The citric acid–MnIII,IVO2(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 MnII. 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 (rmax) along the reaction coordinate follows a Langmuir–Hinshelwood dependence on citrate concentration. Increases in pH decrease rmax and increase the induction time. Adding MnII, ZnII, orthophosphate, or pyrophosphate at the onset of reaction decreases rmax. MnII 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 MnIII,IV, yielding MnII and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from MnII–citrate complexes to surface-bound MnIII,IV, generating MnIII–citrate and MnII. Subsequent intramolecular electron transfer converts MnIII–citrate into MnII and citrate oxidation products.