Synthesis of amorphous-MnO₂/Clinoptilolite and its utilization for NH₄⁺-N oxidation in an anoxic environment

Mediating the anoxic ammonia oxidation with manganese oxide (MnO_x) can reduce the requirements of dissolved oxygen (DO) concentrations in constructed wetlands (CWs) and improve the removal of ammonium nitrogen (NH₄⁺-N). Recent studies that employed natural manganese ore and/or mine waste as substrates in CWs may develop potentially negative environmental effects due to leachates. However, removing NH₄⁺-N by anoxic ammonia oxidation is influenced by the crystal form of MnO_x. In this study, a novel clinoptilolite-based amorphous-MnO₂ (amorphous-MnO₂/clinoptilolite) was synthesized by the sol-gel method as an alternative substrate to improve the efficiency of anoxic ammonia oxidation and reduce the impact of Mn ion leaching. According to the anoxic ammonia oxidation experiment of clinoptilolite, amorphous-MnO₂/clinoptilolite, and manganese ore on NH₄⁺-N, the amounts of NH₄⁺-N removed were 24.55 mg/L/d, 44.55 mg/L/d, and 11.04 mg/L/d, respectively, and the initial NH₄⁺-N concentration was 49.53 mg/L. These results indicated that the amorphous-MnO₂/clinoptilolite had both the adsorption and the anoxic ammonia oxidation performance. The recycling experiment demonstrated that the effect of anoxic ammonia oxygen mediated by amorphous-MnO₂ would not diminish with the gradual saturation of clinoptilolite for NH₄⁺-N. Furthermore, the anoxic ammonia oxidation consumed NH₄⁺-N in the clinoptilolite, which restored the adsorption capacity of the clinoptilolite and simultaneously decreased the leakage of manganese ions in the process, making it environmentally friendly. Therefore, the amorphous-MnO₂/clinoptilolite provided an excellent substrate material for the constructed wetland under an anoxic environment, which greatly improved the nitrogen removal capacity compared to existing substrate materials.