Characteristics of nitrogen and phosphorus adsorption by Mg-loaded biochar from different feedstocks

Herein, biochars from 6 different feedstocks (taro straw, corn straw, cassava straw, Chinese fir straw, banana straw, and Camellia oleifera shell) were produced using magnesium chloride (MgCl₂) as a modifier due to their sorption behavior toward NH₄⁺-N and phosphorus in an aqueous solution. The biochar characteristics were evaluated, including pH, pH_PZC, biochar magnesium content, and total pore volume (PV_tot). The experimental results in terms of the kinetics and equilibrium isotherms showed that the cassava straw and banana straw biochars exhibited the theoretical maximum saturated adsorption capacities of 24.04 mg·g^-1 (NH₄⁺-N) and 31.15 mg·g^-1 (TP), respectively. Biochar produced from these feedstocks had higher magnesium contents and greater total pore volumes, reflecting the significant contributions from magnesium and steric effects. FTIR, XRD, and SEM/EDS analyses demonstrated that NH₄⁺-N and TP sorption mechanisms predominantly involved surface electrostatic attraction, Mg²⁺ precipitates and complexation with surface hydroxyl functional groups.