Adsorption Of Rare Earth Ions On Prussian Blue Colloidal Nanoparticles And The Applications In Low Concentration Rare Earth Recoverying And T₁-weighted Magnetic Resonance Imaging

Prussian blue has excellent performance and it is widely used in many fields, such as electrochemical catalysis, biological sensors, molecule magnetic and hydrogen storage materials. In particular, Prussian blue colloid nanoparticles (PB-CNPs) have excellent adsorbability and dispersion stability due to their high surface energy and porous structure, and can trap with exchangeable hydrate potassium ions. In present study, PB-CNPs were synthesized by a conventional precipitation method and employed as absorbents to adsorbing rare earth ions from low concentration rare earth aquous solution. Furthermore, a novel T1-weighted MRI contrast reagent, Gd3+-loaded PB-CNPs, was prepared thereafter.Firstly, PB-CNPs were prepared by a simple synthetic procedure with which FeCl3.6H2O and K4Fe(CN)6were separately dissolved in aqueous solution at room temperature and mixed under stirring with a molar ratio of1:1.05to1:1.2. PB-CNPs were formed immediaetly and then isolated by adding acetone. After filtrating, washing with acetone and drying in air, PB-CNPs with particle size in the range of10～100nm with D5030～35nm were obtained. These particles can well disperse into water to form colloidal solution with high stability within pH ranging from3to7.5due to their high negative zeta potential,-60mV.Secondly, the as-synhtesized PB-CNPs were employed to adsorb rare earth ions from aqueous solution with the help of a dialysis bag because PB-CNPs do not pass through membrance but simple ions do. When a dialysis bag with PB-CNPs contacted with solution containing rare earth ions, the adsorption was carried out, and the amount of rare earth ions adsorbed onto PB-CNPs were calculated by determing the concentration of rare earth ions in equibrium aqueous solution. With this method, the influence of adsorption time and temperature, solution pH and salt (NaCl) concentration as well as the initial concentration of rare earth ions (co) on the adsorption amount were investigated. The results showed that the as-synthesized PB-CNPs show strong adsorptivity towards rare earth ions in aqueous solution with pH ranging from4to7. The equilibrium data of PB-CNPs adsorbing rare earth ions at selected test temperatures (303,323,343K) follow well with Langmuir isotherm equation. Correspondingly, the Langmuir adsorption constant (KL, L/μmol) and the theoretical maximum adsorption capacity (qm,μmol/mg) for15rare earth elements were obtained and used to calculate thethermodynamic parameters, AHθ, ASθ, AGθ. These data showed that the adsorption of rare earth ions by PB-CNPs is a spontaneous process with enthalpy decreasing and entropy increasing. The KL values for adsorbing different rare earth elements decreased with the increase of lanthanide element atomic number (AN). Two smooth curves intersecting between Sm and Nd are observed in the curve of KL via AN, indicating the adsorbed ions are hydrated and the PB-CNPs show stronger adsorption ability towards light rare earths than heavy rare earths. Furthermore, with the KL values and adsorption capacity data at different adsorbing time, the kinetics of PB-CNPs adsorbing rare earth ions was determined to be followed a pseudo-second order equation. In addition, the experimental data showed that the adsorbed rare earth ions on PB-CNPs can be desorbed by acidic aqueous solutions with pH<2as well as solutions containing salts with high concentration, indicating the adsorbed rare earth ions can be recovered or enriched by a simple ion exchange method.Finally,Gd3+-loaded PB-CNPs were prepared as a potential Ti-weighted MRI contrast reagent because Gd3+can speed up the NMR relaxation rate without causing a significant paramagnetic shift due to its seven unpaired electrons and long electronic relaxation time. Therefore, the effects of weight ration of Gd3+to PB-CNPs, solution pH on the paiticle size and zeta potential were examined and the relaxation efficiency (R1) were determined. It was found that the R1values and particle sizes are increased with the increase of Gd3+loaded amount on PB-CNPs with high dispersion stability, and the optimal Gd3+loaded amount was determined to be ranging from8%to10%. The enhanced contrast effect for T1-weighted MRI by Gd3+-loaded PB-CNPs was explained by the surface adsorption of hydrate gadolinium ions which provide more coordination water molecules for enhancing the relaxation process of protons.