Simultaneous adsorption of ammonia and phosphate by magnesium-doped biochar via different preparation methods: Structure-function relationship

The effects of different preparation methods of magnesium-doped biochar (MgBC) on their physicochemical properties and adsorption performance have always been a research hotspot. MgBC has shown significant potential to simultaneous removal of ammonia (N) and phosphate (P) from wastewater, while the adsorption mechanism remained unclear due to the structure-function relationships in different MgBC. Herein, MgBC prepared via one-step synthesis, pyrolysis-pyrolysis, and hydrolysis-pyrolysis pathways were utilized as typical MgBC adsorbents. Results showed that the MgBC prepared by the hydrolysis-pyrolysis method (MgBC-HT) exhibited large specific surface area (277.26 m 2 /g), well-developed pore structure (pore volume increased by 17.8 times compared to the control group), and excellent uniform dispersion of MgO. Its maximum adsorption capacities for N and P were 6.52 mmol/g and 7.81 mmol/g, respectively. On the contrary, the MgBC prepared via one-step pyrolysis (MgBC-1) and two-step pyrolysis (MgBC-2) exhibited maximum adsorption capacities of 2.53 mmol/g and 4.86 mmol/g for N, and 2.84 mmol/g and 5.67 mmol/g for P, respectively. Quantitative analysis via biochar characterization and batch experiments further explored the adsorption pathways of different MgBC in N and P simultaneous removal. The results showed that struvite precipitation made the highest contribution to the simultaneous adsorption of N and P by MgBC-HT, with approximately 62.3% of N and 61.6% of P removed via this mechanism, while the other mechanisms (hydrogen bonding, surface complexation, and electrostatic interaction) decreased in that order. These findings highlighted the significance of the preparation method for MgBC and provide guidance for further optimization of biochar in the future.