Comparison of engineered nanoparticle transport in columns of three different lengths: Transport experiments and...

This study focuses on the effectiveness of commonly-used 15 cm column lengths for investigating nanoparticle transport in porous media. Experimental tests examined the transport and retention behaviors of two types of nanoparticles, graphene oxide (GO) and titanium dioxide (TiO₂) nanoparticles, in saturated sand columns of different lengths (15, 30 and 45 cm), while considering key environmental factors like ionic strength (IS, 1-50 mM), flow rate (1-3 mL min^-1), and grain size (150-850 μm). In the 15 cm columns, both GO and TiO₂ transport decreased with higher IS and lower flow rate; grain size affected GO and TiO₂ differently. Smaller grain size increased GO retention in the sand columns through straining, thus weakening GO mobility, whereas increased fluid shear force suppressed the ripening of TiO₂, enhancing its migration. Similar environmental effects were noted in longer columns (30 and 45 cm), but fitted transport parameters (S_max and k) and predicted long-term mobility (L_max) indicated that 15 cm columns might underestimate nanoparticle mobility. Blocking and ripening models based on single and multiple observation points to simulate nanoparticle transport and retention showed that predictions aligned well with experimental data. These results indicate that using combinations of columns of different lengths to achieve multiple observation points improves model prediction accuracy; in single-column experiments, the 45 cm and 30 cm columns respectively better predict the mobility range of GO and TiO₂ compared to 15 cm columns.