Hydroxyl radical induced degradation of aromatic molecules

Transcription

1 Hydroxyl radical induced degradation of aromatic molecules 1

2 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 = mol 1 dm 3 s 1 2

3 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 µmol J 1. N 2 O + e aq + H 2 O N 2 + OH + OH k = 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 = mol 1 dm 3 s 1 Light absorption of OH is below 220 nm.

4 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

5 OH addition to phenol Monitoring charge distribution 5

6 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., Radiat. Phys. Chem. 64, ).

7 Rate coefficient determination by build-up 150 Absorbance, arb. unit OH + PhOH OHPhOH d[ OHPhOH] = k [PhOH] [ OH] dt 2 OHPhOH Products 0,0 1,0x10-6 2,0x10-6 Time, s

8 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

9 Methods of rate coefficient determination Pulse radiolysis with competitive technique Thiocianate k 20 OH + SCN OH + SCN 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 [S] [SCN Ethanol C 2 H 5 OH + OH C 2 H 4 OH + H 2 O mol 1 dm 3 s 1 Ferrocyanide OH+[Fe(CN) 6 ] 4 OH +[Fe(CN) 6 ] 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., Int. J. Radiat. Phys. Chem. 3, ]

10 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.

11 Structure dependence of rate coefficients OH + Monosubstituted aromatics log k O H, mol -1 dm 3 s 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 PhCHO PhCOOH PhSO 3 H σ p PhCH 2 OH PhNH 3 + PhCN PhNO 2 PhCOCH 3 PhSO 2 NH 2

12 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.

13 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 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 Chlorobenzene Phenol Ashton, L., Buxton, G.V.; Stuart, C.R., J. Chem. Soc., Faraday Trans. 91, Wojnárovits L., J. Photochem. 24,

14 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

15 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

16 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

17 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

18 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

19 Comparison of aromatic molecule reaction with OH and O 3 3 PhOH p-clphoh 2 log k ozo ne, mol -1 dm 3 s 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

20 Conclusion -The k OH 's fall in the range mol 1 dm 3 s mol 1 dm 3 s 1, the highest values approach the diffusion controlled limit, 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.

21 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

22 Hydroxycyclohexadienyl phenoxyl transition Phenol 3000 Hydroxycyclohexadienyl radical, 5 x magnified ε, mol -1 dm 3 cm Pheno xyl radical Wavelength, nm

23 Hydroxycyclohexadienyl phenoxyl transition ε, mol -1 dm 3 cm Hydroxycyclohexadienyl radical p-cresol Phenoxyl radical Wavelength, nm

24 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 + 1,2x x mol -1 dm 3 s -1 3x 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 ,000 0,005 0,010 0,015 [H 3 O + ], mol dm -3

25 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

26 Peroxy radical forming reaction Aromatic molecule k f, mol -1 dm 3 s -1 k r, s -1 K,mol -1 dm 3 Anisol Toluene Fluorobenzene Benzene Chlorobenzene Benzoic acid ion

27 HO 2 and O 2 - eliminations from peroxy radicals

28 Endoperoxide formation, ring opening

29 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

30 Dose dependence of Chemical Oxygen Demand mmol dm -3 Phenol COD, mg dm air1 air2 N 2 O/O 2 1 N 2 O/O Dose, kgy

31 Connection between Chemical Oxygen Demand and Total Organic Carbon Content 300 A paracetamol diclofenac p-aminophenol 2,6-dichloroaniline B COD, mg dm TOC, mg dm Dose, kgy Dose, kgy

32 Chemical Oxygen Demand Compound Slope, mg dm 3 kgy 1 E Phenol 8.8± o-, m-, p-cresol 8.0± o-, m-, p-chlorophenol 8.0± Ketoprofen 9.0± ,4-Dichlorophenoxy-acetic acid 8.0± Aspirin 8.1± Acetovanillone 6.9± Gallic acid 8.5± Maleic acid 7.4± Fumaric acid 9.0± Diclofenac 9.0± o-, m-, p-aminophenol 4.0± Paracetamol 2.2± ,6-Dichloraniline 5.5± Acid Red 1 5.5±

33 Phenol degradation

34 Paracetamol degradation Phenoxyl (semi-iminoquinone) radical

35 2,6-Dichloroaniline degradation Anilino Radical

36 Diclofenac degradation No anilino or phenoxyl radical

37 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).

38 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 = mol 1 dm 3 s 1 H + O 2 HO 2 k 2 = 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

39 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 ) = 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

40 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

41 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 = mol 1 dm 3 s 1 HO 2 + O 2 + H 2 O H 2 O 2 + O 2 + OH k = mol 1 dm 3 s 1 e aq + H 2 O 2 OH + OH k = mol 1 dm 3 s 1 41

42 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 mol dm -3 AR1 COD, rel. value 50 0 O2 sat O2 sat + HCO2Na Dose, kgy 42

43 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 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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