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. The 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 RIETICA11.

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 (590 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.

Initially, our synthetic attempts focused on Sr_3-xBa_xNbO_5.5 oxides where x = 0, 1, 2 and 3, however only for the three first compositions were single phase samples obtained. X-ray diffraction measurements (Figure 1) demonstrated the three samples to be free of any obvious impurities and have a faced cubic structure with space group (Fm m). Doping with Ba²⁺ significantly increases both the cell volume and the density but decreases the crystallite size. The increase in cell volumes is likely driven by the large ionic size of the Ba²⁺ cation (12 coordinate ionic radius, 1.61 Å). The ionic size of the Sr²⁺ cation (12 coordinate ionic radius, 1.44 Å) is smaller than the Ba²⁺ cation. BaSr₂NbO_5.5 displayed the lowest crystallite size in the series, possibly as a consequence of cation order effects. The materials can be formulated as (BaSr)SrNbO_5.5 in order to emphasize the ordering at the B site between the Sr and Nb cations. In the double perovskite structure, it is anticipated that the two smallest cations will order in the octahedral sites, this ordering being a consequence of the differences in the size and/or charge between the two cations. The largest cation will then occupy the 12-coordinate (cuboctahedral) site. The corresponding ionic radii of Ba²⁺ (12 coordinate ionic radius, 1.61 Å and 6 coordinate ionic radius. 1.35 Å9); Sr²⁺ (1.44 and 1.18 Å9) and Nb⁵⁺ (6 coordinate ionic radius. 0.64 Å9)cationssuggest that the Nb⁵⁺ and one Sr²⁺ cation will occupy the 6-coordinate sites whereas a mixture of Sr²⁺ and Ba²⁺ will occupy the cuboctahedral sites12.

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 formula13 (Equation. 1) where the specific surface area can be calculated using Sauter formula14 (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)

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= 150min is the contact time, V= 50 ml is the volume of MV 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. Ba₂SrNbO_5.5 (72.72 nm, 14.55 m²/g) displayed the highest value of Q_max (46.47(2) mg/g) whereas Sr₃NbO_5.5 (81.91 nm, 14.35 m²/g) exhibited the lowest value of Q_max (8.03(5) mg/g). Q_max for BaSr₂NbO_5.5 (50.45 nm, 22.03 m²/g)is 13.09(2) mg/g. This result suggested an enhancement in the adsorption properties occurs as result of the substitution of Ba²⁺ into the oxide plus the decrease in crystallite size. The decrease in crystallite size leads to an increase in the surface area of particles.

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 two dyes was investigated at 25, 40, 60 and 100^oC. The obtained results are illustrated below in Figure 5. The removal of MV dye increased as temperature increased. For instance, the removal of MV increased from ~64.9% at 25^ᴼC to ~99% at 100^ᴼC when BaSr₂NbO_5.5 was used. This result is agreed with normal expectations, and is a consequence of the increase of adsorption strength and the concentration of active intermediates with temperature. 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 positive and equal to 4.79 and 4.32 kJ/mole for Sr₃NbO_5.5 and BaSr₂NbO_5.5 respectively. The activation energy of the removal was (-0.018 kJ/mole) for Ba₂SrNbO_5.5. This reflects the differences in the strength of the interaction forces between the dye and the oxides.

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.

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 MV using the doped oxides increases as pH increased. The highest removal of MV was recorded at pH= 10 around 98.8 % using Ba₂SrNbO_5.5 where the lowest removal of MV was recorded at pH= 4 around 22% using the same oxide. The removal of MV using Sr₃NbO_5.5 has gradually increased from ~50% to ~75% as pH increased from 2 to 10.The removal of MV reached maximum (~91%) using BaSr₂NbO_5.5 at pH 2 and 10. The removal efficiency of the adsorbents is clearly increases as the acidity decreased.