Engineering applicability of coarse Prunus spinosa biochar for post‑nitrification ammonium polishing in recirculating aquaculture systems

Residual ammonium following biological nitrification remains a critical operational concern in recirculating aquaculture systems (RAS), where even low concentrations can induce chronic toxicity and destabilize system performance. Under post-nitrification conditions, engineering reliability, hydraulic compatibility, and material reusability often outweigh maximum adsorption capacity. This study evaluates the applicability of coarse-grained Prunus spinosa biochar (2–4 mm) as a supplementary polishing medium for residual ammonium control downstream of biological nitrification. Batch adsorption experiments were conducted under RAS-relevant conditions (pH 6.8–7.2; 15–35 °C) to quantify ammonium uptake behavior, selectivity in the presence of competing nitrogen species, and governing kinetic, equilibrium, and thermodynamic mechanisms. The biochar exhibited selective ammonium removal, achieving up to 5% uptake, while nitrite and nitrate removal remained below 1%, preserving nitrification functionality. Ammonium adsorption followed pseudo-second-order kinetics and was well described by the Langmuir model (R2 > 0.98), with moderate monolayer capacities (3.24–3.57 mg g−1). Thermodynamic analysis confirmed spontaneous and endothermic adsorption, with increased favorability at elevated aquaculture-relevant temperatures. To address engineering feasibility beyond batch conditions, short-term continuous-flow fixed-bed column experiments were performed, demonstrating stable hydraulic operation and preferential ammonium removal. Biochar reusability was confirmed over five adsorption–regeneration cycles using 0.1 M NaCl, with 9.1% efficiency retention. Overall, the results demonstrate that coarse Prunus spinosa biochar is a technically viable, reusable, and cost-effective ammonium polishing medium for RAS, providing an application-oriented foundation for future long-term and pilot-scale fixed-bed studies.