Excellent Adsorption–Desorption of Ammonium by a Poly(acrylic acid)-Grafted Chitosan and Biochar Composite for Sustainable Agricultural Development

The discovery of materials as soil conditioners for the improvement of soil nitrogen-use efficiency provides a promising strategy for simultaneously addressing production and pollution problems in agriculture. Herein, adsorption–desorption performance and mechanisms of ammonium (NH4 +–N) by a poly­(acrylic acid)-grafted chitosan and biochar composite (PAA/CTS/BC) were studied to evaluate its potential in soil amendment. Furthermore, mathematical models were further applied to explore the effects of PAA/CTS/BC on controlling soil NH4 +–N loss. Results suggest that PAA/CTS/BC showed a high capacity of NH4 +–N adsorption, with a maximum value of 149.25 mg·g–1 at 25 °C, considerably higher than most of the reported BC-based adsorbents. Fast adsorption of NH4 +–N was completed with an efficiency of >90% within 20 min, primarily attributed to the dominant electrostatic interactions with carboxyl groups. Adsorbed NH4 +–N was capable of being easily re-released with the variation of the liquid-phase concentration because of the weak binding of NH4 +–N with the material. These results demonstrate that the PAA/CTS/BC can be considered as a nitrogen slow-release medium to dynamically maintain a plant-available NH4 +–N concentration in the soil. Finally, the mathematical simulation of NH4 +–N flow in the PAA/CTS/BC-amended soil revealed that soil NH4 +–N loss was effectively reduced by 24.18–31.77% at a 1.57–2.02% material dosage during a 10-week rice cultivation.