Incorporation of iodine into apatite structure: a crystal chemistry approach using Artificial Neural Network

Abstract

Materials with apatite crystal structure have a great potential for incorporating the long-lived radioactive iodine isotope (129 I) in the form of iodide (I −) from nuclear waste streams. Because of its durability and potentially high iodine content, the apatite waste form can reduce iodine release rate and minimize the waste volume. Crystal structure and composition of apatite (A 5 (XO 4) 3 Z) was investigated for iodide incorporation into the channel of the structure using Artificial Neural Network. A total of 86 experimentally determined apatite crystal structures of different compositions were compiled from literature, and 44 of them were used to train the networks and 42 were used to test the performance of the trained networks. The results show that the performances of the networks are satisfactory for predictions of unit cell parameters a and c and channel size of the structure. The trained and tested networks were then used to predict unknown compositions of apatite that incorporates iodide. With a crystal chemistry consideration, chemical compositions that lead to matching the size of the structural channel to the size of iodide were then predicted to be able to incorporate iodide in the structural channel. The calculations suggest that combinations of A site cations of Ag + , K + , Sr 2+ , Pb 2+ , Ba 2+ , and Cs + , and X site cations, mostly formed tetrahedron, of Mn 5+ , As 5+ , Cr 5+ , V 5+ , Mo 5+ , Si 4+ , Ge 4+ , and Re 7+ are possible apatite compositions that are able to incorporate iodide. The charge balance of different apatite compositions can be achieved by multiple substitutions at a single site or coupled substitutions at both A and X sites. The results give important clues for designing experiments to synthesize new apatite compositions and also provide a fundamental understanding how iodide is incorporated in the apatite structure. This understanding can provide important insights for apatite waste forms design by optimizing the chemical composition and synthesis procedure.

Mechanical properties of thermally stable cadmium iodoapatite ceramic by reactive spark plasma sintering technique

“…Thus, the controlling of sintering parameters is very crucial to consolidation process to avoid any iodine loss. This characterization result demonstrates the prediction model of Wang [5].…”

“…But due to their large ionic size (i.e. ionic radii (I -): 1.96 Å), there is a limitation to successfully synthesize iodine bearing apatites [1,2,5]. Consequently, the most synthesized iodoapatites may have limited chemical stability and waste loading.…”

“…Consequently, the most synthesized iodoapatites may have limited chemical stability and waste loading. In order to overcome these limitations, a recent analysis by Wang [5] predicted potential cation and anion site candidate materials by using artificial neural network (ANN) method to incorporate iodine into the potential apatite structure. Several solid-state synthesis routes have been adopted to synthesis various apatite-based waste forms, but due to their long processing temperature, dwell time and slow heating rate, iodine volatilizes [1,2,4].…”

Basic Research Needs for Environmental Management

“…The development of new waste form materials will also be accelerated with the aid of modern computational capabilities. Recently, simulations have been used to design a unique apatite mineral for immobilizing of radioiodine that is stable in geologic disposal, contains high waste loading, and can be fabricated using existing technologies ( Figure 20) (Wang 2015).…”

“…This is an example of using modern modeling tools to design a molecule that would retain not only a specific isotope of iodine, but daughter atoms that are created as a result of radioactive decay. Image courtesy of Jianwei Wang, Department of Geology and Geophysics, Louisiana State University(Wang 2015).…”