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 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 12a). The
overall decrease in adsorption capacity and elution efficiency of AO-Fc
were 1.8 % and 2 % up to seven cycles (Figure 12b,c), 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 loss of adsorbent (Figure
S8) after seven adsorption-desorption cycles of AO-Fc (Figure 12b,c).
Figure 12 Elution efficiency of uranium in 1 M
Na2CO3 and
H2O2 0.1 M (a), elution efficiency and
uranium adsorption capacity of AO-Fc from natural seawater (b,c)
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
onto 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 confirmed
the adsorption was 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.