Redox Behavior of Cu onto Magnetite Nanoparticles

Copper (Cu) oxidation-state distribution controls its toxicity and mobility in natural systems and is affected by its interaction with redox-reactive mineral surfaces such as magnetite. By combining aqueous chemical analysis, X-ray absorption spectroscopy and aqueous speciation modeling, this study provides detailed mechanistic insights regarding Cu redox speciation in solution and at the surface of 10 nm-sized magnetite nanoparticles with varying stoichiometries (0.1 ≤ R = Fe(II)/Fe(III) ≤ 0.5), versus pH (4 to 9) and initial Cu(II) concentration ([Cu(II)]ini = 25 and 500 μM), with 10 mM NaCl. Cu(II) generally prevailed at the magnetite surface, with co-occurring Cu(I) from a smaller extent to an equal extent. Conversely, in the aqueous solution (i.e., after filtration), Cu(I) prevailed at pH > 5, hence evidencing that solution and surface redox speciation may differ. Reduction to Cu(0) was only partial and detected under the most reducing condition (for R = 0.5) at high [Cu(II)]ini, because it was limited by Cu(II)- and Cu(I)-magnetite binding. Significant oxidation Fe(II) to Fe(III) was shown to be responsible for this partial reduction process, which could be overcome by adding Fe(II) ions to recharge the magnetite surface, hence promoting further copper reduction to Cu(0). This study provides fundamental insights into copper–magnetite interactions and enables improved predictions of copper speciation and fate in environmental systems.