Geoscience Reference
In-Depth Information
propionaldehyde indicates that removal is due to reduction of the C=C double
bond and that the hydration of acrolein at this pH is relatively low.
At pH 6.3, acrolein reduction was slower than at a lower pH (4.4). The
incomplete mass balance suggests that either an intermediate accumulated to a
greater extent or adsorption to iron was greater at pH 6.3 (Fig.
16.1
b). Oh et al.
(
2006
) suggest it possible for acrolein to adsorb on iron surfaces through chemical
interaction, due to the p acidity of the C=C double bond, which allows the
compound to bind to transition metals having a filled orbital of p symmetry.
Acrolein reduction by elemental iron is conceptualized as occurring in four
sequential steps: transport from solution to iron surface, adsorption to iron,
reduction of adsorbed acrolein to propionaldehyde, and release of the degradation
product to solution. At pH 7.4, hydration of acrolein is no longer negligible
compared to reduction (Fig.
16.1
c), and the initial lag before propionaldehyde
formation increased to 120 min. Based on these experiments, Oh et al. (
2006
)
suggest that acrolein reduction under acidic or neutral pH and iron presence
involves chemisorption to the iron surface, followed by reduction of the adsorbed
phase.
The degradation of methyl parathion, an organophosphorus pesticide, as
affected by pH was studied in simulated subsurface aqueous solutions containing
hydrogen sulfide and natural organic matter (Guo and Jans
2006
). Methyl para-
thion can reach the subsurface aqueous environment via drainage water, runoff, or
spray drift and may be reduced under the presence of natural organic matter
(which can act as a reducing agent). In a subsurface anoxic environment, favorable
conditions are formed for the development of micropopulations able to reduce
sulfur species such as hydrogen sulfide. Guo and Jans (
2006
) show that the deg-
radation rate of methyl parathion in an aqueous solution, in the presence of
hydrogen sulfide, is slow but increases significantly with the addition of natural
organic matter, such as humic or fulvic acids.
Guo and Jans (
2006
) also examine the effect of pH on the degradation rate
constant, k
0
obs
. Figure
16.2
illustrates that k
0
obs
increases with increasing pH in the
range of pH 5.5-8.3 and drops abruptly at pH between 8.3 and 9.5. The observed
pH dependence indicates that, at higher pH values, the reactive intermediate is not
formed to the same extent. Guo and Jans (
2006
) consider that two reaction
products are produced in a reaction of methyl parathion with hydrogen sulfide, in
an aqueous system containing natural organic matter (NOM). Comparing the same
reactions, but without the NOM, it is observed that the degradation rate constants
are in the same range at pH higher than 8.3. The two postulated reaction mech-
anisms for the degradation are nucleophilic substitution at the methoxy-carbon and
nitro group reduction. The overall degradation rate constant, k
0
obs
, is expressed as
the sum of the nucleophilic substitution and nitro group reduction rate constants.
Desmethyl methyl parathion is the predominant product detected during such
methyl parathion transformation. The presence of 4-nitrophenol, detected in
minute concentrations in this system, may have originated from impurities in the
original pesticide used in the experiment.
