Apatite (Ca5(PO4)3(OH)) is known for freely exchanging its anions and therefore a F-bearing paste is applied to teeth which instigates an exchange between OH- groups from
the apatite structure with F- from the paste. Studies exist of heavy metal adsorption to apatite (Middleburgh and Comans, 1991; Xu and Schwartz, 1994), structural changes of apatites after adsorption experiments (Jeanjean et al., 1994), and cation exchange between metals in solution and Ca from the apatite structure (Suzuki et al., 1981, 1982). Here we report the results of apatite exchange experiments for rare earth elements (REE), actinides, and heavy metals.
Apatite was reacted with aqueous solutions containing a variety of metal cations at pH 4 to 6 at 25oC. The experiments were performed in pH adjusted (using NaOH) nitrate or chloride solutions containing 100-800 ppm of REE, actinides (U, Th), and/or heavy metals (Cd, Pb). The apatite used in the experiments was 0.02-0.05 g of laboratory synthesised apatite with a surface area of 80 m2 g-1. Each experiment took place over 2 to 7 days. The concentration of Ca, P, Na and the reacting metal cations was monitored. The reacting solutions were analysed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and the solids by X-ray Diffraction (XRD), Secondary Ion Mass Spectrometry (SIMS), and X-ray Photoelectron Spectroscopy (XPS).
Results show that the observed interaction process is an ion exchange between Ca from the apatite and the reacting metal cations in solution. The reaction may be represented as
For the synthetic apatite, Pb and the REE show the highest exchange. 40 to 90% of the total mass of Ca from the apatite partitioned into the solutions, with equivalent atomic loss of the reacting metals. Charge balance was always observed i.e. 2La3+ exchanged with 3Ca2+. Of notice is that percentage uptake of the REE varied according to relative concentration in solution i.e. when reacting apatite with a cocktail of rare earths Sm showed the highest intake (similar to most natural apatites) whereas experiments with only one REE at a time showed a maximum uptake for La, indicating that kinetics also play a role in the exchange reaction. Lastly U and Th exchanged with 20% and 8% of the total Ca mass, respectively.
Surface investigations (SIMS, XPS) indicate that the heavy metals and REE are solely exchanged with Ca++ from the structure whereas actinides also adsorb to the mineral surfaces. XRD analysis demonstrate that Ca-apatite has transformed to the Pb-apatite structure after reaction with Pb-bearing solutions. This study demonstrates that apatite is a powerful scavenger of heavy metals and actinides and may be of importance for industrial and radioactive waste disposal.
Jeanjean, J., Vincent, U. & Fedoroff, M., J. Solid State Chem. 108, 68-72 (1994).
Middleburgh, J.J. & Comans, R.N.J., Chem. Geol. 90, 45-53 (1991).
Suzuki, T., Hatsushika, T. & Hayakawa, Y., Chem. Soc., Faraday Trans. 77, 1059-1062 (1981).
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Xu, Y. & Schwartz, F.W., Environ. Sci. Technol. 28, 1472-1480 (1994).