Strontium Hexaferrite / Zinc Oxide Nanocomposites: Study on Photocatalytic Discoloration of Orange II Dye

Orange II is an azo dye widely used in the textile industry. Its discharge into water bodies is harmful to the ecosystem and causes a health hazard. The removal of organic pollutants from wastewater by photocatalyzed discoloration processes has been thoroughly studied. Notwithstanding, it would be interesting to support the catalyst on magnetic materials to facilitate its recovery. In the present work, catalytic discoloration of Orange II was performed using zinc oxide nanopowder in aqueous suspension and novel composites of zinc oxide supported on magnetic materials. We report a one-step method for the synthesis of composites by precipitation of zinc oxide on hard strontium ferrites, which may improve catalyst remotion from water without decreasing its catalytic performance. Composites were structurally and magnetically characterized. The discoloration process, scanned by spectrophotometry for six hours, followed first-order kinetics with a reaction rate constant of 0.0142 1/min and a half-life of 49 min for strontium ferrite/zinc oxide in neutral media at 25 °C. Orange II significant adsorption on catalysts was discarded. The composites showed great stability in aqueous suspension and excellent photocatalytic properties compared with zinc oxide. They can be easily removed from water by magnetic decantation, with a significant recovery factor (70%).


INTRODUCTION
Orange II, also known as 2-Naphthol Orange and Acid Orange 7, is an important example of azo compound dye. It is widely used in wool dyeing and as a colouring agent in printing. Its discharge in water bodies after the industrial operations causes severe damage to aquatic environments and provokes health hazards.
Among the different azo dyes, Orange II was chosen for this study for many reasons. It could be easily prepared and purified in our laboratory. Its concentration in water can be easily measured by spectrophotometry because it has an absorbance maximum within the visible light a ZnO/Zn ferrite composite as a catalyst has been published by Coelho et al. [27]. In comparison, Mandal and Natarajan studied the catalytic properties of a composite of ZnO and a Zn-Fe mixed oxide [28]. Zinc oxide composite catalysts with hematite (Fe2O3) or with elemental iron were tested by Maya-Treviño et al. [29]. All these composite catalysts were prepared with soft ferrites. Notwithstanding, the photocatalyzed discoloration of an azo dye by ZnO supported on hard ferrite particles, as it is informed in this work, would constitute a novel study. To the best of our knowledge, no similar system has been reported yet.

MATERIALS AND METHODS
Both phases of the composites were prepared using wet methods in our laboratory.

Preparation of strontium hexaferrite/zinc oxide composites
Strontium hexaferrite particles were synthesized by a self-combustion method, and the hydrated ZnO was precipitated on the magnetic support.
Particles surface modification by citrate treatment. A certain mass of each ferrite, suspended in sodium citrate solution 0.50 mol/L, was stirred with an ultrasonicator for 2 h at 60 °C to disperse any cluster of particles. By this treatment, the surface of the particles was modified to improve zinc hydroxide (Zn(OH)2) coating. The two samples of modified ferrite particles were centrifuged and washed with acetone.
Zinc oxide precipitation. 10 mL of a zinc acetate dihydrate (Zn(CH3CO2)2·2H2O) 0.55 mol/L solution and 10 mL of ammonium carbonate ((NH4)2CO3) 0.80 mol/L were slowly and simultaneously added dropwise over 20 mL of surface-modified ferrites suspensions at 25 °C. This technique was adapted from a procedure previously published by Hong et al. to obtain a similar material [31].
Since magnetic stirring would cause aggregation of ferrite particles, the suspensions were mixed with nitrogen gas flow during the precipitation. The CO2 released in the reaction also contributed to the stirring.
Due to the ammonium carbonate addition, the change in the pH caused the hydrated zinc oxide (ZnO•xH2O) to precipitate over the modified surface of the ferrite particles. The composites were centrifuged and washed with distilled water, diluted ammonia and absolute ethanol, and dried in a vacuum furnace. Then, they were heated for 4 h at 350 °C, to dehydrate the zinc oxide and gently milled.
These precipitations were done with different mass ratios between ferrite particles (either M00 or M20) and zinc acetate solution to prepare various samples. The names and nominal compositions of these samples are presented in Table 1. Finally, only two composites, labelled Z00 (SrFe11O19/ZnO) and Z20 (Sr0.8Nd0.2Co0.2Fe10.8O19/ZnO), were chosen to carry out the kinetics experiments. All these samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM).

Kinetics measurements
Catalysts particles were suspended in distilled water. Orange II discoloration was scanned by visible light spectrophotometry. All experiments were performed at pH = 7.2.
Preparation of Orange II solution. Orange II (Figure 1) was prepared as its sodium salt by a coupling reaction between beta-naphtol (β-C10H7OH) and sulfanilic acid (para-aminebenzenesulfonic acid) in a sodium nitrite (NaNO2) solution. Dye was recrystallized in ethanol-water, and 100 mg/L solution was made using distilled water.
Kinetics of photocatalyzed discoloration. Kinetics measurements were carried out with composites or ZnO suspensions. Control tests were performed on single ferrite particles or without light exposition. The results of the experiments are shown in Table 2. Samples for discoloration were prepared by mixing 4.00 mL of catalysts suspensions in distilled water (final conc. 1000 mg/L) and 1.00 mL dye solution (final conc. 20 mg/L).
Hydrated zinc oxide reacts either with hydronium or hydroxide ions [32]. Therefore, the pH is stabilized by combined acid-base/solubility equilibria of ZnO in the suspensions [33], which are presented here in equations (1)-(3) as follows:  Samples were placed into glass test tubes (1.5 cm diameter) of high optic quality and attached around the external surface of a borosilicate glass flask (15.0 cm diameter) in a carrousel-like array. A 160 W mixing lamp Philips ML was placed at the center of the flask, and the whole set was put into a thermostat water bath which could be mechanically stirred. The array of reactor tubes can be seen in Figure 2. Sunlight was emulated with this mixing lamp. Its emission combines a continuous spectrum of an incandescent tungsten filament and a line spectrum from a high-pressure mercury source. During the kinetics experiments, the irradiance calculated value at the center of each test tube was 1900 W/m 2 approximately.
Light exposition and extraction of samples. All experiments were performed at 25 °C. Samples were homogenized in darkness by mechanical stirring and, immediately, all the test tubes were attached to the flask. At t = 0, the lamp was switched on; the first tube content was extracted with a syringe and centrifuged for 1 min at approx. 800 g (8000 m/s 2 ). The supernatant solution was put into a quartz cell; its absorbance was measured in a Shimadzu UV-2401 spectrophotometer at its maximum absorbance value within the visible spectrum, λmax = 485 nm (reference: distilled water). This procedure was repeated with the next test tube after fixed intervals up to 240 min. Dye concentration values in each tube were calculated as a function of time using Lambert-Beer's law [35].

RESULTS AND DISCUSSION
Composites were structurally and magnetically characterized. Discoloration kinetics was studied by spectrophotometry.

Structural and magnetic characterization of the composites
XRD, SEM, and VSM were used to study the properties of the catalyst composites.
X-ray Diffractometry and Scanning Electron Microscopy results. The XRD patterns of all the composites, measured in a Philips diffractometer (using CuKα radiation), are shown in Figure 3. The profiles confirm the hexagonal phase of Sr ferrite and the main peaks assigned to ZnO. Strontium hexaferrite, Z00 composite (SrFe11O19/ZnO), and zinc oxide nanoparticles were structurally characterized by SEM in a Zeiss DSM 982 GEMINI microscope. Figure 4 shows the SEM images. The hexagonal ferrite looks like platelets (average particle size 230 nm), while the ZnO nanoparticles are spherical and well dispersed (average particle size 60 nm).  Table 3). Magnetic results of M20/ZnO composite (Z20) are not presented. and zinc oxide is a diamagnetic material, it is possible to ponderate mass composition from MS results. In this way, for the Z00 composite, a ZnO mass fraction of 0.926 was calculated. The enhancement in HC for the Z0L composite is related to the high dilution of the ferrite particles into a non-magnetic material. These composites show good magnetic properties after the one-step preparation method.

Kinetics of Orange II dye photocatalyzed discoloration
Preliminary experiments to explore/discard Orange II adsorption on catalyst were performed. The absorbance of azo dye solutions with and without ZnO nanopowder was measured in darkness. Besides, Orange II stability was tested with M20 ferrite (without ZnO coating) suspended particles. Results are presented in Figure 5. All these profiles show a very small negative slope, so it can be concluded that dye adsorption on ZnO particles or the magnetic support is negligible (< 0.3%). These experiments show that neither ZnO nanoparticles nor Sr ferrite particles diminish Orange II concentration by adsorption processes. only Orange II (20 mg/L) only ZnO (1000 mg/L) only M20 ferrite (1000 mg/L) Figure 5. Orange II adsorption on ZnO or M20 nanoparticles Strontium ferrite/zinc oxide composites kinetics experiments. Figure 6 shows dye absorbance at 485 nm in the centrifuged Z00 composite supernatants divided by the initial value, Abs(t)/Abs0 vs. time. By Lambert-Beer's law, these relative absorbances should be equal to the relation C(t)/C0, between the concentration at any time and the initial concentration at the beginning of the light exposition. Orange II solution behaviour without any catalyst is included for comparison (Experiment #1 - Table 2).
It can be seen in Figure 6 that dye concentration slightly varies in the absence of catalyst (orange symbol), whereas suspended Z00 catalyst particles (blue symbols) cause an exponential decay in the azo dye absorbance. This type of curve would match with a first-order kinetics reaction.   only Orange II (20 mg/L) only ZnO (1000 mg/L) comp. Z00 (1000 mg/L) comp. Z20 (1000 mg/L) Figure 7. Orange II / Z00 or Z20 composites and zinc oxide discoloration with light exposition (for comparison Orange II is included) It can be seen in Figure 7 that Z00 composite and single zinc oxide suspended particles cause similar absorbance exponential decays. The Z20 composite (Nd-Co substituted strontium ferrite) curve presents a similar profile, although its initial slope is less negative than the others.
Kinetics of all photocatalytic reactions agreed with a first-order law, as Marto and other authors had found using only ZnO [8]. The first-order reaction rate constant (k1) and half-life (t½) parameters were calculated from the initial slopes of relative absorbance logarithmic graphs. These values are shown in Table 4. The t½ values for the 1000 mg/L catalyst suspensions were 49 min for Z00 composite and 90 min for Z20 composite. Then, the rate of the reaction catalyzed by composites decreased compared with that of the ZnO suspension with the same nominal concentration (t½ = 43 min).
However, it should be considered that suspensions of Z00 composite have a real mass of zinc oxide 7.4% less than their nominal composition (ZnO mass fraction = 0.926). Therefore, it should be concluded that the decrease in the reaction rate using the composite instead of single ZnO should be lesser than the previously calculated value.
It can be seen in Figure 6 that the initial rate is related to the catalyst concentration up to a value of 250 mg/L. For a higher catalyst concentration (1000 mg/L), the discoloration rates behave similarly for both hexaferrite composites after 180 min, although their initial rates are somewhat different (Figure 7). It is noticeable that the Sr ferrite supported catalyst (blue symbol) presents a similar behaviour as single ZnO nanoparticles (red symbol).
No other analytical technique was used in this work to measure any degradation products from the Orange II. However, as control experiments in darkness showed no significant azo dye adsorption, we proposed that the main discoloration process should be the catalyzed oxidation of the azo group with light exposition. It can also be inferred that photocatalyzed azo dye reaction should be the same with all the tested catalysts, magnetic composites, or suspended ZnO nanopowders.
Effect of catalyst concentration on Orange II discoloration. Figure 8 shows reaction rate constant k1 values obtained from the experiments with ZnO powder and Z00 composite vs. catalyst loading. Within the tested range of catalyst concentration, the value of k1 for the ZnO powder measurements increases linearly with the catalyst load. However, the rate-constant increment in Z00 experiments becomes less steep for amounts of suspended composite greater than 250 mg/L. This behaviour has already been reported and analyzed by Sakthivel et al. [37].
At first, a linear rise in the rate constant with the amount of catalyst is observed. This has been attributed to the increase of active sites on the photocatalyst surface, which results in a greater generation of •OH radicals, that would oxidize more azo dye molecules. But, as Nishio et al. found, for ZnO concentrations between 1000 and 1500 mg/L, the slope substantially decreases, and the rate constant becomes nearly independent of the catalyst load [3].
It has been reported by Kaur et al. that the increment of solution turbidity would hinder light transmission due to the significant scattering, and there would be no longer increment of hydroxide radical generation [38]. Therefore, there is an optimal level of zinc oxide concentration, and a further increase of the catalyst load will not be effective. This optimal value would seem smaller for the Z00 composite (between 250 and 1000 mg/L), probably due to a more intensive scattering of the suspended ferrite particles.

Remotion of the photocatalyst from water suspension
The suspended photocatalysts were removed from dye solutions by magnetic decantation with an iron-neodymium-boron (Fe-Nd-B) alloy magnet and then dried and weighed [39]. Composites were recovered with an average factor of 70%. This magnetic removal was carried out as a lab procedure and has not been tested at an industrial scale. Probably, there would be no severe disadvantages of the usage and recovery of a photocatalyst supported on magnetic particles in an industrial application since similar techniques have already been published and patented [40]. These procedures included the remotion of magnetic particles from seawater [41].

CONCLUSIONS
Composites prepared with ZnO precipitated nanoparticles on hexaferrite magnetic particles (Sr ferrite and Nd-Co substituted Sr ferrite) were successfully synthesized by a one-step coprecipitation method. These composites retain their magnetic properties after preparation at neutral pH. SEM images show that zinc oxide nanoparticles cannot completely coat Sr ferrite particles, although they seem to be steadily attached to the hard magnetic phase.
ZnO nanopowders and the prepared composites were tested as photocatalysts for Orange II dye discoloration in aqueous solution at 25 °C with light exposition at pH = 7.2. Preliminary experiments performed in darkness showed that the adsorption of the azo dye over ZnO or ferrite particles is negligible.
The synthesized composites present good catalyst properties in the photocatalyzed discoloration of the pollutant Orange II compared to single zinc oxide nanoparticles dispersed in water at the same experimental conditions. First-order reaction rate coefficients and half-life values calculated for Sr ferrite/ZnO composites are quite similar to those observed with zinc oxide nanopowder. The Nd-Co substituted Sr ferrite/ZnO catalyst showed the lowest catalytic activity, but its kinetics parameters were good enough to achieve a significant azo dye discoloration.
Composite particles can be easily removed from the water, by magnetic decantation, with a significant recovery factor (70%). These preliminary results indicate that these supported magnetic catalysts can be satisfactorily employed for industrial remediation. The influence of ZnO particle size on the photocatalytic activity of these composites and the re-use of the recovered catalyst particles will be studied in further experiments.

ACKNOWLEDGMENTS
The authors are indebted to Dra. Paula Bercoff for the structural and magnetic characterization of the composites, and to Dra. Nora François for the use of the spectrophotometer. The University of Buenos Aires (grant UBACyT 01-Q603BA) supported this work.