Extraction Of Humic Acid From Soil And Adsorption Of Uranium, Europium And Americium-241 By The Extracted Humic Acid

Humic substances (HSs) are ubiquitous natural macromolecule organic materials. According to their solubility, humic substances are usually subdivided into humic acids (HAs) which are soluble in alkaline media and insoluble in acidic media, fulvic acids (FAs) which are soluble at all pH values and humin, which are insoluble at all pH values. The structures of various humic substances are similar, while some differences are existent in the molecular weight, element and functional groups. However, among the three kinds of humic substances, humic acids are most abundant in natural environment and have been investigated most extensively.Since humic substances contain a variety of functional groups, such as -COOH, -OH, -C=O, -NH₂ and others, they can interact with radioactive nuclides, and affect their speciation, translation, migration-sedimentation, life-utilization, toxicity or other chemical behavior. For decades of years, the growing interest has been invested in the interaction of humic acid with some radioactive elements and the chemical behavior change of the radioactive elements in the presence of humic substances, especially humic acid.In this paper, humic acid (HA) was extracted from the soils in a planning site for disposal of the ²⁴¹Am waste as the procedure recommeded by Interrnational Humic Substance Society (IHSS) with minor modification, and its element composition and Fourier transform infrared (FTIR) Spectrometry were analyzed. Then, the adsorption behavior of U, Eu and ²⁴¹Am by the extracted humic acid and a commercial humic acid product were investigated by the static experimental method. Additionally, The preliminary reduction experiment of U(VI) by the extracted humic acid was performed. It can be noted that the contents of humic acid are different in different soil samples, even the soils in same place but in different depth. For example, the content of HA in upper soil was higher than underground soil, while the content of HA in black land was more than loess, and loess was more than sand soil. However, FTIR spectra analysis indicated that the extracted humic acid and the commercial humic acid product have similar functional groups, such as -CH₃, -CH₂-, -COOH, -C=O, -C=C-.The adsorption experiments showed that the extracted humic acid as well as the commercial humic acid is an efficient adsorbent for U, Eu and ²⁴¹Am. More than 80% of the total U, Eu or ²⁴¹Am in solutions could be adsorbed by 5mg humic acids. U, Eu and ²⁴¹Am uptake on humic acid was a pH-dependent process. Significant differences in adsorption of U, Eu or ²⁴¹Am were observed at 0℃-40℃, especially by the commercial humic acid. Mg²⁺, Al³⁺, Lemon anion, EDTA can inhibit the adsorption of U, Eu and ²⁴¹Am on humic acid, while no significant differences on U, Eu and ²⁴¹Am adsorption were observed in solutions containing K⁺, Cl^- and NO₃^-. Adsorption of ²⁴¹Am could be also inhibited by 2500 multiples of Eu³⁺ or Nd³⁺ to ²⁴¹Am in solution. The relationship between concentrations of. U, Eu and ²⁴¹Am in aqueous solutions and the adsorption capacities can be described by the Langmuir adsorption equation. After adsorption of U and Eu, some peaks (3000-3300cm^-1,1704 cm^-1,138.1 cm^-1,1258 cm^-1) in FTIR spectra of HA became wider and weaker, implying that the adsorption should be involved in complexation process of carboxyl groups of HA with U or Eu.The preliminary experiment showed that UO₂²⁺ could be deoxidized to U⁴⁺ by the extracted humic acid with the reduction ratio of 7.01 in the pH of 3.72. Moreover, the reduced U⁴⁺ would become more stable in the presence of citric acid.