Kinetics and equilibrium adsorption of phosphate on lanthanum oxide supported on activated carbon

Abstract

Understanding the thermodynamics and kinetics of phosphate adsorption is crucial in the design of packed-bed columns for phosphate removal, particularly in clinical applications such as dialysis, in which limited contact time is available. Studies have shown that the kinetics of phosphate adsorption on metal oxides is a two-step process: rapid initial adsorption that occurs in a few minutes to a few hours is followed by slower adsorption that may last for days. In this work, the thermodynamics and kinetics of phosphate adsorption on lanthanum oxide impregnated on activated carbon (La-AC) are investigated to explain its phosphate adsorption behavior. The phosphate adsorption mechanism was investigated by studying (1) the effects of temperature on phosphate adsorption capacity, (2) the dynamics and enthalpy of adsorption using flow microcalorimetry (FMC), (3) the kinetics of phosphate adsorption, and (4) the relationship between the amount of phosphate adsorbed and the amount of hydroxide released. Phosphate adsorption on La-AC followed the Langmuir isotherm. FMC results indicate that phosphate adsorbed through several mechanisms, principally through the formation of monodentate and bidentate inner-sphere surface complexes. A pseudo-second-order kinetic model is typically used to model phosphate adsorption; however, it was found that a kinetic model developed based on the assumption of two parallel surface adsorption reactions fits the measured adsorption kinetics significantly better than the pseudo-second order kinetic model that is almost universally used to describe phosphate adsorption kinetics. • Lanthanum-impregnated activated carbon was investigated as a phosphate adsorbent. • Phosphate adsorption was well described by the Langmuir isotherm. • Phosphate anions adsorbed on lanthanum surface via a parallel path mechanism. • A kinetic model based on the parallel path mechanism was developed. • Parallel path model fit kinetics much better than the pseudo-2nd-order model.