摘要:

Iron (oxyhydr)oxide nanoparticles (IONPs), which are ubiquitous in many natural aquatic and soil systems, can strongly interact with nutrient and contaminant species in the environment through their large specific surface areas and redox reactivity, thus controlling the transport and fate of these elements. Following their formation, IONPs often undergo aggregation and phase transformation processes that collectively determine their long-term environmental stability. The aggregation of IONPs reduces colloidal stability and can lead to deposition and immobilization, whereas stable dispersed colloids can remain mobile and transport associated elements over long distances. The phase transformations of metastable, poorly crystalline IONPs (e.g., ferrihydrite) into more crystalline iron (oxhydr)oxides (e.g., goethite, hematite, and magnetite) profoundly alter particle properties and influence the retention or release of sorbed or structurally incorporated species. This review focuses on IONP aggregation and phase transformation as key processes controlling long-term IONP stability and critically examines how they are influenced by three common environmental factors: metal ions, organic matter (OM), and microbial activity. Metal ions can adsorb to IONP surfaces to modify surface charges or be structurally incorporated to affect IONP crystallography, thereby modulating inter-particle forces and transformation rates. OM can adsorb to IONP surfaces, and, depending on its concentration and molecular characteristics, it can either stabilize particles via electrostatic and/or steric repulsion, or promote aggregation through charge neutralization and bridging effects. Further, organic ligands can also often inhibit IONP transformation or alter transformation pathways by binding to reactive surface sites. Microbial activity influences IONP stability through extracellular polymeric substances (EPS) that coat or bridge particles, and through redox processes that generate or consume Fe(II), thereby either dispersing IONPs or accelerating their transformation into more stable mineral phases. This review summarizes present research on the effects of IONP interactions with metals, organics, and microbes on IONP aggregation and transformation. Such an understanding is crucial for predicting IONP stability and transport in the environment and the long-term cycling of associated organic and inorganic contaminants and nutrients.

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