Computer-based First-principles Kinetic Model for Aqueous Phase Advanced Oxidation Processes: Proof of Reaction Pathway and Byproducts Formation

Transcription

1 Computer-based First-principles Kinetic Model for Aqueous Phase Advanced Oxidation Processes: Proof of Reaction Pathway and Byproducts Formation Daisuke Minikata, Xin Guo, and John Crittenden Brook Byers Institute for Sustainable Systems School of Civil and Environmental Engineering Georgia Institute of Technology 1

2 Various AOPs Technologies 1/2 Proved AOPs H 2 O 2 / UV Advantages Disadvantages Long stability and can be preserved prior to use of H 2 O 2. H 2 O 2 +UV 2 Poor UV absorption of H 2 O 2 Interface of UV with the water matrix Special reactors required for UV illumination Residuals of H 2 O 2 H 2 O 2 / O 3 Suitable for waters with poor UV light transmission Special reactors requirement for UV illumination2 2O 3 +H 2 O 2 2+3O 2 Stripped volatile organics Expensive and inefficient to produce O 3 Residues of gaseous O 3 Difficulty of maintenance (O 3 /H 2 O 2 dosages) Low ph is detrimental TiO 2 / UV Seems to work with waters that have poor UV light transmission Lamps do not need cleaning TiO 2 +UV h + +e cb h + +H 2 O H + + Catalyst fouling Recovery of TiO 2 required for slurry application 2

3 Various AOPs Technologies 2/2 Purifics (UV/TiO 2 ) 0.5 MGD Photo-Cat Purification Low Pressure UV system (UV/H 2 O 2 ) Trojan Tech HiPOx (O 3 /H 2 O 2 ) Applied Process Technologies, Inc 3

4 A Computer-based First-principles Kinetic Model in AOPs Why so important? Experimentally determined kinetic models do not fully predict intermediates and byproducts. Unknown rate constants determined by fitting with experimental observations do not represent mechanisms. We now know the general reaction mechanisms that initiates in the aqueous phase AOPs based on a number of experimental studies. We have a large number of datasets for reaction rate constants in the aqueous phase AOPs (e.g., ). Concern about emerging contaminants Greater than 70 million chemical compounds registered in CAS and commercially available Is it feasible? We now have robust tool (e.g., computational chemistry) and computational resources. We can make it feasible! 4

5 All models are wrong but some are useful. -- George E.P. Box Let s develop some useful ones and make good use of them. -- an optimistic modeler 5

6 H2 A Computer-based First-principles Kinetic Model in AOPs Reaction Pathway Generator (Graph theory) Reaction Rate Constant Predictor (Quantum mechanical calculations and Group Contribution Method) Ordinary Differential Equations (ODEs) Generator and Solver Cl(O)C-CHO + Cl H2O HOOCCHO H2O Cl2C CH(OH)Cl -HCl Cl2C -CHO OO-Cl2C-CHO OCl2C-CHO H2O Input of Parent Contaminant as SMILES HO CHO +C(O)Cl2 HC (OH)2 OOCH(OH)2 HCOOH HCOOH + H HOOCCOOH HO HO C + C - C + - C - H2 H2O C + HCl C + HO dc i /dt = -k ij C i C j C Cl2C=CHCl Cl C(O)Cl2 + CHCl2 C(O)Cl2 +H2 OOCHCl2 Cl2C CHCl2 OOCCl2CHCl2 OCCl2CHCl2 C(O)Cl2 + OCHCl2 +(OH)CHCl2 HC(O)Cl CO + HCl C(OH)Cl2 OOC(OH)Cl2 - Cl Cl2CHC(O)Cl Cl2CHCOOH CCl2COOH OOCCl2COOH OCCl2COOH ClC(O)COOH Reaction Pathway Generator Reduction of Reaction Pathways lnk Reaction Rate Constant Predictor C(O)Cl2 + H HOOCCOOH C(O)Cl2+ COOH OOCOOH C + H ODE Generator and Solver Time-dependent Concentration Profiles t Toxicity Estimator Design and Optimization of Processes C(=C(Cl)Cl)(Cl)Cl G act aq,calc Validation and Accuracy Analysis Profile of Relative Toxicity Flow Conditions in Reactors

7 Group Contribution Method (GCM) for predicting k E a k = Ae - RT Essence is same as the GCM for the gaseous phase developed by Dr. Roger Atkinson (EPA software AOPWIN) Hypothesis Observed experimental reaction rate constant for a given organic compound is the combined rate of all elementary reactions involving, which can be estimated using Arrhenius kinetic expression. The E a consists of two parts based on Benson s thermochemical additivity concept (Benson, 1976): (1) Base part from main reaction mechanisms (i.e., H-atom abstraction, addition to alkenes and aromatic compounds and interaction with S, N, or P-atom-containing compounds). (2) Functional group contribution partially from neighboring and/or next nearest neighboring functional group. 7

8 Reaction mechanisms GCM Results -overall 1/2- Species # of compounds calibrated # of data within EG (%) S.D. from calibration # of compounds predicted # of data within EG (%) S.D. from calibration (1) H-atom abstraction by (2) addition to alkenes (3) addition to aromatic compounds Alkanes, Alcohols, Ethers, Carbonyls, Aldehydes, Esters, Carboxyl compounds, Alkyl halides, cyclo compounds (85%) (58%) 1.1 Unsaturated alkenes (79%) n.a. Benzene, Pyridine, Furan, Imidazole, Triazine derivatives (88%) (72%) 0.53 (4) interaction with S, N, P-containing compounds S (Sulfide, Disulfide, Sulfoxide, Thiol) N (Nitrile, Nitro, Amide, Amine, Nitroso, Nitramine) Phosphorus Urea (74%) (44%) 2.2 Total (83%) (62%) 1.2 Error Goal (EG) = 0.5 k cal /k exp group rate constants and 80 group contribution factors were calibrated. The degrees of freedom was 164. Sample deviation (S.D.) = 1 N 2 ( kexp, i - kcal, i ) / k exp, i N -1 i 1 8

9 k cal (M -1 s -1 ) 1.E E+10 Ideal fit kexp/kcal = 0.5 and E+09 H-atom abstraction (Calibrated) HO addition to alkene (Calibrated) E E E+06 HO interaction with N,S,P (Calibrated) HO addition to aromatic compounds (Calibrated) H-atom abstraction (Prediction) HO interaction with N, S, P (Prediction) HO addition to aromatic compounds (Prediction) E+05 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 1.E k exp (M -1 s -1 ) 10 9 Minakata, Li, Westerhoff, and Crittenden. ES&T 2009, 43, rate constants (83% of 310 data) from calibration were within 0.5 k calc /k exp 2.0. The sample deviation was Degree of Freedom was rate constants (62% of 124) from prediction were within 0.5 k /k

10 GCM has been widely used and accepted well for various community The wide application range in combination with the user-friendliness makes it probably the best currently available estimation tool for OH radical reactions in aqueous solution. Overall, the method of Minakata et al. is currently the most broadly usable method for the prediction of OH radical reaction rates in aqueous solution. Professor Hartmut Herrmann in ChemPhysChem, 2010, 11, GCM provides Microsoft Excel spreadsheet and Compiled Fortran Program as Supplemental Materials in Minakata, Li, Westerhoff, Crittenden, 2009 ES&T 43, , so that any users can download and calculate the HO rate constants for a compound of interest. We are now working on updating the current GCM

11 Challenges Still Remain for Rate Constant Predictions GCM is limited because: 1. Many datasets are required to calibrate parameters 2. Only deal with molecules for which all required group rate constants and group contribution factors have been calibrated before. 3. Limited number of literature-reported experimental rate constants for other reaction mechanisms than 4. Assumed that a functional group has approximately the same interaction properties under a given molecule, so that the GCM disregards the changes of the functional properties that can arise from the intramolecular environment by electronic push-pull effects, hydrogen bond formation, or by steric effects. Application of computational chemistry (quantum mechanical calculations) for predicting reaction rate constants in aqueous phase Looking at reaction energies (transition states) 11

12 A concept of a Linear Free Energy Relationship (LFER) According to a linear free energy relationship (LFER), for the same reaction mechanism, the difference in the free energy of activation of transition states I and R is proportional to the difference in free energy change of the reaction for species I and a reference species R to produce the products, C and D. A + R transition state(s) C + D A + I transition state(s) C + D E E E E 0 a,a*i a,a*r ln I ln E E k - kr - - m'[ Ea,A*I - Ea,A*R ] RT RT E E ( E ) A*I Reference, A*R E ( E 0 ) Reaction coordinate K a, Ri ln kr -ln k 0 rxn, rxn, ln i R - G R - G i R m 0 K a, R0 12

13 Correlation between Reaction Rate Constants and Aqueous Phase Free Energy of Activation ln k - a G b rxn,aq From transition state theory (TST) G G -G rxn,aq aq reactants,aq G aq is a quasithermodynamic quantity, kcal/mol, that indicates the free energy of the transition state G reactants,aq is the molar free energy of reactants, kcal/mol rxn,aq rxn,gas rxn,solvation G G G where,0 0 G G -G - G -G rxn,solvation solvation solvation reactants,solvation reactants,solvation And G aq contains an extra portion of free energy; G extra = RT lnγ(t), where γ(t) is a transmission coefficient that represents the effect of tunneling at temperature T at the transition state 13

14 ln k 25 Linear Free Energy Relationships for H-atom abstraction from a C-H bond by HO Neutral Ionized y = x R² = ln k H y = x r² = Neutral (G3 + COSMO-RS)-ref.16 Ionized (G4 + SMD) - This study G act I, kcal/mol based on experimental G G act aq,calc, kcal/mol based on calculated G Calibration (N=26) 13 out of 14 from prediction was within difference of factor of 5 Minakata and Crittenden, ES&T 2011, 45, Minakata, Song, and Crittenden, ES&T, 2011, 45,

15 Summary of LFERs for Reaction Rate Constant Predictions Reaction mechanisms H-atom abstraction Genetic representations of reaction mechanisms CHR 3 + CR 3 + H 2 O Compound groups Hydrocarbons (alkanes) Oxygenated compounds (Alcohols, Diol, Ether, Ester, Aldehyde, Ketone, Carboxylic) Alkyl halides Coefficient for correlation (lnk =ρδg act aq,calc + σ) (N=number of data) ρ = σ = 21.6 N = 56 addition to alkenes R 2 C=CR' 2 + R 2 (HO)C- CR' 2 Unsaturated alkenes ρ = , σ = 23.2, N = 10 addition to aromatic compounds C 6 H 5 R + HO-C 6 H 5 R Single functional group multi-functional groups (2,3,4,5,6) To be obtained O 2 addition to C-centered radical Peroxyl radical reaction mechanisms CR 3 + O 2 OOCR 3 R HO C R O O R C O + HO 2 (O H + ) R Alkyl carbon centered radicals ρ = , σ = 20.9, N = 36 Aromatic carbon centered radicals ρ = , σ = 13.6, N = 9 Uni-molecular decay (Elimination of HO 2 -/O 2 -) ρ = , σ = 14.5, N = 14 R 2 CHOO + OOCHR' 2 R 2 CHOOOOCHR' 2 Alkyl peroxyl radical (Aliphatics) ρ = , σ = 22.1, N = 9 R 2 C=O + R 2 CHOH + O 2 R 2 CHOO-OOCHR 2 2R 2 C=O + H 2 O 2 2R 2 CHO + O 2 Bi-molecular To be obtained R 2 CHOOCHR 2 + O 2 β-scission, 1,2-H shift, Oxygen-transfer R-C(O)O R + CO 2 CR 3 CR' 2 O CR 3 + R' 2 C=O HR 2 CO C(OH)R 2 + R 2 C=O Alkoxyl radicals To be obtained Hydrolysis R-CHO R-CH(OH) 2 RC (OH) 2 R-(OH) 2 C-OO R-COOH Aldehyde, ketones *R, R'=alkyl or H; n.a. = not available; "-" = not clear; a.v.=available, = unpublished data, = preliminary investigation To be obtained

16 H2 Next Step Input of Parent Contaminant as SMILES C(=C(Cl)Cl)(Cl)Cl HO Cl2C CH(OH)Cl -HCl Cl2C -CHO OO-Cl2C-CHO Cl2C=CHCl Cl Cl2C CHCl2 OOCCl2CHCl2 OCCl2CHCl2 - Cl Reaction Pathway Generator OCl2C-CHO C(O)Cl2 + CHCl2 Cl2CHC(O)Cl Cl(O)C-CHO + Cl H2O HOOCCHO H2O CHO +C(O)Cl2 H2O HC (OH)2 OOCH(OH)2 HCOOH HCOOH + H HOOCCOOH HO HO C + C - C + - C - H2 H2O C + HCl C + HO C(O)Cl2 +H2 OOCHCl2 C(O)Cl2 + OCHCl2 +(OH)CHCl2 HC(O)Cl CO + HCl C(OH)Cl2 OOC(OH)Cl2 Cl2CHCOOH CCl2COOH OOCCl2COOH OCCl2COOH ClC(O)COOH Reduction of Reaction Pathways lnk Reaction Rate Constant Predictor C(O)Cl2 + H HOOCCOOH C(O)Cl2+ COOH OOCOOH C + H dc i /dt = -k ij C i C j C ODE Generator and Solver Time-dependent Concentration Profiles G act aq,calc Validation and Accuracy Analysis t Toxicity Estimator Profile of Relative Toxicity Flow Conditions in Reactors Design and Optimization of Processes

17 Reaction Pathway Generator Graph Theory Approach (Euclid 1735) Mathematic Abstraction Chemical structures are intrinsically graphs that consist of two sets: a set of nodes or vertices), and a set of edges that connect those nodes. Name ID Valence Weight Node C Cl H Edge H Cl C H C O H H H C C Cl H O H A Chemical Tree H H C Cl H H Pointer to the Parent Pointer Array to the Children Array of Bond Types Logics of pathway generator Linear Notation (SMILES) Reactant Graph Match Patterns Products Graph Products Analysis Take Unique Products as New Reactants Reactant/Product relationships Reaction Rules 17 Li and Crittenden, 2009 ES&T 43,

18 General Reaction Rules in AOPs General reaction mechanisms that initiates based on past experimental studies decay Stefan and Bolton, 1998; 1999; 1999; Stefan et al., 1996; 2000; Cooper et al., 2009; Li et al., 2004; 2007; von Sonntag and Schuchmann, 1984; Schuchmann and von Sonntag, 1979

19 Reduction of Pathway using Directed Relation Graph method Directed relation graph (DRG) that represents chemical reaction mechanisms, in which species are represented by nodes (e.g., A, B, C) and reactions are represented by arrows ( ). A k 1 k 2 B C k 3 k 4 D Step 1 Step 2 E C i = Concentration of species, i (i = A, B, C..) k j = Reaction rate constant for reaction j (j = 1,2.) At step1, we calculate direct interaction coefficients (DIC) that represents an importance of a reaction A B. DIC = k 1 C A k 1 C A + k 2 C A If the DIC is smaller than the criteria that is set by an user, the reaction mechanism (A B) will be eliminated and (A C) remains. This elimination process is performed incrementally at every single step. For example, the 0.1% of criteria removes reactions that have < 0.1% of overall reaction rate of reactant of interest from the full reaction mechanism. 19

20 Case Study: Acetone Experimentally Determined Pathway CH 3 COCH 3 Important byproducts CH 2 COCH 3 O 2 OOCH 2 COCH 3 OCH 2 COCH 3 HCHO COCH 3 H 2 O HCOOH CO 2 + H 2 O + HO 2 O 2 CH 3 COOH HOOCCOOH + HCOOH CO 2 + H 2 O + HO 2 CH 2 OOH + CH 2 CO O 2 HCOOH H 2 O CH 3 COOH HOOCCOOH + HCOOH CO 2 + H 2 O + HO 2 20 Stefan, Hoy, Bolton, 1996 ES&T 30,

21 Graph Theory Predicted Pathway CH 3 COCH 3 CH 2 COCH 3 O 2 Important byproducts Manually added reactions OOCH 2 COCH 3 OCH 2 COCH 3 HCHO + COCH 3 CH(OH)COCH 3 H 2 O HCOOH O 2 OOCOCH 3 O 2 OOCH(OH)COCH 3 CO 2- O 2 /H 2 O 2 OCOCH 3 OHCCOCH 3 OHCCOCH 3 H 2 O HOOCCOCH 3 CH 2 COCOOH O 2 OOCH 2 COCOOH H 2 O H 2 O OHCCOCH 3 + CH 3 COCH 2 OH CH(OH)COCH 3 CO 2 + HO 2 HOCH 2 COCH 2 OH + OHCCOCH 2 OH OCH 2 COCOOH OHCCOCOOH OHCCOCOOH + HOCH 2 COCOOH H 2 O OCH 2 COCH 2 OH HCHO + COCOOH CH(OH)COCOOH H 2 O H 2 O CH(OH)COCH 2 OH O O CH(OH)COCOOH O HOOCCOCH 2 OH HOOCCOCOOH O 2 CH(OH)COCH OOCOCOOH OOCH(OH)COCOOH OOCH(OH)COCH 2 OH 2 OH HOOCCOOH HCOOH OOCH(OH)COCOOH O CH(OH)COOH 2 OCOCOOH OHCCOCOOH H 2 O H 2 O OHCCOCH 2 OH CO 2 + HO 2 O 2 OOCH(OH)COCH OHCCOCOOH 2 OH COOH + CO 2 OOCH(OH)COOH O 2 OHCCOCH 2 OH OOCOOH OHCCOOH O 2 OOCH(OH)COCH 3 OHCCOCH 3 CH 2 COCH 2 OH O 2 OOCH 2 COCH 2 OH CH 2 OOH + CH 2 CO H 2 O O 2 HCOOH CH 3 COOH CH 2 COOH O 2 OOCH 2 COOH OHCCOOH + HOCH2COOH OCH 2 COOH HCHO H 2 O HCOOH CO 2 + HO 2 Type of mechanism Number of species Number of reactions HOOCCOOH Full mechanism Reduced mechanism

22 Simplified Graph Theory Predicted Pathway Type of mechanism Number of species Number of reactions Full mechanism Reduced mechanism CH 3 COCH 3 CH 2 COCH 3 O 2 Important byproducts Manually added reactions OOCH 2 COCH 3 Beta scission 1, 2 shift OCH 2 COCH 3 OHCCOCH 3 OHCCOCH 3 + CH 3 COCH 2 OH CH 2 OOH + CH 2 CO H 2 O H 2 O HCHO H 2 O COCH 3 O 2 HOOCCOCH 3 HCOOH OOCOCH 3 HOOCCOCOOH + HCOOH CO 2 + HO 2 OCOCH 3 CO 2 + HO 2 O 2 H 2 O HCOOH CH 3 COOH HOOCCOOH + HCOOH CO 2 + HO 2 22

23 [Major species](mm) Reduction of Acetone Degradation Mechanism in UV/H 2 O 2 AOP Comparison of concentration profiles of major species for experimental data (Stefan, Hoy, Bolton, 1996 ES&T 30, ) and reduced mechanism H2/10 exp acetone exp 2 formic acid exp oxalic acid exp acetic acid exp H2/10 reduced acetone reduced 2 formic acid reduced oxalic acid reduced acetic acid reduced Initial concentration of acetone Initial concentration of Initial concentration of H2 Initial ph 5.9 Wave length of UV Light intensity Reactor type Reaction time 1.02 mm 2.2 mm 15.6 mm 200~300 nm Einstein/L s Completely mixed batch reactor 80 min Type of mechanism Time(min) Number of species Number of reactions Full mechanism Reduced mechanism The criteria for DRG method is 0.1%. Reaction rate constants are obtained from group contribution method (Minakata et al., 2009 ES&T 43, ) and literature including: Nate et al., 1990 J. Phys. Chem. Ref. Data. 19, Nate et al., 1995 J. Phys. Chem. Ref. Data. 25, Buxton et al., 1988 J. Phys. Chem. Ref. Data. 17, von Sonntag et al., 1991 Angeuz. Chem. Int. Ed. Engl. 30, Li et al., 2009 ES&T 43, Stefan et al., 1996 ES&T 30, The rearrangement and beta-scission of peroxyl radical and ketene hydrolysis were added manually because it is not included in this version of the pathway generator. (OOCH 2 COCH 3 = CH 2 OOH + CH 2 CO, CH 2 CO + H 2 O = CH 3 COOH)

24 [Major Species](mM) Reduction of Methane Degradation Mechanism in UV/H 2 O 2 Process Comparison of concentration profiles of major species for full mechanism and reduced mechanism Type of mechanism CH4 full CH2O full CH3OH full HCOOH full C full CH4 reduced CH2O reduced CH3OH reduced HCOOH reduced C reduced Time(min) Number of species Number of reactions Full mechanism Reduced mechanism The criteria for DRG method is 0.1%. Initial concentration of CH4 Initial concentration of Initial concentration of H2 1.1 mm 2.2 mm 14 mm Initial ph 7.56 Wave length of UV Light intensity Reactor type Reaction time 254 nm Einstein/L s Completely mixed batch reactor 30 min Reaction rate constants are obtained from group contribution method (Minakata et al., 2009 ES&T 43, ) and literature including: Nate et al., 1990 J. Phys. Chem. Ref. Data. 19, Nate et al., 1995 J. Phys. Chem. Ref. Data. 25, Buxton et al., 1988 J. Phys. Chem. Ref. Data. 17, von Sonntag et al., 1991 Angeuz. Chem. Int. Ed. Engl. 30, Li et al., 2009 ES&T 43,

25 [Intermediates](mM) [H2](mM) Reduction of Trichloroethene (TCE) Degradation Mechanism in UV/H 2 O 2 Process Comparison of concentration profiles of major species for experimental data (Li, Stefan, Crittenden, 2007 ES&T 41, ) and reduced mechanism Time(min) TCE exp Formic acid exp oxalic acid exp 20 DCA exp TCE reduced formic acid reduced oxalic acid reduced 20 DCA reduced H2 exp H2 reduced Initial concentration of TCE Initial concentration of Initial concentration of H2 Initial ph 5.9 Wave length of UV Light intensity Reactor type Reaction time 1.08 mm 2.2 mm 10.4 mm 200~300 nm Einstein/L s Completely mixed batch reactor 30 min The reason for the inconsistency between experimental and calculated concentration profiles for formic acid and DCA is that the pathway generator does not include the photolysis of TCE and other byproducts. Type of mechanism Number of species Number of reactions Full mechanism Reduced mechanism The criteria for DRG method is 0.01%. Reaction rate constants are obtained from group contribution method (Minakata et al., 2009 ES&T 43, ) and literature including: Nate et al., 1990 J. Phys. Chem. Ref. Data. 19, Nate et al., 1995 J. Phys. Chem. Ref. Data. 25, Buxton et al., 1988 J. Phys. Chem. Ref. Data. 17, von Sonntag et al., 1991 Angeuz. Chem. Int. Ed. Engl. 30, Li et al., 2009 ES&T 43, Li et al., 2007 ES&T 41,

26 H2 Final Step Input of Parent Contaminant as SMILES C(=C(Cl)Cl)(Cl)Cl HO Cl2C CH(OH)Cl -HCl Cl2C -CHO OO-Cl2C-CHO Cl2C=CHCl Cl Cl2C CHCl2 OOCCl2CHCl2 OCCl2CHCl2 - Cl Reaction Pathway Generator OCl2C-CHO C(O)Cl2 + CHCl2 Cl2CHC(O)Cl Cl(O)C-CHO + Cl H2O HOOCCHO H2O CHO +C(O)Cl2 H2O HC (OH)2 OOCH(OH)2 HCOOH HCOOH + H HOOCCOOH HO HO C + C - C + - C - H2 H2O C + HCl C + HO C(O)Cl2 +H2 OOCHCl2 C(O)Cl2 + OCHCl2 +(OH)CHCl2 HC(O)Cl CO + HCl C(OH)Cl2 OOC(OH)Cl2 Cl2CHCOOH CCl2COOH OOCCl2COOH OCCl2COOH ClC(O)COOH Reduction of Reaction Pathways lnk Reaction Rate Constant Predictor C(O)Cl2 + H HOOCCOOH C(O)Cl2+ COOH OOCOOH C + H dc i /dt = -k ij C i C j C ODE Generator and Solver Time-dependent Concentration Profiles G act aq,calc Validation and Accuracy Analysis t Toxicity Estimator Profile of Relative Toxicity Flow Conditions in Reactors Design and Optimization of Processes

27 Relative Toxicity (Concentration/ChV) Aquatic Toxicity Assessment of AOPs Byproducts - Relative Toxicity of TCE Degradation Byproducts - 96-hr Green Algae ChV: most sensitive indicator among the acute and chronic endpoints for fish, daphnia, and green algae (EPA 2005) TQ(Toxicity Quotient): {Concentration 96-hr Green Algae ChV} TQ = Relative toxicity of a mixture TQ or TQ 1 : Toxic and unacceptable (Belden et al. 2007) 50 Green Algae 96-hr ChV (mg/l) TCE 2.9 Formic acid Composite of TCE, formic acid, oxalic acid, MCA, and DCA Oxalic acid MCA DCA *MCA: monochloroacetic acid **DCA: dichloroacetic acid Time (min) 27

28 Relative toxicity (Concentration/ DWEL) Non-carcinogenic Effect Assessment - TCE Degradation Byproducts - DWEL (Drinking Water Equivalent Level): A lifetime exposure concentration protective of adverse non-cancer health effects from drinking water Relative toxicity = [concentration / DWEL] 700 DWEL (mg/l) TCE 0.25 MCA 0.35 DCA Composite of TCE, MCA, and DCA Time (min) *Oxalic acid and Formic acid are not included in the drinking water criteria. 28

29 Relative toxicity Carcinogenic Effect Assessment - TCE Degradation Byproducts - Cancer classification TCE : sufficient evidence in animals and inadequate or no evidence in human (B2) MCA : inadequate information to assess carcinogenic potential (I) DCA : likely to be carcinogenic to humans (L) 10-4 Cancer risk concentration: The concentration of a chemical in drinking water corresponding to an excess estimated lifetime cancer risk of 1 in 10,000. (EPA 2006) Chemical 10-4 Cancer Risk (mg/l) TCE MCA DCA (Concentration / cancer risk) x Time (min) 29

30 Concluding Remarks The computer-based first-principles kinetic model can predict timedependent concentration profiles of parent and major intermediates and byproducts in consistent with experimental results. (Need to add 1,2 shift and beta scission and photolysis reactions) Simplification of reaction mechanisms predicted by the reaction pathway generator is required in order to reduce number of predicted species and reactions. The DRG approach has successfully reduced full reaction mechanisms and concentration profiles of parent and major intermediates and byproducts in the reduced mechanisms are found almost identical to those that are predicted by the full reaction mechanisms. The current pathway generator needs to include special reactions for some parent compounds (e.g., photolysis and re-arragement and betascission of peroxyl radicals and ketene hydrolysis) in order to predict experimentally observed byproducts (e.g., acetate) There are challenges for predicting rate constants for bi-molecular decays of peroxyl radicals. Both experimental and theoretical studies should be coupled to elucidate reaction mechanisms.

31 Acknowledgement National Science Foundation (NSF): and Georgia Tech College of Computing, Office of Information Technology and CEE IT Services for high performance computational resources. High Tower Chair and Georgia Research Alliance at Georgia Tech Brook Byers Institute for Sustainable Systems (BBISS) University of Notre Dame Radiation Center and Department of Energy Our collaborators: Ke Li (UGA), Stefan Mihaela (Calgon Carbon Corp.), Steve Mezyk (California State Univ. Long Beach), and Weihua Song (Fudan Univ.) 31

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