Eco-Friendly Modified Biochar for Phosphate Removal: Adsorption Mechanism and Application in Pressed Vegetable Wastewater Treatment

Phosphorus in wastewater from pressed vegetables is a significant contributor to water pollution, emphasizing the importance of its removal and recycling for ecological management. In this study, an improved method of coprecipitation pyrolysis of Mg(OH)2 and FeCl3•6H2O was used to successfully synthesize iron-magnesium biochar composite (FeMg@BC2) from corn cob. Compared with iron-modified biochar (Fe@BC), magnesium-modified biochar (Mg@BC), and iron–magnesium-modified biochar (FeMg@BC1) prepared by traditional coprecipitation methods, the improved iron–magnesium biochar had higher yield, specific surface area, and crystallinity. The study investigated the impact of modified biochar dosage, initial solution pH, and coexisting ions on the adsorption capabilities of modified biochar for phosphate removal. The results demonstrated that the addition of 1.0 g/L FeMg@BC2 was highly effective in removing phosphate from simulated wastewater when the phosphate concentration was 80 mg/L, achieving a removal rate exceeding 95%. Using an adsorption isotherm model, the maximum phosphate adsorption capacities of Fe@BC, Mg@BC, FeMg@BC1, and FeMg@BC2 were estimated to be 40.76, 46.97, 96.78, and 107.97 mg/g, respectively. Particularly, FeMg@BC2 exhibited superior phosphate adsorption capacity, and its adsorption mechanism mainly included electrostatic attraction, surface precipitation, and ligand exchange. The desorption test of phosphorus-loaded modified biochar revealed that the desorption rates of FeMg@BC2 from simulated wastewater and pressed vegetable wastewater using a 0.5M NaOH solution were 92.2% and 84.8%, respectively. After three cycles of adsorption in pressed vegetable wastewater, the phosphorus removal efficiency of FeMg@BC2 at 1.0 g/L remained about 50%. Therefore, the utilization of FeMg@BC2 for phosphorus recovery from pressed vegetable wastewater showed promising potential.