Study On Absorption And Photodegradation Of Methylene Blue In Aqueous Solution By Cyclotriphosphazene Cross-links

Control and treatment of environment pollution is a major issue, which isattracted attention from all over the world. Among numberous environmentalpollution control technology, the metal oxide semiconductor photocatalytictechnology is an energy efficient and environment friendly way to treat pollution inthis year. Many photocatalyts weakly response to visible light, their applicationis limited. Therefore, the photocatalyst responsing to visible light become a researchhotspot.Hexakis(4-cyanophenyloxy) cyclotriphosphazene and hexakis(4–tetrazolylphenyloxy)cyclotriphosphazene were synthetized from hexachlorocyclotriphosphazene.They were identified by1H-NMR，13C-NMR and31P-NMR. A serial of novelcross-link photocatalysts (PCP12, PCP13and PCP16) were synthetized byhexakis(4-tetrazol-phenyloxy)cyclotriphosphazene and terephthaloyl chloride. Theyare characterized by FT-IR, TG, BET and solid UV.The adsorption properties of photocatalysts of cyclotriphosphazene crosslinkswere studied. Firstly, methylene blue (MB) was substrate, the adsorption amount ofPCP12, PCP13and PCP16were studied under different pH condition. The resultsshow that their adsorption increases with increasing pH, when the pH is greater than11, the adsorption amount remains unchanged. For research the adsorption powerof cyclotriphosphazene crosslinks, the amount of adsorbed Reactive Brilliantnegatively charged (X-3B) was investigated under different pH condition. The resultsshow the amount of adsorption decreases with pH increasing, when the pH is greaterthan7, the adsorption amount is almost zero. Thought cyclotriphosphazenenecrosslinks adsorb positive (MB) and negative(X-3B) dyes under different pHcondition, the attraction between positive and negative is main power of adsorption.Secondly, the time of three cyclotriphosphazene crosslinks photocatalysts reachedsaturated adsorption were investigated. PCP12just needs1minute to saturatedadsorption, PCP13and PCP16respectively need30minutes and90minutes.PCP12exhibits a significant rapid adsorption capacity. Thirdly, the desorption of MBfrom PCP12was investigated. The mixed acetic acid and methanol has higherdesorption rate than HCl solution (pH=1). The content of acetic acid in methanol hasa significant effect on desorption rate, the desorption rate increases at low content,when the volume content is up to15%, the desorption rate is maintained at86%.Finally, the re-adsorption capacity of PCP12was studied. The results show that at thesecond adsorption the amount of adsorption is decreased, because MB stuck in small holes can be desorbed, subsequently, at re-adsorption, the amount of adsorptiondecreases. But at the third time, the amount of adsorption still is greater than75mg/g.So, PCP12has a good re-adsorption capacity.The performance of photocatalyst degradation of cross-link cyclotriphosphazenes was investigated. To compare photodegradation ability of three cyclotri-phosphazene crosslinks, methylene blue and X-3B were degraded under the samecondition. The results show they have different degradation ability, and PCP16showsthe best performance. To degrade the same amount of MB (100mL,10mg/L), PCP12needs the shortest time (9h). To degrade the same amount of X-3B (100mL,7mg/L),after6h irradiation, the residue is the least (38%). Secondly, the intermediate productsin the process of MB photodegradation were identified. Four organic solutions(cyclohexane, ethyl acetate, ethanol and diethyl ether) were used for extraction of theintermediate products in the aqueous layer. The intermediate products werecharacterized by GC-MS, and the inorganic ions were characterized byion chromatography (IC). The aqueous layer contains NO3-and SO42-, which comefrom complete oxidation of the N and S of MB. Thirdly, the active substance inphotodegradation process was investigated. There is hydroxyl radical underirradiation both in water and in MB solution. At the same exposure time (10min), theintensity of hydroxyl radical in water is greater than in MB solution, indicating thedye consumes the hydroxyl radical in degradation process, and the dye does notaccelerate the degradation and does not sensitize catalysts. Finally, according to theintermediate products given by GC-MS and ion chromatography and semiconductorphotocatalyst degradation characteristics, the mechanism of degradation MB bycyclotriphosphazene crosslinks was given.