Simulating the Stability of Colloidal Amorphous Iron Oxide in Natural Water

Considerable uncertainty exists as to whether existing thermodynamic equilibrium solid/water partitioning paradigms can be used to assess the mobility of insoluble manufactured nanomaterials in the aquatic environment. In this work, the traditional Derjaguin–Landau–Verwey–Overbeek theory of colloidal particle stability was examined by using three published expressions for estimating critical coagulation concentrations (CCCs; i.e., the minimum ionic strength needed to induce rapid self-aggregation) via incorporation of diffuse-layer potential estimates obtained from MINTEQA2 geochemical model implementations of an enhanced version of the MIT diffuse-layer model (DLM) and the historical triple-layer model (TLM). Amorphous iron oxide was selected as a test colloid in this assessment and its electrostatic properties were simulated over a pH range of 4 to 10 in 0.7 M seawater, 0.1 M diluted seawater, world average river water, US continental average groundwater and Midwestern US 50th percentile rainwater. Findings from the study included: (1) sources of variation in predictions of the onset of rapid colloidal particle self-aggregation were observed to occur in the following order: aquatic chemistry > selection of DLM vs. TLM diffuse-layer potentials > uncertainties in the Hamaker constant > selection of CCC equation ≈ the distance to the plane of shear (with the DLM), (2) the magnitude of the DLM diffuse-layer potential estimates exceeded that of the TLM estimates, (3) TLM diffuse-layer potential estimates were consistent with a conjecture by Loux and Anderson, Colloids and Surfaces A, 177, 123–131, (2001) that the magnitude of environmental interfacial potentials were likely to be less than RT/F (~25 mV) and DLM estimates were not, and (4) TLM estimates predict that amorphous iron oxide suspensions are likely to be relatively stable only in high pH rainwater; DLM estimates predict relative colloidal iron oxide suspension stability in rainwater and extreme pH river waters. These predictions of rapid aggregation are at least qualitatively in accord with data published in the literature.