Synthesis and phosphate removal performance of La₂O₂CO₃ modified biochar materials

La-CB（Lanthanide-based composite biochar） materials were made by symbiotic co-heating of mugwort biomass and lanthanum solution for the purpose of efficiently removing excess phosphate in wastewater, and according to the effects of different concentrations of LaCl₃ on the adsorption performance of La-CB materials, it was found that La-CB2 materials with medium lanthanum concentrations were suitable for the adsorption study of phosphoric acid. Microscopic analysis used X-ray diffraction (XRD) to analyze the crystal structure of La-CB2 and its phosphorus saturated adsorption (La-CB2-P), and found that a large of La₂O₂CO₃ crystals were formed on the surface of La-CB2, and LaPO₃ crystals were formed after phosphate adsorption. The surface morphology of La-CB and La-CB2 was characterized by transmission electron microscopy SEM-EDS automatic analyzer and analyzed their morphology and size, and it was found that after the modification of lanthanum carbonate, a large of lamellar structures were formed on the surface of mugwort biomass, and there were large nanoscale spherical protrusions on the surface of the sheet structure. Fourier transform infrared spectroscopy was used to characterize the surface functional group properties of materials, and it was proved that La-CB2 formed LaPO₃ by chemical bonding to remove phosphate in water. The effects of material dosage, initial phosphate concentration, adsorption time, pH of initial solution and coexisting ions on the adsorption performance of La-CB2 on the adsorption of phosphate and the germination rate of Lettuce seeds were experimented with the effects of La-CB2-P on the germination rate of lettuce seeds, and the isothermal adsorption curves were fitted by the Langmuir model, Freundlich model and Temkin model, and the kinetic adsorption characteristics were fitted by quasi-primary kinetics, quasi-secondary kinetics and Elovich model. The experimental results show that La-CB2 can efficiently adsorb phosphate at low dosage, the optimal dosage is 1 g/L, the optimal adsorption temperature for phosphate is 25 ℃, the adsorption in the environment of 15 and 35 ℃ is mainly monolayer adsorption, and the re-adsorption phenomenon of the adsorbed molecules at 25 ℃ is transformed into multi-layer adsorption, the adsorption method is mainly chemical adsorption and physical adsorption, and the maximum adsorption capacity simulated by the Langmuir model is 126.82 mg/g. The adsorption equilibrium can be reached within 12 h, and the simulation of the adsorption kinetic model shows the model fit Elovich model > quasi second order model > quasi first order model, which once again proves that La-CB2 is mainly based on chemical adsorption of phosphate, and La-CB2 has the strongest adsorption performance for phosphate in the environment of pH value is 3, and the adsorption performance of La-CB2 is better in acidic to neutral water environment, and the pH value from 3 to 8 could maintain above 85%, and has a buffering effect on the pH of alkaline water environment, the Leaching rate of La³⁺ is 7.6% at pH value 3, and only about 0.029% under other pH conditions, and the DOC dissolution amount of La-CB2 increases and then decreases with the increase of the pH value of the initial solution during phosphorus adsorption. La-CB2 material in different concentrations of CO₃²⁻, HCO₃⁻, SO₄²⁻, NO₃⁻ 4 anionic solutions can maintain strong selective adsorption of phosphate, in humic acid (HA), fulvic acid (FA), sodium acetate (SA), sodium dodecyl sulfate (SDS) and other organic solutions, humic acid can effectively reduce the adsorption performance of La-CB2 on phosphate, the higher the concentration, the stronger the inhibition, but the effect of other organic substances is small. After phosphorus adsorption saturation, La-CB2-P can be used as phosphorus slow-release fertilizer to significantly promote the germination of lettuce, which provides a large theoretical basis for phosphorus adsorption and resource recovery of La-CB2 in nutrient-rich water.