U(VI) adsorption by one-step hydrothermally synthesized cetyltrimethylammonium bromide modified hydroxyapatite-bentonite composites from phosphate‑carbonate coexisted solution

Cetyltrimethylammonium bromide modified hydroxyapatite-bentonite composites (CTAB-HAp-Bent) were first synthesized by one-step hydrothermal method to test its U(VI) adsorption behaviors from phosphate‑carbonate coexisted solution. When pH = 8.0, U(VI) = 50 mg/L, [CO3]T = 2 mmol/L, and [P] = 1.5 μmol/L, CTAB-HAp-Bent achieved the maximum qe values ~108.19 mg/g compared to HAp-Bent (~40.62 mg/g), CTAB-Bent (~81.70 mg/g) and HAp (93.90 mg/g). High alkalinity and ionic strength were unfavorable for U(VI) adsorption. Specifically, qe values increased with an increase of [P] up to 500 μmol/L and then changed little in the ranges from 500 μmol/L to 2000 μmol/L, whereas carbonate suppressed U(VI) adsorption in the whole studied [CO3]T. Pseudo-second-order model, Langmuir isotherm and the thermodynamics deduced a spontaneous endothermic monolayer surface chemical adsorption process. Besides, characterization results suggested that the adsorption mechanisms were involved in the Br− anion exchange and -OH group coordination. Our present work optimized the U(VI) adsorption capacity of bentonite in eutrophic carbonate water, benefiting the research and design of potential modified clay or clay minerals materials for uranium recovery from eutrophic natural uranium-containing water.