Amino-modified porous silica as adsorbents for the removal of aqueous uranium(VI) species: adsorbent design, synthesis, and characterization
Date
2023
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
U(VI), a radionuclides pollution despised in the past decades, has now been frequently detected in various water bodies worldwide with a concentration higher than WHO and EPA. Thus, it sparked interest in developing new approaches to remove it from aqueous solutions. This study produces amino-functionalized porous silica (AFPS) as the adsorbent to remove U(VI) from aqueous solutions. First, mesoporous silica (MPS) materials with different surface areas and average pore sizes were produced by applying urea-formaldehyde resin as the template. After modifying MPS with AEPTES and APTES (AE@MPS and AP@MPS, respectively), the surface properties of the obtained materials are characterized for surface chemical properties via SEM, XPS, NMR, and zeta potential and specific surface area by BET. The surface acidity of AE@MPS and AP@MPS are determined based on electrophoretic mobility measurements, i.e., zeta potential as a function of pH. Second, though used AE@MPS and AP@MPS as adsorbents to remove aqueous U(VI), how the surface properties of adsorbents, such as total pore volume, average pore size, and functional group density will influence its maximum U(VI) adsorption capacity was evaluated and discussed. The result also indicates that, with a given functional group density, AE@MPS and AP@MPS have the highest U(VI) adsorption capacity at a pore size of 4.1 nm and 2.7 nm, respectively. Third, the experimental results are fitted by the Langmuir adsorption isotherm, Potential of Mean Force (PMF) model, and Surface Complex Formation Model (SCFM), as to estimate the adsorption energy ΔG0ads, which consists of specific chemical energy, ΔG0chem, coulombic energy, ΔG0coul, solvation energy, ΔG0solv, and lateral interaction energy, ΔG0lat. From the result of SCFM, for AE@MPS, the values of ΔG0coul and ΔG0chem are close, and both are the main contributor to ΔG0ads; for AP@MPS, the values of ΔG0coul are much larger than ΔG0chem and are the main contributors of ΔG0ads. Fourth, the mechanism of U(VI) adsorption on AE@MPS and AP@MPS was drawn. The U(VI) adsorption capacity of AE@MPS and AP@MPS is greatly affected by pH. At pH < 2, AE@MPS and AP@MPS exhibit no U(VI) adsorption capacity. At 2 < pH < 4, the U(VI) adsorption capacity is mainly brought by SiO-. At 4 < pH < 8, the OH- in the solution rapidly increases. During this phase, U(VI) will desorb from SiO- and form hydrated U(VI) with OH-. Because the structure of hydrated U(VI) is the atomic cluster of UO22+ cation that surrounded by OH- anions. The surface layer of the hydrated U(VI) atomic cluster will be negatively charged, thereby becoming easier adsorbed by the positively charged amino group. Fifth, keep in mind that the adsorption energy distributions of AE@MPS and AP@MPS against aqueous U(VI) are different. By comparing the molecular structure of amino groups on the surface of AE@MPS and AP@MPS, it can be estimated that if the number of amino groups on a single functional branch increase, the proportion of ΔG0chem will increase, leading to larger U(VI) adsorption capacity and more robust selectivity over other aqueous ions. ☐ With the detailed understanding of AE@MPS and AP@MPS as adsorbents against aqueous U(VI), this study is crucial for the design of similar adsorbents.
Description
Keywords
Adsorption, Amino-functionalized, Aquatic chemistry, Porous silica, Uranium