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.