Patent Number: 048083186
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT These and other objects of the invention for recovering and fixing radioactive cesium ions for long-term storage may be met by contacting a feed solution containing the cesium together with other metal ions with a hydrated sodium phlogopite mica which has a c-axis spacing of about 12.23.ANG., whereby the cesium is selectively absorbed by the modified phlogopite, maintaining said contact until sufficient cesium ions are absorbed to reduce the c-axis spacing to at least about 11.58.ANG., and separating the modified phlogopite containing cesium from the feed solution, thereby recovering the cesium ions from the solution and fixing the ions for long-term storage. Alternatively, the modified phlogopite containing the absorbed cesium ions can be heated to at least 150.degree. C. for a period of time sufficient to dehydrate the modified phlogopite thus reducing the c-axis spacing and fixing the cesium ions for long-term storage. The modified phlogopite is prepared by the method described in Clays Clay Miner 14, 69 (1966) incorporated herein by reference. As described therein, naturally occuring phlogopite mica, having the formula: KMg.sub.3 Si.sub.3 AlO.sub.10 (OH).sub.2 is finely ground to about 0.2 to 20 um particle size and contacted with a solution of about 1.0 N NaCl, 0.3 N sodium tetraphenylboron (NaTPB) and 0.01 M ethylenediamine-tetracetic acid (EDTA) for a period of several hours. This results in a complete depletion of K.sup.+ ions from the interlayers of the phlogopite mica and the simultaneous saturation of the interlayers with Na.sup.+ ions along with a monolayer of water molecules. This treatment results in a phase with 12.23.ANG.c-axis (001) spacing as opposed to the original phlogopite mica which has a c-axis spacig of 10.03A. This 12.23.ANG. phase is ideally NaMg.sub.3 Si.sub.3 AlO.sub.10 (OH).sub.2.H.sub.2 O or hydrated sodium phlogopite. Contact between the solution containing cesium ions may take place by passing the solution through a packed bed or column of the modified phlogopite. Alternatively, the modified phlogopite may be mixed with the solution containing the cesium ions and recovered by filtering. The modified phlogopite is very selective for cesium ions and should be able to selectively recover cesium ions from the presence of any other metal ions. The theoretical capacity of the modified phlogopite for cesium ions is about 210 meq/100g. However, the maximum cesium loading which can be attained is about 93.7 meq/100g at which loading a cesium sodium phlogopite mica is formed. This incomplete cesium exchange can be explained by the fact that the interlayer spacing significantly collapses to about 11.58.ANG. when about half of the exchange sites are occupied by cesium. This collapse of the c-axis or interlayer spacing by about 0.65.ANG. is effective in preventing any further exchange of cesium ions from solution. Therefore, just as the cesium ions cannot enter the structure after the initial exchange, the cesium ions that entered the structure cannot escape from the collapsed interlayers, effectively leading to the fixation of the cesium ions. The collapse of the interlayers is explained by dehydration of the ions in the interlayer because of the high charge density of the layers and the low hydration energy of the cesium ions. Once the cesium ions have entered the phlogopite structure, they are fixed and not subject to displacement. However, collapse of the c-axis or interlayer structure and total fixation of the cesium is not believed to occur until the modified phlogopite contains about 80 meq of cesium. The spacing can be reduced and the mica formed when the cesium loading is less than about 80 meq by heating the cesium containing phlogopite to at least 150.degree. C. for a period of time sufficient to partially dehydrate the phlogopite and reduce the c-axis spacing. Generally, a heating time of about an hour, depending on the size of the sample has been found sufficient. Since interlayer collapse is believed caused by dehydration, there is no minimum loading of cesium on the modified phlogopite before fixation can take place by heating. However, a modified phlogopite containing no cesium ions and only sodium ions will require higher temperatures before any dehydration can take place. The following examples are given to illustrate the invention and are not to be taken as limiting the scope of the invention which is defined by the appended claims. EXAMPLE I 5 grams of phlogopite mica was ground to a fine powder having a particle size ranging from about 0.2 to 20.0 um. 5 grams of this powder was contacted with 100 ml of an aoueous solution of about 1.0 N NaCl, 0.3 N sodium tetraphenylboron and 0.01 H ethylene-diamine tetraacetic acid for a period of 24-48 hours. The powder was removed from the solution, washed with water and acetone, dried and characterized by powder x-ray diffraction. The diffraction showed that the c-axis (001) spacing was 12.23.ANG., as shown in FIG. 2. This is compared to the original phogopite mica spacing of 10.03.ANG. shown by FIG. 1. EXAMPLE II A 0.015 gm sample of modified phlogopite of Example I was placed in 15 ml of a CsCl solution containing 26.5 mg Cs per ml for 4 days. The solid and solution phases were separated by centrifugation after equilibration. The solution was analyzed for Cs.sup.+ by atomic absorption spectroscopy (AAS), and the solid phase was characterized by powder x-ray diffraction (XRD). In a similar manner, a number of tests were made with a constant solid solution ratio, but increasing amount of cesium. The cesium exchange solution of modified phlogopite in the presence of pure CsCl solutions showed that a steady state was attained at a cesium loading of 124.5 mq/gm which is the equivalent of 93.7 meq/100 g in the presence of Na.sup.+ released from the interlayers during equilibration. The results are shown in FIG. 4. The K-depleted phlogopite mica has a theoretical exchange capacity of about 210 meq/100 g so that the cesium exchange that occured was incomplete. EXAMPLE III The cesium loaded philogopite of Example II was characterized by powder x-ray diffraction. FIG. 3 is a diffractogram which shows that the c-axis spacing decreased to 11.58.ANG. from 12.23.ANG.. Further examination shows that cesium mica has formed as revealed by the 10.65.ANG.c-axis spacing that can be derived from the d(002), d(003) and d(004) spacings of 5.326.ANG., 3.557.ANG. and 2.661.ANG. respectively. EXAMPLE IV 0.020 gms of the modified phlogoplte as prepared in Example I was contacted with a solution containing 25 ml containing 0.0002 M CsCl, 0.01 M CaCl.sub.2 and 0.04 M NaNO.sub.3 to determining the selectivity of the phlogopite for cesium ions in the presence of excess Na.sup.+ and Ca.sup.++ ions. In a similar manner, like quantities of other cation exchangers known to have an affinity for cesium ions were also tried. The results are shown in Table I. TABLE I ______________________________________ Cesium exchange, K.sub.d (ml/g) Sample 0.01 M CaCl.sub.2 0.04 M NaNO.sub.3 ______________________________________ K-depleted phlogopite mica 664,000 949,000 gamma-zirconium phosphate 27,700 16,000 Mordenite, Nevada 165,000 4,300 Phillipsite, Nevada 34,500 9,800 Clinoptilolite, California 16,600 4,400 ______________________________________ As shown in the Table, the selectivity of the modified phlogopite is much higher than that of the other cation exchangers used for cesium recovery. EXAMPLE V 0.010 gm of the modified phlogopite containg varying amounts of cesium was placed in a solution composed of 10 ml of solution consisting of 0.005 M CaCl.sub.2, 0.001 M MgCl.sub.2, 0.00025 M KCl and 0.001 M NaCl, which are the ions most abundant in natural waters. The phlogopite was allowed to soak for 24 hours before being removed by centrifugation, and dried. The solution was then analyzed by atomic absorption spectroscopy. The results are shown in Table II below. TABLE II ______________________________________ Initial amount of Cs Amount of Cs released, Percentage of exchanged, meq/100 g meq/100 g Cs released ______________________________________ 84.4 0.053 0.06 80.7 0.068 0.08 50.0 0.045 0.09 19.9 0.038 0.19 ______________________________________ As shown by the Table, very little cesium was released. Even the small amounts released appear to have been displaced from the external surfaces because the modified phlogopite has an exchange capacity of about 3 to 4 meq/100 g on the surface. Thus, all the cesium that entered the interlayers of modified phlogopite and most of the surface exchanged cesium was not released by the above treatment. This data clearly shows that the cesium has been immobilized by the modified phlogopite at room temperature without any additional treatment. EXAMPLE VI 0.15 grams of modified phlogopite containing about 50 meq cesium as heated to about 200.degree. C. for one hour in order to fix the cesium within the intelayer. The results of a powder x-ray diffraction are shown in FIG. 5 superimposed on an unheated sample. In a like manner, 0.015 gm sample o modified phlogopite containing 19.9 meq cesium/100 grams was also heated. The results are shown in FIG. 6, also superimposed on an unheated sample. The diffractograms show that the heat treatment at only 200.degree. C. for one hour decreased the c-axis spacing to 10.13-10.14.ANG. from about 12.0.ANG.. This decrease of the c-axis spacing is a result of the dehydration and collapse of the interlayers and thus trapping cesium. As can he seen from the above described specification and examples, the invention for the recovery and storage of cesium in a hydrated, sodium phlogopite mica provide a suitable new material for the decontamination, fixation and long-term storage of cesium.