Review of separation methods for the determination of ammonium/ammonia in natural water

Ammonium (NH4+) and ammonia (NH3) in aquatic ecosystems are of great interest to environmental scientists because they can be used to study the nitrogen cycle and as water quality indicators. Analytical separation methods developed in recent decades have been used widely to determine NH4+ and NH3 in aqueous solutions. This review presents an overview of state-of-the-art separation methods and analytical techniques for determining NH3/NH4+ in natural water, including chromatographic methods, electrophoretic methods, extraction methods, membrane-based gas diffusion methods, membraneless gas diffusion methods, passive sampling methods, and paper-based analytical methods. Common detection techniques that can be used in conjunction with particular separation methods are described, phase-transfer strategies (liquid-liquid, liquid-solid, liquid-membrane-liquid, and liquid-gas-liquid methods) are highlighted, and the strengths and weaknesses of the separation methods are discussed. The outlook, challenges, and expected future developments in the field of separation methods for determining NH4+ and NH3 in natural water are presented.