Advanced environmental chemistry 4

pple bonds) This leads to the formation of a compound called ‘ozonide’ (I): disintegration of ozonide 40  Electrophilic reactions Electrophilic reactions occur in molecular solutions that have a high electronic density and mainly in solutions that contain a high level of aromatic compounds Aromatic compounds that are substituted by electron donors (such as OH and NH2), have a high electronic density on the carbon compounds in ortho and para position Consequentially, in these positions aromatic compounds react actively with ozone 41 Indirect reactions - Contrary to those of ozone, OH-radical reactions are largely selective - Indirect reactions in an ozone oxidation process can be very complex Initiation Radical chain-reaction Termination 42  Initiation The first reaction that takes place is accelerated ozone decomposition by a type of initiator This can be an OH-molecule: 1: O3 + OH-  O2• - + HO2• This radical has an acid/ base equilibrium of pKa = 4,8 Above this value, this radical no longer splits, because it forms a superoxide radical: 2: HO2•  O2•- + H+ (pKa = 4,8) 43  Radical chain-reaction Now, a radical chain-reaction takes place, during which OH-radicals are formed The reaction mechanism is as follows: 3: O3 + O2•-  O3•- + O2 4: O3•- + H+  HO3• (PH < ≈ 8) The OH-radicals that have formed react with ozone according to the following reaction mechanism: 5: OH• + O3  HO4• 6: HO4•  O2 + HO2 44 Initiator Promotor Radical catcher (inhibitor) OH- Humic acid HCO3-/CO32- Fe2+ Primary and secondary alcohols Humic acids H2O2  Aryl-R PO34- Aryl-R Tert-butyl alcohol (TBA) 45 ...  O3•- + O2 4: O3•- + H+  HO3• (PH < ≈ 8) The OH-radicals that have formed react with ozone according to the following reaction mechanism: 5: OH• + O3  HO4• 6: HO4•  O2 + HO2 44 Initiator... an acid/ base equilibrium of pKa = 4, 8 Above this value, this radical no longer splits, because it forms a superoxide radical: 2: HO2•  O2•- + H+ (pKa = 4, 8) 43  Radical chain-reaction Now, a... acid HCO3-/CO32- Fe2+ Primary and secondary alcohols Humic acids H2O2  Aryl-R PO 34- Aryl-R Tert-butyl alcohol (TBA) 45