In situ characterization of aggregates of nanoscale zero-valent iron (nZVI) in water: an engineering aspect

Abstract

Aggregates of nanoscale zero-valent iron (nZVI) are commonly encountered for nZVI in water, especially during practical applications using large quantities of nZVI. Herein, in situ characterization was performed to study the status of nZVI aggregates in aqueous solution. Specifically, the aggregates are studied for their structural and physical properties under typical hydrodynamic conditions corresponding to three widely used unit operations (mechanical mixing, gravitational settling, and compression) in large-scale applications of nZVI, via six techniques including non-intrusive image analysis and particle image velocimetry. The study shows that the aggregates consist of interconnecting chains of iron nanoparticles and are highly porous in structure. The bulk density of the aggregates in water is close to 1.05 g cm⁻³ for the uncompressed aggregates and can reach 1.27 g cm⁻³ after compression; the porosity of the aggregates is close to 99.2% for the uncompressed aggregates and decreases to 95.1% after compression. During mixing, the aggregates remain mostly at tens of microns at the commonly applied mixing intensity (e.g., G at 500–1500 s⁻¹); breakage of the aggregates occurs at more turbulent conditions but they remain at microscale. During settling, the aggregates of size ∼30–300 μm have a settling velocity of nearly 0.05–0.23 mm s⁻¹; the settling aggregates are also characterized for their structural parameters such as aspect ratio, spherical index and fractal dimension. Slurries of the compressed aggregates are shear-thin and have an apparent viscosity of 52–2.8 × 10⁴ mPa s⁻¹ at 1–1000 s⁻¹. The work provides information about the actual status of nZVI in water during its practical application. The acquired quantitative information about the aggregates offers guidance on the design and operation of the facilities using nZVI for environmental applications.