Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 34

perchloroethylene, respectively (Solvay SA, 2002b). Table 1.15 summarizes the susceptibility of the
major solvents to pyrolysis in the presence of metal catalysts and air.
When chlorinated solvents are overheated, a chloride ion gets split off, leading to the formation
of hydrochloric acid. Prolonged contact of solvents with oils, grease, and other soils at elevated
temperatures also causes solvent instability problems (Archer, 1984). For example, thermal break-
down of TCE under conditions of normal use and in the absence of stabilizers probably proceeds
according to the following reactions (Shepherd, 1962):

2ClCH = CCl 2 → (ClCH = CCl 2 ) 2 , (1.1)

(ClCH = CCl 2 ) 2 → HCl + Cl 2 C = CHCC = CCl 2 . (1.2)

1.2.4.4 Acid Breakdown
All halogenated solvents in contact with free-phase moisture and air will slowly react with water in
a process called hydrolysis. Rates of hydrolysis for the chlorinated solvents vary widely. Methyl chlo-
roform has a hydrolysis half-life of a little more than two years, whereas TCE and perchloroethylene
have immeasurably slow hydrolysis half-lives. Hydrolysis produces hydrogen chloride in chlorinated
solvents. If exposed to water, hydrogen chloride produces hydrochloric acid. If not eliminated when
formed, hydrochloric acid will catalyze and increase the rate of hydrolysis and produce acid at a
faster rate. Hydrochloric acid also removes the protective oxide coating on metal surfaces, exposing
fresh metal to the solvent and enabling metal-catalyzed solvent deterioration. In the earliest study of
problems with hydrolysis of chlorinated solvents, Levine and Cass (1939) described carbon tetrachlo-
ride as decomposing as a result of hydrolysis, “splitting of acid and reacting with water.”
Acid can also be introduced into the degreaser solvent from oils, greases, soldering l ux, and
other material present on the work. The oxidation and decomposition of cutting oils dissolved from
the work form acid, usually hydrochloric acid or acetic acid, which then reacts with the solvent
(Starks and Kenmore, 1960b). Acids formed by oxidation of cutting oils or hydrolysis of solvents
can corrode the work and the cleaning equipment itself. In dry-cleaning applications, acids that form
in perchloroethylene can leach dyes and fabric colors; this leaching damages the clothing and makes
solvent recovery difi cult (Cormany, 1977). A source of acid in dry cleaning is sebaceous oils and
other bodily soil on the clothing. These soils contain short-chain, free fatty acids such as butyric acid
and valeric acid that acidify moisture in the cleaning process, which leads to machine corrosion,
odors on the clothing, and undesirable textile effects such as swales. Acidity cannot be tolerated in
dry cleaning, where TCE was used to a limited extent in the 1930s through the 1950s. Free acidity
was also undesirable for TCE when it was used to make decaffeinated coffee (Pitman, 1943).
TCE and perchloroethylene are relatively immune to hydrolysis, however, compared to carbon
tetrachloride or methyl chloroform. Dichloromethane hydrolyzes very slowly (Solvay SA, 2002b).
Table 1.16 provides commonly cited values for hydrolysis half-lives of the major solvents under
ambient conditions in soil and groundwater.

TABLE 1.16
Generalized Abiotic Hydrolysis Half-Lives of the Major Chlorinated Solvents
Solvent Abiotic Hydrolysis Half-Life (years) References

Perchloroethylene 1.3 × 10 6 Jeffers et al. (1989)

Trichloroethylene 1.3 × 10 6 Jeffers et al. (1989)

Dichloromethane 704 Mabey and Mill (1978)

Carbon tetrachloride 41 Jeffers et al. (1989)

Methyl chloroform 2.5 Vogel and McCarty (1987)

Source: Data and citations from Pankow, J.F. and Cherry, J.A., 1996, Dense Chlorinated Solvents and Other DNAPLs in
Groundwater . Waterloo, Ontario, Canada: Waterloo Press.