Characterization of La–Mg-modified palygorskite and its adsorption of phosphate

To further improve the adsorption capacity of palygorskite, this study first prepared lanthanum–magnesium (La–Mg)-modified palygorskite (LMPA) via co-precipitation and heat treatment, and then characterized its properties and adsorption capability for phosphate. The results showed that the LMPA composite was successfully synthesized at a La:Mg molar ratio of 1:3 by co-precipitation and heat treatment at 700 ℃, and exhibited an excellent adsorption capacity for phosphate and good reusability. On the surface of the LMPA composite, La and Mg were well dispersed and the active sites were evenly distributed. The adsorption of phosphate onto the LMPA composite tended to be a nonlinear process and was mainly through surface complexation, in which metal-bound hydroxyl and metal oxide groups played important roles. The pH of the point of zero charge of the composite was 10.1; therefore, the electrostatic attraction of phosphate was promoted through the protonation of the composite's surface at pH < 10. The process of phosphate adsorption by the composite was spontaneous, endothermic, and entropy-increasing. The adsorption of phosphate onto the LMPA composite required a temperature of 25 °C and was influenced by anions, which could form precipitates with lower solubility products than LaPO4 and Mg3(PO4)2 with La and Mg (e.g., HCO- 3). An LMPA dosage of 1.0 g/L could maximize the phosphate equilibrium adsorption capacity. The as-prepared LMPA performed well for phosphate adsorption in real wastewater; however, for practical applications the cost needs to be reduced by optimizing the preparation process, determining its suitability for large-scale production, and selecting appropriate treatment equipment and processes.