Photocatalytic degradation of humic acid in aqueous media using MnFeN-tridopedTiO2 nanoparticles

Abstract
Humic acid (HA) is produced by the decomposition of plant and animal debris in surface water. The presence of HA in the water disinfection process is considered as the main precursor for disinfection by-products. The present study aim was to evaluate photo catalyst degradation of HA, using MnFeN-tridoped TiO2 nanoparticles. The study was conducted in a lab-scale batch photo-catalytic reactor using the interval experimental method. MnFeN-tridopedTiO2 nanoparticles were synthesized by sol-gel method and characterization of nanoparticles was determined by XRD, FTIR, SEM and EDX techniques. The effect of different parameters such as pH (3-11), catalyst dose (0.5-2.5gr coating on glass bed), initial HA (2-50mg/l) on the degradation efficiency of HA were investigated. Also photo catalyst degradation of HA by TiO2، N-dopedTiO2 و FeN-codopedTiO2 nanoparticles was studied in optimal conditions. The results showed that efficiency of photo catalytic degradation of HA in optimum conditions (pH=3, nanoparticle dose of 1.5 gr and the concentration of HA 10 mg) were 86.72%.and 63.05% under ultraviolet and visible light, respectively. The results showed that photo catalytic degradation of HA using MnFeN-tridopedTiO2, increased with increasing catalyst dose and decreased with increasing initial concentration of pollutant and pH. Also the results showed photo catalytic activity of doped TiO2 is higher compared to pure TiO2.
Keywords: Humic acid; Photocatalyst Process; Titanium dioxide
















Introduction
Humic acid (HA) derived from decomposition of plants and animal’s residual and it widely present in surface resources of water. Recently Photocatalytic oxidation based on semiconductors has been widely studied. Photocatalytic processes are a types of AOPs processes that start with the radiation of photons (equal to or greater than energy bands) on the surface of the semiconductor catalyst and generation of electron/hole (e-/h+) in the valence and conduction band respectively. The photons generated holes (h+) and electrons (e-) produced hydroxyl (OH°) and super oxide (O2°) by oxidation and reduction of adsorbed H2O molecules and dissolved oxygen. Therefore, OH° and O2° radical efficiency can removed HA at catalyst surface. TiO2 is considered as one of the best catalyst due to its high photocatalytic activity. However, the broad application of TiO2 have limited because of its large band gap (3.2eV) which requires UV light irradiation for photocatalytic activity. Furthermore, high recombination rate of photogenerated e-/h+ pairs is another drawback that reduces the quantum efficiency of TiO2. Research has shown that doped TiO2 with various metal and nonmetal ions is one of the most promising strategies to solve these problems. The ions dopant in TiO2 can improved the photocatalysis efficiency by decreases the band gap and inhabitation of the e-/h+ pair recombination. In this study, TiO2 doped simultaneously with Fe, Mn and N. MnFeN-tridopedTiO2 was synthesized by so-gel method and photocatalytic activity of the synthesized pure TiO2 and tri-doped TiO2 evaluated by monitoring the degradation of HA as target pollutant.
Material and method
MnFeN-tridoped TiO2 were prepared by a traditional sol–gel method. In a typical procedure, a certain amount of TiCl4 as procedure of TiO2 was added dropwise into deionized water under strong magnetic stirring in water bath room. Then, another solution containing ethanol and certain amount of precursors of nitrogen, iron and manganese were dropwise added to the above solution to form sol. after stirring for 30 minute, drops of ammonium hydroxide were added wisely into above obtained solution to formation of white precipitate and solution was made to settle for twelve hours. Then, precipitate was centrifuged and washed with deionized water. Finally, the precipitate was dried in oven at 200°C for 4 hours and nanoparticles MnFeN-tridopedTiO2 was obtained The effects of pH values (3, 5, 7, 9, 11) nanoparticles dose (0.5, 1, 1.5, 2, and 2.5), initial concentration of humic acid (2-50 mg/l) were investigated as critical parameters. HA concentration was monitored using spectrophotometer at wavelength of 254 nm.

Results and Discussion
Characterization of nanoparticles
The results obtained from the XRD analysis of pure TiO2 and doped TiO2 with N, Fe and Mn nanoparticles indicated that all samples consist of anatase phase as the dominant crystalline phase nanoparticles. The surface morphology of pure and doped TiO2 with N, Fe.N Mn.Fe.N nanoparticles are revealed by SEM micrographs. SEM micrographs of doped and un-doped nanoparticles exhibit particles size in the range of 20–60 nm, which is in agreement with the results obtained from the XRD analyses .These results, confirmed the impurity added to the TiO2 structure by preventing the expansion of the Ti-O-Ti bond decrease the growth of the crystalline particles. Fourier transform infrared (FTIR) analyze of synthesis nanoparticles performed in the wavenumber range of 400–4000 cm-1. The absorption peaks at about 3440-3420 and 1630-1620 are related to stretching vibration of O-H and the bending vibration absorbed water molecules. The bands in the range of 400-800 cm-1 were attributed to the symmetric and asymmetric stretching vibrations of Ti-O-Ti and Ti-O bands.


Effect of operating parameters on the degradation of Humic Acid
The effect of different values of pH (3, 5, 7, 9, 11) on photocatalytic degradation of HA was evaluated in a solution with the same initial concentration of humic acid (10 mg/l), dose of stabilized nanoparticle on the surface of glass bed (1.5g MnFeN-tridopedTiO2) under ultraviolet radiation for 120 minutes. The results showed that, the maximum degradation of HA was obtained at pH=3 (86.72%) and the degradation percentage decreased with increasing pH to 11.
At acidic pH, the surface of the titanium dioxide due to existence of H+, has positive charge. Therefore, removal efficiency of HA increased by electrostatic attraction of the negatively charged HA molecules and positively charged TiO2 surface. In order to evaluate the effect of catalytic dosage on photocatalytic degradation HA, different amounts of nanoparticles were investigated. This study showed that the increase of dosage of nanoparticles, removal efficiency increased. The increase in degradation efficiency with increasing dosage of nanoparticles can be attributed to increasing the available surface area, number of active sites at the catalyst surface and UV trapping that leads to the more electron/hole pairs release and production more oxidizing radicals such as OH° and O2° to degradation of HA. Effect of initial concentration of HA on the efficiency process was tested by various concentrations (10, 20, 30, 40, 60, and 100 mg/L) in the same reaction conditions (1.5g MnFeN-tridopedTiO2 fixed on the glass, pH =3) under ultraviolet radiation for 120 minutes. In this study, with increasing initial concentrations of HA, from 2 to 50 mg / L, the photocatalytic degradation efficiency of MnFeN-tridopedTiO2 nanoparticles decreased from 92% to 30% respectively. In order to evaluate the photocatalytic efficacy of various samples of doped TiO2 nanoparticles (N-doped TiO2, Fe-N-codoped TiO2 and MnFeN-tridopedTiO2) and pure TiO2, photocatalytic degradation of HA took place in the same conditions (10 mg / l humic acid, pH = 3, and 1.5 g catalyst dose) under radiation of ultraviolet radiation. Results showed that the photocatalytic activity of nanoparticles follows the UV/TiO2 <UV/N-dopedTiO2<UV/FeN-codopedTiO2<UV/ MnFeN-tridopedTiO2. Increasing photocatalytic efficacy with substitution of impurity elements (N, Fe, Mn) into the structure of TiO2 relative to pure TiO2 can be attributed to the reducing crystalline nanoparticle size, anatase dominant phase, efficient separation of electrons/holes and reducing their recombination rates in the charge transfer path.

Conclusion
According to the results obtained from this study, it can be concluded that simultaneous doping is an effective way to improve the efficiency of photocatalytic activity of TiO2 in the removal of environmental pollutants such as humic acid in aqueous media. Also, by doping of TiO2 and enhancement of visible light response, the sunlight can be used as a natural source of energy in the photocatalytic process.