Geoscience Reference
In-Depth Information
TABLE 4.40
Hydrolysis Rates at 25
°
C and pH 7 of Selected Halogenated Compounds
Rate Constants (1/s)
K B [OH ],
Base-Catalyzed
Hydrolysis
Reaction Rate
Constant
K N , Neutral-
Catalyzed
Hydrolysis
Reaction Rate
Constant
KH, Pseudo-
First-Order
Reaction Rate
Constant
T 1/2
Half-Lives
Compound
CH 3 F a
7.44 × 10 −10
5.82 × 10 −14
7.44 × 10 −10
30 years
CH 3 Cl a
2.37 × 10 −8
6.18 × 10 −13
2.37 × 10 −8
339 days
CH 3 Br a
4.09 × 10 −7
1.41 × 10 −11
4.09 × 10 −7
20 days
CH 3 I a
7.28 × 10 −8
6.47 × 10 −12
7.28 × 10 −8
110 days
CH 3 CHClCH 3 a
2.12 × 10 −7
2.12 × 10 −7
38 days
CH 3 CH 2 CH 2 Br a
3.86 × 10 −6
3.86 × 10 −6
26 days
CH 2 Cl 2 a
3.2 × 10 −11
2.13 × 10 −15
3.2 × 10 −11
704 years
CHCl 3 b
6.9 × 10 −12
6.9 × 10 −12
3500 years
CHBr 3 b
3.2 × 10 −11
3.2 × 10 −11
686 years
CCl 4 b,c
4.8 × 10 −7
4.8 × 10 −7
7000 years (1 ppm)
C 6 H 5 CH 2 Cl
1.28 × 10 −5
1.28 × 10 −5
15 h
a K H = K N ; K B << K N
b K H = K B ; K N << K B
c Rate second order with respect to [CCl 4 ], K H (1 mol −1 s −1 )
Source: Jorgensen, S.E. and Gromiec, M.J., Mathematical Submodels in Water Quality Systems , Elsevier,
Amsterdam, 1989. With permission.
layers of the aquatic ecosystem will be exhausted. Oxidation of the organic matter,
however, will continue after the dissolved oxygen is exhausted by denitrification
and sulfur reduction. All these processes can, in principle, occur by pure chemical
oxidation, but in general microbiological oxidation plays a far more important role.
The source of free radicals in natural waters is the photolysis of naturally
occurring organic molecules. If a water body is turbid, or very deep, these free
radicals are likely to be generated only near the air-water interface; consequently,
chemical oxidation will be relatively less important.
Oxidation processes are not always effective for all compounds, particularly so
for organochlorines. 161 Although chemical oxidation of natural organic matter is
usually insignificant compared to microbial decomposition, chemical oxidation can
be of great importance for specific, toxic organic compounds. A chain sequence of
processes, some chemical and some biochemical, usually decompose specific organic
compounds. For modeling purposes, it is important to know at least the slowest
reaction and the type of kinetics valid for each because they will determine the
overall decomposition rate. 143
Aquatic systems contain several oxidants, such as radicals, singlet oxygen, and
triplet diradicals, with half-lives of only a few milliseconds, in addition to more
 
 
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