Lanthanum activated palygorskite for selective phosphate separation from aqueous media: Comprehensive understanding of adsorptive behavior and mechanism affected by interfering substances

Abstract

Eutrophication problem in surface waterbodies has attracted increasing attention. Herein, raw palygorskite (PAL) was activated by lanthanum engineering in alkaline surrounding for phosphate removal, and alkaline etching modified its structure and facilitated phosphate migration from bulk solution to active sites. Results indicated that stable and satisfactory phosphate capturing was maintained by as-prepared material within a wide pH range (3–9) and the disturbance of co-existing substances (Cl-, NO3–, HCO3–, SO42–, F- and HA). Further investigation on the role of interfering species in overall adsorption process indicated that phosphate capturing was accelerated rather than hindered at the initial hours, while the ultimate captured amount decreased owing to their competitive occupation on La active sites. Kinetic study suggested the chemisorption of phosphate and isotherm investigation illustrated the multilayer coverage of phosphate on the heterogeneous surface. Thermodynamic study implied spontaneous and endothermic process of phosphate uptake on La-PAL, and cyclic desorption-adsorption experiments demonstrated feasible regeneration and reuse of La-PAL for phosphate separation. Materials characterization demonstrated the formation of amorphous La-O-P inner-sphere complex through ligands exchange (i.e., –CO3–, –OH), whereas the interference of co-existing substances by competitive occupation and overlap on active sites was indicated. The present work could provide comprehensive insight into the role of interfering substances in phosphate adsorption on functionalized materials.