Denitrification of water using ZnO/Cu as the photocatalyst

2.1 ZnO:Cu nanoparticle preparation

All chemicals were analytical reagent grade and used directly without any further purification. Double-distilled water was used throughout this study. The ZnO and Cu precursors used were zinc chloride (ZnCl₂, Merck, Germany) and copper (II) nitrate [Cu(NO₃)₂, Merck, Germany]. Sodium hydroxide (Samchun, South Korea) was used for the precipitation of ZnO from the aqueous solution.

Nanostructure ZnO was prepared by the following method. Five grams of ZnCl₂ was dissolved in 100 ml distilled water at 70°C. Meanwhile, 20 g of sodium hydroxide was dissolved in 100 ml distilled water in a separate vessel. Then, 16 ml of the prepared sodium hydroxide solution was added to the ZnCl₂ solution. The aqueous solution turned into a milky white colloid without any precipitation. The reaction was then allowed to complete for 2 h. Next, the solution was removed by washing with distilled water for five times, after which and zinc oxide was dried at 100°C for 30 min followed by calcination at 500°C, before it finally changed into powder form [26].

ZnO:xCu was prepared by employing the wet impregnation method. In order to prepare the ZnO:xCu, ZnO nano powder was contacted with a solution of copper(II) nitrate with different concentrations (15, 25, 35 weight percent). The prepared catalysts were labeled as ZnO:15%Cu, ZnO:25%Cu and ZnO:35%Cu. The real Cu loading was measured using XRF technique and is reported in Table 1.

2.2 Nano powder characterization

The grain size and crystallinity of the ZnO photocatalysts were studied by X-ray diffraction (XRD, Bruker AXS, D8-Advance, Germany) using Cu Kα (λ = 1.5406 Å) radiation in the region of 2θ = 20°–75°. In addition, the grain or crystalline size (L) was estimated using Scherrer’s formula [27] given by

where L is the crystallite size of pure ZnO, k is a constant (=0.94), λ is the wavelength of the X-ray (Cu Kα = 1.54065 Å), β is the true half-peak width, and θ is the half diffraction angle of the centroid of the peak in degree.

The surface morphology of the nano powders was studied using a field emission scanning electron microscope (FESEM) (Hitachi, Japan). The Fourier transform infrared (FT-IR) (Avantes, Avaspec-2048-TEC, Netherland) spectra of the samples were recorded in the range 4000–400 cm⁻¹ on an FT-IR spectrometer; these are then used for the detection of bonds between the photocatalyst and active phase (Cu). Furthermore, for the elemental analysis and composition of ZnO:xCu catalysts, the X-ray fluorescence (XRF, Bruker AXS, D8-Advance, Germany) technique was employed. In addition, the light absorption of catalysts was evaluated with diffusive reflectance spectroscopy (DRS) analysis.

2.3 Measurement of photocatalytic activity

Ammonium nitrate powder (NH₄NO₃) was dissolved in distilled water at 50 mg·l⁻¹ concentration. The ZnO:xCu photocatalyst was added into 5 ml ammonium nitrate solution and a high pressure mercury lamp (125 W) was used as the light source. The averaged intensity of UV irradiance was 6.8 mW·cm⁻² by measuring with a UV irradiance meter (Camspec, USA). The solution was bubbled with air during irradiation. The UV-Visible spectrophotometer technique was used to measure the concentration of nitrate. The solution was sampled every 30 min and analyzed with this technique at 410 nm wavelength. To interpret the results, a simplified empirical pseudo first-order reaction rate was assumed for the photocatalytic degradation of nitrate [28], [29]

where k is the apparent constant of the reaction rate, and C₀ and C are the initial and final concentrations of nitrate in the solution, respectively.

3.1. Characterization of the nano powder

Figure 1 shows the XRD patterns of the ZnO:xCu nano powders. As can be seen, the main peaks are in 2θ = 32°, 34°, 38°, 47°, 56°, 66° and 69°, thus indicating the presence of ZnO crystals. The crystallite size of ZnO:xCu with different weight percent of Cu can be measured according to Figure 1, and Scherrer equation. So, average crystallite size of ZnO:xCu is 45 nm.

Figure 1:

XRD pattern of the ZnO nano powder.

The FT-IR spectra of ZnO:15%Cu, ZnO:25%Cu and ZnO:35%Cu are shown in Figure 2. Peaks around 3383–1330 cm⁻¹ are assigned to normal polymeric O–H of H₂O in Cu–Zn–O lattice which may belong to moisture included in the atmosphere [30]. The medium to weak bands at 1500–707 cm⁻¹ are attributed to the microstructural features that change with the addition of Cu into the Zn–O lattice. Furthermore, stretching bonds at 542, 498, 434, 506 and 427 belongs to the ZnO compound [30].

Figure 2:

FT-IR spectra for (A) ZnO:15%Cu, (B) ZnO:25%Cu and (C) ZnO:35%Cu.

As mentioned above, the DRS technique was used to evaluate the light absorption and the band gap of the synthesized photocatalysts. The calculation of band gap according to the DRS method can be performed using the Planck equation given by [31]

where h, c and λ are the Planck constant, speed of light and the wave length, respectively. DRS results for ZnO:xCu are shown in Figure 3. As it can be seen, ZnO:15%Cu has the lowest band gap energy (3.3 eV) and ZnO:35%Cu has the highest band gap energy with 4.1 eV energy.

Figure 3:

DRS results for the synthesized ZnO:xCu photocatalysts.

XRF analysis was used to measure the elemental composition of ZnO:xCu photocatalysts. According to the XRF results shown in Table 1, ZnO:15%Cu has 95.6% ZnO and 3.4% (weight percent) Cu, ZnO:25%Cu contains 91.3% ZnO and 6.8% Cu, and ZnO:35%Cu composition is 87.1% ZnO and 10.2% Cu.

3.2 Morphology of the ZnO:xCu photocatalysts

FESEM micrographs were used for observing the morphology of ZnO:xCu photocatalysts. Figure 4 shows the FESEM images of ZnO:xCu nano powders. According to the FESEM micrographs, the particle size of the ZnO photocatalyst is 47 nm, which is in good agreement with the XRD results. The FESEM images indicate that the Cu-incorporated/ZnO nanoparticles exist in a single homogeneous form with narrow size distribution.

Figure 4:

FESEM images of (A) ZnO:15%Cu, (B) ZnO:25%Cu and (C) ZnO:35%Cu.

3.3 Determination of the photocatalytic activity of ZnO:xCu nano powders

The degradation of 5 ml of nitrate solution (50 mg/l) that came into contact with different synthesized photocatalysts under UV irradiation was studied, in order to investigate the photocatalytic activity of ZnO:xCu photocatalysts. Figure 5 shows changes in the concentration of nitrate solution with time upon coming into contact with the ZnO:15%Cu, ZnO:25%Cu and ZnO:35%Cu photocatalysts. Results of the determination of photocatalytic activity indicate that ZnO:25%Cu has the highest and ZnO:15%Cu has the lowest photocatalytic activity. The lower photocatalytic activity of ZnO:15%Cu is due to the low concentration of Cu. In fact, Cu has a key role in the degradation of nitrate in the solution, such that when Cu concentration is high, photocatalytic activity becomes lower due to the decrease in the active sites for activation. The apparent constant rate of reaction k (min⁻¹) is calculated according to Figure 6 and Equation (2). The results are compared with those from a similar work [32] in Table 2. The lowest degradation rate is for ZnO:15%Cu with k = 7.8 × 10⁻² and the highest degradation rate is for ZnO:25%Cu with k = 1.22 × 10⁻¹. A higher value for k in the referenced work (TiO₂ doped with Zn) is due to the addition of formic acid as the electron donor, which helped to improve photocatalytic efficiency [32]. A possible mechanism for photocatalytic degradation of nitrate is shown below [2]:

Figure 5:

Concentration of nitrate in the aqueous solution based on UV-Visible light.

Figure 6:

Changes in concentration of nitrate for the ZnO:xCu photocatalysts.

ZnO:Cu→ZnO:Cu (e−+h+)

H2Oads+h+→ · OH+H+

Cu+NO3−→NO2−+Cuδ+

NO2−+2H+→NH3+H2O

Cuδ++e−→Cu

Due to the absorption of light and electron-hole generation, the ZnO:xCu photocatalyst can activate the Cu active phase to change NO₂^- to NH₃ according to the above mechanism.