cee 697k - college of engineering | umass amherst 6 11 formation of brominated thms acc h cl cl cl c...

9/4/2013

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CEE 697KENVIRONMENTAL REACTION KINETICS

IntroductionDavid A. Reckhow

CEE 697K Lecture #2 1Updated: 4 September 2013

Print version

Lecture #2

Rate Expressions: Bromide + Chlorine case study & lab projectKumar & Margerum paper

Reactions with Chlorine

HOCl + natural organics

(NOM)

Oxidized NOMand inorganic chloride

•Aldehydes

Chlorinated Organics•TOX•THMs•HAAs

Cl

Cl

Cl C H

Br

Cl

Cl C HBr

Cl

Br C H

Br

Br

Br C H

Chloroform Bromodichloromethane Chlorodibromomethane Bromoform

The THMs

The Precursors!

David A. ReckhowCEE 697K Lecture #2

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The Haloacetic Acids

Cl

Cl

Cl C COOH

Br

Cl

Cl C COOHBr

Cl

Br C COOH

Br

Br

Br C COOH

Trichloroacetic Bromodichloroacetic Chlorodibromoacetic TribromoaceticAcid Acid Acid Acid

(TCAA)

Cl

Cl

H C COOH

Br

Cl

C COOHBr

Br

H C COOH

Dichloroacetic Bromochloroacetic DibromoaceticAcid Acid Acid

(DCAA)

H


3

David A. Reckhow

4

Distribution: Variability within a single system

Example: New Haven Service Area DS model

CEE 697K Lecture #2

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David A. Reckhow

5 Sept 11, 1997 22:00

chlorine

New HavenDistribution System

Lake SaltonstallWTP

West RiverWTP

LakeGaillard

WTP

Millrock Basins

Maltby Tank

3,400 pipes2,500 junctions

3 MG

8.7 MG

2.0 mg/L DOC (Treated)pH 71.8 mg/L chlorine doseCEE 697K Lecture #2

David A. Reckhow

6 Sept 11, 1997

22:00

TTHM

CEE 697K Lecture #2

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David A. Reckhow

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Sept 11, 1997 22:00

HAA6

CEE 697K Lecture #2

Single Cell Gel Electrophoresis Genotoxicity PotencyLog Molar Concentration (4 h Exposure)

10-6 10-5 10-4 10-3 10-2

IAA

BA

A

CA

A

DIA

A

TB

AA

DB

AA3,

3-D

ibro

mo-

4-ox

ope

nta

noic

Aci

d

2-B

rom

obut

ene

dio

ic A

cid

2-Io

do-

3-br

omop

rop

enoi

c A

cid

2,3-

Dib

rom

opr

ope

noic

Aci

d

DB

NM

BD

CN

MT

BN

M

TC

NM

BN

M

BC

NM

DB

CN

M

DC

NM

CN

M

Bro

mo

ace

tam

ide

Dib

rom

oace

tam

ide

Tri

brom

opyr

role

MX

Bro

ma

te

EM

S +

Co

ntr

ol

Haloacetic Acids

Halo Acids

Haloacetamides

Halonitromethanes

Other DBPs

DBP Chemical Class

Not Genotoxic: DCAA, TCAA, BDCAA, Dichloroacetamide, 3,3-Dibromopropenoic Acid, 3-Iodo-3-bromopropenoic Acid, 2,3,3,Tribromopropenoic Acid July 2006

Ch

loro

ace

tam

ide

Trih

loro

ace

tam

ide

Iodo

ace

tam

ide

Haloacetonitriles Bro

mo

acet

onitr

ile

Dib

rom

oac

eto

nitr

ile

Bro

mo

chlo

roa

ceto

nitr

ile

Ch

loro

ace

ton

itrile

3,3

-Bro

moc

hlor

o-4-

oxo

pent

ano

ic A

cid

Iod

oace

toni

trile

Tric

hlor

oace

toni

trile

Dic

hlor

oace

ton

itrile

BIA

A

CD

BA

A

BC

AA

Work of Michael Plewa Univ. of Illinois

8

Genotoxicity


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Bromide: THM Formation

Bromide Concentration (mg/L)

0.0 0.4 0.8 1.2 1.6 2.0

Per

cent

of

TT

HM

0

20

40

60

80

100

CHCl3

CHBrCl2

CHBr2Cl

CHBr3

Data from: Minear & Bird, 198096 hours, pH 7.0

5 mg/L Chlorine Dose1 mg/L Humic Acid


10

Impact of Bromide on DHAA Formation

Bromide Concentration (mg/L)

0 1 2 3 4 5

Con

cent

ratio

n ( g

/L a

s C

l- )

0

20

40

60

80

100

CHCl2COOH

CHClBrCOOHCHBr2COOH

pH 7, 25oC, 7 days25 mg/L chlorine dose

2.9 mg/L TOC

From Pourmoghaddas, 1990David A. ReckhowCEE 697K Lecture #2

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Formation of Brominated THMs

A C C

H

ClCl

Cl

C

H

BrCl

Cl

C

H

BrBr

Cl

C

H

BrBr

Br

B

D E

F

HOCl HOCl HOCl

HOCl HOCl

HOCl

HOCl Br HOBr Clk David A. ReckhowCEE 697K Lecture #2

David A. Reckhow

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THM Br/(Cl+Br)

0.0 0.2 0.4 0.6 0.8 1.0

TH

M M

ole

Fra

ctio

n (T

HM

4 o

nly

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

CHCl3CHBrCl2CHBr2ClCHBr3CHCl3 Lab testsCHBrCl2 Lab testsCHBr2Cl Lab testsCHBr3 Lab tests

Full-scale data (small symbols)

from Weinberg et al., 2002 (All 12 plants)

Lab data (large symbols)

from Hua & Reckhow, 2004

Lines are geometric model

Bromo speciation

CEE 697K Lecture #2

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Case study: chlorine + bromide


13

Observed reaction Nucleophilic attack of bromide on oxygen in

hypochlorous acid k2 = 2.95 x103 M-1s-1 at 25ºC

Note that HOCl deprotonates at elevated pHs

From Farkas et al., 1949

Transfer of Cl+ to form intermediate (BrCl)

From Kumar & Margerum, 1987

ClHOBrBrHOCl k2

BrClOHABrHOClHA k 2

3

ClBrBrBrCl fast2

32 BrBrBr fast

Background

Absorbance for monitoring reaction

Known equilibria

David A. Reckhow

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CEE 697K Lecture #2

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Alkaline Experiments


15

Pseudo-1st order Bromide in great excess

Slow reactions Could be monitored directly by

conventional UV spectrophotometer 292 nm peak for hypochlorite

(OCl-) Followed for 4 half-lives Complete from: 1.1 sec – 4 hr

Plot ln(At-A∞) vs time Slope is kobs

Kobs is a linear function of Bromide


16

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pH dependency


17

Overall pH dependence


18

How to explain pH effect on 2nd order rate constant? See also, Brezonik, pg 230; figure 4-20

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Buffer Tests


19

Low pH

High pH

Buffer Effect

Proposed dependence on HA

David A. Reckhow

20

CEE 697K Lecture #2

][][][0 BrHAkHkkk HAHobs

][

][

1

1][

][

0

HK

CHCCHA

a

TT

HKT

a

][

][

][

][ 0

HK

bufferkk

H

kBr

k

a

THAH

obs

12210)15.025.1( sMxkHA

123.19.8 sMkHA

HPO4-2

HCO3-

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High pH only

More generally

David A. Reckhow

21

CEE 697K Lecture #2

][][][0 BrHAkHkkk HAHobs

][]][[][][)]([

0

BrOClHAkHOClkOClk

dt

ICldHAHOCl

]][[][][][

0

BrOClHAkHkkdt

OCldHAH

Acidic Experiments


22

Pseudo-1st order Bromide in great excess

Fast reactions Required high-speed setup Stopped-flow spectrophotometer

Could monitor 2º product 266 nm peak for tribromide (Br3

-)

Followed for 4 half-lives Complete from 0.40-0.01 sec

Plot ln(At-A∞) vs time Slope is kobs

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Stopped-flow


23

Principle

Commercial instruments

Limitations Correct for slow mixing speed (km) For Durrum instrument: km = 1700 s-1

m

obsk

kobs

obs

kk

1

Kobs is still linear with Br-


24

No difference in mechanism?

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Acidic data: impact of acids


25

General and specific acid

Overall pH dependence


26

Three effective zones See also, Brezonik, pg 230; figure 4-20

HOCl

OCl-

H+ + HOCl

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Catalysis: comparison with pKa


27

Bronsted catalysis

Indicates that HA engages in greater donation of proton to OCl- than to HOCl

H3O+

HCO3-

HPO4-2

CH3COOH

CH2ClCOOH

H2O

aAHA KGk

27.075.0

Mechanisms


28

Proposed mechanism

Alternative Less likely

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Temperature: Arrhenius Equation


29

Arrhenius Equation

Where R is the universal gas constant

Transforms to:

/aE RTk Ae8.3145

o

JR

M K

1( ) ( ) a

TELn k Ln A R T

Ln(k)

1/T

1 1a

b a

E

R T TTb Tak k e

Or:

Pizza Kinetics


30

Leenson, 1999 What are the kinetics of

pizza spoilage?

Do they conform to Arrhenius?

J. Chem Ed., 76:4:504-505

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Pizza II


31

Arrhenius plot t~k

Time (min)

0 20 40 60 80 100

Abs

orba

nce

at 2

92 n

m (

cm-1

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Lab Project Example Data #I


32

0.1 M NaOH

What is the Absinf ?

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Time (min)

0 20 40 60 80

Ln (

A2

92-A

29

2in

f)

-6

-5

-4

-3

-2

-1

b[0] -1.5509712892 b[1] -0.0400283854

Lab Project Example Data #II


33

0.1 M NaOH

Set Absinf = 0.085

time vs ln(A-Ainf) Plot 1 Regr

Time (min)

0 20 40 60 80

Abs

orb

an

ce a

t 2

92 n

m (

cm-1

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Lab Project Example Data #III


34

0.05 M NaOH

Set Absinf = ?

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Time (min)

0 20 40 60 80

Ln

(A

292

-A29

2inf

)

-9

-8

-7

-6

-5

-4

-3

-2

-1

b[0] -1.4076822953 b[1] -0.0862394216

Lab Project Example Data #IV


35

0.05 M NaOH

Set Absinf = 0.094

Maybe too high ?Downward curvature

time vs ln(A-Ainf) Plot 1 Regr

Time (min)

0 20 40 60 80

Ln (

A2

92-A

292i

nf)

-7

-6

-5

-4

-3

-2

-1

b[0] -1.6667372496 b[1] -0.0679233226

Lab Project Example Data #V


36

0.05 M NaOH

Set Absinf = 0.092

Looks better, except for final data where relative error is high,Use only earlier data?

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Time (min)

0 10 20 30

Ln

(A

292

-A29

2in

f)

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

b[0] -1.5462593144 b[1] -0.0744460114

Lab Project Example Data #VI


37

0.05 M NaOH

Set Absinf = 0.092

Using only earlier data where relative error is low,Better linearity and estimate of kobs?

Related systems


38

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CEE 697KENVIRONMENTAL REACTION KINETICS

IntroductionDavid A. Reckhow

CEE 697K Lecture #2 39Updated: 4 September 2013

Print version

Lecture #2b

Introduction: Simple Rate Laws Brezonik, pp.31-39


40

Variable Kinetic Order

Any reaction order, except n=1

nnck

dt

dc

11

111

1

nnon

o

tckncc

tkncc nn

on

111

11

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41

Half-lives

Time required for initial concentration to drop to half, i.e.., c=0.5co

For a zero order reaction:

For a first order reaction:

c c kto 2

15.0 ktcc oo k

ct o5.0

21

c c eokt 2

1

5.0kt

oo ecc

k

kt

693.0

)2ln(2

1

Example: Benzyl Chloride


42

Use: Manufacture of benzyl compounds, perfumes,

pharmaceuticals, dyes, resins, floor tiles

Toxicity Intensely irritating to skin, eyes, large doses can cause

CNS depression

Emission 45,000 lb/yr

Fate Benzyl chloride undergoes slow degradation in water to

benzyl alcohol

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Benzyl chloride II


43

25ºC

Sources:•Schwarzenbach et al., 1993, Env. Organic Chemistry•1972, J. Chem.Soc. Chem. Comm. 425-6•1967, Acta Chem. Scand. 21:397-407•1961, J. Chem. Soc. 1596-1604

][][

Akdt

Ad

Benzyl chloride to benzyl alcohol

Nucleophilic substitution SN1 or SN2?

How to distinguish?

Salt effects

CH2Cl CH2OHH2O

HCl

Temperature

0.1ºC 25ºC

K 0.042x10-5 s-1 1.38x10-5s-1

T1/2 19.1 d 0.58 d

Mixed Second Order


44

Two different reactants

Initial Concentrations are different; [A]0≠[B]0 The integrated form is:

Which can be expressed as:

productsBA k 2

dt

Ad

dt

d

Vrate

A

][11

xBxAk

BAkdt

dx

002

2

][][

]][[

tkBA

AB

BA 20

0

00 ][][

][][ln

][][

1

0

0002 ][

][log][][43.0

][

][log

A

BtBAk

B

A

][

][log

B

A

t0

0

][

][log

B

A

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Mixed Second Order


45

Initial Concentrations are the same; [A]0=[B]0

The integrated form is:

Which can be integrated:

productsBA k 2

xAxAk

AAkdt

dx

002

2

][][

]][[

02 ][

12

][

1

Atk

A

][

1

A

t0][

1

A

xBxABA 00 ][][][][

dtkA

AdA 22][

][ tkAA 2

0

2][

1

][

1


46

Pseudo first order

For most reactions, n=1 for each of two different reactants, thus a second-order overall reaction

112 BAcck

dt

dc

115 min109.3 Lmgxk

Many of these will have one reactant in great excess (e.g., B) These become “pseudo-1st

order in the limiting reactant, as the reactant in excess really doesn’t change in concentration

Bc

Ac

productsBA k 2

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0102030405060708090

0 20 40 60 80

Time (min)

Co

nc

en

tra

tio

n


47

Pseudo-1st order (cont.)

Since C2 changes little from its initial 820 mg/L, it is more interesting to focus on CA

CA exhibits simple 1st

order decay, called pseudo-1st order The pseudo-1st order rate

constant is just the “observed rate” or kobs

112 BAcck

dt

dc

tkAoA

obsecc

1

52

min032.0

)820(109.3

xckk Bobs

Example: O3 & Naphthalene


48

How long will it take for ozone (4.8 mg/L dose) to reduce the concentration of naphthalene by 99%?

Used in moth balls and as a chemical intermediate

2nd order reaction; k2 = 3000 M-1s-1

Table 1 in Hoigne & Bader, 1983 [Wat. Res. 17:2:173]

Industrial WW with 0.1mM naphthalene Both reactants are at same (0.1mM)

concentration Therefore, this reduces to a simple 2nd

order reaction

tkAA 2

0][

1

][

1

t300010

1

10

146

min5.5

sec330

000,9903000

t

t

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O3 & Naphthalene (cont.)


49

Contaminated river water (0.001 mM) Now ozone is in great molar excess, so this is a pseudo-1st

order reaction tBkeAA 02 ][0][][

sec4.15

3.0605.4

10300010

10ln

][][][ln

46

8

020

t

t

t

tBkAA

Molecularity of three: 3rd order kinetics


50

Quite improbably, but sometimes happens Three different reactants

Complicated integrated form exists

Two different reactants

Integrated form:

productsCBA k 3

xCxBxAkCBAkdt

dx 00033 ][][][]][][[

productsBA k 32

xBxAkBAkdt

dx 0

203

23 ][2][][][

tkABBA

AB

AA

ABAA3

200

0

0

0

000 ][][2][][

][][ln

][][

][][2][][

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3rd Order (cont.)


51

Only one reactant or Initial Concentrations are the same

The integrated form is:

Which can be integrated:

productsA k 33

xAxAxAk

AAAkdt

dx

0003

3

][][][

]][][[

20

32 ][

16

][

1

Atk

A

2][

1

A

t

20][

1

A

dtkA

AdA 33][

][ tktkAA A 332

02

62][

1

][

1

3rd Order (cont.)


52

Pseudo-2nd order reactionsWhen one of the reactants has a fixed concentration

E.g., present in excess or buffered, or acts catalytically

Like a regular 2nd order reaction with two reactants but observed constant is fundamental rate constant times concentration of the 3rd reactant.

The integrated form:

tCkBA

AB

BA][

][][

][][ln

][][

13

0

0

00

obsk

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Example: chlorate formation


53

Formation of chlorate in concentrated hypochlorite solutions Concern: chlorate is toxicMCLG=0.2 mg/L

Stoichiometry

Is this 3rd order? Be skeptical!

Observed kinetics

So, why is it 2nd order?

ClClOOCl 23 3

23 ][

OClkdt

ClOdobs

Chlorate example (cont.)


54

Answer: this is a reaction pathway composed of two elementary reactions Step #1 Step #2

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

ClClOOCl slow22

ClClOClOOCl fast32

Homework #2 is based on this reaction

H

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55

Reversible reaction kinetics

For a general reversible reaction:

k

qQ + pP bB + aA

k

r

f

And the rate law must consider both forward and reverse reactions:

where,kf = forward rate constant, [units depend on a and b]kb or kr = backward rate constant, [units depend on a and b]CP = concentration of product species P, [moles/liter]CQ = concentration of product species Q, [moles/liter]p = stoichiometric coefficient of species Pq = stoichiometric coefficient of species Q

qQ

pPr

bB

aAf CCkCCkrate


56Reversible 1st order reactions

Stumm & MorganFig. 2.10Pg. 69][][ 21 BkAk

dt

dB

eqKk

k

A

B

BkAkdt

dB

2

1

21

][

][

][][0

Kinetic law

Eventually the reaction slows and, Reactant concentrations

approach the equilibrium values

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Reversible 1st order (cont.)


57

Solution to non-equilibrium reaction period See Brezonik, pg 37-38 for details

Where k* = kf + kr

And:

Where:

tkfr ekkA

kA

*

0*][

1][

k

P A

k

r

f

tk

equ

equ eAA

AA *

][][

][][

equ

equequ

r

f

A

PK

k

k

][

][

Linearized version


58

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