Converting coal fly ash waste into an adsorbent for the removal of ammonia nitrogen is an effective approach. Chemically treated highly porous converted coal fly ash was prepared through hydrothermal synthesis with hydroxide acting as an alkali source for ammonia nitrogen removal from wastewater. The highly porous converted coal fly ash was characterized by scanning electronic microscopy, Fourier transform infrared spectrometry, X-ray powder diffraction, Brunauer-Emmett-Teller analysis, and the results showed that the specific surface area of the highly porous converted coal fly ash had increased significantly from the original 0.15-270 m(2) g(-1). The adsorption isotherms fitted satisfactorily to the Langmuir model, which indicated the adsorption by the chemically treated highly porous converted coal fly ash as a monolayer coverage. The maximum ammonium nitrogen uptake capacity of converted coal fly ash was 139 mg g(-1). Kinetics data were fitted to the pseudo-second-order equation, suggesting that chemisorption, i.e., the process of ion exchange, was the rate determining step.