Cotransport of fullerene nanoparticles and montmorillonite colloids in porous media: Critical role of divalent cations of montmorillonite

While the cotransport of carbon nanoparticles (CNPs) and clay colloids in porous media has been widely studied, the influence of the cation exchange capacity (CEC) of clay colloids on the transport process remains unclear. In this study, batch adsorption and column transport experiments were conducted to investigate the fate and transport of CNPs and clay colloids in quartz sand, with respect to the effect of monovalent-cation exchange capacity (mono-CEC), divalent-cation exchange capacity (di-CEC) and total CEC of clays. Fullerene nanoparticles (nC₆₀) and six types of montmorillonite (ML) with different CEC were selected as modeled CNPs and clay colloids, respectively. Transport behavior of nC₆₀ and ML was characterized using breakthrough curves (BTCs) and fitted with two-kinetic-sites colloid transport model. Results of the adsorption experiments showed a good linear correlation between the deposition of nC₆₀ on the sand surface and the di-CEC of ML. Transport of ML and nC₆₀ was inhibited by each other. The calculated mass recovery of nC₆₀, as well as the fitted maximum deposition capacity and attachment rate coefficients of nC₆₀ exhibited a strong linear relationship with the di-CEC of ML. These results indicate that divalent cations in ML interlayers play a significant role in aggregation between nC₆₀ and ML and their cotransport. Through measurements of the particle size and zeta potentials of sole nC₆₀ and mixtures of ML and nC₆₀, FTIR and XPS analysis of nC₆₀ under different conditions, and a release experiment of nC₆₀ in a sand column, it demonstrated cation bridging (Ca²⁺-π) between nC₆₀ and ML mediated by the divalent cations in ML interlayers. The study highlighted the potential of using di-CEC of clays as an indicator to predict the mobility of nC₆₀ in clay-containing porous media and added insights to the transport behavior of CNPs in porous media.