Effects of ozonation on degradation of Azo Dyes
Date Issued
2006
Date
2006
Author(s)
Hsing, Hao-Jan
DOI
en-US
Abstract
The ozonation of azo dyes both in semibatch reactor and continuous bubble column reactor are investigated in this study. The reaction rate for decolorization is faster than that of total organic carbon (TOC) reduction, and, as TOC removal ratio > 0.9 in semibatch system, the dissolved ozone concentration is nearly not detectable, suggesting that most ozone transfer from gas phase to liquid phase is consumed rapidly in oxidizing the azo dye by breaking the azo bond (N=N). As monitoring the selected parameters during the semibatch experiments, decolorization, and TOC removal were affected by pH, temperature, and ozone dose. Decolorization and AO 6 reduction were completed within 40 minutes of ozonation time for all examined cases. When the ozone dose increased from 9.7 to 33.3 mg/l-min, the decolorization rate constant increased from 0.14 to 0.27 min-1. The increases of k values suggest that decolorization and AO6 removal were affected by ozone dose. For the TOC removal, its reaction rate constants were temperature dependent and the effects of ozone dose and pH were not significant. Sulfate yield was accelerated upon the completion of decolorization. The sulfate yield may be due to the destruction of azo bonds and surplus ozone or OH– turns to attack aromatic rings. Significant increment of sulfate yield was observed during the TOC removal process.
Due to the ability to oxidize organics in water, both ozone alone and O3/UV254 are used to investigate the effectiveness of deolorization and mineralization of AO 6, the target compound. The results show that O3/UV254 system may reach the level of total mineralization of AO6 within 120 min, also revealing that the O3/UV254 system is a powerful treatment to breakdown all organic carbon in solution. During the ozonation, the nitrogen mass balance can be achieved that is due to the nitrogenous compounds are formed, such as N2, NO, and NO2.
In the countercurrent flow bubble column system, TOC, sulfate, and nitrate, but the variations of pH, A597 nm, and color all reach steady constants at 1 HRT. It suggests that color and N=N are easily to be removed comparing to TOC; the sulfate formation is consistent with TOC reduction. To control liquid or gas flow can affect the order of reaction, a pseudo-first order reaction is suggested in the fixed liquid flow rate experiments and a second order reaction may well explain for the fixed gas flow rate conditions. Not only flow patterns affect the RB5 removal and the mineralization of derivatives but also the column height may have influence on the ozone consumption. The extent of decolorization and mineralization decrease as the sampling port height increasing, indicating that the column height may reflect the retention time of ozone gas and the contacting time between ozone and RB 5 in BCR system. The biodegradability is enhanced via ozone treatment that is not proportional to the amount of ozone consumption but all results exhibit the similar increment trends.
Subjects
臭氧化
偶氮性染料
去色
礦化
生物降解
毒性
反應速率常數
Ozonation
Azo dye
decolorization
mineralization
biodegradation
toxicity
reaction rate constant
Type
thesis
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