Removal of Lead (II) Ion from Aqueous Solution Using Ga-Doped ZnO and Ca-Doped ZnO Nanopowder

2.1 Preparation of Nanoparticles and Lead Ion Solution

CZ and GZO were prepared by a sol–gel method. Zinc acetate dihydrate [Zn(CH₃COO)₂⋅2H₂O] and an adequate quantity of calcium chloride hexahydrate [CaCl₂⋅6H₂O] corresponding to [Ca]/[Zn] atomic ratios of 0, 0.01, 0.02, 0.03, 0.04, and 0.05, as precursors in methanol, were used to prepare CZ, whereas GZO was prepared using 16 g of zinc acetate dehydrate as precursor in a 112 mL of methanol. After magnetic stirring at room temperature, the appropriate amount of gallium nitrate hydrate (Ga(NO₃)₃⋅H₂O) corresponding to [Ga]/[Zn] nominal atomic ratios of 0, 0.01, 0.02, 0.03, 0.04, and 0.05 was added. The resulting solutions were placed in an autoclave and dried in the supercritical conditions (ethyl alcohol as cosolvent; Tc = 243 °C; Pc = 63.6 bars). The obtained powder was then treated in a furnace for 2 h at 500 °C in air. Doped Ca samples are named CZ1, CZ2, CZ3, CZ4, and CZ5, whereas Ga-doped samples are called GZO1, GZO2, GZO3, GZO4, and GZO5, according to nominal Ca and Ga loading in each sample.

Lead nitrate was used to prepare the stock solution. The pH of each solution was adjusted with HCl or NaOH (with concentrations 0.1 mol/L) before mixing the adsorbent. The initial lead ion concentration was ranged from 25 to 500 mg/L. Adsorption experiments were carried in an Erlenmeyer flask by taking 10 mg of nanopowder in 25 mL of metal solution at the desired temperature (25 ± 1 °C) and pH. The flasks were agitated on shaker for 24 h, which is more than ample time for adsorption equilibrium. The concentrations of lead before and after adsorption were measured by SPECTRO GENESIS inductively coupled plasma–atomic emission spectrometry. The amount of metal ion adsorbed per unit mass of adsorbent, at equilibrium, was calculated by the following equation:

where C₀ and C_e are the initial and at equilibrium concentration of lead ion (mg/L), V is the solution volume (L), and m is the mass of adsorbent (g).

The removal percentage was calculated by (2):

2.2 Characterization of the Nanoparticles

To identify the crystalline phases of annealed samples, X-ray diffraction (XRD), using the CuKα radiation (λ = 1.5418 Å) of a Bruker D5005 diffractometer, was used. The following Scherrer’s equation was used to calculate the average crystallite size:

where λ represents the wavelength of the X-ray, B is the full width at half maximum of the peak, and θ_B is the maximum of Bragg diffraction peak (in radians).

The morphology of the synthesised sample was observed by scanning electron microscopy (SEM) using a JEOL JSM 5600 LV instrument operating at 20 kV equipped by an Oxford EDX detector and by transmission electron microscopy (TEM) carried out with a JEOL JEM 2010 electron microscope (LaB6 electron gun) operating at 200 kV, equipped by a Gatan 794 Multi-Scan CCD camera for digital imaging. Samples for TEM analysis were prepared by dropping a suspension of the starting powder, dispersed in isopropanol and sonicated, on 400-mesh holey-carbon-coated copper grids.

3.1 Adsorbent Characterizations

Figures 1 and 2 show the X-ray diffractogram spectra of CZ and GZO samples after annealing at 500 °C for 2 h in air. Only the major diffraction peaks of ZnO are shown. No secondary phases are seen within the limit of the XRD measurement, and apparently there is no peak for calcium or gallium oxides or other crystalline phases. The indexed peaks are related to the hexagonal wurtzite structure according to JCPDS database (Card 36–1451). In detail, a slight shift of the XRD peaks towards lower 2-theta degrees was observed upon increasing the Ca and Ga contents as can be seen in the inset of the figures. This behaviour is due to the difference in ion size between zinc (1.37 Å), calcium (1.97 Å), and gallium (1.53 Å) and indicates a local distortion of the wurtzite lattice due to local residual stress inside the nanoparticles. The addition of Ca and Ga in the ZnO matrix causes the reduction of grain size and consequently, according to Scherrer’s equation, the enlargement of peaks (see inset Figs. 1 and 2). This observation proves that Ga and Ca inhibit the grain growth, so they favour the synthesis of smaller ZnO particles. In the other hand, the shift of the peaks towards higher 2-theta can be attributed to the reduction of interplanar distances caused by the stress effect. According to Scherrer’s equation (3), and after correction for instrumental broadening, the average crystallite size is calculated, and it has been estimated to be 55 nm for pure ZnO. For the doped nanoparticles, the average crystallite size varies between 35 and 50 nm for all the samples.

Figure 1:

X-ray diffractograms of pure ZnO and Ca-doped ZnO after annealing at 500 °C.

Figure 2:

X-ray diffractograms of GZO samples after annealing at 500 °C for 2 h.

Figure 3 depicts the SEM micrograph of CZ3 samples after annealing at 500 °C. This figure shows that CZ3 particles are rough porous fine grained, and they have round shape with dimensions varying between 2 and 5 μm. The EDX pattern, shown in Figure 3, confirmed that CZ3 particles consist of the main elements Zn, O, and Ca, and they contain Cl as contaminant. No relevant percentage of any other contaminant species was registered.

Figure 3:

Scanning electron microscopy micrograph of the CZ3 sample.

TEM analysis, presented in Figure 4, shows the size and morphology of the pure ZnO particles and the GZO samples. TEM data are in agreement with XRD data and indicate that the average particle size first decreases slightly with addition of Ga up to 3 at.%, but then become larger for higher Ga loading. TEM images in Figure 4 show that the grains of undoped particles have regular facets, whereas the grains of GZO samples appear to lose their regular facets and become more elongated with increasing gallium content.

Figure 4:

Transmission electron microscopy images of ZnO and Ga-doped ZnO nanoparticles annealed at 500 °C for 2 h in air.

Figure 5 depicts the TEM micrograph of pure ZnO and CZ. It shows that undoped ZnO particles as single grains have an inhomogeneous particle size distribution, with most of them having size in the range between 20 and 80 nm, whereas the incorporation of Ca in ZnO increases the size of the grains as shown in the case of CZ1 and CZ3. We noted also the presence of many particles that are apparently amorphous, and they have irregular form on the surface of CZ3 nanoparticles. These particles were not found on pure ZnO sample, and then it is plausible to assume that they are related to the presence of Ca. It can be suggested that these particles have been formed through the carbonation of the surface during the storage in air of the samples, favoured by the enhanced affinity towards the atmospheric CO₂ of the Ca-doped samples [27].

Figure 5:

Transmission electron microscopy micrographs of (a) undoped ZnO, (b) CZ1, (c) CZ3, and (d) CZ5.

3.2 Adsorption of Lead by Nanomaterials

To study the influence of the presence of Ga and Ca in ZnO on the adsorption of lead from aqueous solution, ZnO was doped by these metals at different concentrations. The results of this study (Fig. 6) show that the adsorption capacity of the particles increases with the incorporation of Ca and Ga in ZnO, and the maximum capacity was obtained with 3 wt.% of Ga and Ca in ZnO.

Figure 6:

Effect of the presence of Ga and Ca in ZnO on the adsorption of lead.

Figure 6 depicts the influence of the presence of Ga and Ca in ZnO on the adsorption of lead from aqueous solution. The adsorptive capacity of lead increased with the presence of Ga and Ca in ZnO, and the maximum of adsorbed Pb concentration per gram of adsorbent is obtained with 3 wt.% of Ga and Ca in ZnO. The effect of the incorporation of Ca and Ga on the adsorption of lead can be related to the structure of the nanoparticles because there is a correlation between the resistivity of the adsorbent and its adsorption capacity. In fact, we have noted in our previous works [26] that the incorporation of Ca and Ga introduces donor levels in ZnO that enhance the electron concentration in the conduction band and involve a decrease of the resistivity of the nanoparticles. The minimum in resistivity was obtained at 3 at.% of Ca and Ga. This effect is accompanied by an improvement in the crystallinity, and it proved that Ca- and Ga-doping atoms are effectively incorporated substitutionally at zinc sites in ZnO lattice. The substitution of the cations of Zn²⁺ by those of Ca²⁺ or Ga³⁺, which acts as donor, leads to the formation of active adsorption sites and participation to the adsorption mechanism. GZO3 and CZ3 are more efficient adsorbents for lead removal than other nanomaterials; for this reason, for all subsequent works, only GZO3 and CZ3 were used to study the effect of different parameters such as pH, contact time, and temperature.

To study the effect of pH on the removal of lead, the masses of GZO3 and CZ3 used in this study are 10 mg, and the initial concentration of Pb²⁺ was fixed at 200 mg L⁻¹. The effect of initial pH on the removal of Pb²⁺ using GZO3 and CZ3 is given in Figure 7. This figure shows that adsorption capacity of GZO3 and CZ3 depends on pH. In fact, when pH increases, the capacity also increases until it reached a maximum, and then it decreases quickly. The maximum sorption capacity of GZO3 and CZ3 is obtained at pH 4 and equal to 457 and 196 mg g⁻¹, respectively. The values of pH are limited to 6 due to the precipitation of lead. At low pH values (acid solution), the sorption of lead is weak because the active sites are protonated due to the competitions between H⁺ and Pb²⁺ for the occupancy of the binding sites [1].

Figure 7:

Effect of initial pH on the removal of lead using GZO3 and CZ3.

The equilibrium adsorption isotherms are promising data because they determine how much adsorbent is required quantitatively for enrichment of an analyte from a given solution. The isotherms of lead adsorption are studied at three temperatures 25 °C, 40 °C, and 55 °C. These isotherms are obtained by fixing the pH of lead solution at 5.4 and varying its initial concentration from 20 to 200 mg/L. The plot of the lead adsorption capacities against its equilibrium concentration and at different temperatures is given in Figure 8 for CZ3 and in Figure 9 for GZO3. Figures 8 and 9 show that, the adsorption capacities of GZO3 and CZ3 increase with temperature. The q_e values in Figure 8 increase quickly at low lead concentration, whereas at high lead concentration, q_e values are slowed down. Figure 9 shows that the q_e values vary linearly with the initial concentration of lead.

Figure 8:

Effect of temperature on the removal of lead using GZO3.

Figure 9:

Effect of temperature on the removal of lead using CZ3.

In the study of the effect of the contact time, the initial concentration of Pb²⁺ was fixed at 200 mg L⁻¹, the mass of adsorbent is 10 mg, and the volume of lead solution is 25 mL. The obtained results of the sorption of Pb²⁺ using CZ3 and GZO3 are shown in Figure 10. We noted, at the beginning of the adsorption, quick increases on the uptake of lead, and the curve of adsorption of lead using GZO3 reached the equilibrium after 60 min, whereas the process of adsorption of lead by CZ3 needs 120 min to attain the equilibrium.

Figure 10:

Effect of contact time for lead removal onto GZO3 and CZ3.

In order to propose these promising materials for the uptake of lead from water, the adsorption capacities of GZO3 and CZ3 were compared to other adsorbents. Table 1 presents, at given temperature and pH, the adsorbed mass of lead per unit mass of different adsorbents (q_e). The comparison of q_e values of GZO3 and CZ3 to other values reported from the literature shows that these nanoparticles are the most efficient adsorbents for lead among the used materials. For this reason, CZ3 and GZO3 can be considered as promising adsorbents for the removal of lead from water and wastewater.