Degradation of bisphenol-A (BPA) by ultrasonic irradiation at 300 kHz was investigated at varying substrate concentrations, pH, hydroxyl radical scavengers and sparge gases. It was found that increases in BPA initial concentration slowed down the reaction rate, but enhanced the sonochemical product yield. Both the rate of reaction and the product yield were adversely affected by pH elevation above the neutral level. The degree of BPA decay was fastest in the presence of air due to the formation of acids and excess radicals, and slowed down as the gas was replaced by argon and oxygen. The impact of large concentrations of hydroxyl radical scavengers such as carbonate and t-butanol was to decrease the rate of degradation, particularly when the scavenger concentration was considerably larger than that of BPA, and the deceleration was more remarkable in the presence of t-butanol than in that of carbonate.
Hydroxyl radical (*OH) scavenging effects of carbonate (CO3(2-)), bicarbonate (HCO3-) and chloride (Cl-) ions on the performance of an ultraviolet light-hydrogen peroxide (UV/H2O2) advanced oxidation process were investigated using a reactive azo dye, C.I. Reactive Red 141 as the model compound. It was found that in the absence of scavengers, complete color removal was possible in 15 min under the experimental conditions employed. The rate of color decay was inhibited remarkably with increasing concentrations of bicarbonate and carbonate species, especially when the distribution of alkalinity favored the dominance of carbonate ions. It was further observed that chloride inhibited the rate only when it was between 100-1250 mM as Cl, and was ineffective at higher concentrations.
Lab-scale degradation of azo dyes with ultrasound (300 kHz), ozone and both was investigated using an aryl-azo-naphthol dye-C.I. Acid Orange 8. It was found that in all schemes color decay was faster than UV absorbance, and the rates followed pseudo-first-order kinetics except for the decay of UV-254 band by ozone. Sonication alone was sufficient for decolorization, but not for UV absorption abatement or mineralization. Ozonation was more effective than ultrasound in bleaching, but not as much for the mineralization of the dye. Combined operation of ultrasound and ozone improved the rate of bleaching and UV absorption decay and remarkably enhanced the mineralization of the dye. This was attributed to increased mass transfer of ozone in solution and its decomposition in the gas phase to yield hydroxyl radicals and other oxidative species. The effect of alpha-methyl substituent at the aryl carbon of the dye was found to decelerate the rate of degradation as a result of weakened intramolecular hydrogen bonding.
Sonochemical degradation of aryl-azo-naphthol dyes represented by C.I. Acid Orange 8 was investigated at 300 kHz to assess the operational parameters and the impacts of rate enhancers (CCl(4)) and rate inhibitors (t-butyl alcohol). It was found that the degradation of the dye was accelerated with increased concentrations of CCl(4) via the accumulation of reactive chlorine species and the hindrance of OH radical combination reactions with atomic hydrogen. The addition of t-butyl alcohol at all test concentrations inhibited the degradation of the dye regardless of the quantity of OH radicals (or H(2)O(2)) in solution. The inhibition was explained by the competition of the dye and t-butyl alcohol at the gas-liquid interface. Finally, the rate of dye degradation in the presence of both reagents at their effective concentrations was found to be considerably slower than that with CCl(4), showing that the formation of reactive chlorine species was remarkably suppressed by the rapid reaction of t-butyl alcohol at the gas-liquid interface.
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