3.2.1 Analysis of optimal pH for uranium adsorption
It is well known the pH value of the adsorption solution markedly affects the uranium adsorption performance of the adsorbent due to the presence of different ionic charges present on the adsorbent surface and the uranium ions. Herein we study the effect of pH on uranium adsorption in the pH range of 3 to 10. From Figure 5a, as we have seen the adsorption capacity increased rapidly with increasing pH 3 to 8, at pH 8 the maximum adsorption capacity of AO-Fc was 411±2 mg reported. With a further increase in pH, the adsorption capacity started to decrease. The optimum pH with maximum adsorption capacity of Fc, CN-Fc, and AO-Fc was 5, 6, and 8 confirming the grafting of amidoxime groups adjusted the pKa value, resulting in shifting of pH to alkaline which is favorable for uranium under seawater conditions. The AO-Fc showed a positive shift towards an optimal pH of 8 towards the alkaline phase whereas the CN-Fc showed a small shift with an optimal pH of 6, confirming the role of amidoxime groups to shift the pKa of the base material. Further, NaCl showed no influence on uranium adsorption of AO-FC, CN-Fc, and Fc (Figure 5b) indicated a high concentration of NaCl in seawater would not affect the uranium adsorption capacity of AO-Fc in seawater conditions.
Figure 5 Effect of solution pH (a) and various concentration of NaCl (b) on uranium adsorption capacity (mg/g)