Evaluating biochar for adsorption of ammonium nitrogen in wastewater:insights into modifications and mechanisms

Ammonium nitrogen (NH 4 + ) is a highly recalcitrant pollutant, leading to severe degradation of aquatic ecosystems and posing serious risks to human health. The application of biochar for NH 4 + removal from wastewater has gained widespread attention. However, its inherent limitations in adsorption capacity present a significant constraint on its broader practical implementation. To address this limitation, various modification techniques have been developed to endow biochar with a range of physicochemical properties. In this review, a systematic investigation was conducted to assess the efficacy of various modification methods on the adsorptive capacity of biochar for NH 4 + in aqueous solutions. Additionally, this review summarizes the adsorption mechanisms which are divided into five categories: hydrogen bonding, pore filling, electrostatic interaction, ion exchange and surface complexation. This review offers valuable insights into the strategies for achieving enhanced adsorption of NH 4 + by modified biochar, along with a comprehensive summary of the associated removal mechanisms.