Magnesium modified algae biochar for phosphorus adsorption: Synthesis, experimental analysis, DFT calculations and regeneration

Abstract

Excessive phosphorus is one of the primary causes of water eutrophication, and adsorption is an effective technology for the reduction of phosphorus concentration. Herein, the magnesium modified algae biochar (Mg@ABB) was successfully prepared by impregnation-pyrolysis process, and the removal efficiency could reach 100 %, accompanied by a unit adsorption capacity of 39.20 mg-P/g under specific conditions: Mg@ABB concentration of 0.1 g/L, an initial solution pH of 8, a surrounding temperature of 25 °C, and an initial phosphorus concentration of 50 mg/L. The adsorption process adhered to the pseudo-first-order kinetic model and Sips equation model, indicating that physisorption controlled the rate of phosphorus adsorption and the adsorption sites on Mg@ABB are heterogeneously distributed. The adsorption thermodynamics demonstrated that the adsorption of phosphate onto Mg@ABB was reversible, and an increase in temperature was unfavorable for the adsorption process. BET, FT-IR and XPS demonstrated that Mg@ABB contained a porous structure and rich functional groups, and magnesium were detected on the surface as the forms of MgO, MgCl₂, and Mg(OH)₂. Density functional theory (DFT) calculations indicated that MgO primarily reacted with phosphorus, and all magnesium compounds exhibited lower binding energies with H₂PO₄⁻, thus suggesting that phosphorus desorption was theoretically more favorable under acidic conditions. In cyclic experiments, after three times of “adsorption-desorption” cycle, the removal efficiency of phosphorus remained above 70 %.