Highly efficient sorption of U(VI) on TiO2 nanosheets supported by amidoxime polyacrylonitrile in a variety of multi-carbonate solutions

Natural carbonate-rich uranium-containing water contributes to the formation of multi-carbonate solutions, including U(VI)–CO3, Mg/Ca–U(VI)–CO3, U(VI)–P–CO3, and U(VI)–X–CO3 (X = F, Cl, Br, I) systems. Herein, a novel composite of TiO2 nanosheets supported by amidoxime polyacrylonitrile (TNSs-PAN-AO) was newly prepared to study the U(VI) sorption in various multi-carbonate solutions. The results showed that the composite achieved the maximum sorption capacity (qe) of 108.4 mg·g−1 in U(VI)–CO3 solution, which was monolayer chemisorption. The competitive complexation of CO32– with U(VI) and the enhanced electrostatic repulsion caused by the transformation of (UO2)2CO3(OH)3– to UO2(CO3)34– species resulted in the relatively low qe values at high pH and [CO3]Total. The U(VI) sorption was suppressed in Mg/Ca–U(VI)–CO3, U(VI)–P–CO3, and U(VI)–X–CO3 solutions, especially in U(VI)–F–CO3, possibly due to the formation of dominant ternary complexes that can preempt the sorption sites of U(VI) on the composite. The desorption experiments showed that the highest desorption rate was 93.78% obtained by 0.1 mol∙L−1 HNO3. The XPS test and DFT calculations indicated that the –C(NH2)=N–OH and Ti–OH groups were the main sorption sites, which can scramble for U(VI) from the formed uranyl complex anions in multi-carbonate solutions. This work presented new insights into the understanding of uranium extraction from multi-carbonate solutions based on the practical U(VI) species, which may be of great significance for the recovery of uranium from natural water.