Page 169 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Oxidation Technologies for Wastewater Treatment: An Overview 143

are created, where the temperature and pressure can reach up to 10,000 K
and 2000 atm; high velocity water jets (100–300 m/s) are also created
(Adewuyi, 2001; Didenko et al., 1999; Pang et al., 2011). Some of the
secondary effects resulting from the cavity collapse are chemical transforma-
tion (chemical bond breakage) that releases highly reactive free radicals,
physical cleaning of solid surfaces, and enhancement in diffusive mass trans-
fer rates. The destruction/oxidation of organic pollutants using cavitation
takes place through two mechanisms: (1) thermal decomposition/pyrolysis
of the volatile pollutant molecule in and around the collapsing bubbles and
•
(2) oxidation of pollutant molecules by reactive free radicals (such as HO ,
• •
O , and HOO radicals) generated during the cavity collapse (Hua and
Hoffmann, 1997). The cavity contains gases and water vapors and other vol-
atile contents that dissociate under cavitating conditions according to the
•
•
reactions such as cleavage of water molecules (into H atoms and OH rad-
icals) and dissolution of oxygen molecules. From the reactions of these enti-
• •
•
ties (O ,H , OH) with each other and with H 2 O and O 2 during the rapid
•
•
quenching phase, HO 2 radicals and H 2 O 2 are formed. These radicals ( OH,
• •
O , and HOO ) then diffuse into the bulk liquid medium where they react
with oxidizable pollutants and oxidize them. The following are the pos-
sible reactions occurring as the result of cavity collapse (Hamadaoui and
Naffrechoux, 2008; Pang et al., 2011):
• •
H 2 O+ ÞÞÞ ! HO +H (3.1)
•
O 2 + ÞÞÞ ! 2O (3.2)
• •
O +H 2 O ! 2HO (3.3)
• •
HO +H ! H 2 O (3.4)
• •
2HO ! O +H 2 O (3.5)
• •
H +O 2 ! HOO (3.6)
•
2HO ! H 2 O 2 (3.7)
•
2HOO ! H 2 O 2 +O 2 (3.8)
Cavitation is classified into four types based on the method of generation:
acoustic, hydrodynamic, optic, and particle. Among these, only acoustic and
hydrodynamic cavitation (HC) have been found to be economically effi-
cient and feasible in bringing about the desired chemical and physical
changes, whereas optic and particle cavitation are typically used for single
bubble cavitation, which fails to induce chemical change in a bulk solution.