Simulating the Selective Adsorption of Pertechnetate to Oxyanion-SAMMS

The environmental remediation of 99Tc contaminated land and water is one of the most difficult challenges facing the nuclear industry. The problem stems from the fact that 99Tc has a long half-life, high fission yield and readily forms the extremely mobile pertechnetate oxyanion, TcO4−, in aqueous solution. Many of the current methods for remediating TcO4− suffer from poor selectivity in the presence of competing anions and are dependent on specific reducing conditions. However, self-assembled monolayers on mesoporous supports (SAMMS) have been proposed as an effective alternative for TcO4− remediation. SAMMS combine a robust inorganic mesoporous support with a highly selective functionalised monolayer that can be tuned to target the contaminant species of interest. When functionalised with a monolayer of Fe3+ complexes it has been shown experimentally to selectively adsorb the monovalent TcO4− preferentially over competing divalent species, such as sulphate (SO42−). In order to tailor the design of oxyanion-SAMMS to optimise its performance, an improved understanding of the cause of this selectivity is required. In this thesis, a novel approach to the construction of realistic models of oxyanion-SAMMS is reported, with the aim of performing simulations to advance our understanding of the use of SAMMS as a TcO4− remediation tool. In order to investigate the properties of SAMMS on many length scales, a multiscale modelling approach was adopted, involving quantum mechanics, atomistic simulations (molecular dynamics and Monte Carlo) and equilibrium calculations based on Poisson-Boltzmann theory. This is the first reported model of SAMMS for use in the aqueous environment and the overall approach could be extended to many other remediation problems. The simulations show that a single Fe3+ complex is selective for SO42− over TcO4− in aqueous solution. This result is contrary to the experimental evidence and implies that the mesoscopic structure of the material must in part be responsible for the observed selectivity. Further simulations suggest Cl− counterions generate a negative electrostatic potential in the SAMMS pore, resulting in significantly larger free energy barriers to pore entry for divalent anions compared to monovalent ones. This observation may have much more general implications for the selective remediation of contaminant ions using porous materials.