Photocatalytic activities of antimony, iodide, and rare earth metals on SnO2 for the photodegradation of phenol under UV, solar, and visible light irradiations

Abstract The tremendous amount of pollutants being dumped into the water has become a major problem across the world and a focus for researchers. These chemicals are toxins that are not easily eliminated. It is difficult to make them retain their transformations under certain conditions. After conversion, they might become more toxic than their parent molecule. There are many ways to withdraw these organic compounds from water sources. The cheapest is to use photocatalytic material oxides such as SnO2 by harnessing sunlight and using it for photocatalytic degradation processes. Photocatalysis by advanced oxidation processes is the most popular and promising method for removing contaminants such as phenol and its intermediates from water. Tin dioxide (SnO2) has been used to detect some toxic gases and is involved in many other technological applications. SnO2 is a strong oxidizing agent and a powerful reducing catalyst. A variety of techniques have been employed to improve the photocatalytic activities of SnO2, including doping. Photodegradation of phenol in the presence of SnO2 nanoparticles (Nps) under UV light irradiation is known to be an effective photocatalytic process. However, phenol photodegradation under solar and visible light irradiation is less effective because of the large band gap of SnO2. In this study, pure SnO2 catalysts were synthesized by a sol-gel method using tin tetrachloride, ethanol, and water. To synthesize SnO2 doped with species containing different ions such as gadolinium (Gd), cerium (Ce), lanthanum (La), neodymium (Nd), iodine (I), and antimony (Sb), different concentrations of these dopants, such as 0.01%, 0.1%, 0.2%, 0.3%, 0.4%, 0.6%, 0.8%, 1.0%, and 1.1%, were mixed and dissolved separately in ethanol and water and then added to the precursor solution. At the final stages, ammonia was added to gel the sol. The sol-gel formed was washed and prepared at a low temperature to obtain the SnO2 Nps. SnO2 powders were characterized by x-ray diffraction, scanning electron microscopy, and transition electron microscopy (TEM) and the specific surface area was estimated by Brunauer–Emmett–Teller analysis. Several analytical techniques were used for phenol and its by-products, such as high-performance liquid chromatography (HPLC), UV-visible light (UV-Vis) spectrophotometry, gas chromatography (GC), capillary electrophoresis, total organic carbon measurements, and Fourier transform infrared spectroscopy (FTIR), and by determining chemical oxygen demand from the pollutant. Results showed a decrease in particle size from 8 to 1.8 nm and an increase in surface area to 58 m2/g upon an increase in different doping contents from 0% to 1.1% as they were incorporated into SnO2. In this study, the optimum parameters were catalyst loading (65 mg/50.00 mL), light intensity (8 W mercury lamp, 300 W xenon lamp, or sunlight on fully sunny days), reaction time (2–3 h), phenol concentration (10 parts per million), 4 L/min of optimal air flow, sampling time (12–13 mins), and sample volume (250.00 mL); the pH of the reaction medium was 5.7. The GC study showed that irradiation of the catalyst by UV light enhanced phenol photodegradation in the first 30 min of the experiment. The UV-Vis investigation of the treated phenol samples indicates that phenol molecules initially transform by-products that also optically absorb in a similar region as phenol. In this study, for photocatalysis experiments on phenol photodegradation, the optimal condition applied under UV light irradiation allowed more than 95% of phenol degradation with SnO2/La 0.6 wt.% after 2 h. Also, 95% of phenol degraded with the photoactivity of SnO2/Sb 0.6 wt.% under solar light irradiation. The same amount of phenol photodegradation was found with SnO2/Gd 0.6 wt.% under visible light irradiation and with the same optimal conditions, changing only the light source. HPLC results showed the intermediates to be in the order: catechol (Cat) > resorcinol > hydroquinone (HQ) > benzoquinone, but the last stages of phenol photodegradation were isopropanol (2-P) and acetic acid (AA). The reaction of phenol photodegradation indicated that it takes place when light radiation photoexcites a catalyst in the presence of oxygen; a hydroxyl radical (•OH) is generated to attack phenol and reacts with OH− to produce Cat or HQ, which upon continuous oxidation breaks it down, leading to the formation of aliphatic acids, finally yielding carbon dioxide (CO2) and water (H2O). In fact, the mineralization process starts early during photocatalytic degradation, because FTIR results showed that the phenol molecules were converted to CO2 in the early stages and continued until all phenol was removed. The change in concentration of phenol affected the pH of the solution owing to the formation of intermediates during the photodegradation of phenol. Clear correlations between results were found from these multiple measurements and a kinetic pathway for the degradation process is proposed. A maximum of 0.02228 min−1 of propanol and a minimum of AA 0.013412 min −1 were recorded.