Geoscience Reference
In-Depth Information
assumes that their maintenance is provided for by the existing organic matter.
Under these conditions, the amount of water-soluble hydrolysis product formed is
proportional to the amount of decomposed parathion, with
dP
=
dt ¼
dc
=
dt
ð
16
:
6
Þ
where c is the concentration of parathion.
Formation of parathion hydrolysis products in remoistened Gilat soil after appli-
cation of parathion at concentrations of 10-160 lg/g dry soil is illustrated in
Fig.
16.37
. These data imply that parathion provides a substance essential for the
growth of a portion of the near-surface microbial population, but that these increased
numbers can be sustained for only a short period. A possible explanation for the short-
lived increase in numbers, in addition to lack of substrate, might be the adverse effect
of the parathion metabolite p-nitrophenol when it reaches a specific concentration.
When parathion reaches a flooded near-surface site, anaerobic conditions
develop. In a study on instantaneous degradation of parathion in anaerobic soils,
Wahid et al. (
1980
) equilibrated parathion with soils previously reduced by
flooding and analyzed contaminant transformation when the soils were kept in a
natural state or made biologically inactive by a pre-autoclaving procedure. The
physicochemical characteristics of the soil used are given in Table
16.9
. The
extent of parathion degradation, as a function of soil properties and period of pre-
flooding, is shown in Fig.
16.38
. The soils were first flooded for different periods
ranging from 0 to 190 days and then equilibrated with parathion for 30 min, after
which parathion degradation was measured. Following flooding, the redox
potential dropped (up to -130 mV) at a faster rate in the Pokkali, organic matter-
rich, soil. Aminoparathion and desethyl aminoparathion were the parathion deg-
radation products determined in the equilibrating solution.
To ascertain whether instantaneous degradation of parathion in the pre-reduced
soils was chemical or biological, parathion was equilibrated with nonsterile and
sterile soils pre-reduced by flooding. Biologically mediated parathion degradation
was determined by analyzing the equilibrating solution after sterilizing the pre-
flooded laterite and Pokkali soils by autoclaving. Soils treated with parathion
immediately prior to flooding served as the aerobic control. Gas chromatograms of
parathion and its degradation products, in nonsterile and sterile samples, are shown
in Fig.
16.39
. In aerobic soil, parathion remained undegraded with recoveries as
high as 92-95 %. In contrast, rapid degradation of parathion occurred in nonsterile
samples of pre-reduced soils. Parathion concentrations declined to 13.8 and
52.1 % in Pokkali and Laterite soils, respectively. Wahid et al. (
1980
) determined
that aminoparathion and desethyl aminoparathion were the parathion degradation
products in the equilibrating solution. Both degradation products are found in pre-
reduced Pokkali soils, but only aminoparathion appears in the pre-reduced Lat-
eritic soil. Parathion was not degraded in autoclaved samples of pre-reduced
samples, proving that anaerobic parathion degradation in a pre-flooded near-sur-
face layer is a biologically mediated process.
