Localized alteration of ferrihydrite natural organic matter coprecipitates following reaction with Fe(II)

Abstract

Abstract Coprecipitation of natural organic matter (NOM) with ferrihydrite (Fh) can alter the trajectory of Fe(II)‐catalyzed transformation. High C/Fe molar ratio Fh‐NOM coprecipitates are expected to resist transformation completely and preserve organic matter (OM). To explore how heterogeneous NOM would influence this process, we reacted low (0.8) and high (1.8) C/Fe molar ratio Fh‐NOM coprecipitates (synthesized with a 13 C‐labeled plant litter extract) with 2 mM Fe(II) for 1 and 14 d under anoxic conditions and examined changes in the solid phase with Mössbauer spectroscopy (MBS) and secondary electron images using scanning transmission electron microscope. Mössbauer spectroscopy data suggests Fe(II) interaction with coprecipitates increases Fe mineral crystallinity, as did lower molar C/Fe ratios and longer aging times. However, de novo crystal phase development was only observed (via secondary electron images) in the Fe(II)‐reacted coprecipitates and only when the organic carbon (OC) loading was low. For example, the low C/Fe coprecipitate developed localized lath‐like crystal phases after 1 d of reaction with Fe(II), while the lath‐like phases only developed in the high C/Fe coprecipitates after they lost some NOM (C/Fe ratio decreased from 1.8 to 0.7) and reacted with Fe(II) for 14 d. In addition, partial exchange of coprecipitate 13 C‐NOM for C in the media (likely the organic PIPES buffer) occurred in all treatments but was accentuated by reaction with Fe(II), suggesting coprecipitate OM composition likely evolves over time, especially when exposed to Fe(II). Our findings suggest that Fe(II) mediated Fh‐NOM transformation is highly dynamic and has the potential to influence the stability of NOM in Fh.