Removal of ammonia nitrogen and phosphate from livestock wastewater by magnesite tailings modified biochar: performance and mechanisms

Currently, magnesium-modified biochar gained significant attention as an adsorbent for removing nitrogen and phosphorus from aquatic environments. In this study, magnesite tailings were innovatively used as magnesium modifiers and rice straw was used as carbon sources, respectively, to prepare a low-cost and efficient magnesia tailing-modified biochar. Adsorption experiments demonstrated that the maximum adsorption capacities for NH₄⁺ and PO₄3⁻ were 138.39 mg/g and 279.42 mg/g, respectively. More importantly, the adsorption performance and removal efficiency for NH₄⁺ and PO₄3⁻ remained stable over a pH range of 3–10. The influences of common anions on adsorption performance were also relatively small, indicating good adsorption capacity and stability of this modified biochar. Characterization analysis, adsorption kinetics models, and isotherm models revealed that the primary mechanisms for NH₄⁺ and PO₄3⁻ adsorption were struvite precipitation, ion exchange, electrostatic attraction, and diffusion. Moreover, the removal efficiencies of NH₄⁺ and PO₄3⁻ from livestock wastewater reached 97.5% and 99.6%, respectively. These results suggested that magnesia tailing-modified biochar exhibited several advantages such as low-cost raw materials, simple preparation, and excellent adsorption performance.