Effects of C/Mn Ratios on the Sorption and Oxidative Degradation of Small Organic Molecules on Mn-Oxides

Manganese (Mn) oxides have a high surface area and redox
potential
that facilitate sorption and/or oxidation of organic carbon (OC),
but their role in regulating soil C storage is relatively unexplored.
Small OC compounds with distinct structures were reacted with Mn(III/IV)-oxides
to investigate the effects of OC/Mn molar ratios on Mn–OC interaction
mechanisms. Dissolved and solid-phase OC and Mn were measured to quantify
the OC sorption to and/or the redox reaction with Mn-oxides. Mineral
transformation was evaluated using X-ray diffraction and X-ray absorption
spectroscopy. Higher OC/Mn ratios resulted in higher sorption and/or
redox transformation; however, interaction mechanisms differed at
low or high OC/Mn ratios for some OC. Citrate, pyruvate, ascorbate,
and catechol induced Mn-oxide dissolution. The average oxidation state
of Mn in the solid phase did not change during the reaction with citrate,
suggesting ligand-promoted mineral dissolution, but decreased significantly
during reactions with the other compounds, suggesting reductive dissolution
mechanisms. Phthalate primarily sorbed on Mn-oxides with no detectable
formation of redox products. Mn–OC interactions led primarily
to C loss through OC oxidation into inorganic C, except phthalate,
which was predominantly immobilized in the solid phase. Together,
these results provided detailed fundamental insights into reactions
happening at organo–mineral interfaces in soils.