3.5 Adsorbent regeneration and elution efficiency
Reusability is an important parameter for the industrial application of
adsorbents that influences the cost of the adsorbent. For the
reusability test, AO-Fc (7 mg) was used in 20 L seawater and the elution
solution (1 M Na2CO3 and
H2O2 0.1 M) was fixed at 3 mL per
milligram of seawater exposed adsorbent. For uranium elution, the
elution efficiency reached 99.9 % within 20 min, indicating the desired
elution solution was better for uranium removal from AO-Fc (Figure 8c).
The overall decrease in adsorption capacity and elution efficiency of
AO-Fc were 1.8 % and 2 % up to seven cycles (Figure 8d and Figure
S12a), confirming the adsorbent can be used for several cycles, which
would significantly decrease the uranium extraction cost. The decrease
in adsorption capacity and elution efficiency might be due to the strong
binding of uranyl ions with active functional groups. The adsorbent
showed excellent structural stability and negligible weight loss of
adsorbent (Figure S12b) after seven adsorption-desorption cycles of
AO-Fc.
Figure 8 High-resolution XPS spectra of U 4f (a), FTIR spectra (c),
elution efficiency of uranium in 1 M
Na2CO3 and
H2O2 0.1 M (c), and uranium adsorption
capacity of AO-Fc in seawater (d)
Conclusion
In conclusion, the new low-cost and facile prepared amidoxime modified
adsorbent were analyzed qualitatively and quantitively and can be
considered as a potential adsorbent for uranium recovery from seawater.
AO-Fc showed significant adsorption capacity and higher uranium
selectivity, which is attributed due to amidoxime groups being present
on the surface of the 2D-open channel. In addition, the addition of
amidoxime groups not only enhanced the adsorption capacity and cracks
the hydrophilicity but also adjusted the adsorbent pKa value.
The BET surface area of AO-Fc was lower as compared to Fc, indicating
the surface area was not a grim factor for the adsorption mechanism. The
uranium adsorption on AO-Fc was strongly dependent on pH value
confirming the adsorption mainly occurred due to surface complexation.
The experimental adsorption simulation by various isotherm models
exhibited that the R2 value of Langmuir isotherm was
close to unity obtained by linear regression. Also, the
qe calculated and the statistical analysis confirmed the
adsorption data were best fitted with the Langmuir isotherm than other
isotherm models.
Kinetic studies showed very fast uranium adsorption and the equilibrium
of adsorption was achieved in 11 days, which was a lower contact time as
compared to the adsorbents reported in Table 5 for uranium extraction
from natural seawater.
The desorption and regeneration showed the adsorbent is economically and
environmentally efficient. The higher adsorption capacity of AO-Fc
towards uranium exhibits that the materials can be potential adsorbents
for uranium extraction under seawater conditions.