Effect of organic C on stable Fe isotope fractionation and isotope exchange kinetics between aqueous Fe(II) and ferrihydrite at neutral pH

Abstract

Low-temperature geochemical cycling of Fe, C, nutrients, and toxic metals in nature are largely regulated by the stability and reactivity of fine-grained ferrihydrite. The presence of impurities such as C and Si in ferrihydrite structure are found to inhibit the rapid transformation of ferrihydrite to more stable iron oxides upon interaction with aqueous Fe(II). Therefore, understanding the factors controlling the reactivity of ferrihydrite, especially in the presence of C, is critical to evaluate the role of ferrihydrite in geochemical cycles. Equilibrium stable isotope fractionations are fundamental thermodynamic properties and therefore equilibrium fractionation in 56Fe/54Fe ratios reflects the nature of Fe bonding in ferrihydrite and its reactivity. In this study, we investigated stable Fe isotope fractionation between aqueous Fe(II) and ferrihydrite-organic matter coprecipitates to evaluate whether previously documented inhibition of mineralogical transformation by organic C is accompanied by changes in Fe isotope fractionation. Experiments conducted using Suwannee River natural organic matter (SRNOM) that was coprecipitated with ferrihydrite (molar C:Fe = 1.2) produced an equilibrium 56Fe/54Fe fractionation of −2.36 ± 0.26‰ between aqueous Fe(II) and ferrihydrite-SRNOM coprecipitates. This fractionation factor significantly differs from that previously determined between Fe(II)aq and pure ferrihydrite (−3.20‰) but is similar to that measured for Fe(II)aq-Si-ferrihydrite (−2.58 ± 0.14‰, molar Fe:Si = 1). Furthermore, the addition of C in the ferrihydrite structure at molar C:Fe ∼1.2 markedly increased the extent of Fe isotope exchange to a similar degree as observed with Si-ferrihydrite with molar Fe:Si ∼1. These observations suggest similar effects on both bonding and reactivity of ferrihydrite upon addition of equimolar C and Si. Due to the coexistence of ferrihydrite with organic matter in nature (e.g., wetlands), these results are important for understanding Fe isotope fractionation and exchange kinetics during mineral-fluid interactions in natural ferrihydrite-bearing systems.