Oxidation of nanoscale zero-valent iron under sufficient and limited dissolved oxygen: Influences on aggregation behaviors

• Two oxidation conditions were designed according to the environmental applications.
• The two oxidized nanoparticles exhibited disparate colloidal behaviors.
• Aggregation formation and aggregation kinetics were elucidated by colloidal theories.
• Dissolved oxygen controlled aggregation by determining composition and magnetization.
• Aggregation morphology decided aggregation kinetics though magnetic.

Oxidations of nanoscale zero-valent iron (nZVI) under aerobic (dissolved oxygen ≈ 8 mg L−1) and anaerobic (dissolved oxygen <3 mg L−1) conditions were simulated, and their influences on aggregation behaviors of nZVI were investigated. The two oxidation products were noted as HO-nZVI (nZVI oxidized in highly oxygenated water) and LO-nZVI (nZVI oxidized in lowly oxygenated water) respectively. The metallic iron of the oxidized nZVI was almost exhausted (Fe0 ≈ 8 ± 5%), thus magnetization mainly depended on magnetite content. Since sufficient dissolved oxygen led to the much less magnetite (∼15%) in HO-nZVI than that in LO-nZVI (>90%), HO-nZVI was far less magnetic (Ms = 88 kA m−1) than LO-nZVI (Ms = 365 kA m−1). Consequently, HO-nZVI formed small agglomerates (228 ± 10 nm), while LO-nZVI tended to form chain-like aggregations (>1 μm) which precipitated rapidly. Based on the EDLVO theory, we suggested that dissolved oxygen level determined aggregation morphologies by controlling the degree of oxidation and the magnitude of magnetization. Then the chain-like alignment of LO-nZVI would promote further aggregation, but the agglomerate morphology of HO-nZVI would eliminate magnetic forces and inhibit the aggregation while HO-nZVI remained magnetic. Our results indicated the fine colloidal stability of HO-nZVI, which might lead to the great mobility in the environment.

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