r - The Hong Kong Polytechnic University

Table 4 Predicted isothermal parameters for Ni(II) and Mn(II) adsorption on loess soil
Langmuir model Freundlich model D-R model
Q
(mg g -1 )
b
(L mg -1 )
R 2
K F
(mg g -1 )
n R 2 q m
(mg g -1 )
K
(mol 2 kJ -2 )
E
(kJ mol -1 )
Ni(II) 13.84 0.037 0.944 1.00 1.91 0.973 39.07 0.005 -9.84 0.987
Mn(II) 7.65 0.022 0.950 0.53 2.03 0.964 19.68 0.006 -9.53 0.973
The monolayer adsorption capacity of loess soil for Ni(II) and Mn(II) estimated by the Langmuir model was
13.84 and 7.65 mg g -1 respectively. The Freundlich constant n for Ni(II) and Mn(II) was 1.91 and 2.03
respectively, greater than unity, indicating some degree of heterogeneity of adsorption system. The q m predicted
with the D-R model was 39.07 and 19.68 mg g -1 for Ni(II) and Mn(II) respectively, larger than the adsorption
capacity Q obtained from the Langmuir model. Because the D-R model describes an ideal state which is almost
impossible to realize. The adsorption energy for Ni(II) and Mn(II) adsorption was estimated to be -9.84 and
-9.53 kJ mol -1 , respectively. The absolute values of the adsorption energy |E| all lied within 8-16 kJ mol -1 ,
implying an ion exchange mechanism.
Desorption of Heavy Metals from Loess
Figure 8 shows the variation of adsorption amount (C s ) of Ni(II) and Mn(II) on loess at different concentration
(C NTA ) of NTA. The initial amount of Ni(II) and Mn(II) on loess was 13.68 and 4.48 mg g -1 respectively. Both
amount of the heavy metals decreased with increasing the concentration of NTA, indicating that NTA
contributed to desorption of heavy metals from loess soil. The variation of desorption efficiencies (R de ) of Ni(II)
and Mn(II) from loess at different C NTA is shown in Figure 9. In general, both R de increased with increasing C NTA .
The desorption ratio of Ni(II) increased to 63 % at C NTA =4 mmol L -1 , and then had a slight decrease at C NTA =5
mmol L -1 . While, the maximum desorption ratio of Mn(II) was 92 % at C NTA =5 mmol L -1 , much more than that
of Ni(II). As can be seen from the result, the initial amount of heavy metal loaded on loess had effect on the
desorption efficiency. The maximum desorption ratio of Mn(II) was greater than that of Ni(II), but the initial
amount of Ni(II) was more than that of Mn(II). Therefore, heavy metal ions could be more easily desorbed from
the loess soil loaded with less heavy metal.
R 2
Figure 8 The amount of Ni(II) and Mn(II) on
loess at different concentration of NTA
Figure 9 Desorption efficiency of Ni(II) and Mn(II)
from loess at different concentration of NTA
CONCLUSIONS
(1) The pseudo-second order kinetics fit the data of Ni(II) and Mn(II) adsorption on loess soil, and the
adsorption rate of Mn(II) was faster than that of Ni(II).
(2) The adsorption capacity of loess soil for Ni(II) and Mn(II) was 13.84 and 7.65 mg g -1 respectively.
(3) The D-R model fit the isothermal data of Ni(II) and Mn(II) adsorption on loess soil best, and the main
adsorption mechanism was ion exchange for both the heavy metals.
(4) The maximum desorption ratio of Ni(II) and Mn(II) from loess soil was 63 % and 92% respectively,
and heavy metal ions could be more easily desorbed from the loess soil loaded with less heavy metal.
ACKNOWLEDGEMENTS
The authors would like to express their sincere gratitude to Research Fund for the Doctoral Program of Higher
Education of China (20090101110075) and Scholarship Award for Excellent Doctoral Student granted by
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