Hydroxyl radical induced degradation of aromatic molecules 1
Poorest source of hydroxyl radical Radiolysis of water H 2 O H 2 O + + e H 2 O H 2 O* H 2 O + + H 2 O H 3 O + + OH e + nh 2 O e aq H 2 O* OH + H e aq + H 3 O + H + H 2 O k = 2.3 10 10 mol 1 dm 3 s 1 2
Hydroxyl radical, properties The OH is oxidizing radical. Reduction potential: 1.9 V In N 2 O saturated solution the yield of OH reacting with solute of 10 3 mol dm 3 concentration G 0.56 µmol J 1. H contributes to reactions with a yield 0.057 µmol J 1. N 2 O + e aq + H 2 O N 2 + OH + OH k = 7 10 9 mol 1 dm 3 s 1 In alkaline solution OH converts to O ion with a pk a of 11.9: OH + OH O + H 2 O k = 1.3 10 10 mol 1 dm 3 s 1 Light absorption of OH is below 220 nm.
OH + aromatic molecule reaction Direct oxidation OH + C 6 H 5 OH C 6 H 5 O + H 2 O OH + C 6 H 5 O C 6 H 5 O + OH Addition forming hydroxycyclohexadienyl radical OH + C 6 H 5 OH OHC 6 H 5 OH H-atom abstraction from side chain OH +CH 3 C 6 H 5 CH 2 C 6 H 5 + H 2 O Aromatic molecule + OH reaction is dominated by addition
OH addition to phenol Monitoring charge distribution 5
Methods of rate coefficient determination Direct method, build-up measurement When the signal growth is followed by rapid decay one tends to underestimate the initial reaction period and as a result overestimate the rate coefficient. The unrealistic rate coefficients, e.g. those which are much higher than the diffusion limited rate coefficient, given in number of cases for reaction of OH with phenols, are suggested for reevaluation. (Schuler R.H., Albarran, G., 2002. Radiat. Phys. Chem. 64, 189 195).
Rate coefficient determination by build-up 150 Absorbance, arb. unit 100 50 0 OH + PhOH OHPhOH d[ OHPhOH] = k [PhOH] [ OH] dt 2 OHPhOH Products 0,0 1,0x10-6 2,0x10-6 Time, s
Rate coefficient determination by build-up OH + PhOH OHPhOH d[ OH] = -k [PhOH] [ OH] = -k [ OH] [ OH] = [ OH] 0 e -k t dt d[ OHPhOH] = k [ OH] = k [ OH] 0 e -k t dt [ OHPhOH] = [ OH] 0 (1 - e -k t ) = [ OHPhOH ] (1 - e -k t ) A OHPhOH = A OHPhOH (1 - e -k t ) 2 OHPhOH Products
Methods of rate coefficient determination Pulse radiolysis with competitive technique Thiocianate k 20 OH + SCN OH + SCN 1.1 10 10 mol 1 dm 3 s 1 SCN + SCN (SCN) 2 k 49 OH + S OHS 1 [(SCN) 1 [(SCN) = + 2 ] 2 ] 0 1 [(SCN) 2 ] 0 ε 500 nm = 7100 mol 1 dm 3 cm 1 k k 49 20 [S] [SCN Ethanol C 2 H 5 OH + OH C 2 H 4 OH + H 2 O 1.9 10 9 mol 1 dm 3 s 1 Ferrocyanide OH+[Fe(CN) 6 ] 4 OH +[Fe(CN) 6 ] 3 9.3 10 9 mol 1 dm 3 s 1 λ max = 410 nm, ε max = 1000 mol 1 dm 3 cm 1 Wilson, R.L., Greenstock, C.L., Adams, G.E., Wageman, R., Dorfman, L.M., 1971. Int. J. Radiat. Phys. Chem. 3, 211 220. ]
Methods of rate coefficient determination Competitive technique using final product measurement The method using steady-state gamma radiolysis and competition with selected compound often gives good agreement in the rate coefficient with the ones determined by direct method. For the good agreement application of low conversion is needed. In rate coefficient determinations for hydroxyl radical generation sometimes Fenton reaction is used, the rate coefficient is obtained by a competitive technique. The Fenton system is rather complex, the results should be considered with some care.
Structure dependence of rate coefficients OH + Monosubstituted aromatics log k O H, mol -1 dm 3 s -1 10.2 10.0 9.8 9.6 9.4 9.2 k d a) PhO - PhNH 2 PhCH 2 COO - PhCH 2 CH 3 PhCH(CH 3 ) 2 PhCOO - PhNHCOCH 3 PhOH PhCH 3 PhCH 2 CH 2 OH PhF PhH PhCl PhCONH 2 + PhCH(CH 3 )NH + 3 PhCH 2 NH 3 - PhBr PhI PhSO 9.0 3 PhCHO PhCOOH PhSO 3 H -1.0-0.8-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 σ p PhCH 2 OH PhNH 3 + PhCN PhNO 2 PhCOCH 3 PhSO 2 NH 2
Hammett substituent constants The values of σ are defined from the ionization constants of benzoic acids: σ = log K X - log K H Where K H is the ionization constant for benzoic acid in water (4.29) at 25 o C and K X is the corresponding constant for meta- or para-substituted benzoic acids.
The diffusion controlled rate coefficient: k diff = 4 π (D PhX + D OH ) (r PhX + r OH ) N 10 3 (mol 1 dm 3 s 1 ) D PhX and D OH diffusion coefficients in m2 s 1 are the reaction radii N is the Avogaro s number r PhX and r OH k diff 1.1 10 10 mol 1 dm 3 s 1 r OH, x 10 9 m r PhX, x 10 9 m D OH, m 2 s 1 D PhX, m 2 s 1 Benzene 0.33 1.4 10 9 0.22 2.31 10 9 Chlorobenzene 0.35 0.4 10 9 Phenol 0.35 0.4 10 9 Ashton, L., Buxton, G.V.; Stuart, C.R., 1995. J. Chem. Soc., Faraday Trans. 91, 1631 1633. Wojnárovits L., 1984. J. Photochem. 24, 341 353.
Separation of diffusion and chemical reaction controlled rate coefficients 1/k OH = 1/k diff + 1/k chem k OH, k, diff mol 1 dm 3 s 1 mol 1 dm 3 s 1 k chem, mol 1 dm 3 s 1 0.2 10 10 1.1 10 10 0.24 10 10 0.4 10 10 0.63 10 10 0.6 10 10 1.3 10 10 0.8 10 10 2.9 10 10 1.0 10 10 1.1 10 11
Structure dependence of rate coefficients OH + Monosubstituted aromatics 11,0 10,8 PhO - PhOH PhCH 3 PhCH 2 OH PhH PhF PhCl b) 10,6 PhNH 2 PhCH(CH 3 )NH 3 + log k chem, mol -1 dm 3 s -1 10,4 10,2 10,0 9,8 9,6 PhCH 2 COO - PhCH 2 CH 3 PhCH(CH 3 ) 2 PhCH 2 CH 2 OH PhCOO - PhNHCOCH 3 + PhCH 2 NH 3 + PhNH 3 PhCONH 2 PhCN PhNO 2 9,4 9,2 - PhBr PhSO PhCOCH 3 PhI 3 PhCHO PhCOOH PhSO 3 H PhSO 2 NH 2-1,0-0,8-0,6-0,4-0,2 0,0 0,2 0,4 0,6 0,8 1,0 σ p
Structure dependence of rate coefficients OH+ p-substituted phenols log k OH, mol -1 dm 3 s -1 10,2 10,0 9,8 9,6 9,4 a) HOPhOCH 3 HOPhNHCOCH k 3 d HOPhOH HOPhI HOPhCH 3 HOPhCH 2 OH HOPhCl PhOH HOPhCOO - HOPhBr HOPhCOCH 3 HOPhNO 2 9,2-1,0-0,8-0,6-0,4-0,2 0,0 0,2 0,4 0,6 0,8 1,0 σ p
Structure dependence of rate coefficients OH + p-substituted phenols 11,2 11,0 HOPhOH HOPhNHCOCH 3 HOPhI b) log k chem, mol -1 dm 3 s -1 10,8 10,6 10,4 10,2 10,0 9,8 HOPhCH 3 HOPhH HOPhCH 2 OH HOPhCOO - HOPhCl 9,6 9,4 HOPhBr HOPhCOCH 3 HOPhNO 2-0,8-0,6-0,4-0,2 0,0 0,2 0,4 0,6 0,8 1,0 σ p
Comparison of OH reaction with aromatic molecules in gas and liquid phases -9,5-10,0 PhNH 2 log k OH, gas, molecule -1 cm 3 s -1-10,5-11,0-11,5-12,0-12,5-13,0 PhCHO PhI PhBr PhNO 2 p-ohphno 2 PhOH OHPhCH 3 PhCH(CH 3 ) 2 PhCH 2 OH PhCH 3 p-ohphcl PhH PhCl 9,4 9,6 9,8 10,0 10,2 10,4 10,6 10,8 11,0 log k chem, mol -1 dm 3 s -1
Comparison of aromatic molecule reaction with OH and O 3 3 PhOH p-clphoh 2 log k ozo ne, mol -1 dm 3 s -1 1 0 PhCHO PhCl PhCH(CH 3 ) 2 PhCH 3 PhCH 2 CH 3 - PhH PhCO 2-1 PhNO 2 PhSO 3-9,4 9,6 9,8 10,0 10,2 10,4 10,6 log k chem, mol -1 dm 3 s -1
Conclusion -The k OH 's fall in the range 2 10 9 mol 1 dm 3 s 1 1 10 10 mol 1 dm 3 s 1, the highest values approach the diffusion controlled limit, 1. 1 10 10 mol 1 dm 3 s 1. -By plotting k OH s as a function of Hammett constants we do not obtain straight lines, due to the partially diffusion controlled character of the reaction. -The log k chem s calculated by the Noyes equation, are linearly correlated with the Hammett constants. -The structure dependence of the electrophile OH + aromatic molecule reaction can be interpreted in terms of electron donating and withdrawing substituent.
OH reaction with aromatic molecules Reaction without stable intermediate complex PhX + OH [PhX + OH] σ-phxoh Reaction with stable intermediate complex k 1 k 3 PhX + OH [π-phxoh ] σ-phxoh k 2 l 1 [ OH] e -l2 t l 2 [ OH] e -l1 t [σ-phxoh ] = + [ OH] l 2 l 1 l 2 l 1 k 1 + k 2 + k 3 = l 1 + l 2 k 1 k 3 = l 1 l 2
Hydroxycyclohexadienyl phenoxyl transition Phenol 3000 Hydroxycyclohexadienyl radical, 5 x magnified ε, mol -1 dm 3 cm -1 2000 1000 Pheno xyl radical 0 300 400 500 600 700 Wavelength, nm
Hydroxycyclohexadienyl phenoxyl transition ε, mol -1 dm 3 cm -1 6000 4000 2000 Hydroxycyclohexadienyl radical p-cresol Phenoxyl radical 0 300 350 400 450 Wavelength, nm
Acid-base catalyzed water elimination from the adduct OH - + p-ch 3 C 6 H 4 (OH) 2 p-ch 3 C 6 H 4 O + H 2 O + OH - 1,6x10 6 4x10 6 H 3 O + + p-ch 3 C 6 H 4 (OH) 2 p-ch 3 C 6 H 4 O + H 2 O + H 3 O + 1,2x10 6 4.9 x 10 10 mol -1 dm 3 s -1 3x10 6 1.8 x 10 8 mol -1 dm 3 s -1 k obsd, s -1 8,0x10 5 k obsd, s -1 2x10 6 4,0x10 5 1x10 6 0,0 0 1x10-5 2x10-5 3x10-5 [OH - ], mol dm -3 0 0,000 0,005 0,010 0,015 [H 3 O + ], mol dm -3
Peroxy radical forming reaction OHPhOH +O 2 k f k r OHPh(OH)OO Products Spin selectivity question R-OO bond energy is in the 25 kj mol -1 range, reversibility Rate coefficients for forward and backward reactions strongly depend on the electron density on the ring Peroxy radicals have light absorption below 300 nm
Peroxy radical forming reaction Aromatic molecule k f, mol -1 dm 3 s -1 k r, s -1 K,mol -1 dm 3 Anisol 8 10 8 3.9 10 4 2.1 10 4 Toluene 4.8 10 8 7.5 10 4 0.64 10 4 Fluorobenzene 4.6 10 8 5.5 10 4 0.84 10 4 Benzene 3.1 10 8 1.2 10 4 2.6 10 4 Chlorobenzene 2.6 10 8 5.5 10 4 0.47 10 4 Benzoic acid ion 2.0 10 8 1.3 10 4 1.5 10 4
HO 2 and O 2 - eliminations from peroxy radicals
Endoperoxide formation, ring opening
Oxygen consumption measurements Oxygen selective electrode Efficiency: O 2 molecule consumed/ OH introduced Compound Formate 0.55 Isopropanol 0.55 t-butanol 0.80 Diethyl ether 0.84 Efficiency
Dose dependence of Chemical Oxygen Demand 450 2 mmol dm -3 Phenol COD, mg dm -3 400 350 air1 air2 N 2 O/O 2 1 N 2 O/O 2 2 300 0 5 10 15 20 Dose, kgy
Connection between Chemical Oxygen Demand and Total Organic Carbon Content 300 A paracetamol diclofenac p-aminophenol 2,6-dichloroaniline 120 100 B COD, mg dm -3 200 100 TOC, mg dm -3 80 60 40 20 00 5 10 15 20 0 0 5 10 15 20 Dose, kgy Dose, kgy
Chemical Oxygen Demand Compound Slope, mg dm 3 kgy 1 E Phenol 8.8±0.6 0.98 o-, m-, p-cresol 8.0±1.2 0.90 o-, m-, p-chlorophenol 8.0±1.2 0.90 Ketoprofen 9.0±1.0 1.0 2,4-Dichlorophenoxy-acetic acid 8.0±0.5 0.9 Aspirin 8.1±0.5 0.9 Acetovanillone 6.9±0.4 0.77 Gallic acid 8.5±0.8 0.94 Maleic acid 7.4±0.7 0.82 Fumaric acid 9.0±0.7 1.0 Diclofenac 9.0±0.3 1.0 o-, m-, p-aminophenol 4.0±1.2 0.45 Paracetamol 2.2±0.8 0.25 2,6-Dichloraniline 5.5±1.0 0.6 Acid Red 1 5.5±0.8 0.6
Phenol degradation
Paracetamol degradation Phenoxyl (semi-iminoquinone) radical
2,6-Dichloroaniline degradation Anilino Radical
Diclofenac degradation No anilino or phenoxyl radical
Summary of degradation efficiency Hydroxyl radical initiated oxidation maleic acid, fumaric acid and phenols proceeds with high efficiency, the oneelectron oxidant OH induces two- to four-electron oxidations. When amino, acetamide or hydrazo groups are attached to the ring the rate is lower, one OH induces only one two electron oxidations. The low rate is due to intermediate radicals which have low reactivity with oxygen (penoxyl, anilino, semi-iminoquinone, hydrazyl).
Contribution of reductive radicals (e aq - and H ) to the oxidative degradation in aerated solution There is a competition between the reaction with O 2 and the solute e aq + O 2 O 2 k 1 = 1.9 10 10 mol 1 dm 3 s 1 H + O 2 HO 2 k 2 = 2.1 10 10 mol 1 dm 3 s 1 HO 2 O 2 + H + pk = 4.8 ± 0.1 e aq + S S k 3 H + S HS k 4 The scavenging capacities (concentration multiplied by of the rate coefficient) determine the main reaction partner 38
Contribution of H to the oxidative degradation in aerated solution In the case of H reactions k 2 [O 2 ] k 4 [S] H reacts with aromatic molecules in radical addition: k 4 (Phenol + H ) = 1.9 10 9 mol 1 dm 3 s 1 +O 2 H + C 6 H 5 OH C 6 H 6 OH OOC 6 H 6 OH H + HOOCCH=CHCOOH HOOCCH 2 - CHCOOH + O 2 HOOCCH 2 -( OO)CHCOOH 39
Contribution of e aq to the oxidative degradation in aerated solution There is a competition between the reaction with O 2 and the solute e aq adds to electron deficient parts of molecules, the anion often undergoes fast protonation: H + e aq + HOOCCH=CHCOOH HOOCCH=CH CO OH +O 2 HOOCCH 2 - CHCOOH HOOCCH 2 -( OO)CHCOOH Cl +O 2 e aq + ClC 6 H 4 OH C 6 H 4 OH OOC 6 H 4 OH 40
Contribution of e aq recations to the oxidative degradation in aerated solution When k 1 [O 2 ] k 3 [S], e aq reacts with O 2 Both the self-termination and reactions with S are slow. In self-termination H 2 O 2 forms, it may contribute to oxidation, e.g. by reacting with e aq and forming OH. 2 HO 2 H 2 O 2 + O 2 k = 8 10 5 mol 1 dm 3 s 1 HO 2 + O 2 + H 2 O H 2 O 2 + O 2 + OH k = 9 10 7 mol 1 dm 3 s 1 e aq + H 2 O 2 OH + OH k = 1.1 10 10 mol 1 dm 3 s 1 41
Contribution of O 2 /HO 2 pair to the oxidative degradation in aerated solution OH + HCO 2 H 2 O + CO 2 CO 2 O 2 CO 2 + O 2 100 2.5 10-3 mol dm -3 AR1 COD, rel. value 50 0 O2 sat O2 sat + HCO2Na 0 100 200 Dose, kgy 42
Contribution of O 2 /HO 2 pair to the oxidative degradation in aerated solution O 2 /HO 2 pair is non-reactive in reactions with aromatics. Little information on the reaction rates. The rate coefficient of the phenol + HO 2 reaction is 5.8 10 2 mol 1 dm 3 s 1. + O 2 HO 2 + C 6 H 5 OH HO 2 C 6 H 5 OH products Similar reactions may also contribute to the degradation of other aromatics. The reductive radicals, e aq and H in oxygenated solution also contribute to the oxidative degradation 43
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