Effects Of Fixed Humic Acid On The Adsorption Of ^152+154Eu??? And ⁷⁵Se???/???

Humic substances?HSs?are ubiquitous in waters and soils and affect significantly the migration of radionuclides in the environment.HSs are usually divided into fulvic acid?FA?,humic acid?HA?and humin based on their solubility in water.HSs not only affect the speciation of radionuclide in aqueous phase by complexing with metal ion,but also influence directly the adsorption of radionuclide on minerals by surface modification.Therefore,the effects of HSs on the environmental behavior of radionuclide and heavy metals have been extensively studied.Purified FA and HA extracted from environmental samples have long been used to study their effects on the adsorption of radionuclide,which is used to account for the contribution of HSs on the adsorption,dispersion and migration of radionuclide in the environment.However,the state of purified and dissolved FA and HA which are introduced into laboratory investigation systems are quite different from the occurrence state of HSs in the environment.In the environment,HSs inevitably interact and complex especially with polyvalent metal ions(such as Fe³⁺and Al³⁺).When hydrolysis and flocculation of polyvalent cations happen,mutual coating of HSs and the hydrolysis products occur and consequently some HSs are fixed in solid media.Due to the complexation with polyvalent metal ions and the ligand exchange reactions with mineral surface sites in HSs fixing processes,the number of the sites on HSs which are free for complexation is significantly decreased.Thereby,different effects of fixed HA/FA and purified and dissolved HA/FA on radionuclide can be expected.However,the difference has not been studied and reported yet.The primary challenge and difficulty of studying fixed HSs lies in the choose of an ideal fixed HSs system.In this dissertation,two systems of fixed HSs were designed to answer the above problem based on the fact that iron and aluminum hydroxides extensively exist in occurrence states of coating with HSs.The adsorbates of Eu???,Se???and Se???were chosen as representative cation and anions to comparatively investigate the effects of fixed and dissolved HA on the adsorption.The main investigations and results of this dissertation are as follows:1.Two series of fixed HS samples with different HA contents were prepared by the hydrolysis of Fe³⁺and Al³⁺in the presence of HA,i.e.,HA was fixed in iron hydroxide and aluminum hydroxide,respectively.The synthesized samples were characterized by X-ray powder diffraction?XRD?,Fourier transform infrared spectroscopy?FT-IR?,scanning electron micrographs?SEM?,transmission electron microscope?TEM?and thermogravimetric analysis?TGA?.The specific surface areas were determined using N₂-BET method.Zeta potential was analyzed as a function of pH.The acid-base properties of the surfaces were studied using potentiometric titrations.It was found that the crystal structure of ferridydrite with fixed HA of 25 and 50 mg/g was the same as ferridydrite without HA,whereas the structure was changed in the presence of 200 mg/g fixed HA.The specific surface area decreased with increasing content of fixed HA.Zeta potential of the samples of fixed HA was lower than ferridydrite without HA.FT-IR results indicated that complexation between Fe³⁺and fixed HA occurred.The results of potentiometric titrations suggested that most of functional groups on fixed HA were saturated by coordination.Similar to ferridydrite system,the mineral constituents of Al?OH?₃ with fixed HA of low HA contents were the same as that without fixed HA,whereas the composition was changed when the amount of fixed HA was 200 mg/g.FT-IR and differential thermal analysis indicated that HA complexed with Al³⁺in the samples of Al?OH?₃ with fixed HA.Similar with the system of ferrihydrite,pH_IEP decreased with increasing fixed HA content.Due to coordination saturation of the most functional groups of the fixed HA,the presence of fixed HA did not change apparently the acid-base property of Al?OH?₃with fixed HA.2.Based on the characterization results,a radiotracer technique was used to comparatively investigate the effects of fixed HA in iron hydroxide and in aluminum hydroxide on the adsorption of ^152+154Eu³⁺,⁷⁵SeO₃^2-and ⁷⁵SeO₄^2-.In addition,the effects of fixed HA on the adsorption of lanthanides in coexistence were studied using an ICP-MS method.It was found that dissolved HA showed a promotion effect in low pH range and an inhibition effect in high pH range,and the effects became more obvious when HA content increased.However,fixed HA showed quite different effects.Fixed HA of 25 and 50 mg/g did not present any effects on the adsorption of ^152+154Eu???and other Ln???.Although fixed HA of 200 mg/g presented similar effects as dissolved HA,the extent of the effects was much less than that of dissolved HA.3.Based on macroscopic results,the mechnisms of the influence of fixed and dissolved HA on lanthanides were studied by using extended X-ray absorption fine structure?EXAFS?and time resolved laser induced fluorescence spectroscopy?TRLFS?.It was found that the surface species of Eu???and Yb???in the presence of fixed HA was the same as that in the presence of dissolved HA,although the macroscopic effects of fixed and dissolved HA on Ln???adsorption were significantly different.The main reason was that the number of free functional groups for Ln???complexation on fixed HA was much less than that on dissolved HA of the same HA content.As a consequence,although the surface species was the same,the significant difference in speciation led to different macroscopic effects of fixed and dissolved HA.4.Based on macroscopic results,the influence of fixed and dissolved HA on selenite(SeO₃^2-)were studied by using X-ray photoelectron spectroscopy?XPS?and attenuated total reflection fourier transform infrared spectroscopy?ATR-FTIR?.It was found that inner-sphere complexation was responsible for SeO₃^2-adsorption whereas out-sphere complexation for SeO₄^2-.Due to the variation in adsorption mechanisms of Se???and Se???,fixed and dissolved HA showed slightly different extent of inhibition effects on the adsorption.The results of this dissertation imply that,as compared to dissolved HA,the effect extent of fixed HA with the same HA content on the adsorption of radionuclides depends on element,oxidation state of the element,fixing medium for HA and the content of fixed HA et al..It is necessary to carry out further study of this subject.