Geoscience Reference
In-Depth Information
Fig. 16.2 Degradation rate
constant
ð
k
0
obs
Þ
for methyl
parathion as a function of pH,
in aqueous 5.0 mM hydrogen
sulfide with and without
natural organic matter
(NOM), at 25 C. Reprinted
with permission from Guo
and Jans (
2006
). Copyright
2006 American Chemical
Society
Tribromoneopentyl alcohol (TBNPA) is an organic halide used by the bromine
industry as a reactive intermediate for high molecular weight flame retardants.
This potential contaminant decomposes in an aqueous solution under basic con-
ditions (pH from 7.0 to 9.5), by a sequence of reactions that release one bromide
ion at each stage. TBNPA has an estimated half-life of about 100 years, it is highly
soluble in water (2 g/L at 25 C), and with its degradation products, it forms an
environmental hazard. Ezra et al. (
2005
) investigated the chemical transformation
of 3-bromo-2, 2-bis(bromomethyl)-propanol (TBNPA) in a laboratory experiment
simulating conditions below an industrial conglomerate located on a fractured
chalk aquitard (Negev, Israel). The natural groundwater under the contaminated
location is saline (magnesium chloride type) with a high content of calcium and
sulfate; it has a pH ranging between 7 and 8 and contains an enormous variety of
organic and inorganic pollutants of industrial origin.
Experimental results of Ezra et al. (
2005
) show that TBNPA slowly decomposes
in an aqueous solution under basic conditions (pH from 7.0 to 9.5) by a sequence of
hydrolysis reactions that release one bromide ion at each stage. The sequence of
three daughter products is BBMO, BMHMO, and DOH. The first product, BBMO
(3,3(bromomethyl) oxetane) contains two equivalent -CH
2
Br groups and two
equivalent CH
2
O fragments bonded to the same carbon atom. The second hydro-
lysis intermediate is BMHMO (3-bromomethyl-3-hydroxymethyloexane). Because
one Br
-
ion is released to solution for each molecule that is decomposed, the last
daughter product, DOH, is a bromine-free compound identified as 2,6-dioxaspiro
3,3 heptane. The pseudo-first-order rate constant of the decomposition of TBNPA
increases linearly with the pH. Analyzing groundwater samples from monitoring
wells by GC-MS, the TBNPA decomposition product BBMO was found, although
it was not among the chemicals produced or used in the industrial complex covering
the area studied. Based on this finding, the authors suggest that the BBMO found in
the studied aquitard is a by-product of spontaneous transformation of TBNPA
which is the most abundant semivolatile organic pollutant at that site.
