Uranium(VI) adsorption from carbonate solutions using cetyltrimethylammonium bromide modified purified-bentonite-MOF composite

A novel cetyltrimethylammonium bromide (CTABr) modified purified-bentonite (p-Bent)-MOF composite (CTABr/p-Bent-Co2(OH)2BDC) was first prepared to study the uranium(VI) adsorption from a variety of carbonate solutions. The experiments showed that at pH 8.0, the high concentration of carbonates and halides significantly suppressed the UVI adsorption. Compared with Co2(OH)2BDC (4.3 ± 0.9 mg·g−1), p-Bent (47.2 ± 3.7 mg·g−1), and CTABr/p-Bent (104.8 ± 0.8 mg·g−1), this ternary composite achieved an improved adsorption capacity of 111.7 ± 1.4 mg·g−1. The XPS spectra and DFT calculations indicated that carbonates and halides can compete for free uranyl ions with the –COOH and –OH groups on the composite surface. Particularly, CO32− and F− can inhibit the UVI adsorption from carbonate solutions by forming uranyl carbonates and uranyl fluorides. The Br− anion exchange and electrostatic attraction were demonstrated to be the dominant mechanisms for UVI adsorption on the composite, together with the –COOH/–OH complexation. This work presents a feasible strategy to prepare new clay-based multicomponent composites for extracting uranium from carbonate solutions, and provides new insight into the understanding of uranium adsorption in natural waters.