Organic Matter Inhibits Redox Activity and Impacts Heterogeneous Growth of Iron (Oxyhydr)oxides on Nano-Hematite

Oxidative mineral growth of goethite (α-FeOOH) on hematite (α-Fe₂O₃) nanoparticles during the oxidation of adsorbed Fe(II) is thermodynamically controlled by mineral surface characteristics and solution conditions. Here, the impact of added organic carbon (OC) on reactivity of ultrafine mineral particles is evaluated. For batch reactors using 0.007 m²/mL hematite, the observed rate constant of 4-chloronitrobenzene reduction to 4-chloroaniline decreases by 4× with the addition of 5 ppm of OC from Suwanee River natural organic matter (SRNOM) and 5× with 10 μM catechol (0.72 ppm of C). Both goethite and hematite are produced, and the fraction of Fe(II) converted to goethite decreases with the addition of SRNOM or catechol. In the absence of added OC, postreaction solids are 17 ± 3 mass % goethite, which decreased to 11 ± 2 mass % and 4 ± 2 mass % with 20 ppm of OC as SRNOM and 20 μM catechol, respectively, and substantial changes in morphology of the goethite product were observed. In the absence of added OC, goethite rods formed at the acute tips of hematite rhombohedra as singular rods ∼50-80 nm long and 10 nm wide. Goethite crystals formed in the presence of 10 μM catechol occurred as 5 to 8 parallel growths measuring 10-50 nm long and 5 nm wide. Batch reactors containing SRNOM had similar results, although the goethite morphology was more irregular. Low-temperature magnetic measurements show that experiments conducted using 20 ppm of SRNOM produced finer grained nano-hematite and goethite than formed in the presence of 20 μM catechol. This study highlights the need for improved methods for characterizing ultrafine mineral phases and demonstrates that organic matter changes the microstructure and morphology of materials formed by oxidative mineral growth and thus how reactive surface area evolves with the extent of reaction.