Adsorption behavior of Eu(III) on partially Fe(III)- or Ti(IV)-coated silica
© Im et al; licensee Springer. 2012
Received: 10 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The adsorption behavior of Eu(III) onto silica surface, which was partially coated with Fe(III) or Ti(IV), was investigated to determine Fe(III) or Ti(IV) effects on the surface reaction of lanthanides on mineral surfaces in groundwater. Compared with a parallel uncoated silica, the Fe(III)-coated silica did not enhance the adsorption of Eu(III). However, enhanced adsorption of Eu(III) on the Ti(IV)-coated silica was observed by increasing the amount of Ti(IV) on the silica surface.
There has been great interest in the immobilizations and adsorption mechanisms of various toxic ions in aqueous solutions by using silica-based sorbents [1–4]. The adsorption reaction of a metal ion onto a metal (hydr)oxide surface is explained in terms of surface complexation. Besides free metal ions, hydrolyzed or complexed species , or even the colloidal species can be adsorbed . Surface precipitation may occur even in a concentration below the surface site saturation . Frequently, experimental evidence indicates surface nucleation of metal hydroxides .
Due to their ubiquity in soils and sediments and high specific surface area, iron or titanium hydroxides (Fe or Ti oxides) around silicate minerals may play a role in the migration of actinides in groundwater. The interactions of actinides on immobile solid surfaces are important processes that determine retardation during transport. Usually, Eu(III) is considered to be an adequate chemical analogue of radiotoxic nuclides, Am(III) and Cm(III). These actinides consist mainly of long-lived nuclides that emit alpha radiation, and their radioactivity continues for several hundred thousands of years .
The following studies have been reported. Fe-modified silica gel was investigated as an adsorbent for humic acids employing an electrostatic binding to Fe and/or in coordination with Fe by direct substitution of OH and Cl on Fe sites . TiO2-coated SiO2 synthesized by hydrolysis and condensation of various silicate and titanate precursors has been actively studied as photocatalysts due to its photocatalytic and photovoltaic effects . Eu(III) sorption onto clay minerals was quantitatively modeled with pH ranging from 3 to 10 using cation exchange reactions for Eu(III)/Na(I) and Eu(III)/Ca(II) .
However, Eu(III) sorption onto Fe(III)- or Ti(IV)-coated silica has not received as much attention as UO22+ sorption. The aim of this paper is to study the sorption of Eu(III) from an aqueous solution on Fe(III)- or Ti(IV)-coated silica to understand the trace radionuclide migration which occurs in groundwater.
The chemicals used in this study including silica (Sigma-Aldrich Corporation, St. Louis, MO, USA; particle size 40 to 63 μm; surface area 550 m2/g), ferric nitrate [Fe(NO3)3·9H2O], titanium butoxide [Ti(OBu)4], 1, 10-phenanthroline [C12H8N2], ethanol [C2H5OH], toluene [C6H5CH3], and europium(III) oxide [Eu2O3] were all of high purity and used as received. Perchloric acid [HClO4], hydroxylamine hydrochloride [NH2OH·HCl], sodium perchlorate [NaClO4], sodium hydroxide [NaOH], sulfuric acid [H2SO4], and hydrofluoric acid [HF] were of analytical grade and used without further purification. NaOH solution was titrated with 0.1 M hydrochloric acid [HCl] standard solution (Merck & Co., Inc., Whitehouse Station, NJ, USA) in the presence of a phenolphthalein indicator.
The dry silica was dispersed in 3.7 M HNO3 for one day and washed with distilled water until the wet silica surface was neutral. Finally, the resulting silica was dried in an oven at 120°C for 6 h and stored in a capped bottle after cooling.
The 11.3 mM Eu2O3 in 20.62 mM HClO4 was prepared as a stock solution for the Eu(III) adsorption tests. The metal-ion concentration of the stock solution was determined with inductively coupled plasma - atomic emission spectrometry [ICP-AES] before diluting for the adsorption experiments. All the solutions were handled under a nitrogen gas flow.
Coated, adsorbed, and desorbed metal concentrations were determined with an ultraviolet and visible [UV-vis] absorption spectrophotometer (Cary 3, Varian, Inc., Santa Clara, CA, USA) and an ICP-AES (ULTIMA 2C, HORIBA Jobin Yvon, HORIBA, Ltd., Minami-ku, Kyoto, Japan). A spectrofluorometer (FS-900CD, Edinburgh Instruments, Livingston, U.K.) was used to obtain appropriate fluorescence spectra.
Partial Fe(III) coating on silica surface
Fe(NO3)3·9H2O (0, 35, 70, 140, 280, and 420 mg) was added with stirring to each silica (50 g) in 500 mL of distilled water. The pH was adjusted to 4.5 with 0.1 M HClO4 or 0.1 M NaOH, and each mixture was stirred for 2 h. The partially Fe(III)-coated silica was glass filtered, washed with a pH 4.5 HClO4 solution and distilled water three times each, and dried at 120°C for 6 h sequentially.
Partial Ti(IV) coating on silica surface
Ti(OBu)4 was slowly added with stirring to each silica (15 g) in C2H5OH and C6H5CH3 (1:1) mixed solution until 0, 5, 10, 20, 100, and 200 mM of Ti(IV) was added in the 50 mL total solution. Then, each mixture was stirred for 2 h. The partially Ti(IV)-coated silica was glass filtered, washed with a C2H5OH and C6H5CH3 (1:1) mixed solution three times, and dried in sequence at 120°C for 6 h.
Eu(III) adsorption onto Fe(III)- or Ti(IV)-coated silica
In each test, 500 mg of dissimilar Fe(III)- or Ti(IV)-coated silica was placed in a 60-mL beaker, and 20 mL of distilled water was added in the beaker. The total volume of each mixture was adjusted to 50 mL, and the final concentration was 0.1 mM Eu2O3 in 0.18 mM HClO4 with a controlling ionic strength with 0.1 M NaClO4. At this point, for the observation of a pH-dependent adsorption, 0.1 M NaOH under the N2 gas flow in order to eliminate the remaining CO2 in the solution, was properly added to each mixture, and the pH went up to 8 for Eu(III) adsorption tests. The mixture in each polyethylene beaker was stirred for more than 30 min until the pH equilibrium was achieved. The mixture was then analyzed by ICP-AES after being filtered through a 0.1-μm pore-sized membrane filter. Fluorescence of Eu ions on Fe(III)- or Ti(IV)-coated silica was obtained from the sediments.
Results and discussion
It has been known that ≡Si-OH in silica (SiO2) is dissociated into surface ≡Si-O- and free H+ at pH > 3, and as the result, the surface is negatively charged, which is appropriate to incorporate electron-deficient metals to the silica surfaces. Here, Fe(III) or Ti(IV) was primarily fixed on the silica surface through the ≡Si-O-Fe or ≡Si-O-Ti structure. In contrast, Eu(III) ion is easily hydrolyzed  and forms insoluble trihydroxide precipitates  and polynuclear hydroxo complexes . The hydrolyzed Eu-OH is assumed to be sorbed into Fe(III)- or Ti(IV)-coated silica in aqueous Eu(III) solutions.
The Ti(IV)-coated silica exhibited a stronger binding toward Eu(III) than the uncoated silica, and the preferential binding is considered due to a higher metal Lewis acidity of Ti than Si. The hard Lewis acid, Eu(III), forms more stable complexes with hydroxyl ligands on relatively harder TiO2 than those on SiO2. The Eu(III) adsorption processes onto the partially Ti(IV)-coated silica involve the combination of Eu(III) hydrolysis and the adsorption of the hydrolysis product, EuIII-OH, to produce ≡Si-O-Ti-OH-EuIII and/or ≡Si-O-Ti-O-EuIII in addition to ≡Si-OH-EuIII and/or ≡Si-O-EuIII, depending on the pH of the prepared solutions .
This study shows an example of foreign ion effects on the adsorption of actinide onto a mineral surface. In the case of the Ti(IV) ion for Eu(III) adsorption onto a silica surface, Ti(IV) enhances the adsorptivity as far as it exists as a surface hydroxide. The enhancement in adsorptivity decreases when the surface hydroxide converts to oxide prior to Eu(III) adsorption. In contrast, Fe(III) coating on silica surfaces did not enhance adsorption of Eu(III), nor were there any changes in fluorescence properties compared with uncoated silica.
This work was supported by the nuclear research and development program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.
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