الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Rare earth elements (REEs) consist of 17 elements of the periodic table including 15 lanthanides along with yttrium and scandium. They are further subdivided into light and heavy rare earth elements on the basis of their atomic number. REEs have unique properties and often termed as ”seeds of technology”. They are widely used in different applications such as metallurgy, electronics, alloys, lasers, magnets, superconductors, catalysis, chemical reagents and nuclear energy. With ever–increasing demand for high–purity REEs, separation of REEs from naturally occurring ores and other mixtures have gained a considerable attention of many scientists all over the world. Few reports were detected dealing with separation of REEs from their aqueous solutions using magnetic hydroxyapatite composites. Therefore, the efforts in this work were directed to synthesize hybrid magnetic hydroxyapatite composites by a co–precipitation method. The synthesized sorbents were characterized using different tools. The potential application of these sorbents in solid phase extraction of rare earth elements; Sm(III), Eu(III) and Tb(III) from their aqueous solutions was evaluated under different batch experimental conditions. Modeling the equilibrium sorption data using different models was carried out with determining the sorption kinetics, isotherms and thermodynamics parameters governing the applied system. The chromatographic separation of Eu(III) and Tb(III) from the magnetic composite (CaHAP/NF) packed column was studied using different eluents. The thesis consists of three chapters summarized as following:
1. Introduction comprises characteristics and different applications of lanthanides. It comprehensively outlines a brief review on extraction and separation of lanthanides from naturally occurring ores and other mixtures using different techniques. The chapter also includes some information about nano–materials as well as the rare earth elements; samarium, europium and terbium. Introduction is ended with a literature survey on separation of Sm(III), Eu(III) and Tb(III) from aqueous solutions using magnetic hydroxyapatite composites.
2. Experimental contains a clear description of chemicals, reagents and apparatus utilized in this work. It comprises preparation of sorbents and radiotracers. The chapter includes a brief description of different conventional techniques that used for characterization of the synthesized sorbents. Experimental also involves a detailed description of both batch and column experiments for sorption of of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents.
3. Results and discussion comprises a discussion of the results obtained from experimental work. The discussion covers: 1) characterization of synthesized sorbents, 2) sorption behavior of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents, 3) modeling the equilibrium sorption data and 4) the chromatographic separation of Eu(III) and Tb(III) from the magnetic composite (CaHAP/NF) packed column.
The physico–chemical characterization of synthesized sorbents in the present work was performed using (BET) surface area measurements, (FT–IR) spectroscopy, X–Ray diffraction, TG–DTA analysis, scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM).
The magnetic hydroxyapatite composites CaHAP/NF, NF/CaHAP and CaHAP–NF have higher values of SBET, VP and DP compared with that of their precursors NF and CaHAP. These properties announce the promising surface characteristics of the magnetic hydroxyapatite composites to be used as novel sorbents in removal of different metal ions. The FT–IR spectroscopic analysis of synthesized sorbents showed that CaHAP/NF, NF/CaHAP and CaHAP–NF exhibited all characteristic peaks of both CaHAP and NF. This strongly confirmed the formation of magnetic hydroxyapatite composites having a variety of surface functionality. XRD patterns of CaHAP–NF, NF/CaHAP, CaHAP/NF magnetic composites exhibited two kinds of diffraction peaks including characteristic peaks of CaHAP and NF. The average crystallite size of synthesized sorbents was estimated from XRD patterns using Scherrer’s equation and the values demonstrated the nano–sized structure of synthesized sorbents. TG–DTA analysis showed weight loss of synthesized sorbents accompanied with endothermic peaks with rising temperature from ambient temperature upto approximately 200 oC due to the release of surface water molecules. While at higher temperatures, the synthesized sorbents remained stable upto 800 oC. Scanning electron microscope (SEM) images of NF showed irregular shapes have rough and porous surface, while that of CaHAP exhibited tiny and asymmetric agglomerates with rough and porous surface. The incorporation of NF with CaHAP structures changed the surface morphology of these structures. The images of CaHAP/NF, NF/CaHAP and CaHAP–NF magnetic composites exhibited symmetric needle and rode like structures on highly rough and porous surface. The magnetic properties of the synthesized magnetic composites were measured using vibrating sample magnetometer (VSM) and the results clarified that the magnetic composites are superparamagnetic materials. They interact with any external applied magnetic field without attaining any magnetism after field removal. Thus, these superparamagnetic particles could be separated from a reaction mixture using an external magnet left behind a clear solution. Also, they did not agglomerate after removal of the external magnetic field. This highlights the high magnetic sensitivity of the synthesized sorbents that is considered as an important feature for the wide applications of such magnetic materials in different fields.
The chemical stability of the synthesized sorbents was investigated under the action of different pH values. The results clearly showed that the sorbents were partly dissoluted when pH value was lower than 3 and completely dissociated in high acidic medium. On contrast, the sorbents were highly stable at pH ≥ 3. Based on these findings, the sorption of studied metal ions on synthesized sorbents was studied in pH higher than 3.
Sorption of Sm(III), Eu(III) and Tb(III) ions onto synthesized sorbents was performed under different experimental conditions to clarify the main factors controlling the sorption process and optimize the separation conditions. Most of Sm(III), Eu(III) and Tb(III) ions were adsorbed on synthesized sorbents after 6 h of contact. Further increase in contact time beyond 6 h up to 48 h did not show any remarkable effects in uptake of Sm(III), Eu(III) and Tb(III) ions which remained almost constant. Accordingly, the equilibrium time was fixed at 6 h for the rest of batch experiments, where all synthesized sorbents except NF have a high affinity towards studied metal ions. Sorption of Sm(III), Eu(III) and Tb(III) ions onto synthesized sorbents was studied at different pH values ranged from 3 to 6 and results illustrated that Sm(III) started to precipitate at pH ~ 4.2, while Eu(III) and Tb(III) precipitation had obviously initiated at pH ~ 3.7. The synthesized sorbents exhibited a high sorption affinity towards Sm(III), Eu(III) and Tb(III) at pH 3.5 that is far from the precipitation limit of studied metal ions and keeps the applied sorbents away from dissolution. Hence, pH 3.5 was chosen as the optimal pH in all experiments of Sm(III), Eu(III) and Tb(III) sorption. The synthesized sorbents exhibited a higher maximum sorption capacity towards Sm(III), Eu(III) and Tb(III) ions compared with other sorbents reported in previous studies. This avouched that the synthesized sorbents are promising candidates can play a vital role in separation and purification of lanthanide elements. The weight of synthesized sorbents that was sufficient for the quantitative removal of studied metal ions was 0.01 g.
The ionic strength had a slight influence on uptake of Sm(III), Eu(III) and Tb(III) onto NF, CaHAP, CaHAP/NF, NF/CaHAP and CaHAP–NF using NaClO4 as a background electrolyte. This confirmed the specific tendency of these sorbents towards studied metal ions and suggested that the sorption of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents was mainly dominated by inner–sphere surface complexation. Retention of Sm(III), Eu(III) and Tb(III) onto NF, CaHAP, CaHAP/NF, NF/CaHAP and CaHAP–NF surfaces was accompanied with a release of detectable concentrations of both Ca(II) and Ni(II) from sorbents to reaction solution without releasing of any amounts of P(III) and Fe(III). This supported the ion exchange as a main mechanism participating in Sm(III), Eu(III) and Tb(III) separation beside other mechanisms. The uptake percent of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents decreased with increasing the initial concentrations of studied metal ions from 20–100 mg L–1. The uptake percent of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents slightly increased with increasing system temperature from 30–60 °C indicating an endothermic nature of sorption process, where a better sorption was achieved at high temperatures. The effect of coexistence of different ions (such as: Cs+, Co2+ and Fe3+) on sorption of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents was investigated at different concentrations of the coexisting ions ranged from 0.0001 – 0.1 mol L–1. The coexistence of Cs+ ions with Sm(III), Eu(III) and Tb(III) ions in aqueous solutions had no effect on their removal efficiency at all studied concentrations of Cs+. While, the presence of Co2+ ions slightly decreased the uptake percent of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents particularly at concentrations of Co2+ higher than 0.001 mol L–1. In contrast, the coexistence of Fe3+ ions with Sm(III), Eu(III) and Tb(III) highly decreased their uptake percent.
Desorption of Sm(III), Eu(III) and Tb(III) from loaded sorbents was performed using organic eluents (0.01M EDTA, 0.001M oxalic acid and 0.001M citric acid) and inorganic eluents (0.1M FeCl3, 0.1M CoCl2 and 0.001M HCl). The maximum desorption of Sm(III), Eu(III) and Tb(III) from synthesized sorbents was occurred using 0.1M FeCl3 and 0.01M EDTA. The efficiency of applied eluents to release Sm(III) from synthesized sorbents has the order: FeCl3 > EDTA > CoCl2 > citric acid > HCl > oxalic acid. The efficiency of applied eluents to release Eu(III) from synthesized sorbents has the order: FeCl3 > EDTA > citric acid > HCl > CoCl2 > oxalic acid, while the order FeCl3 > EDTA > CoCl2 > citric acid > HCl > oxalic acid was revealed for elution of Tb(III). The regeneration efficiency of synthesized sorbents was investigated by successive sorption–desorption cycles of Sm(III), Eu(III) and Tb(III) ions. The loaded sorbents were regenerated using 0.1M FeCl3 and 0.01M EDTA as eluents. The synthesized sorbents could be regenerated and effectively reused for repeated sorption–desorption cycles up to five cycles. A significant decrease was observed in the efficiency when FeCl3 was used as an eluent. While, the uptake of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents was slightly decreased in case of using EDTA as an eluent. This argues the applicability of the synthesized sorbents to be used repeatedly for sorption of Sm, Eu(III) and Tb(III) from aqueous solutions with acceptable efficiencies using EDTA as an eluent.
The data revealed from sorption of Sm(III), Eu(III) and Tb(III) ions onto synthesized sorbents were mathematically treated with the non–linear equations of pseudo–first order, pseudo–second order, Elovich and intra–particle kinetic models. The sorption kinetics of Sm(III), Eu(III) and Tb(III) could be explained more favorably by pseudo–second order model. So, the rate determining step in sorption of samarium, europium and terbium onto synthesized sorbents is a chemisorption process depends on both initial concentration of metal ions and number of active sites in sorbent surface.
Empirical non–linear isotherm models including two, three, four and five– parameter models had been applied to fit experimental sorption data of Sm(III), Eu(III) and Tb(III). The experimental data were well–fitted only Langmuir isotherm model among two–parameter isotherm models. While, all three, four and five–parameter isotherm models exhibited a good fitting with sorption data and were converted to Langmuir isotherm model. This indicated that the sorption isotherms of Sm(III), Eu(III) and Tb(III) ions onto synthesized sorbents could be explained depending on the suppositions of Langmuir isotherm model. Therefore, a chemisorption process was expected to be occurred between the studied metal ions and sorbents’ surfaces. Also, a monolayer of Sm(III), Eu(III) and Tb(III) ions could be predicted to cover the surface of synthesized sorbents as well as the energy of sorption could be the same at all sites.
The thermodynamic parameters ΔSo, ΔHo and ΔGo corresponding to Sm(III), Eu(III) and Tb(III) sorption onto synthesized sorbents were assessed using Van’t Hoff linear equation. The values of thermodynamic parameters indicated that the sorption is an endothermic process takes place spontaneously with the possibility of strong bonding between studied metal ions and sorbents surface.
Fixed bed columns were constructed to individually study the sorption of Eu(III) and Tb(III) onto the magnetic composite CaHAP/NF packed columns. The two rare elements [Eu(III) and Tb(III)] were firstly loaded individually on two similar columns, then eluted using both FeCl3 and EDTA. The breakthrough capacity (Q) of CaHAP/NF for Eu(III) ions in studied column was found to be 67.44 mg g–1. The total recovery of Eu(III) from loaded CaHAP/NF column exhibited the value 94.04 % using 70 mL of 0.1M FeCl3. The elution percent of Eu(III) from CaHAP/NF packed column attained the value 88.39 % using 50 mL of 0.01M EDTA. On the other hand, the breakthrough capacity of CaHAP/NF for Tb(III) ions was found to have the value 59.56 mg g–1. The total recovery of Tb(III) from CaHAP/NF packed column exhibited the value 81.92 % using 70 mL of 0.1M FeCl3. The elution percent of Tb(III) from CaHAP/NF loaded column was found to be 66.61 % using 50 mL of 0.01M EDTA. The possibility of chromatographic separation of Eu(III) from Tb(III) was evaluated by measuring the separation factor α_(Tb )^Eu between Eu(III) and Tb(III). The separation factor between Eu(III) and Tb(III) was only 1.2. This value indicated that separation of Eu(III) from Tb(III) was quite difficult. The chromatographic separation of Eu(III) and Tb(III) from CaHAP/NF packed column was carried out by loading of both Eu(III) and Tb(III) together followed by eluting them using both FeCl3 and EDTA. The breakthrough capacity of CaHAP/NF for Eu(III) ions in studied column was found to have the value 54.15 mg g–1, while that for Tb(III) ions was 43.04 mg g–1. The total recovery of Eu(III) and Tb(III) from CaHAP/NF packed column exhibited the values 91.37 and 76.38 %, respectively using 70 mL of 0.1M FeCl3. The elution percentages of Eu(III) and Tb(III) from CaHAP/NF loaded column recoded 86.23 and 45.55 %, respectively using 50 mL of 0.01M EDTA. EDTA eluted Eu(III) by nearly twofold in comparison with Tb(III) and partially separated them from each other.
Finally, it is constructive to conclude that:
Superparamagnetic hydroxyapatite composites (CaHAP/NF, NF/CaHAP and CaHAP–NF) and their precursors nickel ferrite (NF) and calcium hydroxyapatite (CaHAP) were synthesized via a co–precipitation method. They have promising surface characteristics and high thermal stability promoting them to be effectively applied as solid phase extractants for Sm(III), Eu(III) and Tb(III) from their aqueous solutions.
The synthesized sorbents highly removed Sm(III), Eu(III) and Tb(III) from their aqueous solutions of pH 3.5. They exhibited a higher sorption capacity towards Sm(III), Eu(III) and Tb(III) ions compared with other sorbents reported in previous studies and consequently, they are promising candidates can play a vital role in separation and purification of lanthanide elements.
The ionic strength had a slight influence on uptake of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents using NaClO4 as a background electrolyte. This confirmed the specific tendency of these sorbents towards studied metal ions and suggested that the sorption process was mainly dominated by inner–sphere surface complexation.
The retention mechanism of Sm(III), Eu(III) and Tb(III) ions is shared by more than one mechanism due to the complexity of system. Ion exchange is a main mechanism participating in Sm(III), Eu(III) and Tb(III) separation beside other mechanisms.
The coexistence of Cs+ ions with Sm(III), Eu(III) and Tb(III) ions in aqueous solutions had almost no effect on their removal efficiency. While, the presence of Co2+ ions slightly decreased the uptake percent of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents. In contrast, the coexistence of Fe3+ ions with Sm(III), Eu(III) and Tb(III) highly decreased their uptake percent.
Both FeCl3 and EDTA effectively eluted Sm(III), Eu(III) and Tb(III) from synthesized sorbents.
The synthesized sorbents could be reused repeatedly for sorption of Sm(III), Eu(III) and Tb(III) from aqueous solutions with acceptable efficiencies.
Sorption of Sm(III), Eu(III) and Tb(III) onto synthesized sorbents is a chemical process depends on both initial concentration of metal ions and number of active sites in sorbent surface. A monolayer of Sm(III), Eu(III) and Tb(III) ions could be predicted to cover the surface of synthesized sorbents through a favorable sorption.
Sorption of Sm(III), Eu(III) and Tb(III) is an endothermic process takes place spontaneously with the possibility of strong bonding between studied metal ions and sorbents surface.
Eu(III) was partially separated from Tb(III) using the magnetic composite (CaHAP/NF) as a solid phase and a complexing agent EDTA as an eluent.
On the light of these data, the synthesized sorbents effectively and quantitatively removed studied species from their aqueous solutions. Therefore, they are regarded as promising materials can play a vital role in preconcentration and purification of REEs.