Low-cost Ca/Mg co-modified biochar for effective phosphorus recovery: Adsorption mechanisms, resourceful utilization, and life cycle assessment

Effective recovery of phosphorus from wastewater and reutilization are important for controlling water eutrophication and achieving resourceful utilization of pollutants. In this study, a low-cost, eco-friendly, and highly efficient Ca/Mg co-modified coffee grounds biochar (CMBC) was synthesized in one step for phosphorus adsorption, and the potential application of the phosphorus absorbed CMBC (CMBC-P) as a fertilizer was also explored. The results showed that the surface of CMBC was covered by granular calcium/magnesium oxides with a rougher layered stacking structure. The adsorption capacity of CMBC for phosphorus has been significantly improved compared with that Ca or Mg modification, with the maximum adsorption capacity reached 144.31 mg/g. The mechanisms primarily involve surface precipitation, complexation, ligand exchange, and electrostatic attraction. Density functional theory calculation revealed that the Ca-based active sites were the main binding sites (-6.58 and -6.41 Kcal/mol) for phosphorus compared with that of the Mg (-0.89 Kcal/mol). Natural aging experiment showed that approximately 74.87 % of the adsorbed phosphorus can be released to soil within 60 days and the soil available phosphorus also increased significantly. The CMBC-P also significantly enhanced the germination and growth of plants. Moreover, life cycle assessment revealed that the production of CMBC emitted only 8.38 kg CO2 eq, and a more environmentally friendly approach was also proposed. Cost benefit analysis confirmed the exceptional cost-effectiveness (15.15 g P/$) of CMBC among various adsorbents. Overall, CMBC demonstrated excellent potential for removal phosphorus from water and further use as a phosphorus fertilizer.