Geoscience Reference
In-Depth Information
Table 8.7
Effect of residence time on Pb desorption from a Matapeake soil (Strawn and Sparks
2000 )
Residence time
(days)
Sorbed Pb
(mmol/kg)
Desorbed Pb
(mmol/kg)
Percentage Pb desorbed
(mmol/kg)
1
54.9
27.9
50.8
10
60
28.7
47
32
66.1
30.5
46.1
Table 8.8 Influence of the resident cation on the adsorption of diquat and paraquat by homo-
ionic-exchanged clays (Hays and Mingelgrin 1991 ) indicates data for incomplete cation exchange
Clay
Resident
inorganic cation
CEC
(lEq/g)
Organic adsorption
(lEq/g)
Enthalpy change
DH (kJ/mol)
Diquat
Paraquat
Diquat
Paraquat
Na +
Montmorillonite
999
950
970
-32
-48
K +
850
920
940
-31
-51
Ca 2+
950
940
950
-36
-51
Na +
Vermiculite
1,440
1,210
1,130
26
42
K +
270
230
1.6
-0.9
Ca 2+
1130
(870)
(630)
(-9.2)
(-5.2)
Na +
Illite
230
240
230
-13
-22
K +
ND
220
220
-6.8
-17
Ca 2+
240
230
220
-16
-24
Na +
Kaolinite
30
36
36
0
-11
K +
35
34
1.3
-4.9
Ca +
60
36
36
-18
-25
solid-phase transformation. Xu et al. ( 2008 ) confirm that Ni 2+ adsorption on
montmorillonite and fulvic-humic momtmorillonite complexes may be irrevers-
ible and that the adsorption-desorption pattern is not affected by the presence of
humic substances.
Pesticides are used widely in plant protection practices, by dispersal on agri-
cultural lands, and generally are cationic or nonionic organic compounds. After
application, these products may volatilize, being transported in the groundwater as
solutes or retained on the porous medium solid phase. Pesticide retention is con-
trolled by the molecular properties of the compound, the solid-phase constituents,
and environmental factors. Diquat and paraquat organo-cationic herbicides exhibit
sorption isotherms of the L type according to the Giles et al. ( 1960 ) classification
( Sect. 5.2.2 ) , with the data fitting well to the Langmuir equation. Kookana and
Aylmore ( 1993 ) noted that the sorption capacities of Australian soils studied for
diquat ranged from very high in clay soil (146,400 lmol/kg) to very low in sand
(1,765 lmol/kg). The clay soil showed the highest value of the Langmuir coeffi-
cient, indicating a high bonding energy. Table 8.8 shows the adsorption of these
cationic pesticides by four types of homoionic-exchanged clays: montmorillonite,
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