Adsorption and oxidation of Mn(II) at the organo-ferrihydrite coprecipitate-water interfaces: Implications for Mn geochemical behavior and carbon stability

Adsorption and oxidation of Mn(II) on Fe oxides plays a crucial role in controlling the behavior of Mn in soils. Additionally, Fe oxides are often associated with ubiquitous organic matter (OM) through coprecipitation to form organo-Fe oxide coprecipitates, which improves the stability of OM. However, the adsorption and oxidation of Mn(II) at the coprecipitate-water interfaces and its impact on the stability of OM fraction in the coprecipitates remain unclear. Here, citric acid (CA), a common OM in soil, was used to synthesize CA-ferrihydrite coprecipitates (CFCs), and the interactions between Mn(II) and CFCs were investigated using batch experiments and multiple spectroscopies. Results suggested that the coprecipitated CA promoted Mn(II) adsorption by ferrihydrite under anoxic conditions, but it decreased Mn(II) removal by ferrihydrite under oxic conditions through inhibiting Mn(II) oxidation, particularly at high Mn(II) concentration and CA loading. This was attributed to the fact that the CA suppressed the electron transfer from adsorbed Mn(II) to O 2 by complexing with Mn(II) and increasing the charge transfer resistance of ferrihydrite, and reduced the formed Mn oxides (e.g., hausmannite and manganite). In turn, 8.2%-16.0% of OM in the CFCs was oxidized into CO 2 after Mn(II) removal under oxic conditions. However, the stability of remaining OM in the CFCs was improved, as suggested by the increase of nonextractable C. This was due to additional OM binding sites and physical protection provided by the formed Mn oxides or Mn(II) bridging. These findings facilitate the understanding of Mn(II) adsorption-oxidation at the organo-Fe oxide composite–water interfaces, which aids in assessing the geochemical behavior of Mn and OM stability in soils.