Ionic Strength and Composition Affect the Mobility of Surface-Modified Fe0 Nanoparticles in Water-Saturated Sand Columns

Abstract

The surfaces of nanoscale zerovalent iron (NZVI) used for groundwater remediation must be modified to be mobile in the subsurface for emplacement. Adsorbed polymers and surfactants can electrostatically, sterically, or electrosterically stabilize nanoparticle suspensions in water, but their efficacy will depend on groundwater ionic strength and cation type as well as physical and chemical heterogeneities of the aquifer material. Here, the effect of ionic strength and cation type on the mobility of bare, polymer-, and surfactant-modified NZVI is evaluated in water-saturated sand columns at low particle concentrations where filtration theory is applicable. NZVI surface modifiers include a high molecular weight (MW) (125 kg/mol) poly(methacrylic acid)-b-(methyl methacrylate)-b-(styrene sulfonate) triblock copolymer (PMAA-PMMA-PSS), polyaspartate which is a low MW (2−3 kg/mol) biopolymer, and the surfactant sodium dodecyl benzene sulfonate (SDBS, MW = 348.5 g/mol). Bare NZVI with an apparent ζ-potential of −30 ± 3 mV was immobile. Polyaspartate-modified nanoiron (MRNIP) with an apparent ζ-potential of −39 ± 1 mV was mobile at low ionic strengths (+ and 2+), and had a critical deposition concentration (CDC) of ∼770 mM Na⁺ and ∼4 mM for Ca²⁺. SDBS-modified NZVI with a similar apparent ζ-potential (−38.3 ± 0.9 mV) showed similar behavior (CDC ∼350 mM for Na⁺ and ∼3.5 mM for Ca²⁺). Triblock copolymer-modified NZVI had the highest apparent ζ-potential (−50 ± 1.2 mV), the greatest mobility in porous media, and a CDC of ∼4 M for Na⁺ and ∼100s of mM for Ca²⁺. The high mobility and CDC is attributed to the electrosteric stabilization afforded by the triblock copolymer but not the other modifiers which provide primarily electrostatic stabilization. Thus, electrosteric stabilization provides the best resistance to changing electrolyte conditions likely to be encountered in real groundwater aquifers, and may provide transport distances of 10s to 100s of meters in unconsolidated sandy aquifers at injection velocities used for emplacement.