Prediction of nanomagnetite stoichiometry (Fe(ii)/Fe(iii)) under contrasting pH and redox conditions

Abstract

Magnetite (Fe(III)2Fe(II)O4) nanoparticles are fascinating nanoparticulate minerals for their electronic, magnetic and chemical properties. Ubiquitous, in the environment, they are also among the most used ferromagnetic nanomaterials in environmental, industrial...

Redox Potentials of Magnetite Suspensions under Reducing Conditions

“…The similar potentials and effect of pH with and without added Fe(II) below pH 7.0 are consistent with previous work 36 and suggest that maghemite|Fe(II) aq contributes to poising the potential at low pH regardless of whether Fe(II) is added or not to the magnetite suspensions. Similar potentials at low pH values with and without added Fe(II) are perhaps not all that surprising as below pH 7.0, we and others 36 measure the release of Fe(II) into the aqueous phase from magnetite dissolution and ultimately the potentials measured are likely being set by the equilibria controlling the ratio of aqueous Fe(II) to Fe(III) in solution (Figure S7). 32,35 At pH values above 7.0, however, we observed a slope of −66 ± 3 mV (n = 26 and R 2 = 0.937) which is markedly different from the −179 ± 4 mV observed with 1 mM added Fe(II) (Figure 5).…”

“…The similar potentials with and without added Fe(II), as discussed in more detail below, are most likely caused by magnetite dissolution and the release of Fe(II) to solution at lower pH values (<7.0) as suggested by Jungcharoen et al as well. 36 To evaluate the effect of added aqueous Fe(II) on magnetite potentials, we held the pH constant at 7.1 and measured potentials over a range of Fe(II) aq concentrations from ∼0.2 to 10 mM (Figure 2B). The effect of Fe(II) on E H also displayed Nernstian behavior with an E H vs Fe(II) slope of −54 ± 4 mV (n = 5 and R 2 = 0.984).…”

“…At the lower pH values, the concentration of aqueous Fe(II) increased over time, indicating that net dissolution of Fe(II) from magnetite occurred consistent with previous observations (Figure S5). 16,36,57 To estimate an E H o value, we excluded the data points where precipitation was expected (red markers in Figure S4), as well as the pH 6.5 data points where substantial dissolution was measured (Figure S5), and then performed a multivariate regression using eq 7 with both the pH and Fe(II) slopes held constant at the theoretical values of −59 and −177 mV estimated from Figure 2.…”

“…3,28−30 Magnetite redox properties have been widely explored for these applications, as well as their role in natural and engineered environments relevant to contaminant behavior. 14,17,28,31−34 Several methods have been used to measure or estimate magnetite redox potentials, including combination platinum redox probes, 17,32,35,36 modified and unmodified rotating disc electrodes, 15,16,28,34,37 and colorimetric absorbance measurements. 38 Multiple methods, as well as experimental conditions, have resulted in a wide range of potentials reported for magnetite (from +560 to ∼−500 mV vs SHE over a pH range of 2 to 12 [15][16][17]33,34,39,40 ).…”

“…In addition to the lack of agreement among reported potentials, estimating the potential of magnetite in soils and sediments is further complicated by magnetite being a mixed-valence mineral which can contain Fe 2+ ions in the structure and result in a range of magnetite stoichiometries (x = Fe(II)/Fe(III)) which has been modeled as a magnetite−maghemite solid solution upon dissolution or oxidation. 17,32,36,41 The most common equilibria used to interpret magnetite redox potentials are magnetite|Fe(II) aq , maghemite|magnetite, and hematite|magnetite (eqs 1, 2, and 3). 15,17,32,42 Even among these, the reported standard redox potential (E H o ) estimated from thermodynamic data can vary by up to 210 mV vs SHE.…”