From surface complexation to dissolved ferrous iron accumulation: effects of ferrihydrite crystallinity and pH on catechol reactivity.

At iron (hydr)oxide-organic carbon interfaces, interfacial reactions strongly shape organic matter transformation and Fe speciation in sediment and peatland porewaters. Phenolic compounds are abundant in such environments and can strongly interact with ferrihydrite surfaces. However, it remains unclear whether catechol surface complexation on ferrihydrite is followed by measurable dissolved Fe(II) accumulation and how this response is governed by mineral crystallinity and pH. Here, ferrihydrite samples with different crystallinity (Fh-2L, Fh-4L, and Fh-6L) were used as model minerals to investigate catechol adsorption, dissolved Fe(II) accumulation, and mineral aging responses. Although catechol was adsorbed by all three ferrihydrite samples, measurable dissolved Fe(II) accumulated only in Fh-2L and Fh-4L. At 1440 min, dissolved Fe(II) concentrations reached 1.12 and 0.36 mg L-1 in Fh-2L and Fh-4L, respectively, whereas dissolved Fe(II) in Fh-6L remained below the detection limit (0.03 mg L-1) throughout. pH further regulated dissolved Fe(II) accumulation: in Fh-2L, dissolved Fe(II) peaked at pH 7 (1.254 mg L-1) and decreased to 0.148 mg L-1 at pH 9, whereas in Fh-6L it remained below the detection limit across the entire pH range. Spectroscopy and density functional theory (DFT) indicated inner-sphere catechol complexation, with selected configurations on an idealized oxygen-deficient surface showing stronger adsorption (-2.39/-2.32 eV) than those on the pristine surface (-0.71/-0.75 eV). Accelerated aging experiments further showed that catechol inhibited ferrihydrite ordering. This study distinguishes catechol adsorption from dissolved Fe(II) accumulation and provides mechanistic insight into the crystallinity- and pH-dependent interfacial reactivity of catechol on ferrihydrite.