Development of MgFe layered double hydroxide-modified biochar for efficient nitrate adsorption and potential...

Nitrate (NO₃^-) is one of the major forms of nitrogen element and an essential nutrient for maintenance of crop yield and quality. Suboptimal nitrate use efficiency of traditional fertilizers necessitates the effective retention and slow-release capability, which is crucial for sustainable agriculture and cost saving. Here, the MgFe-layered double hydroxide (MgFe-LDH) assembled onto corn stalk derived biochar (MgFe-LDH@BC) composites were synthesized to serve as an effective slow-release carrier for nitrate. Batch microscopic characterizations demonstrated a nano-sized lamellar structure successfully formed onto the intricate pore skeleton of biochar (BC) with large specific surface area, abundant defect structure and good thermal stability. MgFe-LDH@BC exhibited a remarkable nitrate adsorption capacity of 41.72 mg/g. Anion exchange and electrostatic attraction dominated the adsorption process. After nitrate introducing, MgFe-LDH@BC could continuously release nitrate into water and reach a slow-release performance of 70.96 % within 144 h. The cumulative release rate and release span of nitrate-loaded MgFe-LDH@BC were profoundly influenced by temperature, pH of the aqueous solution and moisture content in soil. It improved the soil nitrate concentration by 198 % (30 % moisture) and 224 % (80 % moisture) relative to the control. Artificial neural network (ANN) model with higher R² values (>0.97) was established and applied to predict the release data. Furthermore, the main framework and crystalline structure of MgFe-LDH@BC remained largely intact after successive steps of nitrate loading, restoring and releasing over a period of 6 days. These results illustrate MgFe-LDH@BC may work as a slow-release carrier for minimizing the nitrate loss to the environment.