Kinetics of the Abiotic Reduction of Polymeric Manganese Dioxide by Nitrite: An Anaerobic Nitrification Reaction

摘要

Manganese oxides are strong environmental oxidants recently found to be involved in the nitrogen cycle. Of the several possible reactions with reduced nitrogen species, the reduction of MnO_2 by nitrite has only received marginal attention. Yet this reaction might explain why nitrification can occur in the absence of O_2, observed in both sediments and water columns. We have determined the stoichiometry of this reaction, as well as the chemical kinetics and the activation parameters, using a soluble polymeric form of MnO_2. The reaction rate decreases with increasing pH and decreasing temperature. The reaction is first order in each reactant with a second order rate constant (k) = 493 M~(-1) min~(-1) at 21.5 ℃ and pH = 5.00. The energy of activation (Ea = 9.370 kJ/mole) and the entropy of activation (△S~(++) = -169.5 J/mole) show the reaction to be associative and diffusion controlled, occurring via an inner-sphere mechanism, likely with O atom transfer from MnO_2 to HNO_2. The reaction is proton assisted and slows down at pH ≥ 5.5 where NO_2~- and MnO_2 (unprotonated and negatively charged) become the dominant species. In natural waters and sediments where anaerobic nitrification has been observed the pH is higher than this. Thus, the thermodynamically favorable reaction will likely proceed by microbial mediation.