Interaction Between Siderite and Goethite Regulates Fe(II) Speciation and Sustains Reduction Capacity

Fe(II) drives many reduction reactions in aquifers, and its reactivity is governed not merely by total abundance but also by its speciation. Siderite is a major reservoir of structural Fe(II) and frequently occurs in close proximity with goethite. But whether such close proximity effects transformation of structural Fe(II) remains unclear. This study demonstrated that coupling siderite dissolution and diffusive transport established a narrow microenvironment between siderite surface and bulk solutions, which has higher aqueous Fe(II) (Fe(II)aq) concentration and increased pH. Close proximity enables goethite to access the microenvironment and form the proton-mediated feedback interaction with siderite, promoting the formation of surface-associated Fe(II). Therefore, the surface-associated Fe(II) pool in mixed systems increased to 109.3–538.7 μM, 2.8–13.7 times higher than that in dual-chamber systems. This increase can be quantitatively predicted from siderite surface area (A) and proton activity (aH+) via a unified relationship (A·aH+0.37). The A determines the number of available reaction sites, and the aH+ governs the reaction rate. Nitrobenzene reduction experiments revealed that surface-associated Fe(II) pool sustained reduction capacity over successive reaction cycles by linking structural Fe(II) and Fe(II)aq. These findings identify interaction between siderite and goethite as an overlooked mechanism for generating a persistent surface-associated Fe(II) pool. The species distribution of Fe(II) determines the durability of abiotic natural attenuation in aquifers.