<p>Ammonium (NH₄⁺) pollution leads to eutrophication and harms biodiversity, aquatic life, and human health. A highly effective adsorbent based on activated biochar from sugarcane waste (SWA) was produced using alkaline-assisted pyrolysis to investigate the NH₄⁺ removal capacity from water. Physicochemical features of SWA adsorbent were characterized using scanning electron microscopy, energy-dispersive X-ray, Fourier transform infrared spectroscopy, nitrogen adsorption–desorption analysis, and pH drift measurements. Batch experiments focused on the effect of contact time, pH, NH₄⁺ initial concentration, temperature, and co-existing cations. Fixed-bed column experiments were also performed to evaluate the adsorption behavior of SWA in continuous-flow sorption. Results show that the activation of biochar led to increases in carboxylic and hydroxyl groups and in biochar surface area, which boosted the adsorption capacity of SWA. NH₄⁺ adsorption by SWA best fits Langmuir, Freundlich, and Redlich-Peterson models, with a Langmuir maximum uptake of 7.19&#xa0;mg/g at 30&#xa0;°C. Kinetic studies showed that NH₄⁺ adsorption follows a pseudo-second-order model, with a coefficient of determination of 0.96. Co-existing cations, including Ca²⁺, Mg²⁺, Mn²⁺, and Fe³⁺, significantly reduced the efficiency of NH₄⁺ removal. NH₄⁺ desorption of laden SWA was most efficient in strongly acidic environments. The Thomas and Yoon-Nelson models effectively described NH₄⁺ adsorption under various concentrations and flow rates in the fixed-bed column. This method of synthesizing activated biochar from sugarcane waste opens up new opportunities for developing biofertilizers as adsorbents for removing and recovering NH₄⁺ from water.</p> Graphical Abstract <p></p>