Characterization of suboxic groundwater colloids using a multi-method approach

Anoxic groundwater colloid properties were measured using a minimally perturbing procedure for sampling, sample preparation and analysis. Analytical methods included atomic force microscopy (AFM), flow field flow fractionation (FlFFF), transmission and scanning electron microscopy (TEM and SEM). Shallow groundwater samples were found to have abundant iron rich nanoparticles (NP) with diameters of 10-30 nm as well as a smaller heterogeneous polydisperse dissolved organic matter (DOM) fraction. AFM results showed NP with average heights of 10 ± 2 nm, which was corroborated by high resolution (HR) TEM and SEM. FlFFF with UV254 nm detection showed particles with number average diffusion coefficients of 2-3 × 10^-10 m^2 s^-1 and hydrodynamic diameters between 1.5-2 nm, probably representing smaller organic macromolecules. Aeration of the samples resulted in extensive agglomeration of NP to form larger (>50 nm) colloids, and the reduction of UV-absorbing material detected in the 0.5-4 nm range. The complementary methods described have potential applications for investigating the fate and transport of NP in suboxic hotspots such as leachate plumes, waste water treatment plants and within the hyporheic mixing zone.