Reactivity of manganese oxide minerals with soil organic matter and lead
There is a paucity of mechanistic information available concerning the reactivity of soil organic matter and Pb(II) on naturally occurring Mn(III,IV) (hydr) oxide minerals. Kinetic and equilibrium studies were conducted with catechol and Pb(II) on birnessite, manganite, and pyrolusite using electron paramagnetic resonance (EPR) spectroscopy, diffuse reflectance spectroscopy (DRS), and x-ray absorption spectroscopy (XAS). The use of in situ spectroscopic tools was valuable in determining reaction mechanisms under conditions representative of natural soil and geochemical environments.
Catechol rapidly dissolved birnessite by a surface-controlled associative mechanism with a pH-independent second-order rate expression based on EPR-stopped flow and temperature experiments. The surface complex formed between birnessite and manganite with catechol was confirmed for the first time by DRS as inner-sphere based on the ligand-to-metal-charge transfer bands at 10812 cm–1 and 12674 cm–1. Surface area and thermodynamic driving force were incapable of describing differences in the observed reduction rates by catechol between the minerals, which decreased in the order birnessite > manganite > pyrolusite.
Available Mn(III) in different coordination environments associated with Mn(III,IV) (hydr) oxide minerals predicted reductive dissolution behavior in the presence of catechol. This finding is extremely novel because historically Mn(II) and Mn(IV) oxidation states have received the most attention.
Lead adsorbed as an inner-sphere surface complex on both birnessite and manganite with no evidence to suggest oxidation or surface precipitation as operative sorption mechanisms based on XAFS spectra. Spectroscopic differences in the chemical structure of adsorbed Pb on birnessite when compared to manganite led to contrasting desorption behavior. The superior ability of birnessite to decrease Pb(II) solubility coupled with its greater reactivity towards soil organic matter are significant findings because it is the most commonly identified solid Mn mineral in natural soil, marine, and geochemical environments. These results can be used to assist in predicting global Mn and C cycling which will benefit production agriculture, the general public, and preserve environmental quality.
0481: Soil sciences