Particle Size Distribution, Concentration, and Magnetic Attraction Affect Transport of Polymer-Modified Fe0 Nanoparticles in Sand Columns

The effect of particle concentration, size distribution (polydispersity) and magnetic attractive forces (Fe-0 content) on agglomeration and transport of poly(styrene sulfonate) (PSS) modified NZVI was studied in water-saturated sand (d(p) = 300 mu m) columns. Particle concentrations ranged from 0.03 to 6 g/L in 5 mM NaCl/5 mM NaHCO3 at a pore water velocity of 3.2 x 10(-4) m/s. Three NZVI dispersions with different intrinsic particle size distributions obtained from sequential sedimentation are compared. The influence of magnetic attraction (Fe-0 content) on NZVI agglomeration and deposition in porous media is assessed by comparing the deposition behavior of PSS-modified NZVI (magnetic) having different Fe-0 contents with PSS-modified hematite (nonmagnetic) with the same surface modifier. At low particle concentration (30 mg/L) all particles were mobile in sand columns regardless of size or magnetic attractive forces. At high concentration (1 to 6 g/L), deposition of the relatively monodisperse dispersion containing PSS-modified NZVI (hydrodynamic radius (R-H) = 24 nm) with the lowest Fe-0 content (4 wt %) is low (attachment efficiency (a) = 2.5 x 10(-3)), insensitive to particle concentration, and similar to PSS-modified hematite. At 1 to 6 g/L, the attachment efficiency of polydisperse dispersions containing both primary particles and sintered aggregates (R-H from 15 to 260 nm) of PSS-modified NZVI with a range of Fe-0 content (10-60%) is greater (a = 1.2 x 10(-2) to 7.2 x 10(-2)) and is sensitive to particle size distribution. The greater attachment for larger, more polydisperse Fe-0 nanoparticles with higher Fe-0 content is a result of their agglomeration during transport in porous media because the magnetic attractive force between particles increases with the sixth power of particle/agglomerate radius. A filtration model that considers agglomeration in porous media and subsequent deposition explains the observed transport of polydisperse PSS-modified NZVI at high concentration.