Ree Adsorption Onto Syntheticδ-MnO₂in Simulated Seawater

With the rapid developments of the technology and the economy, the resourcedemands grow with each passing day, and the resource on the land are scarce, theworld pay more and more attention on the ocean. The ocean ferromanganese nodulecontains many useful elements, which attracted many scholars’ attention.Ferromanganese nodule is a kind of iron and manganese oxides or hydroxides. It iswidely distributed in depth of4000-6000meters ocean bottom surface, about15%ofthe ocean area is covered with ferromanganese nodule. Ferromanganese nodule is oneof the most important deep-sea mineral resources. Studies show that ferromanganesenodule has the highest content of rare earth elements （REE）, which is several timesthan the normal sediments.Compared with the land resources, ocean ferromanganese nodule is a veryimportant and special mineral. It’s very difficult to extract pure minerals, which bringthe great difficulty to study. This research, under the laboratory conditions,synthesized δ-MnO₂, and takes it as a naturalδ-MnO₂substitutes for the adsorption ofREE, studies on adsorption of REE and finds out the mechanism of REE adsorptionon ferromanganese nodules. As a result of the special mineralogy properties offerromanganese nodules, making the ferromanganese nodules in the environment hasthe broad prospect of application. So the study of the mechanism of the REEadsorption on the ferromanganese can help us understand of ferromanganese nodulesbetter,lay the foundation for future research, and provide theoretical basis for theapplication in future.This study uses synthetic δ-MnO₂adsorb REE in simulate seawater, and does theadsorption kinetics experiment and isothermal adsorption experiment, obtains thethermodynamic parameters, it also studies the effects of pH, temperature and saltconcentration on adsorption, get the following conclusions:1.After several experiments, synthetic δ-MnO₂obtained in laboratory is notcrystalline, but in amorphous form. There are some studies show that, the δ-MnO₂in ferromanganese nodules exists in amorphous form, and REE has large ionic radius,which makes it do not fitinto the manganese mineral lattice, prior to bind byamorphous δ-MnO₂. The δ-MnO₂is main mineral in ferromanganese nodules, and theδ-MnO₂occurrence with this amorphous colloidal form. In conclusion, the authorthinks that using the amorphous δ-MnO₂adsorb REE can simulate the adsorption ofREE on ferromanganese nodules better, help us to find out its adsorption mechanism.2. The adsorption kinetic study indicated that, the adsorption process can bedivided into two phases: former30min fast phase and slow phase, a slow phaseprobably need around120min. So the equilibrium time of adsorption is120min.Second-order kinetic equation’s correlation coefficient is more than0.99, First-orderkinetic equation’s correlation coefficients are all above0.88. They can well describeadsorption process.3. The adsorption isotherms conform to Freundlich isotherm, correlationcoefficients are all above0.86. Isotherm cannot fit to Langmuir model.4. The thermodynamic data shows that the adsorption is a physical process whichis spontaneous, exothermic, entropy increasing. The adsorption process containsphysical adsorption and chemical adsorption, the mechanism is complex, but mainlyphysical.5. In this experiment,Δ G°range-15.59～-7.42kJ/mol,ΔH°absolute values inthe range of10.47-16.42kJ/mol,Δ S°<0.The thermodynamic data showed that theadsorption is a spontaneous, exothermic and entropy-reducing process. Theadsorption process includes physical adsorption and chemical adsorption of two parts,the mechanism is very complex, but mainly physical adsorption.6. As pH increases, the adsorption capacity increases slightly. The mechanism ofpH effect on adsorption is more complex, but the pH effect in this experiment is notsignificant. With the increase of temperature, the adsorption capacity is reduced, butthe difference of adsorption capacity under two temperature is not obvious. Saltconcentration increase, the adsorption capacity decline.