Adsorbent materials for ammonium and ammonia removal: A review

Ammonium (NH4+) and ammonia (NH3) are notorious hard-to-treat pollutants, leading to serious deterioration of aquatic ecosystems and significant risks to human health. While adsorption is a promising method to tackle this problem, finding suitable adsorbent materials which are abundant, low-cost and efficient remains a constant challenge. Thus, this review summarizes recent development of important adsorbent materials implemented for NH3/NH4+ removal. Advantages and disadvantages of representative adsorbent materials including bentonite, zeolite, clay, biochar, activated carbon, metal organic framework and their modified forms are compared, and the nature of their adsorption processes are discussed in context of adsorption sites, isotherm models (e.g. Langmuir and Freundlich), kinetic equations (e.g. pseudo-first order, pseudo-second order and intra-particle diffusion) and thermodynamic analysis. Future perspective on the utilization of inexpensive lignite is also conferred. Although both conventional and nanostructured materials face challenges regarding economic cost, energy consumption, secondary pollution and adsorption efficiency, these can be tackled by adopting various of advanced options. Current research on adsorption mechanisms forms a solid basis for the design and development of novel adsorbent materials. We speculate that the pursuit of strategies for effective surface modification of natural abundant resources will lead to a bright future of removal processes suited to low NH3/NH4+ concentration conditions.