Enhancing ammonia oxidation mediated using different crystalline Mn-oxides in an anoxic environment

A recent study found that the nitrogen cycle in marine sediments can occur under oxygen-limited conditions, and this is associated with a reduction of Mn (IV). However, the effect of MnO₂ mediated anoxic ammonia oxidation in different sediments field test results are controversial. In this study, based on the fact that the crystal form and morphology of MnO₂ in marine sediments are affected by geochemistry, α-, β-, γ-MnO₂ and amorphous MnO₂ were prepared to explore the effect of the different MnO₂ crystals on nitrogen removal under oxygen-limited conditions. The experimental results showed that the anoxic ammonia oxidation process was mediated by microorganisms, and the reaction was affected by pH and temperature. The optimal pH was 7 in the range of pH 4-9 and the optimal temperature was 25 °C in the range of 10 °C-40 °C. When the initial concentration of NH₄⁺-N was 50 mg/L, the removal amounts of NH₄⁺-N under an anoxic condition by α-, β-, γ-MnO₂ and amorphous MnO₂ were 18.97 mg/L/d, 6.12 mg/L/d, 10.68 mg/L/d and 24.89 mg/L/d, respectively. During the anoxic oxidation between MnO₂ and NH₄⁺-N, the adsorption process occurred. In addition, the oxidation process produced both NO_x^--N (nitrification reaction) and gaseous nitrogen (ammonia oxidation reaction). The kinetic study showed that the NH₄⁺-N removal process conformed to the pseudo-second-order rate model, and the removal rates were ranked as amorphous MnO₂ > α- > γ- > β-MnO₂. Together, these results showed that the amorphous MnO₂ crystal structure was conducive to improve anoxic ammonia oxidation and nitrogen removal under oxygen-limited conditions.