| Nuclear energy is considered as a feasible solution for the rapidly increasing energy demand and the global warming problem,and these issues have prompted many countries to develop their nuclear industries.The rapid growth of the nuclear power industry brings the intense attention to uranium because of the limited supply of uranium and its various possible harmful effects on the environment.Therefore,uranium separation,recovery,and decontamination from the radioactive waste solutions are of great significance due to utilizing the sustainable uranium resources and the minimal environmental costs.Various acids with different concentrations are commonly used in the mineral ore processing and nuclear fuel cycle to process and reprocess uranium,and it leads to generate massive amounts of aqueous wastes containing uranium?VI?in the different acidic media with the various solution's characteristics.Among the secondary resources of uranium,phosphate rocks are known as an essential sustainable source of uranium because the phosphoric acid?PA?produced from phosphate rocks contains uranium in the range of 50-300 ppm.In addition,a large quantity of radioanalytical acidic wastes containing uranium is produced during the analysis of nuclear materials with different techniques in nitric,sulfuric,phosphoric,and hydrochloric acids mediums in nuclear industries.Therefore,uranium recovery from such these complex waste streams encounters many difficulties due to the high acidities,different complexes,and matrix interference.Although liquid-liquid solvent extraction is widely used in industry,this process is being applied with limitations due to the massive usage of organic solvents and diluents.The solid-phase extraction is one of the effective separation methods because it usually combines the selectivity of solvent extraction with the simplicity and multistage character of column chromatographic system.In comparison with the conventional inorganic and polymeric supports,the silica substrate polymer?SiO2-P?material showed the high surface area and porosity,fast adsorption and desorption kinetics,high mechanical strength,higher thermal and radiation resistance and the outstanding stability in the acidic solutions.Therefore,SiO2-P has been selected as a substrate for the solid-phase extraction in our study to be impregnated by the selected organic extractants to recover uranium from different acidic media.At the first stage of the research,the phosphoric acid solution is considered as the medium of adsorption study since a few researchers work on the uranium recovery form this medium by the adsorption technique.This thesis describes uranium solid-phase extraction from the aqueous phosphoric acid medium using a silica/polymer based HDEHP-TOPO adsorbent.The mixture of di-2-ethyl-hexylphosphoric acid?HDEHP?and tri-n-octylphosphine oxide?TOPO?was impregnated and immobilized into the pores of SiO2-P particles.The various experimental parameters such as the synergistic mixture ratio of extractants,phosphoric acid concentration,the contact time and the temperature were investigated.The results showed that 95%uranium extraction was achieved after 10 min contacting time using HDEHP-TOPO/SiO2-P?mass ratio=3:1w/w?adsorbent from 1 M phosphoric acid solution containing 165 ppm U?VI?,5000ppm Fe?III?,1000 ppm Ca?II?and 1000 ppm Al?III?at 25?.Also,the obtained results displayed the high adsorption capacity for Uranium as high as 48 mg/g at the mentioned conditions.The rate constant(1.62 g mg-1 min-1)was calculated by the pseudo-second-order equation,and it shows that reaction rate was controlled by the chemical adsorption of uranium.Furthermore,HDEHP-TOPO?3:1?/SiO2-P has good selectivity for U?VI?among the co-existing elements?Fe,Al and Ca?.The adsorption isotherms were well described by the Redlich–Peterson equation,closely followed by the Langmuir equation which confirms the mechanism is chemical monolayer adsorption.Obtained thermodynamic parameters??H0.?S0 and?G0?reveal that the U?VI?adsorption to be an exothermic and spontaneous process.By achieving such these good results by this novel adsorbent,we decided to examine other synergistic extractants to introduce a new adsorbent for the same function so that A new macroporous silica-based HDEHP-CMPO impregnated polymeric solid-phase extractant?HDEHP-CMPO?3:1?/SiO2-P?has been synthesized.HDEHP-CMPO/SiO2-P showed a slightly higher adsorption efficiency towards uranium in phosphoric acid concentrations lower than 3 M.The kinetics results indicated that the adsorption rate of HDEHP-CMPO/SiO2-P with the rate constant equals to 58.82 g mg-1 min-1 is much higher than that of HDEHP-TOPO/SiO2-P.While the adsorption isotherm studies revealed the same results as HDEHP-TOPO/SiO2-P with the almost similar maximum adsorption capacity.HDEHP-CMPO/SiO2-P is highly selective towards uranium among other introduced common co-existing elements?Al,Mg,Ca,Ni and Cu?while the iron is co-extracted with uranium in adsorption stage.In desorption process,U?VI?and Fe?III?were separated and desorbed efficiently in a 1 M NH4HCO3 at 25°C and a 1 M H2SO4solution at 55°C,respectively.Moreover,HDEHP-CMPO/SiO2-P adsorbent showed a great stability in high concentrated phosphoric acid solutions.The HDEHP-CMPO/SiO2-P solid-phase extractant also was applied to recover uranium?VI?from HNO3,HCl,and H2SO4 media.The batch experiments showed an excellent uranium adsorption efficiency for the HDEHP-CMPO/SiO2-P in the investigated acidic media with the wide concentration range from the near neutral to high acidic conditions.The adsorption equilibrium data were well described with the Redlich-Peterson isotherm,and the maximum adsorption capacities of the adsorbent for U?VI?in 3 M HNO3,HCl,and H2SO4 media were calculated as 96.55,100.42,and52.77 mg g-1,respectively.Furthermore,the HDEHP-CMPO/SiO2-P showed a significant selectivity for U?VI?among the coexisting elements?Al,Ca,Cu,Mg,Na,and Ni?while the co-extraction of Fe?III?with U?IV?was observed.In the static desorption process,Fe?III?and U?VI?were separated and desorbed efficiently in a 1M H2SO4 solution at 55°C and a 1 M NH4HCO3 at 25°C,respectively.HDEHP-CMPO/SiO2-P also depicted the excellent stability against the nitric,hydrochloric and sulfuric acids solutions which further confirms the capability of this novel chelating adsorbent in uranium recovery from different nuclear waste streams.Finally,to further emphesis on the great capability of this adsorbent for different applications,thorium adsorption from nitric acid medium into HDEHP-CMPO/SiO2-P is investigated by the batch experiments which resulted in high adsorption efficiency?>99%?with a significant distribution coefficient?around 8000 L/kg?in the wide range of concentrations of nitric acid from near pH conditions to the molarity of 9 M.Furthermore,uranium and thorium adsorption behavior in the multiple ion adsorption system showed an excellent adsorption efficiency for both elements,and it can efficiently remove uranium and thorium from nitric acid medium with a wide range of acidity. |
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