Influences Of Common Inorganic Ions On The Photocatalytic Degradation Of Methylene Blue With Ag₃PO₄ Under Visible-Light Irradiation

With the rapid development of economy, environmental pollution problems are more and more serious. As a new environmentally-friendly materials, semiconductor-based photocatalysis could deal with pollutants with the help of sunlight, and opened an effective approach for human to solve environmental pollution problem. Titanium dioxide (TiO2) was recognized as the most promising photocatalyst and used widely owing to its high activity, high stability, low energy-cost and non-toxicity. However, TiO2 has a wide band gap, and it only absorbs UV irradiation, such lower utilization of sunlight badly limits its application in photocatalytic elimination of pollutants.2010, Ye and co-workers reported an active visible light driven photocatalyst silver-orthophosphate (Ag3PO4), it can absorb solar energy with a wavelength shorter than 530 nm. Moreover, quantum efficiencies of this novel photocatalyst could achieve to 90% at wavelengths greater than 420 nm. So Ag3PO4 has a great potential in the decomposition of organic pollutants and O2 evolution from water.Although Ag3PO4 has been proved to be a highly efficient photocatalyst for the decomposition of organics under visible-light, the practical industrial wastewater usually contains considerable amounts of common inorganic ions. The presence of these ions, which might cause effect on the photocatalytic degradation of organic compounds. So the effects of inorganic ions on photocatalytic degradation of dyes were investigated, and the results will provide systematic details and references for the practical application of Ag3PO4 in photocatalytic water decontamination.In this paper, Ag3PO4 was prepared by a simple ion-exchange method in dark. We choose Ag3PO4 as catalyst, azo dye methylene blue (MB) as simulated pollutant, fix the initial concentration of MB, the dosage of the catalyst and irradiation time to implore the adsorption and the photocatalytic degradation of MB on Ag3PO4 in the presence or absence of inorganic ions. The effects of inorganic ions on photocatalytic degradation of MB on AgsPO4 were evaluated by comparing the pseudo-first order rate constants. Results show that:(1) Nanosized Ag3PO4 was synthetized by a simple ion-exchange route, and characterized with scanning electron microscopy, X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The photocatalytic performance of the as-synthesized Ag3PO4 product was evaluated by degradation of MB under visible light irradiation. Result shows that as-prepared Ag3PO4 with a average diameter of 1～5μm, reveals better adsorption of light to 530 nm, as well as good dispersion and purity. And it has an excellent photocatalytic performance, which the photocatalytic degradation rate of MB was 95%.(2) Among NO3-, OH-,NO2-, Cl-,Br-,HCO3-, CO32-, SO42-,SO32-, S2- and PO43-,NO3- and SO42- show negligible effects on adsorption. The anion-exchange reaction between OH- (the original or that from the hydrolysis of CO32-, HCO3-, SO32-, and PO43-)as well as Cl-,Br-, and S2-, and PO43- on Ag3PO4 surface could enhance the adsorption of MB on Ag3PO4, while NO2- weakens the adsorption through competitive adsorption with MB.(3)The effects of these anions on the photocatalytic degradation of MB are highly dependent on their concentrations in aqueous solutions. In the whole range of investigated concentration (0-2 mmol/L), NO3-shows negligible effects; SO42- causes slightly detrimental effects; OH-, NO2-, CO32-, and S2- show obvious negative effects in the whole range of investigated concentration, while HCO3-, Cl-, SO32- and PO43-exhibited positive effects under low concentration and negative effects under high concentration. Br" shows a unique effect that is negative under low concentration and positive under high concentration. At the same time, the influence mechanisms of these inorganic anions on the photocatalytic degradation of MB on Ag3PO4, as well as their enhancement or inhibition on the adsorption, were discussed at fundamental level.(4) The adsorption and the photocatalytic reaction were very sensitive to metal cations except Na+and K+. In general, the adsorption and the degradation rate of MB could be strongly inhibited in the presences of Zn2+, Mn2+, and Cu2+ even at a low concentration of 0.1 mmol·L-1, while Mg2+ and Ca2+ show slight suppressing effects. And Al3+ brings adverse effect to varying degrees. Fe3+ exhibits a unique effect, that is, under low concentration of 0.02～0.1 mmol·L-1, Fe3+ demonstrates slight enhancement on both the adsorption and the degradation of MB, whereas at high concentration level it strongly inhibits the adsorption and the photocatalysis.(5) ICP-AES and TEM analysis suggest that the suppressing effects of the adsorption and the photocatalysis of Mg2+, Ca2+, Zn2+, Mn2+, Cu2+ and Al3+ mainly attributed to the lower solubility product constants of their phosphates. The effect of Fe3+ is not only related to the lower solubility product constants of FePO4 but also the trapping ability of Fe3+ for photo-generated electrons. It is expected that suitable thickness of FePO4 will be positive to enhance the photocatalytic oxidation of MB on Ag3PO4.