Synthesis and characterization of magnetite nanoparticles functionalized with organophosphorus compounds and its application as an adsorbent for La (III), Nd (III) and Pr (III) ions from aqueous solutions

Abstract In this communication, adsorbents based on functionalized magnetite nanoparticles were synthetized, characterized and evaluated as adsorbents for the uptake of La (III), Pr(III) and Nd(III) from aqueous solutions. The functionalization of the magnetite nanoparticles was conducted by coating them in the first step with oleate molecules through a chemisorption process, resulting in a hydrophobic nanomaterial. In the second stage, the hydrophobic material was coated with the organophosphorus extractant CYANEX 272, CYANEX 301 or D2EHPA, resulting in three hydrophilic adsorbent magnetic materials. The structures and surface features of these adsorbents were determined by high-resolution transmission electronic microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analyses, FT-IR, zeta-potential and magnetism techniques. The functionalized magnetite nanoparticles tend to exhibit a spherical morphology averaging a diameter of approximately 7 nm. Additionally, they would be constituted by a core-shell composite, where the core is made of magnetite and the shell is formed by chemisorbed oleate molecules followed by physisorbed organophosphorus extractant layers. Moreover, these functionalized magnetite nanoparticles present a superparamagnetic behavior and have sufficient magnetic saturation values to respond efficiently to an external magnetic field. The favorable adsorption of the three studied lanthanide metallic ions present as free Ln 3+ species was observed in an aqueous solution at a pH of 4, which is consistent with the fact that the functionalized nanoparticles are negatively charged at this pH. All of the functionalized magnetite nanoparticles showed fast adsorption rates, with only 5 to 10 min necessary for reaching adsorption equilibrium. Maximum loading capacities between 5 and 9 mg Ln /g MNP were measured for the three lanthanide ions. However, a slightly better adsorption was seen for La and Pr ions. Likewise, the magnetic nanoparticles showed a slightly higher maximum adsorption capacity when they were functionalized with D2EHPA and CYANEX 301 compared to CYANEX 272. From the discussed results, it is possible to postulate a cooperative-type adsorption mechanism combining chemisorption and physisorption in order to interpret the global adsorption process.