Incorporation of iodine into apatite structure: a crystal chemistry approach using Artificial Neural Network
Materials with apatite crystal structure provide a great potential for incorporating the long-lived radioactive iodine isotope (129I) 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 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 46 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+, Sr2+, Pb2+, Ba2+, and Cs+, and X site cations, mostly formed tetrahedron, of Mn5+, As5+, Cr5+, V5+, Mo5+, Si4+, Ge4+, and Re7+ 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.
Environmentally benign and novel management route for radioactive corrosion products by hydroxyapatite
Abstract A novel and environmentally benign route for the direct solidification of captured corrosion products (Co, Cr, Fe, Ni, Cu, Zn, and Mn) generated from the primary coolant system of nuclear power plants is introduced in this study. Synthesized calcium hydroxyapatite was used to remove individual and mixtures of corrosion products from aqueous solutions. The results show more than a 95% removal of these corrosion products for both cases. The direct solidification of adsorbed corrosion products was done by pressure-less conventional sintering. The sintered matrix revealed a good hardness (3.70 ± 0.60 GPa) and a relative sintered density > 98% after heat treatment at 1150 °C. The measured compressive strength (207.3 ± 9.5 MPa) was significantly higher than the established waste immobilization criteria of the US (3.5 MPa) and Russia (4.9 MPa). The corrosion products consolidated matrix had a normalized leaching rate ranging from 3.4⨯10−2 to 3.1⨯10−6 g/m2/day. Moreover, no additional chemical treatments, additives (gypsum, slaked lime, sodium silicate, etc.), and sophisticated equipment were needed for the adsorption and solidification process. Therefore, the proposed waste management route has no adverse effects on the ecosystem and can be highly efficient to immobilize adsorbed waste and to reduce the secondary waste volume.
Electrochemical detection of copper(II) at a gold electrode modified with a self-assembled monolayer of penicillamine
A penicillamine (PCA) self-assembled monolayer (SAM) was prepared on a gold electrode. It has been found that the modified electrode exhibited a selective response to copper ions. As demonstrated by cyclic voltammetric experiments, the SAM-based electrode showed an attractive ability to preconcentrate efficiently traces of copper(II) from solutions. Under optimum conditions, the anodic peak current was proportional to the concentration of Cu(II) in the range from 8.0 × 10−7 to 1.0 × 10−4 M with a detection limit of 4.0 × 10−7 M. Moreover, this modified gold electrode is also characterized by excellent repeatability, showing a relative standard deviation of 3.2% for nine successive measurements of 1.0 × 10−5 M Cu(II). The PCA/Au SAM gold electrode was used for the determination of Cu(II) in a tap water sample and the results showed a good agreement with the data obtained by atomic emission spectrometry.