CEE 697K ENVIRONMENTAL REACTION KINETICS Lecture #2

Size: px

Start display at page:

Download "CEE 697K ENVIRONMENTAL REACTION KINETICS Lecture #2"

Sharlene McCormick
5 years ago
Views:

1 Updated: 4 September 13 1 Print version CEE 697K ENVIRONMENTL RECTION KINETICS Lecture # Rate Expressions: Bromide + Chlorine case study & lab project Kumar & Margerum paper Introduction

2 Reactions with Chlorine The THMs Cl Cl C H Cl The Precursors! HOCl + natural organics (NOM) Br Cl C H Cl Br Br C H Cl Oxidized NOM and inorganic chloride ldehydes Chlorinated Organics TOX THMs Hs Br Br C H Br Chloroform Bromodichloromethane Chlorodibromomethane Bromoform

3 The Haloacetic cids 3 Cl Cl C COOH Cl Br Cl C COOH Cl Br Br C COOH Cl Br Br C COOH Br Trichloroacetic Bromodichloroacetic Chlorodibromoacetic Tribromoacetic cid cid cid cid (TC) Cl H C COOH Cl H Br C Cl COOH Br H C COOH Br Dichloroacetic Bromochloroacetic Dibromoacetic cid cid cid (DC)

4 Distribution: Variability within a single system 4 Example: New Haven Service rea DS model

5 New Haven Distribution System 5 Sept 11, 1997 : chlorine 3,4 pipes,5 junctions West River WTP 8.7 MG Millroc Basins Maltby Tan 3 MG Lae Gaillard WTP Lae Saltonstall WTP. mg/l DOC (Treated) ph mg/l chlorine dose

6 6 Sept 11, 1997 : TTHM

7 7 Sept 11, 1997 : H6

8 8 Genotoxicity DBP Chemical Class Other DBPs Haloacetamides Haloacetonitriles Halonitromethanes Halo cids Haloacetic cids I B Iodoacetamide Bromoacetamide DBNM Dibromoacetonitrile Iodoacetonitrile Bromoacetonitrile BDCNM TBNM 3,3-Dibromo-4-oxopentanoic cid TCNM Wor of Michael Plewa BNM DBCNM BCNM Univ. of Illinois MX Tribromopyrrole Bromochloroacetonitrile 3,3-Bromochloro-4-oxopentanoic cid DCNM C Chloroacetonitrile Dibromoacetamide Trichloroacetonitrile Chloroacetamide CNM DB DI Dichloroacetonitrile TB BI BC EMS +Control Trihloroacetamide Bromate -Bromobutenedioic cid -Iodo-3-bromopropenoic cid,3-dibromopropenoic cid CDB Single Cell Gel Electrophoresis Genotoxicity Potency Log Molar Concentration (4 h Exposure) Not Genotoxic: DC, TC, BDC, Dichloroacetamide, 3,3-Dibromopropenoic cid, 3-Iodo-3-bromopropenoic cid,,3,3,tribromopropenoic cid July 6

9 Bromide: THM Formation CHCl 3 Data from: Minear & Bird, hours, ph 7. 5 mg/l Chlorine Dose 1 mg/l Humic cid Percent of TTHM 6 4 CHBrCl CHBr Cl CHBr Bromide Concentration (mg/l)

10 Impact of Bromide on DH Formation 1 1 Concentration (µg/l as Cl - ) CHCl COOH CHClBrCOOH ph 7, 5 o C, 7 days 5 mg/l chlorine dose.9 mg/l TOC CHBr COOH Bromide Concentration (mg/l) From Pourmoghaddas, 199

11 Formation of Brominated THMs 11 HOCl HOCl HOCl B C Cl C H Cl Cl H D HOCl E HOCl Cl C Br Cl H F HOCl Br C Br Cl HOCl + Br CEE 697K Lecture HOBr # + Cl Br H C Br Br

12 Bromo speciation 1 1. Full-scale data (small symbols) from Weinberg et al., (ll 1 plants) THM Mole Fraction (THM4 only) Lab data (large symbols) from Hua & Rechow, 4 Lines are geometric model CHCl3 CHBrCl CHBrCl CHBr3 CHCl3 Lab tests CHBrCl Lab tests CHBrCl Lab tests CHBr3 Lab tests THM Br/(Cl+Br)

13 Case study: chlorine + bromide 13 Observed reaction HOCl Nucleophilic attac of bromide on oxygen in hypochlorous acid.95 x1 3 M -1 s -1 at 5ºC Note that HOCl deprotonates at elevated phs From Faras et al., 1949 Transfer of Cl + to form intermediate (BrCl) H + HOCl + Br BrCl 3 From: Kumar & Margerum, Br HOBr + Cl + H fast + Br Br + Cl O + BrCl Br Br fast + Br 3

14 Bacground 14 bsorbance for monitoring reaction Known equilibria

15 laline Experiments 15 Pseudo-1 st order Bromide in great excess Slow reactions Could be monitored directly by conventional UV spectrophotometer 9 nm pea for hypochlorite (OCl - ) Followed for 4 half-lives Complete from: 1.1 sec 4 hr Plot ln( t - ) vs time Slope is obs

16 K obs is a linear function of Bromide 16

17 ph dependency 17

18 Overall ph dependence 18 How to explain ph effect on nd order rate constant? See also, Brezoni, pg 3; figure 4-

19 Buffer Tests 19 Low ph High ph

20 Buffer Effect Proposed dependence on H obs H α C ( ) + H + + H Br H 1 C H C K + T T K T a + + H H a H H 8.9 ± 1.3M s HPO 4-1 HCO 3 - (1.5.15) 1 1 H ± x M s + obs Br H H + H buffer K + H a T +

21 1 High ph only obs ( ) + H + + H Br H H d OCl dt More generally + ( + H + H ) OCl Br H H d Cl( + I) dt ( OCl + HOCl + H OCl ) Br HOCl H

22 cidic Experiments Pseudo-1 st order Bromide in great excess Fast reactions Required high-speed setup Stopped-flow spectrophotometer Could monitor º product 66 nm pea for tribromide (Br 3- ) Followed for 4 half-lives Complete from.4-.1 sec Plot ln( t - ) vs time Slope is obs

23 Stopped-flow 3 Principle Commercial instruments Limitations obs obs ( ) For Durrum instrument: m 17 s -1 obs m Correct for slow mixing speed ( m ) 1

24 K obs is still linear with Br- 4 No difference in mechanism?

25 cidic data: impact of acids 5 General and specific acid

26 Overall ph dependence 6 Three effective zones See also, Brezoni, pg 3; figure 4- H + + HOCl HOCl OCl -

27 Catalysis: comparison with pka 7 Bronsted catalysis G H ( K ) α a HPO 4 - CH ClCOOH H H 3 O + O CH 3 COOH HCO - 3 α.75 α.7 Indicates that H engages in greater donation of proton to OCl - than to HOCl

28 Mechanisms 8 Proposed mechanism lternative Less liely

29 Temperature: rrhenius Equation 9 rrhenius Equation e E / RT a Where R is the universal gas constant Transforms to: E 1 Ln( ) ( ) a T Ln R T Tb R Or: J o M K e Ta Ea 1 1 R Tb Ta Ln() 1/T

30 Pizza Kinetics 3 Leenson, 1999 What are the inetics of pizza spoilage? Do they conform to rrhenius? J. Chem Ed., 76:4:54-55

31 Pizza II 31 rrhenius plot t~

32 Lab Project Example Data #I 3.1 M NaOH.35.3 What is the bs inf? bsorbance at 9 nm (cm -1 ) Time (min)

33 Lab Project Example Data #II 33.1 M NaOH -1 - Set bs inf.85 Ln ( 9-9inf ) b b Time (min)

34 Lab Project Example Data #III 34.5 M NaOH.35.3 Set bs inf? bsorbance at 9 nm (cm -1 ) time vs ln(-inf) Plot 1 Regr Time (min)

35 Lab Project Example Data #IV 35.5 M NaOH -1 Ln ( 9-9inf ) b b Set bs inf.94 Maybe too high? Downward curvature Time (min)

36 Lab Project Example Data #V 36.5 M NaOH -1 - Set bs inf.9 Ln ( 9-9inf ) time vs ln(-inf) Plot 1 Regr b b Loos better, except for final data where relative error is high, Use only earlier data? Time (min)

37 Lab Project Example Data #VI 37.5 M NaOH b b Set bs inf.9 Ln ( 9-9inf ) Using only earlier data where relative error is low, Better linearity and estimate of obs? Time (min)

38 Related systems 38

39 Updated: 4 September Print version CEE 697K ENVIRONMENTL RECTION KINETICS Lecture #b Introduction: Simple Rate Laws Brezoni, pp Introduction

40 Variable Kinetic Order 4 ny reaction order, except n1 dc dt c n n c n1 c n1 o ( n ) t n c c o 1 ( ) 1 1+ n 1 c n1 t ( n 1) n o

41 Half-lives 41 Time required for initial concentration to drop to half, i.e.., c.5c o For a zero order reaction: c c t.5c c t 1 o o o c For a first order reaction: c e o t 1 t.5c c e o o t t 1 1.5c o ln().693

42 Example: Benzyl Chloride 4 Use: Manufacture of benzyl compounds, perfumes, pharmaceuticals, dyes, resins, floor tiles Toxicity Intensely irritating to sin, eyes, large doses can cause CNS depression Emission 45, lb/yr Fate Benzyl chloride undergoes slow degradation in water to benzyl alcohol

43 Benzyl chloride II Sources: Schwarzenbach et al., 1993, Env. Organic Chemistry 197, J. Chem.Soc. Chem. Comm , cta Chem. Scand. 1: , J. Chem. Soc Benzyl chloride to benzyl alcohol H CH Cl O CH OH 5ºC Nucleophilic substitution S N 1 or S N? HCl How to distinguish? Salt effects d dt Temperature.1ºC 5ºC K.4x1-5 s x1-5 s -1 T 1/ 19.1 d.58 d

44 Mixed Second Order 44 Two different reactants Initial Concentrations are different; B The integrated form is: Which can be expressed as: products B + dt d dt d V rate 1 1 ν ξ ( )( ) x B x B dt dx t B B B ln 1 ( ) log.43 log B t B B log B t log B

45 Mixed Second Order 45 Initial Concentrations are the same; B The integrated form is: Which can be integrated: products B + ( )( ) x x dt dx 1 1 t + 1 t 1 x B x B dt d ν t 1 1

Pseudo first order B + products 46 For most reactions, n1 for each of two different reactants, thus a second-order overall reaction c B Many of these will have one reactant in

46 Pseudo first order B + products 46 For most reactions, n1 for each of two different reactants, thus a second-order overall reaction c B Many of these will have one reactant in great excess (e.g., B) 5 3.9x1 Lmg 1 min 1 These become pseudo-1 st order in the limiting reactant, as the reactant in excess really doesn t change in concentration c dc dt c c 1 1 B

47 47 Pseudo-1 st order (cont.) dc dt c c 1 1 B Concentration c obs c o c e B Time (min) obs.3 min t 3.9x1 1 5 (8) Since C changes little from its initial 8 mg/l, it is more interesting to focus on C C exhibits simple 1 st order decay, called pseudo-1 st order The pseudo-1 st order rate constant is just the observed rate or obs

48 Example: O 3 & Naphthalene 48 How long will it tae for ozone (4.8 mg/l dose) to reduce the concentration of naphthalene by 99%? Used in moth balls and as a chemical intermediate nd order reaction; 3 M -1 s -1 Table 1 in Hoigne & Bader, 1983 Wat. Res. 17::173 Industrial WW with.1mm naphthalene Both reactants are at same (.1mM) concentration Therefore, this reduces to a simple nd order reaction t t 99, 33sec 5.5min t 3t

49 O 3 & Naphthalene (cont.) 49 Contaminated river water (.1 mm) Now ozone is in great molar excess, so this is a pseudo-1 st order reaction B t e ln B ( ) 8 3( 4 1 ) 6 ln t t 15.4sec t t

50 Molecularity of three: 3 rd order inetics 5 Quite improbably, but sometimes happens Three different reactants dx dt Complicated integrated form exists Two different reactants dx dt Integrated form: + 3 B + C products ( x)( B x)( C x) 3 B C 3 ( x) ( B x) 3 B 3 ( )( B ) B + ln 3 + B products B ( B ) t 3

51 3 rd Order (cont.) 51 Only one reactant or Initial Concentrations are the same The integrated form is: Which can be integrated: products 3 3 ( )( )( ) x x x dt dx t + 1 t 1 dt d 3 3 ν t t ν

52 3 rd Order (cont.) 5 Pseudo- nd order reactions When one of the reactants has a fixed concentration E.g., present in excess or buffered, or acts catalytically Lie a regular nd order reaction with two reactants but observed constant is fundamental rate constant times concentration of the 3 rd reactant. The integrated form: 1 B ln B B 3 C t obs

53 Example: chlorate formation 53 Formation of chlorate in concentrated hypochlorite solutions Concern: chlorate is toxic MCLG. mg/l Stoichiometry 3OCl ClO3 + Cl Is this 3 rd order? Be septical! Observed inetics So, why is it nd order? d ClO dt 3 obs OCl

54 H Chlorate example (cont.) 54 nswer: this is a reaction pathway composed of two elementary reactions Step #1 Step # OCl OCl slow ClO In multi-step reactions such as these, we say that the overall rate is determined by the slowest step Called the rate-limiting step or RLS Rate law is written based on the RLS Subsequent steps are ignored Prior steps are incorporated as they determine the concentrations of the RLS reactants + Cl fast + ClO ClO3 Cl + Homewor # is based on this reaction

55 Reversible reaction inetics 55 For a general reversible reaction: a+bb f nd the rate law must consider both forward and reverse reactions: rate f C a C r pp + qq where, f forward rate constant, units depend on a and b b or r bacward rate constant, units depend on a and b C P concentration of product species P, moles/liter C Q concentration of product species Q, moles/liter p stoichiometric coefficient of species P q stoichiometric coefficient of species Q b B r C p P C q Q

56 56 Reversible 1 st order reactions Kinetic law db dt 1 B Eventually the reaction slows and, Reactant concentrations approach the equilibrium values Stumm & Morgan Fig..1 Pg. 69 db dt B 1 1 B K eq

57 57 Reversible 1 st order (cont.) Solution to non-equilibrium reaction period See Brezoni, pg for details 1 * { t } + e r f * f r P Where * f + r nd: Where: equ * f r equ ( K ) equ e P t equ equ Linearized version

58 58 To next lecture

CEE 697K ENVIRONMENTAL REACTION KINETICS Lecture #2. Reactions with Chlorine

9/4/3 Updated: 4 September 3 Print version CEE 697K ENVIRONMENTAL REACTION KINETICS Lecture # Rate Expressions: omide + Chlorine case study & lab project Kumar & Margerum paper Introduction Reactions with