Agriculture Reference
In-Depth Information
increased. Because leaching time was greatly shorter than the time for soil-
heavy metal interactions under the natural condition, it was very difficult for
heavy metals from anthropogenic activities to enter in the firm mineral crystal
lattice in soil to form the residual fraction in such a short time. After entering
soils, a part of heavy metals were in the form of exchange fraction, and with
the times, exchange faction would be adsorbed, mineralized by soil and
transformed into other factions. Considering the absolute concentration, the
five fractions of each element were all increased due to element addition into
soil. For Pb, the carbonate fraction was increased greatly. The primary
mechanism for Pb retention in clay soil was through precipitation of the metal
ions with carbonates and amorphous oxides or hydroxides [22]. Yong and
Phadungchewit [23] proved that the presence of carbonates in a soil
contributed significantly to the retention capacity of the soil. The increase of
each non-residual fraction except exchangeable fraction (average 1%) for Cd
was almost equivalent (average 5%). For Cr, the relative concentrations of the
organic and Fe-Mn oxide fractions were significantly increased, especially the
organic fraction (average 36 %). Likewise, through studying the fractions of
heavy metals in sandy and loessial soils, Han and Banin [24]
found that, after
its addition to the soil, Cr was bound to the organic matter fraction. The
organic matter, Fe-Mn oxide and carbonate fraction of As were increased,
obviously. Lund and Fobian [18] found that As was retained by organic matter
and Fe, Al and Mn oxides in contaminated industrial sites.
Table 4. Relative concentrations of the four heavy metals in different
fractions in soil A before and after column leaching tests (%)
Heavy
metal
Original soil
Leached soil
F1
F2
F3
F4
F5
F1
F2
F3
F4
F5
Pb
0.97
1.60
10.4
34.1
52.9
4.10
43.8
14.0
13.3
24.8
Cd
1.40
5.60
21.9
19.6
51.5
3.70
13.2
22.9
25.6
34.6
Cr
0.40
1.16
0.49
15.0
82.9
10.5
25.6
17.0
24.6
22.3
As
0.33
0.55
3.39
1.39
94.3
2.30
17.6
18.6
28.3
33.2
The exchangeable fraction was the first to be brought into solution and
was considered to be easily available for plant uptake. The carbonate fraction
was susceptible to pH changes, and the Fe-Mn oxide fraction was unstable
under low pH conditions. The organic fraction could be degraded under
oxidizing conditions, and the residual fraction was not bioavailable since it
