Aggregation and Sedimentation of Aqueous Nanoscale Zerovalent Iron Dispersions

TLDR

Nanoscale zerovalent iron rapidly transforms many environmental contaminants and is a promising in‑situ remediation agent, but its limited mobility is attributed to rapid aggregation. The study aims to determine how to form stable aqueous dispersions of NZVI for delivery in water‑saturated porous media. The authors used dynamic light scattering, optical microscopy, and sedimentation measurements, along with parallel experiments on magnetite and hematite and extended DLVO theory, to characterize the size and growth of fractal aggregates. They found that aggregation rate rises with particle concentration and magnetization, producing micrometer‑sized aggregates within 10 min and 20–70 μm clusters within 30 min at 60 mg L⁻¹, and that magnetic attraction accelerates aggregation compared to nonmagnetic particles.

Abstract

Nanoscale zerovalent iron (NZVI) rapidly transforms many environmental contaminants to benign products and is a promising in-situ remediation agent. To be effective, NZVI should form stable dispersions in water such that it can be delivered in water-saturated porous media to the contaminated area. Limited mobility of NZVI has been reported, however, attributed to its rapid aggregation. This study uses dynamic light scattering to investigate the rapid aggregation of NZVI from single nanoparticles to micrometer size aggregates, and optical microscopy and sedimentation measurements to estimate the size of inter-connected fractal aggregates formed. The rate of aggregation increased with increasing particle concentration and increasing saturation magnetization (i.e., the maximum intrinsic magnet moment) of the particles. During diffusion limited aggregation the primary particles (average radius = 20 nm) aggregate to micrometer-size aggregates in only 10 min, with average hydrodynamic radii ranging from 125 nm to 1.2 μm at a particle concentration of 2 mg/L (volume fraction(Φ) = 3.2 × 10-7) and 60 mg/L (Φ = 9.5 × 10-6), respectively. Subsequently, these aggregates assemble themselves into fractal, chain-like clusters. At an initial concentration of just 60 mg/L, cluster sizes reach 20−70 μm in 30 min and rapidly sedimented from solution. Parallel experiments conducted with magnetite and hematite, coupled with extended DLVO theory and multiple regression analysis confirm that magnetic attractive forces between particles increase the rate of NZVI aggregation as compared to nonmagnetic particles.