Degradability of synthetic dyestuff by acoustic cavitation: impacts of system conditions and physical /chemical agents

Abstract

The use of ultrasound in environmental applications is a novel advanced oxidation process that is currently being investigated as a method to degrade refractory organic wastes, which can not be degraded by conventional wastewater treatment methods such as biological, chemical and combinations thereof. The application of ultrasound to environmental problems relies on the process of acoustic cavitation: the formation, growth, and implosive collapse of micro bubbles in a liquid. The collapse of such bubbles creates hot spots with temperatures as high as 5000 K, and pressures up to 800 atm, and cooling rates in excess of 1010 K s-1. These conditions are responsible for a variety of physical and chemical effects. Hydroxyl radicals that are formed during the homolytic cleavage of water molecules upon bubble collapse can be utilized to degrade many compounds including persistent environmental pollutants. In addition, radical formation can be enhanced by coupling of ultrasound with oxidants and/or UV light. In this dissertation, the degradation of pure and synthetic dyebath solutions of 9 textile dyes were investigated in three ultrasonic systems (System I: 300 kHz, SystemII: 520 kHz, and System III: 3 x 520 kHz frequency), in the presence and absence of chemical oxidants (ozone, hydrogen peroxide, ferrous ion) and/or UV light. The impacts of system conditions, physical/chemical agents, dye properties and dye-bath matrix on sonolytic destruction of textile dyes were studied. The performance of the systems was assessed and compared with each other by monitoring color, organic matter, toxicity, total dissolved solids, total organic carbon, and chemical oxygen demand degradation, and the increase in biochemical oxygen demand.In case of system comparison, the efficiency of the studied systems with respect to decolorization of the test solutions was such that: System I > System II > System III. System efficiency with respect to the calculated product yield was in the order: System II > System I > System III. Injection of different gasses during ultrasonic irradiation showed that rate of decolorization increased in the sequence: Ar > O2 > Air, and maximum decolorization was obtained with an Ar:O2 gas mixture, at a ratio of 66 per cent Ar to 34 per cent O2.It was found that, the decolorization rate of all dyes was more closely related to their structural properties than to the composition of the dye-baths. Anthraquinone dyes bleached faster than azo dyes. The presence of a-substituents around the -N=N- bond accelerated the decolorization rate. Decolorization of dyes with a single OH substituent in ortho position to their reactive component was faster than those with a second a-substituent such as SO3. Decolorization was decelerated by the formation of ionic sites. Decolorization in dyebath effluents was inhibited only in the presence of sufficiently large carbonate and chloride ions.The abatement in the visible absorption of sonicated dye solutions and dyebaths was always larger than the abatement in their UV absorption. Toxic dyes were detoxified by ultrasound within short contact, but ultrasound was not effective in the overall degradation of the dyes as measured by chemical oxygen demand and total organic carbon of the effluents, unless combined with physical/chemical agents. Sonication of dye solutions in the presence of O3, Fe2+, H2O2, and/or UV irradiation increased degradation yields considerably with respect to those applied individually. The most effective combined scheme was ultrasound/ozone/UV irradiation.Estimated bimolecular rate coefficient of azo dyes with hydroxyl radicals was calculated as 1.22 x 109 M-1s-1. Estimated operational costs of System I, II and III for 45 per cent bleaching of azo dyes were 3.52 USD m-3, 3.37 USD m-3, 9.98 USD m-3, respectively. The cost of O3/US combination in System III was 2.57 USD m-3, and the operation cost of ozonation was 3.60 USD m-3.