminant. The results also highlight the negative impact of possible high concentrations of CO2 on water purification processes. Effects of other added gaseous flows to the reaction mixture are also discussed.
ZnO nano-particles are useful catalyst for complete mineralization of organic contaminants in water. Photo-degradation of organic contaminants with ZnO nano-particles, methyl orange being an example, should be considered for future large scale water purification processes under natural conditions.
Purification of water from hazardous chemicals is an important research area. Organic contaminants, such as industrial dyes, halocarbons and phenol derivatives, are among the main contaminants that demand complete safe removal . Different strategies are being investigated for water remediation, including biological treatment [2, 3], ultra-filtration , adsorption methods  and others. Such methods may not be favored as they may not cause complete mineralization of the organic contaminant. They simply transfer the pollutant from one phase to another . Advanced Oxidation Processes (AOP) have been proposed as alternative routes for water purification. Among those, oxidation via ozone or hydrogen peroxide has been reported as an effective technique [7–11]. Unfortunately, such methods may be costly, as ozonation demands artificial UV radiations, and hydrogen peroxide is not available free of charge. Contaminant complete mineralization with natural solar light seems to be the most practical process for future water purification. A semiconductor photo-catalyst speeds up the action of light by first absorbing photon and producing electrons and holes . With the abundance of costless solar radiations, a low cost catalyst may thus be useful. Different semiconducting materials, in the powder form, have been assessed as photo-catalysts [13, 14]. TiO2 in its anataze form is the most widely used effective photo-catalyst for its high efficiency, photochemical stability, non-toxic nature and low cost. It has been described for degradation of a wide range of organic contaminants [15–22]. Zinc oxide ZnO is a semiconductor with a comparable band gap ~3.2 eV (with wavelength shorter than 400 nm), but has been investigated to a lesser extent in water purification. ZnO is evaluated in many advanced applications such as field-effect transistors, lasers, photodiodes, chemical and biological sensors and solar cells, but to a lesser extent in photo-degradation catalysis [23–26]. One main advantage for ZnO is that it absorbs a larger fraction of solar spectrum, than TiO2 does . The performance of ZnO in degrading a number of organic contaminants has been reported [28, 29]. The quantum efficiency of ZnO nano-particles in photo-degrading organic contaminants process is higher than that of TiO2 [30, 31], due to its higher absorptivity in waves shorter than 400 nm, which accounts to about 5 % of the reaching solar light.
Methyl orange, with molecular structure shown in Fig. 1, is a dye that is believed to be mutagenic . It slightly dissolves in water. Its color changes with pH, from yellow (at pH higher than 4.4) to red (at lower pH values), and therefore it is used as an indicator . Methyl orange is also used as a dye in textile industry . It is an example of the widely spread azo dies , which are resistant to complete biodegradation . For these reasons, methyl orange is commonly used as a model dye to study in environmental cleanup, and this work is no exception.
In earlier study [37, 38], we reported on using commercial ZnO powders as catalysts in photo-degradation of methyl orange. As mentioned above, this work is intended to find optimum conditions for using ZnO nano-particles in methyl orange photo-degradation under natural water conditions, where ZnO is added to contaminated waters and allowed to function on its own under direct solar light.
Hydrochloric acid, sodium hydroxide, and methyl orange were purchased from Merck. ZnO powder was prepared in the lab to obtain small particle sizes, as described earlier [39, 40]. A ZnCl2 solution (250 mL, 0.25 M) was drop-wise added (within 40 min) to NaOH solution (200 mL, 0.90 M) with continuous stirring. The system was then left to settle, and the supernatant was decanted. The resulting precipitate was washed with water many times to remove any remaining ions. Enough amount of distilled water was added to convert the precipitate into slurry. The slurry was centrifuged at 6000 rpm for 10 min, and the supernatant was carefully decanted leaving the solid catalyst which was dried at 120 °C.
The CO2 gas was prepared by adding concentrated HCl solution (5 M) drop-wise to Na2CO3 solid in a stoppered flask with only one outlet. The outlet was connected with a glass tube bubble the CO2 through the reaction mixture at a flow rate 90 mL/L.
A 400 W Osram Tungsten Halogen lamp was used as a source for solar simulator light. The lamp spectrum is a bell curve typical with little (~5 %) in the UV region, just like natural solar light that reaches earth. A light meter (Model lx-102) from Lutron was used to measure the radiation intensity at the reaction mixture surface. A Shimadzu UV-1601 spectrophotometer was used to measure remaining methyl orange concentration using calibration curves, pre-prepared at different pH values, as methyl orange spectra may change with pH value.
The electronic absorption spectra were measured on a Shimadzu UV-1601 spectrophotometer for ZnO powders as suspensions in minimal water amounts. Photoluminescence (PL) Emission Spectra were measured for aqueous suspensions of ZnO powder on a Perkin-Elmer LS50 Luminescence Spectrometer. Excitation wavelength 325 nm was used. XRD patterns were measured on a Philips XRD XPERTPRO diffracto-meter with Cu Kα radiation (λ = 1.5418 Å) located in the labs of Dansuk Industrial Co., LTD., South Korea. Field Emission-Scanning Electron Micrographs (FE-SEM) were measured on a Jeol Model JSM-6700 F microscope, in the labs of Dansuk Industrial Co., LTD. South Korea. Atomic absorption spectra (AAS) were used to measure zinc ions resulting from possible degradation of ZnO. The AAS results were measured on an ICE3000 Thermoscientific Atomic Absorption Spectrophotometer equipped with a zinc lamp.
Photo-catalytic experiments were conducted under direct irradiation from the solar simulator lamp. Water samples pre-contaminated with known concentrations of methyl orange were placed inside a 250 mL beaker. Known nominal amounts of the catalyst ZnO powder were added. The mixture was magnetically stirred with a magnetic bar for 15 min in the dark, to allow adsorption equilibrium and to assess amounts of adsorbed methyl orange on the solid catalyst. The pH of the reaction mixture was controlled by adding drops of dilute HCl or NaOH solutions. The solar simulator lamp, vertically clamped above the solution with an adjustable stand, was then switched on with continuous stirring. The desired irradiation intensity on the mixture surface was achieved by controlling the lamp distance. The reaction time was calculated the time the lamp was switched on. Certain experiments were conducted in duplicate to check the reproducibility of the process.
The reaction progress was followed by measuring the amount of remaining methyl orange with time. This was performed by syringing out small aliquots of reaction mixture at certain times. The aliquots were then centrifuged at high speed (5000 rpm) for 5 min. The liquid phase was then carefully syringed out and analyzed spectrophotometrically at 480 nm.
Results and discussion
Zinc oxide powder characterization
Methyl orange photo-degradation
Exposure of aqueous solutions of methyl orange to solar simulator lamp, in the presence of ZnO nano-powder, caused appreciable de-colorization of methyl orange solution in soundly short times. Control experiments conducted in the dark, using catalyst while keeping other conditions the same, showed no detectable de-colorization. This means that no degradation occurred in the dark, and that the ZnO powder adsorbs only little fraction of the contaminant. Control experiments conducted with irradiation in the absence of ZnO did not show any noticeable de-colorization with prolonged exposure. This indicates the necessity of the ZnO particles to activate the methyl orange degradation. Using a cut-off filter (that blocks light 400 nm and shorter) caused severe lowering in methyl orange degradation. Collectively, these results indicate that light waves shorter than 400 nm are the driving force for the degradation of methyl orange, and that the ZnO particles are needed to observe the degradation process. Degradation is thus due to the shorter wave length tail available in the lamp light. With a band gap more than 3.2 eV, ZnO powder catalyst employs photons with wavelength shorter than 400 nm in the photo-degradation process.
In order to assess the ability of ZnO to photo-degrade aqueous methyl orange under simulated natural conditions different parameters were investigated here. Effects of different reaction parameters, such as pH, catalyst concentration and contaminant concentration, on rate of photo-degradation were reported earlier . The parameters (initial nominal pH, methyl orange concentration and ZnO amount) have been revisited here together with other new parameters (aqueous CO2 concentration, temperature, light intensity and catalyst reuse).
The remaining methyl orange concentrations were plotted with exposure time. The catalytic efficiency is better understood in terms of turnover number, T.N. (degraded molecules/Zn atoms) and quantum yield, Q.Y., (degraded molecules/total incident photons) measured after 30 min exposure to radiation.
Effect of pH on photo-degradation process
Because the amphoteric nature of ZnO, it is necessary to study the effect of the pH value on the methyl orange photo-degradation process. This is also necessary as natural waters normally have pH values in the range 5–8 [56, 57].
The pH value affects the ZnO surface OH groups , the methylene orange and the aqueous solution species. The pH value affects the generation of the oxidizing species (•OH, O2•‾, H2O2 and HO2•) that result in the reaction system [59, 60]. The nature of methyl orange molecule varies with pH value, as stated above. ZnO has a point of zero charge at pH 9.0, above which the ZnO surface is predominantly negatively charged . The electrical properties of the ZnO surface may thus vary with the pH.
Effect of pH on photo-degradation of aqueous methyl orange. Reactions were conducted using methyl orange solutions (100 mL, 10 ppm) and ZnO (0.1 g) at 20 °C under total radiation intensity 19.0 mW/cm2
1.62 × 10−3
1.94 × 10−3
1.17 × 10−3
8.42 × 10−4
6.71 × 10−4
4.36 × 10−4
4.28 × 10−4
2.92 × 10−4
2.12 × 10−4
1.74 × 10−4
In basic media, ZnO becomes Zn(OH)2 form with lower semiconducting properties . Under lower pH conditions the lowering in removal efficiency is possibly due to the dissolution of ZnO into Zn2+ ions , and in highly basic media the ZnO yields zincate ion ZnO2 −2 . The results show that the optimum photo-degradation is close to natural water conditions (neutral to slightly acidic), which adds to the credibility of using ZnO catalyst system. Therefore, unless otherwise stated, all results described here-in-after were obtained under initial nominal pH 7.
Effect of temperature
Effect of temperature on methyl orange photo-degradation. Reactions were conducted using methyl orange solution (100 mL, 20 ppm) at different temperatures, using ZnO catalyst (0.1 g) under total irradiation intensity of 19.0 mW/cm2 at pH ~7
1.86 × 10−3
2.38 × 10−3
2.87 × 10−3
2.80 × 10−3
3.52 × 10−4
4.28 × 10−4
5.44 × 10−4
5.29 × 10−4
If higher temperatures (above 45 °C) are used, the reaction goes slower. This is due to possible escape of the oxygen molecules dissolved inside oxygen. The oxygen molecules are involved with the mechanism of photodegradation reaction. Similar results were observed in earlier reports .
Effect of CO2 and other gas flows
Effect of gas streams on methyl orange photo-degradation. Reactions were conducted using methyl orange solution (100 mL, 20 ppm) under 19.0 mW/cm2 irradiation using ZnO (0.1 g) with continuous stirring at 20 °C
Exposed to air only
N2 flow open system
CO2 and air flows together
2.87 × 10−3
1.72 × 10−3
1.44 × 10−3
0.75 × 10−3
1.03 × 10−3
5.44 × 10−4
3.25 × 10−4
2.72 × 10−4
1.42 × 10−4
1.95 × 10−4
CO2 gas stream may arguably affect photodegradation rate by removing oxygen dissolved in the reaction mixture. This was investigated using two continuous streams of CO2 and air together. Figure 9 shows that using the air (1000 mL/min) stream with the CO2 stream (90 mL/min) did not show significant enhancement in photo-degradation reaction rate. This result further confirms the discussion above, where CO2 captures the free radicals necessary for the photodegradation to occur.
Adding a stream of air alone did not increase the reaction rate. Figure 9 shows that the air stream lowered the reaction rate by about 25 % compared to experiments conducted under normal air. In a well known mechanism , oxygen molecule is assumed to abstract one electron from the excited ZnO particle leading to O2•‾ radical anion species as explained in the equation [e‾ + O2 → O2•‾]. The O2•‾ species is believed to react with the other H2O2 resulting species to yield the chemically active OH. radical as in the equation [H 2 O 2 + O2•‾ → OH• + OH− + O 2 ]. This radical is assumed to react with an organic contaminant molecule and degrade it.
Therefore, the presence of O2 in water is necessary. However, as the results here show, excess of O2 in water inhibits the photodegradation reaction. This is due to the adverse effect of excess O2 which blocks OH• radical as shown in equation [H 2 O 2 + O2•‾ → OH• + OH− + O 2 ].
Adding nitrogen to the reaction mixture together with CO2 practically stopped the reaction progress (Fig. 9). The effect is perhaps dual in nature, where the CO2 behaves as scavenger, as discussed above, while the nitrogen lowers concentration of oxygen that is necessary for the reaction to proceed. Adding a nitrogen flow (240 mL/min) to the reaction mixture, while exposed to atmospheric air, slowed down the process, but did not stop it completely. Nitrogen lowers concentrations of oxygen in the reaction but traces are left therein which are enough for the reaction to occur. All reactions above were conducted under exposure to atmospheric air. This is evident because complete coverage from air while under nitrogen stream caused complete reaction inhibition.
Effect of contaminant concentration
Effect of contaminant concentration on its photo-degradation. Reactions were conducted using different contaminant concentrations in aqueous solution (100 mL), pH ~7, at 20 °C with ZnO (0.1 g) under 19.0 mW/cm2 irradiation intensity
1.73 × 10−3
2.38 × 10−3
2.37 × 10−3
2.29 × 10−3
4.36 × 10−4
4.28 × 10−4
4.48 × 10−4
4.17 × 10−4
Effect of catalyst nominal amount on photo-degradation process
Effect of catalyst amount on methyl orange photo-degradation. Reactions were conducted using methyl orange solution (100 mL, 10 ppm) at 20 °C pH ~7, under 19.0 mW/cm2
3.10 × 10−3
1.68 × 10−3
9.82 × 10−4
9.57 × 10−4
2.93 × 10−4
3.19 × 10−4
3.72 × 10−4
3.62 × 19−4
The QY value showed an increase with increasing nominal catalyst amount at the beginning, but then slightly decreased. Again the lowering in QY is due to the blocking of light by the abundant ZnO particles at the mixture surface which prevent photons from reaching other catalyst sites [14, 37, 53, 54, 65–67]. Agglomeration of smaller ZnO particles into larger ones, in case of higher concentrations, may also play a role, as the total number of surface active sites may decrease . The results indicate that using smaller amounts of catalyst enhanced catalyst efficiency without lowering the average reaction rate or the removal percentage. This is a positive feature of the ZnO catalyst described here, as in case of treating natural waters, smaller amounts of catalyst will be highly favored.
Effect of light intensity on the photodegraation process
Effect of light intensity on methyl orange photo-degradation. Reactions were conducted using two different methyl orange concentrations in 100 mL solution at pH ~7 with ZnO (0.1 g) at 20 °C
6.96 × 10−4
1.5 × 10−3
1.58 × 10−3
1.73 × 10−3
1.42 × 10−3
1.35 × 10−3
6.57 × 10 −4
4.36 × 10−4
7.96 × 10−4
1.86 × 10−3
1.99 × 10−3
2.38 × 10−3
1.62 × 10−3
1.41 × 10−3
8.76 × 10−4
4.28 × 10−4
For the 20 ppm methyl orange concentration, similar behavior occurred, but the irradiation intensity 5.12 mW/cm2 showed highest QY. Collectively the results suggest that it is not necessary to use high irradiation intensities to remove methyl orange from water. This adds to the applicability of using ZnO in natural water purification processes in different environments with different light intensities.
Catalyst re-use experiments
The amount of Zn2+ ions resulting from dissolution of the used ZnO catalyst (with different amounts 0.05–0.3 g per 100 mL solution) during photo-degradation experiments in neutral media was measured by AAS, and was found to be 6 ppm, when the mixture was left overnight. In case of more acidic media the amount was higher, up to 8 ppm. This indicates that only a small fraction of ZnO dissolved under the working conditions. Based on literature  the value for solubility of Zn ions resulting from nano-scale ZnO is about 7 ppm. The WHO recommended upper limit for Zn ions is ~5 ppm . The Zn2+ ions dissolved in this work is not far from the recommended WHO threshold limits. The results add to the credibility of using ZnO in purification of natural waters.
Nano-scale ZnO particles can be effectively used as catalysts for complete mineralization of methyl orange in water with solar simulated light. The catalyst can be effectively used under different working conditions (including temperature and pH) that resemble natural waters, and can thus be investigated at larger scale in natural water purification. Adding streams of air, CO2 gas, and/or N2 gases may affect the reaction progress and may inhibit the reaction.
The thrust of this work has been done at SSERL, Department of Chemistry, ANU. The authors wish to thank the technical staff at ANU for help. XRD and SEM measurements were performed in the laboratories of Dansuk Industrial Co., LTD., South Korea.
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