Characteristics and Stability of Incidental Iron Oxide Nanoparticles during Remediation of a Mining-Impacted Stream

Acid mine drainage (AMD) produces nanoparticulate Fe oxides and sorbed toxic metals, such as Cu and Zn. As an indirect product of human activity, these Fe oxides can be classified as incidental nanoparticles (INPs) and their colloidal aggregates. Research in nanoparticle fate and transport has advanced with the development of single particle inductively coupled plasma-mass spectrometry (spICP-MS), but AMD INPs have received little attention. We examined the characteristics and abundance of Fe oxide INPs in an AMD-impacted stream over the first 6 months of remediation. Fe and Cu INP concentrations were approximately 10⁷ and 10⁵ particles mL^-1, before and after treatment, respectively. Overall, ∼4 Cu-containing INPs were counted for every 100 Fe-containing INPs. We also studied surface chemistry changes during the treatment period using hematite, a model Fe INP, suspended in filtered field waters. Changes in zeta potential and INP size, measured by dynamic light scattering, support that the contaminated stream chemistry (low pH, high ionic strength) promoted rapid aggregation while improved water quality favored stability. However, the water chemistry and INP stability during snowmelt were additionally impacted by electrolyte dilution, the addition of dissolved organic matter, and physical scouring. By linking field measurements to laboratory experiments, this work explores the effects of surface chemistry on AMD-generated INP behavior before and during remediation in a hydrologically dynamic alpine stream. To our knowledge, this is the first investigation of remediation effects on AMD INPs and the first use of spICP-MS as a technique to measure them.