I. Introduction to ozone catalytic oxidation process
Ozone is mainly a strong oxidizing agent in chemical properties. Its oxidizing ability is second only to fluorine, ·OH and O (atomic oxygen). Its oxidizing ability is 1.52 times that of elemental chlorine. In aqueous solution, the reaction mechanism of ozone pollutants mainly includes direct oxidation by ozone and indirect oxidation by free radicals.
Direct oxidation reaction
The direct oxidation reaction between ozone and organic pollutants in water can be divided into two modes:
(1) Electrophilic substitution reaction. Electrophilic substitution reaction mainly occurs at the position with large electron cloud density in the molecular structure. In molecules with substituted phenyl structures such as -OH, -CH3, and -NH2, the electron cloud density of carbon atoms at the ortho and para positions in the benzene ring is large, and the carbon atoms at these positions are easy to undergo electrophilic substitution reaction with ozone.
(2) Dipolar addition reaction. Since ozone molecules have a dipole structure (dipole moment is about 0.55D), when ozone molecules interact with molecules containing unsaturated bonds, dipole addition reaction can be carried out. Generally speaking, the direct oxidation reaction rate of ozone is slow and the reaction is selective, so its efficiency in degrading organic pollutants is low.
Indirect oxidation reaction of free radicals
(1) Indirect oxidation degradation of free radicals can be roughly divided into two stages according to the reaction process: the first stage is the self-decomposition of ozone to produce free radicals. When an initiator such as OH- is present in the solution, the rate of ozone decomposition to produce free radicals can be significantly accelerated. In the second stage, OH reacts with the active structural units in the molecule (such as benzene ring, -OH, -NH2, etc.) and initiates a free radical chain reaction. As the reaction proceeds, the molecular structure is oxidized and broken, decomposed and converted into small organic molecules, such as formic acid, acetic acid, etc., or further these small organic molecules are completely mineralized (with total organic carbon (TOC) as the test indicator) into CO2 and H2O, thereby achieving the purpose of reducing COD (chemical oxygen demand) in the effluent and improving the biodegradability of the treated wastewater.
(2) The reaction selectivity of OH free radicals is very small. When there are multiple pollutants in the water, it will not happen that one substance is degraded while the concentration of another substance remains basically unchanged.
The reaction between ozone and antibiotics in water is relatively complex. In a reaction system, both direct ozone oxidation reaction and indirect free radical oxidation reaction often occur. The pH value of the solution plays a decisive role in the selection of the mechanism of O3 oxidation reaction. In a strong acidic medium, direct oxidation reaction is dominant, while in an alkaline medium, indirect free radical oxidation reaction is dominant.
This process package adds a solid catalyst to the ozone contact reaction tank to further improve the effect of ozone catalytic oxidation.
){kind=link}