Ce(III) nanocomposites by partial thermal decomposition of Ce-MOF for effective phosphate adsorption in a wide pH range

A series of hierarchical micro/nano Ce-based composites were derived from Ce-MOF via thermal treatment in N2 atmosphere. Different from conventional complete decomposed materials, forming cerium oxide in air, Ce-MOF that calcinated in N2 at lower temperatures (400 °C or 500 °C) showed a partial thermal decomposition with high percent content of Ce(III). Even though the complete decomposed products held higher surface areas, the partial decomposed samples exhibited extremely higher phosphate uptake, with working capacity 2–4 times higher than that of ceria. The results implied a predominant effect of different valence states on phosphate removal by Ce-based materials, in which Ce(III) species were demonstrated playing the major role to form binding with phosphate. The maximum adsorption capacity (189.4 mg/g) was achieved by Ce-MOF-500(S) with wide applicable scope of pH ranging from 2 to 12 and great selectivity for phosphate in the presence of competing anions. Remarkably, Ce-MOF-500(S) described obvious enhanced phosphate adsorption ability under alkaline condition. This was due to the fact that the hydrolyzed Ce(III) species brought more active sites in the form of hydroxyl groups for ligand exchange with phosphate. Furthermore, based on the analysis of FTIR, XPS, XRD and zeta potential, electrostatic attraction, ligand exchange and surface precipitation were confirmed as the main adsorption mechanisms for partial decomposed samples, while electrostatic attraction was the main mechanism for complete decomposed samples.