Catalytic oxidation of high-concentration ammonia in groundwater by a naturally formed co-oxide filter film

Abstract

Operation of a sand filter system in treating groundwater produces a high concentration of ammonia (2–2.8 mg L−1), iron (0.9–2 mg L−1) and manganese (1.2–1.7 mg L−1), and on the surfaces of quartz sands, a co-oxide filter film is formed. The removal of ammonia in the filter before and after the treatment of film-coated sands at high temperature and pressure and the transformation of nitrogen in filter bed were studied. The morphology and chemical composition of film-coated sands were studied by SEM and EDS, respectively, and the structure of the film material was characterized by XRD. The specific area and pore properties of quartz sands before and after the formation of the filter film were studied by the BET method. The X-ray photoelectron spectroscopy characterization was introduced to identify the chemsorb sites of dissolved oxygen and ammonia on the film surface. The results show that the film material consisting of iron and manganese oxides was amorphous; both specific area and pore volume of the sands got increased as the film was formed. Removal of ammonia in the filter was a catalytic oxidation process, and 90% of capacity of ammonia treatment was recovered in the 116 h operation after the film-coated sands got treated at high temperature and pressure, which was due to the restoration of the surface structure of the film. The dissolved oxygen was most possibly adsorbed on Mn atoms occurring as Mn2O3 and MnO2, and the Fe atoms occurring as FeO while on the membrane surface, the Mn atoms in states of Mn2O3 and MnO, and the Fe atoms in states of Fe2O3 and FeO may be the chemsorb sites of ammonia.