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 2 Radioactive Waste Disposal Team, Korea Atomic Energy Research Institute

Inorganic Materials Lab. SKKU 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. TcO 4 - and CrO 4 2- are one of hazard elements in the nuclear and industrial wastes, respectively. The sorption reaction of aquatic TcO 4 - or CrO 4 2- 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 TcO 4 - or CrO 4 2- on calcined LDH was proposed to be stepwise ion-exchange reaction: generation of LDH hydroxide, Mg 6 Al 2 (OH) 18, as an intermediate and then replacement of the OH - by TcO 4 - or CrO 4 2- ions. The equilibrium constants (K) for ion-exchange reactions Mg 6 Al 2 (OH) 18 (s) + 2TcO 4 - = Mg 6 Al 2 (OH) 16 (TcO 4 ) 2 (s) + 2OH - or Mg 6 Al 2 (OH) 18 (s) + CrO 4 2- = Mg 6 Al 2 (OH) 16 (CrO 4 )(s) + 2OH - were evaluated by a non-linear least squares fit procedure. The ReO 4 - was used as a TcO 4 - surrogate. Calcined Mg/Al LDH, before and after the sorption reaction with ReO 4 - or CrO 4 2- was characterized to study reaction mechanism. The analyses by powder X-ray diffraction, 27 Al 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 ReO 4 - or CrO 4 2- in the prepared aquatic solution. The detailed XRD analysis for the CrO 4 2- 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.

Inorganic Materials Lab. SKKU  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 -, CrO 4 2-, and PO 4 3- 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. TcO 4 - 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 TcO 4 - or CrO 4 2- with calcined LDH as an inorganic sorbent. Emphasis of the work is placed upon understanding the reaction equilibrium and mechanism. Introduction

Inorganic Materials Lab. SKKU Schematic Representation and General Properties  Schematic representation  General formula of synthetic LDH M(II) 1-x M(III) x (OH) 2 (A m- ) x/m nH 2 O M(II) = Mg, Ni, Zn M(III) = Al, Cr, Fe A m- = exchangeable anion 0.2 ≤ x ≤ 0.4  Anion-exchange property : high anion-exchange capacity: 2  5 meq g -1 Mg 6 Al 2 (OH) 16  (Cl) 2 + CO 3 2- Mg 6 Al 2 (OH) 16  CO 3 + 2Cl -  Memory effect 450  600  C Mg 6 Al 2 (OH) 16  CO 3 Mg 6 Al 2 O 9 + CO 2 + H 2 O A n- Mg 6 Al 2 O 9 + H 2 O Mg 6 Al 2 (OH) 16  A n- + OH -  Use in removing anionic species from wastewater A n- : Cr 2 O 7 2-, CrO 4 2-, HPO 4 2-, TcO 4 -, SO 4 2-, MnO 4 -

Inorganic Materials Lab. SKKU Experimental Sections  Materials & Methods TcO 4 - :0.3M NH 4 TcO 4 / 0.1M NH 4 OH / 0.01M HTcO 4 stock solution ReO 4 - :0.3M NH 4 ReO 4 / 0.01M HReO 4 stock solution CrO 4 2- :0.1 M Na 2 CrO 4. 4H 2 O / 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 TcO 4 - : Liquid Scintillation Analysis (290keV  -radiation of Tc-99) ReO 4 -, CrO 4 2- : UV-Visible Spectroscopy and ICP-AES  Sorption reactor with inert atmosphere and constant temperature

Inorganic Materials Lab. SKKU Sorption Experiments with TcO 4 - and ReO 4 - MO 4 - m LDH Vol. [LDH] 0 * [MO 4 - ] 0 [MO 4 - ] eq [MO 4 - ] sorbed pH 0 pH eq (mg)(ml) (mol/L) (mol/L)(mol/L) (mol/L) TcO E E-59.60E E E E-56.24E E E E-44.46E E E E-32.50E E E E-54.16E E E E-56.43E E E E-51.02E E E E-51.20E E ReO E E-43.29E E E E-41.33E E E E-32.96E E E E-36.75E E E E-32.81E E E E-37.70E E  Mg 6 Al 2 O 9 : M = g/mol

Inorganic Materials Lab. SKKU Sorption Experiments with CrO 4 2- m LDH Vol.[LDH] 0 * [CrO 4 2- ] 0 [CrO 4 2- ] eq [CrO 4 2- ] sorbed [OH - ] 0 [OH - ] eq (mg)(ml)(mol/L)(mol/L)(mol/L)(mol/L)(mol/L)(mol/L) E-31.02E-32.92E-61.02E-38.26E-310.2E E E-34.38E-61.28E-37.75E-310.1E E E-35.12E-61.27E-37.75E-310.1E E E-31.68E-51.51E-37.23E-310.3E E E-32.63E-51.76E-36.71E-39.91E E E-33.28E-51.76E-36.71E-39.93E E E-31.72E-41.87E-36.20E-39.82E E E-31.33E-41.91E-36.20E-310.3E E E-31.33E-41.91E-36.20E-39.84E E E-32.42E-42.06E-35.68E-39.64E E E-32.82E-42.02E-35.68E-39.44E E E-35.25E-42.03E-35.16E-39.01E E E-34.50E-42.10E-35.16E-39.18E E E-36.51E-42.16E-34.65E-38.77E E E-37.22E-42.09E-34.65E-38.63E E E-31.01E-32.05E-34.13E-37.96E E E-31.14E E-33.61E-37.75E E E-31.18E E-33.61E-37.54E E E-31.36E E-33.10E-37.15E E E-31.36E E-33.10E-37.13E E E-31.56E E-32.58E-36.70E E E-31.68E E-32.58E-36.64E E E-31.91E E-32.07E-36.15E E E-32.03E E-31.55E-35.71E E E-32.23E E-31.55E-35.75E E E-32.29E E-31.03E-35.25E E E-32.56E E-35.16E-44.76E E E-32.75E E-36.31E-64.33E E E-32.77E E-36.31E-64.30E E E-32.77E E-36.31E-64.24E E E-32.75E E-36.31E-64.27E-3  Mg 6 Al 2 O 9 : M = g/mol

Inorganic Materials Lab. SKKU Evaluation of Equilibrium Constants for MO 4 - (M: Tc or Re)  Equilibrium reaction K 1 Mg 6 Al 2 (OH) 18 (s) + MO 4 - Mg 6 Al 2 (OH) 17 (MO 4 )(s) + OH - K 2 * Mg 6 Al 2 (OH) 17 (MO 4 )(s) + MO 4 - Mg 6 Al 2 (OH) 16 (MO 4 ) 2 (s) + OH -  Equilibrium constants K 1, K 2 [LDH(MO 4 )(s)] eq [OH - ] eq K 1 =  [LDH(s)] eq [MO 4 - ] eq [LDH(MO 4 ) 2 (s)] eq [OH - ] eq 2 K 2 =  = K 1 K 2  [LDH(s)] eq [MO 4 - ] eq 2  Evaluation of K n [MO 4 - ] eq [MO 4 - ] eq 2 K 1  + 2K 2  [MO 4 - ] sorbed [OH - ] eq [OH - ] eq  =  [LDH] 0 [MO 4 - ] eq [MO 4 - ] eq K 1  + K 2  [OH - ] eq [OH - ] eq K 1 = 1.40  0.11K 2 = 0.47  0.20  Plot of [MO 4 - ] sorbed /[LDH] 0 vs [MO 4 - ] eq /[OH - ] eq

Inorganic Materials Lab. SKKU Evaluation of Equilibrium Constants for CrO 4 2-  Plot of [CrO 4 2- ] sorbed /[LDH] 0 vs [CrO 4 2- ] eq /[OH - ] eq 2  Equilibrium reaction of ion-exchange K Mg 6 Al 2 (OH) 18 (s) + 2 CrO 4 2- Mg 6 Al 2 (OH) 17 (CrO 4 ) 2 (s) + 2OH -  Equilibrium constants K [LDH(CrO 4 )(s)] eq [OH - ] eq 2 K =  [LDH(s)] eq [CrO 4 2- ] eq  Evaluation of K [MO 4 - ] eq K  [CrO 4 2 ] sorbed [OH - ] eq 2  =  [LDH] 0 [MO 4 - ] eq 1 + K  [OH - ] eq 2 - Evaluation of K by non-linear least squares fit: K = 25.3  3.5

Inorganic Materials Lab. SKKU Relative Fraction of Sorbed MO 4 -  Relative fraction of sorbed MO 4 - [LDH(MO 4 )(s)] eq K 1 x n = 1 :  =  [MO 4 - ] sorbed K 1 x + 2K 2 x 2 2[LDH(MO 4 ) 2 (s)] eq 2K 2 x 2 n = 2 :  =  [MO 4 - ] sorbed K 1 x + 2K 2 x 2 where x = [MO 4 - ] eq /[OH - ] eq

Inorganic Materials Lab. SKKU Relative Fraction of LDHs  Relative fraction of LDHs [LDH(s)] eq 1 n = 0 :  =  [LDH] K 1 x + K 2 x 2 [LDH(MO 4 )(s)] eq K 1 x n = 1 :  =  [LDH] K 1 x + K 2 x 2 [LDH(MO 4 ) 2 (s)] eq K 2 x 2 n = 2 :  =  [LDH] K 1 x + K 2 x 2 where x = [MO 4 - ] eq /[OH - ] eq

Inorganic Materials Lab. SKKU Spectroscopic Results of LDHs  FT-IR spectra  27 Al MAS NMR LDH(CO 3 ) calcined LDH LDH(CrO 4 ) LDH(CO 3 ) calcined LDH LDH(CrO 4 ) TdTd OhOh

Inorganic Materials Lab. SKKU Characteristics of Solid Phases  Powder XRD patterns of LDHs LDH(CO 3 ) calcined LDH LDH(ReO 4 ) 2 LDH(OH) 2 Mixture of LDH(OH) 2 & LDH(ReO 4 ) 2

Inorganic Materials Lab. SKKU Schematic Model for Anion-Exchange  Schematic model for anion-exchange  Powder XRD patterns of LDHs Mixture of LDH(OH) 2 & LDH(CrO 4 ) LDH(OH) 2

Inorganic Materials Lab. SKKU Conclusions  The sorption of TcO 4 -, ReO 4 - or CrO 4 2- on calcined LDH is found to be a stepwise ion-exchange reaction: generation of LDH hydroxide, Mg 6 Al 2 (OH) 18, as an intermediate and then replacement of the OH - by TcO 4 -, ReO 4 - or CrO 4 2- ions.  The equilibrium constants for ion-exchange reaction are obtained by a non-linear least squares fit procedure. MO 4 - (M=Tc or Re):K 1 = 1.40  0.11K 2 = 0.47  0.20 CrO 4 2- : K = 25.3  3.5  The results of powder XRD, FT-IR and 27 Al MAS NMR spectroscopy reveal that the layered structure destroyed by calcination is reconstructed after intercalating ReO 4 - or CrO 4 2- in the aqueous salt solution.  The XRD patterns of mixture of LDH(CrO 4 ) and LDH(OH) 2 demonstrate that an intermediate of the sorption process is LDH hydroxide. It is proposed that the CrO 4 2- ion incorporates in the interlayer through a parallel route.