Synthesis of Tributyl Phosphate-Coated Hydroxyapatite for Selective Uranium Removal

Abstract

Efficient and rapid removal of radioactive contaminants is crucial when they are released into the environment following nuclear accidents. Here, different types of apatite were synthesized using tributyl phosphate (TBP) and tested for the removal of uranium from various solutions using different reaction times and uranium concentrations. The uranium adsorption results showed that uranium adsorption reached a steady state within 24 h in tests open to atmosphere at a slightly alkaline pH in different background solutions. TBP-coated hydroxyapatite removed U better than hydroxyapatite alone did. The U removal mechanism was considered as multilayer adsorption, showing the best fit to the Freundlich isotherm. The maximal U adsorption capacity determined from the Langmuir isotherm is 38 mg of U g–1. Because of the high U removal efficiency, even at the high pH used in this study, TBP-coated hydroxyapatite appears to be a promising adsorbent for the removal of U from various waste streams as well as for recovery of U from seawater.

Uptake and Removal of Uranium by and from Human Teeth

“…In the current study, at pH 7, a 94% uranyl uptake was achieved in 1 min, and at pH 6.7 and 7.3, ∼98% uptake was achieved in the initial second. The ability of HAP to nearly quantitatively adsorb uranyl regardless of uranyl concentration demonstrated here is in agreement with the findings of Fuller et al and Kim et al, who performed their studies in solution conditions and content analogous to groundwater. , …”

“…In regard to the effect of equilibration time, this study’s findings are in agreement with Krestou et al, which demonstrated a 90% uranyl uptake at <0.5 h at pH 7.6, as well as with the studies of Kim et al, who observed a near 100% uptake of uranyl from a 0.001 M NaHCO 3 background solution in 1 min . In the current study, at pH 7, a 94% uranyl uptake was achieved in 1 min, and at pH 6.7 and 7.3, ∼98% uptake was achieved in the initial second.…”

“…Due to the formation of these complexes, the solubility of uranium at pH 4–9 is very low (log­[U solubility] ranging from −2 to −8 depending on pH and type of complex), so little of the ion is in solution to be adsorbed . Adsorption appears to be unfavorable, because in this pH range, the HAP surface charge is positive (its isoelectric point ranges from 4.3 to 7.6 and has been stated to be as high as 8.4). ,, The positive surface potential inhibits adsorption of cationic hydroxo species. Neutral complexes are also electrostatically disabled from adsorption onto HAP.…”

“…Neutral complexes are also electrostatically disabled from adsorption onto HAP. Though the positively charged hydroxo-complexes could potentially adsorb onto the negatively charged regions on HAP, they would be hindered from doing so for a different reason: their large size and small mobility . Furthermore, an additional concern to this study is the formation of insoluble calcium carbonate as well as calcium dioxouranium­(VI) carbonate, (CaUO 2 (CO 3 ) 2 ) and CaUO 2 (CO 3 ) 3 [log K = 30.70] at pH 9, which contributes to the apparent lack of visible release of calcium or uptake of uranyl observed at this pH value due to either insolubility or neutral charge .…”

“…To evaluate the mechanism of uranyl uptake by HAP, the data was analyzed using Langmuir and Freundlich isotherm models . The Langmuir isotherm defines uptake as surface monolayer adsorption on the material surface, while the Freundlich isotherm model assumes varying adsorption on heterogeneous sites . The Langmuir isotherm is defined by the equation where C e , (mg/L), defines the uranyl ion concentration in solution at equilibrium, K L (L/mg) is a constant related to adsorption/desorption energy, and q max (mg/g) is the maximum sorption of metal at complete saturation of material.…”