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The nanoparticles tin oxide (SnO 2 ) has been successfully synthesized via a sol-gel method. The asprepared SnO 2 was characterized using thermogravimetric analysis coupled with mass spectroscopy, X-ray diffraction and scanning electron microscope. Batch experiments were carried out to study the adsorption kinetics of Congo red azo dye on SnO 2 . The effect of varying parameters such as contact time, initial dye concentration and adsorbent dose on the adsorption process was investigated. The results showed that the SnO 2 catalysts has high crystallinity with tetragonal rutile structure and average particle size about 13-23 nm. The untreated sample SnO 2 -80 exhibited high efficiency (84.41 %) after 60 min of exposure time, which was 2 times as much as that of SnO 2 -450 sample. The adsorption process was found to be highly dye concentration and adsorbent dose dependent. Pseudo-second-order kinetic model gave the best fit, with highest correlation coefficients (R 2 C 0.99). Regarding the adsorption equilibrium, the experimental results suggest that the Langmuir model was applicable. The formation of hydrogen bond and the electrostatic interaction between the Sn 2? center and the electron rich nitrogen atoms of CR moiety were believed to be the main adsorption mechanism. These findings should be valuable for designing effectively adsorbent material and practical interest in terms of ecology and sustainable development.

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Sr-doped CuBi2O4 micro-particles were successively synthesized via a solid-state technique and were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and UV-vis diffuse reflectance spectroscopy (UV-vis-DRS) techniques. Results show that Sr-doped CuBi2O4 was crystallized with a spinel-type structure and tetragonal crystal system, and the band gap energy was about 1.35 eV. The as-prepared Sr-doped CuBi2O4 treated at 573 °C for 12 h exhibited the highest efficiency, as a result of 97.22 % of CR degradation within 220 min, which is approximately 31 times greater than CR photodegradation when catalyzed by CuBi2O4 (3.13 %) and about 2.3 times superior than that catalyzed by the untreated Sr-doped CuBi2O4 sample (42.08 %). Pseudo-first-order kinetic model gave the best fit, with highest correlation coefficients (R (2) = 0.94-0.97). The Sr-doping and extending reaction time up to 12 h could be effective in producing Sr-doped CuBi2O4 materials that delay electron-hole recombination, thereby increasing the lifetime of the electron electron-hole separation and support the charge carrier transfer to the catalyst surface. On the basis of the calculated energy band positions, superoxide radical anions (O2 (•-)) were the main oxidative species responsible for the photocatalytic degradation of CR dye solution.

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