Investigating the performance of Fe-doped MgO particles in the photocatalytic reduction of nitrate from drinking water

Abstract

Abstract Introduction: Nitrate contamination of water resources as a result of excessive consumption of nitrogenous fertilizers and wastewater discharge is one of the important challenges in providing safe drinking water. Photocatalysis has been investigated increasingly to remove nitrate due to using unlimited sunlight, converting nitrate to harmless nitrogen gas, easy to operate, and applicable in small-scale and remote areas. However, the proposed photocatalysts either have complex and uneconomical synthesis technology or are composed of expensive and toxic elements, which has limited photocatalysis implementation on a large scale. Therefore, this study aims to investigate the performance of pure MgO doped with Fe3+ in the photocatalytic removal of nitrate due to its cheap, abundant, and safe constituent elements. Materials and methods: In the present work, pure and Fe3+ doped MgO were synthesized by precipitation method. Catalyst characterization using XRD, FTIR, SEM-EDX, BET-BJH, XPS, DRS, and Mott-Schottky analyses was carried out to show successful synthesis of the MgO phase, iron doping in the MgO structure, and explain catalysts behavior in the photocatalytic reduction of nitrate. The effect of operational parameters such as photocatalyst dose, hole scavenger amount, initial nitrate concentration, presence of interfering anions, and contact time on process efficiency was also investigated. Finally, the performance of the photocatalytic system was studied in consecutive consumption cycles of optimum catalyst. Results: The photocatalytic reduction of nitrate using pure and iron-doped MgO particles was lower than expected. Besides, the operational parameters did not have an observable effect on the catalyst performance compared to control experiments. In the conditions where the nitrate concentration, catalyst dose, and oxalic acid concentration were 50 mg/L, 0.2 g/L, and 1125 mg/L, respectively, the nitrate conversion rate was only 19.22% after 4 hours of irradiation. Nitrate conversion remained almost unchanged in successive reuses of the optimized catalyst.To explain the behavior of pure and doped MgO in the photoreactor, prepared catalysts were characterized by different techniques. Using XRD patterns, FTIR, and XPS spectra, the successful synthesis of the magnesium oxide phase was confirmed in samples; In addition, EDS and XPS spectra proved the presence of the constituent elements of the catalysts. The results obtained from BET-BJH and SEM studies displayed the production of porous plate-like particles with a high surface area in the range of 241-192 m2/g. According to the DRS analysis, the inclusion of iron in MgO created a short band gap in the region of 2.63-3.09 eV, while for the pure sample, absorption occurred only in the UVC region. Afterwards, using DRS and Mott-Schottky tests the position of CB, VB, and dopant energy levels was determined. Conclusion: The characterization of the synthesized catalysts revealed that the wide band gap and the positive potential of the dopant energy level compared to nitrogen species were the possible factors limiting the performance of pure and doped MgO particles, respectively.