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

Oxidative mineral growth of goethite (alpha-FeOOH) onhematite (alpha-Fe2O3) nanoparticles during the oxidation of adsorbed Fe(II)is thermodynamically controlled by mineral surface characteristics andsolution conditions. Here, the impact of added organic carbon (OC) onreactivity of ultrafine mineral particles is evaluated. For batch reactorsusing 0.007 m2/mL hematite, the observed rate constant of 4-chloronitrobenzene reduction to 4-chloroaniline decreases by 4xwiththe addition of 5 ppm of OC from Suwanee River natural organic matter(SRNOM) and 5xwith 10 mu M catechol (0.72 ppm of C). Both goethiteand hematite are produced, and the fraction of Fe(II) converted togoethite decreases with the addition of SRNOM or catechol. In theabsence of added OC, postreaction solids are 17 +/- 3 mass % goethite,which decreased to 11 +/- 2 mass % and 4 +/- 2 mass % with 20 ppm of OCas SRNOM and 20 mu M catechol, respectively, and substantial changes inmorphology of the goethite product were observed. In the absence of added OC, goethite rods formed at the acute tips of hematiterhombohedra as singular rods similar to 50-80 nm long and 10 nm wide. Goethite crystals formed in the presence of 10 mu M catecholoccurred 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 conductedusing 20 ppm of SRNOM producedfiner grained nano-hematite and goethite than formed in the presence of 20 mu M catechol. Thisstudy highlights the need for improved methods for characterizing ultrafine mineral phases and demonstrates that organic matterchanges the microstructure and morphology of materials formed by oxidative mineral growth and thus how reactive surface areaevolves with the extent of reaction