Uptake and Removal of Uranium by and from Human Teeth

Abstract

Uranium-238 ( 238 U), a long-lived radiometal, is widespread in the environment because of both naturally occurring processes and anthropogenic processes. The ingestion or inhalation of large amounts of U is a major threat to humans, and its toxicity is considered mostly chemical rather than radiological. Therefore, a way to remove uranium ingested by humans from uraniumcontaminated water or from the air is critically needed. This study investigated the uranium uptake by hydroxyapatite (HAP), a compound found in human bone and teeth. The uptake of U by teeth is a result of U transport as dissolved uranyl (UO 2 2+) in contaminated water, and U adsorption has been linked to delays in both tooth eruption and development. In this present work, the influence of pH, contact time, initial U concentration, and buffer solution on the uptake and removal of U in synthetic HAP was investigated and modeled. The influence of pH (pH of human saliva, 6.7−7.4) on the uptake of uranyl was negligible. Furthermore, the kinetics were extremely fast; in one second of exposure, 98% of uranyl was uptaken by HAP. The uptake followed pseudosecond-order kinetics and a Freundlich isotherm model. A 0.2 M sodium carbonate solution removed all the uranyl from HAP after 1 h. Another series of in vitro tests were performed with real teeth as targets. We found that, for a 50 mg/L U in PBS solution adjusted to physiological pH, ∼35% of the uranyl was uptaken by the tooth after 1 h, following pseudo-first-order kinetics. Among several washing solutions tested, a commercially available carbonate, as well as a commercially available fluoride solution, enabled removal of all the uranyl taken up by the teeth.

Investigation of Chelating Agents for the Removal of Thorium from Human Teeth upon Nuclear Contamination

“…And finally, at the equilibrium stage, intraparticle diffusion slows as the Th concentration becomes low in solution. Such a process is in line with our previous intraparticle data of U adsorption onto teeth samples, which also demonstrated a second-order polynomial best fit, 20 and both data sets are consistent with the layered apatite composition of the teeth in which 96% of the enamel and 70% of the dentin is composed of biological apatite. 51 Therefore, it appears that Th possesses an ability similar to that of U to travel down the layered structure of the tooth, aiding its retention.…”

“…In our previous work, we demonstrated through FT-IR data that actinide binding to the tooth occurs via the phosphate component of the tooth's mineral matrix 20 and as Th and its species are overall more cationic than U and its species, it is more electrostatically favorable for Th to bond to phosphate, something which is reflected in the modeling data. The stronger bonding interactions indicated by pseudo-secondorder kinetics, as well as the greater number of surface adsorption sites indicated by the intraparticle data, that Th experiences with the teeth are no doubt contributors to the higher adsorption versus U.…”

“…The proposed mechanism corroborates our previous characterization data showing that actinide binding to the teeth occurs through the phosphate components of the tooth matrix; 20 this, as well as HAP's evident ability to adsorb Th via chemisorption, with extremely fast kinetics and irrespective of solution pH presents good evidence that Th adsorption onto teeth occurs primarily through its HAP components.…”

“…Second, the presence of NaCl would allow the ionic strength of the solution to be kept constant. Finally, TRIS/NaCl is able to prevent the precipitation of Th typically observed in aqueous media at neutral pH, due to the formation of the insoluble species Th­(OH) 4 . ,, The cumulative adsorption of Th over teeth samples as a function of time is displayed in Figure .…”

Removal of Uranium-238, Thorium-232, and Potassium-40 from Wastewater via Adsorption on Multiwalled Carbon Nanotubes

“…Adsorption rates and capacities are high at high temperatures, enabling metal ions to be rapidly adsorbed. 23 When adsorption of radionuclide ions occurs, the enhanced unpredictability at the solid/liquid interface allows for randomness to prevail in the system, 24 as can be seen in Figure 14 . Uranium-238 ions, thorium-232 ions, and potassium-40 ions are all adsorbed more easily as the temperature increases, suggesting an exothermic adsorption process, which may be ascribed to an increase in ion mobility.…”