| Ultrasonic irradiation alone is unable to provide high degradation efficiency tocertain recalcitrant organic contaminants. This is particularly true for hydrophiliccompounds which are usually hard to be decomposed by conventional means as wellas through cavitation phenomenon. Besides, ultrasonic irradiation requires costlyequipment and consumes high amount of energy while not all of the cavitationalenergy can transform into chemical and physical effects. These drawbacks postssevere limitation to success in application in a real wastewater treatment plant. Toovercome the limitation of low degradation efficiency, many efforts have been madeon investigation of various combined ultrasound (US) systems. Of these, applicationof heterogeneous catalysts in ultrasound system has been of most interest, sincethis operating cost could be partially off-set by operating at milder and optimumoperation conditions, which enhance the degradation rate and reduce the reactiontime. So, in this dissertation, different kinds of solid catalysts were applied inultrasound system to enhance the degradation of target pollutants.Firstly, the catalysts of high performance were chosen from zero-valent metals,metal oxides and metal hydroxyl compounds through the sono-catalytic degradationof resorcinol, p-nitrophenol and p-chloronitrobenzene. The most effective catalystand the corresponding target pollutant were determined by comparing the synergyeffect between solid catalysts and ultrasound, which was quantified by calculatingthe enhancement factor. Results show that the highest enhancement factor wasfound in the p-NP removal by US/Zn. And its performance in p-NP degradation andthe synergy mechanism were studied.Data showed that the removal of p-NP by US/Zn favored lower pH and higherzinc dosage. The optimized treatment condition was found to be as follows: initialpH7.0, zinc loading0.6g/L, ultrasonic power120W and solution temperature25℃after a comprehensive study taking cost, equipment corrosion and treatmentefficiency into consideration. Besides, the increase of ionic strength resulted inlower p-NP removal ratio and initial concentration too low or too high could alsoresult in pooer performance of US/Zn system. Results also showed that p-NPremoval dropped by37%when reaction solution was prepared using tap waterinstead of de-ionized water. The performance of recycled zero-valent zinc waslower than new zinc powder.The degradation process of p-NP in the coupled US/Zn system fit thefirst-order reaction kinetics model and the corresponding first-order rate constant was0.075min-1. The addition of radical scavengers such as tert-butyl alcohol,carbonate and bicarbonate could obviously lower the degradation ratio of p-NP,indicating that some p-NP were removed via·OH reaction. When ultrasound wascombined with zero-valent zinc, the constant mechanical washing iduced byultrasound could help to keep Zn active, create more reactive surfaces andaccelerate mass transfer. Meanwhile, the particle effects of zero-valent zinc alsostrengthen the ultrasonic cavitation and enhance denitrification of p-NP. These allcontribute to the remval of p-NP and accelerate the dissolution of zinc and promotethe generation of nitrite and nitrate at the same time. The alternation of p-NPdegradation pathway by US/Zn might also contribute to the synergy mechanismbetween them. Besides, the comparison of the performance of different forms ofzinc also indicated that it was the zero-valent zinc itself that played the major role inthe enhancement of p-NP removal rather than other forms of zinc. |
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