Removal of Neutral Red from Aqueous solutions usingSr2CaBO5.5 (B= Nb+5& Ta+5)

The preparation of samples involved different stoichiometric compositions of Ta₂O₅ (Merck, 99.99%) and CaCO3 or SrCO3 (BDH, 99.98-99.99%). The mixtures were initially ground and preheated at 850°C for 12 h, and then reground and heated at 1100°C for 48 h.

The crystallography of the samples was examined by a PANalytical X’Pert X-ray powder diffraction using Cu Kα radiation (1.5400 Ȧ) and a PIXcel solid-state detector. the operating voltage was 40kV and the current was 30 mA. he samples were measured in lat plate mode at room temperature with a scan range of 10°<2θ<80° and a scan length of 10 mins were used. The structures were refined using the program RIETICA¹⁰.

The absorbance of solutions was determined using ultraviolet visible spectrophotometer (UV/Vis, model Spect-21D) and (190-900 Perkin- Elmer) at maximum wavelength of absorbance (520 nλ). The concentrations of solutions were estimated from the concentration dependence of absorbance fit. The pH measurements were carried out on a WTW720 pH meter model CT16 2AA (LTD Dover Kent, UK) and equipped with a combined glass electrode.

Batch mode removal studies were carried out by varying several parameters such as contact time, pH, temperature and mass of prepared oxide (adsorbent). Essentially, a 50 ml of dye solution with concentration of 10 ppm was taken in a 250 ml conical flask in which the initial pH was adjusted using HCl/NaOH. Optimized amount of adsorbent was added to the solution and stirred using magnetic stirrer for specific time. The oxide samples were separated from solutions using centrifuge 3500 CPM for 5 minutes.

The preparation of an extensive solid state solution allows us to study the effects of altering the chemical composition on the physical properties. The heating regime described above produced crystalline, yellowish coloured, samples. X-ray diffraction measurements (Figure 1) show the samples to be highly crystalline and the data are consistent with these all having a faced cubic structure with space group (Fm3m). The solid state chemistry of Ta⁵⁺ and Nb⁵⁺ are generally believed to be similar, so it is reasonable to expect Sr₂CaTaO_5.5 will display similar behaviour to Sr₂CaNbO_5.5. The cell volume of Sr₂CaNbO_5.5 (559.49 ų) is found to be similar to that for Sr₂CaTaO_5.5 (557.67 ų). This reflects the similarity in the ionic radii of both Nb⁵⁺ (0.64 Å)and Ta⁵⁺ (0.64 Å)cations.

Figure 1.The XRD patterns of Sr2CaNbO5.5 and Sr2CaTaO5.5.

The Average Crystallite size D_p, specific surface area S, lattice strain φ, Lattice parameter a and Cell volume V estimated from X-ray diffraction data are summarised in Table 1. The crystallite size can be calculated using sheerer formula ¹¹ (Equation. 1) where the specific surface area can be calculated using Sauter formula ¹² (Equation.2) in which ρ is the density of the synthesised material.

D_p= (0.94λ)/(β_1/2×cosϴ). (1)

S = 6000/ (D_p ×ρ). (2)

Both the shape and the size of particles are defined by the preparation methods; however the ratio of nucleation to growth rates of particles is also important. Each of these processes depends in turn on variations in the reaction conditions such as the temperature, the nature and concentrations of metal and ligand, and the nature of stabilizer and reducer. The differences in the crystallite size, surface area and density of the two oxides (see table) could be attributed to one of these variations.

The amount of the dye adsorbed by one gram of the oxides (Q) was calculated as following: Q (mg/g) = [(C_i-C_t)×V]/W, where t= 180 min is the contact time, V= 50 ml is the volume of NR solution and W is the mass of oxides. As shown in Figure 4, Q decreases as the mass of adsorbents increased. The maximum capacity of adsorbent Q_max can be estimated from the intercept of the liner fit of 1/Q_t at Y axis. Both Sr₂CaNbO_5.5 (50.4 nm, 13.6 m²/g ) and Sr₂CaTaO_5.5 (43.7 nm, 24.2 m²/g) exhibited a similar value of Q_max(190.5 mg/g) at room temperature. This result is consistent with the similarity in the cell volumes of the two oxides which determined to be (~ 558 ų). The removal capability of Neutral Red is higher than those recorded for Methyl violet when the perovskite oxides BaSr₂NbO_5.5 (~9.3 mg/g)¹³ and Sr₂CaNbO_5.5 (47.39 mg/g )¹⁴ were used as adsorbents.

Figure 4.The effect of adsorbent mass on the removal. The time, volume, concentration and pH of dye solutions are 150min, 50ml, 10ppm and 5.1 respectively.

Temperature has an important impact on the adsorption process. An increase in temperature helps the reaction to compete more efficiently with e^–/H⁺ recombination. The removal of NR dye was investigated at 25, 40, 60 and 100^oC. The obtained results are illustrated below in Figure 5. The removal of NR dye decreased as temperature increased. For instance, the removal of NR decreased from ~94% at 25^ᴼC to ~74% at 100^ᴼC. This result is disagreed with normal expectations, and is probably a consequence of an increase in the thermal energy. It is anticipated that higher temperature induces higher mobility of active sites of adsorbents and the surface area is decreased by increase the temperature¹⁵. The energy of activation (E_a), was calculated from the Arrhenius plot of ln R vs 1000/T. Arrhenius plot shows that the activation energies of the removal are negative and equal to 2.830 and 2.948 kJ/mole for Sr₂CaTaO_5.5 andSr₂CaNbO_5.5 respectively. This reflects the similarity in the strength of the interaction forces between the dye and the two oxides.

Figure 5.The effect of temperature on the MV removal. The time, volume, pH and concentration of dyes solutions are 150min, 50ml, 5.1 and 10ppm respectively

The pH of solutions is a key parameter in dye adsorption. The magnitude of electrostatic charges which are impacted by the ionised dye molecules is controlled by the solution pH. As a result the rate of adsorption will vary with the pH of the medium used. In general, at low solution pH, the percentage of dye removal will decrease for cationic dye adsorption, while for anionic dyes the percentage of removal will increase. This is due to the increase in the positive charge on the solution interface and the adsorbent surface. In contrast, high solution pH is preferable for cationic dye adsorption but shows a lower efficiency for anionic dye adsorption. The positive charge at the solution interface will decrease while the adsorbent surface appears negatively charged ¹⁶.

To study the effect of pH, experiments were carried out at various pH values, ranging from 2 to 10 for constant dye concentration (10 ppm) and adsorbent mass (0.1g). Figure 6 presents the removal of dyes as a function of pH. It was observed that the removal of NR using the two oxides slightly increases as pH inceased. The highest removal of the dye was recorded at pH= 10 around 94 % where the lowest removal was recorded at pH= 2 around 93%. The removal efficiency of the two adsorbents seems to be similar in the acid media where in the basic media small differences (≤ 0.9%) can be noticed.

Figure 6.The effect of pH on the removal of NR. The time, volume and concentration of dyes solution are 180 min, 50ml and 10ppm respectively.