Single and Hybrid Applications of Ultrasound for Decolorization and Degradation of Textile Dye Residuals in Water - Green Chemistry for Dyes Removal from Wastewater - page 282

surface, or furnish metals from their cores upon exposure to ultrasound;
and (iii) those which generate electron-whole pairs and/or reactive oxygen
species on their surface upon sonication.
The first group of solids is made up of a wide range of non-reactive
adsorbents (e.g., activated carbon, fly ash) and the governing mechanism
in the degradation of dyestuff in their presence is sono-sorption of the
solutes on the adsorbent surface with the advantages of ultrasound for
continuous cleaning and disaggregation of the particles and for reducing
the micro-vortex motion of the adsorbate on the surface of the adsorbent
[92,101,102]. The most commonly tested adsorbents for the elimination
of dye residuals in process effluents are natural zeolite, clay/nanoclay, chi-
tosan, sepiolite, limestone, and cellulosic materials [92,95,103-108]. Note
that sono-sorption is considerably more cost-effective for the elimination
of dyestuff than conventional adsorption processes, owing to a lower sor-
bent requirement and a lower cost of adsorbent regeneration.
The second group of solids consists of zero-valent metals, metal oxides
and metallic nanoparticles, all of which are excellent adsorbents with
active heterogeneous reaction sites. Some of the unique benefits that ultra-
sound provides to such catalytic systems are perfect mixing, cleaning, sur-
face improving, disintegrating and disaggregating of the solid particles.
The activities of two such solids, namely zero-valent iron and manganese
dioxide, will be discussed later in this section within the scope of hetero-
geneous catalysis. The third group of solids is made up of semiconductors
and their composites, which owing to their unique properties will also be
given special emphasis in the rest of this chapter.
Heterogeneous sonocatalysis has attracted considerable attention
over the last few years for decomposing and mineralizing synthetic dyes
[15,29,60,85,89,91]. The advantages of the process rise not only from the
presence of surplus nuclei for excess cavity formation, but also from the
turbulent flow conditions that accelerate rates of mass transfer and chemi-
cal reactions at solid surfaces, which are remarkably enlarged via disaggre-
gation of the particles. Among a wide range of catalysts tested in various
laboratories, zero-valent metals, nanopowders, composites of semicon-
ductors, graphite and cast iron have been found considerably effective.
Nanoparticles, particularly of TiO 2 and its composites, play a special
role in catalyzing US-mediated reactions. Figure 7.6 shows relative effica-
cies of nano-TiO 2 and zero-valent copper on the degradation of C.I. Direct
Yellow 9 by short-frequency US [29]. Some of the possible reasons for the
higher performance of TiO 2 than copper are: (i) the slightly positive char-
acter of the semiconductor surface at pH < 6.8 (zero point charge) and
the anionic character of the dye that enables attractive forces between the