Background Art Na-4-mica with theoretical chemical composition of Na4Mg6Al4Si4020F4 xH2O exhibits exceptionally high selectivity and uptake ability for harmful radioactive and heavy metal cations, and high stability.

It is clearly different from both nonswelling phlogopites and expandable montmorillonites because it readily hydrates and swells despite its high negative layer charge. Furthermore, Na-4-mica exhibits high cation-exchange capacity comparable to those of Al-rich zeolites.

It is known that the interlayer region of trioctahedral mica-type 2: 1 layers accommodates unusually large number of hydrated Na and these hydrated Na+ ions undergo irreversible cation-exchange reaction. And it has generated a lot of interest because this characteristic can give the material exceptionally high selectivity and fixation capacity of harmful divalent radioactive and heavy metal cations such as Sr2+, Ba2+, and Ra2+. In the field of environmental purification, it has been a principal issue to decontaminate these cations for a long time.

Although several improved methods have reported since the first synthesis method of swelling Na-4-mica was developed in 1972, all those methods have some drawbacks such as complicated pretreatment and postpreparation procedure and some impurities found in the products. Therefore commercially useful synthesis method of phase-pure Na-4-mica is not known yet.

Recently, a newly reported synthesis method of swelling Na-4-mica known as"all-in-one method"has generated a lot of interest because it largely simplified synthesis process of Na-4-mica. In this method swelling Na-4-mica is synthesized by thermally treating Si source, Al source and NaF mixture at 890 C for about 24 hours.

However, this method also has a drawback in the fact that it is difficult to

obtain pure product through the method. The product contains lots of impurities besides swelling Na-4-mica crystalline phase, because NaF used as a mineralizer and reaction medium has very high melting point of 992 C and high reactivity with other inorganic materials. Furthermore, this method requires complicated postpreparation procedures comprising washing with water- washing with boric acid saturated solution--+ saturation of exchangeable sites by Na+ ions-j washing with water. This postpreparation procedure is not only tedious but also contains possibility of introducing secondary environmental contamination such as discharging corrosive fluorine waste solution.

Therefore it would be very helpful for solving purification problem of harmful bivalent radioactive and heavy metal ions if an effective and simple method for synthesizing phase-pure Na-4-mica without tedious postpreparation procedure.

The present invention has been made in view of the above problems.

Therefore, it is an object of the present invention to provide a novel method for synthesizing phase-pure swelling Na-4-mica without any impurities through a process with simplified postpreparation step, and utilize it in the industrial purpose such as purification of harmful radioactive and heavy metal materials.

Disclosure of the Invention The synthesis method for swelling Na-4-mica according to the present invention basically comprising: thermally treating mixture of Si source, Al source, MgF2 and NaCl with the molar compositions of 1-2 (based on the Si atom): 1~2 (based on the Si atom): 2~4 : 2-6 at the temperature of 600-1000 C for 0. 5-72 hours.

It is desirable to use within the above range to avoid the production of lots of impurities. The most desirable molar ratio of the reactants is 2: 2: 3: 2-6, that is the optimum molar ratio to produce phase-pure swelling Na-4-mica. If the molar ratio is beyond this range, some impurities are produced at the same time. If the treating temperature or treating time is less than the above range, the reaction is insufficient, and if the treating temperature or treating time is more than the above range, lots of impurities are produced together.

The desirable Si source according to the above method includes fumed silica or silica gel, and mixtures thereof, and the desirable Al source includes A1203, bOehmite (AlOOH) or A1 (OH) 3, and mixtures thereof.

More desirably, the synthesis method of phase pure Na-4-mica according to the present invention further comprising the postpreparation step of washing the

synthesized Na-4-mica with water.

This postpreparation step remarkably increases the portion and characteristics of synthesized Na-4-mica crystal.

On the other hand, the Na-4-mica synthesized through the method according to the present invention can be used in decontamination of environmental pollutants by contacting them with radioactive or heavy metal ion contaminated material and adsorbing them.

Phase pure swelling Na-4-mica synthesized by the method according to the present invention has higher uptake capacity of harmful bivalent radioactive and heavy metal cations. That is because the interlayer structure of Na-4-mica synthesized by the method according to the present invention is not deteriorated with the exchange reaction of large cations such as Sr2+ or Ba2+.

In the case of the Na-4-mica synthesized according to the conventional method, the uptake capacity of caion is much lower than its theoretical number because the interlayer structure is collapsed by the exchange reaction with the large cation.

However, phase pure swelling Na-4-mica synthesized by the method according to the present invention does not include anhydrous impurities which grow up between layers, so it is possible to avoid localized collapse of layer, and have much higher and stable cation uptake capacity than that of Na-4-mica synthesized according to the conventional method.

Accordingly, using the phase pure swelling Na-4-mica synthesized by the method according to the present invention, we can remove harmful radioactive and heavy metal ions from the contaminated material with these ions.

Brief Description the Drawings The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing the synthesizing process of swelling Na- 4-mica according to the present invention; Figs. 2a and 2b are powder X-ray diffraction patterns of as-synthesized and washed products obtained in example 2 and comparative example 1 ; Fig. 3 shows the 27A1 NMR spectrum of the product obtained in example 2; Fig. 4 shows the scanning electron micrographs of the products obtained in example 1-3 ;

Fig. 5 is a graph showing the Sr2+ adsorption behavior of the product obtained in example 2.

Best Mode for Carrying Out the Invention Example 1 : Synthesis of swelling Na-4-mica Si02 (fumed silica from Aldrich Chemical Co. ) and AIOOH (bOehmite from Condea Chemie) were employed as Si and Al sources. And Si02, AIOOH, MgF2, and NaCl with molar ratio of 2: 2: 3: 2 (example 1), 4 (example 2), 6 (example 3) were mixed together. And this reactant mixture was well mixed by grinding, then thermally treated in a Pt crucible at 900 C for 15 hours. After cooling to room temperature, the obtained solid (as-synthesized product) was washed several times with deionized water, and dried at 105 C for 12 hours. The resultant Na-4-mica was obtained as the product (washed product).

Fig 1 shows a block diagram of above described procedure. As shown in Fig 1, the postpreparation procedure is greatly simplified according to the present invention.

Comparative Example 1: Synthesis of Na-4-mica according to the all-in-one method Si02, A1203, MgO, and NaF with molar ratio of 3: 2: 2: 7 were mixed together and thermally treated in a Pt crucible at 900 C for 18 hours. Then the resultant material (as-synthesized) was sequentially washed with H3BO3-saturated solution, deionized water and 1N-NaCl solution several times. The resultant solid was dried at 105 C for 12 hours to obtain the resultant product of swelling Na-4- mica (washed product).

The synthesis method of comparative example 1 is more complicated in postpreparation process i. e. , washing process, and produced lots of waste solution.

To compare characteristics of the swelling Na-4-mica obtained from the method of present invention to that of conventional method, the following analytic experiments were carried out using the products of example 1~3 and comparative example 1.

Experiment 1 : XRD patterns of the products To compare the characteristics of the crystal in the products obtained from

example 2 and comparative example 1, powder X-ray diffraction patterns of as- synthesized and washed products obtained from example 2 and comparative example 1 were prepared using X-ray diffraction analysis method, and the results are shown in Figs 2a and 2b respectively.

These XRD patterns were recorded by Ni-filtered Cu Ka radiation at 40kV and 100mA at a scamming speed of 0. 1° 26/min (Phillips, X'pert). The d spacings were determined from Ka 1 radiation (A, =1. 5405A) after ka 2 stripping.

As shown in Fig 2a, as-synthesized products exhibited the main basal peak of hydrated Na-4-mica phase, regardless of the synthesis method. However, the peaks attributed to the anhydrous mica phase (d spacing of 9. 9 A), the impurities and NaF were also observed in the XRD patterns of the comparative example 1 (all-in- one products). In contrast, the as-synthesized product of example 2 did not show any of the peaks attributed to the impurities including anhydrous mica, excluding the peaks assigned to NaCl.

As shown in Fig 2b, XRD patterns of washed products of comparative example 1 (all-in-one products) shows that the postpreparation procedure such as washing with H3BO3-saturated solution, Na+ saturation and washing with water resulted in a phase transition of anhydrous mica to hydrated Na-4-mica, but peaks attributed to impurities at the d spacings of 4.21 and 2.61 A were still remained.

However, the XRD pattern of products of example 2 shows that simple water washing resulted in a complete transition to the 001 peaks of the hydrated Na- 4-mica without any peaks attributed to impurities. NaCl peaks also completely disappeared.

As a result, it is confirmed that it is possible to synthesize swelling Na-4- mica completely without any impurities by the method according to the present invention, without any complicated postpreparation procedure such as washing with H3BO3-saturated solution and Na saturation.

Experiment 2 : 27 Al NMR spectrum Phase pure Na-4-mica (washed product) synthesized in example 2 was examined by 27Al MAS NMR, and the result is shown in Fig 3.

The 27Al spectrum exhibited one strong peak at the chemical shift value of 60.5 ppm that was attributed to tetrahedral Al atoms, but no noticeable peaks near 0 ppm were detected, which indicated the absence of octahedral Al atoms.

This result suggests that all Al atoms are incorporated into tetrahedral sheets of phase pure Na-4-mica and no impurities exist.

Experiment 3: Morphological analysis using electron microscope Scanning electron micrographs of products synthesized in example 1-3 with molar ratio of NaCl/AlOOH 2,4, 6 respectively are shown in Fig 4.

As shown in Fig 4, Na-4-mica synthesized according to the present invention revealed hexagonal crystallites typical of mica morphology, and the morphologies such as granule or sphere that were frequently found in the products synthesized through conventional methods were not detected.

In addition, the crystal size of phase pure Na-4-mica synthesized by the method according to the present invention is significantly affected by the composition of the initial reactant mixture. Especially, larger molar ratio of NaCl/AlOOH led to smaller crystal size as well as more homogeneous particle size distribution. The approximate crystal size of 3, um similar to those of the reported, was obtained when the molar ratio of NaCl/AlOOH was adjusted to the value of 2.0, whereas the increase of the above molar ratio to a value of 6.0 led to a decreased crystal size of approximate 2gm in length.

The above result clearly shows that the crystal size of the Na-4-mica could be manipulated in the range of about 2-5, um, simply by controlling the NaCl content in the initial reactant mixture.

Because Na-4-mica has layered structure, the crystal size greatly influences the cation-exchange knetics of harmful material such as radioactive and heavy metal ions. It was shown that faster cation-exchange reaction could be accomplished by smaller crystal size because of its enlarged surface area. Therefore the fact that it is possible to manipulate the crystal size simply by controlling the NaCl content has significant meaning at the view of industrial applicability Experiment 4: Sr2+ exchange property of swelling Na-4-mica Adsorbed amount of Sr2+ versus equilibrium time was examined to estimate the selectivity and adsorption behavior of the washed product synthesized in Example 2 by the method according to the present invention, and the result is shown in Fig 5.

The uptake isotherm of Fig 5 reveals the experimental result of Sr2+ adsorption behavior in the 2mN Sr2+ solution adjusted to pH 4.0 in the presence of 0.5M NaCl.

Precisely, it was made out by equilibrating 50mg of the NaCl melt product in 20nit of the 2mN Sr2+ solution in the presence of 0.5M NaCl. Sr2+ concentration in the solution was analyzed by atomic absorption spectroscopy (Shimadzu AA6601F) after the solid was removed by filtration. Uptake amounts were

determined by the difference of Sr2+ concentrations between initial and equilibrated solutions at the time.

As shown in Fig 5, the Sr2+ uptake by the Na-4-mica synthesized by the method according to the present invention was found to proceed very slowly and steadily, and even 5 weeks was not enough for the reaction to be equilibrated.

On the other hand, any noticeable structural deterioration by the radio active material uptake was found.

From the above results, it is confirmed that Na-4-mica synthesized by the method according to the present invention is useful to remove radioactive and heavy metal ion from seawater with high salt concentration or atomic waste.

Industrial Applicability According to the present invention, the bulk synthesis of swelling Na-4- mica that does not require tedious postpreparation procedures is possible.

Especially, the swelling Na-4-mica synthesized by the method according to the present invention is useful to remove radioactive and heavy metal from seawater with high salt concentration or atomic waste, because it reveals exceptionally high adsorption capacity of large divalent metal ions.