Stability of coprecipitated natural humic acid and ferrous iron under oxidative conditions

• Influence of humic acid on Fe2 + oxidation is investigated at pH 5 and 8.
• Humic acid can change the pathway formation of Fe oxides from Fe(II) oxidation.
• Maghemite was dominant with small amounts of goethite at pH 8.0.
• Aging improved the crystallinity of Fe oxides at low COOH/Fe(II) ratios.
• Ferrihydrite-HA coprecipitate at pH 5 could favour the carbon stabilisation.

Goethite and lepidocrocite are considered the stable species of Fe oxyhydroxides formed from Fe2 + oxidation in mildly acidic soil environments, while maghemite is formed in mildly alkaline conditions. Complexing ligands, especially humic acid (HA), can change the pathway formation of these iron oxyhydroxides from Fe2 + oxidation. This research aimed to assess the influence of HA on Fe2 + oxidation and its effect on crystalline Fe mineral products. Ferrous iron was added to HA at increasing initial COOH/Fe(II) charge ratios (R) at pHs of 5.0 and 8.0 and aged at 70 °C for 1 month and up to 8 years at room temperature. The precipitated products after aging were analysed using XRD, FT-IR, TEM and AFM. The results indicate that oxidation of Fe2 + at pH 5.0, in the presence of large amounts of HA (R = 0.1) promoted the formation of goethite together with ferrihydrite. Oxidation of Fe2 + in slightly alkaline conditions (pH 8.0) yielded maghemite with small amounts of goethite. Further, long-term aging of HA coprecipitated with Fe2 + perturbed the stacking of the Fe hydroxyl sheets at pH 5.0, favouring the formation of ferrihydrite. At pH 8.0 the presence of humic acid does not influence the crystallinity of the precipitation products and maghemite was still stable after 8 years of aging. This research confirms that the oxidation of Fe2 + in soil and its subsequent hydrolysis is a very common process of Fe hydroxides/oxide formation in acidic soil environments. The type of Fe oxide formed depends strictly on the pH and secondly on the rate of oxidation, especially on the presence and amount of organic compounds that inhibit crystal growth. Humic acid are the main organic molecules in soils, particularly in cool, humid weathering environments where they lead to a decrease in crystalline perfection and promote the formation of ferrihydrite instead of goethite or lepidocrocite. These results provide significant insights into the effect of the mobility of HA in association with Fe minerals.