Structural aspects and mechanisms of anion sorption on HDPy+-modified MX-80 montmorillonite: a Monte Carlo study

MX-80 bentonite has been proposed as filling material and as component of technical barriers in repositories of radioactive waste, because among other favourable properties it also possesses a high sorption capacity for radionuclides in cationic form. The desirable sorption of anionic radionuclides is lacking in natural clay yet, but can be achieved by replacing the inorganic interlayer cations with certain organic cations. Montmorillonite, representing the main sorption pool of bentonite clay, can be saturated with organic cations above the CEC level, still containing at least 1/4 of initial amount of inorganic cations as well as water in the interlayer space. In experiments with HDPy+-modified MX-80 bentonite Bors and al showed that at HDPy+ loading of about 90% CEC bentonite was able to adsorb Cl-, I- and TcO4- ions in the interlayer space. The deciding mechanisms of this sorption, its correlation with changing the arrangement of HDPy+ ions in interlayer space as well as the detailed information about this arrangement itself are not completely understood. However, this knowledge is very important for the targeted optimization of sorption properties of organo-philic bentonite and its applicability under repository conditions. In order to improve our understanding of the involved processes and the underlying structures, we have carried out a series of NPT Monte Carlo simulations of MX-80 montmorillonite with increasing from 8% of CEC to 108% CEC interlayer content of HDPy+ ions. The interlayer contents of Cl-, Na+ ions and water molecules change correspondingly to increase of interlayer content of HDPy+ ions. In these simulations, HDPy+ ion is modelled as a flexible structure, consisting of flexibly joined rigid units (pyridinium ring, CH2 or CH3 groups), so that it can change its conformation. Mineral layers, water molecules, organic and inorganic ions were allowed to move. OPLS-AA force field was used to represent interactions in the system. Layer spacings, interlayer structure and potential energies were sampled for averaging in the thermodynamic equilibrium state. Simulation results are in perfect agreement with experimental assumptions that HDPy+ ions adopt monolayer, bilayer and pseudo-trilayer arrangements depending on their concentration in the interlayer space. Calculated layer spacings, increasing in respond to increasing content and changing arrangements of HDPy+ ions, are in very good agreement with experimental values as well. The results assume that the sorption of Cl- ions is only possible when pseudo-trilayer arrangement of HDPy+ ions is adopted in the interlayer space. Furthermore, analysis of interlayer structure suggests two possible mechanisms of Cl- adsorption by organo-philic montmorillonite and clarifies the function of water in its interlayer space. (authors)