Superselective clay for radium uptake

Abstract

A legacy of the nuclear industry that threatens many areas of the world is contamination by radioactive radium from the solid and liquid wastes left after extracting uranium from its ores. Here we describe a highly charged, synthetic clay that specifically removes radium and immobilizes it at room temperature for safe disposal. This material may prove to be useful in decontaminating drinking water wells and soils of this toxic isotope.

Main

Synthetic cation exchangers have been tailored for decontaminating the environment of radioactive isotopes such as ¹³⁷Cs and ⁹⁰Sr (refs 1–3) and immobilizing them through electrostatic bonding^2,3 at room temperature. There is also a need for selective exchangers for the uptake of radium because not only is this isotope extracted into groundwater from uranium wastes but naturally occurring uranium may also contaminate drinking-water wells. Radium must be filtered out of drinking water to protect the populace from cancer and other health risks^4,5.

Two synthetic micas of nominal composition Na₄Al₄Si₄Mg₆O₂₀F₄·xH₂O ('Na-4-mica') and another two of nominal composition Na₂Al₂Si₆Mg₆O₂₀F₄·xH₂O ('Na-2-mica') were prepared from sol–gel precursors³ (gel method) or by using metakaolinite and MgO as precursors⁶ (kaolinite method) in sodium fluoride at 890 °C. We compared the uptake properties of several natural montmorillonite clays and of these synthetic clays.

Selective uptake of radium was determined using 0.5 M, 1.0 M or 2.0 M NaCl containing ²²⁶RaCl₂ at about 6 × 10⁻⁹ M. These solutions have extremely large equivalent ratios of [Na⁺]/[Ra²⁺] (0.6 × 10⁸, 1.2 × 10⁸ and 2.4 × 10⁸ in 0.5, 1.0 and 2.0 M NaCl solutions, respectively). Aliquots of the solution (20 ml) and sorbent powder (20 mg) in a centrifuge tube were equilibrated at room temperature. After 24 h, the solid and solution phases were separated and the ²²⁶Ra concentration in solution was determined. The selective uptake of radium is expressed as the distribution coefficient K_d.

Table 1 shows that Na-4-mica is the best of all the clays tested at taking up radium from solutions containing 60 to 240 million times more sodium than radium. This mica removed 95.4 ± 0.3% radium from 0.5 M NaCl solution. Its high charge density and special structure of offset layer stacking means that Na-4-mica prefers to take up fewer hydrated monovalent and divalent ions³, which accounts for its superselectivity towards radium.

Table 1 Radium uptake by different clays

To investigate this selective uptake by Na-4-mica, we determined the equilibrium Na/Ba exchange isotherm using barium as a substitute for radium⁷. The shape of this isotherm^2,3 also reveals the nature of the uptake process⁸. Results shown in Fig. 1 indicate that the mica prefers Ba²⁺ in the region of low concentration (initially steep curve), but then exchange stops (flat curve caused by the collapse of the interlayer space)^3,8. This type of interlayer collapse occurs in layered clays because of electrostatic bonding^2,3 with ions of low hydration such as Cs⁺, Rb⁺ and Sr²⁺.

Figure 1

Selective ion-exchange isotherm for Na-4-mica in mixed NaCl/BaCl₂ solutions (barium used as a simulant for radium).

Calculations from the equilibrium isotherm (Fig. 1) show that about 25% of the available barium enters the Na-4-mica interlayers, corresponding to a capacity of 115 mequivalents per 100 g, before collapse of the interlayers prevents further exchange. Radium ion, which is less hydrated than barium ion, should not be taken up to the same extent as barium as it is more likely to be trapped (immobilized) in the interlayers after fractional exchange owing to electrostatic and steric factors.

This special superselective mica could be used to help solve not only the radium-separation problem, but also its fixation and safe disposal.