Basic radical reactions in water treatment by ionizing radiation

Transcription

1 Basic radical reactions in water treatment by ionizing radiation By: László Wojnárovits Selectivity, rate constants, main reactions Do up the buttons again Gomboljuk újra a kabátot! 1

2 General believes: 1. OH is an unselective radical, it reacts with practically diffusion limited rate constant with most of organic molecules. 2. OH decomposes all organic pollutants. 3. The reductive intermediates, e aq and H induce reduction reactions 4. OH is the main reactive intermediate in most of AOP, including radiolysis. 5. In natural waters OH induces the degradation of harmful organic compounds 2

3 Hydroxyl radical ( OH), E( OH/OH ) = 2.1 V Selective radical, or non-selective radical OH addition to phenol. Monitoring charge distribution Selectivity in H-abstraction Alcohol Rate constant (mol -1 dm 3 s -1 ) -CH 3 (%) -CH 2 - (%) -CH- (%) -OH (%) MeOH EtOH PrOH

4 s p k OH s p 0.0 k OH s p k OH

5 Whether the hydroxyl radical degrades all organic compounds, or not? The rate constants of OH reactions with several organic molecules are very low, e.g. with fluorine containing carboxylic acids, which are recalcitrant in OH mediated degradation. For instance: OH + CF 3 COO products k < 10 6 mol 1 dm 3 s 1 OH does not degrade all organic molecules 5

6 Dichloride radical anion (Cl 2 ), E(Cl 2 /2 Cl )=2.1 V +H + -H2O +Cl OH + Cl ClOH (HOClH) Cl Cl2 -H + +H2O -Cl Complexation is fast, in presence of Cl Cl does not react with the organic solutes. In the OH + Cl system below ph 5 Cl 2, above OH dominates Cl 2 reacts in direct oxidation with aromatics with formation of an intermediate radical cation. -2Cl Cl 2 + C 6 H 5 OH [C 6 H 5 OH] + H + + C 6 H 5 O SO 4 + Cl SO Cl k f = mol 1 dm 3 s 1 k r = mol 1 dm 3 s 1

7 Cl 2 s p k OH s p 0.0 k OH s p k OH

8 Carbonate radical anion (CO 3 ), E(CO 3,H + /HCO 3, ph 7) = 1.78 V OH + CO3 2 CO3 + OH OH + HCO3 CO3 +H2O k3 = mol 1 dm 3 s 1 k4 = mol 1 dm 3 s 1 The carbonate radical anion has a planar structure Carbonate radical anion reactions C 6 H 5 NH 2 + CO - 3 CO [C 6 H 5 NH 2 ] + k = mol 1 dm 3 s 1 C 6 H 5 O - + CO - 3 CO C 6 H 5 O k = mol 1 dm 3 s 1

9 CO 3 s p k OH s p 0.0 k OH s p k OH

10 Sulfate radical anion. E(SO 4 /SO 4 2 ) = 2.43 V e aq + S 2 O 8 2 SO 4 + SO 4 2 k = mol 1 dm 3 s 1 Strong electron acceptor, mainly reacts in electron transfer. Electron donating substituent increases, electron withdrawing substituent decreases the rate.

11 SO 4 s p k OH s p 0.0 k OH s p k OH

12 Hydrogen atom. E(H aq+ /H ) = -2.4 H, similarly to OH, is also a pronounced electrophilic radical and thus shows a high regioselectivity in its addition reactions with unsaturated compounds, with saturated molecules H-atom abstraction occurs. H + C 6 H 6 C 6 H mol 1 dm 3 s 1 H + CH 3 OH CH 2 OH +H mol 1 dm 3 s 1 12

13 H s p k OH s p 0.0 k OH s p k OH

14 Hydrated electron. E(aq/e aq ) = -2.9 V The reaction with organic molecules mostly occurs by addition. However, the electron adduct often immediately protonates, in the protonation the same radical is produced as in H addition reactions. Reversible addition +H 2 O e aq + C 6 H 6 C 6 H 6 C 6 H 7 k = mol 1 dm 3 s 1 -OH Dissociative electron capture e aq +C 6 H 5 Cl C 6 H 5 Cl C 6 H 5 +Cl k = mol 1 dm 3 s 1 In e aq and H reactions with organic molecules mostly carbon centered radicals form, just like in OH reactions. 14

15 s p k OH s p 0.0 k OH s p k OH

16 Rate constants of forward (k f ) and reversed (k r ) peroxyl radical forming/dissociation reactions of radicals and equilibrium constants (K). k f R + O 2 ROO Radical k f, mol -1 dm 3 s -1 k r, s -1 K, mol -1 dm 3 Simple alkyl radicals c-c 5 H CH 2 OH Cyclohexadienyl radicals Anisol Toluene Benzene In aerated solutions e aq and H disappear in reaction with dissolved O 2 e aq +O 2 O 2 k = mol 1 dm 3 s 1 k r H + O 2 HO 2 k = mol 1 dm 3 s 1 16

17 Reaction of oxidizing radical (e.g. OH) with organic molecule brings about one-electron oxidation, second oxidation occurs when the radical reacts with O 2. Further oxidations through peroxyl radicals. In reducing radical reactions carbon centered radical reacting with O 2 starts oxidative degradation. Both oxidizing and reducing radicals lead to oxidative degradation. The degradation of the aromatic ring occurs in peroxyl radical reactions. 17

18 Reactions of reactive radicals with water constituents During practical water (potable or wastewater) irradiations dissolved O 2, Cl, HCO 3 /CO 2 3 and the dissolved organic carbon DOC consumes most part of primary water radiolysis radical intermediates. The Cl, HCO 3 /CO 2 3 and DOC concentrations vary in wide ranges: Cl mol dm 3 HCO 3 /CO mol dm 3 DOC mg dm 3 (The organic content of drinking water, water of lakes and rivers is referred to as dissolved organic carbon. DOC mainly consists of fulvic and humic acids). 18

19 Reactions of OH with the main constituents of a model purified water leaving a WWTP Species Concentration k, mol -1 dm 3 s -1 Reaction, % Low ph Reaction, % Neutral ph Cl mol dm HCO mol dm DOC 20 mgc dm * PhX mol dm

20 Reactions of SO 4 with the main constituents of a model purified water leaving a WWTP Species Concentration k, mol -1 dm 3 s -1 Reaction, % Cl mol dm HCO mol dm DOC 20 mgc dm * 33 PhX mol dm

21 Conclusions: 1. OH is a selective radical in addition and abstraction. Reactions are never diffusion controlled, chemical reactivity plays important role. 2. The rate constants with several classes of organic molecules are very low. OH cannot degrade these pollutants. 3. Reductive radicals, e aq and H also contribute to oxidative degradation. 4. In practical water matrices OH mostly reacts with Cl, HCO 3 /CO 3 2 DOC, less than 1% is utilized for degradation of target compounds. 5. Secondary radicals formed in reactions with Cl, HCO 3 /CO 3 2 and DOC are more selective than OH and contribute to the degradation. 6. The dissolved O 2 concentration strongly influences the rate. 7. In surface waters CO 3 is the most abundant radical which strongly influences the degradation of harmful pollutants. 21

22 Some of the old ideas should be reconsidered We have to do up the buttons again. Thank you for your attention 22