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Inorganic Materials Lab. SKKU

Sorption Reaction of Aquatic TcO 4 - or CrO 4 2- on Calcined Mg/Al Layered Double Hydroxide: Reaction Equilibria and Characterization Seog Woo Rhee 1 , Mun Ja Kang 2 and Duk-Young Jung *, 1 1 Department of Chemistry, SungKyunKwan University

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Inorganic Materials Lab. SKKU

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  1. Sorption Reaction of Aquatic TcO4- or CrO42- on Calcined Mg/Al Layered Double Hydroxide: Reaction Equilibria and Characterization Seog Woo Rhee1, Mun Ja Kang2 and Duk-Young Jung*, 1 1 Department of Chemistry, SungKyunKwan University 2 Radioactive Waste Disposal Team, Korea Atomic Energy Research Institute e-mail: dyjung@chem.skku.ac.kr jisanrhee@hanmail.net munkang@nanum.kaeri.re.kr Inorganic Materials Lab. SKKU

  2. Abstract A layered double hydroxide (LDH) is referred to as anionic clay and easily synthesized in the laboratory. The reconstruction reaction of calcined LDH may prove it to be useful for sorbing anionic species from wastes. TcO4- and CrO42- are one of hazard elements in the nuclear and industrial wastes, respectively. The sorption reaction of aquatic TcO4- or CrO42- on calcined Mg/Al LDH was investigated. The calcined LDH was prepared by heating a synthesized Mg/Al LDH to 560 ºC. The batch sorption experiments were carried out in an inert atmosphere and at the constant temperature of 25 ºC. The liquid-solid reaction of TcO4- or CrO42- on calcined LDH was proposed to be stepwise ion-exchange reaction: generation of LDH hydroxide, Mg6Al2(OH)18, as an intermediate and then replacement of the OH- by TcO4- or CrO42- ions. The equilibrium constants (K) for ion-exchange reactions Mg6Al2(OH)18(s) + 2TcO4- = Mg6Al2(OH)16(TcO4)2(s) + 2OH- or Mg6Al2(OH)18(s) + CrO42- = Mg6Al2(OH)16(CrO4)(s) + 2OH- were evaluated by a non-linear least squares fit procedure. The ReO4- was used as a TcO4- surrogate. Calcined Mg/Al LDH, before and after the sorption reaction with ReO4- or CrO42- was characterized to study reaction mechanism. The analyses by powder X-ray diffraction, 27Al MAS NMR and FT-IR spectroscopy were carried out. The XRD pattern of LDH perrhenate or chromate shows the typical reflections for the layer-structured materials. The NMR and FT-IR spectra reveal that the calcined LDH was reconstructed after intercalation of ReO4- or CrO42- in the prepared aquatic solution. The detailed XRD analysis for the CrO42- intercalated material showed that the hydroxyl group on LDH surface is produced along with LDH chromate in the reaction solid. This result implies that the liquid-solid reaction of oxometallate on calcined LDH involves the ion-exchange process.

  3. Introduction  Hydrotalcite is rare but naturally occurring mineral. It has a layered structure and anion-exchange capacity. It is also called layered double hydroxide (LDH).  The layered structure of LDH is destroyed when LDH is calcined. However, this structure is reconstructed with anionic species such as Cl-, CrO42-, and PO43- in aqueous solution. This property of calcined LDH may prove to be useful for sorbing anionic species from industrial and nuclear waste. Technetium-99 is a hazardous element because it exists as appreciable amounts in nuclear waste and has very long half-life. TcO4- anion is highly soluble and mobile.  Chromate anion is one of a toxic element of the industrial wastewater. It is used by industries such as metal platting, leather tanning and textile dyeing.  The purpose of this study is to investigate the solid-liquid reaction of TcO4- or CrO42- with calcined LDH as an inorganic sorbent. Emphasis of the work is placed upon understanding the reaction equilibrium and mechanism.

  4. Schematic Representation and General Properties  Schematic representation  Anion-exchange property : high anion-exchange capacity: 2  5 meq g-1 Mg6Al2(OH)16(Cl)2 + CO32- Mg6Al2(OH)16CO3+ 2Cl-  Memory effect 450600 C Mg6Al2(OH)16CO3 Mg6Al2O9 + CO2 + H2O An- Mg6Al2O9 + H2O Mg6Al2(OH)16An- + OH-  Use in removing anionic species from wastewater An- : Cr2O72-, CrO42-, HPO42-, TcO4-, SO42-, MnO4- General formula of synthetic LDH M(II)1-xM(III)x(OH)2(Am-)x/mnH2O M(II) = Mg, Ni, Zn M(III) = Al, Cr, Fe Am- = exchangeable anion 0.2 ≤ x ≤ 0.4

  5. Experimental Sections  Materials & Methods TcO4- : 0.3M NH4TcO4 / 0.1M NH4OH / 0.01M HTcO4 stock solution ReO4- : 0.3M NH4ReO4 / 0.01M HReO4 stock solution CrO42- : 0.1 M Na2CrO4.4H2O / 0.1M NaOH stock solution Calcined LDH : Mg/Al system, x = 0.25, calcined at 560 C for 3hours Batch experiments with inert atmosphere at 25 C Concentration Determination TcO4- : Liquid Scintillation Analysis (290keV -radiation of Tc-99) ReO4-, CrO42- : UV-Visible Spectroscopy and ICP-AES  Sorption reactor with inert atmosphere and constant temperature

  6. Sorption Experiments with TcO4- and ReO4- • MO4- mLDH Vol. [LDH]0*[MO4-]0 [MO4-]eq [MO4-]sorbed pH0 pHeq • (mg) (ml) (mol/L) (mol/L) (mol/L) (mol/L) • TcO4- 20 247 2.36E-4 1.44E-5 9.60E-6 4.80E-6 5.80 10.59 • 51 247 5.99E-4 1.33E-5 6.24E-6 7.06E-6 5.75 10.87 • 102 247 1.20E-3 1.29E-4 4.46E-5 8.44E-5 5.20 10.85 • 100 246 1.18E-3 1.12E-3 2.50E-4 8.67E-4 5.46 10.72 • 151 247 1.78E-3 1.53E-5 4.16E-6 1.11E-5 5.52 10.92 • 148 253 1.70E-3 1.34E-5 6.43E-6 6.97E-6 11.03 11.29 • 144 250 1.67E-3 1.40E-5 1.02E-5 3.81E-6 11.70 11.72 • 147 250 1.71E-3 1.39E-5 1.20E-5 1.87E-6 12.06 12.10 • ReO4- 204 490 1.20E-3 1.16E-4 3.29E-5 8.27E-5 5.29 10.78 • 204 495 1.20E-3 5.14E-4 1.33E-4 3.81E-4 4.93 10.81 • 205 495 1.20E-3 1.14E-3 2.96E-4 8.44E-4 4.82 10.58 • 203 495 1.19E-3 2.00E-3 6.75E-4 1.32E-3 5.23 10.43 • 204 495 1.20E-3 4.62E-3 2.81E-3 1.81E-3 5.46 9.76 • 206 495 1.21E-3 9.55E-3 7.70E-3 1.85E-3 5.39 9.34 •  Mg6Al2O9 : M = 343.79 g/mol

  7. Sorption Experiments with CrO42- • mLDH Vol. [LDH]0*[CrO42-]0 [CrO42-]eq [CrO42-]sorbed [OH-]0 [OH-]eq • (mg) (ml) (mol/L) (mol/L) (mol/L) (mol/L) (mol/L) (mol/L) • 100 100.0 2.909E-3 1.02E-3 2.92E-6 1.02E-3 8.26E-3 10.2E-3 • 99.5 100.0 2.909E-3 1.28E-3 4.38E-6 1.28E-3 7.75E-3 10.1E-3 • 99.7 100.0 2.909E-3 1.28E-3 5.12E-6 1.27E-3 7.75E-3 10.1E-3 • 99.7 100.0 2.909E-3 1.53E-3 1.68E-5 1.51E-3 7.23E-3 10.3E-3 • 100 100.0 2.909E-3 1.79E-3 2.63E-5 1.76E-3 6.71E-3 9.91E-3 • 99.8 100.0 2.909E-3 1.79E-3 3.28E-5 1.76E-3 6.71E-3 9.93E-3 • 99.5 100.0 2.909E-3 2.04E-3 1.72E-4 1.87E-3 6.20E-3 9.82E-3 • 99.8 100.0 2.909E-3 2.04E-3 1.33E-4 1.91E-3 6.20E-3 10.3E-3 • 99.6 100.0 2.909E-3 2.04E-3 1.33E-4 1.91E-3 6.20E-3 9.84E-3 • 100 100.0 2.909E-3 2.30E-3 2.42E-4 2.06E-3 5.68E-3 9.64E-3 • 99.9 100.0 2.909E-3 2.30E-3 2.82E-4 2.02E-3 5.68E-3 9.44E-3 • 100 100.0 2.909E-3 2.55E-3 5.25E-4 2.03E-3 5.16E-3 9.01E-3 • 99.8 100.0 2.909E-3 2.55E-3 4.50E-4 2.10E-3 5.16E-3 9.18E-3 • 100 100.0 2.909E-3 2.81E-3 6.51E-4 2.16E-3 4.65E-3 8.77E-3 • 99.9 100.0 2.909E-3 2.81E-3 7.22E-4 2.09E-3 4.65E-3 8.63E-3 • 99.5 100.0 2.909E-3 3.06E-3 1.01E-3 2.05E-3 4.13E-3 7.96E-3 • 100 100.0 2.909E-3 3.32E-3 1.14E-3 2.18E-3 3.61E-3 7.75E-3 • 100 100.0 2.909E-3 3.32E-3 1.18E-3 2.14E-3 3.61E-3 7.54E-3 • 99.6 100.0 2.909E-3 3.57E-3 1.36E-3 2.21E-3 3.10E-3 7.15E-3 • 100 100.0 2.909E-3 3.57E-3 1.36E-3 2.21E-3 3.10E-3 7.13E-3 • 100 100.0 2.909E-3 3.83E-3 1.56E-3 2.27E-3 2.58E-3 6.70E-3 • 100 100.0 2.909E-3 3.83E-3 1.68E-3 2.15E-3 2.58E-3 6.64E-3 • 99.7 100.0 2.909E-3 4.09E-3 1.91E-3 2.18E-3 2.07E-3 6.15E-3 • 100 100.0 2.909E-3 4.34E-3 2.03E-3 2.31E-3 1.55E-3 5.71E-3 • 99.8 100.0 2.909E-3 4.34E-3 2.23E-3 2.11E-3 1.55E-3 5.75E-3 • 99.6 100.0 2.909E-3 4.60E-3 2.29E-3 2.31E-3 1.03E-3 5.25E-3 • 99.7 100.0 2.909E-3 4.85E-3 2.56E-3 2.29E-3 5.16E-4 4.76E-3 • 99.7 100.0 2.909E-3 5.11E-3 2.75E-3 2.36E-3 6.31E-6 4.33E-3 • 99.4 100.0 2.909E-3 5.11E-3 2.77E-3 2.34E-3 6.31E-6 4.30E-3 • 99.4 100.0 2.909E-3 5.11E-3 2.77E-3 2.34E-3 6.31E-6 4.24E-3 • 99.7 100.0 2.909E-3 5.11E-3 2.75E-3 2.36E-3 6.31E-6 4.27E-3 • Mg6Al2O9 : M = 343.79 g/mol

  8. Evaluation of Equilibrium Constants for MO4- (M: Tc or Re)  Equilibrium reaction K1 Mg6Al2(OH)18(s) + MO4- Mg6Al2(OH)17(MO4)(s) + OH- K2* Mg6Al2(OH)17(MO4)(s) + MO4- Mg6Al2(OH)16(MO4)2(s) + OH-  Equilibrium constants K1, K2 [LDH(MO4)(s)]eq[OH-]eq K1 =  [LDH(s)]eq[MO4-]eq [LDH(MO4)2(s)]eq[OH-]eq2 K 2 =  = K1K2 [LDH(s)]eq[MO4-]eq2  Evaluation of Kn [MO4-]eq [MO4-]eq2 K1  + 2K2  [MO4-]sorbed [OH-]eq [OH-]eq  =  [LDH]0[MO4-]eq [MO4-]eq2 1 + K1  + K2  [OH-]eq [OH-]eq K1 = 1.40  0.11 K2 = 0.47  0.20 Plot of [MO4-]sorbed/[LDH]0vs [MO4-]eq/[OH-]eq

  9. Evaluation of Equilibrium Constants for CrO42- Plot of [CrO42-]sorbed/[LDH]0vs [CrO42-]eq/[OH-]eq2  Equilibrium reaction of ion-exchange K Mg6Al2(OH)18(s) + 2 CrO42- Mg6Al2(OH)17(CrO4)2(s) + 2OH-  Equilibrium constants K [LDH(CrO4)(s)]eq[OH-]eq2 K=  [LDH(s)]eq[CrO42-]eq  Evaluation of K [MO4-]eq K  [CrO42]sorbed [OH-]eq2  =  [LDH]0[MO4-]eq 1 + K  [OH-]eq2 - Evaluation of K by non-linear least squares fit: K = 25.3  3.5

  10. Relative Fraction of Sorbed MO4-  Relative fraction of sorbed MO4- [LDH(MO4)(s)]eq K1x n = 1 :  =  [MO4-]sorbed K1x + 2K2x2 2[LDH(MO4)2(s)]eq 2K2x2 n = 2 :  =  [MO4-]sorbed K1x + 2K2x2 where x = [MO4-]eq/[OH-] eq

  11. Relative Fraction of LDHs  Relative fraction of LDHs [LDH(s)]eq 1 n = 0 :  =  [LDH]0 1 + K1x + K2x2 [LDH(MO4)(s)]eq K1x n = 1 :  =  [LDH]0 1 + K1x + K2x2 [LDH(MO4)2(s)]eq K2x2 n = 2 :  =  [LDH]0 1 + K1x + K2x2 where x = [MO4-]eq/[OH-] eq

  12. Spectroscopic Results of LDHs  FT-IR spectra  27Al MAS NMR LDH(CrO4) LDH(CrO4) Oh Td calcined LDH calcined LDH LDH(CO3) LDH(CO3)

  13. Characteristics of Solid Phases  Powder XRD patterns of LDHs LDH(ReO4)2 LDH(ReO4)2 Mixture of LDH(OH)2 & LDH(ReO4)2 calcined LDH LDH(CO3) LDH(OH)2

  14. Schematic Model for Anion-Exchange  Powder XRD patterns of LDHs  Schematic model for anion-exchange LDH(CrO4) Mixture of LDH(OH)2 & LDH(CrO4) LDH(OH)2

  15. Conclusions  The sorption of TcO4-, ReO4- or CrO42- on calcined LDH is found to be a stepwise ion-exchange reaction: generation of LDH hydroxide, Mg6Al2(OH)18, as an intermediate and then replacement of the OH- by TcO4-, ReO4- or CrO42- ions.  The equilibrium constants for ion-exchange reaction are obtained by a non-linear least squares fit procedure. MO4- (M=Tc or Re):K1 = 1.40  0.11K2 = 0.47  0.20 CrO42- : K = 25.3  3.5  The results of powder XRD, FT-IR and 27Al MAS NMR spectroscopy reveal that the layered structure destroyed by calcination is reconstructed after intercalating ReO4- or CrO42- in the aqueous salt solution.  The XRD patterns of mixture of LDH(CrO4) and LDH(OH)2 demonstrate that an intermediate of the sorption process is LDH hydroxide. It is proposed that the CrO42- ion incorporates in the interlayer through a parallel route.

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