The citric acid-MnO2(birnessite) reaction. Electron transfer, complex formation, and autocatalytic feedback

Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidation by MnO₂(birnessite), yielding 3-ketoglutarate, acetoacetate, and Mn^II. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concentration decreases the induction period. The maximum rate (r_max) along the reaction coordinate follows a Langmuir-Hinshelwood dependence on citrate concentration. Increases in pH decrease r_max and increase the induction time. Adding Mn^II, Zn^II, orthophosphate, or pyrophosphate at the onset of reaction decreases r_max. Mn^II addition eliminates the induction period, while orthophosphate and pyrophosphate addition increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidation of free citrate by surface-bound Mn^III,IV, yielding Mn^II and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from Mn^II-citrate complexes to surface-bound Mn^III,IV, generating Mn^III-citrate and Mn^II. Subsequent intramolecular electron transfer converts Mn^III-citrate into Mn^II and citrate oxidation products.