Agriculture Reference
In-Depth Information
5.
T
RANSPORT OF
H
EAVY
M
ETALS AND
O
RGANCHLORINE
P
ESTICIDES IN
T
OPSOIL
Polyvinyl chloride (PVC) column with an inner diameter of 3.5 cm and a
length of 30 cm were packed with air-dried soil A on the basis of original bulk
density in the field. Before the leaching experiment, soil A was saturated with
dd H
2
O at ionic strength of 0.01mol·L
-1
CaCl
2
for 24 h. Five soil columns were
leached with the solution containing 100 mg·L
-1
Pb, 100 mg·L
-1
Cd, 10 mg·L
-1
Cr(VI), 10mg·L
-1
As(V) and 50 μg·L
-1
ʱ-BHC respectively at room
temperature (18-20
°
C) and the influent flow was controlled at 2.8 mL·min
-1
.
At certain intervals, the effluents were collected for the measurement of
pollutant concentration.
The results obtained from soil column leaching tests are shown in the form
of breakthrough curve (BTC) fitted by the Yoon-Nelson model [19] (Figure 3).
The rates of the four heavy metals transport in the topsoil followed the order:
Cr(VI) > As(V) > Cd > Pb. Positively charged Pb and Cd were adsorbed by
negative sites on the soil surface[20]. The relative concentrations (C
t
/C
0
) of
Cr(VI) and As(V) in leachates were increased with leaching time. An apparent
plateau of the BTC appeared for Cr(VI) and As(V), which showed that soil A
almost reached saturated adsorption to Cr(VI) and As(V). In comparison to
Cr(VI), the breakthrough point of As(V) was significantly delayed, However,
for Pb, Cd and ʱ-BHC, the concentration in leachates increased much more
slowly and the breakthrough point did not appeared.
Α-BHC migrated slowly in the topsoil. Some studies had demonstrated
that the migration and transformation of organic matter in soil was the
integrated result of adsorption, degradation and other factors. The specific
processes were generally divided into two stages, namely, absorption rising
process and adsorption saturation process. In the first stage, the amount of
microbes was less in the soil, so degradation was weak and adsorption was
dominant. in the second stage, with the biomass of microbial domestication
and proliferation increased, degradation was demonstrated gradually,
adsorption was equilibrium gradually [21]. Because of combined effect of
adsorption and degradation, ʱ-BHC migrated slower in the topsoil.
Combining the results of adsorption with vertical migration, it could be
founded that among all the pollutants studied, the risks of Cr(VI) and As(V)
were greatest to groundwater system, and the risks of Pb and Cd were greatest
to agricultural production.
