Lanthanum-doped ordered mesoporous hollow silica spheres as novel adsorbents for efficient phosphate removal

Abstract

Lanthanum-doped ordered mesoporous hollow silica spheres were fabricated and utilized as phosphate adsorbents for the first time. Our prepared adsorbents exhibited spherical shapes with macropores in the center and ordered 2D hexagonal mesopores in the shell. By increasing the actual loading of La from 4.19 wt% to 22.44 wt%, the BET surface areas of the adsorbents decreased from 987.48 m² g⁻¹ to 420.38 m² g⁻¹, coupled with a decline in average pore diameters and total pore volumes. In the batch adsorption tests, the adsorbents with greater amounts of La loading possessed markedly enhanced adsorption capacities; in contrast, the un-doped pure ordered mesoporous hollow silica particles could hardly adsorb any phosphate in solution. The adsorption isotherms of the La-doped ordered mesoporous hollow silica spheres were represented better by using the Langmuir model than the Freundlich model. The sample with 22.44 wt% La loading exhibited a maximum phosphate capture capacity of 47.89 mg P g⁻¹. In the kinetic study, its phosphate adsorption followed the pseudo-second-order model well. Moreover, a dramatically fast kinetic adsorption was observed in the solution with a low phosphorus concentration of 2 mg P L⁻¹; the adsorption equilibrium was reached in 15 min, with a 99.71% phosphate removal rate. High adsorption capacities were achieved within the pH range 3.0 to 8.0; relatively high selectivity to phosphate was observed in the presence of coexisting anions.