Effect Of Solid Impurities On Photocatalytic Degradation Of Naphthol Blue Black Over TiO₂ And ZnO

Photocatalytic oxidation technology is one of the advanced oxidation technologies, which has important potential applications for water pollution control. Generally, there may be a lot of solid impurities coexisted with photocatalyst in actual water purification process, which may promote or inhibit the activity of photocatalyst. Thus, it's necessary to clarify the effect of solid impurities on the photocatalytic reaction process. It can provide theoretical basis for the development and application of photocatalytic technology.In this paper, we have investigated the influence of the typical particles of clay minerals, metal oxides, organic matter and biological particles on the photocatalytic degradation activity of naphthol blue-black over TiO₂ and ZnO by a kinetic method, and discussed the impact mechanism. The main research contents and results are as follows:(1) The clay minerals used in the experiment including montmorillonite, kaolinite, illite, pyrophyllite and natural quartz sand, were derived from natural products. The iron oxides containingα-FeOOH,α-Fe₂O₃,Fe(OH)₃ gel,α'-AlOOH,γ-Al₂O₃,α-MnO₂,δ-MnO₂,γ-MnOOH,γ-Mn₃O₄ were made by chemical synthesis. Organic matter including humic acid and fulvic acid, were extracted from natural products. Biological particles such as activated sludge were provided by pharmaceutical factory. The solid impurities, pure photocatalysts and the photocatalysts polluted by solid impurities were characterized by XRD, SEM, and UV-Vis DRS techniques.(2) At different pH,δ-MnO₂ had different effects on the photocatalytic activity of TiO₂ for NBB degradation. The coexistence ofδ-MnO₂ had promotive effect on the photocatalytic activity of TiO₂ at pH 3. But the activity of TiO₂ was inhibited severely at pH 6, 9 and 10. The inhibitive effect was in the order of pH10 2 decreased with the increase of pH value; the inhibition effect can be eliminated at pH 3. At pH 6, the manganese oxides exceptα-MnO₂, inhibited the activity of TiO₂ significantly, in the order of the natural manganese mineral <γ-MnOOH <γ-Mn₃O₄ <δ-MnO₂; But under the same conditions, the solid impurities such asγ-MnOOH and natural manganese mineral increased the photocatalytic reaction rate of ZnO; The coexistence ofδ-MnO₂ reduced the rate of photocatalytic degradation of naphthol blue-black, and the time of complete degradation of dyes extended from the original 25 minutes to 35 minutes. Compared with TiO₂/manganese oxides system, ZnO had a strong ability to resist manganese inhibition effect. The UV-Vis DRS results of TiO₂/δ-MnO₂ samples showed thatδ-MnO₂ induced a blue shift of the absorption band edge of TiO₂. SEM images indicated that the size of TiO₂ increased in the presence ofδ-MnO₂, which caused the decreased activity of TiO₂.(3) At pH 6, the selected clay minerals had a weak inhibitive effect on the activity of TiO₂, and montmorillonite had the biggest inhibition, but all reactions were still fitted to first-order dynamics.(4) The degree of inhibition ofα-FeOOH increased slightly with increasing its concentration. At pH 6, the rate constant of the reactions in the presence of iron oxides were less than that of comparative experiment, but the reaction still followed the first order kinetic behavior.(5) At pH 6, the coexistence ofα'-AlOOH andγ-Al₂O₃ made the kinetic curve right-shift. The reaction rate decreased, and the reaction process didn't fit the first order kinetic behavior any more.(6) The effect of humic acid and fulvic acid on the activity of TiO₂ and ZnO were completely different. Humic acid had almost no impact on the time of complete degradation of NBB by TiO₂ and ZnO, and only caused the kinetic curves were downward. But both the catalytic activities of TiO₂ and ZnO were inhibited obviously in the presence of fulvic acid, which was the poison of photocatalysts. The results of DRS showed that the intensity of TiO₂ and ZnO samples contaminated by fulvic acid were lower, and their absorption edge shifted to blue.(7) The addition of activated sludge inhibited the photocatalytic activity of TiO₂, especially at pH 6 and pH 9.