Chloro Compounds in Water Treatment - American Chemical Society

17 C h l o r i n a t i o n a n d the F o r m a t i o n of N - C h l o r o C o m p o u n d s in W a t e r T r e a t m e n t

Downloaded by UNIV OF NORTH CAROLINA on July 13, 2016 | http://pubs.acs.org Publication Date: January 12, 1979 | doi: 10.1021/bk-1978-0082.ch017

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DALE W. MARGERUM, EDWARD T. GRAY, JR., and RONALD P. HUFFMAN Department of Chemistry, Purdue University, West Lafayette, IN 47907

Chlorine is widely used for the disinfection of potable water and wastewater. It is estimated that 0.3 to 0.4 million tons are used annually for sanitary purposes (1). Chlorine and hypochlorous acid (HOCl) are much more effective than hypochlorite ion (OCl ). Thus, for disinfection four times as much as total chlorine is needed at pH 9 than at pH 7. Below pH 7, a 2 X 10 M concentration of HOCl is sufficient for 99.6% virus inactivation, 99.999% coliform kill, and 99.999% cyst disinfection (2). Ammonia, which is present in waste water and even in many drinking and river waters, will react rapidly with HOCl to form monochloramine (NH Cl). Hence NH Cl becomes the principal chlorine disinfectant in any water containing as much as 0.1 mg/L of NH-N and less than 1.0 mg/L of Cl . Additional chlorine will give dichloramine (NHCl ) and trichloramine (NCl ). Figure 1, the decomposition of residual chlorine is accomplished by the destruction of the chloramines (presumably to N and N0 ) with excess chlorine. A typical breakpoint occurs at 8 mg Cl /L per 1 mg N/L. Beyond the breakpoint additional chlorine dosage introduces more HOCl and often N-chloro-organic compounds as well (2). -

-6

2

2

3

2

2

3

-

2

3

2

'Current address: Dept. of Chemistry, U. of Hartford, W. Hartford, CT 06117 Current address: Upjohn Co., P.O.B. 685, Laporte, TX 77571 2

0-8412-0461-6/78/47-082-278$05.00/0 © 1978 American Chemical Society Brinckman and Bellama; Organometals and Organometalloids ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

17.

Chloramines are t o x i c to f i s h . c h l o r i n e causes

Thus, 0.08 ppm residual

f i s h k i l l s and long exposure

NH^Cl are l e t h a l

279

Formation of N-Chloro Compounds

MARGERUM ET A L .

to rainbow t r o u t (3).

and combined c h l o r i n e ( j L c

chloramines)

same order o f magnitude

4).

(2»

to ppb l e v e l s o f

The t o x i c i t i e s o f free are considered to be the

Fresh water fishes

suffer

anoxia

due to c h l o r i n e or chloramine o x i d a t i o n o f hemoglobin to methemo-


globin

(5J.

I t i s important to know the ease o f formation, the

s t a b i l i t y , and r e a c t i v i t y o f chloramines i n order to understand t h e i r environmental

effects.

The present paper i s concerned with some o f the s o l u t i o n chemistry o f chloramines, e s p e c i a l l y the k i n e t i c s o f formation and d i s s o c i a t i o n o f various N-chloro compounds. Morris and his associates

( £ , 7_ 8, 9^ 10) has f

E a r l i e r work by provided impor-

tant basic information about some o f the properties o f c h l o r amines.

It has been p o s s i b l e

use o f stopped-flow

to expand on t h i s base through the

spectrophotometry and by taking advantage o f

the a b i l i t y to form r e l a t i v e l y s t a b l e species.

s o l u t i o n s o f N-chloro

In the preceding paper we have discussed the determina-

t i o n o f accurate e q u i l i b r i u m constants

for the chloramines and the

k i n e t i c s o f d i s p r o p o r t i o n a t i o n o f monochloramines.

We now con-

s i d e r the k i n e t i c s o f t r a n s f e r o f a c t i v e c h l o r i n e to oxygen and nitrogen as well

as i t s t r a n s f e r

between oxygen and nitrogen

compounds. Reactions o f C l . 2

nitrogen from C l reactions.

2

The t r a n s f e r o f C l

i s r a p i d but measurable

+

to oxygen and to

for several

The k i n e t i c s o f h y d r o l y s i s o f C l HOCl + C l " + H

?

different

(eq 1) have been (1)

+

measured by Eigen and Kustin (11) and by L i f s h i t z and PerlmutterHayman (12).

In the present work we needed to know the e q u i l i b -

rium constant f o r eq 1 i n 0.50 M NaC10

4

at 25.0°C

and i n the

process o f t h i s determination obtained the rate constants which are i n s u b s t a n t i a l

agreement with the e a r l i e r s t u d i e s .

Our values

Brinckman and Bellama; Organometals and Organometalloids ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

280

are:

ORGANOMETALS AND ORGANOMETALLOIDS

k

= 28.6 s " ,

= 2.8 χ 1 0

1

1

10"^ M .

M"

4

2

s" , 1

and kj/lc^ = 1.0 χ

It can be seen from these r a t e constants that the

2

equilibration of C l

with HOCl and C l " occurs w i t h i n seconds.

2

The rates o f C l

r e a c t i o n with ammonia, alkylamines and

2

amino acids (eq 2) are very f a s t and, i n f a c t , are nearly at the


Cl

2

+ RNH

Jl!2—•

2

RNHCl + H

diffusion-controlled

limit.

are not r e a c t i v e with C l constants (Table I)

+ Cl"

(2)

The protonated amine s p e c i e s , RNH^ , +

or with HOCl.

2

We have resolved

rate

for the r e a c t i o n o f these a c t i v e c h l o r i n e

species with the free amines by appropriate v a r i a t i o n o f the H and C l " c o n c e n t r a t i o n s .

Once the monochloramines are formed they

also react r a p i d l y with C l RNHCl k

Cl

2

+ RNHCl J £ h

•

(eq 3).

9

Since protonated monochlor-

ά

RNC1 + H 2

+

+ Cl"

(3)

amine, RNH C1 , forms only at very low pH, the rate o f 2

+

o f RNC12 from C l

2

formation

i s not as e a s i l y suppressed i n a c i d i c s o l u t i o n

as i s the rate o f formation o f RNHCl.

As a r e s u l t dichloramines

may be observed as the products o f the c h l o r i n a t i o n o f amines and the rate-determining step i s eq 2 r a t h e r than eq 3. Table I Second-Order Rate Constants ( M ^ s " ) for the C h l o r i n a t i o n o f 1

Amines and Amino A c i d s , 25.0°C .RNH K

ci

.RNH,>

2

κ 2

0

.RNH H0C1 2

α '

K

2

Ammonia

4.0 χ 1 0

9

2.9 X 1 0

6

Methyl amine

2.8 χ 1 0

9

1.9 X 1 0

8

Glycine

—

1.5 X 1 0

9

5.0 X 10

7

Glycyl g l y c i n e

. . .

2.1 X 1 0

9

5.3 X 1 0

6

α-alanine

—

.

1.0 X 1 0

9

5.4 X 10

7

β-alanine

. . .

1.3 X 1 0

9

8.9 X 1 0

7


17.


MARGERUM ET AL.

281

The doubly protonated amino a c i d s , NHgRC00H, are not r e a c +

t i v e with C l

and the concentration o f the completely

2

ed form, Nr^RCOO", i s n e g l i g i b l e Although a monoprotonated

under the a c i d i c conditions

form does react with C l , i t i s


NH^RCOO , but

rather

s p e c i e s , Nr^RCOOH, which i s present as a minor

i s the neutral

species

used.

propos-

2

ed that t h i s i s not the zwi t t e r i o n s p e c i e s ,

component.

unprotonat-

Rate constants c a l c u l a t e d i n terms o f t h i s

i n Table

RNH ° 2

I agree with those found for the free amines and

also are c l o s e to the d i f f u s i o n - l i m i t i n g Reactions o f HOCl.

values.

The t r a n s f e r o f C l

+

from oxygen to

nitrogen i n the r e a c t i o n o f hypochlorous a c i d with ammonia (eq k HOCl + NH

H

3

3 Q )

4)

N H

NH C1 + H 0

C 1

2

(4)

2

i s r a p i d and t h i s i s the main pathway

for monochloramine

t i o n in c h l o r i n a t i o n r e a c t i o n s .

and Morris

taken below pH 6.5 (where NH^

Weil

(6)

formation suppresses

+

used data the rate) and

data taken above pH 12 (where 0C1" formation suppresses to give a rate constant o f 6.2 χ 1 0 our stopped-flow

studies

6

M'^s"

forma

the rate)

for r e a c t i o n 4.

1

In

the r e a c t i o n was measured over the

intermediate pH range as shown i n Figure 2 where ^ ^ j i s a 0

pseudo-first-order

rate constant (eq 5)

using excess

sc

ammonia.

d[NH Cl] ?

" W

- d f —

H

0

C

1

1T

( 5 )

The s o l i d l i n e i n Figure 2 i s c a l c u l a t e d using a value o f 2.8 χ 10

M'V

(0.1 M N a C l O j , 25.0°C) for the second-order rate NH constant, k 3 This value i s based on protonation constants Q *5Q . 1 HOCl 7 ΛΛ -1 6

Kr'^M

1

1

for NH

3

and 1 0

NaClOj at 25.0°C. HOCl and NH

3

are

/ β

^ M

The r e s u l t s

1

for 0C1

a l s o measured i n 0.1 M

confirm the postulate that only

reactants.

The rate constants for HOCl with some a l k y l acids and peptides are summarized i n Table the behavior o f C l , the rate constants 2

of

I.

amines, amino

In c o n t r a s t with

for the HOCl

reactions


ORGANOMETALS


282

AND ORGANOMETALLOIDS

Water Chlorination

Figure 1. Rehtionship between chlorine dosage and residual chlorine for break point chlorination (2)

CHLORINE DOSAGE

Figure 2. Observed firstorder rate constant for the formation of NH Cl from hypochlorous acid (initially 2.0 X 10 M) with excess ammonia (initially 7.35 X 10' M). The solid line is a calculatedfitusing k oci > — 2.8 X WW s' , K„ for NH — 10 Μ-\ and K„ for OCl- = 10 M-\ 25.0°, = 0.10M (NaClOO. g

5

3

H

1

S

NH

1

939

7A4

μ


17.

MARGERUM


E T AL.

283

increase as the b a s i c i t y o f the amines increase as seen i n Figure 3.

A l l the amines except ammonia f a l l

log k 2 with l o g K HOCl " R N H

R N H

2.

along a l i n e c o r r e l a t i n g

This suggests a n u c l e o p h i l i c attack by

the amine on the Cl atom i n HOCl as the rate-determining step i n the r e a c t i o n mechanism. The rate constants for the r e a c t i o n o f monochloramines with Downloaded by UNIV OF NORTH CAROLINA on July 13, 2016 | http://pubs.acs.org Publication Date: January 12, 1979 | doi: 10.1021/bk-1978-0082.ch017

HOCl (eq 6) are given i n Table I I . RNHCl + HOCl

•

The smaller rate

constants

RNC1 + H 0 2

(6)

2

agree with the diminished n u c l e o p h i l i c i t y o f the monochloramines. Figure 4 shows the c o r r e l a t i o n for both RNH and RNHCl.

The slope

2

o f the l o g - l o g

plot is 0.61. Table

II

Protonation Constants and Rate Constants f o r HOCl Reaction with Monochloramines, 0.50 M (NaC10 ), 4

1 0 9

K

H

k

H0Cl

·

M

monochloramine

1.44

150

methylchloramine

1.55

352

N-chloro-3-alanine

0.41

278

N-chloroglycylglycine

-0.67

25.0°C

S

8.7

Below pH 4-5 the a d d i t i o n o f HOCl to RNH s o l u t i o n s 0

results

in RNClp as the observed product rather than RNHCl despite the RNHo fact that the k 2 t e constants are much l a r g e r than the ,RNHCl HOCl HOCl The reason i s that the r a t i o o f RNH /RNH * i s very s m a l l , while the r a t i o o f RNHCl/RNH C1 i s RNHCl DMU ^ large and hence l w - Γ [RNHCl] » k 2[RNH ]. HOCl 2 Reactions o f Monochloramines. The rate o f h y d r o l y s i s o f r a

r

a

t

e

c

o

n

s

t

a

n

t

s

9

0

KNH

H 0 C 1

NH C1 (eq 7) i s r e l a t i v e l y slow. NH Cl ?

k

Η

s ^ for k [ ^ 2 ^ H0

c

2

9

L

J

A rate constant o f 1.9 χ 1 0 "

5

2

NH C1 + H 0 2

+

2θ

a

n

,

NH + HOCl

(7)

3

be c a l c u l a t e d from the formation rate

constant

2




284

Figuré 3. Comparison of second-order rate constants (25.0°C) for the chlorination of amines as a function of their base strength. (Φ) RNH and Cl , (A) amino acids (NH,RCOOH form) and Cl , (O) RNHg and HOCl, and (φ) NH and HOCl g

2

2

S

Figure 4. Dependence of the sec ond-order rate constant for HOCl chlorination of amines and mono chloramines with their base strength, 25.0°C, μ — 0.50U (NaClOt)

9

10

log K ™ H *

log

KH


17.


MARGERUM ET A L .

285

and the e q u i l i b r i u m constant given i n the preceding paper. NH^Cl forms i t i s slow to release HOCl.

a c t i v e c h l o r i n e i n the form o f

Another h y d r o l y s i s r e a c t i o n to give hydroxylamine NH Cl k

2

•

NH 0H + C l "

(8)

o

a s i g n i f i c a n t pathway only i n strong base. report a value o f 6.3 χ 1 0 " M ^ s " 5


(eq 8) i s

2

2ϋ

NH C1 + OH"

Once

1

Anbar and Yagil

for k ^

2

^·

Rate constants

for the h y d r o l y s i s o f monochloramines and dichloramines ing HOCl) are summarized i n Table Table

(13)

(releas

III. III

Rate Constants f o r the Hydrolysis o f Chloramines, 25.0°C,

0.5 M (NaC10 ) 4

k

s"

1

NH C1

1.9 χ 1 0 "

5

N-Cl-B-alanine

2.1 χ 1 0 "

6

CH NHC1

1.6 χ 1 0 "

6

NHC1

6.5 χ 1 0 "

7

N,N-diCl-e-alanine

1.6 χ 1 0 "

9

CH NC1

4.7 χ 1 0 "

8

2

3

2

3

2

As shown i n the preceding paper monochloramines w i l l proportionate

i n a c i d i n accord with eq 9.

RNHCl + RNH C1 2

+

the value o f k p creases.

k

I S

p

i

S

,

decreases

RNC1

2

+ RNH

3

dis

It i s i n t e r e s t i n g

that

(9)

+

as the base strength o f RNH de 0

The c o n t r a s t with the HOCl c h l o r i n a t i o n dependence i s

s t r i k i n g as shown i n Figure 5.

This suggests a s h i f t

i n the r a t e -

determining step to the breaking o f the N-Cl bond a f t e r

nucleo

p h i l i c attack by RNHCl on the c h l o r i n e o f RNH C1 . 0

Is the d i r e c t t r a n s f e r o f Cl another p o s s i b l e ?

+

from one amine nitrogen to

Yes, recent work by Snyder and Margerum (14)

shows that the rate o f c h l o r i n e t r a n s f e r

i n eq 10 i s f a s t e r


than


ORGANOMETALS

8

9

10

AND

ORGANOMETALLOIDS

II

Figure 5. Comparison of the rate constants for monochloramine disproportionation (k ) and the rate constants for the reaction of amines with HOCl (k *) with the basicity of the amines, 25.0°C, μ — 0.50M (NaClOrf. DIS

RNH


17.

M A R G E R U M

NH C1 + NH CH C00" £

2

•

2

the rate o f h y d r o l y s i s . from pH 4 to 9.

287


E T AL.

NH^ + C1NHCH C00"

(10)

2

The observed rate constant i s 1.5 M"^s"^

The observed rate constant i s i n v a r i a n t over

t h i s pH range because the r e a c t i v e species are N H ^ C l

+

and NHoCHo-

C00" whereas the predominant species i n s o l u t i o n are Nf-LCl and


NH

3

CHoCOO .

Protonated monochloramine, NH^Cl

, i s a very

effec

t i v e c h l o r i n a t i n g agent as seen from the data i n Table I V . The Table IV Rate Constants f o r the D i r e c t Transfer o f C h l o r i n e from NH C1 3

+

to Amino Acids and Peptides

(14_), 25.0°C

k, M"

1

s"

glycyl glycine"

1.3 χ 1 0

7

glycine"

2.5 χ Ί Ο

8

β-alanine"

2.6 χ Ι Ο

8

1

NHgCl* ion reacts considerably f a s t e r than HOCl and i s only about an order o f magnitude slower than C l . On the other hand, the + -1 protonation constant to form NH^Cl from NH C1 i s only 28 Μ , so 9

0

the f r a c t i o n o f protonated species present i n neutral pH i s small and the observed second-order rate constants are not large ( £ . £ . 1.5 M"

1

s"

1

for g l y c i n e ) .

Reactions o f Pichloramines. NHC1 + H 0 2

•

2

The h y d r o l y s i s o f NHCl^ (eq 11)

NH C1 + HOCl

i s slower than the h y d r o l y s i s o f NH^Cl. amines (Table I I I ) reactions.

(11)

2

S i m i l a r l y , other d i c h l o r -

have small rate constants f o r t h e i r h y d r o l y s i s

Obviously the i n s t a b i l i t y o f NHC1 i s not due to 2

simple h y d r o l y s i s r e a c t i o n s . The rate constant and e q u i l i b r i u m constant for the d i s p r o p o r t i o n a t i o n r e a c t i o n o f monochloramine given i n the preceding

-3 paper can be used to c a l c u l a t e a rate constant o f 6.4 χ 10 M -1 -3 s for the r e a c t i o n i n eq 12. Therefore greater than 10 M


-1

288

NHC1


2

+ NH

4

•

+

2NH C1 + H 2

(12)

+

concentrations o f ammonia o r other amines could speed the conver sion o f dichloramines to monochloramines.

The e q u i l i b r i u m p o s i

t i o n o f the r e a c t i o n i n eq 12 also s h i f t s to the r i g h t as the pH becomes greater than 6. Other reactions o f NHC1 must take place i n neutral or

basic


0

solutions

i n order to account for the i n s t a b i l i t y reported

this species,

but as y e t these reactions are not well

Discussion.

for

defined.

The k i n e t i c s data for the t r a n s f e r o f C l

+

from nitrogen atoms in amines show that RNHCl and RNClp are ed r a p i d l y and are slow to release Cl

in a c i d .

to and form-

Although NHCl

has often been considered an extremely unstable compound i n

2

regard

to s e l f o x i d a t i o n - r e d u c t i o n , t h i s i s not the case i n a c i d i c s o l u t i o n s i n the absence o f a d d i t i o n a l

HOCl.

and other dichloramines i n neutral

and basic s o l u t i o n s need to be

studied.

The reactions o f NHC1

Monochloramines are r e l a t i v e l y s t a b l e in s l i g h t l y

2

basic

and s l i g h t l y a c i d i c s o l u t i o n s and act as a r e s e r v o i r o f a c t i v e chlorine.

The persistence o f residual

c h l o r i n e i n r i v e r water

has been a t t r i b u t e d to the slow decay o f monochloramine The h a l f l i f e o f t h i s r e s i d u a l half l i f e

(15).

c h l o r i n e i s comparable with the

for the h y d r o l y s i s o f NH C1. 0

Table V summarizes some p o s s i b l e forms i n the c h l o r i n a t i o n o f water. namically unfavorable

reactions o f NH^Cl a f t e r Reaction (a)

thermody-12

(the e q u i l i b r i u m constant i s 6.7 χ 10

M*" ) and would occur only i f other processes 1

r e a c t i o n products.

is

Reaction (b)

cern except i n strong base.

were removing the

i s much too slow to be o f c o n

Reaction (c)

is

thermodynamically

favorable only below pH 6 and i t i s second-order

process i n mono

chloramine so t h i s decay would not be a favorable pathway. s e l f redox decomposition o f NHCl

2

Reaction (d)

The

at higher pH could help to

d r i v e the r e a c t i o n but the rate o f r e a c t i o n (c) pH.

it

i s slower at high

depends on the concentration o f other amines,


17.

M A R G E R U M


E T AL.

289

Table V Possible C l (a)

NH C1 + H 0 0

t (b)


Chloro Compounds in Water Treatment - American Chemical Society

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