Ozone reaction mechanisms
| An ozone process is always based on the effect of direct and indirect reaction mechanisms. This is consequential to the disintegration of ozone in water, into OH-radicals. These radicals are very short-living compounds that have an even stronger oxidation mechanism than that of ozone. This is because the radicals have a high oxidation potential [10,11], see table 1. |
Table 1: redox potential of oxidizing agents *
When the number of OH-radicals in a solution rises, one speaks of an Advanced Oxidation Process (AOP). This unique process causes dissolved solids to be oxidized by both ozone (direct) and OH-radicals (indirect). The ozone oxidation process is represented scematically in figure 1.
Figure 1: reactions of ozone and dissolved solids *
Figure 2: dipolar cyclo addition *
In a protonic solution, such as water, primary ozonide disintegrates into an aldehyde, a keton or a zwitter ion, see figure 3. The zwitter ion will eventually be disintegrated further into hydrogen peroxide and carboxyl compounds.
Figure 3: disintegration of ozonide *
Figure 3: reaction between phenol and ozone *
Advanced oxidation processes
Figure 5: oxidation of pCBA by ozonization and AOP for groundwater (GW) and surface water (SW) (parameters GW: DOC: 1 mg/L, alkalinity 5,2 mM; SW: DOC 3,2 mg/L, alkalinity 3,8 mM. Experiment conditions: pH = 7, T = 11 oC, [O3]0 = 2.1 * 10-5 M, [H2O2]=1*10-5 M, [pCBA] = 0,25 μM) *
This shows various oxidation processes of an ozone-resistant compound (para-chlorobenzenic acid, pCBA): oxidation by means of a conventional ozone process and AOP in groundwater (GW) and surface water (SW). The oxidation rate of pCBA in groundwater is higher for AOP than for conventional oxidation, for surface water this is nearly equal. This is caused by a large capacity of radical catchers (scavenging capacity, see carbonate and bicarbonate) in the surface water, which react with OH-radicals. In these situations, AOP is an inefficient process.
HB + H2O -> H3O+ + B-
The text above shows, that ozone is very selective when it comes to particle oxidation, whereas OH-radicals react with practically any compound. Selectivity is caused by the chemical structure of ozone (dipole structure).
Figure 6: fraction of compounds that react with OH-radicals for drinking water ratois Rc (Oh-ozone). Per: tetra chlorethene; MTBE: methyl tert-butyl ether; GEO: geosmin; ATRA: atrazine; tri: trichlorethene. *
Complete mineralization of compounds is too expensive in most cases and is therefor not achievable by both ozone and AOP. These processes are therefor better applicable for compounds that can be freed of unwanted effects by partial oxidation. This can be color oxidation, taste and scent oxidation, or pre-biodegradation of various organic compounds.